A Minibatch Alternating Projections Algorithm for Robust and Efficient Magnitude Least-Squares RF Pulse Design in MRI.

PurposeSimultaneous multi‐slice techniques are reliant on multiband RF pulses, for which conventional design strategies result in long pulse durations, lengthening echo‐times so lowering SNR for spin‐echo imaging, and lengthening repetition times for gradient echo sequences. Pulse durations can be reduced with advanced RF pulse design methods that use time‐variable selection gradients. However, the ability of gradient systems to reproduce fast switching pulses is often limited and can lead to image artifacts when ignored. We propose a time‐efficient pulse design method that inherently produces gradient waveforms with lower temporal bandwidth.MethodsEfficient multiband RF pulses with time‐variable gradients were designed using time‐optimal VERSE. Using VERSE directly on multiband pulses leads to gradient waveforms with high temporal bandwidth, whereas VERSE applied first to singleband RF pulses and then modulated to make them multiband, significantly reduces this. The relative performance of these approaches was compared using simulation and experimental measurements.ResultsApplying VERSE before multiband modulation was successful at removing out‐of‐band slice distortion. This effectively removes the need for high frequency modulation in the gradient waveform while preserving the benefit of time‐efficiency inherited from VERSE.ConclusionWe propose a time‐efficient RF pulse design that produces gradient pulses with lower temporal bandwidth, reducing image artifacts associated with finite temporal bandwidth of gradient systems.

Current methods for parallel excitation RF pulse design are based on the small-tip-angle approximation, which provides a computationally efficient means of pulse calculation. In general, pulses designed with those methods are inaccurate when scaled to produce large-tip angles, and methods for large-tip-angle pulse design are more computationally demanding. This paper introduces a fast iterative method for large-tip-angle parallel pulse design that is formulated as a small number of Bloch equation simulations and fast small-tip-angle pulse designs, the results of which add to produce large-tip-angle pulses. Simulations and a phantom experiment demonstrate that the method is effective in designing multidimensional large-tip-angle pulses of high excitation accuracy, compared to pulses designed with small-tip-angle methods.

Some advanced RF pulses, like multidimensional RF pulses, are often long and require substantial computation time because of a number of constraints and requirements, sometimes hampering clinical use. However, the pulses offer opportunities of reduced-FOV imaging, regional flip-angle homogenization, and localized spectroscopy, e.g., of hyperpolarized metabolites. Proposed herein is a novel deep learning approach to ultrafast design of multidimensional RF pulses with intention of real-time pulse updates. The proposed neural network considers input maps of the desired excitation region of interest and outputs a single-channel, multidimensional RF pulse. The training library is, e.g., retrieved from a large image database, and the target RF pulses trained upon are calculated with a method of choice. A relatively simple neural network is enough to produce reliable 2D spatial-selective RF pulses of comparable performance to the teaching method. For binary regions of interest, the training library does not need to be vast; hence, reestablishment of the training library is not necessarily cumbersome. The predicted pulses were tested numerically and experimentally at 3 T. Relatively effortless training of multidimensional RF pulses, based on non-MRI-related inputs, but working in an MRI setting still, has been demonstrated. The prediction time of a few milliseconds renders real-time updates of advanced RF pulses possible.

Purpose: Exception gating and dynamic MLC tracking on MRI-gRT systems require segmentation of high temporal resolution images. Multi-band excitation is an emerging MRI acceleration technique that simultaneously excites a short comb of 2D slices, thereby increasing effective temporal resolution. However, the design of multiband RF pulses imposes tradeoffs between slice profiles and the temporal duration of the pulse. The goal of this study was to determine whether slice profiles of shorter duration multiband RF pulses affect organ tracking accuracy for MRI-gRT. Methods: Three-band Shinnar-Le Roux (SLR) RF pulses were designed with slice bandwidths of 1.5kHz and durations ranging from 750 to 3000 µs. The pulses were implemented into a golden angle radial pulse sequence of our own design. Time series of dynamic 2D sagittal cine images of an MRI-compatible dynamic phantom (Model 008M, CIRS) with 2.5 cm superior/inferior target motion were acquired at 3T using each of the RF pulse durations. Images were reconstructed offline in Matlab. The target was contoured on the last image of the time series and tracked through the time series using normalized cross-correlation with a Powell optimizer and B-spline interpolator. Peak-to-peak (P2P) centroid displacements along the superior/inferior dimension were estimated and compared against the phantom motion trajectory. Results: Transition band sizes of the multiband RF slice profiles ranged from 2.1 to 4.6 mm for intended 5 mm slices. P2P centroid displacements ranged from 2.3 to 2.6 cm, demonstrating good agreement with the 2.5 cm phantom displacement, and indicating that slice profile has minimal effect on tracking ability. Conclusion: Slice profile does not affect tracking ability, permitting the use of shorter duration RF pulses to increase temporal resolution during MRI-gRT. Future work will investigate the advantages of multiband excitation for 2D/3D image registration and will combine multiband excitation with in-plane acceleration to further improve temporal resolution. Elekta Instruments, AB

With the development of the high‐field MRI system, the RF field inhomogeneity becomes a challenging problem in high flip angle excitation. In this study, a new RF pulse design method is proposed, which combines the simulated annealing (SA) algorithm and the k‐space method to attain a pulse which can compensate the RF field inhomogeneity effect. The RF pulse envelope is firstly initiated using the k‐space method and a cost function which contains the passband error and stopband error is defined. Then the SA is applied to refine the RF pulse shape to achieve low‐cost value. Finally, under inhomogeneous RF field an optimized RF pulse with improved flip angle accuracy can be obtained. Simulation results show that when compared with the pulse designed only using the k‐space method, the passband error can be decreased by 28%, 40%, and 75% for 90° excitation pulse, refocusing pulse, and 180° inversion pulse, respectively. © 2008 Wiley Periodicals, Inc. Concepts Magn Reson Part B (Magn Reson Engineering) 33B: 75–83, 2008

Equi-ripple design of quadratic-phase RF pulses

Large gains in SNR through the application of Shinnar-Le Roux RF pulse design to short-TR spin-echo fMRI acquisitions at 7T.

To investigate the feasibility of high-dose dobutamine stress (HDDS) imaging using SSFP sequences at 3 T employing patient-adaptive local RF-shimming using a dual-source RF transmission system. 13 Patients underwent a HDDS protocol on a 3 T MRI scanner (Achieva 3.0T-TX, Philips Healthcare), equipped with a dual-source RF transmission system. SSFP cine sequences using patient-adaptive local RF-shimming (RF-S) were compared to cine images acquired without additional shimming. Image quality was evaluated on a 4-point grading scale and number of non-diagnostic segments assessed. Contrast (CN) between myocardium (SIM) and blood pool (SIB) was calculated [(SIB-SIM)/(SIB+SIM)]. Image quality both at rest and maximum stress was significantly improved with RF-S (ED:3.56±0.5 vs. 3.23±0.63; ES:3.4±0.5 vs. 3.1±0.7) compared to no RF-S (ED:2.9±0.72 vs. 2.15±0.78; ES:2.64±0.74 vs. 1.95±0.76; p<0.01). The amount of non-diagnostic segments was significantly reduced when using RF-S at rest and stress (3 vs. 39; 19 vs. 78, p<0.05). All HDDS studies were diagnostic if performed with RF-S (n=13/13) in comparison to conventional shimming (n=5/13). Image contrast was improved for SSFP sequences with RF-S (0.53±0.08) compared to conventional images (0.46±0.09, p=0.06). Patient-adaptive local RF-shimming using a dual-source RF transmission system allows for reliable SSFP imaging in a clinical high-dose dobutamine stress protocol at 3 T. RF-S significantly improves image quality and reduces the number of non-diagnostic myocardial segments.

Improving excitation and inversion accuracy by optimized RF pulse using genetic algorithm

Joint design of minimum duration RF pulses and slice-selective gradient shapes for MRI via time optimal control with strict physical constraints, and its application to simultaneous multislice imaging. The minimization of the pulse duration is cast as a time optimal control problem with inequality constraints describing the refocusing quality and physical constraints. It is solved with a bilevel method, where the pulse length is minimized in the upper level, and the constraints are satisfied in the lower level. To address the inherent nonconvexity of the optimization problem, the upper level is enhanced with new heuristics for finding a near global optimizer based on a second optimization problem. A large set of optimized examples shows an average temporal reduction of 87.1% for double diffusion and 74% for turbo spin echo pulses compared to power independent number of slices pulses. The optimized results are validated on a 3T scanner with phantom measurements. The presented design method computes minimum duration RF pulse and slice-selective gradient shapes subject to physical constraints. The shorter pulse duration can be used to decrease the effective echo time in existing echo-planar imaging or echo spacing in turbo spin echo sequences.

Multi-Slice T2-weighted MR imaging is a fundamental technique in brain imaging but suffers from field inhomogeneities at ultra-high field systems. In this work we present a pulse design method which provides slab-selective universal pulses with phase-coherent excitation profiles over selected brain regions. Applied in a 3D variable flip angle turbo-spin-echo (TSE) sequence, the pulses achieve superior SNR compared to multi-slice TSE imaging and B1 + field inhomogeneity mitigation.

The aim of this study was to evaluate the effect of integrated high-permittivity materials (HPMs) on excitation homogeneity and global specific absorption rate (SAR) for transmit arrays at 7T. A rapid electrodynamic simulation framework was used to calculate L-curves associated with excitation of a uniform 2D profile in a dielectric sphere. We used ultimate intrinsic SAR as an absolute performance reference to compare different transmit arrays in the presence and absence of a layer of HPM. We investigated the optimal permittivity for the HPM as a function of its thickness, the sample size, and the number of array elements. Adding a layer of HPM can improve the performance of a 24-element array to match that of a 48-element array without HPM, whereas a 48-element array with HPM can perform as well as a 64-element array without HPM. Optimal relative permittivity values changed based on sample and coil geometry, but were always within a range obtainable with readily available materials (εr = 100-200). Integration of HPMs could be a practical method to improve RF shimming performance, alternative to increasing the number of coils. The proposed simulation framework could be used to explore the design of novel transmit arrays for head imaging at ultra-high field strength. Magn Reson Med 80:391-399, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Introduction CT coronary angiography (CTCA) is widely applied in coronary artery disease. It is therefore important that imaging protocols are optimised to obtain diagnostic images at the lowest possible radiation dose. We aimed to establish the feasibility of single heart-beat imaging in 320-multidetector CTCA, and assess variables that affect image quality. Methods Consecutive patients (n=249, 38% male) underwent contrast enhanced prospective electrocardiogram-gated single heart-beat CTCA. We assessed images before and after the introduction of sublingual glyceryl trinitrate to the CTCA protocol. Tube current and voltage were selected based on body-mass index (BMI) and acquisition window was selected based on heart rate (70–80% or 30–80% if the heart rate was below or above 65 beats/min respectively). Images were assessed by two trained observers and image quality was graded on a 4-point scale (1, excellent; 4, poor). Obstructive disease was defined as a stenosis of greater than 70% diameter. Results The mean heart rate was 60 beats/min (95% CI 59 to 62), BMI 29 kg/m2 (28, 30), age 58 years (56, 59) and dose-length product 283 mGy cm (266, 301). During scanning, 133(51%) received sublingual glyceryl trinitrate (GTN), 9(4%) had ectopic beats, and 12(5%) had atrial fibrillation. Diagnostic image quality was obtained in 99% with mean image quality 1.4 (1.3, 1.5). For patients with a BMI of 30 or above (n=111) diagnostic image quality was obtained in 109 scans (98%) and excellent image quality was obtained in 78 scans (70%). For patients with a heart rate above 65 bpm (n=58) diagnostic image quality was obtained in 98% (57 scans) and excellent image quality was obtained in 36% (21 scans). The mean dose-length product was 283 mGy cm (266, 301). An increase in heart rate led to an increase in radiation dose to 377 mGy cm (286, 268) for patients with a heart rate of greater than 80 beats/min (p&lt;0.001). A higher radiation dose was also associated with a BMI above 30 (DLP 231 (213, 248) vs 353 (330, 377), p&lt;0.001) and the presence of arrhythmia (DLP 275 (259, 290) vs 384 (301, 467), p 0.013). Age, sex, atrial fibrillation, ectopics, diabetes mellitus (12%) and the presence of obstructive coronary disease were not related to image quality. In multivariate regression analysis a lower heart rate and GTN were associated with improved image quality (β 0.439, p&lt;0.001 and β −0.199, p=0.001 respectively). Figure 1 The effect of heart rate, BMI and arrhythmia on the proportion of CTCA with diagnostic image quality. Conclusions Optimal image quality in single heart-beat 320-multidetector CTCA is achievable at low radiation doses in 99% of unselected patients. Image quality is further improved by lower heart rate and sublingual GTN. This study has established the use of half-segment reconstruction in all patients undergoing wide volume CTCA irrespective of heart rate, ectopic beats and the presence of atrial fibrillation. A widened window of acquisition can be used to improve image quality in patients with a heart rate greater than 65 beats per minute. In addition we have established that the use of GTN improves image quality in 320 multidetector CTCA.

Cardiovascular magnetic resonance (CMR) imaging is a powerful tool for assessing cardiac anatomy and function but remains limited by average image quality due to artefacts and long acquisition times, and complex and often too long breath-holds. Deep learning methods have recently been applied and show potential to shorten scan times by 70-80% while improving image quality, enhancing clinical efficiency. The aim of this study is to summarise the different AI-enabled methods for improving CMR image quality, including scanning time, as a key determinant for artefact reduction. A scoping review was conducted according to PRISMA guidelines. The articles were screened and reviewed by two researchers. A qualitative thematic synthesis was conducted and a CASP-mediated risk of bias assessment was performed. The eligible articles were thirty-one. These articles were thematically categorised in four subgroups, based on emerging themes: scan acceleration, artefact detection, artefact reduction, image reconstruction. A table with significant results for each theme has been presented and results were discussed qualitatively. AI demonstrated consistent improvements across the four subgroups. For scan acceleration, deep learning achieved approximately a 70-80% reduction in scan duration maintaining or even improving image quality. For artefact detection, convolutional neural networks achieved on average a 90% accuracy in detecting artefacts, across multiple metrics, indicating reliable artefact identification and strong agreement with human experts. AI models effectively reduce artefacts and enhance image quality, achieving consistently better reconstruction accuracy, sharper edges, and faster processing compared to conventional methods. Finally, for image reconstruction, generative adversarial networks enhanced structural similarity by approximately 56% (SSIM 0.591 → 0.925). Together, these results illustrate the potential of AI to optimise CMR image quality. AI can be an effective tool in addressing many of the CMR imaging challenges and thus improving image quality.

Summary This case study summarizes the data reprocessing effort that has been applied to seismic data located in the Algerian on-shore over a structurally complex, low porosity, low permeability reservoir and the improvements in final image quality that have been achieved. Subsequent to processing, the seismic data were inverted to rock properties. The final results show seismic data with significant improvements in image quality and structural definition, whilst the inversion to rock properties has enabled lithology and porosity characterization with increased confidence. Furthermore, borehole seismic surveys were acquired and integrated in the 3D volume to design the trajectory of horizontal and side-track wells.