A unifying view on extended phase graphs and Bloch simulations for quantitative MRI

Quantitative MRI methods and learning-based algorithms require exact forward simulations. One critical factor to correctly describe magnetization dynamics is the effect of slice-selective RF pulses. While contemporary simulation techniques correctly capture their influence, they only provide final magnetization distributions, require to be run for each parameter set separately, and make it hard to derive general theoretical conclusions and to generate a fundamental understanding of echo formation in the presence of slice-profile effects. This work aims to provide a mathematically exact framework, which is equally intuitive as extended phase graphs (EPGs), but also considers slice-profiles through their natural spatial representation. We show, through an analytical, hybrid Bloch-EPG formalism, that the spatially-resolved EPG approach allows to exactly predict the signal dependency on off-resonance, spoiling moment, microscopic dephasing, and echo time. We also demonstrate that our formalism allows to use the same phase graph to simulate both gradient-spoiled and balanced SSFP-based MR sequences. We present a derivation of the formalism and identify the connection to existing methods, i.e. slice-selective Bloch, slice-selective EPG, and the partitioned EPG. As a use case, the proposed hybrid Bloch-EPG framework is applied to MR Fingerprinting.

High-Fidelity Control of Spin Ensemble Dynamics via Artificial Intelligence: From Quantum Computing to Imaging

High-fidelity control of spin ensemble dynamics is essential to many fields, spanning from quantum computing to optical, coherent, and nuclear magnetic resonance (NMR) spectroscopy and imaging (MRI). However, attaining robust and high-fidelity quantum spin operations remains an unmet challenge. Using a combination of an evolutionary algorithm and artificial intelligence, we designed time-optimal, radio frequency (RF) pulses with tunable spatial or temporal field inhomogeneity compensation and fidelity for unitary operations up to 0.9999. As a benchmark, we constructed a spin entanglement operator and a programmable quantum state creator for a weakly-coupled two-spin-1/2 system. We achieved high-fidelity transformations under multiple inhomogeneity sources. The newly designed RF pulses are more robust and less prone to imperfection than the commonly used shapes for basic liquid-state NMR experiments and reduce RF artifacts in MRI. This new strategy will enable the design of efficient quantum computing opera-tors as well as new spectroscopic and imaging techniques.

Durability of mitral isthmus ablation with and without ethanol infusion in the vein of Marshall

Background: It is established that ethanol infusion in the vein of Marshall (EIVOM) effectively creates a linear ablation lesion in the mitral isthmus (MI). However, data on the long-term success rates of MI ablation remains limited. Methods and Results: Our cohort consisted of 560 patients with non-paroxysmal atrial fibrillation (AF) who underwent an initial MI ablation. Ablations were performed by only radiofrequency (RF) in 384 patients (RF group) or by RF and EIVOM in 176 patients (EIVOM/RF group). Ethanol of 5 mL was used to perform EIVOM in advance of RF. Following EIVOM, RF pulses were delivered to the lateral MI line. Bidirectional MI block was fully achieved in 353 (first 318, re-do 35) patients of the RF group and 171 (first 128, re-do 43) patients of the EIVOM/RF group (p = 0.09 in the first, 0.10 in the re-do ablation cases). In cases with complete MI line block, recurrent AF or atrial tachycardia (AT) was observed in 130 (37%) patients of the RF group and in 64 (37%) patients of the EIVOM/RF group (log-rank p = 0.12 in the first, 0.30 in the re-do ablation cases). Of the total 194 patients, 112 with drug refractory AF or AT proceeded to the subsequent ablation process. Reconnection of MI block line was observed in 39 (49%) patients in the RF group and 25 (58%) patients in the EIVOM/RF group (p = 0.32). Conclusion: EIVOM effectively ensures MI line block; however, the reconnection rate was similar between the two groups.

Optimized Inner-Volume 3D TSE for High-Resolution Vessel Wall Imaging of Intracranial Perforating Arteries at 7T

The impairment of microvessels can lead to neurologic diseases such as stroke and vascular dementia. The imaging of lumen and vessel wall of perforating arteries requires an extremely high resolution due to their small caliber size. Current imaging techniques have the difficulty in observing the wall of perforating arteries. In this study, we developed a 3D inner-volume (IV) TSE (SPACE) sequence with optimized 2D spatially selective excitation (SSE) RF pulses. The optimized SSE RF pulses were designed through a series of optimization including iterative RF pulse design, trajectory optimization, and phase convention of Carr-Purcell-Meiboom-Gill (CPMG) condition to meet the perforating arteries imaging demands. High resolution of isotropic 0.30 mm within 10 min was achieved for the black- blood images of lenticulostriate artery (LSA). The LSA lumen and vessel wall were imaged by the IV-SPACE sequence simultaneously. Images obtained by the optimized RF pulse has fewer aliasing artifacts from outside of ROI than the traditional pulse. The IV-SPACE images showed clearer delineation of vessel wall and lumen of LSA than conventional SPACE images. IV-SPACE might be a promising method for detecting microvasculopathies of cerebral vascular diseases.

GABA quantification in human anterior cingulate cortex

γ-Aminobutyric acid (GABA) is a primary inhibitory neurotransmitter in the human brain. It has been shown that altered GABA concentration plays an important role in a variety of psychiatric and neurological disorders. The main purpose of this study was to propose a combination of PRESS and MEGA-PRESS acquisitions for absolute GABA quantification and to compare GABA estimations obtained using total choline (tCho), total creatine (tCr), and total N-acetyl aspartate (tNAA) as the internal concentration references with water referenced quantification. The second aim was to demonstrate the fitting approach of MEGA-PRESS spectra with QuasarX algorithm using a basis set of GABA, glutamate, glutamine, and NAA in vitro spectra. Thirteen volunteers were scanned with the MEGA-PRESS sequence at 3T. Interleaved water referencing was used for quantification, B0 drift correction and to update the carrier frequency of RF pulses in real time. Reference metabolite concentrations were acquired using a PRESS sequence with short TE (30 ms) and long TR (5000 ms). Absolute concentration were corrected for cerebrospinal fluid, gray and white matter water fractions and relaxation effects. Water referenced GABA estimations were significantly higher compared to the values obtained by metabolite references. We conclude that QuasarX algorithm together with the basis set of in vitro spectra improves reliability of GABA+ fitting. The proposed GABA quantification method with PRESS and MEGA-PRESS acquisitions enables the utilization of tCho, tCr, and tNAA as internal concentration references. The use of different concentration references have a good potential to improve the reliability of GABA estimation.

Nonhomogeneous force application during typical flutter ablation explains local difficulties in lesion creation

Introduction – The current scientific literature suggests similar or even better catheter contact on caval regions of the cavo-tricuspid isthmus (CTI) compared to the more medial or annular part. Yet CTI ablation can be challenging owing to instability at the inferior vena cava (IVC) edge. No study specifically addressed the issue of catheter-tissue contact on caval/mid/annular regions of the CTI. Methods – Twenty-seven patients (22 men, 67±12 years) underwent typical atrial flutter ablation with a contact force (CF) sensing catheter (Tacticath 75, Abott) and were prospectively included. Operators aimed at optimizing CF for all RF pulses. The product of CF, time (force-time integral; FTI) and delivered power (FTPI) has been proposed as an estimate of lesion size. In a subset of 8 consecutive pts, electrograms (EGM) of all RF lesions were analyzed. Annular CTI was defined as sites showing both atrial and ventricular near-field bipolar EGM. Mid CTI sites had only atrial near field bipolar EGM on both distal and proximal dipoles (or distal only in case of superior-to-inferior approach). Caval sites had near-field bipolar EGM only in distal dipole, while the proximal one was inferiorly located. Results – Complete persistent (at 30 min) CTI block was obtained in all patients. A steerable sheath was used in 12 pts (44%). Procedure duration was 93±30 min, RF delivery time 10±6 min, fluoroscopy time 14±8 min. Mean CF was 15.8±5.9 g. CF was significantly lower (11.1±9.7 g) at the caval CTI than at mid CTI (19.9±11.8 g) and annular CTI (20±12.2 g; p=0.001). CF and FTI were higher during sinus rhythm than during Fl (23.2±15.3 vs 18.5±15.4 g, p=0.04 and 677±432 vs 532±357 gs, p=0.03). Use of a sheath improved mean CF (24.4±12.5 vs 18.4±14.9 g, p=0.01) but this was not significant at caval sites (17.5±11.4 vs 12.6±10.7 g, p=0.31). Conclusion – Applied forces are significantly lower at the IVC edge during CTI ablation. This supports the use of a steerable sheath in challenging cases.

Simulated basis sets for semi-LASER: the impact of including shaped RF pulses and magnetic field gradients

Objective To study the need for inclusion of shaped RF pulses and magnetic field gradients in simulations of basis sets for the analysis of proton MR spectra of single voxels of the brain acquired with a semi-LASER pulse sequence. Materials and methods MRS basis sets where simulated at different echo times with hard RF pulses as well as with shaped RF pulses without or with magnetic field gradients included. The influence on metabolite concentration quantification was assessed using both phantom and in vivo measurements. For comparison, simulations and measurements were performed with the PRESS pulse sequence. Results The effect of including gradients in the simulations was smaller for semi-LASER than for PRESS, however, still noticeable. The difference was larger for strongly coupled metabolites and at longer echo times. Metabolite quantification using semi-LASER was thereby less dependent on the inclusion of gradients than PRESS, which was seen in both phantom and in vivo measurements. Discussion The inclusion of the shaped RF pulses and magnetic field gradients in the simulation of basis sets for semi-LASER is only important for strongly coupled metabolites. If computational time is a limiting factor, simple simulations with hard RF pulses can provide almost as accurate metabolite quantification as those that include the chemical-shift related displacement.

GABA quantification in human anterior cingulate cortex

Abstractγ-Aminobutyric acid (GABA) is a primary inhibitory neurotransmitter in the human brain. It has been shown that altered GABA concentration plays an important role in a variety of psychiatric and neurological disorders. The main purpose of this study was to propose a combination of PRESS and MEGA-PRESS acquisitions for absolute GABA quantification and to compare GABA estimations obtained using total choline (tCho), total creatine (tCr), and total N-acetyl aspartate (tNAA) as the internal concentration references with water referenced quantification. The second aim was to demonstrate the fitting approach of MEGA-PRESS spectra with QuasarX algorithm using a basis set of GABA, glutamate, glutamine, and NAA in vitro spectra. Thirteen volunteers were scanned with the MEGA-PRESS sequence at 3T. Interleaved water referencing was used for quantification, B0 drift correction and to update the carrier frequency of RF pulses in real time. Reference metabolite concentrations were acquired using a PRESS sequence with short TE (30 ms) and long TR (5000 ms). Absolute concentration were corrected for cerebrospinal fluid, gray and white matter water fractions and relaxation effects. Water referenced GABA estimations were significantly higher compared to the values obtained by metabolite references. We conclude that QuasarX algorithm together with the basis set of in vitro spectra improves reliability of GABA+ fitting. The proposed GABA quantification method with PRESS and MEGA-PRESS acquisitions enables the utilization of tCho, tCr, and tNAA as internal concentration references. The use of different concentration references have a good potential to improve the reliability of GABA estimation.

Radiofrequency generator and programmer of pulse sequences for PMR relaxometer

Described the generator of resonance radiofrequency with the step of change 50 kHz in the range 520 MHz and pulse programmer of the pulse sequences for relaxometer of proton magnetic resonance (PMR). Using algorithm of direct digital synthesis (DSP), the RF generator has the ability to produce RF signal with a short switching time and high resolution in frequency and phase. To facilitate the generation of RF pulses, used the pulse programmer implemented in the FPGA directed by the auxiliary controller DSP to generate RF pulses with required sequences and parameters. Realized the testing of the generato r by method of computer modelling in the program Quartus II 12.1 and was shown the opportunities of synthesis of RF-pulses sequences with the phases 0, π/2, π, 3π/2 with resonance frequency tuning.