Undersampled projection reconstruction (PR) is investigated as an alternative method for MRA (MR angiography). In conventional 3D Fourier transform (FT) MRA, resolution in the phase‐encoding direction is proportional to acquisition time. Since the PR resolution in all directions is determined by the readout resolution, independent of the number of projections (Np), high resolution can be generated rapidly. However, artifacts increase for reduced Np. In X‐ray CT, undersampling artifacts from bright objects like bone can dominate other tissue. In MRA, where bright, contrast‐filled vessels dominate, artifacts are often acceptable and the greater resolution per unit time provided by undersampled PR can be realized. The resolution increase is limited by SNR reduction associated with reduced voxel size. The hybrid 3D sequence acquires fractional echo projections in the kx–ky plane and phase encodings in kz. PR resolution and artifact characteristics are demonstrated in a phantom and in contrast‐enhanced volunteer studies. Magn Reson Med 43:91–101, 2000. © 2000 Wiley‐Liss, Inc.

High-spatial-resolution delayed enhancement MR imaging allows noninvasive identification of scar induced by RF ablation following isolation therapy of the PV.

The projection reconstruction (PR) trajectory was investigated for the effect of gradient timing delays between the actual and requested start time of each physical gradient. Radial trajectories constructed with delayed gradients miss the center of k-space in an angularly dependent manner, causing effective echo times to vary with projection angle. The gradient timing delays were measured in phantoms, revealing delays on the x, y, and z gradients which differed by as much as 5 sec. Using this one-time calibration measurement, the trajectories were corrected for gradient delays by addition of compensatory gradient areas to the prephasers of the logical x and y readout gradients. Effective projection-to-projection echo time variability was reduced to less than 1 sec for all imaging orientations. Projection reconstruction (PR) is recently a subject of renewed investigation as a valuable MR imaging trajectory. The sensitivity of PR to off-resonance and B 0 field inhomogeneity described early on (1) has been reduced with more homogeneous magnets and by imaging with high receiver bandwidths, coupled with increased gradient strengths. Gridding reconstruction methods are faster now due to improved computer hardware and the implementations are well known (2). A still serious source of image artifact in images obtained with the PR trajectory arises from miscentered k-space trajectories, to which Cartesian imaging is somewhat immune. For PR, radial lines of kspace are acquired as projections (Np) consisting of Nr readout points. For a miscentered trajectory, the positions of the acquired data are different from the intended positions. Gridding these offset k-space points onto the locations of the intended trajectory results in a reconstruction error.This investigation presents a new solution to the k-space miscentering artifact. The solution follows another proposed for echo-planar imaging (3). Relative delays between the requested and the actual start times of the x, y, and z gradients, shown in Fig. 1 with delays exaggerated for inspection purposes, are assumed to be a cause of k-space miscentering. The effect of different gradient delays on each physical gradient is a miscentering of k-space which varies for each projection angle. First, the relative gradient delays are measured once for a particular scanner, using a homogeneous phantom. This one-time calibration permits calculation of compensatory gradient areas needed to correct these delays for any slice plane rotation. These compensatory areas are added to the gradient readout prephasers and rephasers. Figure 2 shows simulated PR trajectories that result from gradient delays. The trajectories for 16 projections are plotted from 0 -180°. Only the central region of k-space is shown: (kx, ky ) ϭ (Ϯ1, Ϯ1 ) out of a 160 ϫ 160 matrix. The solid dots show the position in k-space at the intended echo time. Typical delays of t x ϭ -3.7 s, t y ϭ ϩ1.5 s, a 160 ϫ 160 pixel field of view, and an 8 s dwell time were used to construct the plot. The trajectories do not align at kr ...

Background-The local environment of delivered mesenchymal stem cells (MSCs) may affect their ultimate phenotype.MR fluoroscopy has the potential to guide intramyocardial MSC injection to desirable targets, such as the border between infarcted and normal tissue. We tested the ability to (1) identify infarcts, (2) navigate injection catheters to preselected targets, (3) inject safely even into fresh infarcts, and (4) confirm injection success immediately. Methods and Results-A 1.5-T MRI scanner was customized for interventional use, with rapid imaging, independent color highlighting of catheter channels, multiple-slice 3D rendering, catheter-only viewing mode, and infarct-enhanced imaging. MRI receiver coils were incorporated into guiding catheters and injection needles. These devices were tested for heating and used for targeted MSC delivery. In infarcted pigs, myocardium was targeted by MR fluoroscopy. Infarct-enhanced imaging included both saturation preparation MRI after intravenous gadolinium and wall motion. Porcine MSCs were MRI-labeled with iron-fluorescent particles. Catheter navigation and multiple cell injections were performed entirely with MR fluoroscopy at 8 frames/s with 1.7ϫ3.3ϫ8-mm voxels. Infarct-enhanced MR fluoroscopy permitted excellent delineation of infarct borders. All injections were safely and successfully delivered to their preselected targets, including infarct borders. Iron-fluorescent particle-labeled MSCs were readily visible on delivery in vivo and post mortem. Conclusions-Precise