Friday, July 9, 2010

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.

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