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The "F-15" protocol is an alternate protocol for furosemide administration that can be used in equivocal cases. The furosemide is given 15 minutes before tracer injection. The peak diuretic response to furosemide response is at 15-18 minutes after intravenous injection, so this approach allows maximum diuresis at the time of maximum tracer excretion. In intermittent hydronephrosis from extrinsic mechanical ureteropelvic junction obstruction, the urinary flow rate may not keep pace with increases in pelvic pressure. In some cases flow can actually decrease at high pressures because the ureteropelvic junction becomes self-obstructing ("kinked") as the pelvis enlarges (Fig. 12).

Processing and Interpretation

As described earlier, regions of interest are drawn around the renal collecting system and pelvis of each kidney and curves are generated from the post-diuretic

Figure 13. Recovery of renal function post pyeloplasty. Early 99mTc-MAG3 static images on a 6 month old infant before (A) and 10 months after a right pyeloplasty (B). There is minimal tracer accumulation in the kidney before the procedure and normal uptake following relief of obstruction.

study. The downslope of the curve from the moment of first action of furosemide to the end of the study is fitted to an exponential function from which is derived the excretion half-time. An unobstructed system will show rapid emptying and a sharp drop in the counts within the regions of interest (Fig. 11). The normal curve will have a concave slope, with a half-time of 10 minutes or less. An obstructed system will show a flat or even rising curve, or a half time of greater than 20 minutes (Fig. 14). In between these easy to recognize extremes are the intermediate curves that require further consideration.

Factors that make interpretation difficult are renal impairment (in which the kidney is incapable of increasing urine output sufficiently to wash out 'hot" urine from the renal collecting system) and large volume renal collecting systems (since the rate of washout will be inversely proportional to the volume). If function is preserved (single kidney GFR of 15 ml/min or greater) in the kidney with a suboptimal response, then an intermediate pattern likely indicates partial obstruction. If the single kidney GFR is less than 15 ml/min, then the study is truly indeterminate. With large volume collecting systems, the F-15 approach may be helpful in a repeat study. If the kidney has normal function, then even partial obstruction may not be functionally significant, and a conservative approach may be warranted.

Clinical Role in Renovascular hypertension

Clinical

Renovascular disease is an important correctable cause of secondary hypertension. While it accounts for less than 1% of mild hypertension, the incidence may be as high as 10-45% in patients with severe hypertension. It is also an increasingly recognized and potentially reversible cause of advanced renal failure. It may be

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