Diffusion tensor imaging focusing on lower cervical spinal cord using 2D reduced FOV interleaved multislice single-shot diffusion-weighted echo-planar imaging: comparison with conventional single-shot diffusion-weighted echo-planar imaging

Abstract

Purpose

To evaluate the performance of diffusion tensor imaging (DTI) of the cervical spinal cord by comparing 2-dimensional standard single-shot interleaved multisection inner volume diffusion-weighted echo-planar imaging (2D ss-IMIV-DWEPI) and conventional 2D ss-DWEPI in a clinical population, focusing on the lower cervical spinal cord.

Materials and Methods

From July to September 2013, a total of 23 patients who underwent cervical spinal MR imaging with DTI were retrospectively enrolled in this study (M:F = 7:16, mean age 45 years, age range 24–76 years). Exclusion criteria were: previous prosthesis insertion (n = 5), syringomyelia on T2-weighted imaging (n = 4), and spinal cord tumor (n = 0). All MRI examinations were performed using 3.0 T imaging with a phased-array spine coil including two different 2D reduced FOV DTI sequences: 2D ss-IMIV-DWEPI (iDTI) and 2D ss-DWEPI without interleaving (cDTI). For quantitative analysis, two musculoskeletal radiologists who were blinded to the sequence measured fractional anisotropy (FA) and apparent diffusion coefficient (ADC) values throughout the whole cervical spinal cord (C1-T1). For qualitative analysis, the readers rated each image based on spinal cord distortion, dural margin delineation, and depiction of intervertebral disc. Quantitative and qualitative evaluations were analyzed separately for upper and lower segments. The t-test was used for quantitative analysis and two-way analysis of variance (ANOVA) and t-tests were performed for qualitative analysis.

Results

FA was significantly higher and ADC was significantly lower on iDTI compared with cDTI (0.679 versus 0.563, respectively, for FA; 631 versus 1026, respectively, for ADC; p < 0.001), and this was consistently observed in the lower segment of the spinal cord. The reviewers rated iDTI as superior in terms of all assessed characteristics. For qualitative analysis, the mean iDTI score was significantly higher than the cDTI score for both the lower and upper segments (p < 0.001).

Conclusion

2D rFOV ss-IMIV-DWEPI demonstrated higher performance than conventional 2D rFOV ss-DWEPI in terms of improving image quality, even in the lower segment of the cervical spinal cord.

Introduction

Diffusion tensor imaging (DTI) provides not only structural integrity information, but also directional information by measuring water molecule diffusion within tissues. Obtained metrics from DTI are fractional anisotropy (FA), the values of apparent diffusion coefficient (ADC), and eigenvalues, which provide information on the scalar properties of the diffuse translation for extracellular water molecules [1], [2], [3], [4]. Previous studies have shown that these metrics reflect the microstructure of the spinal cord and provides visualization of fiber tractography, enabling tracking of the white matter pathways in the brain and spinal cord. Moreover, the application of DTI at the cervical spinal cord allows characterization of microstructural changes including demyelinating disease, infarction, myelopathy, traumatic injury, and spinal cord tumor [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15].

However, in practice, the usefulness of DTI at the cervical spinal cord (CSC) has been impeded by 1) the small dimension of the CSC, 2) partial volume artifacts from surrounding cerebral spinal fluid (CSF) and lipid, 3) motion artifacts from breathing, swallowing, and CSF pulsation, and 4) large bony structures that cause abrupt changes in magnetic susceptibility [16], [17], [18], [19]. For DTI of the CSC, standard single-shot diffusion-weighted echo-planar imaging (ss-DWEPI) is widely used in clinical cervical MR imaging. However, this protocol has a long readout time and low bandwidth in the phase encode direction, and is therefore prone to distortions and motion artifacts. In previous studies, these limitations were emphasized in the lower CSC [11], [20], [21].

Degenerative changes of the cervical spine including spondylosis and disc herniation are known to affect the lower segment because of the relative burden of weight and extensive range of motion [22]. However, several previous studies have emphasized the limitations of cervical DTI of the lower segment. This is mainly because of its location close to the lungs and heart, and negative effects associated with the construction of the surface coil [11], [20], [21]. Over the past decades, there have been several efforts using various EPI-based methods to overcome these limitations, including line scan imaging [23], [24], navigated fast spin-echo [3], propeller-based imaging [25], [26], [27], parallel EPI [28], [29], [30], ZOOM-EPI [31], multichannel coil [32], and more recently, reduced effective field of view (FOV) in the phase encoding direction, an approach that is currently in the limelight. Reducing the FOV in the phase encode (PE) direction enables a drastic shortening of the readout time and also increases the (pseudo)bandwidth in the phase-encoding direction. In addition, geometric distortion in ss-DWEPI is proportional to the FOV in the phase-encoding direction; therefore, susceptibility-related artifacts and pixel misregistration can be reduced in ss-DWEPI [16], [18], [19].

An equally important method is the interleaved multisection inner volume (IMIV) technique, which provides double inversion/refocusing radio-frequency pulses at 2D ss-DWEPI. This allows for acquisition of the entire cervical spinal cord with 1 interleaved image in the sagittal plane [7]. A few reports on the application of IMIV techniques at the CSC have been published [7], [33], [34], [35], but there are no direct comparisons with conventional protocols focusing on the lower CSC.

Therefore, the purpose of this study was to evaluate the performance of DTI in the cervical spinal cord by comparing 2D ss-IMIV-DWEPI (iDTI) and conventional 2D ss-DWEPI (cDTI) in a clinical population, focusing on the lower cervical spinal cord.