The effect of radionuclide bone scans prior to DXA bone mineral densitometry

Full text: There has been debate over whether the presence of radionuclides in patients presenting for dual X-ray absorptiometry examinations will effect their BMD results. This was investigated for the Lunar Expert and Lunar DPXL densitometers using a Lunar Al Spine Phantom with in a water bath. Regions of interest were drawn over a series of 99Tcm-MDP bone scan images to determine the count rate and hence the activity in a region the size of the Lunar Al spine water bath, 28 by 40 cm. Typically, this ROI contained approximately 200 MBq in blood pool images and 50 MBq in delayed images. Additional regions drawn over the lumbar spine, L1-5, suggested activities of between 8 and 10 MBq. The Al spine phantom was scanned 3 times over a range of activities from 0 to 2000 MBq. Additional scans were performed with non-uniform isotope distribution, particularly a hot spine in a cool background and hot kidneys adjacent to the spine in a cool background. A slight linear increase in BMD with activity was noted for each of the densitometers. The percent changes of BMD from the zero activity scans were calculated. The typical uncertainty of the scanners is 0.010 g-cm-2 or for this phantom 0.8%. The scans with non-uniform isotope distribution showed a small difference to the zero activity scans and no difference to those of uniform distribution and similar total activity. Thus, while there is a measurable effect at clinically relevant activities, the magnitude of the effect on BMD is such that it is unlikely to significantly affect the results of a DXA scan