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Chapter 7: Cerebrovascular disease 295

Bleeding, infection, arachnoiditis, exacerbation of spinal various processing which may be performed on the data.
cordcompression,cerebralherniationandspinalcordor However, the main disadvantage is that any metal in the
root injury are all recognised risks, but occur rarely. patient not only causes interference, but can become dis-
lodged. Pacemakers may be reprogrammed by the elec-
tromagnetic pulses, and the magnetic field may induce
CT head
a large current leading to burns so MRI is contraindi-
Computed tomography detects differences in X-ray den- cated. The MRI scanner can also be claustrophobic, and
sitybetweenbone,brain,bloodandCSF.Thereisadiffer- in the case of sick patients, is relatively unaccessible –
ence in healthy tissue and infarcted, infected or oedema- although some units have facilities for ventilation in the
tous tissue. It may be possible to differentiate between MRI scanner.
old lesions and acute lesions – including infarcts and MRI differentiates soft tissues much more clearly than
bleeds. CT.
T1 weighted images: Fat, brain tissue and old hae-

Indications matomas have high signal strength. Bone, CSF, car-
Space-occupying lesions: cerebral tumours (may miss tilage and calcium have low signal strength (i.e. look

low-grade astrocytomas), abscesses. darker).
Cerebral infarction. T2 weighted images: High water content tissues and

Extracerebral or intracerebral haematomas. CSF have high signal strength (i.e. look white).

Subarachnoid haemorrhage (may miss up to 15%). Tumours as small as 5 mm are clearly seen, even in the

Pituitary for tumours, pituitary infarction (although brainstem and cerebellum (these areas with a lot of sur-

MRI is preferred if available). rounding bone are often poorly seen on CT). Acute in-
Intravenous contrast injections show areas of increased farction is seen earlier than on CT, at 6–12 hours. With
blood supply, breakdown of the blood-brain barrier and new techniques (diffusion weighted MRI) changes can
oedema. appear even earlier than this. MRI is excellent at detect-
Lesions under 1 cm in diameter may be missed, al- ing white matter lesions such as demyelination in multi-
though improvements in technology are increasing the ple sclerosis, or intracerebral small vessel disease. Spinal
sensitivity of scans. Small lesions close to the skull may cord MRI is useful for tumours, cord compression and
be missed. vascular malformations. Intravenous gadolinium is used
CT of the skull is also useful for detecting minor frac- as a contrast to demonstrate areas of increased vascular
tures particularly basal skull fractures, which may be dif- supply and oedema. It is also useful in MR angiography
ficult to see on skull X-ray. (MRA), for example when looking for AV malforma-
NewerCT scanners are able to diagnose cerebellar tions.
lesions more reliably than in the past, but MRI is still
preferred if brainstem or cerebellar lesions are sus-
pected. Cerebrovascular disease
Faster scans are now possible – particularly helpful for
patients unwilling or unable to lie flat for long, although
in some cases general anaesthetic may be necessary for Stroke
unco-operative patients.
Definition
Astroke is defined as a sudden onset of non-traumatic
MRI brain and spinal cord focal neurological defect of a vascular aetiology, that ei-
ther causes death or lasts more than 24 hours.
Magnetic resonance imaging uses the magnetic proper-
ties of protons to generate images of tissues.
It has the advantage of not exposing the patient to ion- Incidence
ising radiation (particularly important in young infants, Third commonest cause of death in Western World (1–2
childrenandpregnantmothers).Itisveryversatiledueto per 1000 per year).