Browsing by Author "Sadeghi-Tarakameh, Alireza"

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    ItemOpen Access
    Analysis and mitigation of noise in simultaneous transmission and reception in MRI
    (John Wiley & Sons, Inc., 2021-03-05) Taşdelen, Bilal; Sadeghi-Tarakameh, Alireza; Yılmaz, Uğur; Atalar, Ergin
    Purpose In simultaneous transmission and reception (STAR) MRI, along with the coupling of the excitation pulse to the received signal, noise, and undesired distortions (spurs) coming from the transmit chain also leak into the acquired signal and degrade image quality. Here, properties of this coupled noise and its relationship with the transmit amplifier gain, transmit chain noise density, isolation performance, and imaging bandwidth are analyzed. It is demonstrated that by utilizing a recently proposed STAR technique, the transmit noise can be reduced. The importance of achieving high isolation and careful selection of the corresponding parameters are demonstrated. Theory and Methods A cancellation algorithm, together with a vector modulator, is used for transmit-receive isolation. The scanner is modeled as a pipeline of blocks to demonstrate the noise contribution from each block. With higher isolation, coupled transmit noise can be reduced to the point that the dominant noise source becomes acquisition noise, as in the case for pulsed MRI. Amplifiers with different gain and noise properties are used in the experiments to verify the derived noise-transmit parameter relation. Results With the proposed technique, more than 80 dB isolation in the analog domain is achieved. The leakage noise and the spurs coupled from the transmit chain, are reduced. It is shown that the transmit gain plays the most critical role in determining sufficient isolation, whereas the amplifier noise figure does not contribute as much. Conclusion The transmit noise and the spurs in STAR imaging are analyzed and mitigated by using a vector modulator.
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    ItemOpen 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%.
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    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.
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    ItemOpen 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.