Computerised Tomography

Computerised Tomography (CT), sometimes known as Computer Assisted Tomography (CAT), uses X-radiation to scan the brain or organ of interest. X-rays are produced by chemically unstable substances as a result of a chemical reaction, whereby the substance reaches its chemically stable state. CT scanning allows images of the brain or body to be generated as different tissues will transmit or absorb X-radiation depending on different attenuation coefficients (related to the density) of the various bodily tissues. The patient is placed between an X-ray source and an X-ray detector array. The X-ray tube and detector array trace a circular path around the patient. Multiple samples are taken across a series of orientations around the patient. At each sampled location a profile of different X-ray intensities is obtained. These sampled X-ray profiles are then filter back-projected, or processed using a specific mathematical algorithm, so that a reconstructed image can be produced i.e. a picture which represents the sampled tissue. For detail on the principles behind filtered back projection see Anderson and Gore (1997). CT scanning was introduced by Sir Godfrey Hounsfield in 1972 (Hounsfield 1973), however, it was only about 7 years later that three-dimensional rendering techniques were first used (Herman & Liu 1979) and then pioneered clinically in cranio-facial surgery in Australia (Hemmy 1987; Hemmy et al 1994).

CT scanning has been successfully used for demonstrating abnormalities in bone (particularly around the skull base), detecting acute haemorrhages (following a stroke) and highlighting brain tumours (Fig. 1). On the other hand, the main disadvantage with this technique is that the fine structural details of the soft tissues of the brain are not well seen. Another disadvantage is that X-radiation is used, so radiation safety issues become important. A contrast agent may need to be injected into a peripheral blood vessel for some studies where the integrity of the blood-brain barrier is being investigated (e.g. tumours, acute stroke), hence adding an invasive element to the investigation. This method of investigation may sometimes remain the only safe viable option, as the patient may have previously been implanted with a pacemaker, or other metallic device that prevents them from being safely scanned using magnetic resonance imaging (see below).

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