RANZCR AIT cheat sheet

Attenuation follows I = I0·e^(−μx); the linear attenuation coefficient μ rises with tissue density and effective atomic number and falls as photon energy increases above the relevant k-edge.

CT Hounsfield units are defined as HU = 1000 × (μ_tissue − μ_water)/μ_water, so water is 0 HU, air is approximately −1000 HU, and dense cortical bone typically exceeds +1000 HU.

Photoelectric absorption dominates at low kVp and scales roughly with Z³/E³, giving high soft-tissue contrast but higher dose; Compton scatter dominates at higher energies and depends mainly on electron density.

Half-value layer (HVL) = ln2/μ ≈ 0.693/μ; the HVL of a diagnostic beam increases with added filtration, which hardens the beam and lowers patient skin dose.

MRI: T1 recovery is governed by TR and T1 (short T1 = bright), T2 decay by TE and T2 (long T2 = bright); fluid is dark on T1 and bright on T2, and fat is bright on both fast spin-echo T1 and T2.

ALARA dose quantities: effective dose is measured in sieverts and weights tissue radiosensitivity, while absorbed dose is in gray; a chest radiograph is roughly 0.02 mSv versus approximately 7–10 mSv for a standard CT abdomen/pelvis.

Deterministic effects (skin erythema, cataract) have thresholds and increase in severity with dose; stochastic effects (carcinogenesis) are modelled as no-threshold linear risk, so dose reduction targets both.

Ultrasound axial resolution improves with higher frequency and shorter pulse (fewer cycles), while penetration falls with frequency; Doppler shift Δf = 2·f0·v·cosθ/c, so a 90° angle gives no detectable shift.

Standard normal anatomy landmarks: the carina lies at approximately T4–T5, the aortic arch at T4, the origin of the SMA at L1, renal hila at L1–L2, and the aortic bifurcation at L4.

Chest radiograph review sequence: technical adequacy (rotation, inspiration, penetration), then airway, breathing (lungs/pleura), circulation (heart/mediastinum), diaphragm, bones and soft tissues, and lines/tubes.

Silhouette sign: loss of a normal border indicates adjacent same-density pathology - right middle lobe disease obscures the right heart border, lingular disease the left heart border, and lower-lobe disease the hemidiaphragm.

Air bronchograms imply alveolar (airspace) filling with patent proximal airways, favouring pneumonia or pulmonary oedema over complete bronchial obstruction with collapse.

Pulmonary embolism: CT pulmonary angiography is first-line; a filling defect in a contrast-opacified pulmonary artery is diagnostic, and right heart strain (RV/LV ratio >1) on CT or echo indicates higher risk.

Bowel obstruction versus ileus: dilated small bowel loops (>3 cm) with a transition point and distal collapse suggest mechanical obstruction; uniform dilatation of small and large bowel without transition suggests ileus.

Acute aortic syndromes: an intimal flap separating true and false lumens on CT angiography defines dissection; intramural haematoma shows crescentic high-attenuation mural thickening without a flap.

Bone lesion analysis uses age, site, margin, matrix and periosteal reaction: narrow zone of transition and sclerotic margin favour benignity, while wide transition, cortical destruction and aggressive periosteal reaction favour malignancy or infection.

Stroke imaging: non-contrast CT excludes haemorrhage; CT angiography detects large-vessel occlusion; ASPECTS quantifies early ischaemic change, and perfusion imaging identifies salvageable penumbra.

TNM staging logic: T describes primary tumour extent, N regional nodal involvement, and M distant metastasis; stage grouping combines these into prognostic categories that direct treatment intent.

RECIST 1.1 uses the sum of longest diameters of up to five target lesions (maximum two per organ); complete response requires disappearance of all lesions and normalisation of nodes to short axis under 10 mm.

LI-RADS and Bosniak systems standardise reporting: arterial hyperenhancement with washout and capsule in a cirrhotic liver is LI-RADS 5 (definite HCC), while a Bosniak IV renal cyst shows enhancing solid components.

PI-RADS scoring on multiparametric MRI: a score of 3 or higher in the peripheral zone or 4 or higher in the transition zone generally prompts targeted biopsy.

Thyroid nodules are risk-stratified by ultrasound (TIRADS): solid hypoechoic composition, irregular margins, microcalcifications and taller-than-wide shape raise suspicion and support fine-needle aspiration.

Multi-trauma imaging follows ATLS priorities; whole-body CT with arterial and portal venous phases detects active contrast extravasation, which indicates haemorrhage requiring angiographic embolisation or surgery.

Interventional radiology: transjugular intrahepatic portosystemic shunt (TIPS) treats refractory portal hypertension, and coil or glue embolisation controls acute arterial gastrointestinal bleeding when endoscopy fails.

Radiotherapy and radiology interface: radiation pneumonitis typically appears 1–6 months after thoracic radiotherapy as a geometric opacity confined to the treatment portal, distinguishing it from recurrent tumour.

Contrast safety: iodinated contrast is withheld or premedicated in prior moderate-to-severe reaction, and gadolinium is avoided in severe renal impairment (eGFR below 30) because of nephrogenic systemic fibrosis risk with linear agents.