Kevin F. King

A multispectral three-dimensional acquisition technique for imaging near metal implants

Metallic implants used in bone and joint arthroplasty induce severe spatial perturbations to the B0 magnetic field used for high‐field clinical magnetic resonance. These perturbations distort slice‐selection and frequency encoding processes applied in conventional two‐dimensional MRI techniques and hinder the diagnosis of complications from arthroplasty. Here, a method is presented whereby multiple three‐dimensional fast‐spin‐echo images are collected using discrete offsets in RF transmission and reception frequency. It is demonstrated that this multi acquisition variable‐resonance image combination technique can be used to generate a composite image that is devoid of slice‐plane distortion and possesses greatly reduced distortions in the readout direction, even in the immediate vicinity of metallic implants. Magn Reson Med 61:381–390, 2009.

Imaging near metal with a MAVRIC-SEMAC hybrid.

The recently developed multi‐acquisition with variable resonance image combination (MAVRIC) and slice‐encoding metal artifact correction (SEMAC) techniques can significantly reduce image artifacts commonly encountered near embedded metal hardware. These artifact reductions are enabled by applying alternative spectral and spatial‐encoding schemes to conventional spin‐echo imaging techniques. Here, the MAVRIC and SEMAC concepts are connected and discussed. The development of a hybrid technique that utilizes strengths of both methods is then introduced. The presented technique is shown capable of producing minimal artifact, high‐resolution images near total joint replacements in a clinical setting. Magn Reson Med, 2010.

Magnetic resonance imaging near metal implants.

The desire to apply magnetic resonance imaging (MRI) techniques in the vicinity of embedded metallic hardware is increasing. The soft‐tissue contrast available with MR techniques is advantageous in diagnosing complications near an increasing variety of MR‐safe metallic hardware. Near such hardware, the spatial encoding mechanisms utilized in conventional MRI methods are often severely compromised. Mitigating these encoding difficulties has been the focus of numerous research investigations over the past two decades. Such approaches include view‐angle tilting, short echo‐time projection reconstruction acquisitions, single‐point imaging, prepolarized MRI, and postprocessing image correction. Various technical advances have also enabled the recent development of two alternative approaches that have shown promising clinical potential. Here, the physical principals and proposed solutions to the problem of MRI near embedded metal are discussed. J. Magn. Reson. Imaging 2010;32:773–787.

Designing multichannel, multidimensional, arbitrary flip angle RF pulses using an optimal control approach.

The vast majority of parallel transmission RF pulse designs so far are based on small‐tip‐angle (STA) approximation of the Bloch equation. These methods can design only excitation pulses with small flip angles (e.g., 30°). The linear class large‐tip‐angle (LCLTA) method is able to design large‐tip‐angle parallel transmission pulses through concatenating a sequence of small‐excitation pulses when certain k‐space trajectories are used. However, both STA and LCLTA are linear approximations of the nonlinear Bloch equation. Therefore, distortions from the ideal magnetization profiles due to the higher order terms can appear in the final magnetization profiles. This issue is addressed in this work by formulating the multidimensional multichannel RF pulse design as an optimal control problem with multiple controls based directly on the Bloch equation. Necessary conditions for the optimal solution are derived and a first‐order gradient optimization algorithm is used to iteratively solve the optimal control problem, where an existing pulse is used as an initial “guess.” A systematic design procedure is also presented. Bloch simulation and phantom experimental results using various parallel transmission pulses (excitation, inversion, and refocusing) are shown to illustrate the effectiveness of the optimal control method in improving the spatial localization or homogeneity of the magnetization profiles. Magn Reson Med 59:547–560, 2008.

Accelerated diffusion spectrum imaging in the human brain using compressed sensing

We developed a novel method to accelerate diffusion spectrum imaging using compressed sensing. The method can be applied to either reduce acquisition time of diffusion spectrum imaging acquisition without losing critical information or to improve the resolution in diffusion space without increasing scan time. Unlike parallel imaging, compressed sensing can be applied to reconstruct a sub‐Nyquist sampled dataset in domains other than the spatial one. Simulations of fiber crossings in 2D and 3D were performed to systematically evaluate the effect of compressed sensing reconstruction with different types of undersampling patterns (random, gaussian, Poisson disk) and different acceleration factors on radial and axial diffusion information. Experiments in brains of healthy volunteers were performed, where diffusion space was undersampled with different sampling patterns and reconstructed using compressed sensing. Essential information on diffusion properties, such as orientation distribution function, diffusion coefficient, and kurtosis is preserved up to an acceleration factor of R = 4. Magn Reson Med, 2011.

Regularized sensitivity encoding (SENSE) reconstruction using bregman iterations

In parallel imaging, the signal‐to‐noise ratio (SNR) of sensitivity encoding (SENSE) reconstruction is usually degraded by the ill‐conditioning problem, which becomes especially serious at large acceleration factors. Existing regularization methods have been shown to alleviate the problem. However, they usually suffer from image artifacts at high acceleration factors due to the large data inconsistency resulting from heavy regularization. In this paper, we propose Bregman iteration for SENSE regularization. Unlike the existing regularization methods where the regularization function is fixed, the method adaptively updates the regularization function using the Bregman distance at different iterations, such that the iteration gradually removes the aliasing artifacts and recovers fine structures before the noise finally comes back. With a discrepancy principle as the stopping criterion, our results demonstrate that the reconstructed image using Bregman iteration preserves both sharp edges lost in Tikhonov regularization and fines structures missed in total variation (TV) regularization, while reducing more noise and aliasing artifacts. Magn Reson Med 61:145–152, 2009.

A noniterative method to design large-tip-angle multidimensional spatially-selective radio frequency pulses for parallel transmission

Recently, theoretical and experimental work has shown that parallel transmission of RF pulses can be used to shorten the duration of multidimensional spatially‐selective pulses and compensate for B1 field inhomogeneity. However, all the existing noniterative methods can design only excitation pulses for parallel transmission with a small flip angle (e.g., 30°, or at most 90°) and cannot design large‐tip‐angle inversion/refocusing pulses, because these methods are based on the small‐tip‐angle (STA) approximation of the Bloch equation. In this work, a method to design large‐tip‐angle multidimensional spatially‐selective pulses for parallel transmission is proposed, based on an extension of the single‐channel linear‐class large‐tip‐angle (LCLTA) theory. Design examples of 2D refocusing and inversion parallel transmit pulses and magnetization profiles from Bloch equation simulations demonstrate the strength of the proposed method. A 2D spin‐echo parallel transmission experiment on a slab phantom using a 180° refocusing pulse with an eight‐channel transmit‐only array further validates the effectiveness of the proposed method. Magn Reson Med 58:326–334, 2007.

Joint design of spoke trajectories and RF pulses for parallel excitation

The spoke trajectory is often used in designing multidimensional RF pulses for applications requiring thin slice selection and in‐slice modulation. Ideally, a full set of spokes covering the whole k‐space are desired to generate a given excitation pattern. In practice, however, only a small number of spokes can be used due to the RF pulse length limitation. The spoke locations are, therefore, critical to the performance of the resulting RF pulse and should be in principle optimized jointly with the RF pulse for a given excitation pattern and transmit sensitivities. In this work, we formulate the joint design problem as an optimal spoke selection problem based on the small‐tip‐angle RF pulse design. A sequential selection based algorithm with recursive cost function evaluation is proposed to seek optimized spoke locations to minimize the excitation error. Bloch equation simulations and experimental results on a 3 Tesla scanner equipped with a two‐channel parallel excitation system demonstrate that the proposed method can produce significantly smaller excitation error than conventional methods with high computational efficiency. Magn Reson Med, 2010.

Intravoxel partially coherent motion technique: characterization of the anisotropy of skeletal muscle microvasculature.

To propose a reformulation of the intravoxel incoherent motion (IVIM) technique exploiting the low b‐value diffusion‐weighted imaging regime that can characterize microcirculation of tissues perfused with partially coherent blood flow.

Improving k-t SENSE by adaptive regularization

The recently proposed method known as k‐t sensitivity encoding (SENSE) has emerged as an effective means of improving imaging speed for several dynamic imaging applications. However, k‐t SENSE uses temporally averaged data as a regularization term for image reconstruction. This may not only compromise temporal resolution, it may also make some of the temporal frequency components irrecoverable. To address that issue, we present a new method called spatiotemporal domain‐based unaliasing employing sensitivity encoding and adaptive regularization (SPEAR). Specifically, SPEAR provides an improvement over k‐t SENSE by generating adaptive regularization images. It also uses a variable‐density (VD), sequentially interleaved k‐t space sampling pattern with reference frames for data acquisition. Simulations based on experimental data were performed to compare SPEAR, k‐t SENSE, and several other related methods, and the results showed that SPEAR can provide higher temporal resolution with significantly reduced image artifacts. Ungated 3D cardiac imaging experiments were also carried out to test the effectiveness of SPEAR, and real‐time 3D short‐axis images of the human heart were produced at 5.5 frames/s temporal resolution and 2.4 × 1.2 × 8 mm3 spatial resolution with eight slices. Magn Reson Med 57:918–930, 2007.