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This paper investigates the utility and accuracy of the use of fine-wire thermocouples for time resolved thermometry in turbulent flames. The problem, if course, is that there is no unique time constant in a turbulent flame with widely fluctuating temperature. An analytical formulation is presented which shows that while little error may be expected in determination of r.m.s. values of temperature, there may be a substantial error made in deduction of the mean temperature, under certain circumstances. Experimentally, in a premixed methane-air flame, a comparison is made of a time resolved compensated thermocouple measurement with a simultaneous measurement by molecular Rayleigh scattering, and the results compared with the theoretical predictions. The thermocouple, compensated through a single time constant, behaved admirably, under these conditions. [ABSTRACT FROM PUBLISHER]

Objective: Transcranial focused ultrasound (tFUS) is being explored for neuroscience research and clinical applications due to its ability to affect precise brain regions noninvasively. The ability to target specific brain regions and localize the beam during these procedures is important for these applications to avoid damage and minimize off-target effects. Here, we present a method to combine optical tracking with magnetic resonance (MR) acoustic radiation force imaging to achieve targeting and localizing of the tFUS beam. This combined method provides steering coordinates to target brain regions within a clinically practical time frame., Methods: Using an optically tracked hydrophone and bias correction with MR imaging we transformed the FUS focus coordinates into the MR space for targeting and error correction. We validated this method in vivo in 18 macaque FUS studies., Results: Across these in vivo studies a single localization scan allowed for the average targeting error to be reduced from 4.8 mm to 1.4 mm and for multiple brain regions to be targeted with one transducer position., Conclusions: By reducing targeting error and providing the means to target multiple brain regions within a single session with high accuracy this method will allow further study of the effects of tFUS neuromodulation with more advanced approaches such as simultaneous dual or multi-site brain stimulation.

Purpose: To reduce the inter-scanner variability of diffusion MRI (dMRI) measures between scanners from different vendors by developing a vendor-neutral dMRI pulse sequence using the open-source vendor-agnostic Pulseq platform., Methods: We implemented a standard EPI based dMRI sequence in Pulseq. We tested it on two clinical scanners from different vendors (Siemens Prisma and GE Premier), systematically evaluating and comparing the within- and inter-scanner variability across the vendors, using both the vendor-provided and Pulseq dMRI sequences. Assessments covered both a diffusion phantom and three human subjects, using standard error (SE) and Lin's concordance correlation to measure the repeatability and reproducibility of standard DTI metrics including fractional anisotropy (FA) and mean diffusivity (MD)., Results: Identical dMRI sequences were executed on both scanners using Pulseq. On the phantom, the Pulseq sequence showed more than a 2.5× reduction in SE (variability) across Siemens and GE scanners. Furthermore, Pulseq sequences exhibited markedly reduced SE in-vivo, maintaining scan-rescan repeatability while delivering lower variability in FA and MD (more than 50% reduction in cortical/subcortical regions) compared to vendor-provided sequences., Conclusion: The Pulseq diffusion sequence reduces the cross-scanner variability for both phantom and in-vivo data, which will benefit multi-center neuroimaging studies and improve the reproducibility of neuroimaging studies., (© 2024 International Society for Magnetic Resonance in Medicine.)

Purpose: To compare the performance of multi-echo (ME) and time-division multiplexing (TDM) sequences for accelerated relaxation-diffusion MRI (rdMRI) acquisition and to examine their reliability in estimating accurate rdMRI microstructure measures., Method: The ME, TDM, and the reference single-echo (SE) sequences with six echo times (TE) were implemented using Pulseq with single-band (SB-) and multi-band 2 (MB2-) acceleration factors. On a diffusion phantom, the image intensities of the three sequences were compared, and the differences were quantified using the normalized root mean squared error (NRMSE). For the in-vivo brain scan, besides the image intensity comparison and T2-estimates, different methods were used to assess sequence-related effects on microstructure estimation, including the relaxation diffusion imaging moment (REDIM) and the maximum-entropy relaxation diffusion distribution (MaxEnt-RDD)., Results: TDM performance was similar to the gold standard SE acquisition, whereas ME showed greater biases (3-4× larger NRMSEs for phantom, 2× for in-vivo). T2 values obtained from TDM closely matched SE, whereas ME sequences underestimated the T2 relaxation time. TDM provided similar diffusion and relaxation parameters as SE using REDIM, whereas SB-ME exhibited a 60% larger bias in the 2 > map and on average 3.5× larger bias in the covariance between relaxation-diffusion coefficients., Conclusion: Our analysis demonstrates that TDM provides a more accurate estimation of relaxation-diffusion measurements while accelerating the acquisitions by a factor of 2 to 3.

The use of focused ultrasound to open the blood-brain barrier (BBB) has the potential to deliver drugs to specific regions of the brain. The size of the BBB opening and ability to localize the opening determines the spatial extent and is a limiting factor in many applications of BBB opening where targeting a small brain region is desired. Here we evaluate the performance of a system designed for small opening volumes and highlight the unique challenges associated with pushing the spatial precision of this technique. To achieve small volume openings in cortical regions of the macaque brain, we tested a custom 1 MHz array transducer integrated into a magnetic resonance image-guided focused ultrasound system. Using real-time cavitation monitoring, we demonstrated twelve instances of single sonication, small volume BBB opening with average volumes of 59 ± 37 mm 3 and 184 ± 2 mm 3 in cortical and subcortical targets, respectively. We found high correlation between subject-specific acoustic simulations and observed openings when incorporating grey matter segmentation (R 2  = 0.8577), and the threshold for BBB opening based on simulations was 0.53 MPa. Analysis of MRI-based safety assessment and cavitation signals indicate a safe pressure range for 1 MHz BBB opening and suggest that our system can be used to deliver drugs and gene therapy to small brain regions., Competing Interests: Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2023. Published by Elsevier B.V.)

Localizing the focus during transcranial focused ultrasound procedures is important to ensure accurate targeting of specific brain regions and interpretation of results. Magnetic resonance acoustic radiation force imaging uses the displacement induced by the ultrasound focus in the brain to localize the beam, but the high pressure required to displace brain tissue may cause damage or confounds during subsequent neuromodulatory experiments. Here, reduced apertures were applied to a phased array transducer to generate comparable displacement to the full aperture but with 20% lower free field pressure., (© 2022 Author(s).)

The complexity of the MRI protocol is one of the factors limiting the clinical adoption of MR temperature mapping for real-time monitoring of cardiac ablation procedures and a push-button solution would ease its use. Continuous gradient echo golden angle radial acquisition combined with intra-scan motion correction and undersampled temperature determination could be a robust and more user-friendly alternative than the ultrafast GRE-EPI sequence which suffers from sensitivity to magnetic field susceptibility artifacts and requires ECG-gating. The goal of this proof-of-concept work is to establish the temperature uncertainty as well as the spatial and temporal resolutions achievable in an Agar-gel phantom and in vivo using this method. GRE radial golden angle acquisitions were used to monitor RF ablations in a phantom and in vivo in two sheep hearts with different slice orientations. In each case, 2D rigid motion correction based on catheter micro-coil signal, tracking its motion, was performed and its impact on the temperature imaging was assessed. The temperature uncertainty was determined for three spatial resolutions (1 × 1 × 3 mm 3 , 2 × 2 × 3 mm 3 , and 3 × 3 × 3 mm 3 ) and three temporal resolutions (0.48, 0.72, and 0.97 s) with undersampling acceleration factors ranging from 2 to 17. The combination of radial golden angle GRE acquisition, simultaneous catheter tracking, intra-scan 2D motion correction, and undersampled thermometry enabled temperature monitoring in the myocardium in vivo during RF ablations with high temporal (< 1 s) and high spatial resolution. The temperature uncertainty ranged from 0.2 ± 0.1 to 1.8 ± 0.2 °C for the various temporal and spatial resolutions and, on average, remained superior to the uncertainty of an EPI acquisition while still allowing clinical monitoring of the RF ablation process. The proposed method is a robust and promising alternative to EPI acquisition to monitor in vivo RF cardiac ablations. Further studies remain required to improve the temperature uncertainty and establish its clinical applicability., (© 2022. The Author(s).)

Variable flip angle (VFA) sequences are a popular method of calculating T 1 values, which are required in a quantitative analysis of dynamic contrast-enhanced (DCE) magnetic resonance imaging (MRI). B 1 inhomogeneities are substantial in the breast at 3 T, and these errors negatively impact the accuracy of the VFA approach, thus leading to large errors in the DCE-MRI parameters that could limit clinical adoption of the technique. This study evaluated the ability of Bloch-Siegert B 1 mapping to improve the accuracy and precision of VFA-derived T 1 measurements in the breast. Test-retest MRI sessions were performed on 16 women with no history of breast disease. T 1 was calculated using the VFA sequence, and B 1 field variations were measured using the Bloch-Siegert methodology. As a gold standard, inversion recovery (IR) measurements of T 1 were performed. Fibroglandular tissue and adipose tissue from each breast were segmented using the IR images, and the mean T 1 was calculated for each tissue. Accuracy was evaluated by percent error (%err). Reproducibility was assessed via the 95% confidence interval (CI) of the mean difference and repeatability coefficient ( r ). After B 1 correction, %err significantly ( P < .001) decreased from 17% to 8.6%, and the 95% CI and r decreased from ±94 to ±38 milliseconds and from 276 to 111 milliseconds, respectively. Similar accuracy and reproducibility results were observed in the adipose tissue of the right breast and in both tissues of the left breast. Our data show that Bloch-Siegert B 1 mapping improves accuracy and precision of VFA-derived T 1 measurements in the breast.

Purpose: To investigate the performance of different reconstruction methods for monitoring temperature changes during transcranial magnetic resonance imaging (MRI)-guided focused ultrasound (MRgFUS)., Materials and Methods: Four different temperature reconstruction methods were compared in volunteers (without heating) and patients undergoing transcranial MRgFUS: single baseline subtraction, multibaseline subtraction, hybrid single baseline/referenceless reconstruction, and hybrid multibaseline/referenceless reconstruction. Absolute temperature error and temporal temperature uncertainty of the different reconstruction methods were analyzed and compared., Results: Absolute temperature errors and temporal temperature uncertainty were highest with single baseline subtraction and lowest with hybrid multibaseline/referenceless reconstruction in all areas of the brain. Pulsation of the brain and susceptibility changes from tongue motion or swallowing caused substantial temperature errors when single or multibaseline subtraction was used, which were much reduced when the referenceless component was added to the reconstruction., Conclusion: Hybrid multibaseline/referenceless thermometry accurately measures temperature changes in the brain with fewer artifacts and errors due to motion than pure baseline subtraction methods., (Copyright © 2013 Wiley Periodicals, Inc.)

MR guidance of high intensity focused ultrasound is evolving with each new application. In this paper we describe ongoing research in the MR-guidance aspect of MR-guided focused ultrasound. The structure is divided into the pretreatment/setup phase of the procedure, MR thermometry for monitoring the actual treatment, and methods for assessment and follow-up.

A new MRI-based measurement of the magnetic vector field distribution generated by a transcranial magnetic stimulation (TMS) probe is introduced in this study. Projections of the static magnetic field produced by a DC current in a TMS probe onto the axis of the scanner's main field (B0) can be measured using a conventional B0 field-mapping technique. The remaining Cartesian components can then be calculated from projections obtained at different orientations of the sample and probe. The measured static field can then be scaled to calculate the effects of a dynamic TMS pulse. This article includes a study demonstrating the feasibility of this approach on a phantom and a human subject.

Parallel transmitter techniques are a promising approach for reducing transmitter B1 inhomogeneity due to the potential for adjusting the spatial excitation profile with independent RF pulses. These techniques may be further improved with transmit sensitivity encoding (SENSE) methods because the sensitivity information in pulse design provides an excitation that is inherently compensated for transmitter B1 inhomogeneity. This paper presents a proof of this concept using transmit SENSE 3D tailored RF pulses designed for small flip angles. An eight-channel receiver coil was used to mimic parallel transmission for brain imaging at 3T. The transmit SENSE pulses were based on the fast-k(z) design and produced 5-mm-thick slices at a flip angle of 30 degrees with only a 4.3-ms pulse length. It was found that the transmit SENSE pulses produced more homogeneous images than those obtained from the complex sum of images from all receivers excited with a standard RF pulse., ((c) 2007 Wiley-Liss, Inc.)

Parallel excitation has been introduced as a means of accelerating multidimensional, spatially-selective excitation using multiple transmit coils, each driven by a unique RF pulse. Previous approaches to RF pulse design in parallel excitation were either formulated in the frequency domain or restricted to echo-planar trajectories, or both. This paper presents an approach that is formulated as a quadratic optimization problem in the spatial domain and allows the use of arbitrary k-space trajectories. Compared to frequency domain approaches, the new design method has some important advantages. It allows for the specification of a region of interest (ROI), which improves excitation accuracy at high speedup factors. It allows for magnetic field inhomogeneity compensation during excitation. Regularization may be used to control integrated and peak pulse power. The effects of Bloch equation nonlinearity on the large-tip-angle excitation error of RF pulses designed with the method are investigated, and the utility of Tikhonov regularization in mitigating this error is demonstrated., (Copyright (c) 2006 Wiley-Liss, Inc.)