Intra-scan RF power amplifier drift correction.

PURPOSE: The drift in radiofrequency (RF) power amplifiers (RFPAs) is assessed and several contributing factors are investigated. Two approaches for prospective correction of drift are proposed and their effectiveness is evaluated. METHODS: RFPA drift assessment encompasses both intra-pulse and inter-pulse drift analyses. Scan protocols with varying flip angle (FA), RF length, and pulse repetition time (TR) are used to gauge the influence of these parameters on drift. Directional couplers (DICOs) monitor the forward waveforms of the RFPA outputs. DICOs data is stored for evaluation, allowing calculation of correction factors to adjust RFPAs' transmit voltage. Two correction methods, predictive and run-time, are employed: predictive correction necessitates a calibration scan, while run-time correction calculates factors during the ongoing scan. RESULTS: RFPA drift is indeed influenced by the RF duty-cycle, and in the cases examined with a maximum duty-cycle of 66%, the potential drift is approximately 41% or 15%, depending on the specific RFPA revision. Notably, in low transmit voltage scenarios, FA has minimal impact on RFPA drift. The application of predictive and run-time drift correction techniques effectively reduces the average drift from 10.0% to less than 1%, resulting in enhanced MR signal stability. CONCLUSION: Utilizing DICO recordings and implementing a feedback mechanism enable the prospective correction of RFPA drift. Having a calibration scan, predictive correction can be utilized with fewer complexity; for enhanced performance, a run-time approach can be employed.

Submillimeter balanced SSFP BOLD-functional MRI accelerated with 3D stack-of-spirals at 9.4 T.

PURPOSE: This work aims to improve the speed of balanced SSFP (bSSFP) acquisition with segmented 3D stack-of-spirals for functional brain studies at ultrahigh field. METHODS: Functional experiments were performed with an accelerated 3D stack-of-spirals sequence with water excitation for fat suppression. The resulting data were reconstructed using an iterative algorithm with corrections for system imperfections such as trajectory deviations and B0 inhomogeneity. In the first set of experiments, we evaluated the signal change and stability with respect to echo and TR for a full-field checkerboard stimulus. To demonstrate the high spatio-temporal resolution of the developed method, the results of three optimized protocols at submillimeter resolution (0.6-mm isotropic and 0.8-mm isotropic) and at 1.2 mm isotropic resolution for whole-brain coverage were shown. RESULTS: Water excitation and the model-based iterative reconstruction improved image quality. The BOLD-related signal changes increased with longer TE and longer TR. We observed an increase in thermal noise performance at lower TE and higher TR. However, signal stability deteriorates at higher TE and TR. Therefore, optimized protocols used shorter TE and moderately long TR to maximize the sensitivity and speed. Reproducible activations were detected along the gray-matter gyri in the submillimeter protocols with a median signal change of approximately 4% across subjects. CONCLUSIONS: Three-dimensional stack-of-spirals enables passband balanced SSFP functional imaging at a much higher spatial and temporal scale, compared with conventional spoiled gradient-echo train sequences.

Dynamic parallel imaging at 9.4 T using reconfigurable receive coaxial dipoles.

Parallel imaging is one of the key MRI technologies that allow reduction of image acquisition time. However, the parallel imaging reconstruction commonly leads to a signal-to-noise ratio (SNR) drop evaluated using a so-called geometrical factor (g-factor). The g-factor is minimized by increasing the number of array elements and their spatial diversity. At the same time, increasing the element count requires a decrease in their size. This may lead to insufficient coil loading, an increase in the relative noise contribution from the RF coil itself, and hence SNR reduction. Previously, instead of increasing the channel number, we introduced the concept of electronically switchable time-varying sensitivities, which was shown to improve parallel imaging performance. In this approach, each reconfigurable receive element supports two spatially distinct sensitivity profiles. In this work, we developed and evaluated a novel eight-element human head receive-only reconfigurable coaxial dipole array for human head imaging at 9.4 T. In contrast to the previously reported reconfigurable dipole array, the new design does not include direct current (DC) control wires connected directly to the dipoles. The coaxial cable itself is used to deliver DC voltage to the PIN diodes located at the ends of the antennas. Thus, the novel reconfigurable coaxial dipole design opens a way to scale the dynamic parallel imaging up to a realistic number of channels, that is, 32 and above. The novel array was optimized and tested experimentally, including in vivo studies. It was found that dynamic sensitivity switching provided an 8% lower mean and 33% lower maximum g-factor (for Ry × Rz = 2 × 2 acceleration) compared with conventional static sensitivities.

FastPtx: a versatile toolbox for rapid, joint design of pTx RF and gradient pulses using Pytorch's autodifferentiation.

OBJECTIVE: With modern optimization methods, free optimization of parallel transmit pulses together with their gradient waveforms can be performed on-line within a short time. A toolbox which uses PyTorch's autodifferentiation for simultaneous optimization of RF and gradient waveforms is presented and its performance is evaluated. METHODS: MR measurements were performed on a 9.4T MRI scanner using a 3D saturated single-shot turboFlash sequence for [Formula: see text] mapping. RF pulse simulation and optimization were done using a Python toolbox and a dedicated server. An RF- and Gradient pulse design toolbox was developed, including a cost function to balance different metrics and respect hardware and regulatory limits. Pulse performance was evaluated in GRE and MPRAGE imaging. Pulses for non-selective and for slab-selective excitation were designed. RESULTS: Universal pulses for non-selective excitation reduced the flip angle error to an NRMSE of (12.3±1.7)% relative to the targeted flip angle in simulations, compared to (42.0±1.4)% in CP mode. The tailored pulses performed best, resulting in a narrow flip angle distribution with NRMSE of (8.2±1.0)%. The tailored pulses could be created in only 66 s, making it feasible to design them during an experiment. A 90° pulse was designed as preparation pulse for a satTFL sequence and achieved a NRMSE of 7.1%. We showed that both MPRAGE and GRE imaging benefited from the pTx pulses created with our toolbox. CONCLUSION: The pTx pulse design toolbox can freely optimize gradient and pTx RF waveforms in a short time. This allows for tailoring high-quality pulses in just over a minute.

The possible influence of third-order shim coils on gradient-magnet interactions: an inter-field and inter-site study.

OBJECTIVE: To assess the possible influence of third-order shim coils on the behavior of the gradient field and in gradient-magnet interactions at 7 T and above. MATERIALS AND METHODS: Gradient impulse response function measurements were performed at 5 sites spanning field strengths from 7 to 11.7 T, all of them sharing the same exact whole-body gradient coil design. Mechanical fixation and boundary conditions of the gradient coil were altered in several ways at one site to study the impact of mechanical coupling with the magnet on the field perturbations. Vibrations, power deposition in the He bath, and field dynamics were characterized at 11.7 T with the third-order shim coils connected and disconnected inside the Faraday cage. RESULTS: For the same whole-body gradient coil design, all measurements differed greatly based on the third-order shim coil configuration (connected or not). Vibrations and gradient transfer function peaks could be affected by a factor of 2 or more, depending on the resonances. Disconnecting the third-order shim coils at 11.7 T also suppressed almost completely power deposition peaks at some frequencies. DISCUSSION: Third-order shim coil configurations can have major impact in gradient-magnet interactions with consequences on potential hardware damage, magnet heating, and image quality going beyond EPI acquisitions.

Optimized ultrahigh field parallel transmission workflow using rapid presaturated TurboFLASH transmit field mapping with a three-dimensional centric single-shot readout.

PURPOSE: To evaluate the usage of three-dimensional (3D) presaturated TurboFLASH (satTFL) for B 1 + $$ {\mathrm{B}}_1^{+} $$ and B 0 $$ {\mathrm{B}}_0 $$ mapping on single channel and parallel transmission (pTx) systems. METHODS: B 1 + $$ {\mathrm{B}}_1^{+} $$ maps recorded with 3D satTFL were compared to maps from three other 3D B 1 + $$ {\mathrm{B}}_1^{+} $$ mapping sequences in an agar phantom. Furthermore, individual-channel B 1 + $$ {\mathrm{B}}_1^{+} $$ maps of 18 human subjects were recorded with 3D satTFL using B 1 + $$ {\mathrm{B}}_1^{+} $$ interferometry. A neural network was trained for masking of the maps. RESULTS: Out of the sequences compared satTFL was the only one with a mapping range exceeding well over 90°. In regions with lower flip angles there was high correspondence between satTFL and AFI. DREAM and double angle method also showed high qualitative similarity, however the magnitude differed from the other two measurements. The individual-channel B 1 + $$ {\mathrm{B}}_1^{+} $$ maps were successfully used for pTx pulse calculation in a separate study. CONCLUSION: 3D satTFL can record high-quality B 1 + $$ {\mathrm{B}}_1^{+} $$ maps with a high dynamic range in a short time. Correspondence with AFI maps is high, while measurement duration is reduced drastically.

Reconfigurable dipole receive array for dynamic parallel imaging at ultra-high magnetic field.

PURPOSE: To extend the concept of 3D dynamic parallel imaging, we developed a prototype of an electronically reconfigurable dipole array that provides sensitivity alteration along the dipole length. METHODS: We developed a radiofrequency array coil consisting of eight reconfigurable elevated-end dipole antennas. The receive sensitivity profile of each dipole can be electronically shifted toward one or the other end by electrical shortening or lengthening the dipole arms using positive-intrinsic-negative-diode lump-element switching units. Based on the results of electromagnetic simulations, we built the prototype and tested it at 9.4 T on phantom and healthy volunteer. A modified 3D SENSE reconstruction was used, and geometry factor (g-factor) calculations were performed to assess the new array coil. RESULTS: Electromagnetic simulations showed that the new array coil was capable of alteration of its receive sensitivity profile along the dipole length. Electromagnetic and g-factor simulations showed closely agreeing predictions when compared to the measurements. The new dynamically reconfigurable dipole array provided significant improvement in geometry factor compared to static dipoles. We obtained up to 220% improvement for 3 × 2 (Ry × Rz ) acceleration compared to the static configuration case in terms of maximum g-factor and up to 54% in terms of mean g-factor for the same acceleration. CONCLUSION: We presented an 8-element prototype of a novel electronically reconfigurable dipole receive array that permits rapid sensitivity modulations along the dipole axes. Applying dynamic sensitivity modulation during image acquisition emulates two virtual rows of receive elements along the z-direction, and therefore improves parallel imaging performance for 3D acquisitions.

Double-row 16-element folded-end dipole transceiver array for 3D RF shimming of the whole human brain at 9.4 T.

Homogeneity and longitudinal coverage of transmit (Tx) human head RF coils at ultrahigh field (UHF, ≥7 T) can be improved by 3D RF shimming, which requires using multi-row Tx arrays. Examples of 3D RF shimming using double-row UHF loop transceiver (TxRx) and Tx arrays have been described previously. Dipole antennas provide unique simplicity and robustness while offering comparable Tx efficiency and signal-to-noise ratio to conventional loop designs. Single-row Tx and TxRx human head UHF dipole arrays have been previously described by multiple groups. Recently, we developed a novel type of dipole antenna, a folded-end dipole, and presented single-row eight-element array prototypes for human head imaging at 7 and 9.4 T. These studies have shown that the novel antenna design can improve the longitudinal coverage and minimize peak local specific absorption rate (SAR) as compared with common unfolded dipoles. In this work, we developed, constructed, and evaluated a 16-element double-row TxRx folded-end dipole array for human head imaging at 9.4 T. To minimize cross-talk between neighboring dipoles located in different rows, we used transformer decoupling, which decreased coupling to a level below -20 dB. The developed array design was demonstrated to be capable of 3D static RF shimming and can be potentially used for dynamic shimming using parallel transmission. For optimal phase shifts between the rows, the array provides 11% higher SAR efficiency and 18% higher homogeneity than a folded-end dipole single-row array of the same length. The design also offers a substantially simpler and more robust alternative to the common double-row loop array with about 10% higher SAR efficiency and better longitudinal coverage.

OTUP workflow: target specific optimization of the transmit k-space trajectory for flexible universal parallel transmit RF pulse design.

PURPOSE: To optimize transmit k-space trajectories for a wide range of excitation targets and to design "universal pTx RF pulses" based on these trajectories. METHODS: Transmit k-space trajectories (stack of spirals and SPINS) were optimized to best match different excitation targets using the parameters of the analytical equations of spirals and SPINS. The performances of RF pulses designed based on optimized and non-optimized trajectories were compared. The optimized trajectories were utilized for universal pulse design. The universal pulse performances were compared with subject specific tailored pulse performances. The OTUP workflow (optimization of transmit k-space trajectories and universal pulse calculation) was tested on three test target excitation patterns. For one target (local excitation of a central area in the human brain) the pulses were tested in vivo at 9.4 T. RESULTS: The workflow produced appropriate transmit k-space trajectories for each test target. Utilization of an optimized trajectory was crucial for the pulse performance. Using unsuited trajectories diminished the performance. It was possible to create target specific universal pulses. However, not every test target is equally well suited for universal pulse design. There was no significant difference in the in vivo performance between subject specific tailored pulses and a universal pulse at 9.4 T. CONCLUSIONS: The proposed workflow further exploited and improved the universal pulse concept by combining it with gradient trajectory optimization for stack of spirals and SPINS. It emphasized the importance of a well suited trajectory for pTx RF pulse design. Universal and tailored pulses performed with a sufficient degree of similarity in simulations and a high degree of similarity in vivo. The implemented OTUP workflow and the B0 /B1+ map data from 18 subjects measured at 9.4 T are available as open source (https://github.com/ole1965/workflow_OTUP.git).

The traveling heads 2.0: Multicenter reproducibility of quantitative imaging methods at 7 Tesla.

OBJECT: This study evaluates inter-site and intra-site reproducibility at ten different 7 T sites for quantitative brain imaging. MATERIAL AND METHODS: Two subjects - termed the "traveling heads" - were imaged at ten different 7 T sites with a harmonized quantitative brain MR imaging protocol. In conjunction with the system calibration, MP2RAGE, QSM, CEST and multi-parametric mapping/relaxometry were examined. RESULTS: Quantitative measurements with MP2RAGE showed very high reproducibility across sites and subjects, and errors were in concordance with previous results and other field strengths. QSM had high inter-site reproducibility for relevant subcortical volumes. CEST imaging revealed systematic differences between the sites, but reproducibility was comparable to results in the literature. Relaxometry had also very high agreement between sites, but due to the high sensitivity, differences caused by different applications of the B1 calibration of the two RF coil types used were observed. CONCLUSION: Our results show that quantitative brain imaging can be performed with high reproducibility at 7 T and with similar reliability as found at 3 T for multicenter studies of the supratentorial brain.

Local excitation universal parallel transmit pulses at 9.4T.

PURPOSE: To demonstrate that the concept of "universal pTx pulses" is applicable to local excitation applications. METHODS: A database of B0 / B1+ maps from eight different subjects was acquired at 9.4T. Based on these maps, universal pulses that aim at local excitation of the visual cortex area in the human brain (with a flip angle of 90° or 7°) were calculated. The remaining brain regions should not experience any excitation. The pulses were designed with an extension of the "spatial domain method." A 2D and a 3D target excitation pattern were tested, respectively. The pulse performance was examined on non-database subjects by Bloch simulations and in vivo at 9.4T using a GRE anatomical MRI and a presaturated TurboFLASH B1+ mapping sequence. RESULTS: The calculated universal pulses show excellent performance in simulations and in vivo on subjects that were not contained in the design database. The visual cortex region is excited, while the desired non-excitation areas produce the only minimal signal. In simulations, the pulses with 3D target pattern show a lack of excitation uniformity in the visual cortex region; however, in vivo, this inhomogeneity can be deemed acceptable. A reduced field of view application of the universal pulse design concept was performed successfully. CONCLUSIONS: The proposed design approach creates universal local excitation pulses for a flip angle of 7° and 90°, respectively. Providing universal pTx pulses for local excitation applications prospectively abandons the need for time-consuming subject-specific B0 / B1+ mapping and pTx-pulse calculation during the scan session.

Ultrahigh-resolution quantitative spinal cord MRI at 9.4T.

PURPOSE: To present the results of the first human spinal cord in vivo MRI scans at 9.4T. METHODS: A human brain coil was used to image the human spinal cord at 9.4T. All anatomical images were acquired with a T2 *-weighted gradient-echo sequence. A comparison of the influence of four different B0 shimming routines on the image quality was performed. Intrinsic signal-to-noise-ratio maps were determined using a pseudo-multiple replica approach. Measurements with different echo times were compared and processed to one multiecho data image combination image. Based on the multiecho acquisitions, T2 *-relaxation time maps were calculated. Algorithmic spinal cord detection and gray matter/white matter segmentation were tested. RESULTS: An echo time between 9 and 13.8 ms compromised best between gray matter/white matter contrast and image quality. A maximum in-plane resolution of 0.15 × 0.15 mm2 was achieved for anatomical images. These images offered excellent image quality and made small structures of the spinal cord visible. The scanner vendor implemented B0 shimming routine performed best during this work. Intrinsic signal-to-noise-ratio values of between 6600 and 8060 at the upper cervical spinal cord were achieved. Detection and segmentation worked reliably. An average T2 *-time of 24.88 ms ± 6.68 ms for gray matter and 19.37 ms ± 8.66 ms for white matter was calculated. CONCLUSION: The proposed human brain coil can be used to image the spinal cord. The maximum in-plane resolution in this work was higher compared with the 7T results from the literature. The 9.4T acquisitions made the small structures of the spinal cord clearly visible.

Combined Targeted Analysis of Metabolites and Proteins in Tear Fluid With Regard to Clinical Applications.

PURPOSE: To establish a robust workflow for combined mass spectrometry-based analysis of metabolites and proteins in tear fluid with regard to clinical applicability. METHODS: Tear fluid was taken from 12 healthy volunteers at different time points using specially designed Schirmer strips. Following the liquid extraction of metabolites from standardized punches, the remaining material was processed for bottom-up proteomics. Targeted metabolite profiling was performed adapting a metabolomics kit, which targets 188 metabolites from four different analyte classes. Proteomics was performed of the identical samples targeting 15 tear proteins relevant to ocular health. RESULTS: Sixty metabolites could be consistently determined in all tear samples (98 metabolites were detectable in average) covering acylcarnitines, amino acids, biogenic amines, and glycerophospholipids. Following normalization, the majority of metabolites exhibited intraindividual variances of less than 20%, both regarding different times of sampling, and the individual eye. The targeted analysis of tear proteins revealed a mean intraindividual variation of 23% for the three most abundant proteins. Even extreme differences in tear secretion rates resulted in interindividual variability below 30% for 65 metabolites and two proteins. CONCLUSIONS: The newly established workflow can be used for combined targeted detection of metabolites and proteins in one punch of a Schirmer strip in a clinical setting. TRANSLATIONAL RELEVANCE: Our data about intra- and interindividual as well as intereye variation provide a valuable basis for the design of clinical studies, and for the applicability of multiplexed "omics" to well accessible tear fluid with regard to future routine use.