Distortion and Signal Loss in Medial Temporal Lobe

Background: The 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 Findings: In 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 (36363 mm), a conventional high-resolution EPI (1.561.562 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/Significance: Although 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 functio

High efficiency, low distortion 3D diffusion tensor imaging with variable density spiral fast spin echoes (3D DW VDS RARE)

We present an acquisition and reconstruction method designed to acquire high resolution 3D fast spin echo diffusion tensor images while mitigating the major sources of artifacts in DTI-field distortions, eddy currents and motion. The resulting images, being 3D, are of high SNR, and being fast spin echoes, exhibit greatly reduced field distortions. This sequence utilizes variable density spiral acquisition gradients, which allow for the implementation of a self-navigation scheme by which both eddy current and motion artifacts are removed. The result is that high resolution 3D DTI images are produced without the need for eddy current compensating gradients or B_0 field correction. In addition, a novel method for fast and accurate reconstruction of the non-Cartesian data is employed. Results are demonstrated in the brains of normal human volunteers

Dynamic nuclear polarization and electron spin resonance in paramagnetic solids at high field

Thesis (Ph.D.)--Massachusetts Institute of Technology, Dept. of Physics, 1999.Includes bibliographical references (p. 155-161).Recent advances in high resolution solid state nuclear magnetic resonance spectroscopy have permitted the detailed structural study of large biological systems. The feasibility of many of these experiments is limited by the inherently low sensitivity of solid state NMR, and the implementation of complex multi-dimensional homo- and hetero-nuclear recoupling pulse sequences has been restricted to small model compounds. The primary focus of this thesis is the description of investigations of dynamic nuclear polarization (DNP) at high magnetic field as a means of improving the signal to noise in solid state NMR spectroscopy. DNP transfers the large polarization of unpaired electron spins to nuclei in a process involving sample irradiation at or near the electronic Larmor frequency. Large signal enhancements have been achieved in a nitroxide doped frozen aqueous solution at 5T. The relevant relaxation times governing the transfer process have been measured via the application of various high frequency pulsed electron paramagnetic resonance (EPR) techniques, and a model incorporating cross-relaxation is used to explain the results. It is shown that the electronic and nuclear spin dynamics are consistent with the thermal mixing mechanism of polarization transfer. The high frequency (139.5 GHz, 5T) EPR spectrometer used to perform these experiments is described in detail and several other advancements in the application of high power, high frequency microwave technology to magnetic resonance are also discussed.by Souheil James Inati.Ph.D

Comparison of SNR and distortion: zoomed, full FOV, and low resolution EPI.

<p>(A) SNR, averaged across subjects, as a function of position along the hippocampal axis. A centimeter posterior to the gyrus intralimbicus (GI), SNR follows the expected relationships. First, comparing the two high-resolution acquisitions: the reduced FOV (zoomed) acquisition has the same resolution as the full FOV high-resolution acquisition, but only a quarter of the data points. Because the Fourier transform takes a weighted average of all raw data points to calculate the intensity of each image pixel, the thermal SNR in the final image is directly related to the square root of the number of points in the raw data matrix (assuming equivalent, uncorrelated thermal noise in the source data). Therefore an SNR reduction by a factor of 2 is expected (and observed) for the reduced FOV acquisition (solid lines), relative to the full FOV (dot-dash lines) acquisition. Second, comparing low- and high-resolution data: the low-resolution acquisition has a voxel volume 6 times greater than the high-resolution full FOV acquisition, increasing available signal by 6X, but only 1/4 the data points are acquired for each image (64×64 matrix instead of 128×128), for a √4 reduction in thermal SNR and a net 3X increase in SNR (low-resolution > high-resolution). (B) Average voxel displacement for each of the 3 EPI acquisitions, calculated from the fieldmap for each slice of each ROI. All three acquisitions were acquired in the same scanning session for each subject, so the field distortions are identical in each case. Voxel shift is linearly related to total read-out time, which is shortest for the low-resolution data. But voxels are larger in the low-resolution acquisition, so total displacement is smallest in the zoomed, high-resolution acquisitions.</p

Distortion and signal loss due to through-slice dephasing are evident in the most anterior regions of parahippocampal gyrus (PHG), while the posterior half of the hippocampus (HIPP) and PHG are unaffected by susceptibility artifacts.

<p>(A) Location of slices and ROIs for a representative subject, shown on parasagittal section of the reference anatomy (3D MP-RAGE). (B) Three slices resampled from reference anatomy in anterior MTL to match location and resolution of functional data (zoomed EPI). Red and blue regions of interest are anterior and middle hippocampus; cyan (PR), green (ER) and yellow (pPHG) regions of interest are parahippocampal gyrus. (C) Low resolution EPI acquisition. Thicker slices (3 mm) result in increased signal loss due to through-slice dephasing in PHG (white arrow). (D) High-resolution full FOV EPI images. In the most anterior slice, field gradients in the hippocampus displace signal so the ventricle, rather than the hippocampus, is in the selected region of interest (yellow arrow). Similarly, PHG ROIs contain signal from HIPP (red arrow), and PHG signal is lost. (E) Zoomed high-resolution EPI images. Signal is lost only in anterior PHG (ER and PR); distortion is negligible.</p

Signal drop-out is worse in low-resolution images because thicker slices result in more signal loss due to drop-out, and distortion is worse in full FOV high resolution images due to long read-out times.

<p>(A) Reference anatomy; yellow line is for visual reference and the same in all 4 panels. (B) Conventional low resolution EPI (3 mm isotropic voxels; total read-out, T_RO = 22.5 ms; echo time, TE = 25 ms). Signal from lateral inferior temporal lobe is missing. (C) Full FOV high resolution image (T_RO = 66.6 ms; TE = 37 ms). A combination of signal displacement and signal loss affects lateral temporal cortex. Note tissue signal from medial temporal cortex extends bilaterally below fiducial lines. (D) Zoomed FOV image (T_RO = 16.6 ms; TE = 25 ms). Both distortion and signal loss are minimized.</p