Brian A. Hargreaves

Water–fat separation with IDEAL gradient‐echo imaging

To combine gradient‐echo (GRE) imaging with a multipoint water–fat separation method known as “iterative decomposition of water and fat with echo asymmetry and least squares estimation” (IDEAL) for uniform water–fat separation. Robust fat suppression is necessary for many GRE imaging applications; unfortunately, uniform fat suppression is challenging in the presence of B0 inhomogeneities. These challenges are addressed with the IDEAL technique.

SEMAC: Slice Encoding for Metal Artifact Correction in MRI

Magnetic resonance imaging (MRI) near metallic implants remains an unmet need because of severe artifacts, which mainly stem from large metal‐induced field inhomogeneities. This work addresses MRI near metallic implants with an innovative imaging technique called “Slice Encoding for Metal Artifact Correction” (SEMAC). The SEMAC technique corrects metal artifacts via robust encoding of each excited slice against metal‐induced field inhomogeneities. The robust slice encoding is achieved by extending a view‐angle‐tilting (VAT) spin‐echo sequence with additional z‐phase encoding. Although the VAT compensation gradient suppresses most in‐plane distortions, the z‐phase encoding fully resolves distorted excitation profiles that cause through‐plane distortions. By positioning all spins in a region‐of‐interest to their actual spatial locations, the through‐plane distortions can be corrected by summing up the resolved spins in each voxel. The SEMAC technique does not require additional hardware and can be deployed to the large installed base of whole‐body MRI systems. The efficacy of the SEMAC technique in eliminating metal‐induced distortions with feasible scan times is validated in phantom and in vivo spine and knee studies. Magn Reson Med 62:66–76, 2009.

Metal-Induced Artifacts in MRI

OBJECTIVE The purpose of this article is to review some of the basic principles of imaging and how metal-induced susceptibility artifacts originate in MR images. We will describe common ways to reduce or modify artifacts using readily available imaging techniques, and we will discuss some advanced methods to correct readout-direction and slice-direction artifacts. CONCLUSION The presence of metallic implants in MRI can cause substantial image artifacts, including signal loss, failure of fat suppression, geometric distortion, and bright pile-up artifacts. These cause large resonant frequency changes and failure of many MRI mechanisms. Careful parameter and pulse sequence selections can avoid or reduce artifacts, although more advanced imaging methods offer further imaging improvements.

Characterization and reduction of the transient response in steady-state MR imaging.

Refocused steady‐state free precession (SSFP) imaging sequences have recently regained popularity as faster gradient hardware has allowed shorter repetition times, thereby reducing SSFPs sensitivity to off‐resonance effects. Although these sequences offer fast scanning with good signal‐to‐noise efficiency, the “transient response,” or time taken to reach a steady‐state, can be long compared with the total imaging time, particularly when using 2D sequences. This results in lost imaging time and has made SSFP difficult to use for real‐time and cardiac‐gated applications. A linear‐systems analysis of the steady‐state and transient response for general periodic sequences is shown. The analysis is applied to refocused‐SSFP sequences to generate a two‐stage method of “catalyzing,” or speeding up the progression to steady‐state by first scaling, then directing the magnetization. This catalyzing method is compared with previous methods in simulations and experimentally. Although the second stage of the method exhibits some sensitivity to B1 variations, our results show that the transient time can be significantly reduced, allowing imaging in a shorter total scan time. Magn Reson Med 46:149–158, 2001.

Magnetic resonance imaging of articular cartilage of the knee: comparison between fat-suppressed three-dimensional SPGR imaging, fat-suppressed FSE imaging, and fat-suppressed three-dimensional DEFT imaging, and correlation with arthroscopy.

To compare signal‐to‐noise ratios (S/N) and contrast‐to‐noise ratios (C/N) in various MR sequences, including fat‐suppressed three‐dimensional spoiled gradient‐echo (SPGR) imaging, fat‐suppressed fast spin echo (FSE) imaging, and fat‐suppressed three‐dimensional driven equilibrium Fourier transform (DEFT) imaging, and to determine the diagnostic accuracy of these imaging sequences for detecting cartilage lesions in osteoarthritic knees, as compared with arthroscopy.

Analysis of multiple-acquisition SSFP

Refocused steady‐state free precession (SSFP) is limited by its high sensitivity to local field variation, particularly at high field strengths or the long repetition times (TRs) necessary for high resolution. Several methods have been proposed to reduce SSFP banding artifact by combining multiple phase‐cycled SSFP acquisitions, each differing in how individual signal magnitudes and phases are combined. These include maximum‐intensity SSFP (MI‐SSFP) and complex‐sum SSFP (CS‐SSFP). The reduction in SSFP banding is accompanied by a loss in signal‐to‐noise ratio (SNR) efficiency. In this work a general framework for analyzing banding artifact reduction, contrast, and SNR of any multiple‐acquisition SSFP combination method is presented. A new sum‐of‐squares method is proposed, and a comparison is performed between each of the combination schemes. The sum‐of‐squares SSFP technique (SOS‐SSFP) delivers both robust banding artifact reduction and higher SNR efficiency than other multiple‐acquisition techniques, while preserving SSFP contrast. Magn Reson Med 51:1038–1047, 2004.

Design and analysis of a practical 3D cones trajectory

The 3D Cones k‐space trajectory has many desirable properties for rapid and ultra‐short echo time magnetic resonance imaging. An algorithm is presented that generates the 3D Cones gradient waveforms given a desired field of view and resolution. The algorithm enables a favorable trade‐off between increases in readout time and decreases in the total number of required readouts. The resulting trajectory is very signal‐to‐noise ratio (SNR) efficient and has excellent aliasing properties. A rapid high‐resolution ultra‐short echo time imaging sequence is used to compare the 3D Cones trajectory to 3D projection reconstruction (3DPR) sampling schemes. For equivalent scan times, the 3D Cones trajectory has better SNR performance and fewer aliasing artifacts as compared to the 3DPR trajectory. Magn Reson Med, 2006.

Improved pediatric MR imaging with compressed sensing.

PURPOSE To develop a method that combines parallel imaging and compressed sensing to enable faster and/or higher spatial resolution magnetic resonance (MR) imaging and show its feasibility in a pediatric clinical setting. MATERIALS AND METHODS Institutional review board approval was obtained for this HIPAA-compliant study, and informed consent or assent was given by subjects. A pseudorandom k-space undersampling pattern was incorporated into a three-dimensional (3D) gradient-echo sequence; aliasing then has an incoherent noiselike pattern rather than the usual coherent fold-over wrapping pattern. This k-space-sampling pattern was combined with a compressed sensing nonlinear reconstruction method that exploits the assumption of sparsity of medical images to permit reconstruction from undersampled k-space data and remove the noiselike aliasing. Thirty-four patients (15 female and 19 male patients; mean age, 8.1 years; range, 0-17 years) referred for cardiovascular, abdominal, and knee MR imaging were scanned with this 3D gradient-echo sequence at high acceleration factors. Obtained k-space data were reconstructed with both a traditional parallel imaging algorithm and the nonlinear method. Both sets of images were rated for image quality, radiologist preference, and delineation of specific structures by two radiologists. Wilcoxon and symmetry tests were performed to test the hypothesis that there was no significant difference in ratings for image quality, preference, and delineation of specific structures. RESULTS Compressed sensing images were preferred more often, had significantly higher image quality ratings, and greater delineation of anatomic structures (P < .001) than did images obtained with the traditional parallel reconstruction method. CONCLUSION A combination of parallel imaging and compressed sensing is feasible in a clinical setting and may provide higher resolution and/or faster imaging, addressing the challenge of delineating anatomic structures in pediatric MR imaging.

Recent Advances in MRI of Articular Cartilage

OBJECTIVE MRI is the most accurate noninvasive method available to diagnose disorders of articular cartilage. Conventional 2D and 3D approaches show changes in cartilage morphology. Faster 3D imaging methods with isotropic resolution can be reformatted into arbitrary planes for improved detection and visualization of pathology. Unique contrast mechanisms allow us to probe cartilage physiology and detect changes in cartilage macromolecules. CONCLUSION MRI has great promise as a noninvasive comprehensive tool for cartilage evaluation.

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.