Synopsis

IVIM imaging acquired with two echo times may provide transverse relaxation time of blood fraction, which can be converted to blood oxygen saturation.　We confirmed consistent increases in the renal blood oxygen saturation estimated by this method after an oral water load. IVIM imaging may become a non-invasive method to estimate blood oxygen saturation in kidneys.

PURPOSE

Noninvasive in vivo measurement of oxygen (O₂) saturation in renal blood is desirable for clinical practice. Potential application of intravoxel incoherent motion (IVIM) imaging for kidneys includes estimation of parenchymal blood O₂ saturation¹. Previous studies² showed that renal blood O₂ saturation increases with an oral water load. The purpose of this study is to confirm the ability of IVIM imaging to detect the change in renal blood O₂ saturation with an oral water load.

METHODS

Three healthy volunteers (age, 30-, 41-, and 51-years old) were scanned before and 30-50 min after oral intake of 1.2-1.5 L water with a 3.0T MR system (Achieva d-Stream, Philips Healthcare, Best, the Netherlands) with 32ch ds-Torso and 32ch Posterior coils. FLAIR-DWI and spin-echo (SE)-based DWI with short and long TE were imaged in coronal plane with free breathing. Basic imaging parameters were: slice thickness = 8 [mm], number of slice = 1, acquisition matrix = 65x112, FOV = 230 [mm], number of signal averaged=15, b=0, 250, 500 [s⋅mm^-2]. Imaging parameters of FLAIR-DWI were: TR/ TEshort/TElong = 5500/59/84 [ms] and TI =2000[ms]. Imaging parameters for SE-based DWI were: TR/TEshort/TElong = 3000/59/84 [ms]. A 3-compartment (renal tissue, blood, glomerular filtrates) model was considered.

Definition of parameters

Longitudinal relaxation rates: R_1t for renal tissue, R_1b = 0.58 [s^-1] for blood³, R_1w = 0.234 [s^-1] for water⁴. Transverse relaxation rates: R_2t for renal tissue, R_2b for blood, R_2w for water. Apparent diffusion coefficients: D_t for renal tissue, D* for blood. Constants proportional to proton density: m_0t for renal tissue, m_0b for blood, m0w for water. Signal obtained with DWI can be expressed as the formula A (Figure 1A), where E_r2 = e^–R2 TE, E_b = e^{–bD + K bbDD/6}, E_r1 = e^{–R1 TR}. Signal for IRDWI is expressed as the formula B (Figure 1B). Because m₀ E_r2 always appears, we define m₂ = m₀ E_r2. We adopt a 3-compartment model so that the signal obtained with DWI can be expressed as the formula C (Figure 1C), and that with IRDWI can be expressed as the formula D (Figure 1D).

Data processing

The processing was performed on offline personal computers with Mathematica10 (Wolfram Research, Champaign, IL). IRDW images and DW images with long TE were registered to DW images with short TE using affine transformation. Then, the following processing was performed.

1.　Eight image data sets (DW images and IRDW images with short TE and long TE, b = 250 and 500 for each image) were used to calculate m₀, R_1t, R_2t and D_t; K was set as 0; signals from moving blood and glomerular filtrates were ignored at b≥250.

2.　With parameters obtained from the above 1, 4 image data set (DW images and IRDW images with short TE and long TE, b = 0 for each image) were used to calculate m_0w, m_0b and R_2b.

3.　Blood O₂ saturation (Y) was estimated from R_2b using the formula for τCPMG = 20 ms by Lu, et al⁵.

Statistical analysis

A total of 6 kidneys were analyzed. Regions of interest (ROI) on Y map were set for renal cortical and medullary regions with use of R_1t and m0b maps. Mean values of Y for the ROIs before and after water load were compared using a paired t-test. To accommodate the use of bilateral kidneys per person, the paired t-test was performed using the bootstrapping method with 1000 samples each. Statistical computations were performed using SPSS 22.0 (SPSS, Chicago, IL). P<0.05 indicated statistical significance.

RESULTS

Increase in Y and decrease in R_1t were visually observed after water load in each kidney (Figure 2). Figure 3 shows box-whisker charts of mean values of Y for the ROIs (a) before and (b) after water load. In the medulla, the mean values of Y were significantly elevated after water load (P =0.021). In the cortex, they demonstrated a tendency to be elevated (P = 0.105).

DISCUSSION

Renal blood O₂ saturation estimated by transverse relaxation time of blood fraction with IVIM imaging was elevated after water load. That indicates detection of O₂ saturation in renal blood is feasible. Transverse relaxation time was estimated by use of single-echo two-dimensional DWI acquired with two echo times (59/84 ms). Therefore, R₂ values maybe overestimated because of diffusion on transvers relaxation especially on long-TE acquisition. Consequently, estimated O₂ saturation may be underestimated.

CONCLUSION

Acknowledgements

References

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