Radio nucleotide imaging is based on differences in uptake of radio nucleotides containing radio isotopes when normal and abnormal tissue because of physiological and biological differences, rather than anatomical differences, as with radiographs and MRI and CT scans. The radio isotope emits gamma energy until it decays to a stable state. Gamma rays are detected by gamma camera using a sodium iodide crystal. When the crystal absorbs gamma ray or x-ray, it scintillates, i.e. it emits light. The light is converted to an electrical impulse by a photo multiplier tube and is amplified. This sequence of events allows creation of an image based on intensity and distribution of radioactivity in the body. Routine images of a gamma camera are two-dimensional (plainer). Tomographic images can be obtained by rotating the gamma camera in an elliptical or circular arc around the image body part. This is known as single photon emission computed tomography (SPECT). Computerised reconstruction of the data allows tomographic images to be obtained in axial, coronal and sagittal planes. SPECT scanning does require additional scanning time and is not routinely used.

In bone scanning Technetium 99M is combined with diaphosphonate complexes which carry the Technetium to the bone. The only absolute contra-indication to radio-nucleotide scanning is pregnancy. Technetium 99M has a relatively short physical half life of six hours and a gamma energy of 140 kev. The relatively short half life limits the radiation dose to the body. For evaluation of localised or joint pain, three-phase bone scanning is done. The first phase consists of a radio-nucleotide angiogram in which scans are done every two to five seconds for one to two minutes after the injection. This shows the radio-nucleotide in the blood vessels including flow in the arteries, capillaries and veins. The second phase is a blood pool scan which is done immediately after the first phase. This shows radio-nucleotide in the soft tissues and extra-vascular space. The third phase, known as the delayed phase is done two to four hours after injection and shows radio-nucleotide uptake by bone (???4.22 page 112). By two to four hours 50% or more of the injected dose is taken up by the skeleton. The radio-nucleotide is excreted by the kidneys into the urine. The early phase evaluates the regional vascularity. Increased vascularity can be seen in soft tissue abnormality such as cellulitis and in some soft tissue tumours. Increased vascularity is a non-specific finding present in the early phase of many bone abnormalities including fractures, tumours, infections and other conditions. The most important factor in causing increased uptake in the bone on the delayed scan is increased bone turnover. High bone turnover and new bone formation cause increased uptake of the diaphosphonate complexes.

Bone scanning can be used to detect traumatic fractures within 24 hours of the injury. Initially the flow and blood pool scans test positive showing increased vascularity. The delayed scans can show increased uptake for up to six months or more after a fracture.

Bone bruises or trabecular fractures cause increased vascularity and uptake and maybe associated with ligamentous injuries such as an anterior cruciate ligament injury, a tear which shows bone bruised patterns in the lateral femoral condyle and posterior lateral tibial plateau.

Meniscus tears have been shown to cause increased uptake on a bone scan.

Bone scanning is very sensitive at showing stress fractures from over use such as running, which can occur in the shaft of the tibia, the fibula or the femur. Osteoporotic tibial plateau fractures are also diagnosed well on bone scanning.

Osteoarthritis causes increased uptake of radio nucleotide around the articular surface of the bone because of increased bone turnover. Increased uptake on both sides of a joint is characteristic of osteoarthritis. However, the tibial plateau is usually affected earlier and more severely than the femoral condyle.

In the acute phase of reflex sympathy dystrophy syndrome, there is increased vascularity and diffuse increased uptake. In the sub-acute phase, there is usually normal vascularity and diffuse increased uptake. In the atrophic phase, there may be normal or decreased vascularity and uptake.

Bone scanning is very useful for detecting bone tumours both primary tumours originating within the bone and metastatic spread from a distant tumour.

Bone infection (osteomyelitis) shows increased vascularity and increased uptake on a three phase bone scan. Cellulitis (soft tissue infection), presents with increased vascularity and flow and blood pool scans, but are either normal or only mild increased uptake in the delayed images.

In order to improve this specificity for evaluation and infection, inflammation specific radio-pharmaceuticals have been developed. The two most widely used are Gallium 67 citrate and Indium 111 or Technetium 99M labelled white blood cells. Gallium 67 citrate is taken up in infection owing to a number of factors, but also localises in bone that has increased uptake in tumours, fractures and other areas of increased bone turnover. The comparison of the Gallium scan with bone scan helps increase specificity. Mis-match uptake on the Gallium scan compared to the bone scan is suggestive of infection. Matched bone scan Gallium uptake is indeterminate for infection. Gallium scanning has been largely replaced by radio-labelled white cell scanning.

White cell scanning has a higher specificity for diagnosing infection. With white cell labelled imaging, 50 ml of blood is obtained from the patient, with subsequent separation of the white blood cells. Labelling of the white blood cells occurs with the radio isotope within 15 to 30 minutes. These labelled white blood cells are then re-suspended in the patient's plasma and re-injected. Following Indium 111 labelled injection, white cell labelled injection scans are done at approximately 24 hours after injection. With Technetium 99M labelled white cell imaging begins three hours after injection. Increased uptake is seen where there is localisation of white blood cells resulting from infection or inflammation. The labelling of white cells is time consuming and may be expensive. Some new isotopes have been developed without the need for labelling. Some of these include radio labelled monoclonal anti-granulose like antibodies.

In recent years MRI scanning is largely replaced bone scanning for the diagnosis of localised knee pain.

Last updated 7/04/2007

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