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Radiology for Internal Medicine - Full Study Guide

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Part 1: Imaging Modalities - Overview

1.1 Chest X-Ray (CXR)

• Chest wall contours
• Heart silhouette and great vessels
• Mediastinal outline
• Pulmonary parenchyma and bronchovascular markings
• Costophrenic angles and diaphragm

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1.2 Computed Tomography (CT)

• Hounsfield Units (HU): Quantify tissue density (air = -1000 HU; water = 0; bone ~+400; fat ~-100)
• Lung windows: Best for parenchymal detail (airways, nodules, emphysema)
• Mediastinal windows: Best for soft tissue and vascular structures
• Bone windows: For ribs and vertebrae

• HRCT (High-Resolution CT): Thin slices (1-2 mm); gold standard for interstitial lung disease
• CT Pulmonary Angiography (CTPA): Timed bolus for pulmonary embolism detection - filling defects appear as dark voids in bright vessels
• CECT (Contrast-Enhanced CT): Mediastinal masses, lymph nodes, abscess

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1.3 Magnetic Resonance Imaging (MRI)

• No ionizing radiation - preferred for repeated assessments and pediatric patients
• Superior for chest wall, mediastinum, cardiac structures
• Gadolinium contrast enables perfusion assessment

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1.4 Ultrasound (US)

• Best for pleural effusion detection and guided drainage
• Used for mediastinal cystic vs. solid mass differentiation
• Increasingly used at bedside for pulmonary edema (B-lines = "comet tails" = subpleural fluid)
• More sensitive than CXR for small pleural effusions

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1.5 Nuclear Medicine / PET

• V/Q scan: Ventilation-perfusion mismatch - used for PE diagnosis when CT is contraindicated (e.g., contrast allergy, pregnancy)
• PET/CT with FDG: Metabolic activity - used for lung cancer staging, mediastinal lymphoma, sarcoidosis
• Limitations: False positives (thymic hyperplasia, fibrosing mediastinitis can have high FDG uptake)

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Part 2: Systematic Chest X-Ray Interpretation (The ABCDE Approach)

Cardiac silhouette:

• Normal CTR (Cardiothoracic Ratio): <0.5 on PA film
• Cardiomegaly: CTR >0.5 - causes include LV dilatation (dilated CMP, aortic regurgitation), biventricular failure, pericardial effusion
• "Globular" heart: Large, round - suggests pericardial effusion
• Right border = right atrium; Left border (top to bottom) = aortic knuckle → pulmonary trunk → left atrial appendage → left ventricle

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Part 3: Key Radiological Signs and What They Mean

3.1 Silhouette Sign

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3.2 Air Bronchogram

• Pneumonia (lobar or bronchopneumonia)
• Pulmonary edema
• ARDS
• Lung contusion

Air bronchograms are absent in pleural effusions and mediastinal masses - this is one of the key features distinguishing a mediastinal lesion from a pulmonary one.

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3.3 Kerley B Lines

• Short (1-2 cm), horizontal, peripheral lines at the lung bases
• Represent thickened interlobular septa
• Classic for interstitial pulmonary edema (hydrostatic, e.g., congestive heart failure)
• Also seen in: lymphangitis carcinomatosis, interstitial lung disease, pneumoconiosis

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3.4 Ground-Glass Opacity (GGO) on CT

• Hazy increased density that does not obscure underlying vessels
• Indicates partial filling of alveoli or interstitial thickening
• Causes: pneumonia (atypical/viral), pulmonary edema, organizing pneumonia, drug toxicity, early fibrosis, COVID-19

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Part 4: Common Internal Medicine Conditions - Radiological Findings

4.1 Pleural Effusion

• Bilateral effusions = usually transudates (heart failure, hypoalbuminemia, cirrhosis)
• Right-sided = associated with ascites, heart failure, liver abscess
• Left-sided = pancreatitis, pericarditis, oesophageal rupture, aortic dissection
• Massive effusions = most often malignant (lung or breast metastases)

1. Small: fluid collects subpulmonary - undetectable on erect PA; seen on US or CT
2. ~200-500 mL: blunting of the posterior then lateral costophrenic angles
3. Moderate: homogeneous basal opacification, obliteration of costophrenic angle and hemidiaphragm; meniscus sign (concave superior margin, higher laterally)
4. ~1000 mL: opacity reaches the level of the 4th anterior rib
5. Massive: dense hemithoracic opacification with contralateral mediastinal shift

⚠️ If there is no mediastinal shift with a large effusion: suspect ipsilateral lung collapse or extensive pleural malignancy (e.g., mesothelioma).

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4.2 Lobar Collapse / Atelectasis

• Obstructive (most common in adults): Bronchogenic carcinoma, mucous plugs, foreign body. In a middle-aged/elderly smoker, lobar collapse is bronchogenic carcinoma until proven otherwise.
• Extrinsic compression: Lymph node enlargement, mediastinal mass, aortic aneurysm
• Passive/compressive: Pleural effusion or pneumothorax compressing adjacent lung

• Direct signs (volume loss): Displacement of interlobar fissures, pulmonary vessels and bronchi toward the collapsed lobe
• Indirect signs (compensatory changes): Hyperinflation of adjacent lobes, elevation of ipsilateral diaphragm, tracheal/mediastinal shift toward the collapse, hilar displacement, rib crowding

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4.3 Pneumonia (Consolidation)

• Homogeneous opacity occupying a lobe or segment
• Air bronchograms present (distinguishes from effusion/collapse)
• Lobar distribution: classic for Streptococcus pneumoniae
• Cavitation: Klebsiella (upper lobe, "bulging fissure"), Staph, TB, anaerobes

• Bilateral patchy peribronchial opacities
• Seen with gram-negative organisms, viral pneumonia

• Bilateral diffuse ground-glass / reticular opacities
• Mycoplasma, Legionella, viral (influenza, COVID-19)

• Cavitation (lung abscess, necrotizing pneumonia)
• Pleural effusion / empyema
• Pneumothorax
• Lobar collapse from mucous plugging

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4.4 Pulmonary Edema

• Cardiomegaly (CTR >0.5)
• Pleural effusions (typically bilateral, right > left)
• Engorged lymphatics (Kerley B lines)
• Loss of normally sharp vascular markings

The bat-wing/perihilar "butterfly" pattern is classic for hydrostatic (cardiogenic) pulmonary edema. In ARDS (non-cardiogenic), edema tends to be more peripheral and diffuse.

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4.5 Pneumothorax

• Visible visceral pleural line (thin white line) with absent lung markings beyond it
• Hyperlucent area at the lung apex (most common site on erect film)
• On supine AP film (ICU): deep sulcus sign (abnormally deep lateral costophrenic angle)

• Complete collapse of the lung
• Tracheal and mediastinal shift away from the affected side
• Depressed ipsilateral diaphragm
• Diagnosis is clinical - do not delay treatment for imaging confirmation

• Small: <2 cm rim at apex
• Large: ≥2 cm rim; requires intervention

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4.6 Mediastinal Widening

• Thymoma
• Teratoma / Germ cell tumor
• Thyroid (retrosternal goitre)
• Terrible lymphoma

1. No air bronchogram (vs. pulmonary consolidation which does)
2. Obtuse margins with the lung
3. Distortion/thickening of mediastinal lines
4. Bone abnormalities (rib/vertebral involvement)

• Widened mediastinum >8 cm
• Loss of the aortic knuckle
• Clinical: tearing/ripping interscapular pain, differential blood pressures in arms
• CT aorta with IV contrast = definitive imaging

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4.7 Cardiac Imaging

• Transthoracic echo (TTE): Non-invasive, first line for LV function, valves, pericardium
• Transesophageal echo (TEE): Superior for aortic dissection, endocarditis vegetations, LA thrombus

• High negative predictive value for CAD in low-intermediate risk chest pain
• Calcium scoring (CACS) for cardiovascular risk stratification

• Gold standard for myocardial viability (late gadolinium enhancement)
• Characterizes cardiomyopathies (HOCM, ARVC, infiltrative disease e.g. amyloid, sarcoid)

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Part 5: Abdominal & Other Imaging

5.1 Abdominal X-Ray (AXR)

5.2 Abdominal Ultrasound

• First-line for: gallstones, hydronephrosis, hepatic lesions, ascites, AAA screening
• Limitations: operator-dependent, bowel gas interference

5.3 CT Abdomen/Pelvis

• Best for: acute abdomen, bowel obstruction, appendicitis, pancreatitis, trauma, abscess
• Triple-phase CT liver: arterial/portal/delayed for hepatocellular carcinoma
• CT urogram: urolithiasis, urothelial malignancy

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Part 6: Radiation Safety Principles

• ALARA: As Low As Reasonably Achievable
• MRI and US carry no ionizing radiation - preferred in pregnancy and pediatrics
• Benefits must outweigh risks for every imaging request
• Lead shielding and collimation reduce exposure

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Part 7: Quick-Reference "Must-Know" Signs Table

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Part 8: High-Yield Exam Summary

• First imaging in any chest complaint: CXR
• Gold standard for PE: CTPA
• Gold standard for interstitial lung disease: HRCT
• Gold standard for myocardial viability: Cardiac MRI
• Best for pleural effusion size: Ultrasound (detects as little as 20 mL)
• Middle-aged smoker with lobar collapse: Bronchogenic carcinoma until proven otherwise
• Bilateral pleural effusion without mediastinal shift: Think transudates (heart failure, cirrhosis, nephrotic syndrome)
• Massive effusion with no mediastinal shift: Think mesothelioma or endobronchial obstruction from malignancy
• CT pattern - perihilar ground-glass + Kerley B + cardiomegaly: Congestive heart failure
• CT pattern - peripheral ground-glass + honeycombing: Usual Interstitial Pneumonia (UIP) / IPF

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• Pleural effusion (with the volume thresholds - 200 mL, 500 mL, 1000 mL rules)
• Lobar collapse (by lobe, with the "bronchogenic carcinoma until proven otherwise" rule)
• Pneumonia patterns (lobar vs. bronchopneumonia vs. atypical)
• Pulmonary edema stages (cephalization → Kerley B → bat-wing)
• Pneumothorax (including the supine deep sulcus sign)
• Mediastinal widening with the 4 T's