Mihye Shin

Magnetostatic focal spot correction for x-ray tubes operating in strong magnetic fields using iterative optimization

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∕cm(2). 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∕cm(2), 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.

Design optimization of MR-compatible rotating anode x-ray tubes for stable operation.

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. METHODS 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). CONCLUSIONS 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.

Electrostatic focal spot correction for x-ray tubes operating in strong magnetic fields

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.

TH‐A‐141‐08: Instrument Design to Measure the Optical Properties of Reflectance and Transmittance

PURPOSE To estimate performance of crystaline scintillator-based pixelated detectors depending on the optical properties of the crystal surface (cut, etched, polished) and the septum material between each crystal. We propose a new device design to measure the optical reflectance and transmittance properties of candidate crystal-septum structures. METHODS Using a measurement device including a laser and an arc of photodiode detectors, the reflectance of two sandwich samples that each mimic a pixel was measured: CdWCO4 -glue-ESR-glue-CdWCO4 and CdWO4 -glue-(Al-sputtered-ESR)-glue-CdWO4 . Reflectance was normalized to that of ESR (a multi-layer optical film, highly specular reflector). To provide the required range of incident light angles at the interface of interest, a BGO hemisphere with a high index of refraction (n=2.2) was glued (Meltmount, n=1.7) to the top surface of the sandwich. RESULTS The sandwich structure demonstrated constant reflectivity over all laser angles with reflectances of 95% and 60% for ESR and Al-sputtered-ESR sandwiches respectively. This is because the highest incident angle achieved at the crystal-glue interface was only 25.8 degrees due to the large difference in refractive indices between air (n=1) and CdWO4 (n=2.2), which is below the critical angle for total internal reflection. With the BGO hemisphere added, there are two boundaries where total internal reflection can occur, and three levels of reflectivity were detected with the steps corresponding to the critical angles of 45 degrees and 54 degrees. Normalization is required to remove the influence of the Meltmount and BGO crystal from the measured reflectivities. CONCLUSION The optical reflectance due to surface conditions and septum materials can be accurately measured via the addition of a high-refractive-index hemisphere to a sandwich structure that has equivalent characteristics to the finished pixel matrix. Reflectance and transmittance are both important to develop an efficient pixelated detector, therefore the instrument has been modified and being assembled to measure them. This work is supported by National Institutes of Health (NIH R01 CA138426) and the Richard M. Lucas Foundation. There is no conflict of interest to disclose.

TH-A-141-10: A Piecewise-Focused Pixelated Detector for MV Imaging

PURPOSE For portal imaging, high DQE detectors can be constructed from thick pixelated scintillator arrays that absorb MV x-rays. However, due to beam divergence, MTF and DQE losses can be significant for off-axis elements not focused towards the source. We present a novel focusing approach based on situating a shaped fiber optic plate (FOP) between rectilinear scintillator arrays and an amorphous silicon flat panel imager (AMFPI). METHODS The entire FOP comprises seven wedge-shaped sections that are fused together so that the center of each section points towards the source focal spot which is located 1500 mm away. The arc-shaped FOP directs light from the scintillator assembly to the AMFPI. The scintillator assembly consists of seven identical rectilinear sub-arrays, each with a 1 degree bevel, that are close-packed end-to-end. Each 15mm thick CdWO4 sub-array comprises 66×66 elements with a pixel pitch of 0.784 mm resulting in a piecewise-focused area detector having dimensions 365 mm × 52 mm. RESULTS Monte Carlo simulations of radiative and optical transport with a 6MV source predict a DQE(0) of 23%. With no beam divergence, MTF and DQE values at a spatial frequency of 0.4mm-1 are 0.5 and 15% respectively. Maximum off-axis MTF and DQE losses occur at the subarray edges (divergence angle =1 degree) and are only 6% and 12% respectively at 0.4mm-1 . If the detector were not focused, MTF and DQE losses at 0.4mm-1 would be 30% and 85% respectively. These losses would occur at edge of the detector where the beam divergence is 7 degrees. CONCLUSION A novel approach to creating a focused detector for MV portal, cone-beam or helical CT imaging is presented. Compared to previously proposed designs, all the block arrays are identical and rectilinear scintillator thus reducing costs and simplifying manufacturing processes. Assembly and experimental measurements are underway. NIH Academic-Industrial Partnership NIH RO1 CA138426; Varian Medical Systems.

Reflection properties of scintillator-septum candidates for a pixelated MeV detector

In order to predict and improve the performance of pixelated detectors, it is important to understand the optical properties of the basic unit of the scintillating structure in the detector. To measure one of the essential optical properties, reflectance, we have used a device composed of a laser and photodiode array. We have also developed an analytical model of the optical phenomena based on Snells law and the Fresnel equations to simply analyze measured results and reflectance parameters at the interface. The computed and experimentally measured results typically have good agreement, validating the analytical model and measurements. The optical parameters are used as inputs to GEANT4 [1]. The simulations are then leveraged to optimize an imager design before a prototype is built. The optical reflectance was measured by using relatively inexpensive samples. A sample has scintillator, glue, and septum (reflector) layers, and each sample has a different scintillator surface (polished/rough) and/or reflector [ESR film/aluminum-sputtered (coated) ESR film] condition. A high-refractive-index hemisphere was attached on the top surface of a sample to increase the maximum incidence angle at the scintillator-glue interface from 27° to 52°. The sample including ESR film demonstrated average reflectance approximately 1.3 times higher than that from the sample with aluminum-sputtered ESR film as a reflector, and the polished surface condition showed higher reflectance than the rough-cut surface condition.

Performance Evaluation of an MR-Compatible Rotating Anode X-Ray Tube

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

SU‐D‐218‐03: Resonant Frequency of Rotating Anode X‐Ray Tubes

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.

Resonant Frequency of an MR Compatible Rotating Anode X-Ray Tube

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