Implementing VERSE for Time of Flight RF pulses at 7Tesla: Methodological Considerations
نویسندگان
چکیده
INTRODUCTION. In Time-of-flight (TOF) angiography, fresh blood inflowing spins provide bright intravascular signal while static tissue signal suppression is obtained in the steady state with TR<<T1. At 1.5 and 3T, stronger background suppression is achieved with magnetization transfer (MT), and venous contribution is attenuated with saturation RF pulses. Ramp shaped RF pulses (TONE) can be used to adjust blood velocity sensitivity. Successful TOF can be obtained at 7T, benefiting from higher SNR and longer tissue T1. However, because of SAR constraints, MT cannot be used at 7T and saturation pulses are often skipped [1-3]. SAR constraints also imply using sub-optimal excitation flip angle which in turn limits angiogram quality. These issues can be addressed using VERSE principle [4] to decrease peak RF power. However, VERSE pulses are more sensitive to ΔB0 [4], while ΔB0 increases proportionally with B0. Here, we investigate in simulations and in vivo the impact of off-resonance frequencies on RF excitation profile when applying VERSE at 7T.
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