NORMAL SCANNING PROTOCOL AND DESCRIPTION OF

Patients should arrive at the nuclear medicine department having fasted for at least four hours. This ensures that most tissues are using free fatty acids as their energy source. Diabetic
patients are advised to take their normal insulin or medication prior to arriving at the department. After the staff have made all the necessary patient checks, including correct patient identification and a check of blood glucose level, the injection of radioactive FDG can take place. The patient is advised to lie still for approximately 45 minutes to allow the FDG time to accumulate in metabolically active cells. Any unnecessary patient movement during this uptake period can result in muscular uptake which can cause confusion with later scan interpretation. Patients who are tense during this time often show physiological uptake within the muscles of the neck. Some other patterns of normal uptake are illustrated below and a list of normal and variant uptake is found at the end of this chapter. Following the uptake period, the patient is taken into the scanning room and lies supine on the table. A picture of a GE discovery lightspeed PET/CT scanner is shown in Figure 1.10. The CT scan is performed first, normally without intravenous contrast but increasingly after the administration of oral contrast to outline the normal bowel. The CT scan is normally carried out from the base of skull to mid-thigh level, the so-called half body scan. The reasons for this are:
• Brain metastases are difficult to detect using FDG as any brain
lesion must have an intensity greater than or less than the surrounding
brain tissue to be identified.
• Generally speaking, few tumors that have metastatic deposits
that disseminate to the distal lower limbs.
• There is a decreased radiation burden to the patient.
• There is a considerable amount of time saved if we do not have
to perform a whole body scan.
Whole body scans are carried out in some patient groups. For example, patients with melanoma have a whole body scan from skull vertex to feet. This is because of the widespread and unpredictable lymphatic dissemination that characterises this disease. A similar problem is encountered with the pattern of disease spread in non-Hodgkin’s lymphoma, which often requires a larger scanning volume.Patients with head and neck disease often have scans that include the entire skull, and patients with soft tissue sarcomas may also require additional views. After the CT images are acquired (which only takes a minute or so when using a modern multislice scanner), the patient is then scanned again using the PET component of the machine. The detectors on the PET scanner can identify radioactive emissions from the FDG within the body. A ring of detectors surrounds the patient. This ring is approximately 15cm long, and images are therefore acquired in blocks of 15 cm from the base of the skull to mid-thigh. In most individuals, this area is covered in about 5 blocks (approximately 75cm); taller or shorter individuals will take more or less imaging time. The time required for each 15cm image of the patient is between 3 and 5 minutes. This means that the PET component of the study can take at least 45 minutes to acquire. Any patient movement during this time will degrade the quality
of the images obtained. After the PET scan has been acquired the patient is free to go but is given warnings about exposure to individuals during the next few hours as the radioactivity decays and is excreted from the body.