Comparisons between X-ray photodensitometric and gamma-ray absorptiometric findings of bone mineral measurements, and the evidence of their convertibility.

Fifty-two bone mineral measurements were made in chronic renal failure patients using two different radiologic techniques concurrently: gamma-ray absorptiometry; and x-ray photodensitometry. Although the sites of measurements of the cortical bone mineral mass in radius were different (distal vs. proximal) and different parameters were determined (in units of gm/cm vs. gm/cm2), a moderately good correlation was found between measurements by the two techniques (r = 0.61, p less than 0.001). Using external bone width, it was possible to calculate from photodensitometric measurements the bone mass per unit length (gm/cm). This conversion improved the correlation with absorptiometry values considerably (r = 0.78, p less than 0.001). A similar correlation was found between the trabecular bone mass in the distal radius, measured by absorptiometry, and the cortical bone mass in the proximal radius, determined by photodensitometry (r = 0.79, p less than 0.001), the correlation between the cortical and trabecular bone masses by absorptiometry being only slightly higher (r = 0.84, p less than 0.001). The residual variations not explained by the correlations between the absorptiometric and photodensitometric techniques may be largely due to the real differences of bone mineral masses at the two measuring sites and by the inherent methodological errors.