General Gallium 67 Tumor Imaging:

General Gallium 67 Tumor Imaging:

Chemistry and Pharmacology:

Gallium 67 is produced in a cyclotron by bombarding a zinc target with protons. It has a physical half-life of 78 hours and a biologic half-life of 2 to 3 weeks. Ga-67 decays by electron capture to ground state Zn-67 with the following gamma emissions: 93 keV(40%), 184 keV(24%), 296 keV(22%), 388 keV(7%). The 388 keV emission is generally not used for imaging. Gallium has approximately a 12 hour half-life in blood. The Ga+3 ion resembles the ferric (iron) ion in atomic radius and charge and binds to transferrin (in plasma) [1]. The gallium transferrin complex binds to transferrin receptors and is incorporated intracellularly where it binds to lactoferrin and siderophores (in tissue) [1]. The agent also binds to ferritin.

That which is not protein bound is either cleared by the kidneys or passes into the extravascular space. Saturation of transferrin with iron, as seen clinically in patients with iron overload from repeated transfusions, will result in altered biodistribution of gallium with less liver uptake, greater renal activity, and more rapid clearance. Renal excretion (10-30% of injected dose) occurs mostly during the first 24 hours. This is because gallium citrate is excreted differently than gallium which is bound to transferrin. Renal activity should NOT be seen on 72 hour images. Persistent activity implies renal disease (ATN, renal insufficiency), pyelonephritis, inflammatory nephritis, or severe hepatic failure. Gastrointestinal excretion is the predominant route after 24 hours, primarily through the mucosa of large intestine, but a small amount is cleared via the liver and biliary tract. A laxative may be required prior to later imaging to clear colonic activity.The injected radiotracer must be "carrier free", ie: free of stable gallium. Significant amounts of carrier gallium will change the biologic distribution of Gallium 67 in the body, resulting in increased concentration in the skeleton.

Distribution:

Gallium activity can be seen normally in the following locations:

  • Renal cortex: First 24 hours
  • Liver: Demonstrates greatest uptake of gallium
  • Spleen
  • Bone marrow & skeleton:
  • Gallium is incorporated into the calcium hydroxyapatite crystal as a calcium analog. Marrow activity occurs because of its behavior as an iron analog.
  • May see physeal activity in children
  • Nasopharynx, salivary & lacrimal glands
  • Bowel: Primarily colonic activity on delayed images
  • Breasts: Uptake is especially noted under the stimulation of menses, pregnancy, or BCP's. Gallium also accumulates in breast milk.
  • External genitalia
  • Blood pool (20%)
  • Thymus: May note thymic uptake in children

Gallium Imaging Technique:

The typical dose of Gallium is 10 mCi for neoplasms (Image at 48-72 hours). When performing a total body scan, a slow scan speed (10 cm/min.) is recommended in order to obtain adequate count densities. Spot images should be obtained for 10 minutes or 750,000 counts using three gallium peaks. When performing spots of the abdomen, the liver should be shielded or placed out of the field of view. Concurrent TcSc imaging may be performed to subtract out liver activity. When imaging axillary abnormalities, the arms should be raised above the head to fully expose the axilla.

REFERENCES:
(1) AJR 2011; Kashefi A, et al. Molecular imaging in pulmonary diseases. 197: 295-307

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