In nuclear medicine studies, the planar imaging commonly utilized provides a tracer distribution image in two dimensions. The basic aim of PET scanning is to create an image representing three-dimensional (3D) distribution of tracer, by combining the use of positron-emitting radionuclides and emission CT. Initially, a research tool for a number of years, PET scanning is now emerging as a useful clinical tool in the management of patients with a variety of pathological conditions, although its efficacy for the urological patient is still undetermined.
Certain radioisotopes decay by releasing positrons, which are positively charged electrons. These positrons travel short distances (less than 2.5 mm) and collide with other electrons, which results in the release of two high-energy (511 keV) photons emitted at 180° to each other. The PET scanner comprises several rings of multiple crystal detectors which detect the emitted photon, thereby reconstructing a 3D image of tracer distribution within the body.
One of the advantages of PET scanning is that it utilizes isotopes of elements ubiquitous in the human body and therefore is able to image physiologically important chemicals throughout the body, providing useful functional and metabolic information. Nevertheless, the majority of these have a very short half-life and are impractical for routine clinical use and mainly confined to the research laboratory. Half-lives of 15O, 13N, and 11C are 2, 10, and 20 minutes, respectively.
The mainstay of clinical PET scanning is 18fluorine (half-life 2 h) which is used to produce 18fluorine-2-D-deoxyglucose (18FDG), an analog of glucose. FDG, like glucose, is preferentially transported in tumor cells via specific glucose transporters, due to their inherently increased rate of metabolism and glycolysis. Once within the cell, FDG undergoes phosphorylation by hexokinase to form FDG-6-phosphate, following which it becomes inert and takes no further part in glycolytic pathway. It remains trapped in the tumor cell, and subsequent accumulation will eventually increase tracer activity to levels detectable by the PET scanner. The tracer is excreted through the kidneys. Because small amounts of tracer can be visualized, early tumor detection is possible even before other cross-sectional imaging (like CT or MRI) can detect structural changes. Other tracers, apart from FDG, continue to be developed and tested but remain some way off from entering the clinical setting.
A dedicated full ring PET scanner is the gold standard, but it is expensive. An acceptable alternative is to use a modified multi-head gamma camera. This has the advantage in that it is cheaper and can be used for SPECT imaging. Though costly, a dedicated PET scanner is quicker, more sensitive, has superior resolution compared to the gamma camera, and does not require a collimator. Combining a CT scanner and PET scanner within a single imaging scaffold will provide excellent anatomical as well as functional information and is likely to become the PET imaging technique of the future.
The majority of data on PET scanning arises from studies in brain metabolism and non-urological cancers (e.g., lung, colorectal, head and neck, lymphoma). At present clear guidelines for the use of PET in the management of urological patients do not exist and clinical practice is based on local availability and physician preference. Data assessing PET performance in urological malignancies remains sparse and occasionally conflicting, although further evidence may confirm an emerging role.
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