Meningiomas are benign well defined tumours occurring in characteristic sites related to the meninges. Frequent sites are the falx, parasagittal region and the wing of sphenoid. On computed tomography they appear as homogenous high attenuation lesions with rounded margins. Calcification is present in 16-20%. There is marked enhancement after contrast and no surrounding oedema. Magnetic resonance imaging shows similar appearances with marked enhancement after gadolinium. MRI is not as good as computed tomography because it does not detect small calcifications.
Pituitary tumour : small pituitary tumours will not show changes on plain films and these are best shown by magnetic resonance imaging which is better than computed tomography for examining the pituitary fossa. Microadenomas of the pituitary cause hyperprolactinaemia and are usually less than 1 cm in size. High resolution computed tomography or magnetic resonance is necessary to demonstrate these. However, hyperprolactinaemia is a non-specific sign and there are other causes such as drugs. Imaging is not usually indicated with a serum prolactin measuring less than 1000units.
raniopharyngiomas occur in childhood or adolescence. Calcification occurs in 80% and some can be diagnosed purely from the plain film findings of midline calcification above the pituitary fossa. 15% grow into the pituitary fossa causing enlargement. They are related to the floor of the 3rd ventricle making surgical removal difficult. On computed tomography they are cystic or partially cystic with enhancement in the solid parts after contrast.
Cerebellar tumour: the majority occur in children. Medulloblastoma is the commonest followed by ependymoma. Medulloblastoma on computed tomography shows as a hyperdense central mass growing into the 4th ventricle causing hydrocephalus. It shows intense enhancement after contrast. Magnetic resonance imaging is better for imaging the posterior fossa than computed tomography.
Primary Lymphoma usually appears as a hyperdense lesion on computed tomography showing homogenous enhancement after contrast with little surrounding oedema. It usually occurs deep within the brain in the basal ganglia or paraventricular region
2. VASCULAR LESIONS
This may be:
hypertension in elderly
rupture of angioma
rupture of berry aneurysm
acute leukaemia, anticoagulants
metastases – some may be haemorrhagic – rare
Haemorrhage is well shown on computed tomography. Intracerebral haemorrhage shows as an area of high attenuation i.e. increased whiteness, and shows clearly with a sharp demarcation from the surrounding brain. Blood may rupture into the ventricular system. Haemorrhage is not well seen on magnetic resonance imaging especially if small and computed tomography is the imaging method of choice.
Subarachnoid haemorrhage (SAH)
Computed tomography is the imaging method of choice. This is positive in over 80% of cases. Magnetic resonance imaging is not reliable and will not show small amounts of blood. A negative CT scan does not exclude a subarachnoid haemorrhage as very small leaks may not show. If strongly suspected a lumbar puncture should be performed if the scan is negative.
Signs on computed tomography:
High density (blood) filling in the basal cisterns and subarachnoid space. Appears white. May be very obvious or subtle signs when the bleed is small. The blood is usually larger in amount near the site of the aneurysm and this may indicate which vessel the aneurysm is arising from. Most aneurysms arise from the circle of Willis.