Microfocus X-ray Tubes

Operating the microfocus X-ray source at high power is required to achieve high temporal reso- lution. However, the thermal loading of the anode focal spot is a limiting factor in determining the maximum power of an x-ray tube. In this paper, a transmission window based on polycrystalline diamond, for improved brightness in microfocus X-ray tube was demonstrated. The polycrystalline diamond films which were depos- ited by hot filament chemical vapor deposition (HFCVD) approach revealed good intrinsic thermal properties. We developed a reliable method to fabricate and estimate the windows/target assembly, quantitatively ana- lyzed the anode heat load of the X-ray source operating under the electron beam irradiation, and then tested in a microfocus X-ray tube. It's believed that the experiment and calculation results presented here can be gen- erally applied for the design and the fabrication of a high brightness microfocus X-ray tube employing poly- crystalline diamond windows. KEYWORD: Microfocus X-ray source; Polycrystalline diamond film; Windows/target assembly; HFCVD; Anode heat load.

The design and performance are described of a low-power copper-target X-ray tube with an elongated focus foreshortened to a diameter between 10 and 20 µm. The tube is designed for use with a specularly reflecting ellipsoidal mirror placed between 10 and 20 mm from the source and focusing the X-rays onto the sample, which may be 600 mm distant. The design maximizes the solid angle of collection of the emitted X-rays and thus achieves a high intensity at the sample. The electron beam, of circular cross section, from the gun is focused by an axial magnetic lens and then drawn out by a quadrupole lens to form an elongated spot on the target which is viewed at a small take-off angle. The limits imposed on the maximum intensity at the specimen by aberrations in the lens and by the permissible power dissipation in the target are discussed. The observed intensity is in reasonable agreement with the calculated intensity; it is comparable with that achieved with nonfocusing X-ray optics used with conventional X-ray tubes operated at a power more than 100 times as great.

A microfocus x-ray tube that can generate x rays with the focal spot size less than 5μm has been demonstrated using carbon nanotube (CNT) field emitters. A CNT cathode on a sharp tungsten tip, a magnetic solenoid lens, and a transmission-type x-ray target were adopted for the microfocus x-ray tube. The design characteristics and the operation performance of the microfocus x-ray tube are presented. Due to the small focal spot size, clear x-ray radiographic images of 6μm bars and x-ray images with the magnification factor of higher than 230 were obtained.

A microfocus X-ray tube is useful for performing magnification digital radiography. The 100-µm-focus X-ray generator consists of a main controller, for regulating tube voltage and current, and a tube unit with a high-voltage circuit and a fixed anode X-ray tube. The maximum tube voltage, current, and electric power were 106 kV, 0.5 mA, and 50 W, respectively. Using a 100-µm-thick tungsten filter, the X-ray intensity was 19.8 µGy/s at 1.0 m from the source with a tube voltage of 100 kV and a current of 0.50 mA. Since Kα rays from tungsten targets are absorbed effectively by gadolinium-based contrast media, these rays are useful for performing enhanced angiography. Magnification angiography was performed by threefold magnification imaging using a computed radiography system and the filter at a tube voltage of 100 kV. In angiography of nonliving animals, we observed fine blood vessels of approximately 100 µm with high contrasts.

In this work, we performed a proof-of-concept experiment for phase-contrast x-ray imaging (PCXI) based on a single antiscatter grid and a polychromatic x-ray source. We established a table-top setup which consists of a focused-linear grid having a strip density of 200 lines/inch, a microfocus x-ray tube having a focal-spot size of about 5 μm, and a CMOS-type flat-panel detector having a pixel size of 48 μm. By using our prototype PCXI system and the Fourier demodulation technique, we successfully obtained attenuation, scattering, and differential phase-contrast images of improved visibility from the raw images of several selected samples at x-ray tube conditions of 90 kVp and 0.1 mAs. Further, fusion image (e.g., the attenuation＋the scattering) may have an advantage in displaying details of the sample's structures that are not clearly visible in the conventional attenuation image. Our experimental results indicate that single-grid-based approach seems a useful method for PCXI with great simplicity and minimal requirements on the setup alignment.

X-ray micro-CT (µCT) is an important tool for high-resolution, non-destructive inspection of samples with numerous applications ranging from preclinical imaging to materials science. µCT imaging capabilities can be expanded through the use of small-pixel ( < 100 µm) photon-counting detectors (PCDs) which increase image contrast, reduce image noise, and provide additional insights into material composition through the collected spectral information. Along the same lines, phase-contrast x-ray imaging is increasingly being adopted in laboratory µCT imaging setups to improve the visualization of low-attenuation contrast samples. Along with these technological improvements comes the need for improved modeling and simulation. However, no laboratory µCT simulation tool currently exists that includes a small-pixel PCD. PEPIsim addresses this need, providing a simulation resource that produces photon-counting, spectral, and phase-contrast µCT reconstructions, accurately modelling the response of the small-pixel PCD in addition to incorporating phase-contrast effects. PEPIsim has been benchmarked against experimental images of a custom plastic phantom acquired with a µCT setup featuring a PCD and a micro-focus x-ray tube. Results show that the simulation reproduces the system spectral response (average NMRSE of 2.7%) across a wide range of tube voltages (45–100 kVp). Moreover, good accuracy in terms of measured x-ray attenuation (average relative error (RE) of 1.5%) and SNR (average RE of 6.5%) has been found. Similarly, after phase-retrieval, the simulated images’ SNR is found to be in agreement with experimental images (average RE of 5.5%). Finally, the simulation can also reproduce the fringe visibility due to the edge-enhancement effect (average RE of 5.2%) across a range of magnifications (from 1.3 to 3.7) and source to detector distances (from 40 to 133 cm). Due to its modular structure, PEPIsim can be adapted to any experimental µCT imaging setup, provided that accurate detector and x-ray tube models exist.

Compound refractive lenses (CRL), consisting of a lot number in-line concave microlenses made of low-Z material were studied. Lenses with focal length 109 mm and 41 mm for 8-keV X-rays, microfocus X-ray tube and X-ray CCD camera were used in experiments. Obtained images show intensity distribution of magnified microfocus X-ray source focal spot. Within the experiments, one lens was also used as an objective lens of the X-ray microscope, where the copper anode X-ray microfocus tube served as a source. Magnified images of gold mesh with 5 microns bars were obtained. Theoretical limits of CRL and experimental results are discussed.

THE purpose of this paper is to present some of the factors that have been of importance in recent x-ray tube design and manufacture. To some extent it is a survey of the present status of knowledge about x-ray tubes, with particular reference to progress of the last few years and factors not generally understood. In the attempt to explain, briefly, involved problems in the field of physics and engineering, a certain lack of clarity in some cases is probably unavoidable. If, however, enough clearness can be obtained to convey any appreciable amount of stimulation or value the paper will have served its function. Part I. Design 1. Physical Dimensions The dimensions of an x-ray tube are determined largely by the voltage at which it is to be operated. Provision must be made for the high electrostatic stresses accompanying the voltage, and also for the high velocity electrons made possible by the high voltage and essential in the production of x-rays. It is significant that high velocity electrons are sought deliberately in no other vacuum device, with the exception of the cathode ray tube. Hence, x-ray tubes present problems not found in other vacuum devices. The electrons must be accelerated, focussed, stopped, and resultant heat dissipated. In hitting the focal spot many are not completely stopped, but bounce off with a lower velocity to be attracted to other portions of the tube, such as the envelope, anode shank, and anode arm. The conventional shape of the hot cathode x-ray tube envelope is a spherical bulb having two cylindrical extensions called the anode and the cathode arm (Fig. 1-A). The arms serve the following purposes: (1) Give an overall length in conjunction with the bulb sufficient to prevent spark-over the outside of the tube; (2) give a convenient support for the electrodes; (3) prevent any appreciable number of stray electrons, or positive ions, from getting on the glass near either end of the tube. The last purpose is very important and a consideration of it enters into almost every tube design. The inside surface of the glass envelope near each end of the tube must be relatively free from stray electrons in order to prevent rapid leakage of their negative charge to positive potential, heating of the glass, sparking over its surface, and resulting disturbances in general. This is prevented by the arms because of their relatively long length compared with the constricted space between their inside surfaces. It is possible to prevent stray electrons from getting to any portion of the envelope by means of a design in which the electrons are trapped in a hollow anode. In this case constricted arms are not necessary. Such a design, however, has found little application except for high voltages (4) since in most cases it is complicated and there is little to be gained over the usual design.

To investigate the potential use of the direct magnification (DIMA) technique in dental radiology. Radiographs of anatomical specimens and extracted teeth were prepared under clinical conditions using a microfocus (8 and 16 microns focal diameter) X-ray tube. Entrance dose was measured using an ionization chamber. Image quality was assessed in a single-blind test of pairs of images of extracted teeth with and without caries by ten observers. At 7.3 x magnification the values for exposure time and entrance dose were comparable with those for conventional intraoral radiography, 0.1 and 0.25 s and 1.5 and 2.9 mSv respectively. Diagnostic accuracy using the DIMA images was significantly (P < 0.0001) better compared with dental radiographs. DIMA images with significantly enhanced diagnostic accuracy can be obtained at comparable exposure times and entrance doses to conventional intraoral radiographs.

Grating-based x-ray phase-contrast imaging is a fast developing new modality not only for medical imaging, but as well for other fields such as material sciences. While these many possible applications arise, the knowledge of the noise behavior is essential. In this work, the authors used a least squares fitting algorithm to calculate the noise behavior of the three quantities absorption, differential phase, and dark-field image. Further, the calculated error formula of the differential phase image was verified by measurements. Therefore, a Talbot interferometer was setup, using a microfocus x-ray tube as source and a Timepix detector for photon counting. Additionally, simulations regarding this topic were performed. It turned out that the variance of the reconstructed phase is only dependent of the total number of photons used to generate the phase image and the visibility of the experimental setup. These results could be evaluated in measurements as well as in simulations. Furthermore, the correlation between absorption and dark-field image was calculated. These results provide the understanding of the noise characteristics of grating-based phase-contrast imaging and will help to improve image quality.

A high-resolution in vivo micro-CT system for combining with fluorescence molecular tomography (FMT), was constructed and applied in small animal imaging. The fast scanning micro-CT system designed to provide high-resolution anatomic information and reconstruction priors, consisted of a flat panel detector (FPD) based on amorphous silicon (a-Si) and a micro-focus x-ray tube. The Feldkamp algorithm was adopted in image reconstruction with graphic processing unit (GPU). The system spatial resolution of 13 lp/mm was achieved when the diameter of image field was 6 cm with the system magnification factor of 4. No obvious beam hardening artifact was observed in transaxial image of a water phantom after correction. The contrast-to-noise ratio (CNR) study of various tissue phantoms was also presented. The in-vivo imaging of an anesthetic mouse was performed to demonstrate the feasibility of our system.

Measurement and visualization of the fracture process zone (FPZ) in the silicate composites is essential for understanding how the crack is formed and how it is propagating through the material. Although several time-lapse radiography and tomography experiments during the three-point bending of silicate composites with conventional imaging techniques have been already realized, the crack initiation and propagation is so fast process, that it is very difficult to capture sufficient spatial information in the exact moment when this process is occurring. Nowadays progress in X-ray imaging instrumentation, especially single photon counting CdTe detectors with a fast parallel readout, allows speeding up the acquisition of the tomographic data while keeping the intrinsic resolution of tens of micrometers and high efficiency even for photons with higher energy. Tomography of relatively fast processes is becoming realistic even in laboratory conditions using commercially available microfocus X-ray tubes. As an example of such a fast tomography, results of “on-the-fly” series of tomographic experiments performed on the specimen subjected to continual three-point bending test is presented.

Microtomography with ionising radiations

Rapid high-resolution X-ray diffraction measurement and analysis of MOVPE pHEMT structures using a high-brilliance X-ray source and automatic pattern fitting

Using a microfocus X-ray tube and pixelated energy-resolving detector it is possible to measure the X-ray absorption spectrum of a material with high spatial resolution. Given sufficient energy resolution in the detector it is possible to detect and identify absorption edges which are characteristic to individual chemical elements. Using computed tomography the three dimensional (3D) internal elemental chemistry of an object can be reconstructed. The application of spectroscopic X-ray tomography is demonstrated by mapping distribution of heavy elements inside a mineralised ore sample. We correlate and validate this data with high resolution X-ray tomography and energy-dispersive X-ray spectroscopy data.