Using a modern multidetector computed tomography (MDCT) system, ‘routine’ brain scanning should usually be performed using incremental techniques. Scanning incrementally, rather than helically, gives high quality imaging while affording a lower radiation dose than scanning helically. The radiation dose from an incremental scan is typically 25-30% lower than for a similar volume scanned helically. Additionally, if a patient moves during an incremental scan, only one block of data is corrupted. Many more slices are corrupted for a helical scan.

Scanning incrementally also allows the scanner gantry to be angled to avoid the lens of the eye and to reduce the length of scan required to image the brain. Scanning helically usually means that the gantry cannot be angled, meaning a higher volume is scanned and the lens of the eye cannot be avoided.

With the speed of acquisition currently available there can be no reason not to apply a detailed technique. It is, however, disappointing to observe that when a hospital acquires a new scanner, and after the company demonstrator leaves, the brain imaging technique regresses to one similar to that of the old scanner with thick slices and high contrast images. Radiologists should ensure that thin slices are acquired which are combined to provide images with better spatial resolution and signal to noise, but they can also be reprocessed into the acquired thickness to provide good multiplanar reformation (MPR) if necessary. On a modern scanner, it is possible to reconstruct the combined data down to the acquired submillimetre sections for detailed analysis if necessary, whilst thicker combined sections are sent to the institutional picture archive and communication system (PACS).

If a lesion is seen on a routine scan and the decision to give contrast is made, the following contrast scan should be acquired with a helical technique as described in Chapter 2, or as an angiographic study as appropriate.

In specific situations, where scan information is used as a regular or repeated clinical update, it is possible to use a limited scan of a few thick slices with a markedly reduced radiation dose. This is specifically for follow-up of head injured/postoperative patients and those for intraventricular drainage monitoring. This should be used rarely outwith an interactive neurosurgical environment.

Table 1.1 presents the parameters for a routine scan of the brain. Figure 1.1 is a surview for routine and follow-up brain scans. Incremental, not helical, scanning allows an angulation of the gantry parallel to the floor of the anterior fossa to avoid irradiating the lens of the eye.

Table 1.2 presents the parameters for a follow-up brain scan.

Usually only the simple, axial plane reconstructions are necessary (1.2). To obtain the reverse temporal lobe angle for the assessment of the hippocampus in possible Alzheimer’s disease, take a sagittal plane through the temporal horn then angle the axial plane parallel to the temporal horn (1.19).

Some illustrations of disease/conditions will be given (1.3,1.4,1.5,1.6,1.7,1.8,1.9,1.10,1.11,1.12,1.13,1.14,1.15,1.16,1.17,1.18 and 1.19) but clearly almost any pathology could be shown and the vast majority of such examinations in the above clinical situation will be normal. A negative head scan can be a very reassuring examination and the anticipated and desired clinical result.