Browsing by Subject "RF safety"
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Item Open Access A nine-channel transmit/receive array for spine imaging at 10.5 T: Introduction to a nonuniform dielectric substrate antenna(John Wiley & Sons, Inc., 2021-11-05) Sadeghi-Tarakameh, Alireza; Jungst, S.; Lanagan, M.; DelaBarre, L.; Wu, X.; Adriany, G.; Metzger, G. I.; Moortele, P. F.; Ugurbil, K.; Atalar, Ergin; Eryaman, Y.Purpose: The purpose of this study is to introduce a new antenna element with improved transmit performance, named the nonuniform dielectric substrate(NODES) antenna, for building transmit arrays at ultrahigh- field.Methods: We optimized a dipole antenna at 10.5 Tesla by maximizing the B+1- SAR efficiency in a phantom for a human spine target. The optimization pa-rameters included permittivity variation in the substrate, substrate thickness, antenna length, and conductor geometry. We conducted electromagnetic simu-lations as well as phantom experiments to compare the transmit/receive perfor-mance of the proposed NODES antenna design with existing coil elements from the literature.Results: Single NODES element showed up to 18% and 30% higher B+1- SAR ef-ficiency than the fractionated dipole and loop elements, respectively. The new element is substantially shorter than a commonly used dipole, which enables z- stacked array formation; it is additionally capable of providing a relatively uni-form current distribution along its conductors. The nine- channel transmit/re-ceive NODES array achieved 7.5% higher B+1homogeneity than a loop array with the same number of elements. Excitation with the NODES array resulted in 33% lower peak 10g- averaged SAR and required 34% lower input power than the loop array for the target anatomy of the spine.Conclusion: In this study, we introduced a new RF coil element: the NODES antenna. NODES antenna outperformed the widely used loop and dipole ele-ments and may provide improved transmit/receive performance for future ultra-high field MRI applications.Item Open Access Novel techniques and innovative designs for the RF chain of magnetic resonance imaging scanners(Bilkent University, 2020-07) Tarakameh, Alireza SadeghiIn this dissertation, novel techniques and innovative designs are proposed to serve one of the main goals of MRI, which is achieving the best image quality by improving signal-to-noise ratio (SNR) while confined to the patient’s safety restrictions determined by specific absorption rate (SAR) limits. This dissertation comprises five different contributions to the field. First, the co-simulation method is accelerated using the equivalent circuit model of the intended coil and introduced as a compact novel technique for the fast design of transmit array (TxArray) coils. It is shown that the existing co-simulation method can be accelerated by more than 20-fold for different types of TxArray coils. In another study, a simple strategy is introduced for optimizing the topography of transmit elements to reduce the local SAR at ultra-high field (UHF, B0 ≥ 7 T) MRI. It is shown that the peak local 10g-averaged SAR can be reduced more than 20% for different types of transmit elements at two different field strengths (i.e., 7 T and 10.5 T). As another contribution, confining to the safety limits, the first in vivo human head images at 10.5 T is acquired. To ensure the safety of subjects, the EM simulation model of the radiofrequency (RF) coil is validated using an expanded validation workflow, then, safe RF power levels were calculated. In another study, a novel short dipole-like element with improved SAR performance, non-unform dielectric substrate (NODES) antenna, is introduced which provides an opportunity to design highly-dense TxArray coils for UHF-MRI. Eventually, a nine-channel transmit/receive coil is built and human cadaver spine images are acquired at 10.5 T. In the final study, a new parameter, ultimate intrinsic SAR efficiency (UISARE), is introduced and calculated as the upper bound for SAR performance of the transmit coils at different field strengths. Overall, this dissertation proposes novel techniques and innovative structures for designing the TxArray coils, which mainly contribute to enhancing image quality and patient safety by improving SNR and reducing SAR.Item Open Access Reduction of the radiofrequency heating of metallic devices using a dual‐drive birdcage coil(Wiley, 2013) Eryaman, Yiğitcan; Türk, Esra Abacı; Oto, C.; Algin, O.; Atalar, ErginIn this work, it is demonstrated that a dual-drive birdcage coil can be used to reduce the radiofrequency heating of metallic devices during magnetic resonance imaging. By controlling the excitation currents of the two channels of a birdcage coil, the radiofrequency current that is induced near the lead tip could be set to zero. To monitor the current, the image artifacts near the lead tips were measured. The electric field distribution was controlled using a dual-drive birdcage coil. With this method, the lead currents and the lead tip temperatures were reduced substantially [<0.3°C for an applied 4.4 W/kg SAR compared to >4.9°C using quadrature excitation], as demonstrated by phantom and animal experiments. The homogeneity of the flip angle distribution was preserved, as shown by volunteer experiments. The normalized root-mean-square error of the flip angle distribution was less than 10% for all excitations. The average specific absorption rate increased as a trade-off for using different excitation patterns. Copyright © 2012 Wiley Periodicals, Inc.Item Open Access RF Safety of Active Implantable Medical Devices(John Wiley & Sons, Ltd. All rights reserved., 2019) Silemek, Berk; Açıkel, V.; Atalar, Ergin; Harris, R.K.; Wasylishen, R.L.The radiofrequency (RF) safety of active implantable medical devices (AIMDs) during an magnetic resonance imaging (MRI) scan is discussed in this article. The problem arises from the RF interaction of an AIMD with the MRI scanner is presented. The researchers simulated and modeled to understand the problem. They also developed techniques to resolve the RF safety problem by altering the design of AIMDs. Furthermore, implant friendly imaging solutions are developed. Validated the findings novel in vivo techniques. Clinical investigations are carried out to understand the extent of the problem. In this article, an incomplete summary of the investigations in this field is given.Item Open Access Wireless control of induced radiofrequency currents in active implantable medical devices during MRI(International Society for Magnetic Resonance in Medicine, 2020) Açıkel, V.; Silemek, Berk; Atalar, ErginPurpose To introduce a prototype active implantable medical device (AIMD) for which the induced radiofrequency currents can be controlled wirelessly. Methods The modified transmission line method is used to formulate how the lead‐case impedance of an AIMD affects the temperature rise around the electrode. A prototype AIMD is designed with the aim of controlling the unwanted temperature rise around its electrode during an MRI examination by altering the impedance between the lead and the case of the implant. MRI experiments were conducted with this prototype implant, which also has a built‐in temperature sensor at its electrode. During the experiment, the implant’s lead‐case impedance was controlled using Bluetooth communication with a remote computer, and the lead tip temperature was recorded. Results Ten different lead‐case impedance values and their corresponding tip temperature rises were examined during MRI experiments. The experimental results confirmed that the tip temperature rise can be controlled by varying the lead‐case impedance wirelessly. Conclusion The feedback from the temperature at the AIMD tip, together with variable lead‐case impedance, enables control of the safety profile of the AIMD during an MRI examination.