"CNS Imaging" — Practice

Question 1

A biconvex hyperdense collection that does not cross the coronal suture indicates

Accepted Answer

B

Question 2

Which sequence is best for detecting infarction within the first hour?

Accepted Answer

C

Question 3

A young man struck on the temple loses consciousness briefly, recovers, then deteriorates an hour later. Explain the mechanism and the CT appearance.

Accepted Answer

Extradural haematoma, almost always from a torn middle meningeal artery beneath a temporal bone fracture.

The lucid interval is explained by the mechanism. The initial concussion causes brief loss of consciousness, from which the patient recovers, while arterial bleeding accumulates in the extradural space. Once the volume is sufficient to raise intracranial pressure, consciousness deteriorates again.

The biconvex shape follows from anatomy. The dura is firmly adherent to the inner table at the sutures, so extradural blood cannot spread past them and instead strips the dura inward. It can cross the midline, because the falx is itself a fold of dura and the blood lies superficial to it.

This is a neurosurgical emergency, and outcome is excellent if evacuated promptly, which is why the lucid interval must not be mistaken for recovery.

Question 4

An 80-year-old on warfarin has three weeks of confusion. CT shows effaced sulci on the right with 5 mm midline shift but no obvious collection. Interpret.

Accepted Answer

An isodense subdural haematoma. Between roughly one and three weeks the haemoglobin in a subdural collection degrades to the point where its attenuation matches brain, so the collection becomes nearly invisible on non-contrast CT.

The secondary signs reveal it: effacement of cortical sulci on the affected side, midline shift with no visible cause, ventricular compression, and displacement of the grey-white junction away from the inner table.

The clinical setting fits precisely. Cerebral atrophy in an elderly patient stretches the bridging veins so that trivial or forgotten trauma tears them, and anticoagulation both increases the chance of bleeding and prolongs it.

MRI resolves the diagnosis where CT is equivocal, since blood remains conspicuous on MRI at all ages, and bilateral collections are particularly easy to miss because the midline shift cancels out.

Question 5

A patient had a sudden severe headache eight hours ago. CT is normal. What next, and why?

Accepted Answer

Lumbar puncture, timed appropriately, because a normal CT at this point does not exclude subarachnoid haemorrhage.

CT sensitivity is very high within the first six hours of onset but declines progressively thereafter as blood is diluted and cleared from the cerebrospinal fluid. At eight hours the negative predictive value is no longer sufficient to discharge a patient with a thunderclap headache.

The lumbar puncture is deliberately delayed to around twelve hours from onset, because that is the time needed for haemoglobin released into the cerebrospinal fluid to be metabolised to bilirubin, producing xanthochromia.

Xanthochromia distinguishes true haemorrhage from a traumatic tap, since a traumatic tap introduces fresh blood that has had no time to be metabolised. If haemorrhage is confirmed, CT angiography follows to identify the aneurysm.

Question 6

A patient in India has a first seizure. MRI shows a small cystic lesion with an eccentric bright dot inside it. Identify and outline management.

Accepted Answer

Neurocysticercosis in the vesicular stage, and the eccentric bright dot is the scolex of the larva. The appearance is close to pathognomonic and is one of the few situations in neuroimaging where a single finding establishes a parasitic diagnosis without tissue.

Neurocysticercosis is the commonest cause of adult-onset seizures in much of India, so the pretest probability is high, unlike in most Western settings where a cystic or ring-enhancing lesion would prompt a search for metastasis.

Stage matters for management. In the vesicular stage the larva is alive and the surrounding brain is quiet. As the parasite dies it enters the colloidal stage, the host mounts an inflammatory response, and the lesion develops ring enhancement with surrounding oedema, which is when seizures typically occur. It then involutes through a granular nodular stage to a calcified nodule.

Treatment combines antiepileptic drugs, corticosteroids to control the inflammatory response, and antiparasitic therapy in selected patients, since killing live cysts provokes inflammation and can worsen symptoms without steroid cover. Calcified inactive lesions do not require antiparasitic treatment.

Question 7

A ring-enhancing lesion shows restricted diffusion in its centre. What does this indicate and why?

Accepted Answer

An abscess rather than a necrotic tumour.

Diffusion-weighted imaging measures the freedom with which water molecules move. Pus is thick and cellular, containing inflammatory cells, bacteria, proteins and debris, and that viscosity impedes water movement, so the abscess cavity shows restricted diffusion and appears bright, with corresponding dark signal on the apparent diffusion coefficient map.

The necrotic centre of a high-grade glioma or metastasis contains liquefied tissue of far lower viscosity, so water moves relatively freely and diffusion is not restricted.

This single sequence therefore separates two entities that can look identical on contrast-enhanced imaging, quickly and without additional contrast.

The practical importance is that management diverges completely. An abscess requires drainage and prolonged antibiotics with a search for the source, including endocarditis and a dental or sinus focus, whereas a tumour requires biopsy, staging and a quite different operation.

Question 8

A patient wakes with hemiplegia of unknown onset time. Non-contrast CT is normal. How does advanced imaging change the decision about treatment?

Accepted Answer

Wake-up stroke was historically excluded from thrombolysis because the time of onset is unknown and conventional windows are defined by the clock. Advanced imaging replaces the clock with a direct assessment of tissue.

The underlying idea is the ischaemic penumbra: tissue that is underperfused and functionally silent but not yet infarcted, and therefore salvageable if flow is restored. Diffusion-weighted imaging shows the core that has already infarcted, while perfusion imaging shows the larger region of reduced flow. The difference between them is the mismatch, and a substantial mismatch indicates that a large volume of tissue remains viable.

A related surrogate is the diffusion-FLAIR mismatch, in which a lesion visible on diffusion imaging but not yet on FLAIR implies onset within roughly the last few hours, since FLAIR change takes longer to appear.

The practical consequence is that patients with unknown or extended onset time can be selected for thrombolysis or thrombectomy on the basis of tissue viability rather than on time alone, and CT angiography identifies large vessel occlusion suitable for thrombectomy.

Question 9

A patient with known prostate cancer has back pain and leg weakness for two days. A colleague requests MRI of the lumbar spine only. Comment.

Accepted Answer

The imaging should cover the whole spine, and it should be urgent.

Malignant spinal cord compression is multilevel in a significant proportion of cases, because metastases seed the vertebral column at several sites, and prostate cancer is one of the tumours with a marked predilection for bone. Imaging only the clinically suspected level risks missing a second site of compression, which would alter both the radiotherapy field and any surgical plan.

The clinical localisation is also unreliable. Pain frequently arises at a level different from the compression, and radicular symptoms can be misleading, so the clinically identified level should not define the imaging.

Urgency matters because neurological outcome depends heavily on function at the time treatment begins. A patient who is walking when treated usually continues to walk, whereas a patient who has already lost power rarely regains it.

Alongside imaging, dexamethasone is started immediately unless contraindicated, and oncology and spinal surgical teams are involved without waiting for the scan report.

Practice — "CNS Imaging"

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