ISMRM Practice Talks
May 1st, Building 75 conference room, 4:00 pm
Uniform Wideband Slab Selection with B1+ Mitigation at 7T via Parallel Spectral-Spatial Excitation
Parallel RF (pTx) designs based on small-flip-angle excitations with “spoke”-based trajectories can efficiently mitigate large B1+ inhomogeneities at high field using relatively short slice-selective excitation pulses [1-2]. Unfortunately, such pulses often exhibit anarrow-band off-resonance response and may not be suitable for applications that require B1+ mitigation over a large bandwidth. Proton chemical shift imaging gains SNR and chemical shift dispersion benefits from higher B0, but requires B1+ mitigation over both a specified spectral bandwidth and a spatial FOV. This additional bandwidth constraint presents a challenge for past methods on water-only B1+ mitigations. In this work, we describe a method for general pTx spectral-spatial excitations, and demonstrate the technique on a wideband slice-selective spoke excitation, which is then validated on a water phantom using an 8-channel TX array system on a 7T human MRI scanner.
MRI-guided focused ultrasound-enhanced chemotherapy of 9L rat gliosarcoma: Survival study
We investigated the impact of MRI-guided focused ultrasound-enhanced delivery of doxorubicin to brain on survival in rats with 9L gliosarcoma. In this study, we have shown that targeted drug delivery by MRI-guided focused ultrasound significantly improves survival time in rats with aggressive glioma, compared to chemotherapy alone (/p/ < 0.005). Such demonstrated efficacy in an /in vivo /tumor model represents a large step forward in the development of this technique toward treating patients with disorders of the central nervous system.
Fast Spectroscopy Imaging uniform wideband parallel excitation on 7T
This work combines spiral CSI readouts with parallel RF transmission (pTx) to mitigate B1+ inhomogeneities. We limit this initial demonstration to the low flip-angle domain, and apply “spokes”-based slice selective RF design to an eight channel transmit system at 7T. The 8 transmit channels enable reduced-duration, slice-selective RF pulses that implement excellent B1+ mitigation over 5 cm slab and 600 Hz spectral bandwidth. The goal of this work is to demonstrate efficient spiral CSI encoding with B1+-mitigated spatial-spectral excitation over a spatial FOV and frequencies of interest for 1H brain spectroscopic imaging by spiral CSI acquisitions of the high-SNR water signal shifted to 5 different off-resonance frequencies.
A 16 channel butler matrix for parallel excitation at 7T
Butler matrix is the hardware implementation of the FFT algorithm. In this work we have implemented a 16 channel Butler matrix to drive the most optimal 8 mode of a 16 channel T/R coil.
Electrodynamic constraints on homogeneity and RF power deposition in multiple coil excitations(Young Investigator’s Award Finalist)
This work explores electrodynamic constraints on transmit homogeneity and SAR in the case of fully parallel transmission and RF shimming. Ultimate SAR was computed for various target excitation profiles on a transverse plane through the center of a homogeneous sphere. The behavior with respect to main magnetic field strength, object size and acceleration was investigated in the ultimate case, as well as in the case of finite coil arrays. Ideal current patterns resulting in the lowest possible SAR were calculated and, as expected, increasingly complex current distributions were observed at higher magnetic field strengths.
Design Algorithms for Parallel Transmission in Magnetic Resonance Imaging
April 11th, Conference Room A, 4:00 pm
PI: Elfar Adalsteinsson
In Parallel transmission multiple RF excitation coils are used simultaneously instead of the single RF coil used on conventional MRI. With these multiple coils, RF pulse duration can be significantly shorten/accelerate. This results in the possibility of realizing more complex excitation patterns which can be used in e.g. spatial modulation of the B1+ excitation profile to mitigate RF field inhomogeneity at high-field, and in creating high resolution image of a particular target region.
The focus of this work is on the algorithmic design and implementation of parallel transmission technology. Design improvements and extensions were made to prior parallel RF transmission algorithms resulting in better excitation control, lower RF power requirement, extension to large tip angle excitation designs, and a new algorithm for spectral-spatial excitation that can be use to aid the quantification of brain metabolites, important in neuro-degenerative diseases. In conjunction to this, implementation of parallel transmission technology was accomplished on two MRI systems, at 3 T and 7 T, where experiments were carried out on both phantoms and human subjects to validate the designs and demonstrate the ability of the technique to effectively mitigate B1+ inhomogeneity at high-field.
Single Shot Echo Volumar Imaging (EVI) and Stimulus Induced Rotary Saturation (SIRS) contrast imaging
March 21st, Conference Room A, 4:30 pm
PI: Larry Wald
EVI is an extension of Echo Planar Imaging (EPI) into three dimensions, where a three-dimensional k-space is read out in a single excitation. While Peter Mansfiled has proposed EVI over thirty years ago along with EPI, single shot EVI has remained theory ever since because readouts of the required length were not practically possible. With the advent of parallel detection in MRI accelerated imaging techniques such as SENSE and GRAPPA were introduced and with increasing size of the coil arrays the possible acceleration factors increased as well. At the Martinos center we now have the ability to use acceleration factors of 12 and above, which makes single shot EVI become realistic. I'll show some initial results obtained of single shot acquisitions with volume sizes of up to 64x64x48.
SIRS is a new contrast mechanism which could potentially be used to detect neuronal currents directly using MRI. I'll give a brief overview over previous efforts by other researchers to detect neuronal currents, an introduction on the rotary saturation effect, and explain how it could be used to detect neuronal currents. I will also show some initial phantom results obtained with our SIRS pulse sequence.