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 ...
In MRI of the human brain, subject motion is a major cause of magnetic resonance image quality degradation. To compensate for the effects of head motion during data acquisition, an in-bore optical motion tracking system is proposed. The system comprises two MR compatible infrared cameras that are fixed on a holder right above and in front of the head coil. The resulting close proximity of the cameras to the object allows precise tracking of its movement. During image acquisition, the MRI scanner uses this tracking information to prospectively compensate for head motion by adjusting gradient field direction and RF phase and frequency. Experiments performed on subjects demonstrate robust system performance with translation and rotation accuracies of 0.1 mm and 0.15° respectively.
ACUT 2 E TSE-SSFP is a hybrid between steady state free precession (SSFP) and turbo spin echo (TSE) for bright-blood T2-weighted imaging with signal-to-noise ratio (SNR) and contrastto-noise ratio (CNR) similar to dark-blood TSE. TSE-SSFP uses a segmented SSFP readout during diastole with 180°pulses following a 90°preparation. The 180°refocusing pulses make TSE-SSFP similar to TSE but TSE-SSFP uses gradient moment nulling, whereas TSE uses gradient crushing. TSE-SSFP produced T2-weighted images with minimal T1 weighting. TSE-SSFP and TSE had similar SNR (155.9 ؎ 6.0 vs 160.9 ؎ 7.0; P ؍ NS) for acute myocardial infarction (MI) and twice the SNR of Key words: Edema; infarction; T2; ACUT 2 E; ACUTE; C-TIDE; myocardium; heart T2-weighted turbo spin echo (TSE) (1) cardiac MRI of edema has recently received attention for transplant rejection (2), for differentiating acute versus chronic myocardial infarction (3), for identifying patients with acute myocarditis (4) and for in vivo delineation of the ischemic area at risk in acute myocardial infarction (5). T2-weighted imaging of edema provides complementary information to that obtained by delayed contrast enhanced MRI (6) of infarction and fibrosis. While the current clinical standard T2-weighted dark-blood TSE (7) has been instrumental in imaging edema associated with such pathology, the darkblood preparation leaves residual bright rim blood artifacts next to the endocardium due to inadequate suppression of the signal from the blood pool and is limited by posterior wall signal losses associated with improper timing. Such timing imperfections between the dark-blood preparation and the TSE acquisition result in through-plane motion of the dark-blood prepared slice out of the TSE imaging slice (8). The latter has been recently addressed by specialized techniques aiming to improve acquisition timing (9). With commonly available dark-blood TSE methods decreasing the double inversion slab thickness accentuates the posterior wall signal losses whereas increasing the slab thickness results in more pronounced bright rim artifacts. Steady state free precession (SSFP, a.k.a. true FISP, FI-ESTA, balanced-FFE) (10) with a T2 preparation (T2P-SSFP) was recently introduced to successfully address these issues and improve diagnostic accuracy (11) albeit at reduced signal-to-noise ratio (SNR).Our aim was to develop ACUT 2 E imaging (Acquisition for Cardiac Unified T2 Edema), a TSE-SSFP hybrid method for T2-weighted cardiac imaging of edema associated with acute myocardial infarction. Such a hybrid method could combine the inherent bright-blood contrast of an SSFP acquisition with the T2-weighting and the higher contrastto-noise ratio (CNR) inherent to TSE. We hypothesized that single cardiac phase TSE-SSFP images would possess T2-weighting and bright-blood contrast. We also hypothesized that TSE-SSFP would possess in vivo SNR and CNR similar to that of dark-blood TSE and higher than what can be obtained with T2P-SSFP. We specifically developed this methodology to improve imaging of ...
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