Feasibility of noninvasive quantitative measurements of intrarenal R₂′ in humans using an asymmetric spin echo echo planar imaging sequence

The purpose of this study was to demonstrate the feasibility of an asymmetric spin echo (ASE) single-shot echo planar imaging (EPI) sequence for the noninvasive quantitative measurement of intrarenal R₂′ in humans within 20s. The reproducibility of R₂′ measurements with the ASE-EPI sequence was assessed in nine healthy young subjects in repeated studies conducted over three consecutive days. Moreover, we also evaluated whether the ASE-EPI sequence-measured R₂′ reflected the intrarenal oxygenation changes induced by furosemide in another group of normal human subjects (n=10). Different flow attenuation gradients (b=0, 40 and 80s/mm²) were utilized to examine the impact of the intravascular signal contribution on the estimation of intrarenal R₂′. In the absence of flow dephasing gradients (b=0s/mm²), the computed coefficient of variation (CV) of R₂′ was 21.31±4.52%, and the estimated R₂′ value decreased slightly, but not statistically significantly (p>0.05), after the administration of furosemide in the medullary region. However, CV of R₂′ was much smaller in the presence of flow dephasing gradients (9.68±3.58% with b=40s/mm²and 10.50±3.62% with b=80s/mm²). Moreover, a significant reduction in R₂′ in the renal medulla was obtained (p<0.05 for both b=40s/mm² and b=80s/mm²) after the administration of furosemide, reflecting an increase in oxygen tension in the medullary region. In addition, R₂′ measurements did not differ between the b=40s/mm² and b=80s/mm² scans, suggesting that small diffusion gradients were sufficient to minimize the intravascular signal contribution. In summary, we have demonstrated that renal R₂′ can be obtained rapidly using an ASE-EPI sequence. The measurement was highly reproducible and reflected the expected intrarenal oxygenation changes induced by furosemide.