Rexford D. Newbould

Clinical multishot DW‐EPI through parallel imaging with considerations of susceptibility, motion, and noise

Geometric distortions and poor image resolution are well known shortcomings of single‐shot echo‐planar imaging (ss‐EPI). Yet, due to the motion immunity of ss‐EPI, it remains the most common sequence for diffusion‐weighted imaging (DWI). Moreover, both navigated DW interleaved EPI (iEPI) and parallel imaging (PI) methods, such as sensitivity encoding (SENSE) and generalized autocalibrating parallel acquisitions (GRAPPA), can improve the image quality in EPI. In this work, DW‐EPI accelerated by PI is proposed as a self‐calibrated and unnavigated form of interleaved acquisition. The PI calibration is performed on the b = 0 s/mm2 data and applied to each shot in the rest of the DW data set, followed by magnitude averaging. Central in this study is the comparison of GRAPPA and SENSE in the presence of off‐resonances and motion. The results show that GRAPPA is more robust than SENSE against both off‐resonance and motion‐related artifacts. The SNR efficiency was also investigated, and it is shown that the SNR/scan time ratio is equally high for one‐ to three‐shot high‐resolution diffusion scans due to the shortened EPI readout train length. The image quality improvements without SNR efficiency loss, together with motion tolerance, make the GRAPPA‐driven DW‐EPI sequence clinically attractive. Magn Reson Med 57:881–890, 2007.

Robust GRAPPA-Accelerated Diffusion-Weighted Readout-Segmented (RS)-EPI

Readout segmentation (RS‐EPI) has been suggested as a promising variant to echo‐planar imaging (EPI) for high‐resolution imaging, particularly when combined with parallel imaging. This work details some of the technical aspects of diffusion‐weighted (DW)‐RS‐EPI, outlining a set of reconstruction methods and imaging parameters that can both minimize the scan time and afford high‐resolution diffusion imaging with reduced distortions. These methods include an efficient generalized autocalibrating partially parallel acquisition (GRAPPA) calibration for DW‐RS‐EPI data without scan time penalty, together with a variant for the phase correction of partial Fourier RS‐EPI data. In addition, the role of pulsatile and rigid‐body brain motion in DW‐RS‐EPI was assessed. Corrupt DW‐RS‐EPI data arising from pulsatile nonlinear brain motion had a prevalence of ∼7% and were robustly identified via k‐space entropy metrics. For DW‐RS‐EPI data corrupted by rigid‐body motion, we showed that no blind overlap was required. The robustness of RS‐EPI toward phase errors and motion, together with its minimized distortions compared with EPI, enables the acquisition of exquisite 3 T DW images with matrix sizes close to 5122. Magn Reson Med, 2009.

Propeller EPI in the other direction

A new propeller EPI pulse sequence with reduced sensitivity to field inhomogeneities is proposed. Image artifacts such as blurring due to Nyquist ghosting and susceptibility gradients are investigated and compared with those obtained in previous propeller EPI studies. The proposed propeller EPI sequence uses a readout that is played out along the short axis of the propeller blade, orthogonal to the readout used in previous propeller methods. In contrast to long‐axis readout propeller EPI, this causes the echo spacing between two consecutive phase‐encoding (PE) lines to decrease, which in turn increases the k‐space velocity in this direction and hence the pseudo‐bandwidth. Long‐ and short‐axis propeller EPI, and standard single‐shot EPI sequences were compared on phantoms and a healthy volunteer. Diffusion‐weighted imaging (DWI) was also performed on the volunteer. Short‐axis propeller EPI produced considerably fewer image artifacts compared to the other two sequences. Further, the oblique blades for the long‐axis propeller EPI were also prone to one order of magnitude higher residual ghosting than the proposed short‐axis propeller EPI. Magn Reson Med, 2006.

Foundations of Advanced Magnetic Resonance Imaging

SummaryDuring the past decade, major breakthroughs in magnetic resonance imaging (MRI) quality were made by means of quantum leaps in scanner hardware and pulse sequences. Some advanced MRI techniques have truly revolutionized the detection of disease states and MRI can now— within a few minutes—acquire important quantitative information noninvasively from an individual in any plane or volume at comparatively high resolution. This article provides an overview of the most common advanced MRI methods including diffusion MRI, perfusion MRI, functional MRI, and the strengths and weaknesses of MRI at high magnetic field strengths.

Perfusion mapping with multiecho multishot parallel imaging EPI

Echo‐planar imaging (EPI) is the standard technique for dynamic susceptibility‐contrast (DSC) perfusion MRI. However, EPI suffers from well‐known geometric distortions, which can be reduced by increasing the k‐space phase velocity. Moreover, the long echo times (TEs) used in DSC lead to signal saturation of the arterial input signal, and hence to severe quantitation errors in the hemodynamic information. Here, through the use of interleaved shot acquisition and parallel imaging (PI), rapid volumetric EPI is performed using pseudo‐single‐shot (ss)EPI with the effective T  *2 blur and susceptibility distortions of a multishot EPI sequence. The reduced readout lengths permit multiple echoes to be acquired with temporal resolution and spatial coverage similar to those obtained with a single‐echo method. Multiecho readouts allow for unbiased R  *2 mapping to avoid incorrect estimation of tracer concentration due to signal saturation or T1 shortening effects. Multiecho perfusion measurement also mitigates the signal‐to‐noise ratio (SNR) reduction that results from utilizing PI. Results from both volunteers and clinical stroke patients are presented. This acquisition scheme can aid most rapid time‐series acquisitions. The use of this method for DSC addresses the problem of signal saturation and T1 contamination while it improves image quality, and is a logical step toward better quantitative MR PWI. Magn Reson Med 58:70–81, 2007.

Improving dynamic susceptibility contrast MRI measurement of quantitative cerebral blood flow using corrections for partial volume and nonlinear contrast relaxivity: A xenon computed tomographic comparative study.

To test whether dynamic susceptibility contrast MRI‐based CBF measurements are improved with arterial input function (AIF) partial volume (PV) and nonlinear contrast relaxivity correction, using a gold‐standard CBF method, xenon computed tomography (xeCT).

An auto‐calibrated, angularly continuous, two‐dimensional GRAPPA kernel for propeller trajectories

The k‐space readout of propeller‐type sequences may be accelerated by the use of parallel imaging (PI). For PROPELLER, the main benefits are reduced blurring due to T2 decay and specific absorption ratio (SAR) reduction, whereas, for EPI‐based propeller acquisitions, such as Turbo‐PROP and short‐axis readout propeller EPI (SAP‐EPI), the faster k‐space traversal alleviates geometric distortions. In this work, the feasibility of calculating a two‐dimensional (2D) GRAPPA kernel on only the undersampled propeller blades themselves is explored, using the matching orthogonal undersampled blade. It is shown that the GRAPPA kernel varies slowly across blades; therefore, an angularly continuous 2D GRAPPA kernel is proposed, in which the angular variation of the weights is parameterized. This new angularly continuous kernel formulation greatly increases the numerical stability of the GRAPPA weight estimation, allowing for generation of fully sampled diagnostic quality images using only the undersampled propeller data. Magn Reson Med 60:1457–1465, 2008.

Balanced SSFP imaging of the musculoskeletal system

Magnetic resonance imaging (MRI), with its unique ability to image and characterize soft tissue noninvasively, has emerged as one of the most accurate imaging methods available to diagnose bone and joint pathology. Currently, most evaluation of musculoskeletal pathology is done with two‐dimensional acquisition techniques such as fast spin echo (FSE) imaging. The development of three‐dimensional fast imaging methods based on balanced steady‐state free precession (SSFP) shows great promise to improve MRI of the musculoskeletal system. These methods may allow acquisition of fluid sensitive isotropic data that can be reformatted into arbitrary planes for improved detection and visualization of pathology. Sensitivity to fluid and fat suppression are important issues in these techniques to improve delineation of cartilage contours, for detection of marrow edema and derangement of other joint structures. J. Magn. Reson. Imaging 2007.

Improvements in parallel imaging accelerated functional MRI using multiecho echo-planar imaging†

Multiecho echo‐planar imaging (EPI) was implemented for blood‐oxygenation‐level‐dependent functional MRI at 1.5 T and compared to single‐echo EPI with and without parallel imaging acceleration. A time‐normalized breath‐hold task using a block design functional MRI protocol was carried out in combination with up to four echo trains per excitation and parallel imaging acceleration factors R = 1–3. Experiments were conducted in five human subjects, each scanned in three sessions. Across all reduction factors, both signal‐to‐fluctuation‐noise ratio and the total number of activated voxels were significantly lower using a single‐echo EPI pulse sequence compared with the multiecho approach. Signal‐to‐fluctuation‐noise ratio and total number of activated voxels were also considerably reduced for nonaccelerated conventional single‐echo EPI when compared to three‐echo measurements with R = 2. Parallel imaging accelerated multiecho EPI reduced geometric distortions and signal dropout, while it increased blood‐oxygenation‐level‐dependent signal sensitivity all over the brain, particularly in regions with short underlying T*2. Thus, the presented method showed multiple advantages over conventional single‐echo EPI for standard blood‐oxygenation‐level‐dependent functional MRI experiments. Magn Reson Med 63:959–969, 2010.