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Radiology: A Detailed Overview

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1. What is Radiology?

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2. The Major Imaging Modalities

2.1 Plain Radiography (X-Ray)

• Posteroanterior (PA) and lateral views suffice for most purposes
• Portable/AP radiography used in critically ill patients (has technical limitations vs. PA - magnification, lack of inspiration control, overlying tubes/lines)
• Lateral decubitus - detects small pleural effusions (>10 mL)
• Lordotic view - visualises lung apices without rib overlap

• Dual-energy subtraction (DES): Two acquisitions at different kV exposures, digitally subtracted to produce separate bone- and soft-tissue images, improving lesion conspicuity (see image below)
• Tomosynthesis: Multiple low-dose projections reconstructed as tomographic slices, improving lesion detection vs. conventional radiography

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2.2 Computed Tomography (CT)

From Single-Slice to Multidetector CT (MDCT)

• Spiral (helical) CT was introduced in the early 1990s - replaced sequential single-slice acquisition with volumetric data
• 1998: First multidetector (MDCT) systems launched
• Modern systems: up to 320 active detector rows, rotation times as low as 0.33 seconds/rotation
• Enables single breath-hold whole-thorax imaging, reduces motion artefacts (particularly important in paediatrics), and produces isotropic voxel data for 3D reconstruction

Reconstruction Algorithms (Kernels/Filters)

Window Settings

• Lung windows: Wide windows, low centre - visualise lung parenchyma
• Mediastinal windows: Narrow windows, higher centre - soft tissue/vessel detail
• Manual window adjustment is needed (e.g., when contrast appears too dense and may obscure a pulmonary embolus)

Slice Thickness and Clinical Use

• Narrow sections (0.6-1.25 mm): High-resolution; for nodule characterisation, interstitial lung disease, pulmonary embolism
• Wide sections (2.5-5 mm): Better contrast; mediastinal masses, quick review, lung cancer staging

CT Postprocessing Techniques

Dual-Energy CT (DECT)

• Pulmonary blood volume maps: Surrogate for lung perfusion - identifies perfusion defects in acute/chronic pulmonary embolism
• Virtual unenhanced imaging: Eliminates need for a separate pre-contrast acquisition when evaluating pulmonary nodule enhancement
• Iodine mapping: Differentiates iodinated contrast from calcium/haemorrhage

Dose Reduction Strategies

• Iterative reconstruction (IR): Default in modern CT systems. IR reduces image noise significantly, allowing diagnostically comparable images at lower doses. Especially important in CT lung cancer screening and paediatric patients
• Automatic tube current modulation
• Low kV protocols for vascular studies

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2.3 Ultrasound (US)

• High-frequency linear probe (5-7.5 MHz): Chest wall evaluation
• Sector/phased-array probe (3.5 MHz): Pleural and pulmonary disease

1. Pleural effusion - detects small effusions missed on supine radiographs; identifies septations and echogenicity (suggestive of exudate)
2. Guided aspiration/drainage - guides thoracocentesis for both simple and loculated collections
3. Pneumothorax detection (especially in ICU/supine patients):
   • Loss of lung sliding
   • Presence of the "lung point" on B- and M-mode (pathognomonic - intermittent visualisation of collapsed lung sliding into view at the pneumothorax border)
   • Accentuated reverberation artefacts (A-lines)
   • Absent/decreased comet-tail artefacts (B-lines)
   • "Stratosphere" sign on M-mode (replaces normal "seashore" sign)
4. Alveolar recruitment assessment in ventilated ICU patients with severe respiratory failure
5. Endoscopic/Endobronchial Ultrasound (EUS/EBUS): Cornerstone of mediastinal node staging in lung cancer; allows real-time guided needle biopsy of mediastinal lymph nodes and peri-oesophageal structures

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2.4 Magnetic Resonance Imaging (MRI)

• Low signal-to-noise ratio (lung has few protons)
• Respiratory and cardiac motion artefacts
• Very short T2* decay from heterogeneous magnetic susceptibility at pulmonary interfaces

• Single-shot techniques (HASTE, FASE): Entire k-space filled in one breath-hold; yields T2-weighted images
• 3D GRE sequences (VIBE): T1-weighted, fast, with or without fat saturation
• Respiratory triggering + ECG gating: Mitigates motion
• Navigator echoes: Track diaphragmatic motion for respiratory triggering

• UTE/ZTE sequences: Allow lung parenchyma imaging by capturing very short T2* signal - previously impossible
• Diffusion-Weighted Imaging (DWI): Evaluates tumour cellularity (best for lymphoma, small cell lung cancer); differentiating histological subtypes in mesothelioma
• Hyperpolarised gas MRI (³He, ¹²⁹Xe): Reveals subclinical ventilation defects in obstructive lung disease with far greater sensitivity than spirometry; also provides semiquantitative alveolar volume and surface area measurement

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2.5 Radionuclide Imaging (Nuclear Medicine)

Ventilation-Perfusion (V/Q) Scintigraphy

• Ventilation: Inhaled radiolabelled aerosol or gas (e.g., ⁹⁹ᵐTc-DTPA aerosol, ¹³³Xe)
• Perfusion: IV injection of ⁹⁹ᵐTc-labelled macroaggregated albumin (MAA) - particles lodge in pulmonary capillaries proportional to flow
• Main use: Diagnosis of pulmonary embolism (PE), especially when CT is contraindicated (renal impairment, contrast allergy, pregnancy - lower radiation to gonads)
• Interpreted using PIOPED II criteria

PET and PET-CT

• PET: Detects positron-emitting radiotracers (primarily ¹⁸F-FDG - fluorodeoxyglucose), exploiting the Warburg effect (increased glucose uptake in metabolically active/malignant tissue)
• PET-CT fusion: Combines metabolic information (PET) with anatomical detail (CT) in a single examination
• Established roles:
  • Lung cancer staging (N and M staging, detecting unsuspected extrathoracic metastases)
  • Differentiating malignant from benign pulmonary nodules
  • Assessing treatment response
  • Staging lymphoma
  • Evaluation of solitary pulmonary nodule (SUV >2.5 suspicious for malignancy)
• Limitations: False positives in active infection/inflammation (e.g., sarcoid, aspergillosis); false negatives in low-metabolic tumours (carcinoid, BAC/AIS)

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3. Normal Radiological Anatomy of the Chest

The Lungs and Hila

• The trachea is a midline structure, slightly deviated to the right at the carina. Its right lateral wall forms the right paratracheal stripe (width <4 mm on PA film)
• The carina angle is normally <70°. Widening suggests left atrial enlargement
• Hilar shadows on CXR are predominantly formed by the pulmonary arteries (not lymph nodes). The left hilum is higher than the right in ~97% of patients
• Lung markings extend to the pleural surface; absence suggests pneumothorax

The Mediastinum

• Right paratracheal stripe: <4 mm; widening = lymphadenopathy, thyroid, right-sided aortic arch
• Azygo-oesophageal recess: Concave interface; convexity = subcarinal lymphadenopathy, oesophageal lesion
• Paraspinal lines: Paravertebral soft tissue; widening = haematoma, lymphoma, abscess, aneurysm
• Retrosternal line (lateral view): Anterior mediastinal mass obliterates retrosternal airspace

The Pleura

• PA erect: Blunting of costophrenic angle (>200 mL), meniscus sign, opacification
• Supine (ICU): Haziness of hemithorax (layering fluid) - may be missed
• Subpulmonary effusion: Apparent elevation of hemidiaphragm; distance between lung base and gastric bubble >2 cm (left side)

The Diaphragm

• Right hemidiaphragm sits at the anterior end of the right 6th rib; left is ~1.5 cm lower (due to gastric bubble)
• Eventration vs. true paralysis: fluoroscopy (sniff test) - paradoxical movement indicates paralysis
• Diaphragmatic humps (focal elevation): Most commonly Morgagni or Bochdalek hernias, or anterior fat pad

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4. Key Radiological Patterns and Their Significance

Airspace (Alveolar) Opacification

• Causes: Pneumonia, pulmonary oedema, haemorrhage, aspiration, alveolar proteinosis
• Distribution matters: Perihilar/bilateral (oedema), unilateral/segmental (pneumonia)

Interstitial Patterns

• Ground-glass opacity (GGO): Partial airspace filling; causes include early oedema, pneumocystis, NSIP, atypical pneumonia, COVID-19
• Reticulation: Thickened interlobular septa; IPF, sarcoid, lymphangitic carcinomatosis
• Honeycombing: Destroyed lung architecture in UIP/IPF pattern; subpleural, basal predominant
• Crazy paving: GGO + thickened septa; pulmonary alveolar proteinosis (classic), COVID-19, ARDS

Air Trapping and Emphysema

• Centrilobular emphysema: Focal areas of low attenuation around centrilobular arteriole; smoking-related, upper lobe predominance
• Panlobular emphysema: Uniform destruction of acinus; alpha-1 antitrypsin deficiency, lower lobe predominance
• Paraseptal emphysema: Subpleural, often bullous; associated with spontaneous pneumothorax in young men
• Air trapping: Seen on expiratory CT as persistent low-attenuation lobules (obliterative bronchiolitis, asthma)

Pulmonary Nodules

• Spiculated/irregular border: High risk for malignancy
• Smooth, well-defined, <6 mm: Very low malignancy risk
• Calcification patterns: Popcorn (hamartoma), central/laminar (granuloma - benign), eccentric (potentially malignant)
• Ground-glass nodule: May represent adenocarcinoma-spectrum lesion (AIS/MIA); requires long-term follow-up

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5. Interventional Radiology (IR)

Vascular IR

• Angiography: Diagnostic and therapeutic; coronary, cerebral, peripheral
• Embolisation: Haemostasis (trauma, GI bleeding, haemoptysis, fibroids, arteriovenous malformations)
• Angioplasty and stenting: Renal artery stenosis, peripheral arterial disease, carotid stenosis
• Thrombolysis/thrombectomy: PE (catheter-directed), DVT, ischaemic stroke
• TIPS (Transjugular Intrahepatic Portosystemic Shunt): Portal hypertension management
• TACE/TARE: Transarterial chemoembolisation/radioembolisation for hepatocellular carcinoma

Non-Vascular IR

• Biopsy guidance: CT-/US-guided lung, liver, renal, lymph node, bone biopsies
• Drainage: Percutaneous abscess drainage, nephrostomy, biliary drainage, pleural drainage
• Ablation: Radiofrequency, microwave, cryoablation for liver, lung, renal tumours
• Vertebroplasty/Kyphoplasty: Cement injection for osteoporotic vertebral fractures
• Spinal procedures: Nerve blocks, discography, facet joint injections

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6. Radiation Safety and Dose

• Justification: Benefit must outweigh risk
• Optimisation: Use minimum dose for diagnostic quality
• Dose limitation: Legislative dose limits for workers and public

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7. Subspecialties of Radiology

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8. Emerging Technologies

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