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University of Bonn boosts temporal resolution in MRA with 4D-TRAK

Best Practice
Willinek, Winfried, M.D. Bonn, University of Bonn Germany
von Falkenhausen, Marcus, M.D. Bonn, University of Bonn Germany
Gieseke, J�rgen Philips Healthcare Germany

University of Bonn boosts temporal resolution in 3.0T MRA with 4D-TRAK

University of Bonn (Germany) radiologists are continually pushing to improve MRA's capabilities in studies of the cerebrovasculature, particularly MRA's potential in the characterization of arteriovenous malformations (AVM). Although DSA's subsecond temporal resolution has enabled it to maintain gold standard status in cerebrovascular imaging, Bonn radiologists Winfried Willinek and Marcus von Falkenhausen, and Philips clinical scientist Juergen Gieseke have now also succeeded in achieving high-resolution whole-brain 3D imaging with subsecond MRA temporal resolution. The key at Bonn has been the 4D-TRAK technique, which combines SENSE and CENTRA keyhole techniques, using the center's Achieva 3.0T system. This synergy is resulting in incredible acceleration factors over standard dynamic MRA - 48 times faster routinely and even faster using additional time-saving features.
 Dr. von Falkenhausen, Jürgen Gieseke, Dr. Willinek.
Dr. von Falkenhausen, Jürgen Gieseke, Dr. Willinek.

Combining SENSE and CENTRA keyhole dramatically improves MRA temporal resolution

While MRI has proven adept at non-invasively visualizing AVMs, informing treatment decisions has been better left to modalities such as DSA, which can identify an AVM's feeding arteries and draining veins by virtue of its dynamic capabilities. MRI temporal resolution barriers, however, appear to be eroding. At the University of Bonn, radiologists Winfried Willinek, M.D. and Marcus von Falkenhausen, M.D., and Philips' Juergen Gieseke, Ph.D. have been multiplying their MRA temporal resolution many times over by combining the fast Philips acquisition techniques SENSE and CENTRA keyhole. They maintain superb spatial resolution by implementing the combined technique - 4D-TRAK (Time-Resolved Angiography using Keyhole) - on their Achieva 3.0T system.



  Comparison of 4D-TRAK with DSA in patient with left frontal AVM

  MRA shows the arterial feeders originating from the left anterior

  and left middle cerebral arteries (arrow head) as confirmed by

  DSA (arrow head).


  The superficial venous drainage into the superior sagittal sinus

  is depicted on 4D-TRAK and confirmed by DSA (arrow heads).


Economizing on k-space scanning

CENTRA, Contrast-Enhanced Timing Robust Angiography, is a k-space sampling strategy that optimizes CE-MRA image acquisition during arterial contrast. CENTRA scans the central, contrastdetermining sphere of k-space first, during the first few seconds of arterial contrast. After the arterial phase, acquisition of the peripheral k-space lines can be conducted afterward to obtain the structural, highresolution information.


The CENTRA keyhole method varies the acquisition by updating a small central kspace sphere (e.g. 16% of k-space) at various times during the scan. The efficiency of scanning a fraction of k-space, as though looking through a keyhole, results in an automatic six-fold acceleration of dynamic MRA.


 "If I want 10 dynamics, for example, then the central sphere is updated 10 times, providing different dynamic information in the contrast-over-time curve. Then, the peripheral k-space lines are scanned to obtain the spatial information," Dr. Willinek explains. "So, instead of just pure arterial contrast that basic CENTRA provides, CENTRA keyhole updates the central sphere during several phases."


In evaluating complex pathology, such as the intricate superposition of arteries and veins in an AVM, the CENTRA keyhole approach provides the dynamic window needed to visualize structure and pathology.


"Arteriovenous transit time of contrast agent ranges from one second down to 200 ms, so you need to scan fast to enable identification of feeding arteries - whether it's the anterior cerebral, middle cerebral or posterior cerebral artery, small branches of these arteries or a combination," Dr. Willinek says.


 "Characterizing venous drainage also is important," adds Dr. von Falkenhausen. "We need to determine whether drainage is via smaller internal veins or superficial veins. All of this information is important to plan the intervention, whether it be embolization, neurosurgery or radiation therapy."


To study AVMs, Bonn clinicians perform 29 dynamics requiring 29 updates of the central k-space sphere. Combined with SENSE and scanned on the Achieva 3.0T platform, they are achieving a temporal resolution of 600 ms for each of the 29 dynamics.



  4D-TRAK MIP projections of the brain using CENTRA keyhole (6x acceleration) and

  SENSE (8x acceleration) on an Achieva 3.0T system with an 8-channel SENSE Head coil.

  Temporal resolution of the 50 dynamic scans is 0.66 per volume. Every dynamic scan

  140 slices are acquired with a spatial resolution of 1.1 x 1.4 x 1.1 mm3.



3.0T plus SENSE equal speed with high spatial resolution

Scanning on Achieva 3.0T provides ample signal-to-noise, enabling Bonn investigators to employ high SENSE factors, using the Philips 8-channel SENSE Head coil, while maintaining high spatial resolution.


 "CENTRA keyhole is six times faster than regular MRA, and SENSE provides an eightfold speed increase, making the SENSE and CENTRA keyhole technique 48 times faster than conventional MRA," Dr. Willinek observes. "We can then use Halfscan to reach an acceleration factor of 62. If we then add a smaller rectangular FOV, we calculate the speed to be slightly less than 100 times faster compared to standard MRA."


While the 12 fps temporal resolution of gold standard DSA is still faster than SENSE and CENTRA keyhole, Drs. Willinek, von Falkenhausen and Gieseke are confident that further MR speed increases are possible and that their ongoing study comparing the MR method versus DSA in AVM characterization will demonstrate MR's potential.


"We're doing a good job in clinical practice right now," Dr. Willinek says. "I'm quite sure we're on the right track."

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Nov 26, 2005

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