Appropriate selection of background for 99Tcm-DTPA renography

Since 99Tcm-DTPA is diffusible and not significantly protein bound in plasma, it rapidly enters the extravascular space following injection. Therefore, during the first few minutes of the DTPA renogram, the period on which the measurements of individual kidney glomerular filtration rate and differential function are based, background activity comprises a rising extravascular signal and a falling intravascular signal. The aim of this study was to measure the ratio of these two signals in background present within the renal region of interest (ROI) and compare it with the ratio in a background ROI. An appropriate background ROI is one in which the ratio is equal to that in background in the renal ROI. To pursue this aim, we quantified the rates of change of the intravascular and extravascular activities in background and, by comparing them with the rate of increase of filtered activity, expressed them as GFR equivalents (the intravascular being negative). It is impossible, from a single renogram, to separate the rising extravascular signal from the signal due to filtered activity, and therefore impossible to quantify the extravascular GFR equivalent present in background within the renal ROI. We therefore studied six patients undergoing bone marrow transplantation before and after cyclosporin treatment. By comparing the dynamic renographic data between the two sequential studies, the substantial fall in GFR (from 107 +/- 12 S.D. to 49 +/- 7 ml min-1) permitted separate quantification of the extravascular GFR equivalent in the renal ROI in both studies. Three of the patients were studied on a third occasion after cyclosporin. In two, GFR remained low and these studies were paired with corresponding baseline studies, while in the other it increased and this was compared with the nephrotoxic study, giving a total of nine paired studies between which GFR changed. The ratio of intravascular to extravascular GFR equivalents in a background ROI placed above the kidney was considerably greater, and in a background ROI below the kidney considerably less, than that in the renal ROI. A background ROI which was the difference between the renal ROI and a perirenal ROI, 2 pixels outside the renal ROI along the horizontal and 1 pixel outside along the vertical, gave a ratio almost identical to that of the background within the renal ROI (renal ROI ratio:background ROI ratio = 1.09 +/- 0.17 S.D., n = 18).(ABSTRACT TRUNCATED AT 400 WORDS)