Contrast‐kinetics‐resolved whole‐heart coronary MRA using 3DPR

Slow contrast infusion was recently proposed for contrast‐enhanced whole‐heart coronary MR angiography. Current protocols use Cartesian k‐space sampling with empiric acquisition delays, potentially resulting in suboptimal coronary artery delineation and image artifacts if there is a timing error. This study aimed to investigate the feasibility of using time‐resolved three‐dimensional projection reconstruction for whole‐heart coronary MR angiography. With this method, data acquisition was started simultaneously with contrast injection. Sequential time frames were reconstructed by employing a sliding window scheme with temporal tornado filtering. Additionally, a self‐timing method was developed to monitor contrast enhancement during a scan and automatically determine the peak enhancement time around which optimal temporal frames were reconstructed. Our preliminary results on six healthy volunteers showed that by using time‐resolved three‐dimensional projection reconstruction, the contrast kinetics of the coronary artery system throughout a scan could be retrospectively resolved and assessed. In addition, the blood signal dynamics predicted using self‐timing was closely correlated to the true dynamics in time‐resolved reconstruction. This approach is useful for optimizing delineation of each coronary artery and minimizing image artifacts for contrast‐enhanced whole‐heart MRA. Magn Reson Med 63:970–978, 2010. © 2010 Wiley‐Liss, Inc.     

Nonrigid retrospective respiratory motion correction in whole‐heart coronary MRA

A nonrigid retrospective respiratory motion correction scheme is presented for whole‐heart coronary imaging with interleaved acquisition of motion information. The quasi‐periodic nature of breathing is exploited to populate a 3D nonrigid motion model from low‐resolution 2D imaging slices acquired interleaved with a segmented 3D whole‐heart coronary scan without imposing scan time penalty. Reconstruction and motion correction are based on inversion of a generalized encoding equation. Therein, a forward model describes the transformation from the motion free image to the motion distorted k‐space data, which includes nonrigid spatial transformations. The effectiveness of the approach is demonstrated on 10 healthy volunteers using free‐breathing coronary whole‐heart scans. Although conventional respiratory‐gated acquisitions with 5‐mm gating window resulted in an average gating efficiency of 51% ± 11%, nonrigid motion correction allowed for gate‐free acquisitions, and hence scan time reduction by a factor of two without significant penalty in image quality. Image scores and quantitative image quality measures for the left coronary arteries showed no significant differences between 5‐mm gated and gate‐free acquisitions with motion correction. For the right coronary artery, slightly reduced image quality in the motion corrected gate‐free scan was observed as a result of the close vicinity of anatomical structures with different motion characteristics. Magn Reson Med, 2011. © 2011 Wiley Periodicals, Inc.     

A respiratory self‐gating technique with 3D‐translation compensation for free‐breathing whole‐heart coronary MRA

Respiratory motion remains a major challenge for robust coronary MR angiography (MRA). Diaphragmatic navigator (NAV) suffers from indirect measurement of heart position. Respiratory self‐gating (RSG) approaches improve motion detection only in the head–feet direction, leaving motion in the other two dimensions unaccounted for. The purpose of this study was to extend conventional RSG (1D RSG) to RSG capable of 3D motion detection (3D RSG) by acquiring additional RSG projections with transverse‐motion‐encoding gradients. Simulation and volunteer studies were conducted to validate the effectiveness of this new method. Preliminary comparison was performed between coronary artery images reconstructed from the same datasets using different motion correction methods. Our simulation illustrates that a proper motion‐encoding gradient and derivation method enable accurate 3D motion detection. Results from whole‐heart coronary MRA show that 3D RSG can further reduce motion artifacts as compared to NAV and 1D RSG and enables use of larger gating windows for faster coronary imaging. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Three‐point dixon method enables whole‐body water and fat imaging of obese subjects

Dixon imaging techniques derive chemical shift‐separated water and fat images, enabling the quantification of fat content and forming an alternative to fat suppression. Whole‐body Dixon imaging is of interest in studies of obesity and the metabolic syndrome, and possibly in oncology. A three‐point Dixon method is proposed where two solutions are found analytically in each voxel. The true solution is identified by a multiseed three‐dimensional region‐growing scheme with a dynamic path, allowing confident regions to be solved before unconfident regions, such as background noise. 2π‐Phase unwrapping is not required. Whole‐body datasets (256 × 184 × 252 voxels) were collected from 39 subjects (body mass index 19.8‐45.4 kg/m²), in a mean scan time of 5 min 15 sec. Water and fat images were reconstructed offline, using the proposed method and two reference methods. The resulting images were subjectively graded on a four‐grade scale by two radiologists, blinded to the method used. The proposed method was found superior to the reference methods. It exclusively received the two highest grades, implying that only mild reconstruction failures were found. The computation time for a whole‐body dataset was 1 min 51.5 sec ± 3.0 sec. It was concluded that whole‐body water and fat imaging is feasible even for obese subjects, using the proposed method. Magn Reson Med 63:1659–1668, 2010. © 2010 Wiley‐Liss, Inc.     

Effect of Gd-DTPA induCED susceptibility on single-point Dixon fat/water separation.

A single-point Dixon (SPD) fat/water separation method is proposed for breast dynamic contrast-enhanced MRI (DCE-MRI) in which field inhomogeneities and phase offsets measured prior to the injection of gadopentate dimeglumine (Gd-DTPA) are used to correct the postinjection images. A breast phantom study was conducted to demonstrate this technique, in which varying excess concentrations of Gd-DTPA (0-3 mM) were introduced into a 25-cm(3) breast lesion. The presence of excess Gd-DTPA in the lesion was found to create magnetic field perturbations of up to 0.35 microT per mM excess Gd-DTPA around the lesion. However, these perturbations had a negligible effect on the quality of the fat/water separation for Gd-DTPA concentrations in the range of, and exceeding, those observed in breast tumors following a standard 0.1 mmole/kg injection. Therefore, we conclude that the preinjection phase data is adequate for the correction of the postinjection images in breast exams.     

Dual‐echo Dixon imaging with flexible choice of echo times

In this work, a new two‐point method for water–fat imaging is described and explored. It generalizes existing two‐point methods by eliminating some of the restrictions that these methods impose on the choice of echo times. Thus, the new two‐point method promises to provide more freedom in the selection of protocol parameters and to reach higher scan efficiency. Its performance was studied theoretically and was evaluated experimentally in abdominal imaging with a multigradient‐echo sequence. While depending on the choice of echo times, it is generally found to be favorable compared to existing two‐point methods. Notably, water images with higher spatial resolution and better signal‐to‐noise ratio were attained with it in single breathholds at 3.0 T and 1.5 T, respectively. The use of more accurate spectral models of fat is shown to substantially reduce observed variations in the extent of fat suppression. The acquisition of in‐ and opposed‐phase images is demonstrated to be replaceable by a synthesis from water and fat images. The new two‐point method is finally also applied to autocalibrate a multidimensional eddy current correction and to enhance the fat suppression achieved with three‐point methods in this way, especially toward the edges of larger field of views. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.