Figure 5. Renogram curves and differential renal function. Selected images from the flow and clearance phases are shown in (A). Five regions of interest have been drawn on an early image: left and right kidneys, "background" ROIs placed inferolateral to the two kidneys and an ROI placed over the aorta. These are used to derive a variety of quantitative measures (B) including each kidney's contribution to total renal function expressed as a percentage of the sum of the background corrected counts from the kidney ROIs. The calculation of differential renal function (%DIFF) is performed on data from one of the early images, before any activity has reached the collecting system. Flow (C) and renogram (D) curves are shown. The latter are time activity curves of the net counts within the previously drawn renal ROIs applied to the series of thirty 1 minute images and corrected for the background. (Correction for non-renal background activity is accomplished by subtracting [counts per pixel in background] x [number of pixels in kidney] from total counts in the kidney ROI.) The "time to peak" activity (Tpeak or Tmax) can be read off the graph (4 minutes in this example). This is the time when uptake in the kidney equals excretion. The rate of parenchymal clearance is expressed as the half-time (T 1/2) and the ratio of 20 minute to peak activity (20/MAX).

Figure 6. Cortical curve derived from a 99mTc-MAG3 renal transplant scan in a patient with acute tubular necrosis (ATN). Selected images from the anterior flow and clearance phases show the transplant in the right iliac fossa (A). The region of interest for the kidney includes cortex only, to avoid interference from the collecting system. The "20min/max" ratio can be used as an index of the rapidity of clearance from the renal cortex (B). The values for the maximum renal activity and the activity at 20 minutes can be read off the renogram curve. In this example the 20min/max ratio is high at 0.86. Note the relatively poor clearance from the renal cortex that can be appreciated visually and on the renogram curve (C).

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