Isotope Bone Scanning.

The patient is visualised by injection of a isotope labeled chemical that has an affinity for bone. The isotope is often Technetium 99m. It has a slight resemblence to Calcium, where it can be (temporarily) taken-up by bone matrix. The isotope has a half life of about 6 hours and emits a gamma ray photon of similar penetration to a hard diagnostic x-ray beam. A collimator allows the distribution of light photons in a Sodium Iodide crystal to model the distribution of activity within the patient, relative to a chosen plane of projection. This image of the patient is collected by photo-multipliers, which are arranged in an appropriate plane adjacent to the crystal. Since the timing of any pulse to adjacent detectors and its brightness are used to determine its location, the thickness of the crystal and the hardness of the gamma ray may affect the scintillation and thus the result of this calculation. It explains why some gamma cameras may image certain isotopes better than others. The resulting image is a distribution in space of whatever physiological activity is appropriate to the given radio-pharmaceutical and the clinical state of the patient.

Bone scanning uses a Technetium 99m diphosphonate or polyphosphonate. This becomes fixed to hydroxyapatite in the bone and marks those areas where bone is being laid down. The activity is high at any site where there is a bone reaction to any process or pathology. The diagnosis is made from a combination of pattern and clinical findings and by additional referral to the plain films. Myeloma is one instance where isotope bone scanning may prove unhelpful. You would only get a bone reaction if there were a pathological fracture. The diagnosis can be suspected by unexplained decrease in counts, provided that you have excluded overlying metal, coins in the pocket and joint prostheses, etcetera.

In hyperparathyroidism, the cystic areas of the brown tumours may not show-up with high diagnostic contrast. It is worth remembering that for the unossified matrix in the cystic areas to take up Techetium 99m, the isotope has to get into the lesions, which therefore may not show as active areas.