Synopsis

Keywords: Kidney, Diffusion/other diffusion imaging techniques

The clinical application of triexponential intravoxel incoherent motion analysis in the kidney has been limited because the blurring and misregistration of images caused by respiratory motion during acquisition. To solve this issue, we applied retrospective motion-corrected (MC) reconstruction to free-breathing (FB) DWI of the kidney for accurate triexponential diffusion analysis. Our results showed that triexponential analysis using MC FB-DWI can improve the measurement accuracy and repeatability of triexponential diffusion parameters of the kidney.

INTRODUCTION

Intravoxel incoherent motion (IVIM) analysis with a triexponential model provides information on three diffusion components, including perfusion-related, fast-free, and slow-restricted diffusion.^1,2 Since kidney possess complex tissue structures, including blood vessels, tubules, and connective tissues, it has been illustrated that the DWI signals are well compatible with the triexponential model.³ However, the clinical application of triexponential IVIM analysis of the kidney has been limited because the blurring and misregistration of images caused by respiratory motion during acquisition can deteriorate the measurement accuracy and repeatability.⁴ To solve this issue, we applied retrospective motion-corrected (MC) reconstruction⁵ to free-breathing (FB) DWI of the kidney for accurate triexponential diffusion analysis.

METHODS

Single-shot diffusion echo-planar imaging with FB acquisition was performed on a 3.0 T MRI system to obtain coronal diffusion-weighted images of the kidney. Seven healthy volunteers (all men; mean age, 25.1 ± 5.7 years) were scanned twice to assess the repeatability of measurements. The imaging parameters were as follows: repetition time, 4000 ms; echo time, 62.9 ms; acquisition matrix, 128 × 128; b-values, 1, 10, 30, 50, 100, 200, 400, 600, 800, 1000, and 1200 s/mm²; separate diffusion measures in three orthogonal directions; field of view, 340 mm; slice thickness, 7 mm; number of signals averaged, 3; half-scan factor, 0.803; and sensitivity encoding factor, 2.1. All subjects were asked to fast for at least 4 hours before the scan. After the acquisition, we applied a fast elastic image registration algorithm⁵ to the DWI data to correct the blurring and misregistration of images caused by respiratory motion. Next, voxel-wise estimation of perfusion-related, fast-free, and slow-restricted diffusion coefficients (D_p, D_f, and D_s, respectively), their corresponding fractions (F_p, F_f, and F_s, respectively), and multiplication of D_p and F_p (D_pF_p) were performed by fitting the triexponential model to the MC and motion-uncorrected (non-MC) DWI data. Normalized root-mean-square error (nRMSE) was calculated to evaluate the fitting accuracy. We placed region of interest (ROI) in the renal cortex and determined the triexponential diffusion parameters and nRMSE within the ROI. Repeatability coefficients (RC) were calculated from Bland-Altman plots to assess the repeatability of diffusion parameters. Additionally, to evaluate the respiratory motion-induced variation of diffusion parameters, coefficient of variation for each diffusion parameter in the ROI (CV_ROI) was computed as the standard deviation divided by the mean. Finally, these values were compared between the MC and non-MC schemes.

RESULTS AND DISCUSSION

Representative images of triexponential diffusion parameters and nRMSE obtained with the MC and non-MC schemes are presented in Figure 1. Table 1 presents the diffusion parameters and nRMSE in the renal cortex compared between both schemes. The MC had significantly lower nRMSE compared with the non-MC, indicating that the fitting accuracy of the triexponential analysis can be improved by using the MC.⁵ In addition, the MC showed significantly lower D_p than the non-MC. This result indicates that compared with the other parameters, D_p is more susceptible to signal variations due to respiration-induced image blurring and misregistration.⁶ Table 2 shows RC of each diffusion parameter. The MC improved the repeatability of D_p, F_p, D_pF_p, F_f, and F_s. CV_ROI of diffusion parameters are shown in Table 3. CV_ROI for D_p, F_p, D_pF_p, F_f, D_s, and F_s in the MC were significantly lower than those with the non-MC, suggesting that the MC can improve the respiratory motion-induced variation of these diffusion parameters.

CONCLUSION

The MC FB-DWI of the kidney improved the fitting accuracy and repeatability of triexponential diffusion parameters.

Acknowledgements

No acknowledgement found.