Usefulness and Consideration of Multi-Band EPI using Simultaneous Multi-Slice for Diffusion Kurtosis Imaging at 1.5 Tesla Magnetic Resonance Imaging

Research Article

Austin J Radiol. 2016; 3(4): 1060.

Usefulness and Consideration of Multi-Band EPI using Simultaneous Multi-Slice for Diffusion Kurtosis Imaging at 1.5 Tesla Magnetic Resonance Imaging

Akihiro Kasahara1,2*, Yuiti Suzuki¹, Minoru Mitsuda¹, Kohki Yoshikawa², Masaaki Hori³, Katsuya Maruyama4, Yasushi Watanabe1, Akira Kunimatsu¹, Keiichi Yano¹ and Osamu Abe¹

¹Department of Radiology, University of Tokyo Hospital, Japan

2Graduate Division of Health Sciences, Komazawa University, Japan

3Department of Radiology, Juntendo University School of Medicine, Japan

4SIEMENS Japan K.K, Japan

*Corresponding author: Akihiro Kasahara, Department of Radiology, University of Tokyo Hospital and Graduate Division of Health Sciences, Komazawa University, Bunkyo-ku, Tokyo, Japan

Received: September 22, 2016; Accepted: October 18, 2016; Published: October 20, 2016

Abstract

Multiband Echo Planar Imaging (MB-EPI) was developed as a sequence for the Human Connectome Project of a large-scale clinical trial in the US. Recently, the concomitant use of MB-EPI with functional Magnetic Resonance Imaging (MRI) or diffusion MRI was reported; however, the combination of MB-EPI with Diffusional Kurtosis Imaging (DKI) has not been reported. We investigated the characteristics of Mean Kurtosis (MK) when MB-EPI was applied to DKI in a clinical 1.5 Tesla MRI. We obtained two DKIs, one of an MB-short when acquisition time was shortened and one of an MB-Motion Probing Gradient (MPG) with an increased number of directions at acquisition, similar to a conventional DKI by MB-EPI. These were compared to the reference DKI. An MK map or a quantitative value map of DKI and a tractography of a pyramidal tract were prepared. The latter was designated as the volume of interest, and a tract-specific analysis was conducted to detect statistically significant differences in MK values. When MB-EPI was used (MB-short and MB-MPG), the mean MK value of the pyramidal tract decreased compared to the reference DKI, and both were statistically significant from the DKI. Unlike other studies, we used a 12-channel receiver coil in a 1.5 Tesla MRI. Because the properties of the coil and MRI equipment adversely affect image reconstruction accuracy, this might have led to the statistical significance that we found. Equipment specifications in the imaging environment should be considered when carrying out quantitative evaluations.

Keywords: Multi-band EPI; Simultaneous multi-slice; DWI; DKI; Non- Gaussian diffusion

Abbreviations

MRI: Magnetic Resonance Imaging; DKI: Diffusional Kurtosis Imaging; MK: Mean Kurtosis; MPG: Motion Probing Gradient; DWI: Diffusion Weighted Imaging; QSI: Q-Space Imaging; GRAPPA: Generalized Autocalibrating Partially Parallel Acquisition; MBfactor: Multiband factor; FA: Fractal Anisotropy; ROI: Region Of Interest; VOI: Volume Of Interest; TSA: Tract-Specific Analysis; TR: Repetition Time; L-factor: Leakage factor; SNR: Signal-To-Noise Ratio; g-factor: geometry factor

Introduction

Diffusion-Weighted Imaging (DWI) capturing the signal of the water molecule has been widely applied to collect detailed information of the brain such as extraction of nerve fiber [1-3]. Most water molecule diffusion in vivo hits various endothelial cells, making this restricted diffusion [4,5]. Therefore, DWI can express, to some extent, the characteristics of the diffusion phenomenon in vivo. Recent studies have used techniques, such as Q-Space Imaging (QSI) and Diffusional Kurtosis Imaging (DKI), that express restricted diffusion [6,7]. DKI expresses quantitatively how a water molecule deviates from free diffusion and reportedly can evaluate the actual in vivo fine structure more clearly than DWI. Moreover, clinical application of non-Gaussian diffusion MRI [8] is easier compared with QSI, since imaging at several b-values are available. Raab et al. reported that differential grading of glioma was available in DKI [9]. Acquisition time is longer due to an increased Motion Probing Gradient (MPG) and imaging at multiple b-values (Raab’s DKI is 11 min 57 sec [9]). As a representative technique to accelerate acquisition time in-plane, parallel imaging [10]. On the contrary, the technique of Multiband Echo Planar Imaging (MB-EPI) [11,12] developed in the Human Connective Project [13] with characteristics to shorten acquisition time along the slice direction is now available in clinical equipment. With this technique, multiple slices are simultaneously excited to obtain overwrapped image data; subsequently, these data are separated in individual slice data, using the differences of many coil sensitivities [11]. The clinical application of MB-EPI has been reported [14,15]. In this study, we aimed to investigate the points we should note and the changes appearing in the images when used in combination with DKI and MB-EPI as use in conventional DKI.

Material and Methods

Subjects

Subjects were 13 healthy male volunteers (mean age 29.0 ± 5.39 years). This study was previously approved by the institutional ethics committees of the University of Tokyo Hospital and Komazawa University, and only subjects who signed the written informed consent alone were included in the study.

Equipment and imaging conditions

All MR imaging were performed on a 1.5 Tesla MR scanner (MAGNETOM Avanto, Siemens, Munich, Bavaria, German). Three imaging methods were compared: the conventional method without MB-EPI, which was used as the reference DKI (ref. DKI); a combined MB-EPI with ref. DKI to shorten the acquisition time (MB-short); and MB-MPG with an increased number of MPG directions (50 directions) to obtain a similar acquisition time as the ref. DKI. The version of the MB-EPI sequence used in this study was Release R012 (https://www.cmrr.umn.edu/multiband/#refs). The common parameters for individual imaging included the following: single-shot SE-EPI, FOV 24.5 × 24.5 cm2, matrix size 98 × 98, slice thickness 2.5 mm, slice gap 0 mm, slice sections 60, b-values 0, 1000, and 2000 s/ mm2, δ/Δ 32.7/37.4 msec, generalized autocalibrating partially parallel acquisition (GRAPPA) acceleration factor 2, and an average of 1. In comparison, the Multiband factor (MB-factor) in MB-short and MBMPG was set at 2. The speed-up level along the slice direction in MBEPI is called a MB-factor. The parameters are shown in Table 1.