MAG-3 vs DTPA

	DTPA	MAG-3
Excretion	Filtration 90% extracted from blood by 4 hours	Tubular excretion (95%) and filtration (5%)
Renal function estimate	GFR May underestimate, since some DTPA is bound to plasma proteins	ERPF
Response to captopril in RAS	Decreased activity (since GFR goes down)	Increased activity (since GFR and hence tubular secretion goes down)
Parenchymal visualization	Poor due to short nephrographic phase. Even worse in patients with obstruction or renal impairment, since filtration is impaired	Good
Dose-limiting organ	Bladder	Bladder
Dose	370-740 MBq (10-20 mCi)	370-740 MBq (10-20 mCi)

The difference in excretion is important in understanding post-ACE inhibitor images. In patients with renal artery stenosis, captopril causes a decrease in GFR. If DTPA is used, there is diminished initial uptake of radiotracer with or without prolonged parenchymal transit. The latter is manifested by a delay in time to peak activity and excretion.

Since the drop in GFR doesn't affect MAG-3 uptake, there is adequate initial uptake and secretion. However, the drop in GFR results in decreased urine production and flow and decreased washout of MAG-3, resulting in cortical retention.

References

Mettler FA and Guiberteau MJ. Chapter 10. In Essentials of Nuclear Medicine Imaging. Fifth Edition. Saunders, Philadelphia. 2006. p 294.