Estimation of effective renal plasma flow (ERPF) can be made easily, inexpensively, and accurately by means of a single plasma concentration determination, sampled 44 min after injection of 131I-orthoiodohippurate (OIH). We originally established predictive regression equations based on a series of patients with a wide variety of diseases and a few normal potential kidney donors. The equation best fitting that data was parabolic in form and assumed a negative slope when high ERPF rates were encountered. This problem has been corrected by deriving new equations (parabolic and exponential) based on an expanded series which includes a large number of subjects with high ERPF. Errors of estimation are lower than those of the more classic para-aminohippuric acid (PAH) clearances and well within the exigencies of clinical practice.

A variety of methods have been proposed to estimate the glomerular filtration rate (GFR) from the renal uptake of technetium Tc 99m-DTPA using a gamma camera. To compare alternative methods, we calculated the GFR in several different ways from measurements in 33 patients and compared the results with an independent GFR measurement based on eight-point plasma clearance of ytterbium Yb 169-DTPA. The best agreement was obtained using an algorithm that has not been described previously, in which correction was made for overlap of the kidneys by the liver and spleen. The correlation coefficient was 0.958, and the residual standard deviation was 12.1 ml/min. This method required a single 20-min blood sample as well as the camera data. The best method not requiring a blood sample was significantly less accurate, with a correlation coefficient of 0.837 and a residual standard deviation of 23.1 ml/min. The accuracy of these methods was comparable to that reported for creatinine clearance, the most commonly used estimate of the GFR in current clinical practice.

Several methods are presently available for measuring the mineral content of bone. Those in widespread use include dual-photon absorptiometry and quantitative CT. The feasibilIty of using dual-energy digital chest radiography for determination of the mineral content of posterior ribs on digital chest images was studied by using a prototype unit The resufts showed a significant difference in the mineral density of the posterior ribs of control subjects and those of patients who had osteoporosis (251.1 ± 36 mg Ca2 / cm2 of rib vs 158.8 ± 48 mg Ca2 /cm2, p .01) and a close correlation with values obtained by dual-photon absorptiometry of the lumbar spine (r = .77). The results suggest that this technique Materials and MethodsImages of the chest were obtained in posteroantenor projection with the second-generation Picker scanning-slit dual-energy chest unit (Picker International, Cleveland, OH). The charactenstics of this unit have been described in detail elsewhere [1 -5]. Briefly, an X-ray fan beam is created by a 0.5-mm slit collimator placed between the patient and the focal spot of the Xray tube; the beam is aligned with a 1 .0-mm collimator placed between the patient and the detector array (consisting of 1 024 vertical pixels). The collimators and the detector array are linked mechanically and pivot about the focal spot as the beam scans horizontally across the patient (Fig. 1). A key feature of this device is its linear detector array of 1024 elements. Each element is a low/high atomic sandwich:The first section consists of a low-atomic-number phosphor (optically coupled to a photodiode) that preferentially absorbs the lower-energy photons emerging from the patient; the higher-energy photons penetrate the first section and strike the rear section, which is composed of a high-atomic-number phosphor (also optically coupled to a photodiode) (Fig. 2). In this way, low-and high-energy images are acquired simultaneously. These in turn are summed to obtain a standard digital image (comparable to a conventional radiograph) (Fig. 3) and are processed to obtain soft-tissue (bone cancelled, Fig. 4A) and bone (soft-tissue cancelled, Fig. 4B The posterior ribs were selected as the first site of measurement;they are easily accessible and provide a reproducible location for the parallelogram technique. The efficacy of using the nbs in screening for osteoporosis is suggested by dual-photon absorptiometry studies that have noted a parallel reduction in the mineral content of the ribs and spine in patients who have osteoporosis and in elderly persons [6,7].

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