This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Introduction: Two-dimensional excitation pulses have found numerous uses in MR imaging. Variable-density (VD) 2-D spiral excitation has been investigated as a way to shorten the duration of excitation pulses, limit RF power deposition, and spread out sidelobes. This work proposes a simple method to accomplish this task. Methods and Discussion: The algorithm optimizes the excitation k-space trajectory for variable-density spirals by approximating the trajectory by concentric circles. This same approximation was used by Pauly, et al in describing linear large tip-angle pulses. Because a circle is radially symmetric, its Fourier transform is also radially symmetric and can be expressed one-dimensionally as a Hankel transform. The circle approximation was used to design an inversion pulse with a width of 1 cm. Nominal field of view was chosen to be 10 cm. Variable density trajectories, were described by ta polynomial. A global search was performed, and the polynomial coefficients yielding the minimum value over all trajectories was used in designing the pulse. In order to test the accuracy of the circle approximation, the forward SLR transform was performed for each of the corresponding spiral trajectories, and the maximum sidelobe height was recorded and compared to the value obtained from the circle approximation. Simulations and experimental data verified the expected performance. Acknowledgements: Lucas Foundation, NIH RR009784 an
Showing the most recent 10 out of 446 publications
