Orientation: horizontal slices of the patient are shown as if you are looking towards the patients head from the foot of the bed while they are lying supine (i.e. their left is on your right and vice versa)
Remember free air will rise (appear anteriorly) and free fluid will descend (appear posteriorly) due to the effect of gravity on the supine patient
Contrast vs non-contrast
Types of contrast:
Iodine-based IV contrast: used for opacification of vascular structures and solid abdominal and pelvic organs
Barium- or iodine-based oral or rectal contrast: used for bowel opacification for abdominal/pelvic CT scans
Contrast is not required if looking for: head trauma (acute blood is bright), stroke, bone trauma, parenchymal lung disease (HRCT used), kidney stones (radio-opaque)
CT scans use X-rays and therefore, like a standard radiograph, there are 4 densities (white to black):
Compresses thick slices (7-10mm) of lungs into images so no lung tissue is missed
High-resolution CT (HRCT) chest
Thin slices of lungs (1mm) scanned at wide intervals (1cm)
This gives a detailed image of the lung parenchyma but can miss lung lesions in between the thin slices
Used to look for parenchymal lung disease
CT pulmonary angiogram (CTPA)
Images taken when contrast reaches the pulmonary artery
Used to look for pulmonary emboli (seen as a filling defects)
Common brain pathology
Bleeds/haematomas
Subdural: crescent-shaped, spreads diffusely across affected hemisphere (hyperdense if acute, hypodense if chronic)
Extradural: lens-shaped, does not cross suture lines (hyperdense)
Subarachnoid: hyperdense material in subarachnoid space (can fill sulci, fissures, basal cisterns and ventricles)
Subdural vs. extradural haematoma
Stroke
Ischaemic: hyperdense clot in vessel, loss of grey-white matter differentiation, cortical hypodensity
Haemorrhagic (intracerebral haemorrhage): hyperdense material in brain parenchyma, surrounding hypodense oedema
Lesions
Glioblastoma: irregular thick margins, hypodense necrotic centre, surrounding oedema
Cerebral lymphoma: hyperdense lesion, may be multiple
Toxoplasmosis: ring-enhancing lesion
Cerebral metastasis: may be hyperdense, isodense or hypodense
Look for midline shift and herniation!
Common chest pathology
Pulmonary fibrosis: ‘honeycombing appearance’ due to lots of very small cystic air spaces
Bronchiectasis: dilated bronchioles (>1.5x adjacent pulmonary artery branch – normally the same size), lack of tapering, bronchi within 1cm of pleural surface
