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Blood clearance, urinary excretion, and spinal uptake of 99mTc-HEDP have been measured in ten normal control subjects. Blood clearance from 15 min is biexponential. Approximately 70% of the administered activity was excreted in the urine within 6 hr of injection. Spine-to-background ratios rise sequentially with time. Net normal bone uptake rises little after 2 hr and improved visualization of the normal skeleton therafter is due to reduction of blood background activity. In 13 patients with bone tumors, direct measurement of diphosphonate uptake by the tumor-involved and corresponding normal bone showed a continued rise in diphosphonate levels in the tumor-involved bone due to increased metabolic activity.

Twenty normal volunteers had measurements of 24-hr whole-body retention (WBR) of three structurally related Tc-99m-labeled phosphonate skeletal imaging agents: (1-hydroxyethylidene) diphosphonate (HEDP), methylene diphosphonate (MDP), and hydroxymethylene diphosphonate (HMDP). The average WBR values, reflecting skeletal uptake, were 18.4, 30.3, and 36.6%, respectively. These results clearly illustrate that slight alterations in diphosphonate molecular structure have a significant effect upon specificity for osseous tissue, and thus may affect skeletal image quality and the usefulness of the WBR technique in diagnosing metabolic bone disease.

Digital bone scintigrams were obtained in 19 patients using Tc-99m hydroxyethylidene diphosphonate (HEDP). These were quantitated for skeletal, soft-tissue and renal uptake of tracer using a contrast-enhancement technique to define the regions of interest. Twenty-four hr whole-body retention (WBR) of HEDP was also measured. It was found that approximately 70% of retained diphosphonate was localized in the skeleton at 24 hr, with 26% in soft tissue and 4% in kidneys. However, we have shown that over a wide spectrum of whole-body retention measurements (18 to 70%) the 24-hr soft-tissue component, as a percentage of administered dose, remained relatively constant whereas skeletal tracer accumulation mirrored WBR (r = 0.98, p less than 0.001). The present study indicates that, despite a significant but essentially stable soft-tissue component, measurement of 24-hr WBR of HEDP accurately reflects skeletal metabolism.

Measurement of 24-hr whole-body retention of Tc-99m HEDP, using a shadow-shield, whole-body monitor, has been shown to be a sensitive measure of skeletal metabolism and of value in the diagnosis of metabolic bone disease. A new method of measuring the retention using a gamma camera, with a scanning (fishtail) collimator and patient placed at 2.,3 m distance, has been evaluated in 18 patients undergoing routine bone scans. The patients also had whole-body monitor (WBM), and the two methods correlated well, yielding a regression line GC% = -0.82 + 0.98 WBM%, r = 0.975, p less than 0.001. The limitations to, and repeatability of, the gamma-camera measurements are discussed. This work shows that measurements of whole-body retention can be obtained in any nuclear medicine department possessing a gamma camera with a suitable collimator.