Renography: Methods and Pitfalls

Conventional tracers for renography are Tc-99m-MAG3 (mercaptoacetyltriglycine) and Tc-99m-DTPA (diethyltriaminepentaacetic acid). Others are Tc-99m-sestamibi, Ga-68-EDTA (ethylenediaminetetraacetic acid) and FDG (F-18-fluorodeoxyglucose), the last mentioned because it is not recognised by tubular sodium glucose cotransporter 2 and therefore enters urine. Renography is routinely performed under furosemide challenge, administered 15 min before, at the same time as or 20 min after tracer administration. The renogram comprises 3 phases: perfusion, rising and declining phases. Perfusion phase is important for renal transplants but, in general, not for native kidneys. Measurement of renal perfusion is a separate issue. Split function is measured from the relative gradients of the second phases but optimally from Patlak-Rutland graphical analysis. The third phase - whether present or not - informs on urinary drainage from hydronephrotic kidneys with suspected outflow tract obstruction. Features of obstruction are prolonged parenchymal transit time (PTT), progressively rising renogram and impaired function. PTT is prolonged in obstruction because of increased intratubular pressure and increased fluid reabsorption. It is measured by deconvolution analysis using a region over the left ventricle for blood pool. Judgement by eye is, however, preferable, especially if the contralateral kidney is normal for comparison. Another cause of prolonged PTT is tubular injury which allows increased water and solute reabsorption from tubular lumen. Symmetrical rising renograms are typically seen. Tc-99m-MAG3 secretion into tubular lumen is mediated by multidrug resistance (MDR) transporters so the renogram is exposed to the effects of MDR inhibitors such as chemotherapeutics. Ga-68-has great potential as renographic agent, especially in renal transplant management.