FLOW COMPENSATED SPECTRAL SPATIAL EXCITATION PULSES

点击翻译按钮获取中文摘要

Introduction: Fast spiral acquisitions are very useful in quantitative flow applications because of their short scan times. In these sequences, spectral-spatial excitation pulses are typically employed to reduce blurring from lipids. These pulses excite only water protons within a slice, resulting in minimal signal from lipid protons. However, moving protons experience a phase modulation during spectral-spatial excitation, resulting in decreased signal and problems in flow quantification. We have designed flow compensated spectral-spatial pulses as a solution to this problem. With these new pulses, protons moving at constant velocities experience the same excitation profile as static protons, while lipid signal is still significantly decreased. Methods and Results: To achieve flow compensation during spectral-spatial excitations, rf energy is applied only when the zeroth and first moments of the oscillating gradient are refocused, or nulled. The design of these pulses is limited by the maximum gradient amplitude and slew rate of the system. With standard gradient parameters, flow compensated designs that maintain spectral selectivity only work for relatively thick spatial slab excitations. With our newer gradient system (higher gradient amplitude and faster slew rates), spectral selectivity can be maintained, while achieving a wider range of slice thicknesses. For through-plane flow measurements, velocity encoding can also be incorporated into the flow compensated pulses. This further reduces the gradient duty cycle, and also shortens the minimum TE and TR of the sequence. Summary and Conclusions: Flow compensated spectral-spatial pulse designs are feasible with stronger gradient subsystems. These pulses produce desired excitation profiles for protons moving at constant velocities during the excitation period, while maintaining spectral and spatial selectivity. Compared to non-flow compensated designs, higher signal from moving spins results in better visualization of faster moving flow in vessels, and potentially more accurate determination of through-plane flow measurements in quantitative flow applications.