CHAPTER 1 Evaluation of the Urologic Patient 21
•Because of this agent’s mechanism of renal clearance, it can be used to calculate GFR.
•This agent is dependent on GFR and less useful in patients with renal failure.
Technetium 99m –Dimercaptosuccinic Acid (99m Tc-DMSA)
•Cleared by glomerular filtration and localizes mostly to the renal cortex.
•Most useful for identifying cortical defects and ectopic kidneys and distinguishing between benign and malignant lesions.
•Because 99m Tc-DMSA is retained by the proximal tubular cells, this imaging agent is ideally suited for imaging cortical processes such as acute pyelonephritis and renal scarring.
•No valuable information on the ureter or collecting system can be obtained with 99m Tc-DMSA.
Technetium 99m -Mercaptoacetyltriglycine (99m Tc-MAG3)
•Cleared mainly by tubular secretion and excreted in the urine.
•Used in diuretic scintigraphy for diagnosis of upper tract ob- struction and dynamic renal function (Fig.1.8).
Diuretic Scintigraphy
A renal scan using 99m Tc-MAG3 can provide information regarding differential renal function and obstruction. The patient should be well hydrated the day of the study.
Phamacokinetics. Peak cortical uptake of the 99m Tc-MAG3 radiotracer is typically observed 3 to 5 minutes after intravenous injection, shortly followed by the renal collecting system. By 10 to 15 minutes, the bladder can be visualized as the radiotracer is excreted in the urine.
Phases of Dynamic Renal Imaging. Dynamic renal imaging is performed in the perfusion and functional phases (Fig. 1.9).
•Perfusion phase – Renal plasma blood flow (RPF) to each indi- vidual renal unit is measured and compared with flow within the aorta. A curve with a slow rise to peak suggests poor flow to the kidney and likely underlying poor renal function.
•Functional phase – A comparison of the individual renal curves allows for the determination of relative RPF or renal function. A healthy kidney will spontaneously clear the radiotracer within 15 minutes of initial injection. An obstructed renal unit will show retention of radiotracer in the collecting. Some patients
22 CHAPTER 1 Evaluation of the Urologic Patient
A
B
FIG. 1.8 (A) Technetium 99m -mercaptoacetyltriglycine (99m Tc-MAG3) perfusion images demonstrate normal, prompt, symmetric blood flow to both kidneys. (B) Perfusion time-activity curves demonstrating essentially symmetric flow to both kidneys. Note the rising curve typical of 99m Tc-MAG3 flow studies. Dynamic function images demonstrate good uptake of tracer by both kidneys and prompt visualization of the collecting systems. This renogram demonstrates prompt peaking of activity in both kidneys. The downslope represents prompt drainage of activity from the kidneys. Printout of quantitative data shows the differential renal function to be 47% on the left, 53% on the right. The normal half-life for drainage is less than 20 minutes when 99m Tc-MAG3 is used. The half-life is 5 minutes on the left and 7 minutes on the right, consistent with both kidneys being unobstructed.
CHAPTER 1 Evaluation of the Urologic Patient 23
FIG. 1.9 Normal Technetium 99m -mercaptoacetyltriglycine (99m Tc-MAG3) renogram of a patient with history of hydronephrosis being evaluated for obstruction. In the upper portion of the figure, a series of 2-second–per–frame flow images demonstrate the movement of radiotracer from the site of injection, to the heart, aorta/ renal arteries, and kidneys. A corresponding time-activity curve is shown. The white curve reflects activity in the aorta, and the purple and teal curves reflect radiotracer activity in the kidneys. Note the sharp upstroke of all three lines and that activity in the aorta precedes activity in the kidneys by several seconds. In the lower half of the figure, a series of 2-minute–per–frame images depicts radiotracer activity within the kidneys as it transitions bilaterally into the collecting systems and then drains down the ureters. In the corresponding time-activity curve, activity within the kidneys peaks at approximately 3 to 4 minutes and then washes out, reaching halfpeak approximately 6 to 9 minutes later. The split function of the kidneys is within normal limits, measuring 46% on the left and 54% on the right (red rectangle). No evidence of obstruction is present, and no furosemide is administered.
may experience delayed clearance of radiotracer from the renal pelvis, although they do not have a truly obstructed system (e.g., previously repaired obstructive process such as a ureteropelvic junction obstruction).
•To differentiate these patients from those with obstruction, the diuretic furosemide can be administered when maximum collecting system activity is visualized. The half-time is the time it takes for collecting system activity to decrease by 50% from that at the time of diuretic administration. A postfurosemide half-clearance time of less than 10 minutes is
24 CHAPTER 1 Evaluation of the Urologic Patient
consistent with a patulous nonobstructed system, whereas a half-clearance time of more than 20 minutes is generally consistent with obstruction (Fig. 1.10). A half-clearance time between 10 to 20 minutes is considered indeterminate, and further evaluation is warranted.
A
FIG. 1.10 99m Tc-MAG3 renogram of a patient with right-sided renal obstruction.
(A) In the 2-second–per–frame flow images at the top of the panel, the left kidney appears much better perfused than the right kidney. This is borne out in the time-activity curve in the upper half of the panel in which the teal curve representing the left kidney has a significantly sharper upstroke relative to the purple curve of the right kidney. The white curve of the aorta is irregular and unreliable because of the abnormal course of the aorta caused by the patient’s scoliosis. In the bottom half of the panel, the 2-minute–per–frame images demonstrate normal transit of radiotracer through the left kidney parenchyma and into the collecting system, with drainage to the bladder. This is shown by the teal curve of the left kidney on the time-activity curve. The right kidney, which appears smaller and has a central photopenic area corresponding to a dilated renal pelvis, demonstrates increasing uptake throughout the study with very slow transit into the collecting system. This is shown by the purple curve of the right kidney in the time-activity curve. A markedly abnormal split function is present, measuring 79% on the left and 21% on the right (red rectangle).