Frederick H. Epstein

Adaptive sensitivity encoding incorporating temporal filtering (TSENSE).

A number of different methods have been demonstrated which increase the speed of MR acquisition by decreasing the number of sequential phase encodes. The UNFOLD technique is based on time interleaving of k‐space lines in sequential images and exploits the property that the outer portion of the field‐of‐view is relatively static. The differences in spatial sensitivity of multiple receiver coils may be exploited using SENSE or SMASH techniques to eliminate the aliased component that results from undersampling k‐space. In this article, an adaptive method of sensitivity encoding is presented which incorporates both spatial and temporal filtering. Temporal filtering and spatial encoding may be combined by acquiring phase encodes in an interleaved manner. In this way the aliased components are alternating phase. The SENSE formulation is not altered by the phase of the alias artifact; however, for imperfect estimates of coil sensitivities the residual artifact will have alternating phase using this approach. This is the essence of combining temporal filtering (UNFOLD) with spatial sensitivity encoding (SENSE). Any residual artifact will be temporally frequency‐shifted to the band edge and thus may be further suppressed by temporal low‐pass filtering. By combining both temporal and spatial filtering a high degree of alias artifact rejection may be achieved with less stringent requirements on accuracy of coil sensitivity estimates and temporal low‐pass filter selectivity than would be required using each method individually. Experimental results that demonstrate the adaptive spatiotemporal filtering method (adaptive TSENSE) with acceleration factor R = 2, for real‐time nonbreath‐held cardiac MR imaging during exercise induced stress are presented. Magn Reson Med 45:846–852, 2001. Published 2001 Wiley‐Liss, Inc.

Myocardial Wall Tagging With Undersampled Projection Reconstruction

Azimuthally undersampled projection reconstruction (PR) acquisition is investigated for use in myocardial wall tagging with MR using grid tags to provide increased temporal and spatial resolution. PR can provide the high‐resolution images required for tagging with very few projections, at the expense of artifact. Insight is provided into the PR undersampling artifact, in the context of measuring myocardial motion with tags. For Fourier transform imaging, at least 112 phase‐encodings must be collected to image tagging grids spaced 7 pixels apart. PR requires about 80 projections, a 1.4‐fold reduction in scan time. Magn Reson Med 45:562–567, 2001. Published 2001 Wiley‐Liss, Inc.

Optimization of fast cardiac imaging using an echo-train readout†

Fast gradient‐echo sequences that use an echo‐train readout are becoming more widely used, particularly for imaging the heart. An important issue for these sequences involves determining the optimal duration for the echo‐train readout. In normal volunteer scans and theoretically the echo‐train readout duration was varied from 2.4 to 32.8 msec. Myocardial signal‐to‐noise ratio (SNR), myocardium‐tag signal difference‐to‐noise ratio (SDNR), flow artifact‐to‐noise ratio (FNR), and geometric distortion were measured and/or calculated. Our results showed that to obtain high SNR, SDNR, and data acquisition efficiency while minimizing FNR and geometric distortion, the readout duration should be 10–15 msec at 1.5 T. J. Magn. Reson. Imaging 2000;11:75–80. Published 2000 Wiley‐Liss, Inc.

Low latency temporal filter design for real-time MRI using UNFOLD

To improve real‐time control of interventional procedures such as guidance of catheters, monitoring of ablation therapy, or control of dosage during drug delivery, the image acquisition and reconstruction must be high speed and have low latency (small time delay) in processing. A number of different methods have been demonstrated which increase the speed of MR acquisition by decreasing the number of sequential phase‐encodes. A design and implementation of the UNFOLD method which achieves the desired low latency with a recursive temporal filter is presented. The recursive filter design is characterized for this application and compared with more commonly used moving average filters. Experimental results demonstrate low‐latency UNFOLD for two applications: 1) high‐speed, real‐time imaging of the heart to be used in conjunction with cardiac interventional procedures; and 2) the injection of drugs into muscle tissue with contrast enhancement, i.e., monitoring needle insertion and injection of a drug with contrast enhancement properties. Proof‐of‐concept was demonstrated by injecting a contrast agent. In both applications the UNFOLD technique was used to double the frame rate. Magn Reson Med 44:933–939, 2000.

Functional cardiac MR imaging

Undersampled projection reconstruction acquisitions are investigated for use in functional cardiac MR imaging. 256×256 resolution is obtainable using only 64 projections, with acceptable artifact level. Reduced FOV techniques decrease artifact. Variable angular sampling using projection reconstruction is investigated.