Imaging

The combining of PET and CT dramatically improves both sensitivity and specificity in staging malignant disease and monitoring response to therapy.

Well, we all knew it was going to happen. It was just a matter of time before the need for better speed, quality, and data image attributes would naturally lead to a combining of two stand-alone radiology modalities. The merging of computerized tomography (CT) with positron emission tomography (PET) was an inevitable marriage of form and function. CT provides anatomic images with reasonably good localization of organs and lesions in general, while PET maps both normal and abnormal tissue function. When combined, the two modalities can help both identify and localize functional abnormalities. Currently there are over 800 combined CT/PET scanners in operation worldwide, many of them are being used to stage malignant disease, as well as to monitor response to therapy.

Proponents of this hardware fusion cite that both sensitivity and specificity are improved when both testing technologies are combined, making the CT/PET test more accurate than either CT or PET alone. The synergistic advantage of adding CT to PET is that the attenuation correction needed for PET can also be derived from the CT data, an advantage not obtainable by integrating PET and magnetic imaging resonance (MRI). CT/PET examination is 25% - 30% faster than PET alone and using standard attenuation correction methods. This leads to higher patient throughput and a more comfortable patient examination— typically taking 30 minutes or less.

Historical Perspective
PET has historically been used in oncology settings due to the useful nature of the information it provides. The properties of positron decay, permit accurate imaging of the distribution of positron-emitting radiopharmaceuticals and has been used to characterize various physiologic and pathologic states. For example, PET has been used to identify such disorders as Alzheimers disease and epilepsy, and cardiac disorders such as coronary artery disease and myocardial status. However, the most popular use for PET remains for oncology purposes utilizing radiopharmaceutical fluorine 18 flourodeoxyglucose (FDG). This agent is currently the most common one in use for PET imaging. FDG is a glucose analogue and, since normal tissue utilize glucose for energy, FDG is taken up by cells and then trapped intracellularly in this pathway. Since most tumors tend to favor this pathway, they tend to appear more active on imaging than normal tissue. Interpreting metabolic activity alone can also be misleading since results can be confounded by other normal conditions such as exercise, muscle uptake, cardiac uptake in a non-fasting state, and routine uptake in highly metabolic brown fat cell sites.

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