Keywords: RF Arrays & Systems, RF Arrays & Systems, Transmit radiation, Birdcage coil, Faraday cage

Motivation: Eliminating the Faraday cage would lower installation costs by ~2X and facilitate deployment of MRI in diverse settings, but requires reducing the Tx system’s electromagnetic (EM) radiation.

Goal(s): We employ a parallel transmit (pTx) array and EM absorbers to reduce RF-radiation from a 3T MRI operating without a shielded room, ensuring operation within regulatory limits.

Approach: We model a 3T MRI with a CP body coil and design a 16-channel pTx array and EM-absorber to reduce E-field radiation. Performance is assessed with pTx pulse optimization and L-curve analysis.

Results: We demonstrate a 2270-fold reduction in radiation compared to the birdcage coil.

Impact: By successfully mitigating RF-radiation from MRI systems operated without Faraday cages, this research advances cost-effective MRI installations in diverse clinical/research environments, encouraging further exploration of pTx technology for controlling electromagnetic fields both inside/outside of the body.

Keywords: RF Pulse Design & Fields, Heart, parallel transmission, 7T, ultra-high field MRI

Motivation: Parallel transmission (pTx) pulses with fixed gradient trajectory such as k_T-points have successfully demonstrated to benefit cardiac imaging at 7T. However, it is possible to optimize pTx pulses without restrictions regarding RF and gradient waveforms.

Goal(s): Our aim was to optimize first pTx pulses with free RF and gradient trajectory for cardiac imaging.

Approach: We designed tailored free pulses for 35 heart subjects and compared them in terms of flip angle (FA) homogeneity and applied energy dose to k_T-point pulses.  

Results: Comparable performance in FA homogeneity was obtained for both pulse types, but the free pulses additionally achieved a reduction of SED.

Impact: Generating homogeneous excitation while complying with RF power deposition regulations is a challenge for cardiac imaging at 7T. Designing pTx pulses with free RF and gradient waveforms allows for reduced SED and similar homogeneity compared to k_T-point pulses.

Keywords: Parallel Transmit & Multiband, Parallel Transmit & Multiband

Motivation: Last year we demonstrated the utility of our new parallel transmit spatial-spectral pulse design for robust water excitation, but only using Bloch simulations without experimental proof.

Goal(s): Our goal here was to validate our method via phantom and human scans at 7 Tesla.

Approach: We validated our design method by collecting 3D GRE data in a water-fat phantom and in the human brain. All data were obtained on a Siemens Terra using the commercial Nova 8-channel transmit coil.

Results: Our method outperformed existing approaches, producing uniform water excitation across the whole brain with nearly complete fat suppression even in the challenging areas.

Impact: Validated in humans at 7 Tesla, our design method provides an effective solution for volumetric uniform water excitation, eliminating the need for additional fat saturation and holding a promise to many applications including high-resolution functional MRI at ultrahigh field.

Keywords: Parallel Transmit & Multiband, Parallel Transmit & Multiband

Motivation: Subject-specific pTx methods can counteract excitation inhomogeneities in UHF body imaging, but require lengthy calibration times. Calibrationless universal approaches exist but yield reduced homogeneity for similar SAR levels.

Goal(s): Our aim was to evaluate the possibility of shortening the calibration time by accelerating the B₁⁺-mapping process.

Approach: For 35 heart subjects, tailored pTx pulses were designed on B₁⁺-maps with different undersampling and their performance was compared to universal approaches regarding flip angle homogeneity and SAR.

Results: Tailored pulses designed on accelerated, undersampled B₁⁺-maps (TA=30s) result in improved homogeneity compared to universal pulses or in a roughly 2.5-fold reduction of SAR at comparable homogeneity.

Impact: Subject-specific pTx pulses optimized on accelerated, undersampled B₁⁺-maps allow for improved homogeneity or reduced SAR compared to universal pulses. Reduced SAR could benefit body imaging at 7T, since SAR limitations often prevent the optimal choice of acquisition parameters at UHF.

Keywords: Parallel Transmit & Multiband, Parallel Transmit & Multiband

Motivation: Optimization of parallel transmission (pTx) pulse design with hard constraints on SAR will benefit from faster approaches.

Goal(s): We seek to incorporate hard quadratic constraints for pTx using a physics-driven deep learning (DL) approach.

Approach: We unroll an extension of the log-barrier method to enforce SAR constraints, while learning the optimal gradient step sizes using a neural network. This strategy accelerates optimization with fewer steps, while not sacrificing performance.

Results: Preliminary results show that our method is faster than traditional techniques like CVXPY with similar performance.

Impact: Our proposed method reduces the time-consuming optimization used in conventional pTx and may lead to improvements especially for real-time UHF applications.

Keywords: RF Pulse Design & Fields, High-Field MRI

Motivation: In ultra-high field MRI, transmit magnetic field (B1+) inhomogeneities are affecting kidney imaging. 

Goal(s): Our goal was evaluate dynamic kT-point pulses for homogenous excitation in bilateral renal imaging.

Approach: Channel-wise B1+ maps of the kidneys were acquired for 15 subjects. Universal and individual kT-point pulses with different number of subpulses were calculated and evaluated. For one subject anatomical images were acquired with phase shims and dynamic kT-point pulses.

Results: While fixed-phase shims still suffer from flip-angle inhomogeneity, universal and subject-specific dynamic pulses with non-selective 5kT-point trajectories improves flip-angle homogeneity in the kidney and enable imaging with homogeneous excitation. 

Impact: Our study evaluated universal and subject-specific kT-point pulses for kidney MRI at 7T. kT-point pulses substantially improve flip-angle homogeneity and universal pulses enable calibration-free imaging at ultra-high field, promising advances in high-resolution renal imaging.

Keywords: RF Pulse Design & Fields, High-Field MRI, motion

Motivation: Low-field motion-tracking methods are insufficient at ultrahigh-field as motion also affects flip-angle .

Goal(s): Our goal was to develop a method that can redesign pulses for ultrahigh-field MRI within 1 second.

Approach: We implemented a method to rapidly recalculate the post-motion basis-functions needed for pulse design and complemented it with dictionary matching to reduce pulse computation times a of small-tip-angle multi-spoke multi-slice parallel-transmit pulse design method.

Results: With basis-functions recalculated in 0.13 seconds/slice and pulses reoptimized in 0.18 seconds/slice, multi-slice multi-spoke parallel-transmit pulses can be redesigned in runtime using the proposed method. Redesigned pulses significantly reduce motion-induced error, yielding consistent excitation with pre-motion excitation.

Impact: A pulse design method is developed that can redesign practical parallel-transmit pulses in under a second. It can correct for motion-related flip angle distortions at ultrahigh-field and will help facilitate scanning of patients who cannot remain still (e.g., paediatric, dementia).

Keywords: RF Pulse Design & Fields, RF Pulse Design & Fields, Reduced FOV; 3D EPI;

Motivation: Universal pulses have been shown to be robust to B1⁺ inhomogeneity for brain imaging at 7T, without time-consuming online design. A similar approach may be useful for designing multidimensional RF pulses for the 3D prostate imaging at 3T.

Goal(s): To design a universal multidimensional refocusing pulse and demonstrate its potential use for reduced field-of-view imaging in the prostate.

Approach: A three-dimensional universal refocusing pulse was designed using 6 subjects and validated in 12 subjects and in vivo prostate imaging at 3T.

Results: The proposed 3D universal refocusing pulse achieved similar performance on seen and unseen subjects. We successfully acquired reduced field-of-view prostate images.

Impact: Our simulation and in vivo results demonstrated the potential to design one universal 3D refocusing pulses for most subjects in prostate imaging.

Keywords: RF Pulse Design & Fields, RF Pulse Design & Fields

Motivation: Adiabatic pulses are popular for wideband inversion or refocusing, but limited by SAR in high-field. Conventional modulated pulses are limited in bandwidth by peak voltage requirements.

Goal(s): Develop an intuitive wideband pulse design method compatible with limited peak voltage for ultra-high field applications.

Approach: We construct a “multiband” pulse covering the desired bandwidth. We use time-shifting to reduce the peak voltage.

Results: We illustrate by designing a 4.4kHz bandwidth pulse, which has similar peak amplitude but better B₁ robustness than a root-flipped SLR pulse. We use this pulse for semi-LASER MRS in a phantom, giving similar spectra to an adiabatic hypersecant pulse.

Impact: We introduce a time-shifted multiband method to design wideband pulses that is fast and intuitive to construct and avoids the tradeoff between RF peak amplitude and bandwidth. These pulses are useful for inversion or refocusing in twice-refocused sequences like semi-LASER.

Keywords: RF Pulse Design & Fields, RF Pulse Design & Fields

Motivation: Voxel-wise objective function with auto-differentiation for RF pulse optimization has become prevalent. While benefit from the spatial flexibility of desired pattern, conventional fixed-point representation of a matrix fed into the voxel-wise objective function leads to sub-optimal and undesired resultant profile at courser resolution. 

Goal(s): This abstract aims to address the issues of fixed-point artifact in the resultant excitation/refocusing profile.

Approach: We proposed a method called stochastic-offset-approach which assigns random spatial offsets to each point centered at voxel. The objective function is evaluated on this modified set of points. 

Results: The results show that our proposed method achieve superior performance compared with fixed-point method. 

Impact: This method would significantly speed up and improve the performance of optimizations with voxel-wise objective function evaluated at fixed points under coarser resolution. 

Keywords: Multiple Sclerosis, Parallel Transmit & Multiband

Motivation: 3D TSE sequences at 7T suffer from poor homogeneity, signal dropouts and local SAR limits.

Goal(s): Clinically acceptable image quality using scalable dynamic parallel transmit (pTx) pulses under SAR-constraints

Approach: Prior to the scan, a dictionary of preoptimized, symmetric pTx pulses is built. At the scanner, the best pulse for that subject is identified and serves as initialization for a then fast individual optimization constrained to maximum SAR, maximum voltage and temporal symmetry. 

Results: Clinically acceptable image homogeneity is achieved with two different pulses at the expense of 0.4ms/0.9ms longer pulse duration and 220%/14% higher SAR than the commonly used 1Tx pulse.

Impact: A workflow to design customized and arbitrarily scalable pTx pulses is demonstrated, enabling homogeneous 3D TSE imaging of the head with variable flip angles at 7T. Various flip angle trains can be applied as flexibly as on 1Tx systems.

Keywords: RF Pulse Design & Fields, Quantitative Imaging, T1rho ; pulsed spinlock;pulse sequence;

Motivation: Conventional T1rho techniques require sufficiently long TSL (time of spinlock) to ensure reliable T1rho quantification, However, maximum TSL allowed in clinical MR scanners is often limited by SAR (specific absorption rate) and RF amplifier to ensure patient safety and prevent damage to the scanners.

Goal(s): Our proposed toggling inversion preparation pulsed spinlock mitigates the problem by employing a train of spinlock pulses with a gap duration between two pulses.

Approach: We Confirmed our conjectures using simulation, phantom, and in vivo experiment.

Results: Our approach can achieve reliable T1rho quantification using longer TSL compared to the conventional spinlock technique.

Impact: The Proposed method has potential to enable T1rho imaging of tissue with relatively long relaxation time and at MRI system where continus spin-lock is challenging.

Keywords: RF Pulse Design & Fields, RF Pulse Design & Fields, Multiphoton, SMS, SAR, PINS, MultiPINS

Motivation: Elevated specific absorption rate (SAR) levels pose a significant challenge for simultaneous multislice (SMS) imaging especially at high field strengths.

Goal(s): Multiphoton excitation has recently been proposed for reduced-SAR SMS applications. We aim to evaluate the SAR benefit of multiphoton SMS imaging compared to PINS and MultiPINS.

Approach: We show how pulse parameters affect the efficiencies of multiphoton SMS, PINS, and MultiPINS compared to conventional SMS through simple calculations and simulations. Additionally, we implement a multiphoton SMS spin-echo sequence in vivo at 3T.

Results: Multiphoton SMS is more SAR-efficient than PINS and MultiPINS for short pulse durations and thin slices under slew-rate constraints.

Impact: Multiphoton excitation makes short pulse durations and thin slices possible for SMS applications under SAR and slew-rate constraints. This subsequently enables the acquisition of high-quality imaging data for both scientific and medical purposes while reducing scan times.

Keywords: RF Pulse Design & Fields, RF Pulse Design & Fields

Motivation: Adiabatic BIR-4 pulses, employed for $$$B_1^+$$$ robust spectroscopic excitation, exhibit frequency-dependent modulations in received signals, especially at low tilt angles. These modulations interfere with accurate quantitative analysis.

Goal(s): The study aims to examine BIR-4 overshoots at low $$$B_1^+$$$ and introduces optimal control RF pulses to mitigate these issues.

Approach: We investigate and compare BIR-4 and optimal control RF pulses in simulations, phantom experiments, and in-vivo ³¹P spectroscopy.

Results: We show that optimal control RF pulse design is imperative for obtaining accurate quantitative data. Optimal control RF pulses have the potential to significantly improve in-vivo ³¹P magnetic resonance spectroscopy.

Impact: Optimal control pulses offer precise excitation, surpassing BIR-4 under low flip angles and challenging transmit conditions. This ensures a stable magnetization steady-state, vital for accurate quantitative analysis in applications such as enzymatic exchange rate measurement via magnetization transfer spectroscopy.

Keywords: RF Pulse Design & Fields, Cardiovascular

Motivation: Large field-of-views in body imaging, e.g. coronary magnetic resonance angiography (MRA), are a limiting factor for short scan times, but this can be mitigated with outer volume suppression (OVS) and high parallel imaging acceleration rates.

Goal(s): Evaluate the use of adiabatic pulses for OVS in accelerated coronary MRA.

Approach: Two single-band adiabatic pulses and a dual-band adiabatic (DBA) pulse were optimized in terms of stop-band flip angle and evaluated in spoiled GRE-based coronary MRA.

Results: Adiabatic pulses demonstrate thorough signal suppression in stop-bands and a significant reduction of fold-over artifacts in vivo, with DBA bearing the promise of slightly improved homogeneity.

Impact: Single and dual-band adiabatic pulses provide robust outer volume suppression in accelerated coronary imaging, successfully mitigating residual fold-over artifacts.

Keywords: RF Pulse Design & Fields, RF Arrays & Systems

Motivation: There is a lack of tools for predicting the component values for such irregular wireless volume coils.

Goal(s): The goal is to propose a novel co-simulation method that accurately predicts the capacitance distribution of irregular volume coils.

Approach: We validated this method using various shapes of irregular volume coils, including bottle-shaped, dome-shaped, and elliptical coils, at both 1.5 T and 3 T.

Results: The consistency between the simulated and practical capacitance further confirms the accuracy and reliability of the proposed probe-based co-simulation methods.

Impact: This co-simulation method can guide fabricating wireless irregular volume coils and can be extended to other types of wireless coils.