Thorax and Myocardium
Uptake by the heart muscle is highly variable, not only from one patient to another but even in the same patient imaged on different occasions. To understand why this is true, it is helpful to look at the myocardium’s 50% to 70% of total energy is obtained from the oxidation of fatty acids, with the rest being primary obtained from carbohydrates like glucose and lactate. However, the percentage of each energy source contributes is affected by many factors:
- Nutrition
- Hormones
- Level of myocardial work
- Level of myocardial blood flow
Particularly relevant to PET is that under fasting conditions, myocardial fatty acid metabolism is the predominant energy source. This scenario occurs because plasma insulin levels fall, resulting in increased decomposition of fat cells from adipose tissue and the subsequent increase of fatty acid levels in the plasma. More fatty acids are delivered to the myocardium, which decreases the myocardium’s need for glucose metabolism. Conversely, after an individual eats, plasma insulin levels inhibits fat breakdown within adipose tissue; consequently, plasma fatty acid levels decrease, which in turn decrease myocardial fatty acid metabolism. As a consequence, glucose becomes a primary source of energy.
However, despite patient fasting, many variations in update can be seen. These range from absent to very intense uptake within the left ventricle. A less common observation is intense uptake within the right ventricle.
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| Image showing physiologic left ventricular on (A) CT, (B) PET, (C) PET/CT fusion images. |
Thymus PET Scan
In children, thymus uptake is normal appearing as an inverted shape. In adults, increased FDG activity in the thymus after chemotherapy or radiotherapy is a normal variant. This phenomenon is called thymic rebound.
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| Image showing thymic rebound in a postchemotherapy patient on (A) CT, (B) PET, (C) PET/CT fusion images. |
Breast PET Scan
FDG uptake in the breast varies. There is increased uptake in women who have dense breast or in women who receive hormonal therapy. Increased FDG activity is normally seen in the nipples. In addition, there is increased FDG uptake in the breasts of women who are lactating.
Lung PET CT
Chronic obstructive pulmonary disease necessitates excessive contraction of accessory muscles for expiration. This results in increased intercostal uptake that can be misinterpreted as metastases to the ribs or as bone marrow uptake. Hyperventilation may induce uptake in the diaphragm.
Gastrointestinal Scan
It is unclear why FDG uptake occurs in the digestive tract. Gastric activity has a characteristic J-shape within the left upper abdomen, and uptake can be either faint or intense.
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| PET/CT image of normal stomach wall uptake. Actual PET/CT images are always displayed in color. |
FDG uptake is mostly seen in the large bowel and to a lesser extent in the small bowel and stomach. Activity in the cecum is usually higher than that in the other colonic segments. Attemps to use laxatives, antimicrobials, smooth muscle relaxants or glucagon to reduce bowel activity and therefore reduce uptake have met with variable success and are generally not recommended. Uptake in the bowel can be seen as either faint or intense and in a diffuse pattern or concentrated in the right lower quadrant. Diffuse uptake in the bowel is often associated with normal findings at colonoscopy, whereas focal uptake may indicate inflammation. There is also a risk that high bowel activity in normal patients or in patients with inflammatory bowel disease may mask celiac, mesenteric, and iliac lymph node uptake. Focal colonic uptake should be further evaluated by colonoscopy. Also, the patient’s age affects the degree of uptake in the bowel, which further complicates interpretations, activity increase until approximately age 60, and thereafter bowel activity decreases.
