CT has been the cornerstone of oncologic imaging for over 20 years but lacks the ability to show differences in physiology. PET has incomparable abilities to determine the metabolic activity of tissues but needs the assistance of higher-resolution, anatomic information that it cannot provide. CT is the easiest and the highest-resolution tomographic modality to integrate into PET imaging. The combination of the two offers the best of both the worlds in an integrated data set and thus improves diagnostic accuracy and localization of many lesions.
Characteristic of malignant cells:
Point to be noted for diabetic patients
Hyperglycemia and hyperinsulinemia are very important considerations when preparing a patient for a PET study with FDG. High blood levels of glucose will compete with FDG for uptake by the tumor. High levels of insulin will push FDG, along with glucose, into skeletal muscle and myocardium, increasing the image background and decreasing the availability of the radioactive glucose tracer for uptake by the tumor. Optimal imaging conditions can usually be achieved in euglycemic patients with a 6- to 8-hour fast. This fasting is essential for the quality of the test; altered tracer bio-distribution caused by ingestion of even a small meal shortly before a FDG injection can impair the visualization of malignant lesions.
For diabetic patients, the situation is more complex. Several protocols are used by different PET centers to optimize the chances of a good study. A non–insulin-dependent diabetic can be studied early in the morning after an overnight fast if his or her early morning, fasting blood sugars routinely are below 150 – 180 mg/dL.
Bone marrow activation is a major issue in the oncology population because many of the patients are receiving Granulocyte colony stimulating factors or have rebounding bone marrow after a course of chemotherapy. PET usually has very high sensitivity for splenic or osseous metastases, but in a patient who has received GcSF, the marrow and spleen show greatly increased activity that can obscure such lesions. Similar, though usually less intense, uptake can be seen in patients with anemia or those recovering from chemotherapy. In general, PET should be delayed for at least 1 week after administration of short-acting cytokines and up to 3 weeks after administration of long-acting cytokines or chemotherapy.
Because of these and other challenges, referring physicians should provide clinical information with the request for a PET scan to assist the interpreting physician in providing the most accurate information possible in patients.
Some of the important factors include:
Purpose that it serves:
For optimal scan results PET-CT Scan should be scheduled:
Dr. Arun Gera, Senior Consultant & Coordinator, Dept of Nuclear Medicine, Dharamshila Narayana Superspeciality Hospital, Delhi
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