Design of a close proximity hybrid x-ray/MR system [electronic resource]
Keywords: Bioengineering.

Using hybrid x-ray/MR (XMR) systems for image guidance during interventional procedures can enhance the diagnosis and treatment of neurologic, oncologic, cardiovascular, and other disorders. XMR suites have become more available, with various vendors offering dual-modality solutions. These systems combine the three-dimensional imaging capabilities and excellent soft tissue contrast provided by MR with the high spatial/temporal resolution and accurate device tracking provided by x-ray. To eliminate system compatibility concerns, the suites typically have long travel distances between MR and x-ray components. As a result, switching between modalities requires shuttling the patient several meters from one system to the other. Because patients typically have critically placed monitoring systems and intravenous lines for drug delivery and anesthesia, the cumbersome shuttling process impedes repeated switching between the modalities. To circumvent the hurdles associated with alternating between modalities, we proposed a close proximity hybrid system design in which a c-arm fluoroscopy unit is placed immediately adjacent to a closed bore MR system with a minimum distance of 1.2 meters between the x-ray and MR imaging field of views. Placing the x-ray system so close to the MR bore requires an x-ray tube capable of operating in a relatively strong MR fringe field environment. 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 my work, I have developed (1) techniques to correct for the altered electron trajectories and (2) a novel motor design that eliminates the reduced rotation speed of the anode. Altering electron trajectories between the cathode and anode affects the location, size, and shape of the x-ray tube focal spot on the anode. I proposed a combination of approaches to control the trajectories. First, I derived an active magnetic shielding design using constrained optimization techniques that minimizes power consumption and heat deposition in the external deflection coils. I then adapted my shielding design to include rare earth permanent magnets, to further reduce the power and size requirements of the coils. Finally, I designed a split-focusing cup that controls the electron trajectories via electrostatic mechanisms, providing an alternative that is more space efficient and MR-compatible. High rotation speed of the anode is needed for sufficient instantaneous heat loading on the target area, to achieve the needed x-ray tube output. There is currently no available motor design to rotate the anode in the expected magnetic environment. To solve this problem, I designed a new motor that operates efficiently within the fringe field. The design is analogous to a modified three-pole brushed DC motor, with the radial component of the MR fringe field replacing the permanent magnet stator field used in conventional brushed DC motors. The motor support bearings provide rotating electrical contacts, while feedback signals from a position sensor control electrical commutation. A vacuum compatible prototype of the proposed motor design was assembled, and its performance was evaluated at various field strengths and orientations. Combining the control mechanisms for the electron trajectories with the new motor design yields a robust x-ray tube capable of operating in fringe fields with magnitudes on the order of 0.1 to 0.2 T.