Magnetic resonance in medicine : official journal of the Society of Magnetic Resonance in Medicine
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Off-resonance artifacts hinder the wider applicability of echo-planar imaging and non-Cartesian MRI methods such as radial and spiral. In this work, a general and rapid method is proposed for off-resonance artifacts correction based on data convolution in k-space. The acquired k-space is divided into multiple segments based on their acquisition times. ⋯ The technique was demonstrated in phantom and in vivo studies for radial, spiral and echo-planar imaging datasets. For radial acquisitions, the proposed method allows the self-calibration of the field map from the imaging data, when an alternating view-angle ordering scheme is used. An additional advantage for off-resonance artifacts correction based on data convolution in k-space is the reusability of convolution kernels to images acquired with the same sequence but different contrasts.
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Chemical exchange saturation transfer (CEST) is a technique to indirectly detect pools of exchangeable protons through the water signal. To increase its applicability to human studies, it is needed to develop sensitive pulse sequences for rapidly acquiring whole-organ images while adhering to stringent amplifier duty cycle limitations and specific absorption rate restrictions. In addition, the interfering effects of direct water saturation and conventional magnetization transfer contrast complicate CEST quantification and need to be reduced as much as possible. ⋯ Fortunately, the limited width of the direct water saturation signal could be exploited to fit it with a Lorentzian function allowing CEST quantification. Amide proton transfer effects ranged between 1.5% and 2.5% in selected white and grey matter regions. This power and time-efficient 3D pulsed CEST acquisition scheme should aid endogenous CEST quantification at both high and low fields.
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An alternative to the standard echo-planar spectroscopic imaging technique is presented, spectroscopic imaging using concentrically circular echo-planar trajectories (SI-CONCEPT). In contrast to the conventional chemical shift imaging data, the sampled data from each set of concentric rings were regridded into Cartesian space. Usage of concentric k-space trajectories has the advantage of requiring significantly reduced slew rates than echo-planar spectroscopic imaging, allowing for the collection of higher spectral bandwidths and opening the door for high-bandwidth echo-planar styled spectroscopic imaging at higher magnetic fields. ⋯ The feasibility of using concentric k-space trajectories is demonstrated, and the spatial profiles and representative spectra are compared with the standard echo-planar spectroscopic imaging technique in a gray matter phantom containing metabolites at physiological concentrations and healthy human brain in vivo. The symmetric nature of the concentric circles also reduces the number of required excitations for a given resolution by a factor of two. Possible artifacts and limitations are discussed.
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It has been suggested that, high b-value diffusion weighted MRI improves the sensitivity and specificity of these images to tissue microstructure when compared with "clinical" b-value diffusion weighted MRI (b ≈ 1000 s/mm(2)). However, it suffers from poor signal to noise ratio - leading to longer acquisition times and therefore more motion artifacts. Together with the orientational sensitivity of the diffusion weighted MRI signal, the contrast at different b-values and different gradient directions is significantly different. ⋯ Here, we suggest a framework based on both experimental data (diffusion tensor MRI) and simulations (using the composite hindered and restricted model of diffusion framework) to correct the motion induced misalignments and artifacts of high b-value diffusion weighted MRI. This approach was evaluated using visual assessment of the registered diffusion weighted MRI and the composite hindered and restricted model of diffusion analysis results, as well as residual analysis to assess the quality of the composite hindered and restricted model of diffusion fitting. Both qualitative and quantitative results demonstrate an improvement in fitting the data to the composite hindered and restricted model of diffusion model following the suggested registration framework, thereby, addressing a long-standing problem and making the correction of motion/distortions in data collected at high b-values feasible for the first time.
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The use of high-permittivity materials has been shown to be an effective method for increasing transmit and receive sensitivity in areas of low-signal intensity in the brain at high field. Results in this article show that the use of these materials does not increase the intercoil coupling for a phased array receive coil, does not have any detrimental effects on the B(0) homogeneity within the brain, and does not affect the specific absorption rate distribution within the head. Areas of the brain close to the pads exhibit significant increases (>100%) in transmit field efficiency, but areas further away show a less pronounced (~10%) decrease due to the homogenization of the transmit field and the loss introduced by the dielectric pads.