Performance of a static‐anode/flat‐panel x‐ray fluoroscopy system in a diagnostic strength magnetic field: A truly hybrid x‐ray/MR imaging system

Abstract

Minimally invasive procedures are increasing in variety and frequency, facilitated by advances in imaging technology. Our hybrid imaging system (GE Apollo flat panel, custom Brand x-ray static anode x-ray tube, GE Lunar high-frequency power supply and 0.5 T Signa SP) provides both x-ray and MR imaging capability to guide complex procedures without requiring motion of the patient between two distant gantries. The performance of the x-ray tube in this closely integrated system was evaluated by modeling and measuring both the response of the filament to an externally applied field and the behavior of the electron beam for field strengths and geometries of interest. The performance of the detector was assessed by measuring the slanted-edge modulation transfer function (MTF) and when placed at zero field and at 0.5 T. Measured resonant frequencies of filaments can be approximated using a modified vibrating beam model, and were at frequencies well below the 25 kHz frequency of our generator for our filament geometry. The amplitude of vibration was not sufficient to cause shorting of the filament during operation within the magnetic field. A simple model of electrons in uniform electric and magnetic fields can be used to estimate the deflection of the electron beam on the anode for the fields of interest between 0.2 and 0.5 T. The MTF measured at the detector and the DQE showed no significant difference inside and outside of the magnetic field. With the proper modifications, an x-ray system can be fully integrated with a MR system, with minimal loss of image quality. Any x-ray tube can be assessed for compatibility when placed at a particular location within the field using the models. We have also concluded that a-Si electronics are robust against magnetic fields. Detailed knowledge of the x-ray system installation is required to provide estimates of system operation.

Study of increased radiation when an x‐ray tube is placed in a strong magnetic field

“…3 We observed that the x-ray tube output increased by more than 30% when the x-ray source was placed in a 0.5 T magnetic field. When high energy electrons strike the target in an x-ray tube, a substantial number of electrons are backscattered.…”

“…The off-focal radiation, if not confined near the focal spot area, can degrade the MTF. This effect may partially account for the change in MTF previosuly measured at the patient position when the x-ray source was placed in a magnetic field of 0.2 T. 3 Since the magnetic field also affects the primary electrons, 5 careful analysis is needed to differentiate the impacts on MTF from these two factors.…”

Closed bore XMR (CBXMR) systems for aortic valve replacement: Investigation of rotating‐anode x‐ray tube heat loadability

“…Percutaneous aortic valve replacement is currently guided using x-ray fluoroscopy and x-ray angiography, which provide high-quality images of catheters, guidewires, the prosthetic valve, the coronary arteries, and the aorta in real time and without motion artifacts. 12 Unfortunately, no soft tissue anatomy of the heart can be visualized in fluoroscopic images, so complications such as cardiac tamponade from tissue perforation have occurred due to the difficulty of positioning and deploying the prosthetic valve. 13 Thus, the technique is still limited and has only been attempted as a last resort on a small number of patients worldwide.…”

“…24 It was shown that the performance of an appropriately modified flat-panel detector was also unaffected by the fringe field. 12 Therefore, this article will investigate the effect of induction motor performance on x-ray tube heat loadability, which has not yet been studied. If anode rotation frequency decreases and eddy current heating in the rotor increases as a result of the x-ray tube induction motor being in a magnetic field, this will limit the heat loadability of the x-ray tube.…”

“…Hybrid x-ray/MRI ͑XMR͒ systems combine an x-ray imaging system with an MRI scanner into a single imaging system to provide complementary information for improved clinical benefit. 12,[14][15][16][17][18][19][20][21][22] MRI can provide the anatomical information that is lacking in the x-ray images. We believe that a new hybrid x-ray/MRI ͑XMR͒ system design known as a closed bore XMR ͑CBXMR͒ ͑Ref.…”