The Experts below are selected from a list of 360 Experts worldwide ranked by ideXlab platform
Rebecca Fahrig - One of the best experts on this subject based on the ideXlab platform.
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electrostatic focal spot correction for X Ray Tubes operating in strong magnetic fields
Medical Physics, 2014Co-Authors: Prasheel Lillaney, Mihye Shin, Waldo Hinshaw, Rebecca FahrigAbstract:Purpose: A close proXimity hybrid X-Ray/magnetic resonance (XMR) imaging system offers several critical advantages over current XMR system installations that have large separation distances (∼5 m) between the imaging fields of view. The two imaging systems can be placed in close proXimity to each other if an X-Ray Tube can be designed to be immune to the magnetic fringe fields outside of the MR bore. One of the major obstacles to robust X-Ray Tube design is correcting for the effects of the MR fringe field on the X-Ray Tube focal spot. Any fringe field component orthogonal to the X-Ray Tube electric field leads to electron drift altering the path of the electron trajectories. Methods: The method proposed in this study to correct for the electron drift utilizes an eXternal electric field in the direction of the drift. The electric field is created using two electrodes that are positioned adjacent to the cathode. These electrodes are biased with positive and negative potential differences relative to the cathode. The design of the focusing cup assembly is constrained primarily by the strength of the MR fringe field and high voltage standoff distances between the anode, cathode, and the bias electrodes. From these constraints, a focusing cup design suitable for the close proXimity XMR system geometry is derived, and a finite element model of this focusing cup geometry is simulated to demonstrate efficacy. A Monte Carlo simulation is performed to determine any effects of the modified focusing cup design on the output X-Ray energy spectrum. Results: An orthogonal fringe field magnitude of 65 mT can be compensated for using bias voltages of +15 and −20 kV. These bias voltages are not sufficient to completely correct for larger orthogonal field magnitudes. Using active shielding coils in combination with the bias electrodes provides complete correction at an orthogonal field magnitude of 88.1 mT. Introducing small fields (<10 mT) parallel to the X-Ray Tube electric field in addition to the orthogonal field does not affect the electrostatic correction technique. However, rotation of the X-Ray Tube by 30° toward the MR bore increases the parallel magnetic field magnitude (∼72 mT). The presence of this larger parallel field along with the orthogonal field leads to incomplete correction. Monte Carlo simulations demonstrate that the mean energy of the X-Ray spectrum is not noticeably affected by the electrostatic correction, but the output fluX is reduced by 7.5%. Conclusions: The maXimum orthogonal magnetic field magnitude that can be compensated for using the proposed design is 65 mT. Larger orthogonal field magnitudes cannot be completely compensated for because a pure electrostatic approach is limited by the dielectric strength of the vacuum inside the X-Ray Tube insert. The electrostatic approach also suffers from limitations when there are strong magnetic fields in both the orthogonal and parallel directions because the electrons prefer to stay aligned with the parallel magnetic field. These challenging field conditions can be addressed by using a hybrid correction approach that utilizes both active shielding coils and biasing electrodes.
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performance evaluation of an mr compatible rotating anode X Ray Tube
ASME 2013 International Mechanical Engineering Congress and Exposition, 2013Co-Authors: Mihye Shin, Waldo Hinshaw, Prasheel Lillaney, Rebecca FahrigAbstract:The key technical innovation needed for close proXimity hybrid X-Ray/MR (XMR) imaging systems is a new rotating anode X-Ray Tube motor that can operate in the presence of strong magnetic fields. In order for the new motor design to be optimized between conflicting design requirements, we implemented a numerical model for evaluating the dynamics of the motor. The model predicts the amount of produced torque, rotation speed, and time to accelerate based on the Lorentz force law; the motor is accelerated by the interaction between the magnetic moments of the motor wire loops and an eXternal magnetic field. It also includes an empirical model of bearing friction and electromagnetic force from the magnetic field. Our proposed computational model is validated by eXperiments using several different magnitudes of eXternal magnetic fields, which averagely shows an agreement within 0.5 % error during acceleration. We are using this model to improve the efficiency and performance of future iterations of the X-Ray Tube motor.Copyright © 2013 by ASME
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resonant frequency of an mr compatible rotating anode X Ray Tube
ASME 2012 International Mechanical Engineering Congress and Exposition, 2012Co-Authors: Mihye Shin, Waldo Hinshaw, Prasheel Lillaney, Rebecca FahrigAbstract:Switching between X-Ray and MR imaging with the systems in close proXimity can facilitate accurate image-guided interventional procedures using their complementary advantages. Conventional X-Ray Tubes use induction motors whose functionality fails within MR fringe field environments; thus we developed a novel MR compatible X-Ray Tube motor.Vibrations from the structural instability of a motor can lead to mechanical failure, as well as image artifacts due to shaking X-Ray focal spot on the anode target; hence it is important for proper X-Ray Tube motor operation to identify the resonant frequencies that cause large amplitude vibrations. The ability to model rotor dynamics and how they change with the motor design is valuable because it allows us to optimize the motor structure. A finite element model has been developed and validated by eXperiments measuring the acceleration change and the power spectrum of the motor response.Copyright © 2012 by ASME
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magnetostatic focal spot correction for X Ray Tubes operating in strong magnetic fields using iterative optimization
Medical Physics, 2012Co-Authors: Prasheel Lillaney, Mihye Shin, Steven M Conolly, Rebecca FahrigAbstract:Purpose: Combining X-Ray fluoroscopy and MR imaging systems for guidance of interventional procedures has become more commonplace. By designing an X-Ray Tube that is immune to the magnetic fields outside of the MR bore, the two systems can be placed in close proXimity to each other. A major obstacle to robust X-Ray Tube design is correcting for the effects of the magnetic fields on the X-Ray Tube focal spot. A potential solution is to design active shielding that locally cancels the magnetic fields near the focal spot. Methods: An iterative optimization algorithm is implemented to design resistive active shielding coils that will be placed outside the X-Ray Tube insert. The optimization procedure attempts to minimize the power consumption of the shielding coils while satisfying magnetic field homogeneity constraints. The algorithm is composed of a linear programming step and a nonlinear programming step that are interleaved with each other. The coil results are verified using a finite element space charge simulation of the electron beam inside the X-Ray Tube. To alleviate heating concerns an optimized coil solution is derived that includes a neodymium permanent magnet. Any demagnetization of the permanent magnet is calculated prior to solving for the optimized coils. The temperature dynamics of the coil solutions are calculated using a lumped parameter model, which is used to estimate operation times of the coils before temperature failure. Results: For a magnetic field strength of 88 mT, the algorithm solves for coils that consume 588 A/cm2. This specific coil geometry can operate for 15 min continuously before reaching temperature failure. By including a neodymium magnet in the design the current density drops to 337 A/cm2, which increases the operation time to 59 min. Space charge simulations verify that the coil designs are effective, but for oblique X-Ray Tube geometries there is still distortion of the focal spot shape along with deflections of approXimately 3 mm in the radial and circumferential directions on the anode. Conclusions: Active shielding is an attractive solution for correcting the effects of magnetic fields on the X-Ray focal spot. If eXtremely long fluoroscopic eXposure times are required, longer operation times can be achieved by including a permanent magnet with the active shielding design.
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su d 218 03 resonant frequency of rotating anode X Ray Tubes
Medical Physics, 2012Co-Authors: Mihye Shin, Waldo Hinshaw, Prasheel Lillaney, Rebecca FahrigAbstract:Purpose: To evaluate a new rotating anode X‐Ray Tube from the resonant frequency point of view for stable and safe operation, and to validate a finite element model for insight into X‐Ray Tube rotor dynamics and vibration. Methods: The 3‐dimensional FEM model of the X‐Ray Tube motor has been developed using ANSYS and COMSOL. The resultant resonant frequency from the FEM simulation is substantiated by eXperiments. During deceleration of the X‐Ray Tube, an accelerometer and a corresponding amplifier send the time domain vibration response to a spectrum analyzer which generates the power spectrum. In the frequency domain analysis, a peak signifies large vibrations at that frequency. To corroborate the FEM model, the resonant frequency of the motor assembly without the anode attached was also measured. Lastly, a rough estimate of the resonant frequency can also be observed in angular speed curves which are obtained utilizing a quadrature position sensor.Results: The first mode resonance is eXpected at 20.3 Hz from the FEM simulation. This result matches closely with the peak at 22.2 Hz in the power spectrum and the location of the abrupt decreasing acceleration (slope) in the speed curve at 22 Hz. Without the anode, the FEM simulation result of 35.1 Hz is equal to the first peak at 35.1 Hz, and the angular acceleration is suddenly reduced at 34.4 Hz. Conclusions: For image‐guided interventional procedures using a hybrid system, the X‐Ray Tube should create fluX at various times requiring repeatedacceleration and deceleration of the motor. Hence it is ideal that the resonant frequency is higher than operational speed, although alternatively the motor could accelerate through the resonant frequency quickly. Design improvements to modify the location of resonance of our motor assemblyare underway using the verified FEM model. NIH R01 EB007626, Richard M. Lucas Foundation
Prasheel Lillaney - One of the best experts on this subject based on the ideXlab platform.
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electrostatic focal spot correction for X Ray Tubes operating in strong magnetic fields
Medical Physics, 2014Co-Authors: Prasheel Lillaney, Mihye Shin, Waldo Hinshaw, Rebecca FahrigAbstract:Purpose: A close proXimity hybrid X-Ray/magnetic resonance (XMR) imaging system offers several critical advantages over current XMR system installations that have large separation distances (∼5 m) between the imaging fields of view. The two imaging systems can be placed in close proXimity to each other if an X-Ray Tube can be designed to be immune to the magnetic fringe fields outside of the MR bore. One of the major obstacles to robust X-Ray Tube design is correcting for the effects of the MR fringe field on the X-Ray Tube focal spot. Any fringe field component orthogonal to the X-Ray Tube electric field leads to electron drift altering the path of the electron trajectories. Methods: The method proposed in this study to correct for the electron drift utilizes an eXternal electric field in the direction of the drift. The electric field is created using two electrodes that are positioned adjacent to the cathode. These electrodes are biased with positive and negative potential differences relative to the cathode. The design of the focusing cup assembly is constrained primarily by the strength of the MR fringe field and high voltage standoff distances between the anode, cathode, and the bias electrodes. From these constraints, a focusing cup design suitable for the close proXimity XMR system geometry is derived, and a finite element model of this focusing cup geometry is simulated to demonstrate efficacy. A Monte Carlo simulation is performed to determine any effects of the modified focusing cup design on the output X-Ray energy spectrum. Results: An orthogonal fringe field magnitude of 65 mT can be compensated for using bias voltages of +15 and −20 kV. These bias voltages are not sufficient to completely correct for larger orthogonal field magnitudes. Using active shielding coils in combination with the bias electrodes provides complete correction at an orthogonal field magnitude of 88.1 mT. Introducing small fields (<10 mT) parallel to the X-Ray Tube electric field in addition to the orthogonal field does not affect the electrostatic correction technique. However, rotation of the X-Ray Tube by 30° toward the MR bore increases the parallel magnetic field magnitude (∼72 mT). The presence of this larger parallel field along with the orthogonal field leads to incomplete correction. Monte Carlo simulations demonstrate that the mean energy of the X-Ray spectrum is not noticeably affected by the electrostatic correction, but the output fluX is reduced by 7.5%. Conclusions: The maXimum orthogonal magnetic field magnitude that can be compensated for using the proposed design is 65 mT. Larger orthogonal field magnitudes cannot be completely compensated for because a pure electrostatic approach is limited by the dielectric strength of the vacuum inside the X-Ray Tube insert. The electrostatic approach also suffers from limitations when there are strong magnetic fields in both the orthogonal and parallel directions because the electrons prefer to stay aligned with the parallel magnetic field. These challenging field conditions can be addressed by using a hybrid correction approach that utilizes both active shielding coils and biasing electrodes.
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performance evaluation of an mr compatible rotating anode X Ray Tube
ASME 2013 International Mechanical Engineering Congress and Exposition, 2013Co-Authors: Mihye Shin, Waldo Hinshaw, Prasheel Lillaney, Rebecca FahrigAbstract:The key technical innovation needed for close proXimity hybrid X-Ray/MR (XMR) imaging systems is a new rotating anode X-Ray Tube motor that can operate in the presence of strong magnetic fields. In order for the new motor design to be optimized between conflicting design requirements, we implemented a numerical model for evaluating the dynamics of the motor. The model predicts the amount of produced torque, rotation speed, and time to accelerate based on the Lorentz force law; the motor is accelerated by the interaction between the magnetic moments of the motor wire loops and an eXternal magnetic field. It also includes an empirical model of bearing friction and electromagnetic force from the magnetic field. Our proposed computational model is validated by eXperiments using several different magnitudes of eXternal magnetic fields, which averagely shows an agreement within 0.5 % error during acceleration. We are using this model to improve the efficiency and performance of future iterations of the X-Ray Tube motor.Copyright © 2013 by ASME
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resonant frequency of an mr compatible rotating anode X Ray Tube
ASME 2012 International Mechanical Engineering Congress and Exposition, 2012Co-Authors: Mihye Shin, Waldo Hinshaw, Prasheel Lillaney, Rebecca FahrigAbstract:Switching between X-Ray and MR imaging with the systems in close proXimity can facilitate accurate image-guided interventional procedures using their complementary advantages. Conventional X-Ray Tubes use induction motors whose functionality fails within MR fringe field environments; thus we developed a novel MR compatible X-Ray Tube motor.Vibrations from the structural instability of a motor can lead to mechanical failure, as well as image artifacts due to shaking X-Ray focal spot on the anode target; hence it is important for proper X-Ray Tube motor operation to identify the resonant frequencies that cause large amplitude vibrations. The ability to model rotor dynamics and how they change with the motor design is valuable because it allows us to optimize the motor structure. A finite element model has been developed and validated by eXperiments measuring the acceleration change and the power spectrum of the motor response.Copyright © 2012 by ASME
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magnetostatic focal spot correction for X Ray Tubes operating in strong magnetic fields using iterative optimization
Medical Physics, 2012Co-Authors: Prasheel Lillaney, Mihye Shin, Steven M Conolly, Rebecca FahrigAbstract:Purpose: Combining X-Ray fluoroscopy and MR imaging systems for guidance of interventional procedures has become more commonplace. By designing an X-Ray Tube that is immune to the magnetic fields outside of the MR bore, the two systems can be placed in close proXimity to each other. A major obstacle to robust X-Ray Tube design is correcting for the effects of the magnetic fields on the X-Ray Tube focal spot. A potential solution is to design active shielding that locally cancels the magnetic fields near the focal spot. Methods: An iterative optimization algorithm is implemented to design resistive active shielding coils that will be placed outside the X-Ray Tube insert. The optimization procedure attempts to minimize the power consumption of the shielding coils while satisfying magnetic field homogeneity constraints. The algorithm is composed of a linear programming step and a nonlinear programming step that are interleaved with each other. The coil results are verified using a finite element space charge simulation of the electron beam inside the X-Ray Tube. To alleviate heating concerns an optimized coil solution is derived that includes a neodymium permanent magnet. Any demagnetization of the permanent magnet is calculated prior to solving for the optimized coils. The temperature dynamics of the coil solutions are calculated using a lumped parameter model, which is used to estimate operation times of the coils before temperature failure. Results: For a magnetic field strength of 88 mT, the algorithm solves for coils that consume 588 A/cm2. This specific coil geometry can operate for 15 min continuously before reaching temperature failure. By including a neodymium magnet in the design the current density drops to 337 A/cm2, which increases the operation time to 59 min. Space charge simulations verify that the coil designs are effective, but for oblique X-Ray Tube geometries there is still distortion of the focal spot shape along with deflections of approXimately 3 mm in the radial and circumferential directions on the anode. Conclusions: Active shielding is an attractive solution for correcting the effects of magnetic fields on the X-Ray focal spot. If eXtremely long fluoroscopic eXposure times are required, longer operation times can be achieved by including a permanent magnet with the active shielding design.
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su d 218 03 resonant frequency of rotating anode X Ray Tubes
Medical Physics, 2012Co-Authors: Mihye Shin, Waldo Hinshaw, Prasheel Lillaney, Rebecca FahrigAbstract:Purpose: To evaluate a new rotating anode X‐Ray Tube from the resonant frequency point of view for stable and safe operation, and to validate a finite element model for insight into X‐Ray Tube rotor dynamics and vibration. Methods: The 3‐dimensional FEM model of the X‐Ray Tube motor has been developed using ANSYS and COMSOL. The resultant resonant frequency from the FEM simulation is substantiated by eXperiments. During deceleration of the X‐Ray Tube, an accelerometer and a corresponding amplifier send the time domain vibration response to a spectrum analyzer which generates the power spectrum. In the frequency domain analysis, a peak signifies large vibrations at that frequency. To corroborate the FEM model, the resonant frequency of the motor assembly without the anode attached was also measured. Lastly, a rough estimate of the resonant frequency can also be observed in angular speed curves which are obtained utilizing a quadrature position sensor.Results: The first mode resonance is eXpected at 20.3 Hz from the FEM simulation. This result matches closely with the peak at 22.2 Hz in the power spectrum and the location of the abrupt decreasing acceleration (slope) in the speed curve at 22 Hz. Without the anode, the FEM simulation result of 35.1 Hz is equal to the first peak at 35.1 Hz, and the angular acceleration is suddenly reduced at 34.4 Hz. Conclusions: For image‐guided interventional procedures using a hybrid system, the X‐Ray Tube should create fluX at various times requiring repeatedacceleration and deceleration of the motor. Hence it is ideal that the resonant frequency is higher than operational speed, although alternatively the motor could accelerate through the resonant frequency quickly. Design improvements to modify the location of resonance of our motor assemblyare underway using the verified FEM model. NIH R01 EB007626, Richard M. Lucas Foundation
Sung Oh Cho - One of the best experts on this subject based on the ideXlab platform.
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a vacuum sealed miniature X Ray Tube based on carbon nanoTube field emitters
Nanoscale Research Letters, 2012Co-Authors: Sung Hwan Heo, Hyun Jin Kim, Jun Mok Ha, Sung Oh ChoAbstract:A vacuum-sealed miniature X-Ray Tube based on a carbon nanoTube field-emission electron source has been demonstrated. The diameter of the X-Ray Tube is 10 mm; the total length of the Tube is 50 mm, and no eXternal vacuum pump is required for the operation. The maXimum Tube voltage reaches up to 70 kV, and the X-Ray Tube generates intense X-Rays with the air kerma strength of 108 Gy·cm2 min−1. In addition, X-Rays produced from the miniature X-Ray Tube have a comparatively uniform spatial dose distribution.
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a microfocus X Ray Tube based on a microstructured X Ray target
Nuclear Instruments & Methods in Physics Research Section B-beam Interactions With Materials and Atoms, 2009Co-Authors: Aamir Ihsan, Sung Hwan Heo, Sung Oh ChoAbstract:We present a novel concept to develop a microfocus X-Ray Tube based on a microstructured X-Ray target that is irradiated with a nonfocused electron beam. X-Ray emissions from the microstructured targets with various morphologies were studied using Monte-Carlo simulation code MCNP5. The calculations revealed that the microstructured targets are quite capable of minimizing the effective X-Ray spot size compared with those of conventional transmission-type X-Ray targets. Based on the simulation results of X-Ray brightness, optimum geometric parameters were derived for the microstructured targets with different morphologies. Moreover, the stability of the microstructured target against heat loads delivered by an electron beam was also investigated under both the continuous and pulsed operation modes. From the analysis, the limitations of the maXimum allowable electron beam currents for the stable operation of the X-Ray targets are presented. The combination of the microstructured targets and nonfocused electron beam allows the miniaturization of a microfocus X-Ray Tube by eliminating the needs for massive and compleX focusing devices.
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transmission type microfocus X Ray Tube using carbon nanoTube field emitters
Applied Physics Letters, 2007Co-Authors: Sung Hwan Heo, Aamir Ihsan, Sung Oh ChoAbstract:A microfocus X-Ray Tube that can generate X Rays with the focal spot size less than 5μm has been demonstrated using carbon nanoTube (CNT) field emitters. A CNT cathode on a sharp tungsten tip, a magnetic solenoid lens, and a transmission-type X-Ray target were adopted for the microfocus X-Ray Tube. The design characteristics and the operation performance of the microfocus X-Ray Tube are presented. Due to the small focal spot size, clear X-Ray radiographic images of 6μm bars and X-Ray images with the magnification factor of higher than 230 were obtained.
Mihye Shin - One of the best experts on this subject based on the ideXlab platform.
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electrostatic focal spot correction for X Ray Tubes operating in strong magnetic fields
Medical Physics, 2014Co-Authors: Prasheel Lillaney, Mihye Shin, Waldo Hinshaw, Rebecca FahrigAbstract:Purpose: A close proXimity hybrid X-Ray/magnetic resonance (XMR) imaging system offers several critical advantages over current XMR system installations that have large separation distances (∼5 m) between the imaging fields of view. The two imaging systems can be placed in close proXimity to each other if an X-Ray Tube can be designed to be immune to the magnetic fringe fields outside of the MR bore. One of the major obstacles to robust X-Ray Tube design is correcting for the effects of the MR fringe field on the X-Ray Tube focal spot. Any fringe field component orthogonal to the X-Ray Tube electric field leads to electron drift altering the path of the electron trajectories. Methods: The method proposed in this study to correct for the electron drift utilizes an eXternal electric field in the direction of the drift. The electric field is created using two electrodes that are positioned adjacent to the cathode. These electrodes are biased with positive and negative potential differences relative to the cathode. The design of the focusing cup assembly is constrained primarily by the strength of the MR fringe field and high voltage standoff distances between the anode, cathode, and the bias electrodes. From these constraints, a focusing cup design suitable for the close proXimity XMR system geometry is derived, and a finite element model of this focusing cup geometry is simulated to demonstrate efficacy. A Monte Carlo simulation is performed to determine any effects of the modified focusing cup design on the output X-Ray energy spectrum. Results: An orthogonal fringe field magnitude of 65 mT can be compensated for using bias voltages of +15 and −20 kV. These bias voltages are not sufficient to completely correct for larger orthogonal field magnitudes. Using active shielding coils in combination with the bias electrodes provides complete correction at an orthogonal field magnitude of 88.1 mT. Introducing small fields (<10 mT) parallel to the X-Ray Tube electric field in addition to the orthogonal field does not affect the electrostatic correction technique. However, rotation of the X-Ray Tube by 30° toward the MR bore increases the parallel magnetic field magnitude (∼72 mT). The presence of this larger parallel field along with the orthogonal field leads to incomplete correction. Monte Carlo simulations demonstrate that the mean energy of the X-Ray spectrum is not noticeably affected by the electrostatic correction, but the output fluX is reduced by 7.5%. Conclusions: The maXimum orthogonal magnetic field magnitude that can be compensated for using the proposed design is 65 mT. Larger orthogonal field magnitudes cannot be completely compensated for because a pure electrostatic approach is limited by the dielectric strength of the vacuum inside the X-Ray Tube insert. The electrostatic approach also suffers from limitations when there are strong magnetic fields in both the orthogonal and parallel directions because the electrons prefer to stay aligned with the parallel magnetic field. These challenging field conditions can be addressed by using a hybrid correction approach that utilizes both active shielding coils and biasing electrodes.
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performance evaluation of an mr compatible rotating anode X Ray Tube
ASME 2013 International Mechanical Engineering Congress and Exposition, 2013Co-Authors: Mihye Shin, Waldo Hinshaw, Prasheel Lillaney, Rebecca FahrigAbstract:The key technical innovation needed for close proXimity hybrid X-Ray/MR (XMR) imaging systems is a new rotating anode X-Ray Tube motor that can operate in the presence of strong magnetic fields. In order for the new motor design to be optimized between conflicting design requirements, we implemented a numerical model for evaluating the dynamics of the motor. The model predicts the amount of produced torque, rotation speed, and time to accelerate based on the Lorentz force law; the motor is accelerated by the interaction between the magnetic moments of the motor wire loops and an eXternal magnetic field. It also includes an empirical model of bearing friction and electromagnetic force from the magnetic field. Our proposed computational model is validated by eXperiments using several different magnitudes of eXternal magnetic fields, which averagely shows an agreement within 0.5 % error during acceleration. We are using this model to improve the efficiency and performance of future iterations of the X-Ray Tube motor.Copyright © 2013 by ASME
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resonant frequency of an mr compatible rotating anode X Ray Tube
ASME 2012 International Mechanical Engineering Congress and Exposition, 2012Co-Authors: Mihye Shin, Waldo Hinshaw, Prasheel Lillaney, Rebecca FahrigAbstract:Switching between X-Ray and MR imaging with the systems in close proXimity can facilitate accurate image-guided interventional procedures using their complementary advantages. Conventional X-Ray Tubes use induction motors whose functionality fails within MR fringe field environments; thus we developed a novel MR compatible X-Ray Tube motor.Vibrations from the structural instability of a motor can lead to mechanical failure, as well as image artifacts due to shaking X-Ray focal spot on the anode target; hence it is important for proper X-Ray Tube motor operation to identify the resonant frequencies that cause large amplitude vibrations. The ability to model rotor dynamics and how they change with the motor design is valuable because it allows us to optimize the motor structure. A finite element model has been developed and validated by eXperiments measuring the acceleration change and the power spectrum of the motor response.Copyright © 2012 by ASME
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magnetostatic focal spot correction for X Ray Tubes operating in strong magnetic fields using iterative optimization
Medical Physics, 2012Co-Authors: Prasheel Lillaney, Mihye Shin, Steven M Conolly, Rebecca FahrigAbstract:Purpose: Combining X-Ray fluoroscopy and MR imaging systems for guidance of interventional procedures has become more commonplace. By designing an X-Ray Tube that is immune to the magnetic fields outside of the MR bore, the two systems can be placed in close proXimity to each other. A major obstacle to robust X-Ray Tube design is correcting for the effects of the magnetic fields on the X-Ray Tube focal spot. A potential solution is to design active shielding that locally cancels the magnetic fields near the focal spot. Methods: An iterative optimization algorithm is implemented to design resistive active shielding coils that will be placed outside the X-Ray Tube insert. The optimization procedure attempts to minimize the power consumption of the shielding coils while satisfying magnetic field homogeneity constraints. The algorithm is composed of a linear programming step and a nonlinear programming step that are interleaved with each other. The coil results are verified using a finite element space charge simulation of the electron beam inside the X-Ray Tube. To alleviate heating concerns an optimized coil solution is derived that includes a neodymium permanent magnet. Any demagnetization of the permanent magnet is calculated prior to solving for the optimized coils. The temperature dynamics of the coil solutions are calculated using a lumped parameter model, which is used to estimate operation times of the coils before temperature failure. Results: For a magnetic field strength of 88 mT, the algorithm solves for coils that consume 588 A/cm2. This specific coil geometry can operate for 15 min continuously before reaching temperature failure. By including a neodymium magnet in the design the current density drops to 337 A/cm2, which increases the operation time to 59 min. Space charge simulations verify that the coil designs are effective, but for oblique X-Ray Tube geometries there is still distortion of the focal spot shape along with deflections of approXimately 3 mm in the radial and circumferential directions on the anode. Conclusions: Active shielding is an attractive solution for correcting the effects of magnetic fields on the X-Ray focal spot. If eXtremely long fluoroscopic eXposure times are required, longer operation times can be achieved by including a permanent magnet with the active shielding design.
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su d 218 03 resonant frequency of rotating anode X Ray Tubes
Medical Physics, 2012Co-Authors: Mihye Shin, Waldo Hinshaw, Prasheel Lillaney, Rebecca FahrigAbstract:Purpose: To evaluate a new rotating anode X‐Ray Tube from the resonant frequency point of view for stable and safe operation, and to validate a finite element model for insight into X‐Ray Tube rotor dynamics and vibration. Methods: The 3‐dimensional FEM model of the X‐Ray Tube motor has been developed using ANSYS and COMSOL. The resultant resonant frequency from the FEM simulation is substantiated by eXperiments. During deceleration of the X‐Ray Tube, an accelerometer and a corresponding amplifier send the time domain vibration response to a spectrum analyzer which generates the power spectrum. In the frequency domain analysis, a peak signifies large vibrations at that frequency. To corroborate the FEM model, the resonant frequency of the motor assembly without the anode attached was also measured. Lastly, a rough estimate of the resonant frequency can also be observed in angular speed curves which are obtained utilizing a quadrature position sensor.Results: The first mode resonance is eXpected at 20.3 Hz from the FEM simulation. This result matches closely with the peak at 22.2 Hz in the power spectrum and the location of the abrupt decreasing acceleration (slope) in the speed curve at 22 Hz. Without the anode, the FEM simulation result of 35.1 Hz is equal to the first peak at 35.1 Hz, and the angular acceleration is suddenly reduced at 34.4 Hz. Conclusions: For image‐guided interventional procedures using a hybrid system, the X‐Ray Tube should create fluX at various times requiring repeatedacceleration and deceleration of the motor. Hence it is ideal that the resonant frequency is higher than operational speed, although alternatively the motor could accelerate through the resonant frequency quickly. Design improvements to modify the location of resonance of our motor assemblyare underway using the verified FEM model. NIH R01 EB007626, Richard M. Lucas Foundation
Norbert J Pelc - One of the best experts on this subject based on the ideXlab platform.
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investigation of electron trajectories of an X Ray Tube in magnetic fields of mr scanners
Medical Physics, 2007Co-Authors: Rebecca Fahrig, Steven M Conolly, Norbert J PelcAbstract:A hybrid X-Ray/MR system combining an X-Ray fluoroscopic system and an open-bore magnetic resonance (MR) system offers advantages from both powerful imaging modalities and thus can benefit numerous image-guided interventional procedures. In our hybrid system configurations, the X-Ray Tube and detector are placed in the MRmagnet and therefore eXperience a strong magnetic field. The electron beam inside the X-Ray Tube can be deflected by a misaligned magnetic field, which may damage the Tube. Understanding the deflection process is crucial to predicting the electron beam deflection and avoiding potential damage to the X-Ray Tube. For this purpose, the motion of an electron in combined electric ( E ) and magnetic ( B ) fields was analyzed theoretically to provide general solutions that can be applied to different geometries. For two specific cases, a slightly misaligned strong field and a perpendicular weak field, computer simulations were performed with a finite-element method program. In addition, eXperiments were conducted using an open MRI magnet and an inserted electromagnet to quantitatively verify the relationship between the deflections and the field misalignment. In a strong ( B ≫ E ∕ c ; c : speed of light) and slightly misaligned magnetic field, the deflection in the plane of E and B caused by electrons following the magnetic field lines is the dominant component compared to the deflection in the E × B direction due to the drift of electrons. In a weak magnetic field ( B ≤ E ∕ c ) , the main deflection is in the E × B direction and is caused by the perpendicular component of the magnetic field.
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robust X Ray Tubes for use within magnetic fields of mr scanners
Medical Physics, 2005Co-Authors: Zhifei Wen, Rebecca Fahrig, Norbert J PelcAbstract:A hybrid system that combines an X-Ray fluoroscopic system and a magnetic resonance (MR) system can provide physicians with the synergy of eXquisite soft tissue contrast (from MR) and high temporal and spatial resolutions (from X Ray), which may significantly benefit a number of image-guided interventional procedures. However, the system configuration may require the X-Ray Tube to be placed in a magnetic field, which can hinder the proper functioning of the X-Ray Tube by deflecting its electron beam. From knowledge of how the magnetic field affects the electron trajectories, we propose creating another magnetic field along the cathode-anode aXis using either solenoids or permanent magnets to reduce the deflection of the electron beam for two cases: a strong and slightly misaligned field or a weak field that is arbitrary in direction. Theoretical analysis is presented and the electron beam is simulated in various eXternal magnetic fields with a finite element modeling program. Results show that both correction schemes enhance the robustness of the X-Ray Tube operation in an eXternally applied magnetic field.
