In the past, the gallium scan was the
gold standard for
lymphoma staging, until it was replaced by
positron emission tomography (PET) using
18F-fluorodeoxyglucose (FDG). 67Ga-citrate 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. 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-67 imaging is still in use, but may be completely replaced by
Fluorodeoxyglucose18F-fluorodeoxyglucose PET imaging in the future. In infections, the gallium scan has an advantage over indium leukocyte imaging 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.
Mechanism The body generally handles Ga3+ as though it were
ferric iron (Fe3+), and thus the free 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. 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). Gallium-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.
Common indications • Whole-body survey to localize source of fever in patients with fever of unknown origin. • 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 gallium-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). Note that all of these conditions are also seen in PET scans using gallium-68.
Technique 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 (CT) 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 dose is around 150
megabecquerels. Imaging should not usually be sooner than 24 hours as 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. ==Gallium PSMA scan==