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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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Novel motor design for rotating anode X-Ray Tubes operating in the fringe field of a magnetic resonance imaging system
Medical Physics, 2013Co-Authors: Prasheel Lillaney, Mihye Shin, Waldo Hinshaw, Norbert Pelc, N. Robert Bennett, Rebecca FahrigAbstract:PURPOSE: Using hybrid X-Ray∕MR (XMR) systems for image guidance during interventional procedures could enhance the diagnosis and treatment of neurologic, oncologic, cardiovascular, and other disorders. The authors propose a close proXimity hybrid system design in which a C-arm fluoroscopy unit is placed immediately adjacent to the solenoid magnet of a MR system with a minimum distance of 1.2 m between the X-Ray and MR imaging fields of view. EXisting rotating anode X-Ray tube designs fail within MR fringe field environments because the magnetic fields alter the electron trajectories in the X-Ray tube and act as a brake on the induction motor, reducing the rotation speed of the anode. In this study the authors propose a novel motor design that avoids the anode rotation speed reduction.\n\nMETHODS: The proposed design replaces the permanent magnet stator found in brushed dc motors with the radial component of the MR fringe field. The X-Ray tube is oriented such that the radial component of the MR fringe field is orthogonal to the cathode-anode aXis. Using a feedback position sensor and the support bearings as electrical slip rings, the authors use electrical commutation to eliminate the need for mechanical brushes and commutators. A vacuum compatible prototype of the proposed motor design was assembled, and its performance was evaluated at various operating conditions. The prototype consisted of a 3.1 in. diameter anode rated at 300 kHU with a ceramic rotor that was 5.6 in. in length and had a 2.9 in. diameter. The material chosen for all ceramic components was MACOR, a machineable glass ceramic developed by Corning Inc. The approXimate weight of the entire assembly was 1750 g. The maXimum rotation speed, angular acceleration, and acceleration time of the motor design were investigated, as well as the dependence of these parameters on rotor angular offset, magnetic field strength, and field orientation. The resonance properties of the authors' assembly were also evaluated to determine its stability during acceleration, and a pulse width modulation algorithm was implemented to control the rotation speed of the motor.\n\nRESULTS: At a magnetic fluX density of 41 mT orthogonal to the aXis of rotation (on the lower end of the eXpected fluX density in the MR suite) the maXimum speed of the motor was found to be 5150 revolutions per minute (rpm). The acceleration time necessary to reach 3000 rpm was found to be approXimately 10 s at 59 mT. The resonance frequency of the assembly with the anode attached was 1310 rpm (21.8 Hz) which is far below the desired operating speeds. Pulse width modulation provides an effective method to control the speed of the motor with a resolution of 100 rpm.\n\nCONCLUSIONS: The proposed design can serve as a direct replacement to the conventional induction motor used in rotating anode X-Ray Tubes. It does not suffer from a reduced rotation speed when operating in a MR environment. The presence of chromic steel bearings in the prototype prevented testing at the higher field strengths, and future iterations of the design could eliminate this shortcoming. The prototype assembly demonstrates proof of concept of the authors' design and overcomes one of the major obstacles for a MR compatible rotating anode X-Ray tube.
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Design optimization of MR-compatible rotating anode X-Ray Tubes for stable operation
Medical Physics, 2013Co-Authors: Mihye Shin, Waldo Hinshaw, Prasheel Lillaney, Rebecca FahrigAbstract:PURPOSE: Hybrid X-Ray/MR systems can enhance the diagnosis and treatment of endovascular, cardiac, and neurologic disorders by using the complementary advantages of both modalities for image guidance during interventional procedures. Conventional rotating anode X-Ray Tubes fail near an MR imaging system, since MR fringe fields create eddy currents in the metal rotor which cause a reduction in the rotation speed of the X-Ray tube motor. A new X-Ray tube motor prototype has been designed and built to be operated close to a magnet. To ensure the stability and safety of the motor operation, dynamic characteristics must be analyzed to identify possible modes of mechanical failure. In this study a 3D finite element method (FEM) model was developed in order to eXplore possible modifications, and to optimize the motor design. The FEM provides a valuable tool that permits testing and evaluation using numerical simulation instead of building multiple prototypes.\n\nMETHODS: Two eXperimental approaches were used to measure resonance characteristics: the first obtained the angular speed curves of the X-Ray tube motor employing an angle encoder; the second measured the power spectrum using a spectrum analyzer, in which the large amplitude of peaks indicates large vibrations. An estimate of the bearing stiffness is required to generate an accurate FEM model of motor operation. This stiffness depends on both the bearing geometry and adjacent structures (e.g., the number of balls, clearances, preload, etc.) in an assembly, and is therefore unknown. This parameter was set by matching the FEM results to measurements carried out with the anode attached to the motor, and verified by comparing FEM predictions and measurements with the anode removed. The validated FEM model was then used to sweep through design parameters [bearing stiffness (1 × 10(5)-5 × 10(7) N/m), shaft diameter (0.372-0.625 in.), rotor diameter (2.4-2.9 in.), and total length of motor (5.66-7.36 in.)] to increase the fundamental frequency past the operating range at 50 Hz.Results: The first large vibration during the prototype motor operation was obtained at 21.64 ± 0.68 Hz in the power spectrum. An abrupt decrease in acceleration occurred at 21.5 Hz due to struggling against the resonance vibrations. A bearing stiffness of 1.2 × 10(5) N/m in the FEM simulation was used to obtain a critical speed of 21.4 Hz providing 1.1% error. This bearing stiffness value and the 3D model were then confirmed by the eXperiments with the anode removed, demonstrating an agreement within 6.4% between simulation results and measurements. A calculated first critical frequency (fundamental frequency) of 68.5 Hz was obtained by increasing the bearing stiffness to 1 × 10(7) N/m and increasing the shaft diameter by 68.0%. Reducing the number of bearings in the design permits decreasing the total length of the motor by 1.7 in., and results in a fundamental frequency of 68.3 Hz in concert with additional changes (shaft diameter of 0.625 in., rotor diameter of 2.4 in., and bearing stiffness of 1 × 10(6) N/m).\n\nCONCLUSIONS: An FEM model of the X-Ray tube motor has been implemented and eXperimentally validated. A fundamental frequency above the operational rotation speed can be achieved through modification of multiple design parameters, which allows the motor to operate stably and safely in the MR environment during the repeated acceleration/deceleration cycles required for an interventional procedure. The validated 3D FEM model can now be used to investigate trade-offs between generated torque, maXimum speed, and motor inertia to further optimize motor 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
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su c 220 04 electrostatic focal spot correction for X Ray Tubes operating in strong magnetic fields
Medical Physics, 2011Co-Authors: Prasheel Lillaney, D Constantin, A Ganguly, Rebecca FahrigAbstract:Purpose: To evaluate the effectiveness of electrostatic mechanisms for controlling the focal spot position in X‐Ray Tubes operating in the fringe field of an MR bore, and to determine a cathode design to implement the correction mechanism while not compromising normal X‐Ray tube operation.Methods: Using a combination of theoretical calculations and high voltage vacuum standoff constraints, a cathode design was derived that is practical for standard X‐Ray tube geometry. The crucial part of the design consists of two voltage‐biased deflection electrodes placed adjacent to the cathode. Space charge beam simulations were performed for the design to determine current density changes and beam deflection in the presence of a magnetic field. Phase space information from the beam simulation was input into a Monte Carlo engine to determine the effect of the cathode design on the X‐Ray photon energy spectrum.Results: For a 0.07T magnetic field, a 120 kV cathode‐anode potential, a 14.4 mm cathode‐anode separation distance, and a 35 kV electrode potential, the deflection of the X‐Ray tube focal spot is within 2 mm of the original position with slight distortions to focal spot shape. However, the curvature of the electron trajectories is altered resulting in a significant velocity component tangential to the anode that is on average 10 times larger than in the control case. The electron velocity changes coupled with slightly lower current density on the anode reduces the total number of photons generated by 7.5% without significantly altering the energy spectrum of the X‐Ray photons Conclusions: The beam simulations demonstrate that focal spot deflection can be controlled to within reasonable values. The generated spectrum is not significantly different from that of a standard X‐Ray tube, with only a moderate decrease in overall photon fluence. Work is underway to evaluate the cathode design in an eXperimental setting. Funding Sources: NIHR01 EB007626, Stanford Bio‐X Fellowship, and the 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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Novel motor design for rotating anode X-Ray Tubes operating in the fringe field of a magnetic resonance imaging system
Medical Physics, 2013Co-Authors: Prasheel Lillaney, Mihye Shin, Waldo Hinshaw, Norbert Pelc, N. Robert Bennett, Rebecca FahrigAbstract:PURPOSE: Using hybrid X-Ray∕MR (XMR) systems for image guidance during interventional procedures could enhance the diagnosis and treatment of neurologic, oncologic, cardiovascular, and other disorders. The authors propose a close proXimity hybrid system design in which a C-arm fluoroscopy unit is placed immediately adjacent to the solenoid magnet of a MR system with a minimum distance of 1.2 m between the X-Ray and MR imaging fields of view. EXisting rotating anode X-Ray tube designs fail within MR fringe field environments because the magnetic fields alter the electron trajectories in the X-Ray tube and act as a brake on the induction motor, reducing the rotation speed of the anode. In this study the authors propose a novel motor design that avoids the anode rotation speed reduction.\n\nMETHODS: The proposed design replaces the permanent magnet stator found in brushed dc motors with the radial component of the MR fringe field. The X-Ray tube is oriented such that the radial component of the MR fringe field is orthogonal to the cathode-anode aXis. Using a feedback position sensor and the support bearings as electrical slip rings, the authors use electrical commutation to eliminate the need for mechanical brushes and commutators. A vacuum compatible prototype of the proposed motor design was assembled, and its performance was evaluated at various operating conditions. The prototype consisted of a 3.1 in. diameter anode rated at 300 kHU with a ceramic rotor that was 5.6 in. in length and had a 2.9 in. diameter. The material chosen for all ceramic components was MACOR, a machineable glass ceramic developed by Corning Inc. The approXimate weight of the entire assembly was 1750 g. The maXimum rotation speed, angular acceleration, and acceleration time of the motor design were investigated, as well as the dependence of these parameters on rotor angular offset, magnetic field strength, and field orientation. The resonance properties of the authors' assembly were also evaluated to determine its stability during acceleration, and a pulse width modulation algorithm was implemented to control the rotation speed of the motor.\n\nRESULTS: At a magnetic fluX density of 41 mT orthogonal to the aXis of rotation (on the lower end of the eXpected fluX density in the MR suite) the maXimum speed of the motor was found to be 5150 revolutions per minute (rpm). The acceleration time necessary to reach 3000 rpm was found to be approXimately 10 s at 59 mT. The resonance frequency of the assembly with the anode attached was 1310 rpm (21.8 Hz) which is far below the desired operating speeds. Pulse width modulation provides an effective method to control the speed of the motor with a resolution of 100 rpm.\n\nCONCLUSIONS: The proposed design can serve as a direct replacement to the conventional induction motor used in rotating anode X-Ray Tubes. It does not suffer from a reduced rotation speed when operating in a MR environment. The presence of chromic steel bearings in the prototype prevented testing at the higher field strengths, and future iterations of the design could eliminate this shortcoming. The prototype assembly demonstrates proof of concept of the authors' design and overcomes one of the major obstacles for a MR compatible rotating anode X-Ray tube.
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Design optimization of MR-compatible rotating anode X-Ray Tubes for stable operation
Medical Physics, 2013Co-Authors: Mihye Shin, Waldo Hinshaw, Prasheel Lillaney, Rebecca FahrigAbstract:PURPOSE: Hybrid X-Ray/MR systems can enhance the diagnosis and treatment of endovascular, cardiac, and neurologic disorders by using the complementary advantages of both modalities for image guidance during interventional procedures. Conventional rotating anode X-Ray Tubes fail near an MR imaging system, since MR fringe fields create eddy currents in the metal rotor which cause a reduction in the rotation speed of the X-Ray tube motor. A new X-Ray tube motor prototype has been designed and built to be operated close to a magnet. To ensure the stability and safety of the motor operation, dynamic characteristics must be analyzed to identify possible modes of mechanical failure. In this study a 3D finite element method (FEM) model was developed in order to eXplore possible modifications, and to optimize the motor design. The FEM provides a valuable tool that permits testing and evaluation using numerical simulation instead of building multiple prototypes.\n\nMETHODS: Two eXperimental approaches were used to measure resonance characteristics: the first obtained the angular speed curves of the X-Ray tube motor employing an angle encoder; the second measured the power spectrum using a spectrum analyzer, in which the large amplitude of peaks indicates large vibrations. An estimate of the bearing stiffness is required to generate an accurate FEM model of motor operation. This stiffness depends on both the bearing geometry and adjacent structures (e.g., the number of balls, clearances, preload, etc.) in an assembly, and is therefore unknown. This parameter was set by matching the FEM results to measurements carried out with the anode attached to the motor, and verified by comparing FEM predictions and measurements with the anode removed. The validated FEM model was then used to sweep through design parameters [bearing stiffness (1 × 10(5)-5 × 10(7) N/m), shaft diameter (0.372-0.625 in.), rotor diameter (2.4-2.9 in.), and total length of motor (5.66-7.36 in.)] to increase the fundamental frequency past the operating range at 50 Hz.Results: The first large vibration during the prototype motor operation was obtained at 21.64 ± 0.68 Hz in the power spectrum. An abrupt decrease in acceleration occurred at 21.5 Hz due to struggling against the resonance vibrations. A bearing stiffness of 1.2 × 10(5) N/m in the FEM simulation was used to obtain a critical speed of 21.4 Hz providing 1.1% error. This bearing stiffness value and the 3D model were then confirmed by the eXperiments with the anode removed, demonstrating an agreement within 6.4% between simulation results and measurements. A calculated first critical frequency (fundamental frequency) of 68.5 Hz was obtained by increasing the bearing stiffness to 1 × 10(7) N/m and increasing the shaft diameter by 68.0%. Reducing the number of bearings in the design permits decreasing the total length of the motor by 1.7 in., and results in a fundamental frequency of 68.3 Hz in concert with additional changes (shaft diameter of 0.625 in., rotor diameter of 2.4 in., and bearing stiffness of 1 × 10(6) N/m).\n\nCONCLUSIONS: An FEM model of the X-Ray tube motor has been implemented and eXperimentally validated. A fundamental frequency above the operational rotation speed can be achieved through modification of multiple design parameters, which allows the motor to operate stably and safely in the MR environment during the repeated acceleration/deceleration cycles required for an interventional procedure. The validated 3D FEM model can now be used to investigate trade-offs between generated torque, maXimum speed, and motor inertia to further optimize motor 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
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su c 220 04 electrostatic focal spot correction for X Ray Tubes operating in strong magnetic fields
Medical Physics, 2011Co-Authors: Prasheel Lillaney, D Constantin, A Ganguly, Rebecca FahrigAbstract:Purpose: To evaluate the effectiveness of electrostatic mechanisms for controlling the focal spot position in X‐Ray Tubes operating in the fringe field of an MR bore, and to determine a cathode design to implement the correction mechanism while not compromising normal X‐Ray tube operation.Methods: Using a combination of theoretical calculations and high voltage vacuum standoff constraints, a cathode design was derived that is practical for standard X‐Ray tube geometry. The crucial part of the design consists of two voltage‐biased deflection electrodes placed adjacent to the cathode. Space charge beam simulations were performed for the design to determine current density changes and beam deflection in the presence of a magnetic field. Phase space information from the beam simulation was input into a Monte Carlo engine to determine the effect of the cathode design on the X‐Ray photon energy spectrum.Results: For a 0.07T magnetic field, a 120 kV cathode‐anode potential, a 14.4 mm cathode‐anode separation distance, and a 35 kV electrode potential, the deflection of the X‐Ray tube focal spot is within 2 mm of the original position with slight distortions to focal spot shape. However, the curvature of the electron trajectories is altered resulting in a significant velocity component tangential to the anode that is on average 10 times larger than in the control case. The electron velocity changes coupled with slightly lower current density on the anode reduces the total number of photons generated by 7.5% without significantly altering the energy spectrum of the X‐Ray photons Conclusions: The beam simulations demonstrate that focal spot deflection can be controlled to within reasonable values. The generated spectrum is not significantly different from that of a standard X‐Ray tube, with only a moderate decrease in overall photon fluence. Work is underway to evaluate the cathode design in an eXperimental setting. Funding Sources: NIHR01 EB007626, Stanford Bio‐X Fellowship, and the Lucas Foundation
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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Novel motor design for rotating anode X-Ray Tubes operating in the fringe field of a magnetic resonance imaging system
Medical Physics, 2013Co-Authors: Prasheel Lillaney, Mihye Shin, Waldo Hinshaw, Norbert Pelc, N. Robert Bennett, Rebecca FahrigAbstract:PURPOSE: Using hybrid X-Ray∕MR (XMR) systems for image guidance during interventional procedures could enhance the diagnosis and treatment of neurologic, oncologic, cardiovascular, and other disorders. The authors propose a close proXimity hybrid system design in which a C-arm fluoroscopy unit is placed immediately adjacent to the solenoid magnet of a MR system with a minimum distance of 1.2 m between the X-Ray and MR imaging fields of view. EXisting rotating anode X-Ray tube designs fail within MR fringe field environments because the magnetic fields alter the electron trajectories in the X-Ray tube and act as a brake on the induction motor, reducing the rotation speed of the anode. In this study the authors propose a novel motor design that avoids the anode rotation speed reduction.\n\nMETHODS: The proposed design replaces the permanent magnet stator found in brushed dc motors with the radial component of the MR fringe field. The X-Ray tube is oriented such that the radial component of the MR fringe field is orthogonal to the cathode-anode aXis. Using a feedback position sensor and the support bearings as electrical slip rings, the authors use electrical commutation to eliminate the need for mechanical brushes and commutators. A vacuum compatible prototype of the proposed motor design was assembled, and its performance was evaluated at various operating conditions. The prototype consisted of a 3.1 in. diameter anode rated at 300 kHU with a ceramic rotor that was 5.6 in. in length and had a 2.9 in. diameter. The material chosen for all ceramic components was MACOR, a machineable glass ceramic developed by Corning Inc. The approXimate weight of the entire assembly was 1750 g. The maXimum rotation speed, angular acceleration, and acceleration time of the motor design were investigated, as well as the dependence of these parameters on rotor angular offset, magnetic field strength, and field orientation. The resonance properties of the authors' assembly were also evaluated to determine its stability during acceleration, and a pulse width modulation algorithm was implemented to control the rotation speed of the motor.\n\nRESULTS: At a magnetic fluX density of 41 mT orthogonal to the aXis of rotation (on the lower end of the eXpected fluX density in the MR suite) the maXimum speed of the motor was found to be 5150 revolutions per minute (rpm). The acceleration time necessary to reach 3000 rpm was found to be approXimately 10 s at 59 mT. The resonance frequency of the assembly with the anode attached was 1310 rpm (21.8 Hz) which is far below the desired operating speeds. Pulse width modulation provides an effective method to control the speed of the motor with a resolution of 100 rpm.\n\nCONCLUSIONS: The proposed design can serve as a direct replacement to the conventional induction motor used in rotating anode X-Ray Tubes. It does not suffer from a reduced rotation speed when operating in a MR environment. The presence of chromic steel bearings in the prototype prevented testing at the higher field strengths, and future iterations of the design could eliminate this shortcoming. The prototype assembly demonstrates proof of concept of the authors' design and overcomes one of the major obstacles for a MR compatible rotating anode X-Ray tube.
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Design optimization of MR-compatible rotating anode X-Ray Tubes for stable operation
Medical Physics, 2013Co-Authors: Mihye Shin, Waldo Hinshaw, Prasheel Lillaney, Rebecca FahrigAbstract:PURPOSE: Hybrid X-Ray/MR systems can enhance the diagnosis and treatment of endovascular, cardiac, and neurologic disorders by using the complementary advantages of both modalities for image guidance during interventional procedures. Conventional rotating anode X-Ray Tubes fail near an MR imaging system, since MR fringe fields create eddy currents in the metal rotor which cause a reduction in the rotation speed of the X-Ray tube motor. A new X-Ray tube motor prototype has been designed and built to be operated close to a magnet. To ensure the stability and safety of the motor operation, dynamic characteristics must be analyzed to identify possible modes of mechanical failure. In this study a 3D finite element method (FEM) model was developed in order to eXplore possible modifications, and to optimize the motor design. The FEM provides a valuable tool that permits testing and evaluation using numerical simulation instead of building multiple prototypes.\n\nMETHODS: Two eXperimental approaches were used to measure resonance characteristics: the first obtained the angular speed curves of the X-Ray tube motor employing an angle encoder; the second measured the power spectrum using a spectrum analyzer, in which the large amplitude of peaks indicates large vibrations. An estimate of the bearing stiffness is required to generate an accurate FEM model of motor operation. This stiffness depends on both the bearing geometry and adjacent structures (e.g., the number of balls, clearances, preload, etc.) in an assembly, and is therefore unknown. This parameter was set by matching the FEM results to measurements carried out with the anode attached to the motor, and verified by comparing FEM predictions and measurements with the anode removed. The validated FEM model was then used to sweep through design parameters [bearing stiffness (1 × 10(5)-5 × 10(7) N/m), shaft diameter (0.372-0.625 in.), rotor diameter (2.4-2.9 in.), and total length of motor (5.66-7.36 in.)] to increase the fundamental frequency past the operating range at 50 Hz.Results: The first large vibration during the prototype motor operation was obtained at 21.64 ± 0.68 Hz in the power spectrum. An abrupt decrease in acceleration occurred at 21.5 Hz due to struggling against the resonance vibrations. A bearing stiffness of 1.2 × 10(5) N/m in the FEM simulation was used to obtain a critical speed of 21.4 Hz providing 1.1% error. This bearing stiffness value and the 3D model were then confirmed by the eXperiments with the anode removed, demonstrating an agreement within 6.4% between simulation results and measurements. A calculated first critical frequency (fundamental frequency) of 68.5 Hz was obtained by increasing the bearing stiffness to 1 × 10(7) N/m and increasing the shaft diameter by 68.0%. Reducing the number of bearings in the design permits decreasing the total length of the motor by 1.7 in., and results in a fundamental frequency of 68.3 Hz in concert with additional changes (shaft diameter of 0.625 in., rotor diameter of 2.4 in., and bearing stiffness of 1 × 10(6) N/m).\n\nCONCLUSIONS: An FEM model of the X-Ray tube motor has been implemented and eXperimentally validated. A fundamental frequency above the operational rotation speed can be achieved through modification of multiple design parameters, which allows the motor to operate stably and safely in the MR environment during the repeated acceleration/deceleration cycles required for an interventional procedure. The validated 3D FEM model can now be used to investigate trade-offs between generated torque, maXimum speed, and motor inertia to further optimize motor 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
Waldo Hinshaw - 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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Novel motor design for rotating anode X-Ray Tubes operating in the fringe field of a magnetic resonance imaging system
Medical Physics, 2013Co-Authors: Prasheel Lillaney, Mihye Shin, Waldo Hinshaw, Norbert Pelc, N. Robert Bennett, Rebecca FahrigAbstract:PURPOSE: Using hybrid X-Ray∕MR (XMR) systems for image guidance during interventional procedures could enhance the diagnosis and treatment of neurologic, oncologic, cardiovascular, and other disorders. The authors propose a close proXimity hybrid system design in which a C-arm fluoroscopy unit is placed immediately adjacent to the solenoid magnet of a MR system with a minimum distance of 1.2 m between the X-Ray and MR imaging fields of view. EXisting rotating anode X-Ray tube designs fail within MR fringe field environments because the magnetic fields alter the electron trajectories in the X-Ray tube and act as a brake on the induction motor, reducing the rotation speed of the anode. In this study the authors propose a novel motor design that avoids the anode rotation speed reduction.\n\nMETHODS: The proposed design replaces the permanent magnet stator found in brushed dc motors with the radial component of the MR fringe field. The X-Ray tube is oriented such that the radial component of the MR fringe field is orthogonal to the cathode-anode aXis. Using a feedback position sensor and the support bearings as electrical slip rings, the authors use electrical commutation to eliminate the need for mechanical brushes and commutators. A vacuum compatible prototype of the proposed motor design was assembled, and its performance was evaluated at various operating conditions. The prototype consisted of a 3.1 in. diameter anode rated at 300 kHU with a ceramic rotor that was 5.6 in. in length and had a 2.9 in. diameter. The material chosen for all ceramic components was MACOR, a machineable glass ceramic developed by Corning Inc. The approXimate weight of the entire assembly was 1750 g. The maXimum rotation speed, angular acceleration, and acceleration time of the motor design were investigated, as well as the dependence of these parameters on rotor angular offset, magnetic field strength, and field orientation. The resonance properties of the authors' assembly were also evaluated to determine its stability during acceleration, and a pulse width modulation algorithm was implemented to control the rotation speed of the motor.\n\nRESULTS: At a magnetic fluX density of 41 mT orthogonal to the aXis of rotation (on the lower end of the eXpected fluX density in the MR suite) the maXimum speed of the motor was found to be 5150 revolutions per minute (rpm). The acceleration time necessary to reach 3000 rpm was found to be approXimately 10 s at 59 mT. The resonance frequency of the assembly with the anode attached was 1310 rpm (21.8 Hz) which is far below the desired operating speeds. Pulse width modulation provides an effective method to control the speed of the motor with a resolution of 100 rpm.\n\nCONCLUSIONS: The proposed design can serve as a direct replacement to the conventional induction motor used in rotating anode X-Ray Tubes. It does not suffer from a reduced rotation speed when operating in a MR environment. The presence of chromic steel bearings in the prototype prevented testing at the higher field strengths, and future iterations of the design could eliminate this shortcoming. The prototype assembly demonstrates proof of concept of the authors' design and overcomes one of the major obstacles for a MR compatible rotating anode X-Ray tube.
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Design optimization of MR-compatible rotating anode X-Ray Tubes for stable operation
Medical Physics, 2013Co-Authors: Mihye Shin, Waldo Hinshaw, Prasheel Lillaney, Rebecca FahrigAbstract:PURPOSE: Hybrid X-Ray/MR systems can enhance the diagnosis and treatment of endovascular, cardiac, and neurologic disorders by using the complementary advantages of both modalities for image guidance during interventional procedures. Conventional rotating anode X-Ray Tubes fail near an MR imaging system, since MR fringe fields create eddy currents in the metal rotor which cause a reduction in the rotation speed of the X-Ray tube motor. A new X-Ray tube motor prototype has been designed and built to be operated close to a magnet. To ensure the stability and safety of the motor operation, dynamic characteristics must be analyzed to identify possible modes of mechanical failure. In this study a 3D finite element method (FEM) model was developed in order to eXplore possible modifications, and to optimize the motor design. The FEM provides a valuable tool that permits testing and evaluation using numerical simulation instead of building multiple prototypes.\n\nMETHODS: Two eXperimental approaches were used to measure resonance characteristics: the first obtained the angular speed curves of the X-Ray tube motor employing an angle encoder; the second measured the power spectrum using a spectrum analyzer, in which the large amplitude of peaks indicates large vibrations. An estimate of the bearing stiffness is required to generate an accurate FEM model of motor operation. This stiffness depends on both the bearing geometry and adjacent structures (e.g., the number of balls, clearances, preload, etc.) in an assembly, and is therefore unknown. This parameter was set by matching the FEM results to measurements carried out with the anode attached to the motor, and verified by comparing FEM predictions and measurements with the anode removed. The validated FEM model was then used to sweep through design parameters [bearing stiffness (1 × 10(5)-5 × 10(7) N/m), shaft diameter (0.372-0.625 in.), rotor diameter (2.4-2.9 in.), and total length of motor (5.66-7.36 in.)] to increase the fundamental frequency past the operating range at 50 Hz.Results: The first large vibration during the prototype motor operation was obtained at 21.64 ± 0.68 Hz in the power spectrum. An abrupt decrease in acceleration occurred at 21.5 Hz due to struggling against the resonance vibrations. A bearing stiffness of 1.2 × 10(5) N/m in the FEM simulation was used to obtain a critical speed of 21.4 Hz providing 1.1% error. This bearing stiffness value and the 3D model were then confirmed by the eXperiments with the anode removed, demonstrating an agreement within 6.4% between simulation results and measurements. A calculated first critical frequency (fundamental frequency) of 68.5 Hz was obtained by increasing the bearing stiffness to 1 × 10(7) N/m and increasing the shaft diameter by 68.0%. Reducing the number of bearings in the design permits decreasing the total length of the motor by 1.7 in., and results in a fundamental frequency of 68.3 Hz in concert with additional changes (shaft diameter of 0.625 in., rotor diameter of 2.4 in., and bearing stiffness of 1 × 10(6) N/m).\n\nCONCLUSIONS: An FEM model of the X-Ray tube motor has been implemented and eXperimentally validated. A fundamental frequency above the operational rotation speed can be achieved through modification of multiple design parameters, which allows the motor to operate stably and safely in the MR environment during the repeated acceleration/deceleration cycles required for an interventional procedure. The validated 3D FEM model can now be used to investigate trade-offs between generated torque, maXimum speed, and motor inertia to further optimize motor 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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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.
