Influence of extrinsic factors on bone scintigraphy an experimental and clinical study

Bone scintigraphy takes advantage of the strong affinity of technetiated diphosphonates to bone. These compounds accumulate particularly at sites of new bone formation which occurs as a response to various conditions affecting bone. The main field of application has been in the assessment of metastatic bone disease, but the method is also used in other conditions. The image quality in bone scintigraphy largely depends on the activity ratio between bone and surrounding soft tissues. This ratio may be low in certain pathological conditions, or because of drug interaction or failure of radiopharmaceutical control. Often, however, there is no apparent reason for a poor image quality, and the mechanisms governing the distribution of bone-seeking agents are not completely elucidated.

The aim of this study was to shed light on various extrinsic measures that may influence the distribution of bone-seeking agents. The main part of the study was performed in an experimental system. Mice exposed to various extrinsic measures were injected with 99Tcm-hydroxymethylene diphosphonate (HDP), a commonly used bone-seeking agent. The animals were later sacrificed and the radioactivity of different dissected organs was assessed in a scintillation counter. Several extrinsic measures induced a decreased bone activity and an increased activity in soft tissues. This unfavourable distribution of the bone-seeking agent was achieved after a limited oral phosphate load, after drug interference with the sympathetic nervous system and after disturbances in fluid balance. An opposite effect, corresponding to an improved image quality of the potential bone scan, was achieved after a prolonged phosphate load. The study indicated that excretion of activity to the GI-tract is a normal finding.

Changes in the extracellular fluid volume and hormonal changes affecting the rate of bone turnover may partly explain the results in this study. However, it has not been possible to entirely explain the complex underlying mechanisms behind the findings. No simple measure has been found that is suitable for routine use in order to improve the image quality. The factors influencing the activity distribution in an unfavourable direction need to be further studied to evaluate their significance in clinical practice.

As a clinical implication of the experimental findings, the influence of hydration on the image quality was studied in normal humans. This was also a test of the long-lived but unproven conception that increased diuresis caused by hydration reduces the soft tissue activity and thereby improves the image quality in bone scintigraphy. No such effect was achieved in the present study. It is concluded that hydration can be avoided when it is problematic for the patient. However, as hydration reduces the radiation dose to the urinary bladder wall it may be justified, at least in younger individuals.