Positron emission tomography (PET Scan)
Positron emission tomography, also called PET imaging or a PET scan, is a type of imaging that uses small amounts of radioactive material called radiopharmaceuticals or radiotracers.
Low-dose radionuclides (atoms with an unstable nucleus and hence emitting energy in the form of gamma rays) are produced by a cyclotron (an atom smashing radiation unit) and then chemically bound to a molecule such as glucose (a simple sugar molecule) and injected into the patient before the scan begins. The radioactive glucose then accumulates in the brain and tumour, where it gives off gamma rays detectable by a device called a gamma camera.
The PET scanner containing the gamma camera rotates around the patient's head, detecting the rays emitted by the radioactive molecules and hence measuring brain activity as it metabolises (converts) the glucose into energy, indicated by concentrations of the radioactive glucose. By measuring metabolic rates, PET scanning also gains information about blood flow and oxygen use, and hence levels of malignancy or grade in brain tumours.
Results are recorded onto a computer, which produces a colour-coded moving picture or a greyscale image of the brain. Highly 
Malignant brain tumours consume glucose at a higher rate than normal brain, and so degrees of malignancy can be detected using this method. For example, areas of the brain which emit lots of gamma rays may be shown as red spots ('hot spots') on the picture on the computer monitor. Areas which emit low levels of gamma rays may be shown as blue ('cold spots'). Various other colours may be used for 'in between' levels of gamma rays emitted.
The use of PET is limited because cyclotrons are scarce; there are very few PET scanners in the UK and so this type of scan is used mainly for research purposes. In some centres, PET images can be combined with computed tomography (CT) or magnetic resonance imaging ( MRI) to produce more detailed images, a practice known as image fusion or co-registration.
This combined information can be superimposed on one image, leading to more precise information. Some manufacturers are now making PET/CT scanning units that are able to perform both types of imaging studies at the same time.
Interpretation of Scans
A Neuroradiologist interprets the computer images produced by CT, MRI and PET scans. The pictures help establish a tentative diagnosis of tumour type and grade, indicate changes in your tumour through comparison with previous scans, or give more detailed information if a more accurate form of scanning is undertaken than on previous occasions. However, only examination of an actual sample of tumour tissue gained through a biopsy or surgical resection can provide an exact diagnosis of tumour type and grade.
Sources:
ABTA’s Primer for Brain Tumours downloaded 17.5.09 from http://neurosurgery.mgh.harvard.edu/abta/primer.htm
Colorado Springs Imaging, downloaded 18.5.09 from http://www.coloradospringsimaging.com/index.php
PatientInfo UK, downloaded 18.5.09 from http://www.patient.co.uk/showdoc/27000361/
Radiology Consultants of North Dallas, downloaded 18.5.09 from http://www.rcnd.com/AboutUs.html
Radiology Info, downloaded 17.5.09 from http://www.radiologyinfo.org/en/info.cfm?PG=headct
Image Source: http://en.wikipedia.org/wiki/Pet_scan
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