Fluoroscopy is a radiological technique that provides real-time x-ray viewing in interventional and angiographic procedures. In fluoroscopic procedures, there are several issues have to be solved, such as a risk of radiation exposure to the patients and operators and low image qualities by motion blur. To lower the radiation dose and motion blur, most of fluoroscopic systems provide a pulse-mode operation. However, conventional systems adopt filament-based thermionic analog x-ray tubes that generate relatively longer x-ray pulses than a few milliseconds due to intrinsic difficulty in modulating electron emissions, thus still have many problems of motion blur for fast objects, unnecessary x-ray radiation, and mismatched frame rate to the moving objects. In this work, we tried to solve these problems by suggesting an adaptively variable frame-rate fluoroscopy with an ultra-fast digital x-ray tube (DXT) based on carbon nanotube (CNT) field electron emitters.
We first fabricated a vacuum-sealed CNT DXT and its monoblock with a power generator for the fluoroscopic system. Ultra-short and high-frequency x-ray pulses of up to 500 ns at 1 MHz was achieved by the direct control of electron emission through an active current-control unit. X-ray pulse frames from the CNT DXT with a tube voltage of 120 kV and current of 20 mA were adaptively modulated in the range of 1-30 Hz according to the motion of objects, greatly improving temporal resolution with a reduced radiation dose. The adaptively variable frame-rate fluoroscopy could pave the way for both reducing x-ray doses and improving temporal and spatial resolution.
Cone-beam breast computed tomography (CBCT) would be a promising modality in screening and diagnosis of breast, providing complete 3-dimensional images with little painful compression of breast during the imaging compared to conventional mammography and tomosynthesis. To date, all CBCT systems including a commercial one by Koning have been utilizing a typical filament-based x-ray tube. However, the filament-based x-ray tube even in a grid type has strict limitation in time resolution, of longer than few milliseconds, with a limited dose rate to cause a large motion blur in CBCT projection images. Micro-calcifications of less than 1 mm in early breast cancer could be hardly distinguished by using conventional CBCT systems. We tried to solve this problem by adopting a fast digital x-ray tube based on carbon nanotube (CNT) field emitters. We, for the first time, developed a rotational anode x-ray tube with CNT emitters for advanced CBCTs. The x-ray tube consisted of CNT paste-emitters and a rotating anode made of W/Re target, and was fully vacuum-sealed with a glass envelope. Ultra-short x-ray pulses of less than sub-ms with a moderate high current of more than 200 mA and a focal spot of ~0.3 in nominal value was successfully obtained. We performed preliminary studies on CBCT imaging using the digital x-ray tube and achieved 300 projection images for 10 s, great reducing motion blurs in the images. It is expected that the CNT digital x-ray tube developed improves CBCT imaging greatly and then promotes CBCT modality in breast screening and diagnosis.
There have been many efforts to develop x-ray sources using field electron emitters instead of conventional thermionic cathodes for digital controlling of x-rays in medical imaging. Specially, portable x-ray systems need a miniature x-ray tube with less-power consumption, easy insulation of high voltage and light shielding of x-rays. Carbon nanotube (CNT) has attracted much attention as the most promising field emitter due to its geometric high aspect ratio, high physical and chemical inertness. To date, however, CNT field emitters have not been satisfactorily incorporated into a fully vacuum-sealed x-ray tube due to their instability and/or unreliability. We successfully developed a fully vacuum-sealed, miniature x-ray tube with CNT emitters for portable dental x-ray systems. The x-ray tube was designed in a triode configuration with a self-electron focusing gate and reliable CNT emitters, and was fully vacuum-sealed within a miniaturized volume of 15 mm in diagonal and 65 mm in length, very tiny and small as compared with conventional thermionic one. The nominal focal spot size of the x-ray tube is 0.4 mm with an operational tube voltage of 65 kV and a current of 3 mA, which offers quite good x-ray images of a human tooth phantom. No heating the miniature x-ray tube for electron emission leads to easy insulation of high tube voltage and light shielding of x-rays, giving a compact and light portable x-ray system. Furthermore, digital operation of the x-ray tube through an active-current control could provide a commercial lifetime along with pretty good stability.
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