Gallium salts are taken up by tumors, inflammation, and both acute and chronic infection, allowing these pathological processes to be imaged. 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.
Gallium scan showing panda (A) and lambda (B) patterns, considered specific for sarcoidosis in the absence of histological confirmation.
In the past, the gallium scan was the gold standard for lymphoma staging, until it was replaced by positron emission tomography using fludeoxyglucose (FDG). 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.
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.
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, leukocytelactoferrin, bacterialsiderophores, inflammatory proteins, and cell-membranes in neutrophils, both living and dead.
Lactoferrin is contained within leukocytes. Gallium may bind to lactoferrin and be transported to sites of inflammation, or binds to lactoferrin released during bacterial phagocytosis at infection sites (and remains due to binding with macrophage receptors). 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). Uptake is thought to be associated with a range of tumour properties including transferring receptors, anaerobic tumor metabolism and tumor perfusion and vascular permeability.
Whole-body survey to localize source of fever in patients with fever of unknown origin.
Evaluation of patients who are not candidates for WBC scans (WBC count less than 6,000).
Note that all of these conditions are also seen in PET scans using the gallium-68.
The main (67Ga) technique uses scintigraphy to produce two-dimensional images. After the tracer has been injected, images are typically 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. 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.
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).
In a normal scan, uptake of gallium is seen in wide range of locations which do not indicate a positive finding. These typically include soft tissues, liver, and bone. Other sites of localisation can be nasopharyngeal and lacrimal glands, breasts (particularly in lactation or pregnancy), normally healing wounds, kidneys, bladder and colon.
Gallium PSMA scan
CT scan (left) and Gallium PSMA PET scan (right) of patient with prostate cancer metastases in the bones
Gallium PSMA scanning is recommended primarily in cases of biochemical recurrence of prostate cancer, particularly for patients with low PSA values, and in patients with high risk disease where metastases are considered likely.
An intravenous administration of 1.8–2.2 megabecquerels of 68Ga-PSMA per kilogram of bodyweight is recommended. Imaging should commence approximately 60 minutes after administration with an acquisition from mid-thigh to the base of the skull.
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. It decays by electron capture, then emits de-excitation gamma rays that are detected by a gamma camera. Primary emission is at 93 keV (39% abundance), followed by 185 keV (21%) and 300 keV (17%).:64 For imaging, multiple gamma camera energy windows are used, typically centred around 93 and 184 keV or 93, 184, and 296 keV.
Radiochemistry of gallium-68
Gallium-68 is produced from decay of Germanium-68, which has a 270.8 day half-life. Use of a generator means a supply of 68Ga can be produced easily with minimal infrastructure, for example at sites without a cyclotron, commonly used to produce other PET isotopes.
It decays by positron emission and electron capture into Zinc-68. Maximum energy of positron emission is at 1.9 MeV.:65
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