Ultra Fast T2-weighted TSE sequences using Flip Angle Sweep with Half-Fourier and SENSE at 3T

Purpose: Due to their insensitivty to susceptibility effects Turbo Spin-echo (TSE)-based sequences appear to be very attractive for high field applications, where susceptibility effects can impair the image quality. However their application at 3T interfere with high RF power deposition (exceed the specific absorption rate SAR limits for patient safety), especially single shot TSE sequences suffer from RF-deposition. It is well known that the SAR reduction can be obtained by using parallel imaging (SENSE) and half Fourier, which decrease the RF-deposition by echo train shortening. Nevertheless, very often a shorter acquisition time is requested. Here we present a new method, which is designed to provide high spatial resolution T2-w. images in a very short scan time by reducing SAR considerably. 5 volunteers and >150 patients were examined with this new method at 3T using two different TE'S (80 ms, 100 ms)

Methods: SAR can be reduced by the application with lower flip angles RF-refocusing pulses. Different variable flip angle refocusing techniques are published [1,2,3, 4] which avoids signal loss using lower refocusing flip angles. We combined flip angle sweep (FAS) with half Fourier and SENSE. This method varies the refocussing angle along the echo train and was implemented on a clinical 3.0T system (Intera, Philips Medical Systems, Best, NL) and evaluated in 5 volunteers (contrast dependency with FAS variation with 15° increments from 30° –180°, all with SENSE-factor 1, 2, 3, 4. In 70/30 patients with TE=80 ms/100 ms we measured the contrast of the WM to different anatomical structures (Pons, Nucleus ruber, Substantia nigra, Caput nucleus caudate, Putamen, Thalamus, gray matter frontal, Cortex temporal and frontal) in comparison to a routine TSE sequence. Protocol description: 24 axial slices covering the brain, resolution with TE=80 ms, TR=4199 ms, and 1×1×5mm, TR=3575 ms, fat suppression (SPAIR). An 8-element SENSE head coil was used. The scan time for the routine sequence was 109 seconds and for the final new designed sequence was about 8.4s with TE 80 ms and 10s with TE 100 ms. In addition, this technique was applied on 70 patients with higher reolution (0.89×0.89×2.0mm), 21 slices, scan time 28s instead 135s for conventional TSE sequence, and 30 patients with the same resolution, but 40–60 slices (about 1min., conv. TSE 7min.) in coronar or sagittal orientation.

Results: Blurring artifacts impaired the diagnostic image quality with SF <3. The optimal FAS value was found at 60° due to SNR and CNR in comparison to the conventional sequence. The contrast behaviour of different anatomical structures were similar for both sequences (ultra fast and conventional sequence). The image quality of the conventional TSE sequence is more sensitive to patient movement. Depending on the resolution and number of slices the scan time is between 6–12 times faster than the conventional TSE sequence.

Conclusions: We have presented and successfully implemented a new method, FAS combined with SENSE, which enables T2-weighted imaging with high spatial resolution and very short scan time. This reduced the scan about a factor between 6–12. This new method produce images with similar contrast and diagnostic quality although the SNR is lower compared to the conventional sequence. FAS with SENSE is a very useful technique in the clinical routine especially with a high number of slices and at non cooperative patient.

References: [1] Alsop DC: The sensitivity of low flip angle RARE imaging. Magn Reson Med 1997; 37:176–184; [2] Hennig J, Scheffler K, Hyperechoes. Magn Reson Med 2001; 46: 6–12; [3] Hennig J, Weigel M, Scheffler K. Magn Reson Med 2003; 49: 527–535; [4] Gieseke-J;Kuhl-CK;von-Falkenhausen-M;Blömer-R;Gür-O;van-Yperen-G;Schild-HH;Lutterbey-G. Ultra Fast T2-weighted TSE sequences with Flip Angle Sweep and SENSE at 3T .ISMRM: 2004