Nuclear Medicine Imaging

In nuclear medicine imaging, a radioactive source is injected into the patient. The patient is placed in a detector array, and the radiation emitted from the body is measured. The detector array is sometimes configured similar to that for CT. Other systems utilize a two dimensional array of detectors. The major difference between CT and this type of study, is that with CT the radiation source's location is external and known, while in nuclear medicine, the source is internal and its distribution unknown.

Nuclear medicine studies allow for very flexible imaging. Radioactive tracers can be attached to hundreds of different substances. By tagging molecules that seek specific sites in the body, it is possible to image many physiologic processes. For example, it is possible to tag glucose with radioactive isotopes and image its distribution, which indicates the metabolic process of cells. Nuclear medicine studies produce images with fairly low resolution and a high amount of noise. This is due to the inability to use high radiation doses because the substance will stay within the body and circulate through the entire body.

The two most common types of nuclear medicine studies are Single Photon Emission Computed Tomography (SPECT) and Positron Emission Tomography (PET). SPECT studies utilize radiotracers which emit photons while decaying. PET studies utilize radiotracers that produce positron-electron pairs. PET is more flexible because it is easier to produce a wide variety of radioactively tagged substances.

The output of a SPECT or PET scanner is typically a set of 10-30 transaxial slices. Each slice is typically 5-10mm thick and contains pixels 5-10mm in size. There may be gaps between the slices. This type of study produces images which have few anatomic cues. The images mimic physiologic activity which may or may not be easily relatable to the underlying anatomy.

Medical Imaging Techniques
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