Introduction Method Results Discussion References - ismrm

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(1) S Conolly, D Nishimura, A Macovsky, G Glover. Variable Rate Selective Excitation. J Mag Reson 2000; 78:440-458. (2) J Pauly, P Le Roux, D Nishimura, ...
Clinical 3T SAR Reduction Using VERSE Pulses Y. Zur1, J. Hugg1, A. Montag1, D. Outmezguine1, R. Busse2 1

GE Medical Systems, Haifa, Israel, Israel, 2Applied Science Laboratory, GE Medical Systems, Menlo Park, CA, United States

Introduction The increased SNR at 3T versus 1.5T, which scales roughly as the field strength, comes at the price of higher SAR, which scales as the square of the field strength. Over the past several years, 3T scanners have become increasingly used for routine clinical scanning. In order to achieve the same slice coverage as in routine scanning at 1.5 T, while staying within the SAR regulatory limits, it is necessary to modify the standard 1.5T imaging protocols. We have reduced the transmitted power in the spin echo and fast spin echo pulse sequences by replacing the standard excitation and refocusing pulses with VariablE Rate Selective Excitation (VERSE) pulses. In our optimization scheme, regulatory compliance and image quality equal to that of 1.5 T were treated as absolute constraints. We varied pulse modulation parameters and flip angles so as to strike a balance between SNR, contrast, and scan coverage.

Method The VERSE technique is a modulation of a standard rf pulse envelope to reduce the peak B1, accompanied by a modulation of the slice select gradient to compensate for the change in the excitation profile (1). SAR reduction is achieved because the SAR is proportional to the time integral of B12. Our first step was to design rf pulses, which met the slice profile requirements, using the SLR method (2). Next, bandwidth-limiting filters were applied. Finally, VERSE scaling and time interpolation was applied to the rf pulse envelopes and the slice select gradient patterns. For off-center slices the synthesizer frequency was also modulated to match the gradient patterns. The parameters of the scaling function determined the savings in either time or power. Figure 1 shows the scaling function in our implementation of the FSE pulse. A similar function has been used in the SE sequence. Experiments were performed on a GE Signa 3T scanner as well as on a GE Signa Twin 1.5T scanner. In both cases gradient maximum was 40 mT/m with slew rate 150 mT/m/ms. verse

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Figure 2

Results Total SAR in the SE sequence has been reduced by a factor of 2.77. In the FSE implementation SAR has been reduced up to a factor of 4 depending on the refocusing flip angle (3). The rf pulse durations were left unchanged. Figure 2 shows a comparison of images between standard and VERSE pulsed for the SE sequence on the 3T scanner. The VERSE sequence image is on the left. Images acquired using phantoms have shown quantitatively that VERSE pulses cause no degradation in either SNR or resolution.

Discussion We have shown clinically that VERSE can provide substantial SAR reduction. A potential drawback of VERSE is that off-resonant tissue (e.g. fat or chemical shift) is differentially excited, which may affect image contrast. The clinical implications are currently under investigation.

References (1) S Conolly, D Nishimura, A Macovsky, G Glover. Variable Rate Selective Excitation. J Mag Reson 2000; 78:440-458. (2) J Pauly, P Le Roux, D Nishimura, A Macovski, Paremeter Relations for SLR Design Algorithm. IEEE Trans Med Imag 1991;10:53-65 (3) DC Alsop.The Sensitivity of Low Flip Angle Rare Imaging. Magn Reson Med 1997; 37:176-184.

Proc. Intl. Soc. Mag. Reson. Med. 11 (2003)

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