Brain scan

Brain scans are done to help investigate the cause of persistent headache, confusion, fits, memory problems, or focal weakness in the arms or legs. They can help in confirming or ruling out the presence of lesions (e.g. toxoplasmosis, non-Hodgkin's lymphoma) or central nervous system disease (e.g. progressive multifocal leukoencephalopathy, or PML).

Brain scans are performed using magnetic resonance imaging (MRI), computed tomography (CT) or computed axial tomography (CAT), positron emission tomography (PET), and single photon emission computed tomography (SPECT). These techniques directly or indirectly image the structure and function of the brain. Structural imaging is used to diagnose tumours or injury. Functional imaging is used to diagnose metabolic diseases and lesions (such as Alzheimer’s).

In HIV care, MRI and CT/CAT scans are mainly used and the choice is often dictated by available hospital facilities and cost. Both methods are non-invasive and painless, as are PET and SPECT.

MRI uses magnetic fields and radio waves to produce brain structure images without the use of radiation. It renders brain surface and subsurface images with a high degree of anatomical detail. Originally, MRIs could only provide information on the physical appearance of the brain, including assessment of water content, inflammation, and bleeding. Functional MRI (fMRI) imaging provides the same structural information, but functioning (brain activity) information as well. MRIs can produce cross-sectional images in any direction. Some people feel claustrophobic in an MRI machine. It helps if the patient closes their eyes while the 'bed’ is sliding into the MRI tube. Some people are more comfortable if they are first given some type of sedation.

CT or CAT scans take a series of X-rays from many angles. Normally, this technique is used for viewing brain injuries, assessing ventricle size, and evaluating tissue damage caused by swelling. The scan takes just a few minutes to complete.

PET measures emissions from radioactively labelled metabolically active chemicals that have been injected into the bloodstream. When these chemicals reach the brain, two- and three-dimensional images are produced that map neurotransmitter activity. This test can also be completed in moments and is useful in the diagnosis of tumours, strokes, and other diseases that can cause dementia.

SPECT is somewhat similar to PET, but uses gamma ray emitting radioisotopes and a camera to record data. An injection of radioactive tracer is taken up by the brain and cerebral blood flow is reflected. SPECT is mainly used for imaging epilepsy or diagnosing brain disease, such as dementia.

Brain scans are useful in diagnosing problems such as toxoplasmosis, lymphoma, HIV-associated dementia, or PML (progressive multifocal leukoencephalopathy). Brain scans are also done to monitor the effect and response to treatment.