‘Stroke’ is a clinical diagnosis characterized by a sudden onset of acute neurological symptoms of presumed vascular origin. This chapter illustrates some of the common and not so common appearances in acute ischaemic stroke.

The role of CT in the modern medical setting has become imperative with the advent of thrombolytic treatment for ischaemic infarction of under 3 hours’ duration.

There are three principle goals in stroke imaging and MDCT is useful in each area:

This section will cover the first two of the above issues. It is not the intention simply to illustrate this with case after case of acute ischaemic infarction (this is well covered in standard neuroradiological texts), but rather to show how MDCT can offer benefits in certain aspects of infarction identification. Up to 75% of trial patients with proximal MCA occlusion have identifiable ischaemic changes on CT at 3-6 hours after the stroke onset.

It may be difficult to identify the very early changes of ischaemia on CT. If the eyes are included on the scan and they are deviated to one side, it is more than likely that the patient has a total anterior circulation stroke affecting the hemisphere towards which the eyes are deviated. When identified, look extra carefully for either a dense artery sign, or subtle loss of the grey matter definition in the insula and/or basal ganglia, on the ipsi-lateral side.

Always look for a hyperdense artery. This is most regularly reported in the MCA, but it can be identified in any first or second order vessel including the ICA, branch MCA, basilar, and posterior cerebral arteries. Thin section acquisition, e.g. 1-2 mm, has been shown to increase the identification of vessel occlusion and early ischaemic loss of grey/white matter differentiation, so this should be used routinely if possible.

One of the new exciting and useful benefits of MDCT is the ability to offer routine perfusion brain imaging. The role of perfusion imaging in the management of acute stroke is still under debate. In our centre it is currently used as a troubleshooting tool. Presently, all patients receive an immediate CT brain scan. In cases where there is doubt as to the patency of the vessels, MDCTA is performed. This is probably the most appropriate additional technique. CT perfusion is used when there is doubt as to the time of onset of symptoms or where a second infarct or extension of the first infarct may have occurred. We currently operate on a 3 hour time limit for thrombolysis, but this is likely to increase to 6 hours as more data become available.

It is not within the remit of this book to give a full technical description of perfusion CT. The essential technique is to scan the same volume of brain repeatedly
during a high infusion rate injection of iodinated contrast. The aim is to generate sequential images which contain precontrast, arterial uptake, and washout before recirculation can occur. This is then postprocessed, usually with a deconvultional technique to obtain perfusion parameters on a voxel by voxel basis. The parameters in routine clinical use are the mean transit time (MTT), cerebral blood flow (CBF), time to peak (TTP), and cerebral blood volume (CBV). These are intimately related on a physiological level. Again, a conclusion on which parameters are ‘best’ or most suitable has not been reached. Interested readers are directed to the many articles on the topic, of which a few starter papers are given in the References section.

Most current techniques use a single slab of tissue, in the region of 4 cm width for a 64-slice scanner. This allows for a very short interval between scans and allows an attempt at quantitative imaging. Recently, ‘jog’ or ‘shuttle’ modes have been introduced, wherein after the initial scan, the gantry moves to an adjacent slab, another scan is performed, the gantry returns to the original position and the sequence is repeated. This allows qualitative scanning over a large volume (up to 8 cm width). Unfortunately the interscan interval is prolonged. The quantitative data are therefore unreliable and should not be used. If a purely qualitative assessment is required then the jog/shuttle mode is fast and reliable. If more detailed qualitative data are required then standard perfusion techniques should be utilized.