The SPECT side of the technology is useless without the phosphate-compound and gamma-emitting radiopharmaceuticals chemically designed to be taken up by bone. SPECT scanning is highly interventional because it not only aids in bone pathology, but also forecasts possible paths of disease and appropriate treatment.

"Functional changes on the bone's surface predispose structural changes. These changes may take weeks and even months to appear," says Dr. Jeffrey P. Norenberg, Executive Director of the National Association of Nuclear Pharmacies and Associate Director of the New Mexico Center for Isotopes in Medicine at the University of New Mexico, Albuquerque.

SPECT/CT enables precise imaging of bone metastases and infection before either has a chance to enter more advanced, destructive stages. SPECT scanning can also guide biopsy procedures and orthopedic surgery and provide a vehicle for administering bone cancer radiotherapy treatments, says Norenberg.

The radiopharmaceuticals used in SPECT bone scans are comprised of the radioisotope technetium-99m and one of two phosphate compounds, methylene diphosphonate (MDP) and hydroxymethylene diphosphonate (HDP). Their generic drug names are medronate and oxidronate, says Dr. Richard Green, Director of Radiopharmacy Practice for the central-Ohio based Cardinal Health, a leading supplier of radiopharmaceuticals.

In clinical studies, both have been shown to be equally effective in bone scintigraphy and are used "almost interchangeably," says Norenberg. "The vast majority of bone surveys, unless there is an interruption in supply, are conducted using MDP or HDP."

Technically, there is currently a shortage of technetium-99m's parent isotope, molybdenum, but the centralized nuclear pharmacy system in the U.S. is able to support the majority of SPECT procedure volume, says Green.

"I would like to have more molybdenum, but we are able to supply about 85% to 90% of our customers' demands," he says.

Once injected with the radioisotope, patients undergoing a SPECT or SPECT/CT procedure can be scanned at multiple stages of agent uptake, often from 30 minutes to 24 hours later, which helps physicians determine specific disease states.

In a typical bone scan there are "hot spots" - areas where there is very high uptake of the agent, which indicates increased bone formation, or osteoblastic activity, and "cold spots," where bone breakdown, or osteolytic lesions, appears to have no labeling whatsoever, says Green.

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