Fast online spectral-spatial pulse design for subject-specific fat saturation in cervical spine and foot imaging at 1.5 T

OBJECTIVE

To compensate subject-specific field inhomogeneities and enhance fat pre-saturation with a fast online individual spectral-spatial (SPSP) single-channel pulse design.

METHODS

The RF shape is calculated online using subject-specific field maps and a predefined excitation k-space trajectory. Calculation acceleration options are explored to increase clinical viability. Four optimization configurations are compared to a standard Gaussian spectral selective pre-saturation pulse and to a Dixon acquisition using phantom and volunteer (N = 5) data at 1.5 T with a turbo spin echo (TSE) sequence. Measurements and simulations are conducted across various body parts and image orientations.

RESULTS

Phantom measurements demonstrate up to a 3.5-fold reduction in residual fat signal compared to Gaussian fat saturation. In vivo evaluations show improvements up to sixfold for dorsal subcutaneous fat in sagittal cervical spine acquisitions. The versatility of the tailored trajectory is confirmed through sagittal foot/ankle, coronal, and transversal cervical spine experiments. Additional measurements indicate that excitation field (B1) information can be disregarded at 1.5 T. Acceleration methods reduce computation time to a few seconds.

DISCUSSION

An individual pulse design that primarily compensates for main field (B0) inhomogeneities in fat pre-saturation is successfully implemented within an online "push-button" workflow. Both fat saturation homogeneity and the level of suppression are improved.

Water irradiation devoid pulses enhance the sensitivity of 1H,1H nuclear Overhauser effects

The nuclear Overhauser effect (NOE) is one of NMR spectroscopy's most important and versatile parameters. NOE is routinely utilized to determine the structures of medium-to-large size biomolecules and characterize protein-protein, protein-RNA, protein-DNA, and protein-ligand interactions in aqueous solutions. Typical [¹H,¹H] NOESY pulse sequences incorporate water suppression schemes to reduce the water signal that dominates ¹H-detected spectra and minimize NOE intensity losses due to unwanted polarization exchange between water and labile protons. However, at high- and ultra-high magnetic fields, the excitation of the water signal during the execution of the NOESY pulse sequences may cause significant attenuation of NOE cross-peak intensities. Using an evolutionary algorithm coupled with artificial intelligence, we recently designed high-fidelity pulses [Water irrAdiation DEvoid (WADE) pulses] that elude water excitation and irradiate broader bandwidths relative to commonly used pulses. Here, we demonstrate that WADE pulses, implemented into the 2D [¹H,¹H] NOESY experiments, increase the intensity of the NOE cross-peaks for labile and, to a lesser extent, non-exchangeable protons. We applied the new 2D [¹H,¹H] WADE-NOESY pulse sequence to two well-folded, medium-size proteins, i.e., the K48C mutant of ubiquitin and the Raf kinase inhibitor protein. We observed a net increase of the NOE intensities varying from 30 to 170% compared to the commonly used NOESY experiments. The new WADE pulses can be easily engineered into 2D and 3D homo- and hetero-nuclear NOESY pulse sequences to boost their sensitivity.

Analytical models of probe dynamics effects on NMR measurements

This paper provides a detailed analysis of three common NMR probe circuits (untuned, tuned, and impedance-matched) and studies their effects on multi-pulse experiments, such as those based on the Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence. The magnitude of probe dynamics effects on broadband refocusing pulses are studied as a function of normalized RF bandwidth. Finally, the probe circuit models are integrated with spin dynamics simulations to design hardware-specific RF excitation and refocusing pulses for optimizing user-specified metrics such as signal-to-noise ratio (SNR) in grossly inhomogeneous fields. Preliminary experimental results on untuned probes are also presented.

Constant gradient elastography with optimal control RF pulses

This article presents a new motion encoding strategy to perform magnetic resonance elastography (MRE). Instead of using standard motion encoding gradients, a tailored RF pulse is designed to simultaneously perform selective excitation and motion encoding in presence of a constant gradient. The RF pulse is designed with a numerical optimal control algorithm, in order to obtain a magnetization phase distribution that depends on the displacement characteristics inside each voxel. As a consequence, no post-excitation encoding gradients are required. This offers numerous advantages, such as reducing eddy current artifacts, and relaxing the constraint on the gradients maximum switch rate. It also allows to perform MRE with ultra-short TE acquisition schemes, which limits T₂ decay and optimizes signal-to-noise ratio. The pulse design strategy is developed and analytically analyzed to clarify the encoding mechanism. Finally, simulations, phantom and ex vivo experiments show that phase-to-noise ratios are improved when compared to standard MRE encoding strategies.

Analytical optimization of active bandwidth and quality factor for TOCSY experiments in NMR spectroscopy

Active bandwidth and global quality factor are the two main metrics used to quantitatively compare the performance of TOCSY mixing sequences. Active bandwidth refers to the spectral region over which at least 50 % of the magnetization is transferred via a coupling. Global quality factor scores mixing sequences according to the worst-case transfer over a range of possible mixing times and chemical shifts. Both metrics reward high transfer efficiency away from the main diagonal of a two-dimensional spectrum. They can therefore be used to design mixing sequences that will function favorably in experiments. Here, we develop optimization methods tailored to these two metrics, including precise control of off-diagonal cross peak buildup rates. These methods produce square shaped transfer efficiency profiles, directly matching the desirable properties that the metrics are intended to measure. The optimization methods are analytical, rather than numerical. The two resultant shaped pulses have significantly higher active bandwidth and quality factor, respectively, than all other known sequences. They are therefore highly suitable for use in NMR spectroscopy. We include experimental verification of these improved waveforms on small molecule and protein samples.

Efficient high-resolution RF pulse design applied to simultaneous multi-slice excitation

RF pulse design via optimal control is typically based on gradient and quasi-Newton approaches and therefore suffers from slow convergence. We present a flexible and highly efficient method that uses exact second-order information within a globally convergent trust-region CG-Newton method to yield an improved convergence rate. The approach is applied to the design of RF pulses for single- and simultaneous multi-slice (SMS) excitation and validated using phantom and in vivo experiments on a 3T scanner using a modified gradient echo sequence.

Design and optimization of pulsed Chemical Exchange Saturation Transfer MRI using a multiobjective genetic algorithm

Pulsed Chemical Exchange Saturation Transfer (CEST) MRI experimental parameters and RF saturation pulse shapes were optimized using a multiobjective genetic algorithm. The optimization was carried out for RF saturation duty cycles of 50% and 90%, and results were compared to continuous wave saturation and Gaussian waveform. In both simulation and phantom experiments, continuous wave saturation performed the best, followed by parameters and shapes optimized by the genetic algorithm and then followed by Gaussian waveform. We have successfully demonstrated that the genetic algorithm is able to optimize pulse CEST parameters and that the results are translatable to clinical scanners.