Abstract
BackgroundThe medial temporal lobe (MTL) contains subregions that are subject to severe distortion and signal loss in functional MRI. Air/tissue and bone/tissue interfaces in the vicinity of the MTL distort the local magnetic field due to differences in magnetic susceptibility. Fast image acquisition and thin slices can reduce the amount of distortion and signal loss, but at the cost of image signal-to-noise ratio (SNR).Methodology/Principal FindingsIn this paper, we quantify the severity of distortion and signal loss in MTL subregions for three different echo planar imaging (EPI) acquisitions at 3 Tesla: a conventional moderate-resolution EPI (3×3×3 mm), a conventional high-resolution EPI (1.5×1.5×2 mm), and a zoomed high-resolution EPI. We also demonstrate the advantage of reversing the phase encode direction to control the direction of distortion and to maximize efficacy of distortion compensation during data post-processing. With the high-resolution zoomed acquisition, distortion is not significant and signal loss is present only in the most anterior regions of the parahippocampal gyrus. Furthermore, we find that the severity of signal loss is variable across subjects, with some subjects showing negligible loss and others showing more dramatic loss.Conclusions/SignificanceAlthough both distortion and signal loss are minimized in a zoomed field of view acquisition with thin slices, this improvement in accuracy comes at the cost of reduced SNR. We quantify this trade-off between distortion and SNR in order to provide a decision tree for design of high-resolution experiments investigating the function of subregions in MTL.
Highlights
As neuroscientists and psychologists begin to ask more detailed questions regarding the functional organization of substructures in the medial temporal lobe (MTL), more sophisticated and accurate high-resolution imaging of these structures is desirable. (For reviews regarding structure and function of MTL subregions, which include the amygdala, hippocampal subfields and entorhinal, perirhinal and parahippocampal cortices, see [1,2,3,4].) for these studies, the geometry of the brain and skull around inferior temporal regions means that image distortion and signal loss due to susceptibility-induced magnetic field gradients can be quite large and hamper interpretability of conventional resolution data
To quantify effects of distortion on subregions in MTL, regions of interest were delineated on the 3D MP-RAGE volume data set for each subject and translated to the echo planar imaging (EPI) data and field map acquired during the same scanning session
Distortion was assessed by voxel displacement maps calculated from the field map data; signal loss was quantified by the measured signal-to-noise ratio (SNR) in each of the EPI acquisitions
Summary
As neuroscientists and psychologists begin to ask more detailed questions regarding the functional organization of substructures in the medial temporal lobe (MTL), more sophisticated and accurate high-resolution imaging of these structures is desirable. (For reviews regarding structure and function of MTL subregions, which include the amygdala, hippocampal subfields and entorhinal, perirhinal and parahippocampal cortices, see [1,2,3,4].) for these studies, the geometry of the brain and skull around inferior temporal regions means that image distortion and signal loss due to susceptibility-induced magnetic field gradients can be quite large and hamper interpretability of conventional resolution data. (For reviews regarding structure and function of MTL subregions, which include the amygdala, hippocampal subfields and entorhinal, perirhinal and parahippocampal cortices, see [1,2,3,4].) for these studies, the geometry of the brain and skull around inferior temporal regions means that image distortion and signal loss due to susceptibility-induced magnetic field gradients can be quite large and hamper interpretability of conventional resolution data. Functional imaging artifacts can be minimized by fast imaging techniques (e.g. parallel imaging), but these often are available at the cost of reduced signal-to-noise ratio (SNR). Not all subregions of MTL are affected by EPI image artifacts This means that the optimal balance between distortion and SNR will be different for each experiment. Fast image acquisition and thin slices can reduce the amount of distortion and signal loss, but at the cost of image signal-to-noise ratio (SNR)
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
