Neuroimaging interpretation: CT, MRI, vascular imaging
Imaging questions ask you to choose the right modality for the clinical question, name the sign, and interpret it in context rather than in isolation.
CT
Non-contrast CT detects acute haemorrhage as hyperdensity and bone well. Early ischaemic signs include loss of the insular ribbon, obscured lentiform nucleus, hyperdense MCA (dot sign in M2), and sulcal effacement. ASPECTS scores the MCA territory from 10 (normal) down to 0. Hyperdensity can also be calcification, contrast, or a dense vessel. CT is fast and available but insensitive to posterior fossa and early cortical lesions. CT venography diagnoses venous thrombosis (empty delta sign). CT perfusion identifies core and penumbra for late-window thrombectomy.
MRI
DWI with ADC is the most sensitive sequence for acute infarction (bright DWI, dark ADC); restricted diffusion also occurs in abscess pus, cellular tumours, and CJD. FLAIR shows oedema and demyelination; T1 shows anatomy; T2 shows fluid and pathology; GRE/SWI shows microbleeds, haemosiderin and calcification. Gadolinium enhancement indicates blood-brain barrier breakdown and is used for active demyelination and tumours. Ring enhancement: smooth thin ring with restricted diffusion centrally suggests abscess; thick irregular ring suggests glioblastoma or metastasis. PRES shows posterior vasogenic oedema without restricted diffusion. Cortical ribboning on DWI suggests CJD.
Vascular imaging
CTA and MRA identify occlusion, stenosis, dissection (string sign, mural haematoma on T1 fat-sat), and aneurysm. Digital subtraction angiography remains the reference standard and allows treatment. Carotid Doppler assesses stenosis; transcranial Doppler detects vasospasm after subarachnoid haemorrhage and raised velocities in sickle cell disease. MR venography diagnoses venous sinus thrombosis.
Worked example
A patient arrives 4 hours after sudden aphasia and right hemiparesis. Non-contrast CT shows no blood and ASPECTS 9; CTA demonstrates a left M1 occlusion; CT perfusion shows a small core and large penumbra. The imaging selects the patient for thrombectomy and permits thrombolysis if clinical criteria are met. A large established hypodensity with low ASPECTS would argue against intervention.
Traps
A normal CT does not exclude ischaemic stroke, subarachnoid haemorrhage after 6 hours, or posterior fossa lesions. T2 shine-through on DWI must be checked against ADC. Hyperdensity is not automatically haemorrhage. Pacemakers and metallic foreign bodies require MRI safety checks. Renal impairment limits gadolinium and iodinated contrast.