Renal Scan: DTPA

Dynamic renal scintigraphy with technetium-99m diethylenetriaminepentaacetic acid (DTPA, pentetate) measures the filtration function of each kidney. DTPA is filtered by the glomeruli and neither secreted nor reabsorbed by the tubules, and about 20% is extracted at each pass.

The uptake in the first 2 to 3 minutes reflects glomerular filtration, and the washout reflects drainage.

Adults receive 100 to 300 MBq by bolus injection, and children a weight based dose with a minimum of about 20 MBq. The camera is placed behind the patient (in front for a transplant or pelvic kidney), and images are collected from the time of injection.

A flow phase of 1 to 2 seconds in each frame for the first minute is followed by frames of 15 to 20 seconds for 20 to 30 minutes.

The study gives perfusion curves, the time to peak, the washout curve (renogram), the relative function of each kidney, and the glomerular filtration rate.

Furosemide (20 to 40 mg intravenously in adults, 0.5 to 1 mg for each kg in children) is added when obstruction is the question. Captopril is added for suspected renovascular hypertension. The effective dose is about 1 to 2 mSv.

Chronic kidney disease affects about 10% of adults. Obstructive uropathy is a common and reversible cause of kidney failure, and renovascular disease accounts for 1 to 5% of hypertension.

DTPA is the preferred tracer when an absolute filtration rate is needed with the images, and the split function is accurate to within about 5 points when kidney function is reasonable.

Indications.

• Relative function of each kidney: before nephrectomy or pyeloplasty, in living donor assessment, and in follow up of reflux or obstruction.
• Obstruction at the pelvi ureteric junction or ureter (with a diuretic).
• Glomerular filtration rate, absolute and for each kidney, when plasma clearance methods are not available.
• Renal transplant: perfusion, acute tubular necrosis, rejection, urine leak, vascular occlusion and lymphocele.
• Renovascular hypertension: study before and after an ACE inhibitor.
• Renal trauma: perfusion, urine leak and devascularisation.
• Differentiating obstruction from acute tubular necrosis in acute kidney injury.
• Function of an ectopic, horseshoe or duplex kidney.
• Function after pyeloplasty or stenting.
• Indirect radionuclide cystography for reflux in children able to void on request.

Contraindications and precautions.

• Pregnancy: avoid unless essential. Breast feeding: usually no interruption, or a brief one.
• The patient is well hydrated (500 mL of water, or 10 mL for each kg in a child, 30 to 60 minutes before). Dehydration reduces the washout and may mimic obstruction.
• The patient empties the bladder before the study. A catheter is used in children, in retention, in reflux studies and in high pressure bladders.
• ACE inhibitor challenge: withhold ACE inhibitors for 3 to 7 days and diuretics for 2 to 3 days. Caution in bilateral renal artery stenosis or a solitary kidney, because of acute renal failure.
• Furosemide: caution in allergy to sulphonamides, in hypovolaemia and in severe hypokalaemia.
• Blood pressure is monitored after captopril. A fall in blood pressure of 20 mmHg or more is common and can distort the images.
• Children may need feeding, swaddling or sedation.

Preparation and technique.

The patient lies supine or sits against the camera. The field includes both kidneys, the aorta and the bladder, or the iliac fossa for a transplant. The tracer is injected as a tight bolus through a good vein, and a poor bolus makes perfusion and uptake curves unreliable.

Regions are drawn around each kidney and an adjacent background. Relative function is calculated from the uptake in the 1 to 2 or 2 to 3 minutes after the tracer reaches the kidney, with background subtraction.

The glomerular filtration rate is estimated from the uptake, corrected for the depth of each kidney, or from plasma samples.

Diuretic timing differs between centres. Furosemide may be given at the start of the study, 15 to 20 minutes into the study when the collecting system is full, or 20 minutes before the tracer.

Imaging continues for a further 20 minutes, and the patient may stand and void before the final images.

Normal appearances.

Aortic and renal perfusion appear within seconds of each other, and the kidneys become clearer over 1 to 2 minutes.

Activity peaks at 3 to 5 minutes, then the tracer drains from the cortex into the pelvis, and the bladder is seen by 5 to 10 minutes. The collecting system is empty after the diuretic and voiding.

Relative function is 45 to 55% for each kidney. Time to peak is under about 5 minutes, and the washout half time after diuretic is under 10 minutes. Normal glomerular filtration rate in young adults is about 90 to 120 mL/min/1.73 m2, falling with age.

Interpretation.

• Obstruction: activity in the pelvis rises or stays flat after the diuretic, and the washout half time is over 20 minutes. A half time of 10 to 20 minutes is equivocal and needs correlation with function and clinical state.
• Dilated, non-obstructed system: prompt washout after diuretic, with normal or near normal function.
• Poor function: low uptake, delayed peak and a low glomerular filtration rate. When function is low, the diuretic may not produce a diuresis, and the test is non-diagnostic.
• Renovascular hypertension: after an ACE inhibitor, uptake falls, the time to peak lengthens, cortical retention appears and the relative function of the affected kidney falls by 10 points or more compared with a baseline study.
• Transplant with acute tubular necrosis: good perfusion with poor uptake and retained activity. Acute rejection: reduced perfusion and uptake. Vascular occlusion: absent perfusion. Urine leak: activity outside the kidney. Obstruction: a dilated pelvis with persistent retention. Lymphocele: a cold area compressing the kidney or ureter.
• Renal trauma: perfusion defects, extravasation of tracer, or no flow in a devascularised kidney.
• Reflux (indirect study): a sudden rise in ureteric activity during voiding.
• Unequal split function: under 40% in a kidney with obstruction signifies impaired function, and a fall of 5 to 10 points on serial studies shows deterioration.
• Congenital anomalies: ectopic, horseshoe, crossed and duplex kidneys, with the function of each moiety.
• Glomerular filtration rate stages chronic kidney disease. A donor candidate should have close to equal function in the two kidneys, with a normal total.

Critical values and findings needing immediate action.

• Absent perfusion of a transplant or a traumatised kidney: urgent surgical or vascular review.
• High grade obstruction with fever or sepsis: urgent decompression by nephrostomy or stent and antibiotics.
• Urine leak after transplant or trauma: urology review and drainage.
• Acute renal failure after an ACE inhibitor in renal artery stenosis: stop the drug, give fluids and refer.
• Marked fall in function on serial studies in an obstructed kidney: urgent surgical planning.
• Severe reaction to the diuretic or tracer (rare): supportive treatment.

Limitations and artefacts.

• Dehydration, a full bladder and high pressure in the bladder delay washout and mimic obstruction.
• Poor bolus, extravasation at the injection site and patient motion degrade the curves.
• Region of interest placement, background subtraction and kidney depth change relative function and glomerular filtration rate.
• A large, compliant pelvis can retain tracer after a normal diuretic response.
• Low kidney function reduces the signal and the response to the diuretic. DTPA has a low extraction compared with the tubular agent, so images are poor in advanced failure and in neonates.
• Overlap with the liver, spleen or bowel activity affects background.
• The timing and dose of the diuretic change the washout half time, and each centre needs its own reference values.
• Reflux during voiding is missed by the indirect method if the child cannot void on request.
• Anatomical detail is poor, and ultrasound or CT is needed for the cause of obstruction.
• The glomerular filtration rate from the camera is less exact than plasma clearance.

Special groups.

• Neonates and infants: renal function is immature in the first month, so studies for antenatal hydronephrosis are done at 4 to 6 weeks. The tubular agent is preferred.
• Children: weight based dose, bladder catheter in selected cases, and sedation only if needed.
• Patients with transplants: serial studies to separate causes of falling function.
• Patients with a solitary kidney: caution with the ACE inhibitor challenge.
• Pregnant patients: avoid, and use ultrasound.
• Regions: obstruction from stones, schistosomiasis, tuberculosis and posterior urethral valves is frequent, and kidney failure often presents late. Scintigraphy is available in few centres, so ultrasound, creatinine and plasma clearance estimates are used.

Follow up tests.

• Renal ultrasound with Doppler.
• CT or MRI urography for the cause of obstruction.
• Repeat scan with the tubular agent when function is poor.
• Cortical scan for scarring.
• Creatinine, estimated glomerular filtration rate, urinalysis and urine culture.
• Micturating cystourethrogram or direct radionuclide cystography.
• Renal artery Doppler, CT angiography or catheter angiography for renovascular disease.
• Referral to nephrology, urology or transplant services.