Seung Kyun Lee

Peripheral nerve stimulation characteristics of an asymmetric head-only gradient coil compatible with a high-channel-count receiver array

To characterize peripheral nerve stimulation (PNS) of an asymmetric head‐only gradient coil that is compatible with a commercial high–channel‐count receive‐only array.

On shimming approaches in 3T breast MRI

A comparative study is presented, analyzing quantitatively the impact of 15 shim strategies on the homogeneity of the main magnetic field over the three‐dimensional breast region in 3T MRI. The results obtained in 12 female volunteers, spanning a wide range of body and breast types, indicate that the inclusion of the back and heart in the shim region of interest leads to considerable decrease in field homogeneity, and needs to be avoided. Comparison between shim strategies using volumetric B0 maps, covering the entire breast region, and 1–6 plane B0 maps indicate only minimally reduced performance for the latter. Interestingly, however, no single shim strategy relying on a limited number of B0 maps as input was found to work best in all volunteers. This was attributed to the limited capability of a small number of B0 maps to capture the B0 variability existent within breast. On the average, a rectangular shim region of interest, encompassing the breast region alone, worked best for the cohort studied here. Magn Reson Med, 2013.

High slew-rate head-only gradient for improving distortion in echo planar imaging: Preliminary experience.

To investigate the effects on echo planar imaging (EPI) distortion of using high gradient slew rates (SR) of up to 700 T/m/s for in vivo human brain imaging, with a dedicated, head‐only gradient coil.

High slew-rate head-only gradient for improving distortion in echo planar imaging

To investigate the effects on echo planar imaging (EPI) distortion of using high gradient slew rates (SR) of up to 700 T/m/s for in vivo human brain imaging, with a dedicated, head‐only gradient coil.

Gradient pre-emphasis to counteract first-order concomitant fields on asymmetric MRI gradient systems

To develop a gradient pre‐emphasis scheme that prospectively counteracts the effects of the first‐order concomitant fields for any arbitrary gradient waveform played on asymmetric gradient systems, and to demonstrate the effectiveness of this approach using a real‐time implementation on a compact gradient system.

B0 concomitant field compensation for MRI systems employing asymmetric transverse gradient coils

Imaging gradients result in the generation of concomitant fields, or Maxwell fields, which are of increasing importance at higher gradient amplitudes. These time‐varying fields cause additional phase accumulation, which must be compensated for to avoid image artifacts. In the case of gradient systems employing symmetric design, the concomitant fields are well described with second‐order spatial variation. Gradient systems employing asymmetric design additionally generate concomitant fields with global (zeroth‐order or B0) and linear (first‐order) spatial dependence.

Technical Note: Compact three‐tesla magnetic resonance imager with high‐performance gradients passes ACR image quality and acoustic noise tests

PURPOSE A compact, three-tesla magnetic resonance imaging (MRI) system has been developed. It features a 37 cm patient aperture, allowing the use of commercial receiver coils. Its design allows simultaneously for gradient amplitudes of 85 millitesla per meter (mT/m) sustained and 700 tesla per meter per second (T/m/s) slew rates. The size of the gradient system allows for these simultaneous performance targets to be achieved with little or no peripheral nerve stimulation, but also raises a concern about the geometric distortion as much of the imaging will be done near the systems maximum 26 cm field-of-view. Additionally, the fast switching capability raises acoustic noise concerns. This work evaluates the system for both the American College of Radiologys (ACR) MRI image quality protocol and the Food and Drug Administrations (FDA) nonsignificant risk (NSR) acoustic noise limits for MR. Passing these two tests is critical for clinical acceptance. METHODS In this work, the gradient system was operated at the maximum amplitude and slew rate of 80 mT/m and 500 T/m/s, respectively. The geometric distortion correction was accomplished by iteratively determining up to the tenth order spherical harmonic coefficients using a fiducial phantom and position-tracking software, with seventh order correction utilized in the ACR test. Acoustic noise was measured with several standard clinical pulse sequences. RESULTS The system passes all the ACR image quality tests. The acoustic noise as measured when the gradient coil was inserted into a whole-body MRI system conforms to the FDA NSR limits. CONCLUSIONS The compact system simultaneously allows for high gradient amplitude and high slew rate. Geometric distortion concerns have been mitigated by extending the spherical harmonic correction to higher orders. Acoustic noise is within the FDA limits.

Gradient nonlinearity calibration and correction for a compact, asymmetric magnetic resonance imaging gradient system.

Due to engineering limitations, the spatial encoding gradient fields in conventional magnetic resonance imaging cannot be perfectly linear and always contain higher-order, nonlinear components. If ignored during image reconstruction, gradient nonlinearity (GNL) manifests as image geometric distortion. Given an estimate of the GNL field, this distortion can be corrected to a degree proportional to the accuracy of the field estimate. The GNL of a gradient system is typically characterized using a spherical harmonic polynomial model with model coefficients obtained from electromagnetic simulation. Conventional whole-body gradient systems are symmetric in design; typically, only odd-order terms up to the 5th-order are required for GNL modeling. Recently, a high-performance, asymmetric gradient system was developed, which exhibits more complex GNL that requires higher-order terms including both odd- and even-orders for accurate modeling. This work characterizes the GNL of this system using an iterative calibration method and a fiducial phantom used in ADNI (Alzheimers Disease Neuroimaging Initiative). The phantom was scanned at different locations inside the 26 cm diameter-spherical-volume of this gradient, and the positions of fiducials in the phantom were estimated. An iterative calibration procedure was utilized to identify the model coefficients that minimize the mean-squared-error between the true fiducial positions and the positions estimated from images corrected using these coefficients. To examine the effect of higher-order and even-order terms, this calibration was performed using spherical harmonic polynomial of different orders up to the 10th-order including even- and odd-order terms, or odd-order only. The results showed that the model coefficients of this gradient can be successfully estimated. The residual root-mean-squared-error after correction using up to the 10th-order coefficients was reduced to 0.36 mm, yielding spatial accuracy comparable to conventional whole-body gradients. The even-order terms were necessary for accurate GNL modeling. In addition, the calibrated coefficients improved image geometric accuracy compared with the simulation-based coefficients.

Lightweight, compact, and high-performance 3T MR system for imaging the brain and extremities: FOO et al.

To build and evaluate a small‐footprint, lightweight, high‐performance 3T MRI scanner for advanced brain imaging with image quality that is equal to or better than conventional whole‐body clinical 3T MRI scanners, while achieving substantial reductions in installation costs.

Distortion correction in diffusion-weighted imaging of the breast: Performance assessment of prospective, retrospective, and combined (prospective + retrospective) approaches

To compare the effectiveness of prospective, retrospective, and combined (prospective + retrospective) EPI distortion correction methods in bilateral breast diffusion‐weighted imaging (DWI) scans.