Shengzhen Tao

Integrated image reconstruction and gradient nonlinearity correction

To describe a model‐based reconstruction strategy for routine magnetic resonance imaging that accounts for gradient nonlinearity (GNL) during rather than after transformation to the image domain, and demonstrate that this approach reduces the spatial resolution loss that occurs during strictly image‐domain GNL‐correction.

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.

NonCartesian MR image reconstruction with integrated gradient nonlinearity correction

PURPOSE To derive a noniterative gridding-type reconstruction framework for nonCartesian magnetic resonance imaging (MRI) that prospectively accounts for gradient nonlinearity (GNL)-induced image geometrical distortion during MR image reconstruction, as opposed to the standard, image-domain based GNL correction that is applied after reconstruction; to demonstrate that such framework is able to reduce the image blurring introduced by the conventional GNL correction, while still offering effective correction of GNL-induced geometrical distortion and compatibility with off-resonance correction. METHODS After introducing the nonCartesian MRI signal model that explicitly accounts for the effects of GNL and off-resonance, a noniterative gridding-type reconstruction framework with integrated GNL correction based on the type-III nonuniform fast Fourier transform (NUFFT) is derived. A novel type-III NUFFT implementation is then proposed as a numerically efficient solution to the proposed framework. The incorporation of simultaneous B0 off-resonance correction to the proposed framework is then discussed. Several phantom and in vivo data acquired via various 2D and 3D nonCartesian acquisitions, including 2D Archimedean spiral, 3D shells with integrated radial and spiral, and 3D radial sampling, are used to compare the results of the proposed and the standard GNL correction methods. RESULTS Various phantom and in vivo data demonstrate that both the proposed and the standard GNL correction methods are able to correct the coarse-scale geometric distortion and blurring induced by GNL and off-resonance. However, the standard GNL correction method also introduces blurring effects to corrected images, causing blurring of resolution inserts in the phantom images and loss of small vessel clarity in the angiography examples. On the other hand, the results after the proposed GNL correction show better depiction of resolution inserts and higher clarity of small vessel. CONCLUSIONS The proposed GNL-integrated nonCartesian reconstruction method can mitigate the resolution loss that occurs during standard image-domain GNL correction, while still providing effective correction of coarse-scale geometric distortion and blurring induced by GNL and off-resonance.

Partial fourier and parallel MR image reconstruction with integrated gradient nonlinearity correction

To describe how integrated gradient nonlinearity (GNL) correction can be used within noniterative partial Fourier (homodyne) and parallel (SENSE and GRAPPA) MR image reconstruction strategies, and demonstrate that performing GNL correction during, rather than after, these routines mitigates the image blurring and resolution loss caused by postreconstruction image domain based GNL correction.

The effect of concomitant fields in fast spin echo acquisition on asymmetric MRI gradient systems

To investigate the effect of the asymmetric gradient concomitant fields (CF) with zeroth and first‐order spatial dependence on fast/turbo spin‐echo acquisitions, and to demonstrate the effectiveness of their real‐time compensation.

Image-based gradient non-linearity characterization to determine higher-order spherical harmonic coefficients for improved spatial position accuracy in magnetic resonance imaging

PURPOSE Spatial position accuracy in magnetic resonance imaging (MRI) is an important concern for a variety of applications, including radiation therapy planning, surgical planning, and longitudinal studies of morphologic changes to study neurodegenerative diseases. Spatial accuracy is strongly influenced by gradient linearity. This work presents a method for characterizing the gradient non-linearity fields on a per-system basis, and using this information to provide improved and higher-order (9th vs. 5th) spherical harmonic coefficients for better spatial accuracy in MRI. METHODS A large fiducial phantom containing 5229 water-filled spheres in a grid pattern is scanned with the MR system, and the positions all the fiducials are measured and compared to the corresponding ground truth fiducial positions as reported from a computed tomography (CT) scan of the object. Systematic errors from off-resonance (i.e., B0) effects are minimized with the use of increased receiver bandwidth (±125kHz) and two acquisitions with reversed readout gradient polarity. The spherical harmonic coefficients are estimated using an iterative process, and can be subsequently used to correct for gradient non-linearity. Test-retest stability was assessed with five repeated measurements on a single scanner, and cross-scanner variation on four different, identically-configured 3T wide-bore systems. RESULTS A decrease in the root-mean-square error (RMSE) over a 50cm diameter spherical volume from 1.80mm to 0.77mm is reported here in the case of replacing the vendors standard 5th order spherical harmonic coefficients with custom fitted 9th order coefficients, and from 1.5mm to 1mm by extending custom fitted 5th order correction to the 9th order. Minimum RMSE varied between scanners, but was stable with repeated measurements in the same scanner. CONCLUSIONS The results suggest that the proposed methods may be used on a per-system basis to more accurately calibrate MR gradient non-linearity coefficients when compared to vendor standard corrections.

WE-FG-206-01: Magnetization-Prepared Shells Trajectory with Automated Gradient Waveform Design

PURPOSE The 3D non-Cartesian shells trajectory uses helical spirals to sample concentric spherical shells in k-space. The ordering of the shells sampling can be flexibly arranged to adapt to magnetization-prepared (MP) type of acquisition. The sampling of the shells around the center of k-space can be tuned to when the peak contrast between the white matter and gray matter is reached. The prototype of the sequence (MP-SHELLS) was previously implemented with a manually prescribed empirical gradient waveform. Recently we have implemented an automated trajectory and view ordering design supporting flexibly selected imaging parameters. The purpose of this work is to evaluate the performance of the automated MPSHELLS and compare it with a clinically used Cartesian MP-RAGE sequence. METHODS The k-space is segmented into several groups of concentric shells with each shell sampled by a group of interleaves following pre-defined helical spiral trajectories. The number of interleaves for each shell was determined to satisfy the Nyquist criteria. For each interleave, the gradient waveform is automatically generated via a time-optimal waveform design strategy following the sampling requirement. To enable magnetization-preparation, the acquisition order for the readouts from different shells was automatically arranged according to the time-dependent, white-gray matter contrast. The pulse sequence was tested on a GE 3T scanner. Under an IRB-approved protocol, a healthy volunteer was scanned with the MP-Shells sequence and compared with a clinical MP-RAGE sequence. RESULTS The in vivo results showed that improved gray/white matter contrast was achieved with the MP-SHELLS acquisition as compared to MP-RAGE. The calculated CNR values for the MP-SHELLS and MP-RAGE image are 21.81 and 15.02, respectively. The acquisition time for the MP-SHELLS is 5:10 compares to MP-RAGE acquisition time 9:13. CONCLUSION We have demonstrated a fully automated MP-SHELLS design with superior gray/white matter contrast and shorter acquisition time than the conventional Cartesian MP-RAGE. Funding support: NIH R01EB010065.

Magnetization-prepared shells trajectory with automated gradient waveform design

To develop a fully automated trajectory and gradient waveform design for the non‐Cartesian shells acquisition, and to develop a magnetization‐prepared (MP) shells acquisition to achieve an efficient three‐dimensional acquisition with improved gray‐to‐white brain matter contrast.