Renal Scan: MAG3
Dynamic renal scintigraphy with technetium-99m mercaptoacetyltriglycine (MAG3) shows renal perfusion, tubular function and urinary drainage.
MAG3 is bound to plasma protein and removed mainly by secretion in the proximal tubule, with an extraction of 50 to 60% at each pass, much higher than for the filtration agent. It gives clearer images, particularly when function is reduced and in infants.
Adults receive 70 to 200 MBq by bolus injection, and children a weight based dose with a minimum of about 15 MBq.
Images are taken from the time of injection, with a flow phase of 1 to 2 seconds in each frame for 60 seconds, then 10 to 20 seconds in each frame for 20 to 30 minutes.
The camera is behind the patient, or in front for a transplant.
The study gives perfusion, time to peak, relative function, washout half time and, with a calibration, an estimate of effective renal plasma flow. The effective dose is about 0.7 mSv, and the bladder wall receives the largest share of dose, so hydration and frequent voiding matter.
Antenatal hydronephrosis is found in 1 to 5% of pregnancies, and pelvi ureteric junction obstruction affects about 1 in 1000 to 2000 live births. Kidney transplantation needs repeated assessment of function in the first weeks.
MAG3 is the first choice for diuretic renography, for neonates and for kidneys with reduced function, and separates obstruction from simple dilation with good accuracy.
Indications.
- Obstruction of the pelvi ureteric junction or ureter, with a diuretic.
- Antenatal hydronephrosis: relative function and drainage after birth.
- Follow up after pyeloplasty, ureteric reimplantation or stenting.
- Relative function when kidney function is reduced.
- Renal transplant: perfusion, tubular necrosis, rejection, drug toxicity, obstruction, urine leak and vascular complications.
- Renovascular hypertension, with an ACE inhibitor.
- Acute kidney injury: obstruction against acute tubular necrosis.
- Renal trauma and suspected renal artery occlusion.
- Function of duplex, ectopic and horseshoe kidneys.
- Assessment before nephrectomy or living donation.
Contraindications and precautions.
- Pregnancy: avoid unless essential. Breast feeding: usually no interruption, or a brief one.
- Hydration before the study: 500 mL of water for adults, 10 to 15 mL for each kg in children, or intravenous fluid in infants. The patient voids before starting.
- Bladder catheter in infants, in retention, and when bladder pressure is high or reflux is suspected.
- ACE inhibitor challenge: captopril 25 to 50 mg an hour before the tracer, with blood pressure monitored. Withhold ACE inhibitors for 3 to 7 days before the baseline study, and diuretics for 2 to 3 days.
- Furosemide: caution in sulphonamide allergy, hypovolaemia and hypokalaemia. The dose is 20 to 40 mg in adults and about 1 mg for each kg in children, up to 40 mg.
- Sedation is rarely needed. Infants are fed and wrapped.
- Allergy to the tracer is very rare.
Preparation and technique.
The patient is well hydrated and the bladder is empty. The camera covers both kidneys and the bladder. A tight bolus is injected, and the first minute of images records perfusion. Renal regions and background regions are drawn on summed images.
Relative function is taken from the uptake between 1 and 2 minutes, or 1 and 2.5 minutes, after the tracer reaches the kidney, with background subtraction. The residual cortical activity at 20 and 30 minutes, expressed as a fraction of the peak, is a useful measure of drainage in infants.
For diuretic renography, furosemide is given when the pelvis is full (usually 15 to 20 minutes after injection), or 20 minutes before the tracer in some centres. The bladder is emptied by catheter or voiding, and images are taken for 20 more minutes.
The patient stands or walks briefly to help drainage if the washout is delayed, and a post-void image is taken.
