Bone mineral density (BMD) and soft-tissue composition of the total body and major subregions were measured with dual-energy x-ray absorptiometry (DEXA). Total body scans were made in 12 young adults (6 male, 6 female) on five occasions at both a medium speed (20 min) and a fast speed (10 min). There were no significant differences in mean results or in precision errors between the two speeds. The precision errors (1 SD) for total body BMD, percent fat in soft tissue (% Fat), fat mass, and lean tissue mass were less than 0.01 g/cm2, 1.4%, 1.0 kg, and 0.8 kg, respectively. These results corresponded to a relative error of 0.8% for total body BMD and 1.5% for lean body mass. Regional BMD and soft-tissue values (arms, legs, trunk) were determined with slightly higher precision errors. Skeletal mineral was 5.8 +/- 0.5% of lean tissue mass (r = 0.96, p less than 0.001). DEXA provides precise composition analysis with a low radiation exposure (less than 0.1 microGy).

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We tested a dual-energy bone densitometer (LUNAR DPX) that uses a stable x-ray generator and a K-edge filter to achieve the two energy levels. A conventional scintillation detector in pulse-counting mode was used together with a gain stabilizer. The densitometer normally performs spine and femur scans in about 6 minutes and 3 minutes, respectively, with adequate spatial resolution (1.2 x 1.2 mm). Total body scans take either 10 minutes or 20 minutes. The long-term (6 months, n = 195) precision of repeat measurement on an 18-cm thick spine phantom was 0.6% at the medium speed. Precision error in vivo was about 0.6, 0.9 and 1.5% for spine scans (L2-L4) at slow, medium and fast speeds, while the error was 1.2 and 1.5 to 2.0%, respectively, for femur scans at slow and medium speed. The precision of total body bone density was 0.5% in vitro and in vivo. The response to increasing amounts of calcium hydroxyapatite was linear (r = 0.99). The densitometer accurately indicated (within 1%) the actual amount of hydroxyapatite after correction for physiological amounts of marrow fat. The measured area corresponded exactly (within 0.5%) to that of known annuli and to the radiographic area of spine phantoms. There was no significant effect of tissue thickness on mass, area, or areal density (BMD) between 10 and 24 cm of water. The BMD values for both spine and femur in vivo correlated highly (r = 0.98, SEE = .03 g/cm2) with those obtained using conventional 153Gd DPA. Similarly, total body BMD correlated highly (r = 0.96, SEE = .02 g/cm2) with DPA results.

Sunhinary. A survey has been ii)ade of the p)rol)erties of cori wlitoc(hon(rlia in .swelling and contraction. The mitochoni(lria swell spontalneouisly in KCl but not in sucrose.Aged mitochondria will swell rapidly in sucrose if treate(l Nvith citrate or ED'FA. Swelling does not impair oxidative phosphorylation if bovine ser-niiii albumin is p)resellt. ( 10) studie(l the relationshil) between swelling and( su ccinate oxidationi of lupine mitoclhonidria uisinig both l)acked volunm.e anid light-scatterinig measuremlienits.\Vhile packed volume measuremlienits indicated that succiniate somiietimes prevente(l osmlotically induced swelling and induced the mitochondria to contract, parallel optical studies indicated that succinate in-(luced the mlitochondria to sewell. Osmliotic swelling brought abouit either anl activation or inhibition of succiniate oxidation depending onl the concentration of succinlate used. These studies l)ointed out the need for a more complete characterization of swelling and contraction phenomenia in p)lanit miitochondria. Lyons and Pratt (18)