RANZCR AIT flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Hounsfield unit formula

HU equals 1000 multiplied by (tissue attenuation minus water attenuation) divided by water attenuation. Water is 0 HU, air about minus 1000 HU, and cortical bone usually above 1000 HU.

Photoelectric versus Compton

Photoelectric absorption dominates at low kVp and scales with atomic number cubed, giving high contrast. Compton scatter dominates at higher energy and depends on electron density, reducing contrast but dominating dose in thick body parts.

Half-value layer

The thickness of a specified material that reduces beam intensity to half, equal to ln2 divided by the linear attenuation coefficient. Added filtration increases HVL, hardening the beam and lowering skin dose.

T1 versus T2 weighting

Short TR and short TE give T1 weighting, where fat is bright and fluid dark. Long TR and long TE give T2 weighting, where fluid is bright. Fat remains relatively bright on fast spin-echo T2.

Effective dose units

Effective dose, in sieverts, sums tissue-weighted absorbed doses to express overall stochastic risk. A chest radiograph delivers roughly 0.02 mSv, while CT abdomen and pelvis delivers about 7 to 10 mSv.

Doppler shift equation

The frequency shift equals twice the transmitted frequency times flow velocity times cosine of the insonation angle, divided by the speed of sound. Flow at 90 degrees produces no detectable shift.

Stochastic versus deterministic effects

Deterministic effects such as skin erythema and cataract have dose thresholds and worsen with increasing dose. Stochastic effects such as carcinogenesis are modelled as no-threshold probabilistic risks proportional to dose.

Ultrasound axial resolution

Axial resolution is the minimum separation of reflectors along the beam and improves with higher frequency and shorter pulses containing fewer cycles. Higher frequency also reduces penetration depth.

Carina vertebral level

The tracheal bifurcation, or carina, lies at approximately the T4 to T5 vertebral level in adults. The aortic arch lies at about T4, and the aortic bifurcation is near L4.

Silhouette sign

Loss of a normal radiographic border indicates adjacent same-density pathology. Right middle lobe consolidation obscures the right heart border, and lower lobe disease obscures the hemidiaphragm.

Air bronchogram significance

Air-filled bronchi outlined against opacified lung indicate alveolar filling with patent proximal airways, favouring pneumonia or pulmonary oedema rather than complete obstruction with collapse.

CT pulmonary angiogram clot

A filling defect within a contrast-opacified pulmonary artery on CT pulmonary angiography diagnoses pulmonary embolism. Right ventricular to left ventricular ratio above one suggests right heart strain.

Mechanical obstruction versus ileus

Small bowel loops over 3 cm with a transition point and distal collapse suggest mechanical obstruction. Uniform dilatation of small and large bowel without a transition point suggests paralytic ileus.

Aortic dissection CT sign

An intimal flap separating true and false lumens on CT angiography defines aortic dissection. Intramural haematoma instead shows crescentic high-attenuation mural thickening without a visible flap.

Benign bone lesion features

A narrow zone of transition with a sclerotic margin, no cortical destruction, and a non-aggressive periosteal pattern favour a benign bone lesion. Wide transition and cortical breach favour malignancy or infection.

ASPECTS score purpose

The Alberta Stroke Program Early CT Score quantifies early ischaemic change on non-contrast CT across ten regions. A lower score indicates more extensive infarction and poorer expected outcome after reperfusion.

Oedema versus consolidation

Pulmonary oedema typically produces bilateral perihilar airspace opacity with septal lines, pleural effusions, and cardiomegaly. Consolidation is more often lobar and segmental with air bronchograms from infection.

Pneumothorax radiograph signs

A visible pleural line with absent lung markings beyond it indicates pneumothorax. Tension physiology adds mediastinal shift away from the affected side and ipsilateral diaphragmatic depression.

Small bowel obstruction signs

Dilated central small bowel loops with valvulae crossing the full width, multiple air-fluid levels, and a transition point on CT indicate mechanical small bowel obstruction rather than ileus.

Free air on erect film

Subdiaphragmatic crescentic lucency on an erect chest or abdominal radiograph indicates pneumoperitoneum. CT is more sensitive and can localise the perforation site.

TNM staging meaning

T describes the extent of the primary tumour, N regional lymph node involvement, and M distant metastasis. Stage grouping combines these into prognostic categories guiding curative or palliative treatment.

RECIST target lesions

RECIST 1.1 selects up to five target lesions, maximum two per organ, measured by longest diameter. Response is classified by percentage change in the summed diameters from baseline.

LI-RADS 5 criteria

In a cirrhotic liver, arterial hyperenhancement with non-peripheral washout, enhancing capsule, or threshold growth in a lesion over 10 mm constitutes LI-RADS 5, indicating definite hepatocellular carcinoma.

Bosniak IV renal cyst

A Bosniak IV cystic renal mass shows enhancing solid components or thick irregular enhancing walls and septa. It is considered malignant and generally warrants surgical resection.

PI-RADS biopsy threshold

PI-RADS assigns suspicion from 1 to 5 on multiparametric prostate MRI. Scores of 3 or higher in the peripheral zone and 4 or higher in the transition zone generally prompt targeted biopsy.

TIRADS suspicious features

Solid hypoechoic composition, irregular margins, microcalcifications, taller-than-wide shape, and extrathyroidal extension raise thyroid nodule suspicion on ultrasound and support fine-needle aspiration.

Active contrast extravasation

On trauma CT, focal high-attenuation contrast outside vessels that enlarges on delayed phases indicates active bleeding. It predicts failure of conservative management and supports angiographic embolisation or surgery.

TIPS indication

A transjugular intrahepatic portosystemic shunt decompresses the portal system and treats refractory variceal bleeding or refractory ascites. Encephalopathy is the principal complication limiting its use.

Radiation pneumonitis pattern

Radiation pneumonitis appears one to six months after thoracic radiotherapy as opacity with straight, non-anatomic borders confined to the treatment portal, helping distinguish it from tumour recurrence.

Gadolinium renal restriction

Gadolinium-enhanced MRI is avoided in severe renal impairment with eGFR below 30 because of nephrogenic systemic fibrosis risk, particularly with earlier linear agents. Macrocyclic agents carry lower risk.

Adrenal incidentaloma workup

Adrenal masses under 10 HU on unenhanced CT are lipid-rich benign adenomas. Higher attenuation requires washout imaging or biochemistry to exclude pheochromocytoma and functioning or malignant lesions.

Pulmonary nodule follow-up

Solid pulmonary nodules are managed by size and risk: nodules under 6 mm in low-risk patients usually need no routine follow-up, while larger or growing nodules require interval CT or PET-CT.