Purpose

To perform slice selective excitation with dramatically reduced SAR.

Introduction

Patient heating due to SAR forces tradeoffs in MRI protocols such as reduced flip angles and increased imaging time. SAR constraints are severe at high field strengths, but are significant even at 1.5T field strength. We present a method combining custom transmit coil design and RF pulse software with two-channel parallel transmission that reduces SAR for axial slice selective excitation by a factor of 2 without affecting excitation slice profile or time. Our coil design is also suitable for conventional use as a single-channel volume excitation coil.

Methods

Our method of reduced SAR slice selective excitation uses an array of two transmit coils, each of which has a transverse magnetic field profile with relatively uniform amplitude over the imaging volume but with opposite approximately linear phase along the longitudinal direction (1). These coils are coaxial around the same imaging volume. This configuration is illustrated in Figure 1, which shows two separately driven microstrip coils, each with four elements, which have the field profiles shown in Figure 2. Each of these coils is driven with a separate slice-selective RF waveform, such as a Hamming windowed sinc. The waveforms are scaled to half of the desired total flip and are offset relative to one another by a time increment Δt which, in combination with the slice selection gradient, corresponds to linear phase accrual equal to the intrinsic linear phase difference of the coils. The RF waveforms for the two coils are shown in Figure 3.

Results

The B and E fields of the two-coil linear phase array were calculated by FDTD simulation (XFDT, Remcom) and used to perform RF simulation of slice selective excitation using each coil individually with conventional RF waveform design, and both coils simultaneously with the proposed method. Figure 2 shows the magnitude and angle of the transverse component of B determined by FDTD simulation at the center of each coil along its length. (For more details, see (1)). Figure 3 shows the RF excitations applied simultaneously to the two coils and the resulting slice profile of excitation along the center of the coils. Figure 4 shows the SAR corresponding to conventional slice selective excitation using just one of the coils, representing conventional excitation with a single volume coil, and with both coils simultaneously using the new method. SAR was calculated based on FDTD simulation of the electric field of both coils. SAR averaged over the imaging volume is decreased by a factor of 2.06 with the new method compared to single coil excitation.

Discussion

Many methods have been employed to reduce SAR in slice selective excitation/refocusing including increased RF pulse length, variable rate selective excitation (VERSE) (2), and decreased flip angle for refocusing pulses in turbo-spin-echo imaging (3). These methods result in undesirable consequences including longer imaging time, decreased effective pulse bandwidth causing misregistration of fat and water excitation profiles, and contamination of T2 weighting caused by stimulated echoes from trains of refocusing pulses less than 180 degrees. Our proposed method has none of these drawbacks, achieving slice profile and RF pulse length identical to conventional RF excitation but with half the SAR. This effect arises from the combination of the two linear phase coil field profiles, which can combine constructively over parts of the imaging volume and destructively over other parts during the course of the RF excitation. Current results are based on FDTD simulation as we do not have access to a two-channel RF system. Future work will include hardware implementation and verification of the simulation results.

Conclusion

A novel method of parallel transmission for axial slice selective excitation utilizing a set of two coils with linear phase variation along the through-slice direction (1) allows decrease in SAR by a factor of two for axial slice selective excitation with no increase in excitation time, no change in excitation bandwidth, and no change in slice profile compared to conventional slice selective excitation.

Acknowledgements

No acknowledgement found.