18 F-FDG PET/CT and Nuclear Medicine for the Evaluation of Breast Cancer

PET and PET/CT Scanners

In positron decay, a nuclide transforms one of its core protons into a neutron and emits a positron (β ⁺ ). The β ⁺ will annihilate with an atomic electron and they will convert their mass into energy and produce a pair of photons (called annihilation photons) that travels in opposite directions exactly 180 degrees apart from each other. The radiotracers used in this imaging technique, such as ¹⁸ F-FDG, are positron emitters. The equipment measures the two annihilation photons that are produced after positron emission. This is the key difference from the conventional radiotracers used in nuclear medicine that only emit single photons in any direction at each decay event. Accordingly, PET scanners are designed as a ring of multiple pairs of photon detectors arranged 180 degrees apart from each other with the patient lying in the middle of this ring. These pairs of photon detectors, which are made of either bismuth germanium oxide, gadolinium orthosilicate, or lutetium orthosilicate crystals, are electronically linked such that they accept only pairs of photons that arrive at both detectors at exactly the same time point and reject scattered photons that arrive at incongruent time points ( Fig. 11.1 ; Box 11.1 ). This design allows for superior photon sensitivity and spatial resolution, because a lead-based collimator, such as those used in conventional nuclear medicine scanners, is no longer needed to reduce scattered photons that hit the camera face at tangential angles.

PET/CT scanners are integrated units with the individual PET and CT components nearly identical to their stand-alone counterparts. Integrated PET/CT has the added advantage of using the CT data for attenuation correction (see Box 11.1 ) and anatomic localization and, additionally, more sophisticated PET/CT fusion software can be used for interpretation. CT-based attenuation correction, however, may introduce artifacts on PET when imaging high-density materials such as bowel contrast and metallic prostheses. The field of view of a modern PET/CT scanner is approximately 16 to 18 cm wide (see Box 11.1 ). The bed is advanced into the scanner in six to seven increments for a patient to be scanned from the base of the skull to the midthigh. An additional six to seven increments are required for scanning the extremities. Typically, each increment, termed a bed position , requires 1 to 5 minutes of acquisition time before moving on to the next section of the body. The acquisition time is dependent on a variety of factors including patient weight and girth, scanner sensitivity, and amount of injected radiotracer. In contrast to conventional cross-sectional imaging, scanning starts at the pelvis and moves cranially to minimize the amount of urine accumulation in the bladder.

¹⁸ F-Fluorodeoxyglocose

¹⁸ F-FDG is the primary radiotracer in clinical use for oncologic PET and PET/CT scanning, including breast cancer. Once injected, ¹⁸ F-FDG is transported into cells via the glucose transporter system (GLUT) and quickly phosphorylated by hexokinase II. After phosphorylation, it is trapped within the cell where it decays with a physical half-life of 110 minutes. Typically, imaging commences at 45 to 90 minutes after injection of tracer to allow for adequate time for tumor accumulation and background washout from renal clearance ( Box 11.2 ).

Patient preparation before and after ¹⁸ F-FDG injection is critical for high-quality scanning (see Box 11.2 ). Because ¹⁸ F-FDG is a glucose analog, increased serum levels of insulin and/or glucose may alter the biodistribution of ¹⁸ F-FDG. High levels of either endogenous or injected insulin cause significant uptake of FDG in muscle and greatly decrease the sensitivity of the scan. For this reason, diabetic patients who have used regular insulin within 4 hours of FDG injection usually require rescheduling, as do patients who have eaten a meal within 8 hours. A less significant and somewhat controversial factor is that high levels of circulating glucose may potentially compete with FDG at tumor sites, decreasing sensitivity of the scan as well. Patients should be instructed to fast for at least 4 to 6 hours before the intravenous administration of ¹⁸ F-FDG so that PET images are not affected by circulating glucose as well as endogenous insulin. Most institutions reschedule the study if a patient’s blood glucose level is greater than 150 to 200 mg/dL. Detailed procedure guidelines for tumor imaging with ¹⁸ F-FDG PET/CT are provided by the Society of Nuclear Medicine and Molecular Imaging (SNMMI) ( ).

Accurate interpretation requires knowledge of the physiology of ¹⁸ F-FDG as well as the limitations of PET scanning. Box 11.3 summarizes the pearls in the interpretation of ¹⁸ F-FDG PET/CT for breast cancer assessment. Greater ¹⁸ F-FDG uptake levels suggest a greater likelihood of malignancy; however, it is not reliable when used alone. Equally important is learning to differentiate focal from nonfocal lesions through experience (there is no clinically used quantitative parameter for focality), with focal lesions more likely to be malignant than benign.