On the effects of gating in diffusion imaging of the brain using single shot EPI
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Keywords: Diffusion tensor imaging; Magnetic resonance imaging; Cardiac gating;
Triggering; Artifacts Magnetic Resonance Imaging 19 (2001) 1125-1128
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Magnetic Resonance Imaging 19 (2001) 1125–1128
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On the effects of gating in diffusion imaging of the brain using single shot EPI
Stefan Skare*, Jesper L.R. Andersson
Department of Clinical Neuroscience, MR Center, SE-171 76 Stockholm, Sweden Received 1 April 2001; accepted 17 June 2001
Abstract To reduce image artifacts in diffusion tensor imaging using single shot EPI, cardiac gating may be performed to prevent brain motion induced signal void in the DWI data. In this work the necessity of performing cardiac gating with single shot echo planar imaging has been explored using F statistics of the variance in DWI data. Peripheral measurement of the cardiac cycle has been employed because of its greater convenience, and hence use, compared to ECG gating. Four volunteers have been studied. Six different trigger delays have been evaluated in the range 18 –500 ms relative the peripheral pulse wave. Brain motion was found significant in and inferior to the corpus callosum area. A trigger delay of 500 ms was found to be optimal. However, a long delay allows only for one slice per R-R interval. Therefore, a minimum trigger delay may be the best choice in terms of SNR per unit of experiment time. © 2001 Elsevier Science Inc. All rights reserved.
Keywords: Diffusion tensor imaging; Magnetic resonance imaging; Cardiac gating; Triggering; Artifacts
1. Introduction Diffusion tensor imaging (DTI) requires high SNR DTI data for the estimation of all elements of the diffusion tensor. To obtain optimal SNR in the DTI data several issues have been investigated in the literature. These include e.g. optimal diffusion gradient schemes [1,2]. In any diffusion experiment, minimizing artifacts due to different types of macroscopic motion is of critical importance. For nonsingle shot EPI sequences, navigator echoes are frequently used to remove e.g. patient head motion. It has been shown that for various multi shot sequences in the brain, cardiac gating is also of great importance [3– 8]. For single shot EPI, patient motion can generally be neglected during the acquisition window, which implies that navigator correction is usually unnecessary. However, it may still be of importance to perform gating even though single shot techniques are used, because even in this time frame brain motion [9,10] of some parts of the brain is large compared to molecular Brownian motion leading to signal voids in the diffusion weighted image. This leads to artifactually high
* Corresponding author. Tel.: 66. E-mail address: stefan@skare.se (S. Skare). 46 8 517 761 18; fax: 46 8 517 732
estimates of the apparent diffusion coefficient (ADC). A similar tissue motion effect has been shown in detail in for the abdomen [11]. For the brain Brockstedt et al. [12] have shown that cardiac gating is beneficial, but no thorough analysis has yet been performed. In this work we have investigated the need for gating the diffusion weighted single shot EPI sequence for brain studies. We have also examined where in the brain gating is most critical and which optimal trigger delay after the pulse wave that minimizes the variance in the DWI data.
2. Materials and methods The experiments were performed on a 1.5T GE Signa Echospeed (GE Medical Systems, Milwaukee, WI, USA). Four healthy volunteers were scanned using a diffusion weighted single shot spin echo EPI sequence. In order to get a reasonable total experiment time, the diffusion weighted (DW) gradients were applied in one direction only, the z or through slice direction. Hence, our data set does not allow for estimation of the DT parameters but rather demonstrates the test-retest reliability of a single component measurement. The b value was set to 1000 s/mm2. Common imaging parameters were TE 95 ms, FOV 24 cm, 128
0730-725X/01/$ – see front matter © 2001 Elsevier Science Inc. All rights reserved. PII: S 0 7 3 0 - 7 2 5 X ( 0 1 ) 0 0 4 1 5 - 5
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