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Register a new userFlexible water excitation for fat-free MRI at 3 Tesla using lipid insensitive binomial off-resonant RF excitation (LIBRE) pulses
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Added by
Jessica A.M. Bastiaansen
Publication date
2019
fields
Physics
and research's language is
English
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Purpose: To develop a robust and flexible low power water excitation pulse that enables effective fat suppression at high magnetic field strength. Methods: A water excitation method that uses spatially non-selective pulses was optimized in numerical simulations, and implemented and tested in phantoms and healthy volunteers at 3T. The lipid insensitive binomial off-resonant excitation (LIBRE) pulse comprises two low power rectangular sub-pulses that have a variable frequency offset, phase offset and duration. The capability and extent of LIBRE fat suppression was quantitatively compared with conventional fat saturation (FS) and water excitation (WE) techniques. Results: LIBRE enables simultaneous water excitation and near complete fat suppression in large volumes at 3T as demonstrated by numerical simulations, and experiments. In phantoms and in human subjects, the frequency responses matched well with those from the numerical simulation. Comparing FS and WE, LIBRE demonstrated an improved robustness to magnetic field inhomogeneities, and a much more effectively suppressed fat signal. This applied for a range of pulse durations and pulses as short as 1.4 ms. Conclusion: A flexible water excitation method was developed that shows robust, near complete fat suppression at 3T.
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Robust and homogeneous lipid suppression is mandatory for coronary magnetic resonance angiography (MRA) since coronary arteries are commonly embedded in fat. However, effective large volume lipid suppression becomes challenging when performing radial whole-heart coronary MRA and the problem may even be exacerbated at increasing magnetic field strengths. Incomplete fat suppression also generates artifacts, and may affect advanced motion correction methods. The aim was to evaluate a recently reported lipid insensitive MRI method for self-navigated coronary MRA at 3T. Lipid insensitive binomial off resonant excitation (LIBRE) radiofrequency (RF) excitation pulses were included into a self-navigated 3D radial GRE coronary MRA sequence at 3T. LIBRE was compared against conventional fat saturation (FS) and binomial 1-180{deg}-1 water excitation (WE). First, fat suppression of all techniques was numerically characterized using Matlab and experimentally validated in phantoms and in legs of human volunteers. Subsequently, free-breathing self-navigated coronary MRA was performed using the LIBRE pulse as well as FS and WE in ten volunteers. Results obtained in the simulations were confirmed by the experimental validations as LIBRE enabled near complete fat suppression for 3D radial imaging in vitro and in vivo. For self-navigated whole-heart coronary MRA at 3T, fat SNR was significantly attenuated using LIBRE compared with conventional FS. LIBRE increased the RCA vessel sharpness significantly (37 +/- 9% (LIBRE) vs. 29 +/- 8% (FS) and 30 +/- 8% (WE), both p<0.05) and led to a significant increase in the measured RCA vessel length to (83 +/- 31 mm (LIBRE) vs. 56 +/- 12 mm (FS) and 59 +/- 27 (WE) p<0.05). LIBRE enables robust large volume fat suppression and significantly improves coronary artery image quality at 3T compared to the use of conventional fat suppression and water excitation.
Here we propose double-coil setup to allow high signal-to-noise ratio broad-range heteronuclear magnetic resonance imaging experiments: two independent coils, one of them tuned to $^{1}$H frequency to perform anatomical $^{1}$H imaging, and another one, metamaterial-inspired coil, tuned to the X-nucleus frequency. In this work our goal was to design a broad-range X-nuclei coil to cover $^{2}$H, $^{11}$B, $^{13}$C, $^{23}$Na, $^{7}$Li and $^{31}$P nuclear magnetic resonance frequencies, and to combine it with $^{1}$H coil in one setup. The system was designed for 11.7 T scanner, i.e., with 76-203 MHz frequency tuning range for the X-nuclei and tuned to 500 MHz for the proton coil. X-nuclei coil operates via excitation of the fundamental eigenmode of an array of parallel non-magnetic wires. The excitation of the array is provided via non-resonant feeding loop inductively coupled to the resonator. In order to tune the X-coil over such a wide range, both structural capacitance and inductance of the coil were made variable; narrow range tuning of the $^{1}$H coil is achieved via conventional tuning-matching circuit. Here, the design principle and setup tunability were investigated in simulations and experimentally.
Dixon MRI is widely used for body composition studies. Current processing methods associated with large whole-body volumes are time intensive and prone to artifacts during fat-water separation performed on the scanner, making the data difficult to analyse. The most common artifact are fat-water swaps, where the labels are inverted at the voxel level. It is common for researchers to discard swapped data (generally around 10%), which can be wasteful and lead to unintended biases. The UK Biobank is acquiring Dixon MRI for over 100,000 participants, and thousands of swaps will occur. If those go undetected, errors will propagate into processes such as abdominal organ segmentation and dilute the results in population-based analyses. There is a clear need for a fast and robust method to accurately separate fat and water channels. In this work we propose such a method based on style transfer using a conditional generative adversarial network. We also introduce a new Dixon loss function for the generator model. Using data from the UK Biobank Dixon MRI, our model is able to predict highly accurate fat and water channels that are free from artifacts. We show that the model separates fat and water channels using either single input (in-phase) or dual input (in-phase and opposed-phase), with the latter producing improved results. Our proposed method enables faster and more accurate downstream analysis of body composition from Dixon MRI in population studies by eliminating the need for visual inspection or discarding data due to fat-water swaps.
It is widely recognized that a physical system can only respond to a periodic driving significantly when the driving frequency matches the normal mode frequency of the system, which leads to resonance. Off-resonant phenomena are rarely considered because of the difficulty to realize strong coupling between physical systems and off-resonant waves. Here we examine the response of a magnetic system to squeezed light and surprisingly find that the magnons are maximally excited when the effective driving frequency is several orders of magnitude larger than the resonant frequency. The generated magnons are squeezed which brings the advantage of tunable squeezing through an external magnetic field. Furthermore, we demonstrate that such off-resonant quasi-particle excitation is universal in all the hybrid systems in which the coherent and parametric interaction of bosons exists and that it is purely a quantum effect, which is rooted in the quantum fluctuations of particles in the squeezed vacuum. Our findings may provide an unconventional route to study off-resonant phenomena and may further benefit the use of hybrid matter-light systems in continuous variable quantum information.
Optical pumping of He-3 produces large (hyper) nuclear-spin polarizations independent of the magnetic resonance imaging (MRI) field strength. This allows lung MRI to be performed at reduced fields with many associated benefits, such as lower tissue susceptibility gradients and decreased power absorption rates. Here we present results of 2D imaging as well as accurate 1D gas diffusion mapping of the human lung using He-3 at very low field (3 mT). Furthermore, measurements of transverse relaxation in zero applied gradient are shown to accurately track pulmonary oxygen partial pressure, opening the way for novel imaging sequences.
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