Gallium 67 scan

A gallium scan or gallium 67 scan (also called "gallium imaging") is a type of nuclear medicine study that uses a gallium-67 (⁶⁷Ga)radioactive tracer to obtain images of a specific type of tissue, or disease state of tissue. Gallium salts like gallium citrate and gallium nitrate are used. The form of salt is not important, since it is the freely dissolved gallium ion Ga³⁺ which is active.^[1]

Gallium is taken up by tumors, inflammation, and both acute and chronic infection, allowing these pathological processes to be imaged by a gamma camera, SPECT camera, or SPECT/CT. Gallium is particularly useful in imaging osteomyelitis that involves the spine, and in imaging older and chronic infections that may be the cause of a fever of unknown origin.^[2][3]

Mechanism

The body generally handles Ga³⁺ as though it were ferric iron (Fe-III), and thus the free isotope ion is bound (and concentrates) in areas of inflammation, such as an infection site, and also areas of rapid cell division. Gallium (III) (Ga³⁺) binds to transferrin, leukocyte lactoferrin, bacterial siderophores, inflammatory proteins, and cell-membranes in neutrophils, both living and dead.^[4]

This relatively nonspecific gallium binding allows sites with tumor, inflammation, and both acute and chronic infection to be imaged by nuclear scan techniques.

General uses in medicine

In the past, the gallium scan was the gold standard for cancer diagnosis and staging, until it was replaced by positron emission tomography using fludeoxyglucose. Gallium imaging is still used to image inflammation and chronic infections, and it still sometimes locates unsuspected tumors as it is taken up by many kinds of cancer cells in amounts that exceed those of normal tissues. Thus, an increased uptake of gallium-67 may indicate a new or old infection, an inflammatory focus from any cause, or a cancerous tumor.

Ga-67 acts as an iron analogue, binding to any proteins that bind iron. Initially it binds to transferrin. Assuming adequate blood flow, the complex diffuses through loose endothelial junctions of capillaries at sites of inflammation and enters the extracellular fluid.

Leukocytes migrate to sites of inflammation and degranulate, releasing large quantities of iron-binding lactoferrin. Ga-67 has higher affinity for leukocyte lactoferrin than it does for serum transferrin, and so follows the leukocyte concentration. The isotope may thus be used in detecting abscesses that provoke a leukocyte response, including abscesses that are "sterile" (free of bacteria, or of living bacteria).

Ga-67 also attaches to the siderophore molecules of bacteria themselves, and for this reason can be used in leukopenic patients with bacterial infection (here it attaches directly to bacterial proteins, and leukocytes are not needed).

It has been suggested that gallium imaging may become an obsolete technique, with indium leukocyte imaging and technetium antigranulocyte antibodies replacing it as a detection mechanism for infections. For detection of tumors, especially lymphomas, gallium imaging is still in use, but may be replaced by fludeoxyglucose PET imaging in the future.

In infections, the gallium scan has an advantage over indium leukocyte imaging (also called indium-111 white blood cell scan) in imaging osteomyelitis (bone infection) of the spine, lung infections and inflammation, and for chronic infections. In part this is because gallium binds to neutrophil membranes, even after neutrophil death. Indium leukocyte imaging is better for acute infections (where neutrophils are still rapidly and actively localizing to the infection), and also for osteomyelitis that does not involve the spine, and for abdominal and pelvic infections. Both the gallium scan and indium leukocyte imaging may be used to image fever of unknown origin (elevated temperature without an explanation). However, the indium leukocyte scan will image only the 25% of such cases which are caused by acute infections, while gallium will also localize to other sources of fever, such as chronic infections and tumors.^[5]

Common indications (specific uses) of gallium-67 imaging

• Whole-body survey to localize source of fever in patients with Fever of Unknown Origin (FUO).
• Detection of pulmonary and mediastinal inflammation/infection, especially in the immunocompromised patient.
• Evaluation and follow-up of active lymphocytic or granulomatous inflammatory processes such as sarcoidosis or tuberculosis.
• Diagnosing vertebral osteomyelitis and/or disk space infection where Ga-67 is preferred over labeled leukocytes.
• Diagnosis and follow-up of medical treatment of retroperitoneal fibrosis.
• Evaluation and follow-up of drug-induced pulmonary toxicity (e.g. Bleomycin, Amiodarone)
• Evaluation of patients who are not candidates for WBC scans (WBC count less than 6,000 and/or poor IV access).

Note that all of these conditions are also seen in PET scans using the less common positron-emitting isotope gallium-68, which has the same chemical characteristics as Ga-67. See gallium-68 generator.

Radiochemistry of gallium-67

Gallium-67 citrate is produced by a cyclotron. Charged particle bombardment of enriched Zn-68 is used to produce gallium-67. The gallium-67 is then complexed with citric acid to form gallium citrate. The half life of gallium-67 is 78 hours.^[6] It decays by electron capture, then emits de-excitation gamma rays that are detected by a gamma camera.

Gallium-67 photopeaks:

• Energy Abundance
• 93 keV 40%
• 184 keV 20%
• 300 keV 17%
• 393 keV 5%

Technique

The main technique uses scintigraphy to produce two-dimensional images. After the tracer has been injected, images are taken by a gamma camera at 24, 48, and in some cases, 72, and 96 hours later. Each set of images takes 30–60 minutes, depending on the size of the area being imaged. The resulting image will have bright areas that collected large amounts of tracer, because inflammation is present or rapid cell division is occurring. Single photon emission computed tomography (SPECT) images may also be acquired. In some imaging centers, SPECT images may be combined with computed tomography scan using either fusion software or SPECT/CT hybrid cameras to superimpose both physiological image-information from the gallium scan, and anatomical information from the CT scan.

A common injection doses is around 150 megabecquerels.^[7] Imaging should not usually be sooner than 24 hours - high background at this time produces false negatives. Forty-eight-hour whole body images are appropriate. Delayed imaging can be obtained even 1 week or longer after injection if bowel is confounding. SPECT can be performed as needed. Oral laxatives or enemas can be given before imaging to reduce bowel activity and reduce dose to large bowel; however, the usefulness of bowel preparation is controversial.^[8]

10% to 25% of the dose of gallium-67 is excreted within 24 hours after injection (the majority of which is excreted through the kidneys). After 24 hours the principal excretory pathway is colon. The "target organ" (organ that receives the largest radiation dose in the average scan) is the colon (large bowel).

Areas where Ga-67 normally localizes include: liver (site of highest uptake), bone marrow, spleen, salivary glands, nasopharynx, lacrimal glands, breast uptake (especially in pregnant and lactating women), kidneys and bladder (in the first 24 hours - faint uptake can still be normal for up to 72 hours), mild diffuse lung uptake (at 24 hours or less)

See also

• Indium scan
• Radiology
• Nuclear Medicine
• Gallium-68 generator.
• Gallium scan (redirects to this article, but may also refer to gallium-68 PET scan)

References

External links

1. http://www.theabr.org/
2. http://www.med.harvard.edu/JPNM/