Browsing by Subject "Radiofrequency"

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    Approximate fourier domain expression for bloch-siegert shift
    (John Wiley and Sons Inc., 2015) Turk, E. A.; Ider, Y. Z.; Ergun, A. S.; Atalar, Ergin
    Purpose: In this study, a newsimple Fourier domain-based analytical expression for the Bloch-Siegert (BS) shift-based B1 mapping method is proposed to obtain |B1+| more accurately while using short BS pulse durations and small off-resonance frequencies.Theory and Methods: A new simple analytical expression for the BS shift is derived by simplifying the Bloch equations. In this expression, the phase is calculated in terms of the Fourier transform of the radiofrequency pulse envelope, and thus making the off- and on-resonance effects more easily understandable. To verify the accuracy of the proposed expression, Bloch simulations and MR experiments are performed for the hard, Fermi, and Shinner-Le Roux pulse shapes.Results: Analyses of the BS phase shift-based B1 mapping method in terms of radiofrequency pulse shape, pulse duration, and off-resonance frequency show that |B1+| can be obtained more accurately with the aid of this new expression.Conclusions: In this study, a new simple frequency domain analytical expression is proposed for the BS shift. Using this expression, |B1+| values can be predicted from the phase data using the frequency spectrum of the radiofrequency pulse. This method works well even for short pulse durations and small offset frequencies.
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    Simultaneous radiofrequency (RF) heating and magnetic resonance (MR) thermal mapping using an intravascular MR imaging/RF heating system
    (John Wiley & Sons, 2005-06) Qiu, B.; El-Sharkawy, A.-M.; Paliwal, V.; Karmarkar, P.; Gao, F.; Atalar, Ergin; Yang, X.
    Previous studies have confirmed the possibility of using an intravascular MR imaging guidewire (MRIG) as a heating source to enhance vascular gene transfection/expression. This motivated us to develop a new intravascular system that can perform MR imaging, radiofrequncy (RF) heating, and MR temperature monitoring simultaneously in an MR scanner. To validate this concept, a series of mathematical simulations of RF power loss along a 0.032-inch MRIG and RF energy spatial distribution were performed to determine the optimum RF heating frequency. Then, an RF generator/amplifier and a filter box were built. The possibility for simultaneous RF heating and MR thermal mapping of the system was confirmed in vitro using a phantom, and the obtained thermal mapping profile was compared with the simulated RF power distribution. Subsequently, the feasibility of simultaneous RF heating and temperature monitoring was successfully validated in vivo in the aorta of living rabbits. This MR imaging/RF heating system offers a potential tool for intravascular MR-mediated, RF-enhanced vascular gene therapy.
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    Specific absorption rate reduction using nonlinear gradient fields
    (Wiley, 2013) Kopanoglu, E.; Yilmaz, U.; Gokhalk, Y.; Atalar, Ergin
    The specific absorption rate is used as one of the main safety parameters in magnetic resonance imaging. The performance of imaging sequences is frequently hampered by the limitations imposed on the specific absorption rate that increase in severity at higher field strengths. The most well-known approach to reducing the specific absorption rate is presumably the variable rate selective excitation technique, which modifies the gradient waveforms in time. In this article, an alternative approach is introduced that uses gradient fields with nonlinear variations in space to reduce the specific absorption rate. The effect of such gradient fields on the relationship between the desired excitation profile and the corresponding radiofrequency pulse is shown. The feasibility of the method is demonstrated using three examples of radiofrequency pulse design. Finally, the proposed method is compared with and combined with the variable rate selective excitation technique. © 2012 Wiley Periodicals, Inc.
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    Superconductivity in high-frequency fields
    (Overseas Publishers Association, 1998-01-15) Kulik, I. A.
    Fundamentals of BCS and GLAG theories of superconductivity are reviewed with a focus on high.;,frequency properties of bulk superconductors, superconducting films and superconducting cavities. Superconductivity as a macroscopic quantum coherent state. Supercurrents and persistent currents. Condensate and excitations. Complex penetration depth and RF losses. Mechanisms of Q degradation in superconducting cavities at increasing a.c. field amplitude. Depairing effects and vortex nucleation mechanisms. Surface superconductivity and tilted vortices. Material parameters and factors responsible for ultimate performance of superconducting resonators.
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    Theory of Q-degradation and nonlinear effects in Nb-coated superconducting cavities
    (Gordon and Breach - Harwood Academic, 1998) Kulik, I. A.; Palmieri, V.
    Amplitude-dependent absorption of RF power in superconducting cavity with a wall of normal metal (Cu ) covered by a film of superconductor (Nb ) is determined by three factors: (1)depairing effect of high-current density on superconducting energy gap; (2)increase of electromagnetic power penetrating to normal metal and Drude-absorbed in it, at higher RF amplitude; (3)heating of both superconducting coating and normal substrate resulting in increase of quasiparticle excitation density at higher RF power (P). Cavity Q-factor is calculated as a hnction of RF amplitude A = fi and is shown to follow a relationship 1n(Q/ Q,) = -const-P" with a=l at low temperature T < T* and a=1/2 at T > T* where T* is characteristic temperature T* X TcS I d, S is the London penetration depth and d the thickness of superconducting coating.