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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).

Renal physiology Rate (GFR)

Non-imaging methods can be used to calculate the clearance of tracers. If 99mTc-DTPA is used, the GFR can be calculated. After injection, the tracer is assumed to mix rapidly in the blood leading to a uniform plasma concentration. The tracer then diffuses out of the plasma into the extracellular fluid (ECF) and eventually the intravascular and extracellular compartments reach equilibrium. The concentration of tracer in the plasma declines over time because of diffusion into the ECF and renal excretion. For example, Figure 8 is a plot of activity concentration over time on a semi-log graph after the injection of 99mTc-DTPA into a patient. The figure is

Figure 7. Calculation of pelvic emtying in response to diuretic. Sequential 1 minute images are shown in (A). Furosemide was injected at 12 minutes. Visually, the right renal pelvis is seen to empty while there is slower emptying from the left renal pelvis. Pelvic emptying in response to diuretic can be quantified by taking data from regions of interest drawn around the collecting systems (B). An exponential curve is fitted to the excretion portion of the curve and the half-time of excretion is determined (C). In this case, the right renal pelvis empties with a half-time of 7 minutes and the left with a half-time of 17 minutes. (A half-time of < 10 minutes is normal while > 20 minutes is associated with obstruction. A half-time of between 10 and 20 minutes is indeterminate and indicates the need for close follow-up.)

Figure 7. Calculation of pelvic emtying in response to diuretic. Sequential 1 minute images are shown in (A). Furosemide was injected at 12 minutes. Visually, the right renal pelvis is seen to empty while there is slower emptying from the left renal pelvis. Pelvic emptying in response to diuretic can be quantified by taking data from regions of interest drawn around the collecting systems (B). An exponential curve is fitted to the excretion portion of the curve and the half-time of excretion is determined (C). In this case, the right renal pelvis empties with a half-time of 7 minutes and the left with a half-time of 17 minutes. (A half-time of < 10 minutes is normal while > 20 minutes is associated with obstruction. A half-time of between 10 and 20 minutes is indeterminate and indicates the need for close follow-up.)

a computer-generated graph, similar to what would be obtained by taking multiple plasma samples over several hours. The rate of decline measured from these samples can be used to calculate GFR.

In clinical practice, it is not practical to take multiple plasma samples over 6-8 hours. One popular method of GFR calculation uses two plasma samples obtained one and three hours after 99mTc-DTPA injection, and fits the data into a regression equation derived from an eight sample 2-compartment model. An imaging approach

Figure 8. Computer-generated example of a plasma disappearance curve. The y-axis is logarithmic, i.e., the same as a graph on semi-log paper. The early steep part of the curve (A) represents a combination of renal excretion of tracer and tracer diffusing from the intravascular fluid compartment. The shallower portion of the curve (B) represents renal excretion of activity from the extracellular fluid. Analysis of the second portion of the curve allows for calculation of the clearance of the material (which in the case of 99mTc-DTPA is the GFR).

Figure 8. Computer-generated example of a plasma disappearance curve. The y-axis is logarithmic, i.e., the same as a graph on semi-log paper. The early steep part of the curve (A) represents a combination of renal excretion of tracer and tracer diffusing from the intravascular fluid compartment. The shallower portion of the curve (B) represents renal excretion of activity from the extracellular fluid. Analysis of the second portion of the curve allows for calculation of the clearance of the material (which in the case of 99mTc-DTPA is the GFR).

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