Noninvasivie assesment of renal oxygenation plays a major role in the evaluation of renal function and has attracted considerable attention in recent years (1-2). This study demonstrates the feasibility of using a susceptibility-based MRI technique for measuring renal oxygen extraction fraction (OEF) change under the influence of carbogen (97% O₂, 3% CO₂) breathing.

Animals: This study was approved by the local institutional review board for experimental animal studies. Eight New Zealand White rabbits (weight range 2.5-3.5 kg) were included in this study.

MRI Methods: Renal images were carried out on a 3.0 Tesla Philips Achieva MR scanner (Philips Medical Systems, Best, Netherlands), using a 16 channel Knee coil. For OEF estimation, a triple-echo ASE sequence with 32 varied echo shifts was
implemented to acquire the source images (Fig.1). Detailed imaging
parameters were: FOV = 340 × 220 mm2, matrix size =
112 × 72, TR = 2000 ms, TE1/TE2/TE3 = 65/93/121 ms, slice thickness = 6 mm, SENSE factor = 2, readout bandwidth=303.97 kHz.

Study Protocol: During the whole experiments, rabbits were anesthetized with 1% isoflurane delivered by a calibrated vaporizer. A block design was applied for gas administration: 10 min room air, 10 min carbogen, 10 min room air, and 10 min carbogen. During each gas challenge, two seperated ASE scans were conducted with an interval of 5 minutes. Blood samples (0.5 mL) were withdrawn from the auricular veins of five rabbits for measurement of pO₂ at four time points: (a,b) immediately after 5 minutes of air inhalation and (c,d) after 5 minutes of carbogen inhalation. All the blood samples were collected into a capillary tube for blood gas analysis (ABL 700 series, Radiometer, Copenhagen, Denmark).

Quantitative Analysis: Measurement of renal OEF was
derived from a theoretical model proposed by Yablonskiy and Haacke (3). A nonlinear least-squares curve fitting function was used to fit this model. Within-session and between-day reproducibility of the OEF measurement were evaluated by coefficient of variation (CV), which was calculated as the standard deviation divided by the mean of the intrarenal OEF from two scans. Pearson correlation coefficient was utilized to characterize the correlation strength between MRI based renal OEF and pO₂. Paired two-sided Student t-test was employed to assess statistical differences between the inhalation of room air and carbogen. P < 0.05 was considered to be statistically significant.

A representative coronal T2-weighted image of renal is shown in Fig.2a. The corresponding spin echo image and images acquired at asymmetric echo with τ = -18 ms and τ = 13 ms of the ASE sequence are demonstrated in Fig.2 (b-d). Representative OEF maps from of one rabbit and the average renal OEF values under respiratory challenges are displayed in Fig.3 and Fig.4. It was shown that renal oxygenation was clearly influenced by carbogen breathing in both cortex and medulla. During room air breathing, relatively higher OEF was seen in the renal medulla than cortex (0.34 ± 0.03 vs 0.32 ± 0.04 for Scan 1 (P < 0.05), and 0.34 ± 0.04 vs 0.31 ± 0.02 for Scan 2 (P < 0.05)). The measured renal OEF under room air breathing was in good agreement with previous studies (4) using the same susceptibility-based method. After averaging the OEF values under the same conditions, the OEF decrease due to carbogen breathing was 13.1% (95% confidence interval: 11.3–15.5%) in the cortex and 12.7% (10.5-14.2%) in the medulla, but no statistical difference was seen between cortex and medulla (P = 0.78).

The within-session CVs of OEF measurement under room air breathing were 8.05% in the cortex and 5.02% in the medulla, while the between-day CVs of OEF measurement are 8.45% in the cortex and 5.02% in the medulla. The results of blood pO₂ and the corresponding renal OEF are shown in Fig.5 (N = 5). The average pO₂ was 88.8 ± 23.5 mmHg (Air1) and 127.2 ± 48.1 (Air2) during room air breathing, and it rose up to 340.2 ± 51.5 mmHg (Carbogen1) and 419.25.2 ± 74.1 mmHg (Carbogen2) during carbogen breathing. Scatterplot shows negative correlation between reduced OEF and elevated pO₂ (r = 0.68 (P < 0.05) in cortex, and r = 0.64 (P < 0.05) in medulla.

This study proposed a method to evaluate renal oxygenation noninvasively, with good scan-rescan reproducibility. Significant decrease of renal OEF was found during carbogen challenge. Furthermore, the efficacy of this susceptibility-based method was proved by blood pO2 measurement.