Browsing by Author "Eryaman, Y."
Now showing 1 - 10 of 10
Results Per Page
Sort Options
Item Open Access Comments on "Ensuring Safety of Implanted Devices Under MRI Using Reversed Polarization"(Wiley, 2011-10-24) Eryaman, Y.; Hersek, S.; Atalar, ErginItem Open Access Design of internal MRI coils using ultimate intrinsic SNR(Springer, 2009-09-27) Eryaman, Y.; Öner, Y.; Atalar, ErginObject: Internal MRI coils have important applications in diagnostic and interventional studies. Since they can be placed very close to the region of interest in the body, they are favored over external coils in applications where high-resolution images are required. In this paper it is demonstrated that ultimate intrinsic SNR (UISNR) and the optimum coil sensitivity solutions can be used to make new coil designs with higher intrinsic SNR. Materials and methods: In this study, UISNR, which is the maximum attainable value of the intrinsic SNR, is used as a measure of performance and as a design criterion. As an example, a novel endorectal MRI coil is designed. The design is tested with phantom and patient studies. Results: An endorectal coil is built to demonstrate the effectiveness of the design strategy. ISNR of the endorectal coil approximates the UISNR to 72%. Conclusion: An internal coil design method that takes advantage of the UISNR and optimum coil sensitivity calculations was presented. This method can also be used to design better internal MRI coils for different applications.Item Open Access Effect of field strength on RF power deposition near conductive leads: a simulation study of SAR in DBS lead models during mri at 1.5 T-10.5 T(Public Library of Science, 2023-01-26) Kazemivalipour, Ehsan; Sadeghi-Tarakameh, A.; Keil, B.; Eryaman, Y.; Atalar, Ergin; Golestanirad, L.Background Since the advent of magnetic resonance imaging (MRI) nearly four decades ago, there has been a quest for ever-higher magnetic field strengths. Strong incentives exist to do so, as increasing the magnetic field strength increases the signal-to-noise ratio of images. However, ensuring patient safety becomes more challenging at high and ultrahigh field MRI (i.e., ≥3 T) compared to lower fields. The problem is exacerbated for patients with conductive implants, such as those with deep brain stimulation (DBS) devices, as excessive local heating can occur around implanted lead tips. Despite extensive effort to assess radio frequency (RF) heating of implants during MRI at 1.5 T, a comparative study that systematically examines the effects of field strength and various exposure limits on RF heating is missing. Purpose This study aims to perform numerical simulations that systematically compare RF power deposition near DBS lead models during MRI at common clinical and ultra-high field strengths, namely 1.5, 3, 7, and 10.5 T. Furthermore, we assess the effects of different exposure constraints on RF power deposition by imposing limits on either the B1+ or global head specific absorption rate (SAR) as these two exposure limits commonly appear in MRI guidelines. Methods We created 33 unique DBS lead models based on postoperative computed tomography (CT) images of patients with implanted DBS devices and performed electromagnetic simulations to evaluate the SAR of RF energy in the tissue surrounding lead tips during RF exposure at frequencies ranging from 64 MHz (1.5 T) to 447 MHz (10.5 T). The RF exposure was implemented via realistic MRI RF coil models created based on physical prototypes built in our institutions. We systematically examined the distribution of local SAR at different frequencies with the input coil power adjusted to either limit the B1+ or the global head SAR. Results The MRI RF coils at higher resonant frequencies generated lower SARs around the lead tips when the global head SAR was constrained. The trend was reversed when the constraint was imposed on B1+. Conclusion At higher static fields, MRI is not necessarily more dangerous than at lower fields for patients with conductive leads. Specifically, when a conservative safety criterion, such as constraints on the global SAR, is imposed, coils at a higher resonant frequency tend to generate a lower local SAR around implanted leads due to the decreased B1+ and, by proxy, E field levels.Item Open Access Evaluation of internal MRI coils using ultimate intrinsic SNR(John Wiley & Sons, 2004) Çelik, H.; Eryaman, Y.; Altıntaş, A.; Abdel-Hafez, I. A.; Atalar, ErginThe upper bounds of the signal-to-noise ratio (also known as the "ultimate intrinsic signal-to-noise ratio" (UISNR)) for internal and external coils were calculated. In the calculation, the body was modeled as a dielectric cylinder with a small coaxial cylindrical cavity in which internal coils could be placed. The calculated UISNR values can be used as reference solutions to evaluate the performance of internal MRI coils. As examples, we evaluated the performance of a loopless antenna and an endourethral coil design by comparing their ISNR with the UISNR.Item Open Access Improving radiofrequency power and specific absorption rate management with bumped transmit elements in ultra-high field MRI(Wiley, 2020) Sadeghi-Tarakameh, Alireza; Adriany, G.; Metzger, G. J.; Lagore, R. L.; Jungst, S.; DelaBarre, L.; Van de Moortele, P. F.; Uğurbil, K.; Atalar, Ergin; Eryaman, Y.Purpose: In this study, we investigate a strategy to reduce the local specific absorption rate (SAR) while keeping constant inside the region of interest (ROI) at the ultra‐high field (B0 ≥ 7T) MRI. Methods: Locally raising the resonance structure under the discontinuity (i.e., creating a bump) increases the distance between the accumulated charges and the tissue. As a result, it reduces the electric field and local SAR generated by these charges inside the tissue. The at a point that is sufficiently far from the coil, however, is not affected by this modification. In this study, three different resonant elements (i.e., loop coil, snake antenna, and fractionated dipole [FD]) are investigated. For experimental validation, a bumped FD is further investigated at 10.5T. After the validation, the transmit performances of eight‐channel arrays of each element are compared through electromagnetic (EM) simulations. Results: Introducing a bump reduced the peak 10g‐averaged SAR by 21, 26, 23% for the loop and snake antenna at 7T, and FD at 10.5T, respectively. In addition, eight‐channel bumped FD array at 10.5T had a 27% lower peak 10g‐averaged SAR in a realistic human body simulation (i.e., prostate imaging) compared to an eight‐channel FD array. Conclusion: In this study, we investigated a simple design strategy based on adding bumps to a resonant element to reduce the local SAR while maintaining inside an ROI. As an example, we modified an FD and performed EM simulations and phantom experiments with a 10.5T scanner. Results show that the peak 10g‐averaged SAR can be reduced more than 25%.Item Open Access In vivo human head MRI at 10.5T: a radiofrequency safety study and preliminary imaging results(Wiley, 2020) Sadeghi-Tarakameh, Alireza; DelaBarre, L.; Lagore, R. L.; Torrado-Carvajal, A.; Wu, X.; Grant, A.; Adriany, G.; Metzger, G. J.; Van de Moortele, P.-F.; Uğurbil, K.; Atalar, Ergin; Eryaman, Y.Purpose: The purpose of this study is to safely acquire the first human head images at 10.5T. Methods: To ensure safety of subjects, we validated the electromagnetic simulation model of our coil. We obtained quantitative agreement between simulated and experimental and specific absorption rate (SAR). Using the validated coil model, we calculated radiofrequency power levels to safely image human subjects. We conducted all experiments and imaging sessions in a controlled radiofrequency safety lab and the whole‐body 10.5T scanner in the Center for Magnetic Resonance Research. Results: Quantitative agreement between the simulated and experimental results was obtained including S‐parameters, maps, and SAR. We calculated peak 10 g average SAR using 4 different realistic human body models for a quadrature excitation and demonstrated that the peak 10 g SAR variation between subjects was less than 30%. We calculated safe power limits based on this set and used those limits to acquire T2‐ and ‐weighted images of human subjects at 10.5T. Conclusions: In this study, we acquired the first in vivo human head images at 10.5T using an 8‐channel transmit/receive coil. We implemented and expanded a previously proposed workflow to validate the electromagnetic simulation model of the 8‐channel transmit/receive coil. Using the validated coil model, we calculated radiofrequency power levels to safely image human subjects.Item Open Access Miniaturized fiber-optic transmission system for MRI signals(John Wiley & Sons, Inc., 2008) Memis, O. G.; Eryaman, Y.; Aytur, O.; Atalar, ErginConventional MRI instruments transmit received MRI signals through electrical cables. Although this design has proved to be effective over the years, we report a fiber-optic system that addresses the needs of recent developments in MRI technology. One of these technologies is phased array coils with a high number of elements, where total size of interconnections is a primary problem, and other problem is internal MRI coils, where there is a need for improvements in safety. The Miniature Fiber-Optic Transmission (FOT) System was developed to address these issues. The system consists of a receiver coil with active detuning, a low-noise preamplifier, and a laser diode connected to a photodetector with fiber-optic cabling. The overall noise figure of the system is lower than 1 dB. Total power consumption is 50 mW, and the device is switchable with another fiber-optic line, which can also control active detuning. A prototype device was tested in a GE 1.5 Tesla MRI scanner, and several images were acquired with a signal to noise ratio similar to coaxial cabling. We believe that this design will reduce the cabling problems of arrays and enable placement of internal coils into body cavities with no safety hazard to the patient, such as electrical shock or burns.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 Reduction of implant RF heating through modification of transmit coil electric field(WILEY, 2010-12-08) Eryaman, Y.; Akin, B.; Atalar, ErginIn this work, we demonstrate the possibility to modify the electric-field distribution of a radio frequency (RF) coil to generate electric field-free zones in the body without significantly altering the transmit sensitivity. Because implant heating is directly related to the electric-field distribution, implant-friendly RF transmit coils can be obtained by this approach. We propose a linear birdcage transmit coil with a zero electric-field plane as an example of such implant-friendly coils. When the zero electric-field plane coincides with the implant position, implant heating is reduced, as we demonstrated by the phantom experiments. By feeding RF pulses with identical phases and shapes but different amplitudes to the two orthogonal ports of the coil, the position of the zero electric-field plane can also be adjusted. Although implant heating is reduced with this method, a linear birdcage coil results in a whole-volume average specific absorption rate that is twice that of a quadrature birdcage coil. To solve this issue, we propose alternative methods to design implant-friendly RF coils with optimized electromagnetic fields and reduced whole-volume average specific absorption rate. With these methods, the transmit field was modified to reduce RF heating of implants and obtain uniform transmit sensitivity.Item Open Access A workflow for predicting temperature increase at the electrical contacts of deep brain stimulation electrodes undergoing MRI(John Wiley & Sons, Ltd, 2022-06-04) Tarakameh, A.R.; Zulkarnain, N.I.H.; He, X.; Atalar, Ergin; Harel, N.; Eryaman, Y.Purpose: The purpose of this study is to present a workflow for predicting the radiofrequency (RF) heating around the contacts of a deep brain stimulation (DBS) lead during an MRI scan. Methods: The induced RF current on the DBS lead accumulates electric charge on the metallic contacts, which may cause a high local specific absorption rate (SAR), and therefore, heating. The accumulated charge was modeled by imposing a voltage boundary condition on the contacts in a quasi-static electromagnetic (EM) simulation allowing thermal simulations to be performed with the resulting SAR distributions. Estimating SAR and temperature increases from a lead in vivo through EM simulation is not practical given anatomic differences and variations in lead geometry. To overcome this limitation, a new parameter, transimpedance, was defined to characterize a given lead. By combining the transimpedance, which can be measured in a single calibration scan, along with MR-based current measurements of the lead in a unique orientation and anatomy, local heating can be estimated. Heating determined with this approach was compared with results from heating studies of a commercial DBS electrode in a gel phantom with different lead configurations to validate the proposed method. Results: Using data from a single calibration experiment, the transimpedance of a commercial DBS electrode (directional lead, Infinity DBS system, Abbott Laboratories, Chicago, IL) was determined to be 88 Ω. Heating predictions using the DBS transimpedance and rapidly acquired MR-based current measurements in 26 different lead configurations resulted in a <23% (on average 11.3%) normalized root-mean-square error compared to experimental heating measurements during RF scans. Conclusion: In this study, a workflow consisting of an MR-based current measurement on the DBS lead and simple quasi-static EM/thermal simulations to predict the temperature increase around a DBS electrode undergoing an MRI scan is proposed and validated using a commercial DBS electrode. © 2022 The Authors. Magnetic Resonance in Medicine published by Wiley Periodicals LLC on behalf of International Society for Magnetic Resonance in Medicine.