Normal F-18 FDG Distibution in PET/CT Imaging
This radiotracer, which enters cells through glucose transporters and thus represents glycolytic rate of cells, is used to find many forms of cancers.
PET imaging is based on detecting coincident gamma photos from annhiliation events, thus providing higher resolution images than single photon imaging.
Go back to Oncology (PET) imaging.
PET imaging is based on detecting coincident gamma photos from annhiliation events, thus providing higher resolution images than single photon imaging.
Go back to Oncology (PET) imaging.
Normal Biodistribution
According to Essentials of Nuclear Imaging, Physiologic FDG distribution is typically:
20-40% into urine by 2nd hour
7% into brain
4.5% into liver
3.3% into heart
1.7% into red bone marrow
1.3% into kidneys
0.9% into lungs
Cerebral cortex, basal ganglia and thalami (i.e., the grey matter) have high FDG activity because these tissues are very metabolically active and use glucose as their primary substrate.
Vocal cords at rest are mildy to not FDG-avid. But talking can cause the vocal cords to become FDG-avid.
Tonsils (especially palatine tonsils), lymphoid tissue at Waldeyer ring, and parotid and submandibular glands have some FDG activity, sometimes high if they are part of reactive inflammation. As with vocal cords, this FDG activity is symmetric.
Thyroid is typically not FDG-avid, but mild diffuse FDG activity is within normal.
Thymus is FDG-avid in children and adults up to ~30 years. Thymic FDG activity may be seen due to thymic rebound after chemotherapy.
Skeletal muscle at rest (and at basal levels of insulin) has low FDG activity. Increased muscular FDG activity may be seen at the shoulders and upper back due to patient’s tension secondary to positioning. Increased FDG activity can also be seen in the diaphragmatic crura, intercostals muscles, psoas muscles, paravertebral muscles, forearms and muscles of mastication.
Brown fat (especially in the supraclavicular regions) can be FDG-avid if stimulated in adults, and is more often seen in children.
Left ventricular myocardium has variable (i.e., high to low and in-between) FDG activity. Myocardial cells use fatty acids as their primary metabolic substrate at rest, but glucose becomes the primary substrate if blood glucose levels are high (causing increased insulin levels) or oxygen demand is increased beyond supply (ischemia), prompting anaerobic metabolism. Intercollated patches of intense and fainter FDG activity is within normal as different foci of myocardium switch between fatty acids and glucose metabolism. Compared to the LV, the right ventricular myocardium has faint FDG activity.
Aortic wall FDG activity is minimal, but can be increased in older adults due to macrophages in atherosclerotic plaques.
Lungs have low and diffuse FDG activity (which is more visible on non-AC images versus attenuation corrected images).
Breast FDG activity is mild to moderate in young women and postmenopausal women on hormone replacement therapy. Dense breasts have greater FDG activity versus fatty breasts. FDG activity may be intense in lactating breast tissue.
Bowel FDG activity varies in intensity and location. Focally high FDG activity in the colonic mucosa can be within normal, though colonic FDG activity tends to be diffuse and highest in the ascending and cecal portions. Rest of GI tract: Esophogeal FDG activity tends to be mild and uniform, and stomach FDG activity can be much greater than liver activity especially if contracted or hiatal hernia. Small bowel activity is less than colon normally.
Liver FDG activity is heterogenous or “patchy.” Benign lesions like focal nodular hyperplasia can be FDG-avid.
Urinary FDG activity is intense, so activity in the kidneys, ureters and bladder will be intense. This is because up to 40% of administered FDG is excreted by the kidneys within the first 2 hours. FDG accumulates in the renal calyces, pelvis and ureters. Ureteral activity may be discontinuous due to peristalsis.
Pelvic organ FDG activity is mild to minimal (but may be increased in the uterine during menstruation). FDG activity in the testes is mild. Faint FDG activity may be seen in the penis.
Bone marrow FDG activity intensity is similar to liver but more homogenous, which is mildly above blood pool FDG activity. FDG activity in the bone marrow may be increased in anemic patients or patients on medications like G-CSF. Splenic FDG activity is low but can be increased by stimulation, similar to bone marrow. Lymph node FDG activity is faint, except when extravasation occurs and infiltrated FDG concentrates in a downstream node.
Source: Mettler FA and Guilberteau MJ. Essentials of Nuclear Imaging. 6th ed. Saunders, 2012.
20-40% into urine by 2nd hour
7% into brain
4.5% into liver
3.3% into heart
1.7% into red bone marrow
1.3% into kidneys
0.9% into lungs
Cerebral cortex, basal ganglia and thalami (i.e., the grey matter) have high FDG activity because these tissues are very metabolically active and use glucose as their primary substrate.
Vocal cords at rest are mildy to not FDG-avid. But talking can cause the vocal cords to become FDG-avid.
Tonsils (especially palatine tonsils), lymphoid tissue at Waldeyer ring, and parotid and submandibular glands have some FDG activity, sometimes high if they are part of reactive inflammation. As with vocal cords, this FDG activity is symmetric.
Thyroid is typically not FDG-avid, but mild diffuse FDG activity is within normal.
Thymus is FDG-avid in children and adults up to ~30 years. Thymic FDG activity may be seen due to thymic rebound after chemotherapy.
Skeletal muscle at rest (and at basal levels of insulin) has low FDG activity. Increased muscular FDG activity may be seen at the shoulders and upper back due to patient’s tension secondary to positioning. Increased FDG activity can also be seen in the diaphragmatic crura, intercostals muscles, psoas muscles, paravertebral muscles, forearms and muscles of mastication.
Brown fat (especially in the supraclavicular regions) can be FDG-avid if stimulated in adults, and is more often seen in children.
Left ventricular myocardium has variable (i.e., high to low and in-between) FDG activity. Myocardial cells use fatty acids as their primary metabolic substrate at rest, but glucose becomes the primary substrate if blood glucose levels are high (causing increased insulin levels) or oxygen demand is increased beyond supply (ischemia), prompting anaerobic metabolism. Intercollated patches of intense and fainter FDG activity is within normal as different foci of myocardium switch between fatty acids and glucose metabolism. Compared to the LV, the right ventricular myocardium has faint FDG activity.
Aortic wall FDG activity is minimal, but can be increased in older adults due to macrophages in atherosclerotic plaques.
Lungs have low and diffuse FDG activity (which is more visible on non-AC images versus attenuation corrected images).
Breast FDG activity is mild to moderate in young women and postmenopausal women on hormone replacement therapy. Dense breasts have greater FDG activity versus fatty breasts. FDG activity may be intense in lactating breast tissue.
Bowel FDG activity varies in intensity and location. Focally high FDG activity in the colonic mucosa can be within normal, though colonic FDG activity tends to be diffuse and highest in the ascending and cecal portions. Rest of GI tract: Esophogeal FDG activity tends to be mild and uniform, and stomach FDG activity can be much greater than liver activity especially if contracted or hiatal hernia. Small bowel activity is less than colon normally.
Liver FDG activity is heterogenous or “patchy.” Benign lesions like focal nodular hyperplasia can be FDG-avid.
Urinary FDG activity is intense, so activity in the kidneys, ureters and bladder will be intense. This is because up to 40% of administered FDG is excreted by the kidneys within the first 2 hours. FDG accumulates in the renal calyces, pelvis and ureters. Ureteral activity may be discontinuous due to peristalsis.
Pelvic organ FDG activity is mild to minimal (but may be increased in the uterine during menstruation). FDG activity in the testes is mild. Faint FDG activity may be seen in the penis.
Bone marrow FDG activity intensity is similar to liver but more homogenous, which is mildly above blood pool FDG activity. FDG activity in the bone marrow may be increased in anemic patients or patients on medications like G-CSF. Splenic FDG activity is low but can be increased by stimulation, similar to bone marrow. Lymph node FDG activity is faint, except when extravasation occurs and infiltrated FDG concentrates in a downstream node.
Source: Mettler FA and Guilberteau MJ. Essentials of Nuclear Imaging. 6th ed. Saunders, 2012.
Basic Principles
FDG is an analog of glucose, by far the most commonly used substrate in metabolic processes throughout the body.
The biodistribution of FDG is affected by blood glucose levels, less because of competition with glucose for entry into cells, and more because of the effect of insulin on glucose transport. The purpose of the 4 to 6-hour fast before F-18 FDG administration is to guarantee that blood insulin levels are at basal levels. The disadvantage of having higher than basal levels of insulin is FDG uptake in skeletal muscle, which creates artifact.
In general, FDG should be below 200 mg/dL and ideally below 150 mg/dL. Also, insulin administration should be avoided for 4 hours before FDG administration, although diabetic patients (especially insulin-dependent patients) will pose challenges to these recommendations.
FDG is mostly excreted by the kidneys, and not reabsorbed like glucose is. Thus, good hydration is recommended before FDG administration to improve target-to-background ratios. FDG-concentrated urine in the bladder rarely poses a problem in interpreting closeby tissues due to reconstruction of MIP (maximum intensity projection) and CT images.
Usually, 10 to 25 mCi (370 to 925 MBq) of F-18 FDG is administered intravenously to the patient. Imaging is performed 45 to 60 minutes after injection.
The biodistribution of FDG is affected by blood glucose levels, less because of competition with glucose for entry into cells, and more because of the effect of insulin on glucose transport. The purpose of the 4 to 6-hour fast before F-18 FDG administration is to guarantee that blood insulin levels are at basal levels. The disadvantage of having higher than basal levels of insulin is FDG uptake in skeletal muscle, which creates artifact.
In general, FDG should be below 200 mg/dL and ideally below 150 mg/dL. Also, insulin administration should be avoided for 4 hours before FDG administration, although diabetic patients (especially insulin-dependent patients) will pose challenges to these recommendations.
FDG is mostly excreted by the kidneys, and not reabsorbed like glucose is. Thus, good hydration is recommended before FDG administration to improve target-to-background ratios. FDG-concentrated urine in the bladder rarely poses a problem in interpreting closeby tissues due to reconstruction of MIP (maximum intensity projection) and CT images.
Usually, 10 to 25 mCi (370 to 925 MBq) of F-18 FDG is administered intravenously to the patient. Imaging is performed 45 to 60 minutes after injection.