Pet Variables

Nuclear Medicine

Nuclear Medicine, along with medicine in general, is always changing. The current revolution taking place in nuclear medicine is among the most exciting. The development of whole-body Positron Emission Tomography (PET), together with the imaging agent F18 FDG (flurodeoxyglucose) enables the accurate non-invasive detection and staging of cancer. PET was developed in the late 1960’s, but recent developments have resulted in its recognition as an invaluable clinical test for tumor imaging. PET and nuclear medicine are unique and different than other imaging modalities because they do not have the unique ability to image functional processes, such as tumor metabolic activity, in vivo.

The basic principles of PET are based on the detection of photons emitted from the patient have the administration of the short-lived radiopharmaceutical F18 or FDG (a sugar analogue). These photons are detected by the PET scanner and allow for the reconstruction of a three dimensional image of the glucose metabolism in the body. Because most malignant tumors exhibit a high glucose metabolic activity, the image to image glucose metabolism non-invasively is important. PET scan images can be displayed in the following ways: transaxial, coronal or sagittal image planes. These images result in 3D images of the body which can be used to localize both normal and abnormal processes, which is unattainable by other imaging modalities. Modalities such as MRI and CT cannot distinguish between malignant disease and most times invasive biopsies are needed to asses the nature of the tumor.

Whole Body PET-scans detect the sites of radioactive glucose metabolism. Since we are imaging the areas of metabolism with the radioactive glucose, certain variables can cause false positives. Increased exercise before scanning may cause elevated areas of glucose metabolism in the body, as would consuming a meal high in sugar before scanning. Patients are usually asked to fast after midnight the day of the test and when the patient arrives for their scan, their blood sugar level is taken to be sure it is within normal ranges. The normal sites of elevated glucose metabolism are: brain, heart and malignant tumor cells. The brain and heart will always show up on a PET scan with increased uptake because the brain uses the glucose as “fuel” and the heart burns glucose. Malignant tumors will also show on PET scans since they feed on the glucose, hence why we are able to diagnose malignancy by doing PET scans as opposed to other diagnostic modalities.

Just like every other diagnostic procedure, PET is not accurate 100% of the time. It is very important for the PET scanning staff and the interpreting physician to have a detailed patient history prior to reading the PET scan. A physician trained in reading PET scans with a thorough knowledge of patient history can minimize the number of false positive and false negative readings. The following are just some of a list of potential medical causes of false positive readings: recent radiotherapy, inflammatory process, repair process, recent surgery, muscle effects including stress, injection site and lymph nodes proximal to injection, cecum, bowel and stomach uptake, tonsillar and thymic uptake in children and adolescents and granulomatous disease.