[Measurement of lumbar bone density by biphotonic absorptiometry. Contribution of a multidetector system]

The in-vitro and in-vivo performances of a biphotonic absorptiometer with multidetector (ORIS ODC 200/SOPHA Medical France r) were compared with those of a first generation device (LAb 22a/NOVO Denmark) with a single detector. The reproducibility of both devices was tested in-vitro on a hydroxyapatite powder ghost template and a human spine enclosed in polymethyl-metacrylate; the best variation coefficients, for results in grams per cm2, are 2.11 and 1.35 p. cent for the hydroxyapatite powder and 1 and 1.36 p. cent for the spine with the single detector device and the multidetector apparatus, respectively. Influence of the soft tissues thickness measured with an aluminum tube immersed under increasing water depths (5 to 20 cm), shows decreased values in g/cm2 of 0.2 p. cent per cm of water with the multidetector apparatus, and 0.67 p. cent; for the single detector device. In vivo, the reproducibility was evaluated on 5 control patients, 3 with repositioning (CV:ORIS = 1.97 p. cent; CV:NOVO = 2.59 p. cent), and two without repositioning (CV:ORIS = 0.26 p. cent; CV:NOVO = 0.60 p. cent). Besides, the correlation between the values obtained with both devices in 62 patients is highly significant (9 = 0.95), with values with the multidetector apparatus exceeding by 13.7 p. cent those obtained with the single detector device. This excellent correlation permits to adjust the results and especially the normal reference values from one apparatus to the other.