Research articleFeasibility of postoperative 3-tesla diffusion tensor imaging in cervical spondylotic myelopathy: A comparison of single-shot EPI and multi-shot EPI
Introduction
Diffusion tensor imaging (DTI) is a magnetic resonance imaging (MRI) modality, which is based on proton mobility and offers information on water diffusion [1]. It provides fractional anisotropy (FA) and apparent diffusion coefficient (ADC) values. The FA values correspond to the global anisotropy of the analyzed structure. They reflect the tissue integrity, including the white matter fibers, from which fiber tractography is derived [1,2].
Previous studies have revealed changes in the diffusion characteristics of DTI in various spinal cord pathologies, including spondylotic myelopathy, demyelinating disease, vascular malformation, trauma, and tumor, even in cases where conventional MRI show no abnormal findings [[3], [4], [5], [6]]. Many studies have demonstrated that in cervical spondylotic myelopathy (CSM), DTI values and tractography findings correlate with disease pathology and functional impairments [[7], [8], [9], [10], [11]]. For example, Maki et al. showed that FA values measured on corresponding maps of the spinal cord along the lateral and posterior column strongly correlate with motor dysfunction of the lower extremities [8]. Cui et al. [7] suggested that lower mean FA values in diffusion tensor tractography indicate spinal cord damage in patients with CSM. A meta-analysis demonstrated a tendency for increased ADC and decreased FA values in patients with spondylotic myelopathy [12].
In contrast to the increased use of DTI in patients with CSM, its use in patients with CSM undergoing surgery has been very limited. This is because of the significant image distortion in single-shot echo-planar imaging (SS-EPI), the most frequently used technique for spine diffusion imaging, which is due to B0 field inhomogeneities caused by the large susceptibility variation near metal instruments as well as near vertebrae. SS-EPI also suffers from motion artifacts arising from swallowing, breathing, and cerebrospinal fluid pulsation, which inhibit visualization of the small cross-sectional areas of the spinal cord [13]. In addition, higher magnetic fields, worse B0 inhomogeneities, image distortion, and motion artifacts make it hard to evaluate 3-T MRI scans in patients with metal implants. Although only few reports have evaluated postoperative DTI in patients with CSM, revealing its clinical relevance in patients with surgical decompressions, postoperative DTI using 3-T scanners is still challenging despite increased use of 3-T MR [[14], [15], [16]].
Interleaved multi-shot EPI (MS-EPI) offers the possibility of obtaining spine diffusion images with improved resolution and reduced distortion [17]. To correct phase variation between subsets of k-space data in MS-EPI, recently developed image reconstruction using image-space sampling (IRIS) functions for MS-EPI DTI (Phillips Healthcare, Best, The Netherlands) were shown to significantly reduce motion artifacts, while obtaining additional navigator echo [18]. Thus, we speculated that MS-EPI using IRIS may improve image quality of 3-T MRI in patients undergoing surgery due to CSM.
We performed a prospective cross-sectional study to compare postoperative DTI quality between the established SS-EPI sequence and MS-EPI using IRIS in 3-T MR in patients with CSM. We also evaluated the correlation between DTI index and image quality, and the clinical impact of patients’ symptoms and operation methods.
Section snippets
Subjects
This study was approved by an institutional review board. Informed consent was obtained from all subjects. In this study, we prospectively enrolled patients with CSM scheduled for MRI at one month after cervical operations in the Department of Orthopedic Surgery during the period from December 2017 to March 2018. DTI scans were additive to the scheduled MR at 1 month postoperatively to evaluate the acceptable decompression and postoperative complications. The inclusion criterion was the patient
Qualitative variables for cord visualization
The results of the qualitative analysis are presented in Table 2. Visualization of the proximal spinal cord segment was not significantly different between SS- and MS-EPI sequences in both group 1 (SS-EPI with 6 vs MS-EPI with 6 diffusion gradients) and 2 (SS-EPI with 15 vs MS-EPI with 6 diffusion gradients). In the metallic segment, MS-EPI with 6 diffusion gradients showed significantly less distortion and better cord visualization in both group 1 and 2, when compared with SS-EPI sequences (
Discussion
In this article we compared postoperative high tesla DTIs with SS- and MS-EPI protocols in patients with CSM, which focused on the image distortion and success of fiber tractography. We also evaluated the influence of operation methods to MS-EPI images.
Although MS-EPI provides a strong benefit in greatly reducing susceptibility artifacts by decreasing the readout time between subsets of k‐space data, the high number of motion artifacts is a main issue of this sequence because of its phase
Disclosures
No funding has been received for the conduct of this study and/or preparation of this manuscript.
Declaration of Competing Interest
The authors declare that there are no conflicts of interest.
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