01 Imaging Modalities Overview
Diagnostic imaging is the most powerful clinical tool for non-invasive diagnosis, surveillance, and procedural guidance. Every clinician — not just radiologists — must be able to select the right study, understand its limitations, and interpret the most common findings. Imaging modalities are distinguished by the physical energy used to generate contrast: ionizing radiation (radiographs, CT, fluoroscopy, nuclear medicine), magnetic fields and radiofrequency (MRI), or mechanical sound waves (ultrasound). Each produces a characteristic appearance and is suited to specific anatomy and pathology.
Why This Matters
Selecting the wrong modality leads to missed diagnoses, unnecessary radiation, contrast harm, and delayed care. A physician who understands modality strengths can order appropriately, interpret independently, and communicate meaningfully with radiology colleagues. The imaging report is a draft — the treating clinician integrates it with the patient.
Modality Comparison
| Modality | Energy | Strengths | Limitations | Radiation |
|---|
| Radiograph (X-ray) | Ionizing photons | Fast, cheap, portable, screens bones/lungs | Low soft-tissue contrast, 2D projection | Low (CXR ≈ 0.1 mSv) |
| CT | Ionizing photons (rotating) | Rapid, high spatial resolution, excellent for trauma, vascular | Radiation dose, IV contrast risk | Moderate–high (5–15 mSv) |
| MRI | Magnetic field + RF | Superb soft-tissue contrast, multiplanar, no radiation | Slow, expensive, claustrophobia, implants | None |
| Ultrasound | Mechanical sound waves | Portable, real-time, no radiation, pediatric/OB | Operator-dependent, limited by gas/bone | None |
| Nuclear medicine | Gamma emission (radiotracer) | Physiologic / functional information | Low spatial resolution, radiation | Variable (1–25 mSv) |
| Fluoroscopy | Continuous X-ray | Dynamic, procedural guidance, GI studies | Cumulative radiation, operator skill | Variable, can be high |
Density & Signal Language
Each modality has a language for describing contrast. Radiographs and CT describe density (radiopaque/white = dense; radiolucent/black = lucent). MRI describes signal intensity (hyperintense = bright; hypointense = dark) on a given sequence. Ultrasound describes echogenicity (hyperechoic = bright; hypoechoic = dark; anechoic = black, typically fluid). Using precise language prevents ambiguity in handoffs and report dictation.
The five densities on plain film are: air (black), fat (dark gray), soft tissue/water (gray), bone/calcium (white), metal (bright white). Memorizing this hierarchy lets you identify foreign bodies, calcifications, and lipomatous lesions at a glance.
Role of the Radiologist and the Clinician
Imaging interpretation is a shared responsibility. The radiologist describes findings, generates differential diagnoses, and communicates urgent results directly. The clinician provides the context (symptoms, exam, labs, prior studies) that determines which findings matter and how they change management. The best diagnoses emerge from a dialogue, not a one-way report. Clinicians who review their own imaging alongside the radiologist develop sharper diagnostic skills and catch important subtleties that context-free reads may miss.
Common Requisition Pitfalls
| Pitfall | Consequence | Prevention |
|---|
| Vague indication ("pain") | Radiologist under-scrutinizes the actual concern | Provide focused clinical question and exam findings |
| Wrong modality | Misses diagnosis, wastes radiation/time | Consult ACR Appropriateness Criteria or radiology |
| No prior studies available | Cannot compare stability vs new findings | Ensure PACS access before ordering |
| No allergy / renal information | Contrast reactions or nephropathy | Check eGFR and allergy history |
| Incorrect laterality | Wrong side imaged | Confirm before signing order |
02 X-Ray & CT Physics
Radiograph Formation
X-rays are generated when high-energy electrons strike a tungsten anode. The beam passes through the patient and is attenuated differentially by tissues of different atomic number and density. The unattenuated photons strike the detector, producing a 2D projection image. Attenuation increases with higher atomic number (bone, iodine) and thicker tissue. The image represents a superimposition of all structures along the beam path — this is both the strength (efficient survey) and limitation (loss of depth) of radiography.
CT Physics
Computed tomography uses a rotating X-ray tube and detector array to acquire projections from multiple angles around the patient. Reconstruction algorithms (filtered back projection or iterative methods) compute a cross-sectional image in Hounsfield units (HU), which quantify tissue attenuation relative to water.
| Tissue | Hounsfield Units | Appearance |
|---|
| Air | −1000 | Black |
| Fat | −100 to −50 | Dark gray |
| Water / CSF | 0 | Gray |
| White matter | 20–30 | Gray |
| Gray matter | 35–45 | Slightly brighter |
| Muscle / soft tissue | 30–60 | Gray |
| Acute blood | 50–80 | Hyperdense |
| Iodinated contrast | 100–500+ | Bright white |
| Bone / calcification | 400–1000+ | Bright white |
| Metal | >3000 | Streak artifact |
CT Windowing
CT images store >4000 HU of data, but the human eye can only discriminate about 30 shades of gray. Windowing selects a range (window width) around a center (window level) to optimize contrast for the tissue of interest. Common windows:
| Window | Level (HU) | Width (HU) | Use |
|---|
| Lung | −600 | 1500 | Lung parenchyma, nodules, emphysema |
| Mediastinum / soft tissue | 40 | 400 | Heart, vessels, lymph nodes |
| Bone | 400 | 1800 | Cortex, trabeculae, fractures |
| Brain | 40 | 80 | Gray/white differentiation, blood |
| Subdural (stroke) | 80 | 200 | Subtle subdural or subarachnoid blood |
| Liver | 60 | 150 | Hepatic lesions, contrast enhancement |
Always review chest CT in both lung and mediastinal windows. A nodule invisible on mediastinal windowing may be obvious on lung windows, and vice versa for lymphadenopathy. Missing this step is the most common reason residents miss findings.
CT Artifacts to Recognize
| Artifact | Cause | Mitigation |
|---|
| Beam hardening / streak | Dense objects (metal, contrast) | Dual-energy reconstruction, metal artifact reduction |
| Motion | Patient movement, breathing | Breath-hold, faster scan, sedation |
| Partial volume | Voxel contains multiple tissue types | Thinner slices |
| Ring artifact | Detector miscalibration | Technical service |
| Photon starvation | Insufficient dose in dense body regions | Increased mAs, body habitus compensation |
03 MRI Physics & Sequences
MRI exploits the magnetic properties of hydrogen protons in tissue. In the main magnetic field (B0, typically 1.5 or 3 Tesla), protons align and precess at the Larmor frequency. A radiofrequency pulse tips them out of alignment; as they relax back, they emit a signal detected by coils. Two relaxation constants determine tissue contrast:
| Constant | Mechanism | Weighted Sequence |
|---|
| T1 (longitudinal) | Return of magnetization to B0 axis | T1-weighted — anatomy |
| T2 (transverse) | Dephasing in xy plane | T2-weighted — pathology, fluid |
Signal Characteristics by Sequence
| Tissue | T1 | T2 | FLAIR | DWI |
|---|
| Water / CSF | Dark | Bright | Dark (suppressed) | Dark |
| Fat | Bright | Intermediate–bright | Bright | Variable |
| Acute infarct | Iso | Bright (hours–days) | Bright | Bright (cytotoxic edema) |
| White matter | Bright | Dark | Dark | Dark |
| Gray matter | Intermediate | Intermediate | Intermediate | Intermediate |
| Blood (subacute) | Bright (methemoglobin) | Bright | Bright | Variable |
| Calcium / cortical bone | Dark | Dark | Dark | Dark |
| Melanin / proteinaceous | Bright | Variable | Variable | Variable |
T1 vs T2 Mnemonic
"Water is bright on T2, fat is bright on both." To identify a sequence at a glance: look at CSF. Dark CSF = T1. Bright CSF = T2. If CSF is dark but brain lesions are bright → FLAIR (fluid-attenuated inversion recovery).
Key MRI Sequences
| Sequence | Contrast | Primary Use |
|---|
| T1 | Fat bright, water dark | Anatomy, fat identification, subacute blood |
| T2 | Water bright | Edema, inflammation, tumor, cysts |
| FLAIR | CSF suppressed, edema still bright | Periventricular MS plaques, SAH, stroke edema |
| DWI / ADC | Restricted diffusion bright on DWI, dark on ADC | Acute infarct (minutes–days), abscess, hypercellular tumor |
| GRE / SWI | Susceptibility artifact from iron/blood | Microhemorrhages, cavernomas, chronic hemosiderin |
| STIR | Fat suppressed, fluid bright | Bone marrow edema, MSK inflammation |
| Post-contrast T1 (gadolinium) | Enhancement = blood-brain barrier breakdown, vascularity | Tumors, infection, MS active plaques |
| MRA / MRV | Flow-sensitive | Vessels without contrast |
A bright DWI lesion with a dark ADC map (true restricted diffusion) is pathognomonic for acute ischemic stroke within minutes to hours of onset. "T2 shine-through" (bright on both DWI and ADC) is not real restriction and should not be called stroke.
04 Ultrasound & Doppler Physics
Ultrasound uses high-frequency (2–18 MHz) sound waves emitted and received by a transducer. Sound reflects at acoustic impedance boundaries; the time delay is used to map depth, and echo amplitude determines pixel brightness. Higher frequencies give better resolution but less penetration; low-frequency probes (2–5 MHz) image deep structures (abdomen, OB), while high-frequency probes (7–15 MHz) image superficial structures (thyroid, vessels, MSK).
Echogenicity Lexicon
| Term | Appearance | Examples |
|---|
| Anechoic | Black, no echoes | Simple cyst, bladder urine, ascites, bile |
| Hypoechoic | Darker than reference tissue | Hematoma, abscess, some tumors |
| Isoechoic | Same echogenicity as reference | Iso-to-liver renal lesion |
| Hyperechoic | Brighter than reference | Fat, gas, calcification, hemangioma |
| Echogenic with shadowing | Bright with posterior dark shadow | Gallstones, kidney stones, bone |
| Posterior acoustic enhancement | Bright behind fluid-filled structure | Simple cysts, bladder |
Doppler Ultrasound
The Doppler effect measures frequency shifts from moving reflectors (red cells) to quantify flow. Color Doppler overlays flow direction on B-mode; pulsed Doppler measures velocity at a specific site. Used to evaluate vascular stenosis, DVT, arterial insufficiency, testicular torsion, and fetal wellbeing.
Ultrasound cannot see through bone or gas. Abdominal US is limited by bowel gas (why patients fast before gallbladder studies). The "acoustic window" is the reason liver, bladder, and gravid uterus are ideal because they transmit sound well.
Ultrasound Artifacts with Diagnostic Value
| Artifact | Appearance | Utility |
|---|
| Acoustic shadowing | Dark shadow posterior to reflector | Gallstones, kidney stones, calcifications, bone |
| Posterior enhancement | Bright area behind fluid | Confirms cystic nature of lesions |
| Comet tail | Reverberation echoes | Adenomyomatosis, crystals |
| Ring-down | Parallel echo reflections | Gas bubbles |
| Mirror image | Reflected duplicate structure | Above diaphragm (identifies liver border) |
| Twinkle artifact | Color Doppler flicker behind object | Urinary stones |
05 Contrast Agents & Safety
Iodinated (CT) Contrast
Intravenous iodinated contrast opacifies vessels and enhancing tissues based on iodine's high atomic number. Modern low- or iso-osmolar non-ionic agents have substantially lower adverse event rates than older high-osmolar ionic agents. Contraindications and considerations:
| Concern | Mechanism | Management |
|---|
| Contrast-induced nephropathy (CIN) | Direct tubular toxicity, renal vasoconstriction | Screen eGFR; hydrate if eGFR <30; hold metformin if risk of AKI |
| Anaphylactoid reaction | Non-IgE mediated (usually); prior reaction is biggest risk factor | Pre-medicate with steroids + diphenhydramine; have epinephrine ready |
| Shellfish / iodine "allergy" | Myth — no cross-reactivity | Treat as general contrast allergy risk; not a contraindication |
| Thyroid storm (hyperthyroid) | Iodine load precipitates storm in untreated Graves | Avoid in active hyperthyroidism; monitor if essential |
| Extravasation | Soft tissue injury at IV site | Elevation, warm compresses; surgical consult if compartment syndrome |
Gadolinium (MRI) Contrast
Gadolinium chelates shorten T1 relaxation, producing bright enhancement on post-contrast T1 images. Gadolinium crosses disrupted blood–brain barriers, highlighting tumors, infection, and active demyelination. Key safety issue: nephrogenic systemic fibrosis (NSF) in patients with severe renal failure (eGFR <30); modern macrocyclic agents carry very low risk but caution persists. Gadolinium deposition in dentate nucleus and globus pallidus has been demonstrated but clinical significance is unclear.
Ultrasound Contrast
Microbubble agents (e.g., perflutren) enhance vascular and lesion characterization on ultrasound, with virtually no renal or thyroid toxicity. Used for liver lesion characterization, echocardiography, and vesicoureteral reflux studies.
Pre-Medication Protocol (Elective)
For patients with prior moderate/severe iodinated contrast reaction: Prednisone 50 mg PO at 13, 7, and 1 hour before scan plus diphenhydramine 50 mg PO/IV 1 hour before. Emergent alternative: hydrocortisone 200 mg IV plus diphenhydramine 50 mg IV immediately and every 4 hours until scan. Pre-medication does not eliminate but significantly reduces reaction risk.
Severity of Contrast Reactions
| Severity | Symptoms | Management |
|---|
| Mild | Urticaria, mild itching, flushing, limited nausea | Observation, diphenhydramine if needed |
| Moderate | Diffuse urticaria, mild bronchospasm, facial edema, vasovagal | Diphenhydramine, bronchodilators, fluids, oxygen |
| Severe | Laryngeal edema, severe bronchospasm, shock, cardiac arrhythmias | Epinephrine (IM 0.3 mg), IV fluids, oxygen, airway support, code team |
06 Radiation Safety & ALARA
ALARA (As Low As Reasonably Achievable) is the foundational principle of radiation safety. Every study should be justified (will it change management?), optimized (appropriate technique and dose), and limited (no repeats without cause). Pediatric patients are disproportionately sensitive to radiation and require weight-based dose reduction.
Effective Dose Reference
| Study | Approx. Effective Dose (mSv) | Equivalent |
|---|
| Chest radiograph (PA + lateral) | 0.1 | 10 days background |
| Mammogram | 0.4 | 7 weeks background |
| Abdominal radiograph | 0.7 | 4 months background |
| CT head | 2 | 8 months background |
| CT chest | 7 | 2 years background |
| CT abdomen / pelvis | 10 | 3 years background |
| CT angiography (PE or coronary) | 10–15 | 3–5 years background |
| PET/CT | 14–25 | 5–8 years background |
| Fluoroscopic procedure (cath, TIPS) | 5–20+ | Highly variable |
MRI Safety Zones
The strong static magnetic field is always on. Ferromagnetic objects become projectiles. Implanted devices must be screened: older pacemakers and ICDs are often unsafe (newer MR-conditional devices are acceptable with protocols); cochlear implants, aneurysm clips, metallic foreign bodies in the eye are contraindications unless proven MR-safe. Gadolinium and heat deposition from RF are additional considerations.
Always ask women of childbearing age about pregnancy before CT or fluoroscopy. For urgent imaging in pregnancy: ultrasound and MRI (without gadolinium if possible) are preferred. When CT is essential, shielding and dose optimization minimize fetal exposure; a single CT abdomen delivers ~25 mGy to the fetus, below the 100 mGy threshold of concern.
MRI Zones
| Zone | Access | Notes |
|---|
| Zone I | Public access | Outside MR environment |
| Zone II | Supervised public | Screening and history occur here |
| Zone III | Restricted | MR-trained personnel only; no unscreened entry |
| Zone IV | Scanner room | Magnet is always on; only cleared patients and personnel |
Gadolinium Classes
| Class | Stability | NSF Risk |
|---|
| Macrocyclic (gadoteridol, gadobutrol, gadoterate) | Highest | Lowest |
| Linear ionic (gadobenate, gadoxetate) | Intermediate | Intermediate |
| Linear non-ionic (gadodiamide, gadoversetamide) | Lowest | Highest (largely withdrawn) |
07 Systematic CXR Approach (A–E)
The chest radiograph is the most frequently ordered imaging study in medicine and the most diagnostically high-yield for the cost. Interpretation discipline is the difference between finding and missing pathology. Always use the same system every time. The classic mnemonic is ABCDEF or the traditional A–E approach.
Technical Assessment First
| Element | Check For | Normal Finding |
|---|
| Projection | PA vs AP vs lateral | PA preferred (heart not magnified) |
| Rotation | Medial clavicle ends equidistant from spinous process | No rotation |
| Inspiration | Count ribs above diaphragm | 8–10 posterior / 5–6 anterior ribs visible |
| Penetration | Thoracic spine faintly visible through heart | Adequate exposure |
| Angulation | Clavicles flat vs bowed up | Orthogonal to detector |
A–E Systematic Review
| Letter | Focus | Key Findings |
|---|
| A — Airway | Trachea midline, carina, main bronchi | Deviation (mass, atelectasis, pneumothorax), narrowing |
| B — Bones & soft tissues | Ribs, clavicles, spine, scapulae, soft tissue | Fractures, lytic/blastic lesions, subcutaneous emphysema |
| C — Cardiac silhouette | Heart size, contour, chamber enlargement | Cardiothoracic ratio <50% on PA (normal) |
| D — Diaphragm | Contour, levels, free air | Right slightly higher than left; no subdiaphragmatic air |
| E — Everything else | Lung fields, pleura, mediastinum, hila, corners | Symmetry, opacities, lines/tubes, review hidden areas |
Hidden Areas — Don't Miss Zones
Always explicitly check: apices (Pancoast tumor, small pneumothorax), behind the heart (retrocardiac pneumonia, hiatal hernia), costophrenic angles (small effusion blunting), below the diaphragm (free air), and bones (clavicle, rib, humerus fractures). Systematic review of these zones catches findings obscured by overlapping structures.
The silhouette sign: loss of a normal interface between two structures of different densities indicates adjacent pathology. Loss of the right heart border = right middle lobe; loss of the left heart border = lingula; loss of the diaphragm = lower lobe. This lets you localize pneumonia on a single frontal view.
08 Pulmonary Parenchymal Patterns
Lung opacities are categorized by distribution (focal vs diffuse), anatomic pattern (airspace vs interstitial vs nodular), and location. Recognizing patterns narrows the differential efficiently.
Airspace vs Interstitial Disease
| Pattern | Findings | Differential |
|---|
| Airspace / consolidation | Fluffy opacities, air bronchograms, segmental/lobar | Pneumonia, pulmonary edema, hemorrhage, aspiration, ARDS |
| Interstitial / reticular | Lines, Kerley B lines, honeycombing | Pulmonary edema (early), ILD, lymphangitic spread |
| Nodular | Well-defined round opacities | Metastases, granulomas, infection |
| Reticulonodular | Mixed lines and nodules | Sarcoidosis, hypersensitivity pneumonitis, miliary TB |
| Ground-glass | Hazy increased density without vascular obscuration (best on CT) | Early edema, infection, PCP, hemorrhage, NSIP |
Common CXR Disease Patterns
| Diagnosis | Classic Findings |
|---|
| Lobar pneumonia | Dense lobar consolidation, air bronchograms, silhouette sign |
| Bronchopneumonia | Patchy, bilateral, lower-lobe predominant opacities |
| Cardiogenic pulmonary edema | Cardiomegaly, cephalization, Kerley B lines, perihilar "bat wing" edema, effusions |
| ARDS | Bilateral diffuse airspace opacities, normal heart size, no effusions |
| COPD / emphysema | Hyperinflation, flattened diaphragms, increased retrosternal airspace, bullae |
| Atelectasis | Volume loss, fissural shift, elevated hemidiaphragm, mediastinal shift toward lesion |
| Lung mass | Discrete opacity >3 cm, spiculated margins, associated adenopathy or effusion |
| Miliary TB | Innumerable 1–3 mm nodules diffusely distributed |
| Sarcoidosis (stage I–II) | Bilateral hilar lymphadenopathy ± reticulonodular opacities |
Cephalization (upper-lobe vascular redistribution) is the earliest sign of pulmonary venous hypertension on an upright CXR. It precedes Kerley lines and overt alveolar edema and often correlates with a PCWP of 12–18 mmHg.
Atelectasis Subtypes
| Type | Mechanism | Example |
|---|
| Obstructive (resorptive) | Airway blockage; distal alveoli resorb gas | Mucus plug, endobronchial tumor, foreign body |
| Compressive | External compression on lung | Pleural effusion, pneumothorax, mass |
| Passive / relaxation | Loss of apposition to chest wall | Pneumothorax recoil |
| Cicatrizing | Fibrosis pulling lung in | TB, radiation, chronic fibrosis |
| Adhesive | Surfactant deficiency | ARDS, neonatal RDS, PE |
| Subsegmental (linear) | Hypoventilation | Post-operative, splinting from pain |
09 Pleura, Mediastinum & Cardiac Silhouette
Pleural Disease
| Finding | Appearance | Clinical |
|---|
| Pleural effusion (small) | Blunting of costophrenic angle, meniscus sign (>200 mL) | CHF, pneumonia, malignancy |
| Large effusion | Opacification, mediastinal shift away, layered fluid on decubitus | Parapneumonic, empyema, hemothorax |
| Pneumothorax | Visceral pleural line, absent lung markings peripheral to it | Spontaneous, traumatic, iatrogenic |
| Tension pneumothorax | Mediastinal shift away, flattened/inverted ipsilateral diaphragm | Clinical emergency — decompress before imaging |
| Hydropneumothorax | Air–fluid level in pleural space | Post-traumatic, post-drainage, bronchopleural fistula |
| Pleural thickening / calcification | Smooth or nodular density along pleura | Prior empyema, asbestos (calcified plaques, basilar) |
Mediastinum & Hilum
The mediastinum is divided into anterior, middle, and posterior compartments, each with characteristic pathology. Widened mediastinum (>8 cm on supine AP) in trauma raises concern for aortic injury. Hilar enlargement is caused by vessels, lymphadenopathy, or masses.
| Compartment | Structures | Masses ("4 T's") |
|---|
| Anterior | Thymus, lymph nodes, vessels | Thymoma, Teratoma, Thyroid (retrosternal), Terrible lymphoma |
| Middle | Heart, trachea, great vessels, nodes | Lymphadenopathy, bronchogenic cyst, aortic aneurysm |
| Posterior | Esophagus, descending aorta, spine, nerves | Neurogenic tumors (schwannoma), esophageal masses, aortic disease |
Cardiac Silhouette
On PA film, the cardiothoracic ratio (max cardiac width / max thoracic width) should be <0.5. AP films magnify the heart and inflate the ratio (do not call cardiomegaly on AP). Chamber enlargement clues: left atrial enlargement → splayed carina (>90°), double density behind right heart border, posterior displacement on lateral. Left ventricular enlargement → rounded, laterally displaced apex. Right heart enlargement → uplifted apex, filling of retrosternal space on lateral.
Don't call cardiomegaly on a supine AP film. AP magnification and lack of inspiration enlarge the cardiac silhouette. If the patient is stable, order a PA film before committing to the diagnosis.
Cardiac Chamber Enlargement Signs
| Chamber | PA Film | Lateral Film |
|---|
| Left atrium | Splayed carina (>90°), double density behind right heart, straightened left heart border | Posterior displacement of esophagus with barium |
| Left ventricle | Rounded, laterally and inferiorly displaced apex | Posterior extension past IVC line |
| Right atrium | Prominence of right heart border | Difficult to assess |
| Right ventricle | Uplifted apex ("boot" shape) | Filling of retrosternal clear space |
10 Lines, Tubes & Common Disease Patterns
Every post-procedural or ICU chest radiograph must confirm correct positioning of lines and tubes and screen for complications (pneumothorax, malposition, hemorrhage).
Line and Tube Positioning
| Device | Correct Position | Complications to Assess |
|---|
| Endotracheal tube | Tip 3–5 cm above carina (T2–T4 level) | Right mainstem intubation, extubation, esophageal |
| Central venous catheter | Tip at cavoatrial junction (just above right atrium) | Pneumothorax, arterial puncture, malposition |
| PICC line | Tip in lower SVC / cavoatrial junction | Malposition (azygos, IJ, subclavian) |
| Swan-Ganz catheter | Tip in right or left pulmonary artery (not peripheral) | PA rupture, knotting |
| Nasogastric tube | Tip below diaphragm in stomach, side port below GE junction | Pulmonary malposition (tube in bronchus/lung) |
| Chest tube | Along pleural space; all side holes inside thorax | Last side hole outside chest (not draining) |
| Pacemaker leads | RV apex ± RA appendage ± coronary sinus (CRT) | Lead fracture, dislodgment, perforation |
Line Position Disaster
A central line that appears to travel laterally (outside expected SVC course) or curves back on itself may be in a branch vessel or arterial. Do not infuse vesicants or TPN until position is confirmed. The most dangerous malposition is right atrial (perforation risk) or contralateral brachiocephalic vein.
Post-Operative Chest
Post-cardiac surgery films should be scrutinized for: sternal wires (complete, fractured, migration), mediastinal widening (bleeding, hematoma), pneumothorax, pleural effusion, atelectasis (especially left lower lobe), pericardial effusion, and position of chest tubes and lines. A rapidly increasing mediastinal contour after cardiac surgery suggests acute hemorrhage requiring urgent re-exploration.
Post-Procedure Complications
| Procedure | Imaging Complication to Watch For |
|---|
| Central line placement | Pneumothorax, hemothorax, arterial puncture, malposition |
| Thoracentesis | Pneumothorax, re-expansion edema |
| Paracentesis | Hemoperitoneum, bowel perforation |
| Lumbar puncture | Epidural / spinal hematoma (if anticoagulated) |
| Biopsy (lung) | Pneumothorax, hemorrhage, air embolism |
| Liver biopsy | Hemorrhage, pneumothorax, bile peritonitis |
11 Lung Nodules & Fleischner Criteria
A pulmonary nodule is a rounded opacity ≤3 cm surrounded by lung; larger lesions are masses. Solitary pulmonary nodules are common incidental findings. Characterization relies on size, margin, density, growth, and patient risk factors.
Benign vs Malignant Features
| Feature | Benign | Malignant |
|---|
| Size | <6 mm | >8–10 mm |
| Margin | Smooth, well-defined | Spiculated, lobulated |
| Calcification | Central, laminated, popcorn (hamartoma), diffuse | Eccentric, stippled, or none |
| Growth (doubling time) | <20 days (infection) or >400 days (benign) | 30–400 days |
| Density | Solid with fat (hamartoma) | Part-solid / ground-glass (adenocarcinoma) |
| Associated | None | Lymphadenopathy, effusion, bone lesions |
Fleischner Society Guidelines (2017)
| Nodule Type | Low-Risk Patient | High-Risk Patient |
|---|
| Solid, <6 mm, single | No routine follow-up | Optional CT at 12 months |
| Solid, 6–8 mm, single | CT at 6–12 months | CT at 6–12 months, then 18–24 months |
| Solid, >8 mm | CT at 3 months, PET/CT, or biopsy | CT at 3 months, PET/CT, or biopsy |
| Subsolid ground-glass <6 mm | No routine follow-up | Optional CT at 2–4 years |
| Subsolid ground-glass ≥6 mm | CT at 6–12 months, then every 2 years through 5 years | Same |
| Part-solid ≥6 mm | CT at 3–6 months; if unchanged and solid <6 mm, annual ×5 | Same — biopsy if solid >6 mm |
Fleischner criteria apply only to incidental nodules in adults ≥35, not to screening or cancer staging. A nodule in an active cancer patient is treated as metastatic until proven otherwise. Fleischner is for true incidentalomas.
Nodule Calcification Patterns
| Pattern | Appearance | Implication |
|---|
| Central / bull's eye | Dense central nidus | Benign (granuloma) |
| Laminated / concentric | Onion-skin rings | Benign (healed granuloma) |
| Diffuse / solid | Entirely calcified | Benign (old granuloma) |
| Popcorn | Clumped chondroid | Benign (hamartoma) |
| Eccentric / stippled | Irregular, off-center | Indeterminate / possibly malignant |
12 PE CTA & Interstitial Lung Disease
CT Pulmonary Angiography for PE
CT pulmonary angiography (CTPA) is the gold standard for diagnosing acute pulmonary embolism. Intravenous iodinated contrast is timed to peak pulmonary artery opacification. A positive study shows a filling defect (low-attenuation area) within a contrast-filled vessel, which may be occlusive or non-occlusive, central or peripheral.
| Finding | Significance |
|---|
| Saddle embolus | Clot straddling the bifurcation of main pulmonary arteries — large clot burden |
| RV dilation (RV:LV >0.9) | RV strain, worse prognosis, consider thrombolysis |
| Septal bowing | Interventricular septum pushed into LV — severe RV failure |
| Pulmonary infarction | Peripheral wedge-shaped opacity (Hampton hump) |
| Chronic PE | Webs, bands, eccentric calcified mural thrombi, mosaic perfusion |
Interstitial Lung Disease Patterns
| Pattern | Features | Differential |
|---|
| UIP (usual interstitial pneumonia) | Peripheral, basilar, subpleural, honeycombing, traction bronchiectasis | IPF, connective tissue disease, asbestosis |
| NSIP (non-specific interstitial pneumonia) | Ground-glass, peripheral reticular, subpleural sparing | CTD-associated (scleroderma), hypersensitivity, drug |
| Hypersensitivity pneumonitis | Upper-lobe ground-glass, centrilobular nodules, mosaic air trapping | Bird fancier, farmer's lung |
| Sarcoidosis | Perilymphatic nodules, upper-lobe, hilar/mediastinal adenopathy, fibrosis in end-stage | Granulomatous disease |
| Organizing pneumonia (COP) | Peripheral, subpleural, or peribronchial consolidation; reverse halo | Cryptogenic, post-infectious, drug |
Emphysema Subtypes on CT
Centrilobular (upper-lobe predominant) — smoking; small central lucencies within secondary pulmonary lobules. Panlobular (lower-lobe) — α1-antitrypsin deficiency. Paraseptal (subpleural) — risk factor for spontaneous pneumothorax in young adults. Pattern recognition on HRCT drives the diagnostic pathway.
Low-dose CT (LDCT) is the only validated lung cancer screening test. USPSTF recommends annual LDCT for adults 50–80 with ≥20 pack-year history and current smoking or quit within 15 years. Lung-RADS is the reporting system for screening LDCT.
Bronchiectasis Features
| Finding | Description |
|---|
| Signet ring sign | Bronchus larger than adjacent pulmonary artery |
| Lack of tapering | Bronchi remain same caliber toward periphery |
| Tram tracks | Parallel thick-walled airways on radiograph |
| Mucus plugging | Impacted airways (finger-in-glove) |
Causes include post-infectious (classic), cystic fibrosis (upper lobe), ABPA (central, finger-in-glove mucus), primary ciliary dyskinesia (with situs inversus in Kartagener), and immunodeficiency. HRCT pattern (upper vs lower, central vs peripheral) narrows etiology.
13 Abdominal Radiograph & KUB
The abdominal radiograph is a low-cost screening tool with limited sensitivity but high specificity for certain findings. The acute abdominal series includes supine abdomen, upright abdomen, and upright chest (for free air). A KUB (kidneys, ureters, bladder) is a supine film used primarily to track stones, catheters, and stents.
Key Abdominal X-Ray Findings
| Finding | Appearance | Diagnosis |
|---|
| Small bowel obstruction | Dilated small bowel (>3 cm), air–fluid levels at different heights, "string of pearls," valvulae conniventes across lumen | SBO (adhesions, hernia, tumor) |
| Large bowel obstruction | Dilated colon (>6 cm; cecum >9 cm), haustral markings partially across | LBO (cancer, volvulus, stricture) |
| Sigmoid volvulus | "Coffee bean" sign, inverted U arising from pelvis | Sigmoid volvulus (elderly, bedbound) |
| Cecal volvulus | Dilated cecum in LUQ, "coffee bean" pointing toward LUQ | Cecal volvulus (younger adults) |
| Free intraperitoneal air | Subdiaphragmatic lucency on upright CXR, Rigler sign (air on both sides of bowel wall), football sign (supine) | Perforated viscus |
| Toxic megacolon | Colonic dilation >6 cm, loss of haustra, thumbprinting | C. difficile, IBD, ischemic colitis |
| Pneumatosis intestinalis | Linear air within bowel wall | Ischemia (adults), NEC (neonates) |
| Abdominal calcifications | Rim (AAA), multiple (gallstones, phleboliths), branching (staghorn calculus) | Varies by location |
Free air under the diaphragm on an upright CXR is the most sensitive plain-film sign of perforation (detects as little as 1–2 mL). If the patient cannot stand, order a left lateral decubitus abdominal film — free air rises over the liver. Always include a CXR in the acute abdominal series.
Classic Abdominal Radiograph Signs
| Sign | Description | Diagnosis |
|---|
| Rigler sign | Air outlining both sides of bowel wall | Pneumoperitoneum |
| Football sign | Air outlining falciform ligament on supine film | Large pneumoperitoneum |
| Coffee bean sign | Dilated bowel loop appearing as coffee bean | Sigmoid or cecal volvulus |
| Target / bull's eye | Concentric rings in RLQ | Intussusception |
| Double bubble | Two air-filled bubbles (stomach + duodenum) | Duodenal atresia (neonate) |
| String of pearls | Small gas bubbles between valvulae conniventes | Small bowel obstruction |
| Thumbprinting | Bowel wall thickening imprints | Ischemic or infectious colitis |
| Ground-glass abdomen | Diffuse opacification | Massive ascites |
14 Abdominal CT — Acute Abdomen
CT with IV contrast is the workhorse for evaluating adult abdominal pain. Oral contrast is used selectively (less often now with modern multidetector scanners). Non-contrast CT is reserved for renal stones and patients who cannot receive iodinated contrast.
Common Acute Findings
| Diagnosis | CT Findings |
|---|
| Acute appendicitis | Appendiceal dilation >6 mm, wall thickening, periappendiceal fat stranding, appendicolith, abscess if perforated |
| Diverticulitis | Sigmoid diverticula with wall thickening and pericolonic fat stranding; complications: abscess, fistula, perforation |
| Acute pancreatitis | Pancreatic enlargement, peripancreatic stranding/fluid; necrosis = non-enhancing pancreas; pseudocyst late |
| Small bowel obstruction | Dilated proximal loops, decompressed distal, transition point; closed loop / ischemia = pneumatosis, portal venous gas, wall thickening, non-enhancement |
| Mesenteric ischemia | Bowel wall thickening, pneumatosis, mesenteric edema, arterial occlusion or SMV thrombus |
| Abdominal aortic aneurysm (AAA) | Aortic diameter ≥3 cm (infrarenal); rupture: retroperitoneal hematoma, periaortic fat stranding, crescent sign |
| Renal / ureteral stone | Hyperdense calculus, hydroureter, perinephric stranding, secondary hydronephrosis |
| Pyelonephritis | Striated nephrogram, wedge-shaped low attenuation, perinephric stranding, abscess in severe |
| Abscess | Rim-enhancing fluid collection with gas locules |
| Cholecystitis | Wall thickening (>3 mm), pericholecystic fluid, stones, sonographic Murphy (better on US) |
CT Contrast Phases
| Phase | Timing after Injection | Primary Use |
|---|
| Non-contrast | Before contrast | Stones, calcifications, baseline HU |
| Arterial | 25–35 seconds | Vascular (dissection, active bleed), HCC detection |
| Portal venous | 60–80 seconds | Standard abdominal CT, liver metastases |
| Delayed / equilibrium | 3–10 minutes | Hemangioma fill-in, urinary excretion (CT urogram) |
Triple-Phase Liver CT
Used for suspected hepatocellular carcinoma: HCC shows arterial enhancement followed by washout on portal venous/delayed images, with a pseudocapsule. This pattern (LI-RADS 5) is diagnostic without biopsy in cirrhotic patients. Hemangiomas show peripheral nodular enhancement with centripetal fill-in; metastases typically remain hypovascular.
15 Abdominal Ultrasound & FAST
Right Upper Quadrant Ultrasound
The first-line test for suspected biliary disease, liver lesions, and unexplained RUQ pain. Patients fast 8 hours to distend the gallbladder.
| Finding | Appearance | Clinical |
|---|
| Cholelithiasis | Echogenic, shadowing, mobile intraluminal focus | Gallstones |
| Acute cholecystitis | Wall >3 mm, pericholecystic fluid, sonographic Murphy sign, stones | Gallbladder inflammation |
| Choledocholithiasis | Dilated CBD >6 mm, intraductal stone | CBD stone — ERCP or MRCP |
| Hepatic steatosis | Increased hepatic echogenicity relative to kidney | Fatty liver |
| Cirrhosis | Nodular contour, caudate hypertrophy, splenomegaly, ascites, varices | End-stage liver disease |
| Hepatic hemangioma | Well-defined hyperechoic lesion | Benign — confirm with MRI if uncertain |
FAST Exam (Focused Assessment with Sonography for Trauma)
Rapid bedside ultrasound in blunt abdominal trauma to detect free intraperitoneal fluid. Four views:
| View | Looks For |
|---|
| Perihepatic (Morison pouch) | Fluid between liver and right kidney (most sensitive) |
| Perisplenic | Fluid in splenorenal recess, subphrenic |
| Pelvic (rectovesical / pouch of Douglas) | Fluid in most dependent pelvic space |
| Subxiphoid / pericardial | Pericardial effusion, tamponade |
The eFAST (extended FAST) adds bilateral thoracic views for pneumothorax (absent lung sliding, lung point) and hemothorax.
FAST has high specificity (>95%) but moderate sensitivity (~80%) for free fluid. A negative FAST does not exclude injury in a stable patient — consider CT. In an unstable patient with positive FAST, proceed directly to the OR; CT wastes critical time.
Point-of-Care Ultrasound (POCUS)
| Exam | Application | Key Finding |
|---|
| Cardiac (echo) | Pericardial effusion, global LV function, RV strain | Tamponade, hypokinesis, dilated RV in PE |
| Lung | Pneumothorax, pulmonary edema, consolidation | Absent lung sliding, B-lines, hepatization |
| IVC | Volume status | Collapsibility index >50% suggests low CVP |
| Aorta | AAA screening | Diameter ≥3 cm |
| Renal | Hydronephrosis screening | Dilated pelvis and calyces |
| Procedural guidance | Central line, thoracentesis, paracentesis | Real-time needle visualization, safer access |
16 Hepatobiliary MRI & MRCP
MRI is superior to CT for lesion characterization in the liver and biliary tree. Liver MRI uses T1, T2, in-phase/out-of-phase, DWI, and dynamic post-contrast sequences. Hepatobiliary agents (gadoxetate) provide additional delayed hepatocyte uptake information for focal lesion characterization.
MRCP (Magnetic Resonance Cholangiopancreatography)
Heavily T2-weighted sequence where static fluid (bile, pancreatic juice) is markedly bright while background tissue is suppressed. Non-invasive alternative to ERCP for evaluating biliary anatomy. Detects choledocholithiasis, strictures, biliary tree variants, pancreas divisum, chronic pancreatitis, and PSC (beaded bile ducts).
| Liver Lesion | MRI Characteristics |
|---|
| Simple cyst | T1 dark, T2 very bright, no enhancement |
| Hemangioma | T2 very bright, peripheral nodular discontinuous enhancement with centripetal fill-in |
| FNH (focal nodular hyperplasia) | Iso-intense, arterial enhancement, central scar bright on T2, retains gadoxetate |
| Adenoma | Fat/hemorrhage on T1, variable enhancement, drops out on delayed hepatobiliary phase |
| HCC | Arterial enhancement, washout, capsule, T2 mild-moderate bright |
| Metastasis | T2 moderate bright, hypovascular rim enhancement (most), restricted diffusion |
On in-phase / out-of-phase MRI, hepatic steatosis causes signal drop on out-of-phase images (fat and water cancel in the same voxel). This is the most sensitive imaging test for fatty liver disease and quantifies steatosis non-invasively.
LI-RADS Major Features (HCC)
| Feature | Definition |
|---|
| Arterial phase hyperenhancement | Unequivocally brighter than background liver in arterial phase |
| Non-peripheral washout | Reduced enhancement below background in portal venous or delayed phase |
| Enhancing capsule | Peripheral rim of enhancement in portal venous or delayed phase |
| Threshold growth | ≥50% size increase in ≤6 months |
| Size | Diameter of observation |
Applied only in patients with cirrhosis or chronic HBV. LR-5 lesions (definite HCC) can be diagnosed and treated without biopsy in the appropriate clinical setting.
17 Renal & Urinary Tract Imaging
Modality Selection
| Indication | First-Line Modality |
|---|
| Suspected renal stone | Non-contrast CT (stone protocol) |
| Hematuria workup | CT urography (multiphase) |
| Hydronephrosis screening | Ultrasound |
| Renal mass characterization | Multiphase CT or MRI (Bosniak) |
| Renal artery stenosis | Doppler US, MRA, CTA |
| Bladder / urothelial cancer | CT urography, cystoscopy |
| Pyelonephritis complicated | Contrast CT |
Bosniak Classification of Renal Cysts
| Category | Features | Malignancy Risk | Management |
|---|
| I | Simple cyst, thin wall, no septa, no enhancement | ~0% | No follow-up |
| II | Few thin septa, fine calcification | ~0% | No follow-up |
| IIF | Multiple thin septa or minimal wall thickening | ~5% | Follow with imaging |
| III | Thickened or nodular walls/septa with measurable enhancement | ~50% | Surgery or biopsy |
| IV | Enhancing soft tissue components adjacent to cyst | ~90% | Surgery |
Renal Stone Imaging
Non-contrast CT has near 100% sensitivity for urinary stones regardless of composition (uric acid stones are radiolucent on plain film but visible on CT). Stones <5 mm typically pass spontaneously; stones 5–10 mm may require intervention; >10 mm usually require procedural removal. Key CT findings: hyperdense calculus, hydroureter, perinephric stranding (indicating obstruction), and the "tissue rim" sign distinguishing ureteral stone from phlebolith.
A ureteral stone at the ureterovesical junction can mimic a phlebolith. The "tissue rim sign" (soft-tissue edema surrounding the stone) and associated hydroureter distinguish stone from phlebolith on non-contrast CT.
Adrenal Incidentaloma Evaluation
| Characteristic | Benign (Adenoma) | Concerning |
|---|
| Non-contrast HU | <10 HU (lipid-rich) | >10 HU |
| Absolute washout | >60% at 15 min | <60% |
| Relative washout | >40% | <40% |
| Size | <4 cm | ≥4 cm — surgical consideration |
| Borders | Smooth, homogeneous | Irregular, heterogeneous |
| Growth on interval imaging | Stable | >1 cm/year |
All adrenal incidentalomas need biochemical workup for hormonal excess (pheochromocytoma, Cushing syndrome, hyperaldosteronism) regardless of imaging features. Imaging alone does not exclude functional tumors.
18 Pelvic, Obstetric & Prostate Imaging
Pelvic Ultrasound
Transabdominal provides a global view (requires full bladder as acoustic window); transvaginal gives superior resolution of uterus, endometrium, ovaries, and adnexa. First-line for suspected ovarian pathology, pelvic pain, abnormal bleeding, and early pregnancy.
| Finding | Features |
|---|
| Simple ovarian cyst | Anechoic, thin wall, posterior enhancement, <5 cm benign |
| Hemorrhagic cyst | Complex internal echoes, fishnet/reticular pattern, resolves over weeks |
| Endometrioma | Homogeneous low-level echoes ("ground glass") |
| Dermoid / mature teratoma | Hyperechoic with acoustic shadowing (fat, calcium, hair) |
| Ovarian torsion | Enlarged ovary, decreased/absent Doppler flow, heterogeneous edema |
| Ectopic pregnancy | Extra-uterine gestational sac, adnexal ring of fire, empty uterus with positive β-hCG, free fluid |
| Uterine fibroid | Hypoechoic, well-defined, may shadow |
| Endometrial thickening | Postmenopausal >4 mm requires biopsy |
Obstetric Ultrasound
| Gestational Age | Expected Findings |
|---|
| 5 weeks | Gestational sac (intrauterine) |
| 6 weeks | Yolk sac, fetal pole, cardiac activity |
| 8–12 weeks | Nuchal translucency screening |
| 18–22 weeks | Anatomy scan, placental location, amniotic fluid |
| 28–40 weeks | Growth, biophysical profile, presentation |
Prostate MRI & PI-RADS
Multiparametric prostate MRI (T2, DWI, dynamic contrast enhancement) is used for detection, localization, staging, and post-treatment surveillance of prostate cancer. PI-RADS v2.1 assigns a 1–5 score indicating probability of clinically significant cancer: 1–2 (benign), 3 (equivocal), 4–5 (likely/highly likely clinically significant cancer → targeted biopsy).
Discriminator Zone Rules
Peripheral zone: DWI is dominant (cancer restricts diffusion against bright T2 background). Transition zone: T2 is dominant (cancer is homogeneous low T2 against heterogeneous benign prostatic hyperplasia). This zonal approach is core to PI-RADS scoring.
Testicular Ultrasound
| Finding | Features | Clinical |
|---|
| Testicular torsion | Asymmetric decreased/absent intratesticular flow on Doppler; edema in late phase | Surgical emergency — 6 hour window |
| Epididymitis | Enlarged epididymis with hyperemia | Medical management (antibiotics) |
| Hydrocele | Anechoic fluid surrounding testicle | Common, usually benign |
| Varicocele | Dilated pampiniform plexus, "bag of worms," flow on Valsalva | Left >> right; right-sided new = r/o retroperitoneal mass |
| Testicular tumor | Intratesticular hypoechoic mass with flow | Seminoma, non-seminomatous germ cell tumors |
| Epididymal cyst / spermatocele | Anechoic lesion in epididymal head | Benign |
19 Fracture Description & Principles
Every fracture description must be systematic and precise to communicate with the orthopedic surgeon. The components are: bone, location, pattern, displacement, angulation, rotation, and associated findings.
Fracture Description Checklist
| Element | Options |
|---|
| Bone & location | Proximal / mid / distal third; metaphyseal, diaphyseal, epiphyseal |
| Open vs closed | Open = communication with skin disruption |
| Pattern | Transverse, oblique, spiral, comminuted, segmental, torus/buckle, greenstick, impacted |
| Displacement | Percentage of cortical width, direction of distal fragment |
| Angulation | Degrees, direction of distal fragment apex |
| Rotation | Assessed clinically; may show on joint films |
| Shortening | Overlap of fragments |
| Intra-articular | Involvement of joint surface |
| Associated | Dislocation, soft tissue, NVI compromise |
Salter-Harris Classification (Pediatric Physeal Injuries)
| Type | Description | Mnemonic | Prognosis |
|---|
| I | Slip through physis only | Straight across | Best |
| II | Physis + metaphysis (Thurston-Holland fragment) | Above physis | Good (most common) |
| III | Physis + epiphysis (intra-articular) | Lower | Variable |
| IV | Metaphysis + physis + epiphysis | Through everything | Poor (growth arrest) |
| V | Compression / crush of physis | ERasure / crush | Worst (growth arrest) |
Children's bones deform before they break. Torus (buckle) and greenstick fractures are unique to pediatric immature bone. Physeal injuries may be radiographically occult; compare to the contralateral side when uncertain, and recognize that tenderness over the physis in a child is a fracture until proven otherwise.
20 Regional Fractures & Dislocations
Common Fractures by Region
| Region | Fracture / Injury | Key Feature |
|---|
| Wrist | Colles (distal radius, dorsal angulation) | "Dinner fork" deformity, FOOSH in elderly |
| Wrist | Smith fracture | Distal radius with volar angulation (reverse Colles) |
| Wrist | Scaphoid fracture | Snuffbox tenderness, AVN risk — may be occult; repeat in 10–14 days or MRI |
| Hand | Boxer fracture | 5th metacarpal neck, angulation after punch |
| Forearm | Monteggia | Proximal ulna fracture with radial head dislocation |
| Forearm | Galeazzi | Distal radius fracture with DRUJ dislocation |
| Elbow | Supracondylar humerus (pediatric) | Posterior fat pad (sail sign) = occult fracture |
| Shoulder | Anterior dislocation | Humeral head anteroinferior, Hill-Sachs / Bankart lesions |
| Shoulder | Posterior dislocation | "Lightbulb" sign on AP; seizure, electrocution |
| Hip | Femoral neck fracture | Shortened, externally rotated; AVN risk high |
| Hip | Intertrochanteric | Extracapsular; better blood supply, less AVN |
| Knee | Tibial plateau fracture | Lipohemarthrosis on cross-table lateral |
| Ankle | Weber / Lauge-Hansen classification | Location of fibular fracture relative to syndesmosis |
| Ankle | Maisonneuve | Proximal fibula fracture with syndesmotic injury (check knee to ankle) |
| Foot | Jones fracture | 5th metatarsal base; poor healing |
| Foot | Lisfranc injury | Tarsometatarsal malalignment; subtle but devastating |
| Spine | Compression fracture | Vertebral height loss, wedge shape (osteoporosis) |
| Spine | Jefferson fracture | C1 burst fracture (axial load) |
| Spine | Hangman fracture | C2 pars interarticularis, hyperextension |
| Spine | Odontoid (dens) fracture | Type II at base is unstable; common in elderly falls |
| Spine | Chance fracture | Flexion-distraction, seat belt mechanism |
Occult Fracture Clues
A normal-looking radiograph with clinical suspicion still warrants concern: fat pad sign at the elbow (occult radial head or supracondylar), lipohemarthrosis at the knee (intra-articular fracture), scaphoid tenderness (occult scaphoid), pubic ramus fracture (look for contralateral posterior ring injury). Get MRI or CT when suspicion is high.
Joint Dislocation Essentials
| Joint | Direction & Mechanism | Associated Injury |
|---|
| Glenohumeral | Anterior (95%) — abduction + external rotation | Hill-Sachs, Bankart, axillary nerve |
| Glenohumeral | Posterior — seizure, electrocution | Reverse Hill-Sachs, "lightbulb" sign |
| Elbow | Posterior — FOOSH | Coronoid, radial head fractures |
| Hip | Posterior (90%) — dashboard injury | Sciatic nerve, acetabular fracture, AVN |
| Hip | Anterior (10%) — forced abduction | Femoral artery / nerve |
| Knee | Any direction — high-energy | Popliteal artery injury — surgical emergency |
| Patella | Lateral (most common) | Medial retinaculum tear |
| Ankle | Usually associated with fracture | Syndesmotic injury, tibiotalar ligaments |
A posterior knee dislocation is a vascular emergency. Up to 40% have popliteal artery injury. Even if pulses are present, perform an ABI and consider CTA, because intimal injuries may evolve over hours to days into thrombosis.
21 Arthritis, Bone Tumors & MSK MRI
Arthritis Radiographic Features
| Feature | Osteoarthritis | Rheumatoid Arthritis |
|---|
| Distribution | DIP, PIP, 1st CMC, weight-bearing joints | MCP, PIP, wrists, symmetric |
| Joint space | Asymmetric narrowing | Symmetric narrowing |
| Osteophytes | Prominent | Absent |
| Erosions | Absent | Marginal erosions |
| Cysts | Subchondral cysts | Periarticular cysts |
| Density | Subchondral sclerosis | Periarticular osteopenia |
| Soft tissue | Normal | Fusiform soft tissue swelling |
Other Arthropathies
Gout: punched-out erosions with overhanging edges, tophi, preserved joint space until late. Psoriatic arthritis: "pencil-in-cup" deformity, DIP involvement, periostitis, sausage digit. Ankylosing spondylitis: sacroiliitis, bamboo spine, syndesmophytes. CPPD / pseudogout: chondrocalcinosis of menisci, triangular fibrocartilage, pubic symphysis.
Bone Lesion Characterization
| Feature | Benign | Malignant |
|---|
| Margin | Narrow zone of transition, sclerotic rim | Wide zone of transition, permeative |
| Periosteal reaction | Solid, thick | Sunburst, onion-skinned, Codman triangle |
| Matrix | Variable | Osteoid or chondroid matrix |
| Soft tissue mass | Absent | Present |
MSK MRI Key Indications
| Study | Key Findings |
|---|
| Knee MRI | ACL tear (empty lateral notch, increased T2 signal), meniscal tear (linear increased signal reaching articular surface), bone contusions, collateral ligaments |
| Shoulder MRI | Rotator cuff tear (full-thickness = fluid across), labral tears (SLAP, Bankart), AC joint |
| Hip MRI | AVN (early marrow edema), labral tears, stress fractures, occult fracture |
| Spine MRI | Disc herniation, cord compression, stenosis, cord lesions, infection (discitis), tumor |
| Foot/ankle MRI | Ligament injuries, osteochondral lesions, stress fractures, Morton neuroma |
Spinal cord compression is a neurosurgical emergency. Any patient with severe back pain plus neurologic deficit or new urinary retention needs emergent MRI of the entire spine (not just one level) to identify all sites of compression before initiating treatment.
Benign vs Aggressive Bone Lesions
| Lesion | Typical Age | Location | Features |
|---|
| Osteochondroma | <20 | Metaphysis (long bone) | Cortex and medulla continuous with lesion |
| Enchondroma | 20–40 | Phalanges, long bones | Chondroid matrix, lucent, rings-and-arcs |
| Fibrous dysplasia | Variable | Any bone | Ground-glass matrix, expansile |
| Giant cell tumor | 20–40 | Epiphysis, around knee | Lucent, eccentric, subarticular; locally aggressive |
| Osteosarcoma | 10–20 | Metaphysis, around knee | Osteoid matrix, sunburst periostitis, Codman triangle |
| Ewing sarcoma | 10–20 | Diaphysis | Onion-skin periostitis, permeative, soft-tissue mass |
| Chondrosarcoma | 40–60 | Pelvis, proximal femur | Chondroid matrix, cortical destruction |
| Multiple myeloma | >50 | Axial skeleton, skull | Punched-out lytic lesions; poor uptake on bone scan |
| Metastasis | >40 | Axial skeleton | Lytic (breast, lung, thyroid, renal), blastic (prostate), mixed |
22 CT Head & Intracranial Hemorrhage
Non-contrast CT head is the first-line imaging in acute neurologic presentations: trauma, stroke, altered mental status, severe headache. It is fast, widely available, and exquisitely sensitive for acute blood (hyperdense on CT).
Intracranial Hemorrhage Subtypes
| Type | Location | Shape | Cause | Key Features |
|---|
| Epidural hematoma | Between skull and dura | Biconvex "lens" | Arterial (middle meningeal), trauma | Does not cross sutures; lucid interval; surgical emergency |
| Subdural hematoma | Between dura and arachnoid | Crescentic | Venous (bridging veins), trauma, elderly, anticoagulation | Crosses sutures, not midline; acute bright, chronic dark |
| Subarachnoid hemorrhage (SAH) | Subarachnoid space (sulci, cisterns) | Follows CSF spaces | Aneurysm rupture (non-traumatic), trauma | "Worst headache of life," thunderclap; CTA for aneurysm |
| Intraparenchymal (ICH) | Brain tissue | Round / irregular | Hypertension (BG, thalamus, pons, cerebellum), amyloid (lobar), AVM, tumor | Mass effect, midline shift, IVH extension |
| Intraventricular (IVH) | Ventricles | Layered dependent | Extension of ICH/SAH, premature infants | Obstructive hydrocephalus risk |
Blood Density Over Time (CT)
| Age | Appearance |
|---|
| Hyperacute (<6 h) | Hyperdense (50–80 HU) |
| Subacute (days to weeks) | Isodense to brain (easy to miss) |
| Chronic (weeks+) | Hypodense, approaching CSF density |
Mass Effect Signs
Sulcal effacement, ventricular compression, midline shift (measure at septum pellucidum), subfalcine herniation (cingulate under falx), uncal herniation (medial temporal lobe over tentorium), tonsillar herniation (cerebellar tonsils through foramen magnum). Measure midline shift at the septum pellucidum relative to a line drawn between the anterior and posterior attachments of the falx.
Traumatic Brain Injury Patterns
| Pattern | Mechanism | Imaging |
|---|
| Contusion | Coup / contrecoup against skull | Hemorrhagic focus at frontal / anterior temporal lobes |
| Diffuse axonal injury (DAI) | Rotational / deceleration shear | Small hemorrhages at gray-white junction, corpus callosum, brainstem; best on SWI/GRE |
| Cerebral edema | Secondary injury | Loss of gray-white differentiation, sulcal effacement, tight basal cisterns |
| Skull fracture | Direct impact | Lucent line on CT; look for depressed fragments and air |
| Basilar skull fracture | Severe head trauma | Air in sinuses or temporal bone, battle sign, raccoon eyes, CSF rhinorrhea |
23 Ischemic Stroke Imaging
Stroke imaging has three goals: (1) exclude hemorrhage (non-contrast CT), (2) identify large vessel occlusion amenable to thrombectomy (CTA), (3) quantify salvageable penumbra (CT perfusion or MRI).
Early CT Signs of Ischemic Stroke
| Sign | Finding | Timing |
|---|
| Hyperdense MCA sign | Bright M1 segment (acute thrombus) | Immediate |
| Loss of insular ribbon | Loss of gray-white differentiation at insula | First hours |
| Loss of basal ganglia definition | Obscuration of lentiform nucleus | First hours |
| Sulcal effacement | Cortical swelling obliterating sulci | First hours |
| Parenchymal hypodensity | Frank low attenuation in vascular territory | 6–24 h |
ASPECTS Score
Alberta Stroke Program Early CT Score evaluates 10 MCA territory regions; start at 10 and subtract 1 for each region with early ischemic change. ASPECTS ≤7 correlates with worse outcomes and greater hemorrhagic transformation risk with thrombolytics; ASPECTS ≥6 is commonly used as threshold for thrombectomy eligibility.
CTA & CT Perfusion
CTA identifies large vessel occlusions (ICA, M1, basilar) that are targets for mechanical thrombectomy. CT perfusion differentiates core infarct (non-salvageable) from ischemic penumbra (salvageable): core shows decreased CBV and CBF with increased MTT; penumbra shows preserved CBV but increased MTT. The mismatch (penumbra − core) represents tissue at risk that could be saved by reperfusion.
MRI for Stroke
DWI is the most sensitive imaging for acute ischemic stroke, detecting changes within minutes. Restricted diffusion (bright DWI, dark ADC) is seen within 30 minutes and persists for 7–10 days. DWI/FLAIR mismatch (bright DWI, still-normal FLAIR) suggests stroke onset <4.5 hours and is used to guide thrombolysis in wake-up strokes.
In a patient with suspected stroke, order non-contrast CT + CTA head and neck as a single protocol. This rules out hemorrhage, identifies LVO for thrombectomy, and assesses the neck vessels for a source — in one trip to the scanner.
Stroke Territories and Clinical Correlation
| Vessel | Territory | Classic Deficit |
|---|
| ACA | Medial frontal lobe, parasagittal | Contralateral leg weakness > arm |
| MCA — superior division | Lateral frontal, motor cortex | Contralateral face/arm weakness, Broca aphasia (L) |
| MCA — inferior division | Lateral temporoparietal | Wernicke aphasia (L), neglect (R) |
| PCA | Occipital lobe, thalamus | Contralateral homonymous hemianopia |
| Basilar / brainstem | Pons, midbrain, cerebellum | Cranial nerve + crossed motor deficits, locked-in |
| Lacunar | Deep perforators (BG, thalamus, pons) | Pure motor, pure sensory, ataxic hemiparesis |
| Watershed | Border zones (ACA–MCA, MCA–PCA) | "Man in the barrel" proximal weakness after hypotension |
24 MRI Brain & Spine
Common Brain MRI Findings
| Pathology | Signal Characteristics |
|---|
| Acute infarct | DWI bright, ADC dark; FLAIR bright after hours; T2 bright |
| MS plaques | Periventricular T2/FLAIR hyperintensities perpendicular to ventricles (Dawson fingers); active plaques enhance |
| Glioblastoma | Heterogeneous, rim enhancement with central necrosis, vasogenic edema, crosses corpus callosum (butterfly) |
| Meningioma | Extra-axial, dural tail, homogeneous enhancement, T1 iso, T2 iso-bright |
| Vestibular schwannoma | CP angle mass, enhances, extends into internal auditory canal ("ice cream cone") |
| Metastases | Multiple, gray-white junction, ring enhancement, marked vasogenic edema |
| Abscess | Ring enhancement, restricted diffusion centrally (distinguishes from tumor) |
| Cavernous malformation | Popcorn T2 with hemosiderin rim (dark on GRE/SWI) |
Spine MRI
| Finding | Appearance |
|---|
| Disc herniation | Focal disc material extending beyond endplates, may compress nerve root or thecal sac |
| Central canal stenosis | Loss of CSF around cord/cauda equina, disc-osteophyte, ligamentum flavum hypertrophy |
| Neural foraminal stenosis | Obliteration of perineural fat, nerve root compression |
| Cord compression | Cord deformity, T2 hyperintensity from edema/myelomalacia |
| Discitis / osteomyelitis | Disc T2 bright, endplate erosions, marrow edema, enhancement; epidural phlegmon/abscess |
| Cauda equina syndrome | Mass (disc, tumor, hematoma) compressing cauda at L1–S2 |
| MS (cord) | Short-segment T2 hyperintensity, dorsal cord, enhancing if active |
| Transverse myelitis | Long-segment central cord T2 hyperintensity >2 vertebral levels |
Ring-enhancing brain lesions differential (MAGIC DR): Metastasis, Abscess, Glioblastoma, Infarct (subacute), Contusion, Demyelination (tumefactive MS), Radiation necrosis. DWI helps: bacterial abscess restricts diffusion centrally, tumors do not.
Herniation Syndromes
| Type | Anatomy | Clinical |
|---|
| Subfalcine | Cingulate gyrus under falx cerebri | Contralateral leg weakness (ACA compression) |
| Uncal (transtentorial) | Medial temporal lobe over tentorium | Ipsilateral CN III palsy (blown pupil), contralateral hemiparesis |
| Central transtentorial | Downward displacement of diencephalon | Cushing reflex, progressive brainstem dysfunction |
| Tonsillar | Cerebellar tonsils through foramen magnum | Medullary compression, cardiopulmonary arrest |
| Upward transtentorial | Posterior fossa mass pushing up | Midbrain compression, obstructive hydrocephalus |
Hydrocephalus Patterns
| Type | Mechanism | Imaging |
|---|
| Communicating | Impaired CSF resorption | All ventricles enlarged |
| Obstructive (non-communicating) | Blockage within ventricular system | Dilation proximal to obstruction |
| Ex-vacuo | Brain atrophy | Ventricular enlargement proportional to sulcal prominence |
| Normal pressure (NPH) | Unknown | Ventriculomegaly out of proportion to sulci, triad of gait/cognition/urinary |
25 Vascular Imaging & CTA
Aortic Imaging
| Condition | CTA Findings |
|---|
| Aortic dissection | Intimal flap separating true from false lumen; Stanford A (ascending) = surgical emergency; Stanford B (descending) = usually medical |
| Intramural hematoma | Crescentic high-attenuation aortic wall thickening on non-contrast, no flap |
| Penetrating aortic ulcer | Contrast-filled outpouching through intima into media with surrounding hematoma |
| Thoracic aortic aneurysm | ≥4 cm; monitor, repair threshold 5.5 cm ascending |
| Abdominal aortic aneurysm | ≥3 cm; repair threshold 5.5 cm (men), 5.0 cm (women) or rapid growth |
| Ruptured AAA | Periaortic hematoma, crescent sign, retroperitoneal blood; hemodynamic compromise |
| Traumatic aortic injury | Pseudoaneurysm at ligamentum arteriosum (isthmus), mediastinal hematoma |
Peripheral Vascular Imaging
| Study | Use | Notes |
|---|
| Carotid duplex ultrasound | Carotid stenosis screening | Peak systolic velocity correlates with stenosis; confirm significant disease with CTA/MRA |
| Venous compression ultrasound | DVT diagnosis | Non-compressibility of vein = DVT (most sensitive finding) |
| CTA chest (PE) | Pulmonary embolism | First-line when D-dimer positive or high pre-test probability |
| CTA abdomen/pelvis runoff | Peripheral arterial disease, mesenteric ischemia | Alternative to angiography |
| MRA | Vessels without iodinated contrast | Renal artery, intracranial, aortic |
Dissection Classification
Stanford A involves the ascending aorta — regardless of where the tear started — and is a surgical emergency due to risk of rupture into the pericardium, coronary involvement, and aortic insufficiency. Stanford B involves only the descending aorta; initial management is medical (strict BP and HR control) with endovascular repair for complications or progression.
DVT Ultrasound Technique
Compression ultrasound assesses proximal veins (common femoral, femoral, popliteal) with sequential compression. Normal veins collapse completely under gentle pressure; the presence of non-compressible material within the vein is diagnostic of thrombus. Augmentation and color Doppler add flow information for partial or non-occlusive thrombus. Calf veins are less commonly evaluated but become important in recurrent symptoms. A negative proximal US with positive D-dimer may require serial imaging or whole-leg evaluation.
Carotid Doppler Velocity Criteria
| Stenosis | Peak Systolic Velocity (cm/s) | ICA/CCA Ratio |
|---|
| Normal | <125 | <2.0 |
| 50–69% | 125–230 | 2.0–4.0 |
| ≥70% | >230 | >4.0 |
| Near occlusion | Variable / low | Variable |
| Total occlusion | No flow | N/A |
26 Nuclear Medicine & Fluoroscopy
Common Nuclear Medicine Studies
| Study | Radiotracer | Use |
|---|
| Bone scan | 99mTc-MDP | Metastases, stress fractures, osteomyelitis, Paget disease |
| V/Q scan | 99mTc-MAA (perfusion), Xe-133 (ventilation) | PE when CT contraindicated; PIOPED criteria (high/intermediate/low probability) |
| HIDA scan | 99mTc-IDA analogs | Cholecystitis (non-visualization of gallbladder), bile leak, biliary atresia |
| MAG3 renal scan | 99mTc-MAG3 | Differential renal function, obstruction, renovascular HTN (with captopril) |
| MUGA | 99mTc-labeled RBCs | LVEF, cardiotoxicity monitoring (pre/post chemotherapy) |
| Myocardial perfusion (SPECT) | 99mTc-sestamibi, 201Tl | Ischemia, viability, stress/rest imaging |
| Thyroid uptake & scan | 123I or 99mTc-pertechnetate | Hyperthyroidism (Graves vs toxic nodule vs thyroiditis), nodule evaluation |
| PET/CT | 18F-FDG | Oncology staging/response, FUO, sarcoid, infection |
| Gastric emptying | 99mTc-labeled meal | Gastroparesis |
| Meckel scan | 99mTc-pertechnetate | Meckel diverticulum (ectopic gastric mucosa) |
Fluoroscopic Studies
| Study | Contrast | Use |
|---|
| Barium swallow / esophagram | Barium (or water-soluble if perforation suspected) | Dysphagia, reflux, stricture, Zenker diverticulum, achalasia (bird beak) |
| Upper GI series | Barium | Ulcers, masses, hiatal hernia, gastric outlet obstruction |
| Small bowel follow-through | Barium | Crohn disease, small bowel tumors |
| Barium enema | Barium (single or double contrast) | Colonic mass, volvulus (diagnostic & therapeutic), intussusception reduction |
| VCUG | Iodinated water-soluble | Vesicoureteral reflux (pediatric UTI) |
| Hysterosalpingogram (HSG) | Iodinated | Fertility workup (tubal patency) |
If bowel perforation is suspected, use water-soluble contrast (Gastrografin) rather than barium. Barium extravasation into the peritoneum causes severe chemical peritonitis and fibrosis. Water-soluble contrast is safely resorbed if it leaks.
PET/CT Interpretation
18F-FDG PET measures glucose metabolism. Cancer cells typically show increased uptake; SUVmax is the standardized uptake value. An SUVmax >2.5 in a pulmonary nodule is suspicious for malignancy. PET is integrated with CT anatomy (PET/CT) or MRI (PET/MRI) for localization. Key uses: lung cancer staging, lymphoma response assessment, head and neck cancer, colorectal recurrence, melanoma, and fever of unknown origin.
| False Positive on FDG-PET | False Negative on FDG-PET |
|---|
| Infection / inflammation (TB, sarcoid) | Low-grade indolent tumors |
| Brown fat (neck, supraclavicular) | Mucinous carcinoma |
| Muscle activity | Bronchoalveolar carcinoma |
| Post-surgical change | Small lesions (<1 cm) — resolution limit |
| Radiation-induced pneumonitis | Hyperglycemia (reduces tumor avidity) |
V/Q Scan Interpretation (PIOPED)
A ventilation/perfusion scan compares regional ventilation (inhaled Xe-133 or Tc-DTPA aerosol) to perfusion (IV Tc-MAA). A segmental perfusion defect with normal ventilation ("mismatch") in a vascular distribution is consistent with PE. Reported as normal, very low, low, intermediate, or high probability. Useful when iodinated contrast is contraindicated (renal failure, severe allergy) or in pregnancy (lower fetal dose than CTPA).
27 Modality Selection & Appropriateness
The ACR Appropriateness Criteria are evidence-based guidelines matching clinical scenarios to the most appropriate imaging studies. The goal is to minimize radiation, contrast, cost, and wasted time while maximizing diagnostic yield.
First-Line Modality by Presentation
| Clinical Scenario | First-Line Imaging |
|---|
| Acute chest pain, r/o MI | CXR; consider coronary CTA or stress test |
| Acute chest pain, r/o PE | CTA chest (PE protocol) |
| Acute chest pain, r/o dissection | CTA chest/abdomen/pelvis |
| Suspected pneumonia | CXR PA and lateral |
| Adult RLQ pain (appendicitis) | CT abdomen/pelvis with contrast |
| Pediatric RLQ pain | Ultrasound; MRI if inconclusive |
| Pregnant RLQ pain | Ultrasound; MRI if inconclusive |
| RUQ pain (gallbladder) | Ultrasound |
| Flank pain (renal colic) | Non-contrast CT (stone protocol) |
| Suspected AAA | Ultrasound (screening); CTA (definitive) |
| Acute stroke | Non-contrast CT + CTA head/neck ± CT perfusion |
| Severe headache / SAH | Non-contrast CT; LP if negative; CTA if positive |
| First seizure | MRI brain (non-urgent); CT if acute |
| Trauma blunt abdominal, stable | CT abdomen/pelvis with contrast |
| Trauma blunt abdominal, unstable | FAST at bedside |
| Back pain with red flags | MRI spine |
| Suspected DVT | Venous compression US |
| Acute scrotum | Testicular ultrasound with Doppler |
| First-trimester bleeding | Transvaginal ultrasound + β-hCG |
| Suspected cholangitis | Ultrasound; MRCP or ERCP |
| Breast lump | Diagnostic mammogram + US |
When NOT to Image
| Scenario | Why Not |
|---|
| Uncomplicated low back pain <6 weeks | Imaging does not change management in absence of red flags |
| Simple syncope with normal exam | Head CT has very low yield |
| Uncomplicated headache with normal exam | Neuroimaging not indicated |
| Acute sinusitis | Clinical diagnosis |
| Rib contusion without red flags | Imaging rarely changes management |
Red Flags for Back Pain Imaging
Image immediately for: saddle anesthesia, urinary retention, fecal incontinence, progressive neurologic deficit, history of cancer, fever with back pain, IV drug use, significant trauma, or unexplained weight loss. Absent red flags, 6 weeks of conservative management is appropriate before imaging.
Trauma Imaging Protocols
| Trauma Scenario | Study |
|---|
| Blunt polytrauma ("pan-scan") | CT head, C-spine, chest/abdomen/pelvis with contrast |
| Penetrating torso | CT chest/abdomen/pelvis with IV ± triple contrast |
| Blunt head | Non-contrast CT head; CTA if high-risk mechanism or fracture at skull base |
| Blunt neck (C-spine clearance) | CT cervical spine; MRI if neurologic deficit or ligamentous concern |
| Pelvic trauma | CT pelvis with contrast; angiography for active extravasation |
| Extremity trauma | Plain radiographs; CT or MRI for complex fractures or occult injury |
Pediatric Considerations
Children have higher radiosensitivity and longer lifetime to develop radiation-induced cancer. Always ask: (1) Can we use ultrasound or MRI instead? (2) Can we use reduced dose protocols? (3) Can we limit scan coverage? Image Gently and Image Wisely are national campaigns emphasizing these principles. In children with suspected appendicitis, ultrasound is first-line; MRI is a good alternative when US is non-diagnostic, avoiding CT radiation entirely.
28 High-Yield Pearls & Report Terminology
Common Radiology Report Terms
| Term | Meaning |
|---|
| Consolidation | Airspace filling with fluid/cells/material, air bronchograms visible |
| Ground-glass opacity | Hazy increased density without obscuring vessels (CT term) |
| Reticular | Linear network pattern, suggests interstitial disease |
| Tree-in-bud | Small airway impaction, infection or aspiration |
| Honeycombing | Clustered cystic spaces, end-stage fibrosis |
| Mosaic attenuation | Heterogeneous lung density, air trapping or small-vessel disease |
| Fat stranding | Increased density of fat, indicates inflammation |
| Free fluid | Non-loculated fluid in peritoneum, pelvis, pleural space |
| Rim enhancement | Peripheral enhancement of a lesion with non-enhancing center (abscess, necrotic tumor) |
| Washout | Lesion hyperenhances early then becomes hypointense relative to background (HCC) |
| Restricted diffusion | Bright DWI + dark ADC (acute stroke, abscess, cellular tumor) |
| T2 shine-through | Bright on DWI from long T2, not true restriction |
| Mass effect | Compression / displacement of adjacent structures |
| Nondisplaced | Fracture with fragments in anatomic alignment |
| Clinical correlation recommended | Finding is non-specific; caller should integrate with clinical picture |
High-Yield Modality Pairings
| Scenario | Best Modality | Why |
|---|
| Acute stroke <6 h | CT + CTA, then MRI DWI | Rule out bleed, identify LVO, confirm infarct |
| Biliary colic | Ultrasound | Sensitive for stones, no radiation |
| Flank pain, hematuria | Non-contrast CT stone protocol | >99% stone detection |
| Small bowel obstruction | CT with contrast | Transition point, grade, ischemia |
| Occult scaphoid | MRI | Most sensitive for radiographically occult fracture |
| Rotator cuff tear | MRI or ultrasound | Tear size, retraction, muscle atrophy |
| First seizure workup | MRI brain | Structural lesions not seen on CT |
Rapid-Fire Clinical Pearls
When ordering emergency CT for suspected PE, always check renal function and the D-dimer in low pre-test probability patients. A negative D-dimer in a low/moderate probability patient can avoid CT entirely. The PERC rule further identifies patients who need no testing at all.
The "reverse halo" or atoll sign (central ground-glass surrounded by denser consolidation) is classically associated with organizing pneumonia but also occurs in invasive fungal infection, sarcoidosis, and pulmonary infarct. Pattern recognition narrows but rarely finalizes the diagnosis — integrate with clinical context.
Imaging does not replace the physical exam. A tender abdomen with peritoneal signs mandates surgical evaluation regardless of negative imaging; a normal CT with equivocal exam still warrants observation. The patient in front of you — not the image — drives the decision.
Every CXR should be compared to the patient's prior film when available. A new finding is alarming; an old unchanged finding is reassuring. Always ask for priors before finalizing your read.
Structured reporting improves clarity, reduces missed findings, and supports downstream data mining. Reporting templates for stroke, PE, prostate MRI, liver lesions, and lung cancer screening are standard of care and should be used when available.
Air bronchograms mean the airways are patent and the surrounding lung is airless (filled with fluid, pus, blood, or cells). This is why pneumonia and pulmonary edema show air bronchograms but endobronchial obstructions do not.
Acute blood on CT is bright (50–80 HU) because of the iron in hemoglobin. On MRI, acute blood is complex: hyperacute is T1 iso and T2 bright (oxyhemoglobin); acute is T1 iso and T2 dark (deoxyhemoglobin); early subacute is T1 bright and T2 dark; late subacute is T1 and T2 bright; chronic is T1 and T2 dark (hemosiderin rim).
MRI is contraindicated with old pacemakers, cochlear implants, metallic foreign bodies in the eye, and some aneurysm clips. Always screen rigorously before sending a patient to the scanner — the magnet is always on, and accidents are catastrophic.
For suspected cord compression, order MRI of the entire spine (cervical, thoracic, lumbar) not just the suspected level. Metastatic disease frequently has multiple sites of involvement, and missing one level changes surgical planning and radiation fields.
In pregnancy, ultrasound and MRI (without gadolinium) are preferred. When CT or nuclear medicine is essential for maternal health, do not withhold it — a missed PE or appendicitis is far more dangerous than the radiation dose of a single study. Shield the abdomen and minimize dose when possible.
A negative FAST in a stable trauma patient does NOT exclude intra-abdominal injury. FAST is a decision tool for the unstable patient: if positive, go to the OR; if negative but unstable, keep looking. The stable patient should still receive CT when indicated.
Contrast extravasation at the IV site usually resolves with conservative care (elevation, warm compresses). Large volumes (>100 mL) or signs of compartment syndrome require surgical consultation. Document the event, reassure the patient, and update the allergy list if indicated.
"Incidentaloma" is not a diagnosis. Every incidental finding has a management algorithm — adrenal nodules (non-contrast HU <10 = adenoma), thyroid nodules (TI-RADS), liver lesions (size and density), and lung nodules (Fleischner). Know the major frameworks.
Communicate critical findings directly. If the radiology report mentions a new pneumothorax, mass, hemorrhage, or cord compression, the radiologist must call a clinician and document the conversation. As the receiving clinician, acknowledge the call, document your acknowledgment, and act on the finding without delay.
Always include the clinical question and relevant history on the imaging requisition. A radiologist reading "abdominal pain" vs "68 y/o with sudden-onset LLQ pain, fever, WBC 16" will produce very different levels of scrutiny. Good communication improves the read.
The biggest source of diagnostic error in radiology is not missing a finding — it's misinterpreting a finding that was seen. Protect against this by always describing what you see (objective) before concluding what it means (subjective). If an objective finding does not fit the conclusion, re-examine the conclusion.
Interpretation Strategy
For every study: (1) Confirm identity, indication, and technique. (2) Use a systematic approach (ABC, A–E, or region-by-region). (3) Describe each finding objectively (size, location, characteristics). (4) Compare to priors. (5) Generate a differential based on pattern. (6) Communicate the clinical impact. (7) Recommend next steps when appropriate. These seven steps apply to every modality and every anatomic region — master them and you will read imaging confidently across the full breadth of clinical medicine.