INFLUENCE OF FAT ON ULTRASOUND MEASUREMENTS OF THE OS CALCIS

Measurements of the speed-of-sound (SOS) and of the broadband ultrasou nd attenuation (BUA) on the os calcis were recently proposed to assess osteoporotic fragility. Velocity and attenuation were measured throug h the heel which can be divided in three phases including hydroxyapati te, soft tissue, and fat. The aim of this study was to evaluate the in fluence of fat composition and heel width on SOS and BUA. This influen ce was determined from both in vitro investigations examining fat samp les, phantoms, and cadaver heels, and in vivo ones observing adult vol unteers as well as a wide sample section of healthy elderly women. Ult rasound velocities on various fat samples were significantly lower tha n those on distilled water (-65 m/second to -123 m/second). The excisi on of the surrounding soft tissue from cadaver heels made SOS steadily increase whereas the insertion of a 10 mm piece of lard in the latera l face of cadavers' and volunteers' heels os calcis lowered SOS about 30 m/second. Furthermore, a difference of SOS was estimated at 15 m/se cond for a 12.5% variation of the marrow fat weight. Among 334 elderly and healthy women aged 75 and over, a significant negative correlatio n was found between SOS and heel width (r = -0.27; P < 0.0001). On the other hand, fat composition had no significant effect on BUA measurem ent, and no significant relationship was found between BUA and heel wi dth. This study demonstrates that an increase of heel width and fat th ickness provides an underestimation of os calcis SOS, but has no signi ficant effect on BUA.