To the Editor: We read with interest the article entitled “In Vivo Imaging of the Inner Ear at 7 T MRI: Image Evaluation and Comparison with 3 T” from Van Egmond et al. (1). As referenced by the authors, we have previously reported on the development of an imaging protocol at 7 T for inner ear imaging using high-permittivity dielectric pads. Our research group has studied the importance of geometry, positioning, and contents of these pads to overcome B1 field inhomogeneities present at high field (2,3). Specifically, because of the intrinsic asymmetry of the B1 field, different geometries were needed on either side of the head, and a sex-specific design was needed to account for anatomic differences between males and females (2). Neither of these aspects was discussed in the study of Van Egmond et al. (1). Therefore, we are interested in the design details of the unilaterally applied dielectric pad. Furthermore, we like to emphasize that it is the elliptical shape of the head that leads to the typical B1 inhomogeneities at the location of the inner ear (4) and not the air-bone-fluid interfaces, as mentioned by Van Egmond et al. (1). In our studies, we successfully applied our protocol in a patient population (n = 13) with detailed evaluation of 24 anatomic structures imaged at 3 T and 7 T, as published in the American Journal of Neuroradiology (5). We found a significant improvement of visualization of 11 anatomic structures scanned at 7 T compared with 3 T. Seven of these structures were also evaluated by Van Egmond et al. (1), but a fair comparison with our results cannot be made because no statistical tests were performed and the resolution between the 7-T protocols differs notably. One detail of the study by Van Egmond et al. (1) that drew our attention is the marginal increase in in-plane resolution with the transition from 3- to 7-T images. Besides a reduction in slice thickness from 1.0 to 0.5 mm, the in-plane resolution was only increased from 0.5 × 0.6 mm2 to 0.5 × 0.5 mm2. It is not clear whether this marginal increase in resolution has any potential in yielding a more detailed depiction of the inner ear microstructures, as suggested by the authors in the introduction. Indeed, the acquisition time was more than tripled (from 4.32 to 15 min). Especially from a clinical perspective, it can be questioned whether this time cost is justified for a minor gain in diagnostic value, as confirmed also by the authors; no additional anatomic information was obtained in the 7-T images. In contrast, in our study, the acquisition time was also increased, albeit by a much smaller fraction, from 6 to 10 hours; however, this was motivated by an eightfold increase in resolution (0.6–0.3 mm3) and a significant improvement in the visualization of small anatomic structures of the inner ear (5). The choice of imaging resolution in the current study therefore needs more explanation by the authors to justify their approach. Annerie M. A. van der Jagt, M.D. Department of Otorhinolaryngology Leiden University Medical Center Leiden, The Netherlands [email protected] Wyger M. Brink Andrew Webb, Ph.D. Department of Radiology Leiden University Medical Center Leiden, The Netherlands Johan H. M. Frijns, M.D., Ph.D. Department of Otorhinolaryngology and Leiden Institute for Brain and Cognition Leiden University Medical Center Leiden, The Netherlands Berit M. Verbist, M.D., Ph.D. Department of Radiology Leiden University Medical Center Leiden and Department of Radiology Radboud University Medical Center Nijmegen, The Netherlands The authors disclose no conflicts of interest. This research was financially supported by Advanced Bionics.
Abstract Direct spectroscopic quantification of small molecules using low cost, low field (< 0.1 Tesla) large bore portable magnets is not possible using conventional techniques due the presence of strong homonuclear coupling which results in complicated spectral patterns with resonances separated by much less than the achievable spectral linewidth. In contrast, a method using the signals from a Carr-Purcell-Meiboom-Gill (CPMG) train, in which the data are Fourier transformed in this indirect dimension, can produce so-called J-spectra in which several distinct spectral features can be distinguished. In this work we evaluate this technique to quantify the amount of alcohol (ethanol) in intact bottles of wines or spirits.
This accessible yet in-depth textbook describes the step-by-step processes involved in biomedical device design. Integrating microfabrication techniques, sensors and digital signal processing with key clinical applications, it covers: the measurement, amplification and digitization of physiological signals, and the removal of interfering signals; the transmission of signals from implanted sensors through the body, and the issues surrounding the powering of these sensors; networks for transferring sensitive patient data to hospitals for continuous home-monitoring systems; tests for ensuring patient safety; the cost-benefit and technological trade-offs involved in device design; and current challenges in biomedical device design. With dedicated chapters on electrocardiography, digital hearing aids and mobile health, and including numerous end-of-chapter homework problems, online solutions and additional references for extended learning, it is the ideal resource for senior undergraduate students taking courses in biomedical instrumentation and clinical technology.
This accessible yet in-depth textbook describes the step-by-step processes involved in biomedical device design. Integrating microfabrication techniques, sensors and digital signal processing with key clinical applications, it covers: the measurement, amplification and digitization of physiological signals, and the removal of interfering signals; the transmission of signals from implanted sensors through the body, and the issues surrounding the powering of these sensors; networks for transferring sensitive patient data to hospitals for continuous home-monitoring systems; tests for ensuring patient safety; the cost-benefit and technological trade-offs involved in device design; and current challenges in biomedical device design. With dedicated chapters on electrocardiography, digital hearing aids and mobile health, and including numerous end-of-chapter homework problems, online solutions and additional references for extended learning, it is the ideal resource for senior undergraduate students taking courses in biomedical instrumentation and clinical technology.
High-permittivity dielectric pads, i.e., thin, flexible slabs, usually consisting of mixed ceramic powders and liquids, have been previously shown to increase the magnetic field at high and ultra high-fields in regions of low efficiency of transmit coils, thus improving the homogeneity of images. However, their material parameters can change with time, and some materials they contain are bio incompatible. This article presents an alternative approach replacing ceramic mixtures with a low-cost and stable artificial dielectric slab. The latter comprises a stack of capacitive grids realized using multiple printed-circuit boards. Results in this article show that the proposed artificial dielectric structure can obtain the same increase in the local transmit radiofrequency magnetic field distribution in a head phantom at 7 T as the conventional dielectric pad.
In this work, we experimentally demonstrate an increase in the local transmit efficiency of a 1.5 T MRI scanner by using a metasurface formed by an array of brass wires embedded in a high permittivity low loss medium. Placement of such a structure inside the scanner results in strong coupling of the radiofrequency field produced by the body coil with the lowest frequency electromagnetic eigenmode of the metasurface. This leads to spatial redistribution of the near fields with enhancement of the local magnetic field and an increase in the transmit efficiency per square root maximum specific absorption rate in the region-of-interest. We have investigated this structure in vivo and achieved a factor of 3.3 enhancement in the local radiofrequency transmit efficiency.
Motivation: Inner-Shield in Halbach-array magnet effect RF coil transmit efficiency and signal-to-noise ratio(SNR). With this, finding a good trade of between magnet diameter and SNR is important. Goal(s): Find the proper magnet diameter with respect to the RF coil transmit efficiency. Approach: Simulations of the transmit efficiency of three RF coils used for neuroimaging on a 46 mT Halbach-array point-of-care MRI system have been performed in terms of analyzing the coil to RF shield distance located inside the magnet. Results: Results show that a distance of 1 cm results in a 50% lower transmit/receive efficiency than a 3 cm gap. Impact: This means that slightly larger magnets may have higher signal-to-noise even though the B0 field is lower.
Three-dimensional contrast source inversion-electrical properties tomography (3-D CSI-EPT) is an iterative reconstruction method that estimates the electrical properties of tissue from transmit field magnetic resonance data. However, in order to bring 3-D CSI-EPT into practice for complex tissue structures and to understand the origin and effect of errors, insight in the sensitivities of reconstruction accuracy to the major error-sources is necessary. In this paper, different strategies for implementing 3-D CSI-EPT, including their iterative structure, are presented, of which the regularized implementation shows the most potential to be used in practice. Moreover, the influence of initialization, noise, stopping criteria, incident fields, B1-maps, transceive phase and domain truncation are discussed. We show that of all these different error-sources, initialization, accurate coil models and domain truncation have the most dramatic effect on electrical properties reconstructions in practice.
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