Original contributionImprovement of renal diffusion-weighted magnetic resonance imaging with readout-segmented echo-planar imaging at 3 T
Introduction
Diffusion-Weighted MR Imaging (DWI) has shown promising results to differentiate pathological from healthy tissues in renal tumors [1], transplant rejection [2], pyelonephritis [3], ureteral stone obstruction [4] and renal artery stenosis [5]. DWI techniques used in the abdomen rely on single-shot diffusion-weighted echo-planar imaging (ss-EPI) which is sensitive to image artifacts [6]. The trade-off between resolution and signal-to-noise ratio for the large FOV used in the abdomen imposes an increased matrix size and therefore a longer EPI readout time. The sequence becomes more sensitive to in-plane geometric distortions caused by the off-resonance water protons in areas where a significant difference in susceptibility exists. In renal DW applications, severe distortions are present at the bowel (filled with air) and tissue interface.
One solution to improve the distortions in diffusion MRI is to use a ‘Readout Segmentation Of Long Variable Echo-trains’ (RESOLVE) MR sequence in combination with parallel imaging technique, such as GRAPPA, GeneRalized Autocalibrating Partially Parallel Acquisitions. The RESOLVE strategy is based on a segmentation of k-space into several shots along the readout direction in order to shorten the echo spacing. RESOLVE combined with parallel imaging was previously introduced by Porter et al. for acquiring high-resolution DW images with low susceptibility based image distortion and T2⁎ blurring in the brain [7]. This strategy has been validated in non-triggered applications, such as in head and breast imaging to reduce sensitivity to susceptibility artifacts [8], [9], [10]. RESOLVE outperformed ss-EPI for analysis of the pediatric brain in regions prone to geometric distortions, such as the orbit, the skull base, and the posterior fossa [11]. By comparison with a conventional ss-EPI sequence, RESOLVE has improved the lesion-to-background contrast and the categorization between benign and malignant breast lesions [12]. For these reasons, we hypothesized that RESOLVE could improve the robustness against artifacts that are a consequence of the long k-space sampling in renal DWI. We proposed in this study an implementation of a respiratory-gated RESOLVE protocol for kidney DWI as well as a comparison of image quality with a different implementation of ss-EPI in a phantom, healthy volunteers and chronic kidney disease (CKD) patients.
Section snippets
Subjects
Twenty healthy volunteers, comprising 11 females and 9 males, with a mean age of 28.8 ± 4.7 years (range, 23–39 years) and ten patients, comprising 4 females and 6 males, with a mean age of 55 ± 16 (ranges 26–78 years), were recruited after informed consent. Healthy volunteers enrolled in this study had no known kidney disease. All patients were chronic kidney disease (CKD) patients, comprising 1 native kidney and 9 allograft patients. The patients' characteristics are summarized in Table 1. The study
Geometric distortions
The level of geometric distortions in the phantom is shown in Fig. 1a. The strongest deformation of the phantom borders was observed with the reference EPI, followed by the HR-EPI as quantified by the deviation from the corner of the phantom on the undistorted GRE reference image. The RESOLVE images were characterized by the smallest geometrical deformation among all the sequences. The maximum distance in the phase-encoding direction between the phantom edges of the GRE image and the reference
Discussion
Our goal was to study the potential of readout-segmented echo-planar imaging with a fivefold segmented k-space acquisition for renal DWI. An improved image quality from the RESOLVE sequence over ss-EPI sequences was observed in both healthy volunteers and CKD patients. The respiratory-gated RESOLVE significantly reduced diffusion artifacts and the other hurdles encountered with the use of ss-EPI. Whereas such improvements have already been demonstrated in the brain and breast [8], [9], [10],
Conclusions
In conclusion, despite a longer scan time, RESOLVE enhanced significantly the quality of renal diffusion-weighted images by reducing the image distortion and blurring and by improving the difference in SI and ADC between the renal cortex and medulla in healthy volunteers. The performance of the RESOLVE was also demonstrated in CKD patients with a disappearance of the cortico-medullary ADC difference. These improvements justify further clinical studies of the potential of RESOLVE for diffusion
Acknowledgements
This work was supported by grants from the Clinical Research Center of the Medicine Faculty of Geneva University and Geneva University Hospital and the Louis-Jeantet Foundation (CRC) and in part by the Centre for Biomedical Imaging (CIBM) of EPFL, University of Geneva and the University Hospitals of Geneva and Lausanne, as well as the Leenaards and Insuleman foundation.
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