18F-FDG: [18]F-flourodeoxyglucose (FDG) Imaging
18F -FDG is a glucose analog with replacement of the oxygen in C-2 position with 18-fluorine. Though it behaves as glucose in many situations, there are some important differences that should be understood.
Uptake
Just as glucose, FDG is actively transported into the cell mediated by a group of structurally related glucose transport proteins (GLUT). Once intracellular, glucose and FDG are phosphorylated by hexokinase as the first step toward glycolysis. Normally, once phosphorylated glucose continues along the glycolytic pathway for energy production. FDG however cannot enter glycolysis and becomes effectively trapped intracellularly as FDG-6-Phosphate. Tumor cells display increased number of glucose transporters, particularly GLUT-1 and GLUT-3, as well as higher levels of hexokinase, isoforms type I and II. Tumor cells are highly metabolically active (high mitotic rates) ,and favor the more inefficient anaerobic pathway adding to the already increased glucose demands. These combined mechanisms allow for tumor cells to uptake and retain higher levels of FDG when compared to normal tissues.
FDG is not cancer specific and will accumulate in areas with high levels of metabolism and glycolysis. Therefore increased uptake can be expected in sites of hyperactivity (muscular, nervous); active inflammation (infection, sarcoid, arthritis, etc.); tissue repair, etc.
Patient Preparation (for oncologic imaging)
Diet: Preferably overnight fasting until scan time or at least 4-6 hours fasting prior to scanning.
Glucose levels: plasma glucose level should be checked prior to injection. Less than 150 mg/dl glucose level is desirable. If greater than 200 mg/dl, can wait, recheck, and inject once below 200 mg/dl.
Normal administered dose
10-20 mCi IV injection
Scanning
Scanning should begin approximately 60 minutes following injection. This interval allows adequate time for intracellular uptake and trapping of FDG, and for its clearance from the blood while minimizing the loss of activity due to decay. Tumors however, may continue to concentrate the tracer with time and the background will continue to decrease. By contrast some inflammatory lesions may show wash out activity with time.
Scan time: The total scan time can be from 30-90 minutes depending on the technique, technology, and patient size.. Each bed position can take 4-10 minutes each. Normally whole body scans are obtained from the base of the skull to the proximal thighs.
F18-FDG Radiation Absorbed Dose estimates
Principle Radiation Emission Data - F-18
(Physical half-life = 109.8 minutes) |
| Radiation (keV) |
Mean % per disintegration |
Mean energy |
| Positron |
100 |
250 |
| Gamma ± |
200 |
511 |
Dosimetry - F-18-Fluorodeoxyglucose
(NUREG/CR-6345, page 9, September 18, 1992) |
| Organ |
mGy/MBq |
rad/mCi |
| Adrenals | 1.3 x 10-2 | 4.9 x 10-2 |
| Brain | 1.9 x 10-2 | 7.0 x 10-2 |
| Breasts | 9.2 x 10-3 | 3.4 x 10-2 |
| Gallbladder Wall | 1.4 x 10-2 | 5.0 x 10-2 |
| LLI Wall | 1.7 x 10-2 | 6.1 x 10-2 |
| Small Intestine | 1.4 x 10-2 | 5.1 x 10-2 |
| Stomach | 1.3 x 10-2 | 4.7 x 10-2 |
| ULI Wall | 1.3 x 10-2 | 4.9 x 10-2 |
| Heart Wall | 6.0 x 10-2 | 2.2 x 10-1 |
| Kidneys | 2.0 x 10-2 | 7.4 x 10-2 |
| Liver | 1.6 x 10-2 | 5.8 x 10-2 |
| Lungs | 1.7 x 10-2 | 6.4 x 10-2 |
| Muscle | 1.1 x 10-2 | 4.2 x 10-2 |
| Ovaries | 1.7 x 10-2 | 6.3 x 10-2 |
| Pancreas | 2.6 x 10-2 | 9.6 x 10-2 |
| Red Marrow | 1.3 x 10-2 | 4.8 x 10-2 |
| Bone Surface | 1.2 x 10-2 | 4.3 x 10-2 |
| Skin | 8.4 x 10-3 | 3.1 x 10-2 |
| Spleen | 3.7 x 10-2 | 1.4 x 10-1 |
| Testes | 1.3 x 10-2 | 4.8 x 10-2 |
| Thymus | 1.2 x 10-2 | 4.4 x 10-2 |
| Thyroid | 1.0 x 10-2 | 3.9 x 10-2 |
| Urinary Bladder Wall | 1.9 x 10-1 | 7.0 x 10-1 |
| Uterus | 2.3 x 10-2 | 8.5 x 10-2 |
| Effective Dose Equivalent |
mSv/MBq |
rem/mCi |
| 3.0 x 10-2 |
1.1 x 10-1 |