Brian Hargreaves, Postdoctoral Faculty Sponsor
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To develop short water suppression sequences for 7 T magnetic resonance spectroscopic imaging, with mitigation of subject-specific transmit RF field ( B1+) inhomogeneity.Patient-tailored spiral in-out spectral-spatial saturation pulses were designed for a three-pulse WET water suppression sequence. The pulses' identical spatial subpulses were designed using patient-specific B1+ maps and a spiral in-out excitation k-space trajectory. The subpulse train was weighted by a spectral envelope that was root-flipped to minimize peak RF demand. The pulses were validated in in vivo experiments that acquired high resolution magnetic resonance spectroscopic imaging data, using a crusher coil for fast lipid suppression. Residual water signals and MR spectra were compared between the proposed tailored sequence and a conventional WET sequence.Replacing conventional spectrally-selective pulses with tailored spiral in-out spectral-spatial pulses reduced mean water residual from 5.88 to 2.52% (57% improvement). Pulse design time was less then 0.4 s. The pulses' specific absorption rate were compatible with magnetic resonance spectroscopic imaging TRs under 300 ms, which enabled spectra of fine in plane spatial resolution (5 mm) with good quality to be measured in 7.5 min.Tailored spiral in-out spectral-spatial water suppression enables efficient high resolution magnetic resonance spectroscopic imaging in the brain. Magn Reson Med 79:31-40, 2018. © 2017 International Society for Magnetic Resonance in Medicine.
View details for DOI 10.1002/mrm.26683
View details for Web of Science ID 000417926300003
View details for PubMedID 28370494
View details for PubMedCentralID PMC5623606
To design multichannel large-tip-angle kT-points and spokes radiofrequency (RF) pulses and gradient waveforms for transmit field inhomogeneity compensation in high field magnetic resonance imaging.An algorithm to design RF subpulse weights and gradient blip areas is proposed to minimize a magnitude least-squares cost function that measures the difference between realized and desired state parameters in the spin domain, and penalizes integrated RF power. The minimization problem is solved iteratively with interleaved target phase updates, RF subpulse weights updates using the conjugate gradient method with optimal control-based derivatives, and gradient blip area updates using the conjugate gradient method. Two-channel parallel transmit simulations and experiments were conducted in phantoms and human subjects at 7 T to demonstrate the method and compare it to small-tip-angle-designed pulses and circularly polarized excitations.The proposed algorithm designed more homogeneous and accurate 180° inversion and refocusing pulses than other methods. It also designed large-tip-angle pulses on multiple frequency bands with independent and joint phase relaxation. Pulses designed by the method improved specificity and contrast-to-noise ratio in a finger-tapping spin echo blood oxygen level dependent functional magnetic resonance imaging study, compared with circularly polarized mode refocusing.A joint RF and gradient waveform design algorithm was proposed and validated to improve large-tip-angle inversion and refocusing at ultrahigh field.
View details for DOI 10.1002/mrm.25739
View details for Web of Science ID 000370593700025
View details for PubMedID 25916408
View details for PubMedCentralID PMC4624053