Purpose

Hypoxia plays an important role in renal ischemia-reperfusion injury (IRI), but lack of sensitive and non-invasive detection methods, especially in the early stage^[1-3]. So far, a number of studies have reported that T2’mapping could be used in acute ischemic stroke^{[4, 5]}. However, it remains unknown whether T2’ mapping could serve as a noninvasive biomarker to assess the renal oxygen. Therefore, the main goal of this study was to explore if T2’mapping can assess the changes of oxygenation in rabbits with IRI using an oxygen-dependent pharmacological maneuver and to compare R2’(1/T2’) parameters with quantitative pathological scores of oxygen.

Materials and Methods

Animal model
In the animal care committee–approved study, forty rabbits were divided into four groups according to the time of renal pedicle clipping randomly: 45min for the mild IRI group (n=10), 60 min for the moderate (n=10) and 75min for the severe group(n=10) and another ten rabbits were set as the sham group (without IRI operation). Intravenous furosemide (FU) was administered 24 hours after IRI.
MRI experiment
All rabbits were performed five times (IRI_pre, IRI_24h, FU_5min, FU_10min and FU_15min) with a 3.0-Telsa MR (750w, GE Healthcare, USA). Multi-echo FGRE sequence was used to acquire T2* maps. The corresponding scan parameters were of field-of-view = 14cm ×14cm, matrix size = 1.1 mm ×1.1 mm×4 mm, repetition time (TR)= 700ms, echo of time (TE): 3.6ms/ 7.5ms/ 11.3ms/ 15.2ms/ 19.1ms/ 22.9ms/ 26.8ms, flip angle = 45° ,The scan time was 1 minute 44 seconds. Multi-echo FSE sequence was used to acquire T2 maps. The field-of-view and matrix size were the same as above, TR = 971ms, TE = 6.1ms/13.4ms/20.7ms/28.0ms /35.3ms/42.6ms/49.9ms, the scan time was 4 minutes 11 seconds.
Data analysis
All data were analyzed with a vendor-provided T2 & T2* fitting software on ADW 4.7 workstation (GE Healthcare). With mono-exponential model, the corresponding T2 and T2* maps were obtained. The R2’ was then calculated by R2*- R2. Dynamic Renal R2’values were measured in the cortex and inner or outer medulla at five time points. The quantitative scores of oxygen were assessed on the HIF-1α immunohistochemical staining and pyramonidazole immunofluorescence staining^[6].
All statistical analyses were performed in SPSS software. The embedded One-way ANOVA was used to test the difference of R2’ values at different time in the three stripes. In addition, Spearman correlation analysis was employed to assess the relationship between quantitative scores of oxygen and R2’ values. Significance threshold was set as p<0.05.

Results

Compared to the sham group, R2’ values increased significantly at 24 hours after IRI in the outer medulla [sham group (19.31 ± 1.21)/s; mild group (20.05 ± 1.26)/s, P < 0.001; moderate group (25.38 ± 1.38)/s, P < 0.001; severe group (25.79 ± 1.10)/s, P < 0.001) and in the inner medulla (sham group (8.20±1.36)/s; mild group (11.70±1.50)/s, P < 0.001; moderate group (13.21±0.79)/s, P < 0.01; severe group (14.73±1.73)/s，P < 0.001, respectively]. However, comparable T2’ values were found in the cortex among these four groups. Furosemide led to a significant decrease of R2’ values in the inner medulla in the sham group, mild and moderate group [R2’ decrease: (5.45±2.14)/s, (4.60±0.93)/s, (2.63± 0.65)/s,respectively, P < 0.001 each ], and R2’ decrease in the outer medulla was as follows [(11.17±4.33)/s, (7.80±0.74)/s, (3.83± 0.79)/s, respectively, P < 0.001 each) (Fig 1),whereas no significant R2’ decrease was found in the severe group (P＞ 0.05). In addition, quantitative scores of oxygen revealed significant difference among these four groups on the HIF-1α staining or pyramonidazole staining in the outer medulla (P < 0.001 each) (Fig 2). R2’ values at the time points FU_15min and R2’ difference (before and after injecting furosemide) were respectively significantly correlated with renal oxygen scores of the pyramonidazole staining (r = 0.71, -0.90, respectively, P < 0.001) or HIF-1α staining (r =0.81, -0.84, respectively, P < 0.001).
In this study, we mainly investigated the T2’ mapping in evaluating the renal oxygen with a pharmacological maneuver. R2’ parameters had been found closed correlated with quantitative scores of oxygen, indicating that T2’ mapping could assess the change of renal oxygen. The technique can be used for early diagnosis and monitoring of renal IRI in human.

Conclusion

T2’ mapping can evaluate the renal oxygenation in the early phase of IRI,especially in the outer mudulla, and may serve as a noninvasive and quantitative biomarker for IRI.

Acknowledgements

No acknowledgement found.