Introduction and imaging methods

Computed tomography (CT) and magnetic resonance imaging (MRI) are the mainstays of cerebral imaging. Skull radiography now plays very little part in diagnosis, being largely replaced by multislice CT.

Non- or minimally invasive angiography performed using CT (CT angiography) or MRI (magnetic resonance angiography) has resulted in invasive catheter angiography being reserved for a few special diagnostic indications or as part of an interventional, (therapeutic), procedure.

Anatomical detail is far better displayed by MRI than by CT, although both are valuable in clinical practice.

With T1-weighted (T1W) MR images, grey matter is of lower signal intensity (darker) than white matter (Fig. 1.1 ). On T2-weighted (T2W) images, including T2-FLAIR sequences, the reverse is true (Fig. 1.2).

With CT, somewhat paradoxically, white matter is depicted as darker grey than grey matter (Fig. 1.3 ). The explanation is that CT is an X-ray investigation. White matter contains lipid as part of myelin, which is relatively radiolucent.

The appearance of myelinated tracts on MRI is rather more variable and will be inl uenced by the pulse sequence used. In perhaps its simplest form, the lipid in subcutaneous fat is typically high signal (white) on both T1 and T2 MR sequences.