Toward a practical protocol for human optic nerve DTI with EPI geometric distortion correction

Abstract

Purpose: To develop a practical protocol for diffusion tensor imaging (DTI) of the human optic nerve with echo planar imaging (EPI) geometric distortion correction. Materials and Methods:A conventional DTI protocol was modified to acquire images with fat and cerebrospinal fluid (CSF) suppression and field inhomogeneity maps of contiguous coronal slices covering the whole brain. The technique was applied to healthy volunteers and multiple sclerosis patients with and without a history of unilateral optic neuritis. DTI measures and optic nerve tractography before and after geometric distortion correction were compared. Diffusion measures from left and right or from affected and unaffected eyes in different subject cohorts were reported. Results:The image geometry after correction closely resembled reference anatomical images. Optic nerve tractography became feasible after distortion correction. The diffusion measures from the healthy volunteers were in good agreement with the literature. Statistically significant differences were found in the fractional anisotropy and orthogonal eigenvalues between affected and unaffected eyes in optic neuritis patients with poor recovery. The diffusion measures before and after geometric distortion correction were not significantly different. For cohorts without optic neuritis, the difference between diffusion measures from left and right eyes was not statistically significant. Conclusion:The proposed technique could provide a practical DTI protocol to study the human optic nerve. MAGNETIC RESONANCE DIFFUSION TENSOR IMAG-ING (DTI) (1) has become a well-known clinical research tool to investigate brain tissue microstructure in vivo. DTI provides quantitative measures reflecting the integrity of the axonal fiber tracts in the central nervous system (CNS) via observation of water diffusion anisotropy. This information allows tractography of the fiber bundles suggesting connections between cortical regions. DTI has been applied to study changes associated with neurodevelopmental disorders and changes due to trauma and neurodegenerative diseases (2,3).DTI of the human optic nerve has been explored by many research groups (4 -17). As a pure white matter tract and most of its course being structurally isolated from the rest of the CNS, the optic nerve can serve as a unique structure to study the changes in the CNS due to multiple sclerosis (MS) and optic neuritis (ON). For example, acute inflammation with visual impairment is a first clinical presentation of CNS demyelination. About 20%-30% of patients with this symptom will develop MS and nearly 50% of the MS patients will develop ON (18). With a reliable assessment of visual function and the quantitative measurements from DTI, the impact of injuries and diseases to the optic nerves can be closely studied. However, DTI of the optic nerves is challenging because of their small diameter, confounding signals from surrounding fat tissue and cerebrospinal fluid (CSF), magnetic susceptibility gradients due to adjacent sinuses and bony structu...

Development of a high-resolution fat and CSFsuppressed optic nerve DTI protocol at 3T: application in multiple sclerosis

“…In both figures, the ROI positions (four pixels in size) are indicated in red. Healthy control mean optic nerve MD and FA values measured using the standard acquisition protocol (ii) were consistent with previous measurements made at 1.5T (Chabert et al, 2005;Trip et al, 2006;WheelerKingshott et al, 2006) and 3T (Kolbe et al, 2009;Naismith et al, 2009;2010;Smith et al, 2011;Techavipoo et al, 2009;Wang et al, 2011;Xu et al, 2007). …”

“…Another approach was proposed by Andersson et al (2003) to correct for susceptibility distortions in SE-EPI images; this required two sets of data: one acquired with the phase-encoding direction reversed, plus a B0 map, however, it may not be feasible for application in the optic nerve. Techavipoo et al (2009) used a conventional single-shot SE-EPI protocol with parallel imaging factor 2.5 and additional field inhomogeneity maps covering the whole brain with a voxel size of 1.5mmx1.5mm, and 3mm slices, and found that optic nerve tractography became feasible with geometric distortion correction. However, DTI measures were not significantly different when calculated with or without the use of distortion correction.…”

“…Although for white matter diffusion acquisitions the optimal b-value is approximately 1000ms -2 , much lower b-values are typically used in optic nerve DTI studies [e.g. (Kolbe et al, 2009;Naismith et al, 2010;Techavipoo et al, 2009;Wheeler-Kingshott et al, 2002; in order to maximize SNR and because of the motion sensitivity of DTI and the constant motion of the optic nerve. It has also previously been shown that, for optic nerve DTI, the acquisition of repeated averages of the same diffusion direction are necessary in order to minimize the effect of optic nerve motion between image acquisitions (Wheeler-Kingshott et al, 2000).…”

Diffusion Tensor Imaging of the Optic Nerve in Subacute Anterior Ischemic Optic Neuropathy at 3T

“…29 However, our measurements of FA, ʈ , and Ќ were not lower than those in other reports. 20 This may be ascribed to a higher b-value (b ϭ 1000 s/mm 2 ) and resolution (0.90 ϫ 0.90 ϫ 2 mm 3 ) with fat suppression to increase the accuracy of measurement, twice-refocused SE with a bipolar gradient in the EPI sequence to reduce eddy currents, and parallel imaging (GRAPPA, IPAT factor ϭ 2) to accelerate the image acquisition and reduce distortion. Second, cardiac gating was not implemented in the current protocol, so the protocol may contain errors from different cardiac cycles.…”

“…Chabert et al 19 used another type of acquisition method (non-Care-Purcell-Meiboom-Gill fast spin-echo) and obtained results very similar to those of Wheeler-Kingshott. 17 A conventional DTI protocol was modified by Techavipoo et al 20 to acquire images with fat and CSF suppression and field inhomogeneity maps of contiguous coronal sections covering the whole brain. The technique was applied to healthy volunteers and patients with multiple sclerosis with and without a history of unilateral optic neuritis.…”