Fig 8 - uploaded by John Alan Rowlands
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Simulated focal spot comparison between no external field (0 Gauss) and a 1000 Gauss external field in Y, with least squares optimized coil correction along Y.
Source publication
In order to achieve a truly hybrid, high quality X-ray/MR system one must have a rotating anode x-ray source as close as possible to the bore of the high-field MR magnet. Full integration between a closed bore MR system and an x-ray fluoroscopy system presents two main challenges that must be addressed: x-ray tube motor operation and efficiency in...
Context in source publication
Context 1
... order to constrain the current density value of the coil farther from the electron beam, the inner radius of this coil was reduced from 60mm to 30mm. The results for this simulation are shown in figure 8. With this configuration Gauss coil correction for V mm the deflection of the center of mass of the focal spot was again reduced to less than 1 mm and no additional changes in orientation were observed. ...
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Citations
... This simplification has been used in a prior work by Wen et al. 16 and experimentally validated. 36 The final volume was a hexahedron with x, y, and z dimensions of 90, 18, and 25 mm, respectively. The dimensions were chosen to be sufficiently large enough such that boundary conditions applied to the exterior planes of the model did not affect the results. ...
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.
... external fields and can utilize the MR fringe field to generate torque. 13 Work has also been performed on developing correction mechanisms for the changes in the focal spot. By using active shielding local cancellation of the MR fringe field in the region between the anode and cathode can be achieved, preserving the shape and position of the focal spot on the anode target. ...
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.
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.
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.
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.
Hybrid closed bore x‐ray/MRI systems are being developed to improve the safety and efficacy of percutaneous aortic valve replacement procedures by harnessing the complementary strengths of the x‐ray and MRI modalities in a single interventional suite without requiring patient transfer between two rooms. These systems are composed of an x‐ray C‐arm in close proximity (≈1m) to an MRI scanner. The MRI magnetic fringe field can cause the electron beam in the x‐ray tube to deflect. The deflection causes the x‐ray field of view to shift position on the detector receptacle. This could result in unnecessary radiation exposure to the patient and the staff in the cardiac catheterization laboratory. Therefore, the electron beam deflection must be corrected. The authors developed an active magnetic shielding system that can correct for electron beam deflection to within an accuracy of 5% without truncating the field of view or increasing exposure to the patient. This system was able to automatically adjust to different field strengths as the external magnetic field acting on the x‐ray tube was changed. Although a small torque was observed on the shielding coils of the active shielding system when they were placed in a magnetic field, this torque will not impact their performance if they are securely mounted on the x‐ray tube and the C‐arm. The heating of the coils of the shielding system for use in the clinic caused by electric current was found to be slow enough not to require a dedicated cooling system for one percutaneous aortic valve replacement procedure. However, a cooling system will be required if multiple procedures are performed in one session.
