Accelerated Methods for RF Pulse Design in UHF MRI
نویسنده
چکیده
In ultra high field magnetic resonance imaging (UHF MRI), the complex interaction of the electromagnetic wave used for excitation with the dielectric load of the human body results in spatially varying magnetic and electric fields that are highly dependent on intersubject anatomical variations. Based on these spatially varying, subject-dependent magnetic and electric field maps, we must design a radiofrequency (RF) pulse that (a) achieves some desired excitation pattern with an RMS deviation (termed FAI) that is less than a specified tolerance and (b) minimizes the maximum amount of power deposited at any point in the body (termed pkSAR). As the field maps for a specific scan are known only once the patient has been inserted into the scanner, RF pulse design must take place in real time. The particular RF coil for the application of interest utilizes parallel transmission whereby multiple independent channels each transmit a sinusoidal RF wave with some amplitude and phase that combine to form the total electric and magnetic fields. In addition to the magnetic field map, the excitation profile depends on gradient encoding based on a set of predetermined spatial frequencies. The goal in RF pulse design is to determine what complex voltage (amplitude and phase) must be applied to each channel during each discrete gradient encoding step to best address the above criteria.
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