RF Excitation Pulse Research Articles

Interleaved flow-sensitive dephasing (iFSD): Toward enhanced blood flow suppression and preserved T1 weighting and overall signals in 3D TSE-based neuroimaging.

To develop and validate a 3D turbo spin-echo (TSE)-compatible approach to enhancing black-blood (BB) effects while preserving T1 weighting and overall SNR. Following the excitation RF pulse, a 180° RF pulse sandwiched by a pair of flow-sensitive dephasing (FSD) gradient pulses in the phase- (y) and partition-encoding (z) directions, respectively, is added. The polarity of FSD gradients in z direction is toggled every TR, achieving an interleaved FSD (iFSD) configuration in y-z plane. The technique was optimized and evaluated in 18 healthy volunteers and 32 patients with neurovascular disease or brain metastases. Comparisons were made among TSE with and without one of BB preparations: iFSD, delay alternating with nutation for tailored excitation, and motion-sensitized driven equilibrium. iFSD-TSE achieved the best blood flow suppression indicated by venous sinus SNR and parenchyma-to-sinus contrast-to-noise ratio (CNR). iFSD-TSE yielded slightly lower white matter SNR (106.6 ± 32.9) and white-to-gray matter CNR (27.3 ± 8.1) compared to TSE (111.4 ± 31.5 and 28.6 ± 8.8), which were significantly higher than those of delay alternating with nutation for tailored excitation-prepared TSE (84.3 ± 25.0 and 16.8 ± 4.8) and motion-sensitized driven equilibrium-prepared TSE (77.3 ± 26.6 and 15.9 ± 5.3). At the neurovascular wall lesions, iFSD-TSE yielded the highest wall-to-lumen CNR among the three sequences with a BB preparation, all of which significantly outperformed TSE. iFSD-TSE effectively suppressed slow-flow artifacts that otherwise mimicked an atherosclerotic lesion or strongly contrast-enhancing vessel wall. In diagnosing brain metastases, iFSD allowed for highest inter-reader agreement (κ 0.75) and shortest reading time. iFSD is a promising approach compatible with 3D TSE for robust blood flow suppression and preserved T1 weighting and overall SNR.

Robust dual-angle measurement in magnetization transfer spectroscopy by time-optimal control.

Magnetization transfer spectroscopy relies heavily on the robust determination of relaxation times of nuclei participating in metabolic exchange. Challenges arise due to the use of surface RF coils for transmission (high variation) and the broad resonance band of most X nuclei. These challenges are particularly pronounced when fast mapping methods, such as the dual-angle method, are employed. Consequently, in this work, we develop resonance offset and robust excitation RF pulses for 31P magnetization transfer spectroscopy at 7T through ensemble-based time-optimal control. In our approach, we introduce a cost functional for designing robust pulses, incorporating the full Bloch equations as constraints, which are solved using symmetric operator splitting techniques. The optimal control design of the RF pulses developed demonstrates improved accuracy, desired phase properties, and reduced RF power when applied to dual-angle mapping, thereby improving the precision of exchange-rate measurements, as demonstrated in a preclinical in vivo study quantifying brain creatine kinase activity.

A vendor-neutral EPI sequence for hyperpolarized 13C MRI.

To develop a flexible, vendor-neutral EPI sequence for hyperpolarized 13C metabolic imaging. An open-source EPI sequence consisting of a metabolite-specific spectral-spatial RF excitation pulse and a customizable EPI readout was created using the Pulseq framework. To explore the flexibility of our sequence, we tested several versions of the sequence including a symmetric 3D readout with different spatial resolutions for each metabolite (1.0 cm3 and 1.5 cm3). A multichamber phantom constructed with a Shepp-Logan geometry, containing two chambers filled with either natural abundance 13C compounds or hyperpolarized (HP) [1-13C]pyruvate, was used to test each sequence. For experiments involving HP [1-13C]pyruvate, a single chamber was prefilled with nicotinamide adenine dinucleotide hydride and lactate dehydrogenase to facilitate the conversion of [1-13C]pyruvate to [1-13C]lactate. All experiments were performed on a Siemens Prisma 3T scanner. All the sequence variations localized natural-abundance 13C ethylene glycol and methanol to the appropriate compartment of the multichamber phantom. [1-13C]pyruvate was detectable in both chambers following the injection of HP [1-13C]pyruvate, whereas [1-13C]lactate was only found in the chamber containing nicotinamide adenine dinucleotide hydride and lactate dehydrogenase. The conversion rate from [1-13C]pyruvate to [1-13C]lactate (kPL) was 0.01 s-1 (95% confidence interval [0.00, 0.02]). We have developed and tested a vendor-neutral EPI sequence for imaging HP 13C agents. We have made all of our sequence creation and image reconstruction code freely available online for other investigators to use.

Comparison of driven equilibrium and standard spin-echo sequence in MR microscopy: Analysis of signal dependence on RF pulse imperfection and diffusion

Rapid MR imaging of slowly relaxing samples is often challenging. The most commonly used solutions are found in multi spin-echo (RARE) sequences or gradient-echo (GE) sequences, which allow faster imaging of such samples with multiple acquisitions of k-space lines per excitation or imaging with very short repetition times (TRs). Another solution is the use of a spin-echo (SE) sequence superimposed with a driven equilibrium Fourier transform (DEFT) method. Such a (DE-SE) imaging sequence has two refocusing RF pulses that produce two spin-echoes. In the first echo, the signal is acquired from the k-space line, and in the second echo, a 90° RF pulse is applied, typically 180° out of phase with respect to the excitation RF pulse. This last RF pulse allows almost complete magnetization reversal back to the longitudinal orientation with minimal magnetization loss. The DE-SE sequence and its RARE variant are widely used in clinical imaging, but its use in MR microscopy has some peculiarities related to the usually less perfect RF pulse flip angles and diffusion. In this study, their effects are first theoretically analyzed and later verified by experiments on test samples performed on a 9.4 T system for MR microscopy. Experiments on a water-filled tube for TE = 3.4 ms and TR = 25–200 ms showed that the DE-SE sequence produces about 10 times more signal than the SE sequence in this TR range. Finally, the performance of the DE-SE sequence compared to the SE sequence was demonstrated on a biological sample. The presented DE-SE sequence has been shown to be effective for rapid imaging of samples with long T1 relaxation times in MR microscopy and can also be considered as a suitable method for rapid proton density weighed imaging of materials.

Optimal Protocol for Contrast-enhanced Free-running 5D Whole-heart Coronary MR Angiography at 3T.

Free-running 5D whole-heart coronary MR angiography (MRA) is gaining in popularity because it reduces scanning complexity by removing the need for specific slice orientations, respiratory gating, or cardiac triggering. At 3T, a gradient echo (GRE) sequence is preferred in combination with contrast injection. However, neither the injection scheme of the gadolinium (Gd) contrast medium, the choice of the RF excitation angle, nor the dedicated image reconstruction parameters have been established for 3T GRE free-running 5D whole-heart coronary MRA. In this study, a Gd injection scheme, RF excitation angles of lipid-insensitive binominal off-resonance RF excitation (LIBRE) pulse for valid fat suppression and continuous data acquisition, and compressed-sensing reconstruction regularization parameters were optimized for contrast-enhanced free-running 5D whole-heart coronary MRA using a GRE sequence at 3T. Using this optimized protocol, contrast-enhanced free-running 5D whole-heart coronary MRA using a GRE sequence is feasible with good image quality at 3T.

The effect of and correction for through-slice dephasing on 2D gradient-echo double angle mapping.

To show that variations through slice and slice profile effects are two major confounders affecting 2D dual angle maps using gradient-echo signals and thus need to be corrected to obtain accurate maps. The 2D gradient-echo transverse complex signal was Bloch-simulated and integrated across the slice dimension including nonlinear variations in inhomogeneities through slice. A nonlinear least squares fit was used to find the factor corresponding to the best match between the two gradient-echo signals experimental ratio and the Bloch-simulated ratio. The correction was validated in phantom and in vivo at 3T. For our RF excitation pulse, the error in the factor scales by approximately 3.8% for every 10 Hz/cm variation in along the slice direction. Higher accuracy phantom maps were obtained after applying the proposed correction; the root mean square error relative to the gold standard decreased from 6.4% to 2.6%. In vivo whole-liver maps using the corrected map registered a significant decrease in gradient through slice. inhomogeneities varying through slice were seen to have an impact on the accuracy of 2D double angle maps using gradient-echo sequences. Consideration of this confounder is crucial for research relying on accurate knowledge of the true excitation flip angles, as is the case of mapping using a spoiled gradient recalled echo sequence.

Probing the “Dead-Time” in NMR by Combining Single Pulse and Solid Echo Experiments Followed by a Global Model Fit Analysis

The main question addressed in this work is how to probe the “effective dead-time” in an NMR instrument, i.e., the time needed to blank the receiver after an rf-pulse excitation to prevent damage to the receiver and to avoid any distortion of the NMR signal being sampled. The strategy is to design a suitable FID-model to fit the single pulse excitation (SPE) and solid echo pulse (SEPS) data (on solid Tricosane) using a Global model-fit analysis technique. The derived dead time is discussed with respect to sampling temperature (25–40 °C) and—in particular—with respect to the number of SEPS data involved in the Global fit analysis by applying the Bayesian Information Criterion (BIC) in combination with more traditional statistical analysis. It is concluded that the “effective dead time” can be determined within a standard error of less than 2.5%.

Volumetric T2 -weighted spin echo imaging with improved SNR using localized quadratic encoding and a spiral readout trajectory.

To demonstrate T2 -weighted (single-echo) spin-echo (SE) imaging with near-optimal acquisition efficiency by applying SNR-efficient RF slice encoding and spiral readout. A quadratic-phase (frequency swept) excitation RF pulse replaced the conventional excitation in T2 -weighted SE sequence to excite a thick slab that is internally spatially encoded by a variable phase along the slice direction. Highly overlapping slabs centered at every desired slice location were acquired in multiple passes, such that the entire imaging volume was excited by contiguous slabs in any given pass. Following 90° excitation, each slab was refocused with a conventional 180° RF to produce a SE signal, followed by a spiral in-out readout. A noise-insensitive reconstruction removed the quadratic phase in the spatial frequency domain, yielding desired slice resolution and improved SNR. Increasing the RF frequency sweep (hence, excitation width) allowed more frequent encoding of each slice over the multiple passes, improving final image SNR, until crosstalk ensued at excessive slab widths compared to their center-to-center spacing. With an optimized slab width, the proposed technique used all passes to acquire every prescribed slice, with substantially improved SNR over conventional SE or 2D-turbo-spin-echo (TSE) scans. Quantitative SNR measurements indicated similar SNR as 3D-TSE, but radiologist scoring favored 3D-TSE, mainly because of spiral-related artifacts and possibly because of regularized reconstructions in 3D-TSE. Using SNR-efficient slice excitation scheme and spiral readout helped eliminate SNR and temporal inefficiencies in conventional T2 -weighted imaging, yielding SNR independent of TR or number of passes.

Design and development of a novel flexible ultra-short echo time (FUSE) sequence.

To present the validation of a new Flexible Ultra-Short Echo time (FUSE) pulse sequence using a short-T2 phantom. FUSE was developed to include a range of RF excitation pulses, trajectories, dimensionalities, and long-T2 suppression techniques, enabling real-time interchangeability of acquisition parameters. Additionally, we developed an improved 3D deblurring algorithm to correct for off-resonance artifacts. Several experiments were conducted to validate the efficacy of FUSE, by comparing different approaches for off-resonance artifact correction, variations in RF pulse and trajectory combinations, and long-T2 suppression techniques. All scans were performed on a 3 T system using an in-house short-T2 phantom. The evaluation of results included qualitative comparisons and quantitative assessments of the SNR and contrast-to-noise ratio. Using the capabilities of FUSE, we demonstrated that we could combine a shorter readout duration with our improved deblurring algorithm to effectively reduce off-resonance artifacts. Among the different RF and trajectory combinations, the spiral trajectory with the regular half-inc pulse achieves the highest SNRs. The dual-echo subtraction technique delivers better short-T2 contrast and superior suppression of water and agar signals, whereas the off-resonance saturation method successfully suppresses water and lipid signals simultaneously. In this work, we have validated the use of our new FUSE sequence using a short T2 phantom, demonstrating that multiple UTE acquisitions can be achieved within a single sequence. This new sequence may be useful for acquiring improved UTE images and the development of UTE imaging protocols.

A non-linear triangular split-ring based metaresonator for targeted scanning at 1.5T MRI

In magnetic resonance imaging (MRI), RF signals are initially transmitted to stimulate the body protons which eventually release the electromagnetic energy while returning back to their original states. The image resolution and scanning efficiency of MRI can be improved by enhancing the magnetic fields received from the patient’s body using metamaterials. The major limitation of linear metamaterials is that they amplify RF magnetic fields both during transmission and reception phases. This requires modification of the RF excitation pulses during the transmission phase. Further, local increase of transmitted power poses a potential threat of tissue-heating and high specific absorption rate (SAR) values in addition to perturbing the transmit field homogeneity. In order to circumvent these problems, we propose a self-adaptive metaresonator which has the capability of self-detuning itself during transmission of RF pulses during MRI scans. A triangular split-ring based metaresonator is designed for maximum thirty-fold SNR improvement in 1.5T MRI. Switching diodes have been employed for switching on and off the magnetic field enhancement by the metaresonator. During transmission phase when the switching diodes get turned on, the metaresonator is detuned. During reception phase when the switching diodes get turned off, the metaresonator is tuned to 63.8 MHz which is the Larmor frequency of 1.5T MRI. The proposed metaresonator is thin and compact which enables its easy placement in the multi-element arrays of clinical MRI.

Open-source myocardial T1 mapping with simultaneous multi-slice acceleration: Combining an auto-calibrated blipped-bSSFP readout with VERSE-MB pulses.

Enabling fast and accessible myocardial T1 mapping is crucial for extending its clinical application. We introduce Open-MOLLI-SMS combining simultaneous multi-slice (SMS) with auto-calibration and variable-rate selective excitation (VERSE)-multiband pulses to obtain all slices in a fast single-shot T1 mapping sequence. Open-MOLLI-SMS was developed by integrating SMS with the open-source method Open-MOLLI previously implemented in Pulseq. Three methods were integrated for Open-MOLLI-SMS: (1) auto-calibration blip patterns to ensure consistency between the data and coil information; (2) a blipped-balanced SSFP (bSSFP) readout to induce controlled aliasing in parallel imaging shifts without disturbing the bSSFP frequency response; and (3) a VERSE-multiband pulse for minimizing the achievable TR and the specific absortion rate (SAR) impact of SMS. Two (SMS2) or three (SMS3) slices were excited simultaneously and encoded with an in-plane acceleration factor of 2. Experiments were performed in the International Society for Magnetic Resonance in Medicine/National Institute of Standards and Technology phantom and five healthy volunteers. Phantom results show accurate T1 estimates for reference values between 400 to 2200 ms. Artifacts were visible for Open-MOLLI-SMS3 but not replicated in vivo. In vivo Open-MOLLI-SMS (T1 SMS2 = 993 ± 10 ms; T1 SMS3 = 1031 ± 17 ms) provided similar values to mean T1 single-band Open-MOLLI estimates (T1 Open-MOLLI = 1005 ± 47 ms). Open-MOLLI-SMS2 provided the closest estimates to the reference. This proof-of-principle implementation study demonstrates the feasibility of speeding up T1 -mapping acquisitions and increasing coverage by combining auto-calibration strategies with a blipped-bSFFP readout and VERSE multiband RF excitation pulses. The proposed methodology was built on the Open-MOLLI mapping sequence, which provides a fast means for prototyping and enables open-source sharing of the method.

Quantification of cross-relaxation in downfield 1 H MRS at7T in human calf muscle.

The purpose of this study was to characterize the 1 H downfield MR spectrum from 8.0 to 10.0ppm of human skeletal muscle at 7T and determine the T1 and cross-relaxation rates of observed resonances. We performed downfield MRS in the calf muscle of 7 healthy volunteers. Single-voxel downfield MRS was collected using alternately selective or broadband inversion-recovery sequences and spectrally selective 90° E-BURP RF pulse excitation centered at 9.0ppm with bandwidth = 600 Hz (2.0ppm). MRS was collected using TIs of 50-2500 ms. We modeled recovery of the longitudinal magnetization of three observable resonances using two models: (1) a three-parameter model accounting for the apparent T1 recovery and (2) a Solomon model explicitly including cross-relaxation effects. Three resonances were observed in human calf muscle at 7T at 8.0, 8.2, and 8.5ppm. We found broadband (broad) and selective (sel) inversion recovery T1 = mean ± SD (ms): T1-broad,8.0ppm = 2108.2 ± 664.5, T1-sel,8.0ppm = 753.6 ± 141.0 (p = 0.003); T1-broad,8.2ppm = 2033.5 ± 338.4, T1-sel,8.2ppm = 135.3 ± 35.3 (p < 0.0001); and T1-broad,8.5ppm = 1395.4 ± 75.4, T1-sel,8.5ppm = 107.1 ± 40.0 (p < 0.0001). Using the Solomon model, we found T1 = mean ± SD (ms): T1-8.0ppm = 1595.6 ± 491.1, T1-8.2ppm = 1737.2 ± 963.7, and T1-8.5ppm = 849.8 ± 282.0 (p =0.04). Post hoc tests corrected for multiple comparisons showed no significant difference in T1 between peaks. The cross-relaxation rate σAB = mean ± SD (Hz) of each peak was σAB,8.0ppm = 0.76 ± 0.20, σAB,8.2ppm = 5.31 ± 2.27, and σAB,8.5ppm = 7.90 ± 2.74 (p < 0.0001); post hoc t-tests revealed the cross-relaxation rate of the 8.0ppm peak was significantly slower than the peaks at 8.2 ppm (p =0.0018) and 8.5ppm (p =0.0005). We found significant differences in effective T1 and cross-relaxation rates of 1 H resonances between 8.0 and 8.5ppm in the healthy human calf muscle at 7T.

Imaging gas-exchange lung function and brain tissue uptake of hyperpolarized 129 Xe using sampling density-weighted MRSI.

Imaging of the different resonances of hyperpolarized 129 Xe in the brain and lungs was performed using a 3D sampling density-weighted MRSI technique in healthy volunteers. Four volunteers underwent dissolved-phase hyperpolarized 129 Xe imaging in the lung with the MRSI technique, which was designed to improve the point-spread function while preserving SNR (1799 phase-encoding steps, 14-s breath hold, 2.1-cm isotropic resolution). A frequency-tailored RF excitation pulse was implemented to reliably excite both the 129 Xe gas and dissolved phase (tissue/blood signal) with 0.1° and 10° flip angles, respectively. Images of xenon gas in the lung airspaces and xenon dissolved in lung tissue/blood were used to generate quantitative signal ratio maps. The method was also optimized and used for imaging dissolved resonances of 129 Xe in the brain in 2 additional volunteers. High-quality regional spectra of hyperpolarized 129 Xe were achieved in both the lung and the brain. Ratio maps of the different xenon resonances were obtained in the lung with sufficient SNR (> 10) at both 1.5 T and 3T, making a triple Lorentzian fit possible and enabling the measurement of relaxation times and xenon frequency shifts on a voxel-wise basis. The imaging technique was successfully adapted for brain imaging, resulting in the first demonstration of 3D xenon brain images with a 2-cm isotropic resolution. Density-weighted MRSI is an SNR and encoding-efficient way to image 129 Xe resonances in the lung and the brain, providing a valuable tool to quantify regional spectroscopic information.

Rapid 3D absolute B1 + mapping using a sandwiched train presaturated TurboFLASH sequence at 7 T for the brain and heart.

To shorten the acquisition time of magnetization-prepared absolute transmit field (B1 + ) mapping known as presaturation TurboFLASH, or satTFL, to enable single breath-hold whole-heart 3D B1 + mapping. SatTFL is modified to remove the delay between the reference and prepared images (typically 5 T1 ), with matching transmit configurations for excitation and preparation RF pulses. The new method, called Sandwich,is evaluated as a 3D sequence, measuring whole-brain and gated whole-heart B1 + maps in a single breath-hold. We evaluate the sensitivity to B1 + and T1 using numerical Bloch, extended phase graph, and Monte Carlo simulations. Phantom and in vivo images were acquired in both the brain and heart using an 8-channel transmit 7 Tesla MRI system to support the simulations. A segmented satTFL with a short readout train was used as a reference. The method significantly reduces acquisition times of 3D measurements from 360 s to 20 s, in the brain, while simultaneously reducing bias in the measured B1 + due to T1 and magnetization history. The mean coefficient of variation was reduced by 81% for T1 s of 0.5-3s compared to conventional satTFL. In vivo, the reproducibility coefficient for flip angles in the range 0-130° was 4.5° for satTFL and 4.7° for our scheme, significantly smaller than for a short TR satTFL sequence, which was 12°. The 3D sequence measured B1 + maps of the whole thorax in 26 heartbeats. Our adaptations enable faster B1 + mapping, with minimal T1 sensitivity and lower sensitivity to magnetization history, enabling single breath-hold whole-heart absolute B1 + mapping.

Characterizing Off-center MRI with ZTE

PurposeTo maximize acquisition bandwidth in zero echo time (ZTE) sequences, readout gradients are already switched on during the RF pulse, creating unwanted slice selectivity. The resulting image distortions are amplified especially when the anatomy of interest is not located at the isocenter. We aim to characterize off-center ZTE MRI of extremities such as the shoulder, knee, and hip, adjusting the carrier frequency of the RF pulse excitation for each TR. MethodsIn ZTE MRI, radial encoding schemes are used, where the distorted slice profile due to the finite RF pulse length rotates with the k-space trajectory. To overcome these modulations for objects far away from the magnet isocenter, the frequency of the RF pulse is shifted for each gradient setting so that artifacts do not occur at a given off-center target position. The sharpness of the edges in the images were calculated and the ZTE acquisition with off-center excitation was compared to an acquisition with isocenter excitation both in phantom and in vivo off-center MRI of the shoulder, knee, and hip at 1.5 and 3T MRI systems. ResultsDistortion and blurriness artifacts on the off-center MRI images of the phantom, in vivo shoulder, knee, and hip images were mitigated with off-center excitation without time or noise penalty, at no additional computational cost. ConclusionThe off-center excitation allows ZTE MRI of the shoulder, knee, and hip for high-bandwidth image acquisitions for clinical settings, where positioning at the isocenter is not possible.

Interleaved binomial kT -points for water-selective imaging at 7T.

We present a time-efficient water-selective, parallel transmit RF excitation pulse design for ultra-high field applications. The proposed pulse design method achieves flip angle homogenization at ultra-high fields by employing spatially nonselective -points pulses. In order to introduce water-selection, the concept of binomial pulses is applied. Due to the composite nature of -points, the pulse can be split into multiple binomial subpulse blocks shorter than half the precession period of fat, that are played out successively. Additional fat precession turns, that would otherwise impair the spectral response, can thus be avoided. Bloch simulations of the proposed interleaved binomial -points pulses were carried out and compared in terms of duration, homogeneity, fat suppression and pulse energy. For validation, in vivo MP-RAGE and 3D-EPI data were acquired. Simulation results show that interleaved binomial -points pulses achieve shorter total pulse durations, improved flip angle homogeneity and more robust fat suppression compared to available methods. Interleaved binomial -points can be customized by changing the number of -points, the subpulse duration and the order of the binomial pulse. Using shorter subpulses, the number of -points can be increased and hence better homogeneity is achieved, while still maintaining short total pulse durations. Flip angle homogenization and fat suppression of interleaved binomial -points pulses is demonstrated in vivo at 7T, confirming Bloch simulation results. In this work, we present a time efficient and robust parallel transmission technique for nonselective water excitation with simultaneous flip angle homogenization at ultra-high field.

B1 field map synthesis with generative deep learning used in the design of parallel-transmit RF pulses for ultra-high field MRI

Spoke trajectory parallel transmit (pTX) excitation in ultra-high field MRI enables B1+ inhomogeneities arising from the shortened RF wavelength in biological tissue to be mitigated. To this end, current RF excitation pulse design algorithms either employ the acquisition of field maps with subsequent non-linear optimization or a universal approach applying robust pre-computed pulses. We suggest and evaluate an intermediate method that uses a subset of acquired field maps combined with generative machine learning models to reduce the pulse calibration time while offering more tailored excitation than robust pulses (RP).The possibility of employing image-to-image translation and semantic image synthesis machine learning models based on generative adversarial networks (GANs) to deduce the missing field maps is examined. Additionally, an RF pulse design that employs a predictive machine learning model to find solutions for the non-linear (two-spokes) pulse design problem is investigated.As a proof of concept, we present simulation results obtained with the suggested machine learning approaches that were trained on a limited data-set, acquired in vivo. The achieved excitation homogeneity based on a subset of half of the B1+ maps acquired in the calibration scans and half of the B1+ maps synthesized with GANs is comparable with state of the art pulse design methods when using the full set of calibration data while halving the total calibration time. By employing RP dictionaries or machine-learning RF pulse predictions, the total calibration time can be reduced significantly as these methods take only seconds or milliseconds per slice, respectively.

Safety for Human MR Scanners at 7T

After introduction of the first human 7 tesla (7T) system in 1999, 7T MR systems have been employed as one of the most advanced platforms for human MR research for more than 20 years. Currently, two 7T MR models are approved for clinical use in the U.S.A. The approval facilitated introduction of the 7T system, summing up to around 100 worldwide. The approval in Japan is much awaited. As a clinical MR scanner, the 7T MR system is drawing attention in terms of safety.Several large-sized studies on bioeffects have been reported for vertigo, dizziness, motion disturbances, nausea, and others. Such effects might also be found in MR workers and researchers. Frequency and severity of reported bioeffects will be presented and discussed, including their variances. The high resonance frequency and shorter RF wavelength of 7T increase the concern about the safety. Homogeneous RF pulse excitation is difficult even for the brain, and a multi-channel parallel transmit (pTx) system is considered mandatory. However, pTx may create a hot spot, which makes the estimation of specific absorption rate (SAR) to be difficult. The stronger magnetic field of 7T causes a large force of displacement and heating on metallic implants or devices, and the scan of patients with them should not be conducted at 7T. However, there are some opinions that such patients might be scanned even at 7T, if certain criteria are met. This article provides a brief review on the effect of the static magnetic field on humans (MR subjects, workers, and researchers) and neurons, in addition to scan sound, SAR, and metal implants and devices. Understanding and avoiding adverse effects will contribute to the reduction in safety risks and the prevention of incidents.

Rapid 2D variable flip angle method for accurate and precise T1 measurements over a wide range of T1 values.

PurposeTo perform dynamic T 1 mapping using a 2D variable flip angle (VFA) method, a correction for the slice profile effect is needed. In this work we investigated the impact of flip angle selection and excitation RF pulse profile on the performance of slice profile correction when applied to T 1 mapping over a range of T 1 values.MethodsA correction of the slice profile effect is proposed, based on Bloch simulation of steady‐state signals. With this correction, Monte Carlo simulations were performed to assess the accuracy and precision of 2D VFA T 1 mapping in the presence of noise, for RF pulses with time‐bandwidth products of 2, 3 and 10 and with flip angle pairs in the range [1°‐90°]. To evaluate its performance over a wide range of T 1, maximum errors were calculated for six T 1 values between 50 ms and 1250 ms. The method was demonstrated using in vitro and in vivo experiments.ResultsWithout corrections, 2D VFA severely underestimates T 1. Slice profile errors were effectively reduced with the correction based on simulations, both in vitro and in vivo. The precision and accuracy of the method depend on the nominal T 1 values, the FA pair, and the RF pulse shape. FA pairs leading to <5% errors in T 1 can be identified for the common RF shapes, for T 1 values between 50 ms and 1250 ms.Conclusions2D VFA T 1 mapping with Bloch‐simulation‐based correction can deliver T 1 estimates that are accurate and precise to within 5% over a wide T 1 range.

A robust broadband fat-suppressing phaser T2 -preparation module for cardiac magnetic resonance imaging at 3T.

Designing a new T2 -preparation (T2 -Prep) module to simultaneously provide robust fat suppression and efficient T2 preparation without requiring an additional fat-suppression module for T2 -weighted imaging at 3T. The tip-down radiofrequency (RF) pulse of an adiabatic T2 -Prep module was replaced by a custom-designed RF-excitation pulse that induces a phase difference between water and fat, resulting in a simultaneous T2 preparation of water signals and the suppression of fat signals at the end of the module (a phaser adiabatic T2 -Prep). Numerical simulations and in vitro and in vivo electrocardiogram (ECG)-triggered navigator-gated acquisitions of the human heart were performed. Blood, myocardium, and fat signal-to-noise ratios and right coronary artery vessel sharpness were compared against previously published adiabatic T2 -Prep approaches. Numerical simulations predicted an increased fat-suppression bandwidth and decreased sensitivity to transmit magnetic field inhomogeneities using the proposed approach while preserving the water T2 -Prep capabilities. This was confirmed by the tissue signals acquired in the phantom and the in vivo images, which show similar blood and myocardium signal-to-noise ratio, contrast-to-noise ratio, and significantly reduced fat signal-to-noise ratio compared with the other methods. As a result, the right coronary artery conspicuity was significantly increased. A novel fat-suppressing T2 -Prep method was developed and implemented that showed robust fat suppression and increased vessel sharpness compared with conventional techniques while preserving its T2 -Prep capabilities.