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

Sammanfattning: Influence of Extrinsic Factors on Bone Scintigraphy An Experimental and ClinicalStudy by Marika Cronhjort, M.D. Department of Surgical Sciences, Division for Diagnostic Radiology, Karolinska Hospitaland Institute, S-171 76, Stockholm, Sweden Bone scintigraphy takes advantage of the strong affinity of technetiated diphosphonatesto bone. These compounds accumulate particularly at sites of new bone formation whichoccurs as a response to various conditions affecting bone. The main field of applicationhas been in the assessment of metastatic bone disease, but the method is also usedin other conditions. The image quality in bone scintigraphy largely depends on theactivity ratio between bone and surrounding soft tissues. This ratio may be low incertain pathological conditions, or because of drug interaction or failure of radiopharmaceuticalcontrol. Often, however, there is no apparent reason for a poor image quality, andthe mechanisms governing the distribution of bone-seeking agents are not completelyelucidated. The aim of this study was to shed light on various extrinsic measures that mayinfluence the distribution of bone-seeking agents. The main part of the study wasperformed in an experimental system. Mice exposed to various extrinsic measures wereinjected with 99Tcm-hydroxymethylene diphosphonate (HDP), a commonly used bone-seekingagent. The animals were later sacrificed and the radioactivity of different dissectedorgans was assessed in a scintillation counter. Several extrinsic measures induceda decreased bone activity and an increased activity in soft tissues. This unfavourabledistribution of the bone-seeking agent was achieved after a limited oral phosphateload, after drug interference with the sympathetic nervous system and after disturbancesin fluid balance. An opposite effect, corresponding to an improved image qualityof the potential bone scan, was achieved after a prolonged phosphate load. The studyindicated that excretion of activity to the GI-tract is a normal finding. Changes in the extracellular fluid volume and hormonal changes affecting the rateof bone turnover may partly explain the results in this study. However, it has notbeen 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 improvethe image quality. The factors influencing the activity distribution in an unfavourabledirection need to be further studied to evaluate their significance in clinical practice. As a clinical implication of the experimental findings, the influence of hydrationon the image quality was studied in normal humans. This was also a test of the long-livedbut unproven conception that increased diuresis caused by hydration reduces the softtissue activity and thereby improves the image quality in bone scintigraphy. No sucheffect was achieved in the present study. It is concluded that hydration can be avoidedwhen it is problematic for the patient. However, as hydration reduces the radiationdose to the urinary bladder wall it may be justified, at least in younger individuals. Keywords. Bone scintigraphy, bisphosphonates, biodistribution, animal Stockholm 1997 ISBN 91-628-2747-2