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k-t BLAST and k-t SENSE promise 5-8 times faster cine, dynamic MRI

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Tsao, Jeffrey, Ph.D. Zurich, University and ETH Zurich Switzerland

k-t BLAST and k-t SENSE promise 5-8 times faster cine and dynamic MRI

 

The fast acquisition and reconstruction strategies of k-t BLAST and k-t SENSE (both works-in-progress) will radically alter the way clinicians conduct cine and dynamic MRI, in both real-time and breath hold scans. The techniques - developed with Philips' support at the Institute for Biomedical Engineering, University and ETH Zurich - exploit the fact that typical image series display many correlations in space and time. These similarities or redundancies enable the acquisition of a greatly reduced data set. The results are 5-8-fold accelerations in acquisition speed, providing dramatic increases in spatial and temporal resolution and throughput, and radically lowered scan times and artifacts. The potential of k-t methods for dynamic cardiac and other real-time studies is enormous.

To provide images of an object in motion, conventional dynamic MRI acquires a full data set for reconstructing each time frame separately. However, this strategy has rarely been fast enough to obtain an instant snapshot due to the speed limitations of MR or concerns about peripheral nerve stimulation. To overcome this, in recent years attention has been focused on "smarter" acquisition techniques, most notably Philips SENSET, which leads the way in parallel imaging. Now this focus on efficient data sampling has yielded a further spectacular advance: k-t BLAST (k-t space Broad-use Linear Acquisition Speed-up Technique) and k-t SENSE (k-t space SENSitivity Encoding) techniques for dynamic MRI. k-t BLAST and k-t SENSE use single or multiple receiver coils. Both techniques are compatible with any pulse sequence.

 

kt-BLAST and kt-SENSE explained
kt-BLAST and kt-SENSE explained

 

"Instead of regarding a set of dynamic images as completely different, independent time frames, k-t BLAST and k-t SENSE algorithms regard all of these images as one entity," says Jeffrey Tsao, Ph.D. from the Institute. "In so doing, you can exploit any connections or redundancies among all the images over a time period, to accelerate acquisition."

When applying these techniques to the heart - which beats and is moved by breathing - image pixels in the relatively static chest wall, for example, would be shared by all the time frames in an acquisition. Conversely, while the heart is quite mobile, its motion is periodic and generally smooth, enabling predictions of frame-to-frame changes. The more predictable the changes, the faster acceleration factors are possible. In a k-t BLAST or k-t SENSE study, these correlations in k-space and time - which determine the acquisition acceleration factor - are ascertained from an initial set of low-resolution training images. The training data set and subsequent diagnostic acquisition data set are then sent to the reconstruction software.

Several ways to exploit the benefits

Such dramatic accelerations are particularly striking in time-resolved 3D scanning of the heart, for example. "With the k-t methods, this now takes about 20-25 sec. (20 phases, 20 slices), versus 1:30-2:40 min. in an unaccelerated scan. Scanning any faster in a conventional way sacrifices the number of cardiac phases or spatial resolution," Dr. Tsao says.

 

"Another example is 2D quantitative flow imaging, where k-t BLAST or k-t SENSE enables a complete scan time of 10-15 sec., whereas previously you would need about a minute, which is clearly too long for a breath hold. These high acceleration factors let us re-think applications that were not possible before."

 

Alternatively, instead of reducing acquisition time, the k-t methods can be used to increase temporal resolution in real-time, untriggered, free-breathing cardiac studies, featuring more diagnostic information. "We can comfortably achieve 2 x 2 mm in-plane resolution at about 30 frames/sec. [fps] using the Balanced FFE sequences in combination with these techniques, or even 70 fps if you use an EPI sequence," he says.

 

In addition to cine or dynamic cardiac imaging, the k-t techniques are also appropriate for any cine or dynamic application, including abdominal, speech and kinetic examinations. Philips scientists and IBTZ are actively investigating these methods further, but it is already clear that k-t BLAST and k-t SENSE offer huge potential in further revolutionizing fast, dynamic MRI.

 

 

Acknowledgement: Jeffrey Tsao is a recipient of a postdoctoral fellowship from the Canadian Institutes of Health Research (CIHR).

Reference

Tsao J, Boesiger P, Pruessmann KP.

k-t BLAST and k-t SENSE: Dynamic MRI with high frame rate exploiting

spatiotemporal correlations.

Magn Reson Med 50, 1031-1042, 2003



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