The spatial-resolution properties in a digital radiographic system can be quantified by means of a “presampling” modulation transfer function (MTF). The theory and technique for determining the presampling MTF are illustrated based upon our recent research. With this technique, the presampling MTF, which includes the geometric unsharpness, the detector unsharpness, and the unsharpness of the sampling aperture, can be measured beyond the Nyquist frequency. An image of a slightly angulated slit is employed in order to obtain Fourier transforms of line spread functions (LSFs) at different alignments of the slit relative to the sampling coordinate. The presampling OTF up to 2ⁿ (n: positive integer) times the Nyquist frequency can be determined by averaging of the 2ⁿ Fourier transforms of 2ⁿ LSFs which are obtained from 2ⁿ shifts of the sampling coordinates at increments of Δx/2ⁿ(Δx: pixel size). The presampling MTF can then be calculated form the absolute value. As an example, the presampling MTFs for a digital subtraction angiographic (DSA) system are measured in two orthogonal directions for three different image-intensifier modes. It is also demonstrated that an “overall” MTF of a digital imaging system may incorrectly indicate the resolution properties of the system if the aliasing effect is not negligible, and thus the overall MTF of a digital system cannot be used in the same manner as that for an analog imaging system.

抄録全体を表示

An improvement of the triple-flash x-ray generator having variable spectra for biomedical radiography is described. This generator consisted of the following components: a high-voltage generating unit, a voltage divider unit, three high-voltage pulsers, a triple-parallel impulse switching system utilizing air gap pulsers for the main gas gaps, a high-power gas diode, a turbo molecular pump, and an x-ray to having cold cathodes. The pulse condensers of the pulsers were charged to the same or different energies by using a voltage divider unit and were connected to the x-ray tube through a high-power gas diode. The pulse widths were about 5Ons, and the minimum time interval between two pulses was about 100μs. The x-ray intensity was less than 1×10^-5C/kg at 1m per pulse, and the effective, focal spot size was determined by the diameter of the anode rod. The triple exposure of pulsed x-rays having variable spectra and time intervals was obtained.

抄録全体を表示

We measured the “digital” characteristic curve, which relates the pixel value to the relative x-ray exposure, in a Fuji computed radiographic system (FCR-101). The time-scale sensitometric technique was applied to the sensitometry in the digital system due to the absence of the reciprocity-law-failure effect. The calibration curve showing the relationship between the relative x-ray exposure and the selected time in the x-ray generator was measured with a fluorescence meter and was used for determination of the relative x-ray exposure. It was found at two facilities that the digital characteristic curve obtained by the time-scale method was in good agreement with that obtained by inverse-square method. Moreover, the curves measured with different imaging parameter settings of the FCR system were comparable at two facilities. It was shown, at extremely larger value settings of imaging parameters, that the curve shape was nonlinear at higher and/or lower pixel values due mainly to the property of the electronic component such as photomultiplier. The use of the time-scale technique in the characteristic curve determination makes the measurements easier and shorter

抄録全体を表示

The X-ray spectra when the X-ray tube for diagnostic was operated under the fluoroscopic condition, were measured by using a lithium-drifted silicon detector.
The spectra were obtained by correcting the counts efficiency of silicon, the absorption of air, and the original measured figures. The absorption spectra of storage phosphors (Imaging plates; Ips) were measured through one-shot dual energy subtraction method. From the X-ray spectra measured under the combination of Lucite absorbers and copper filters with various thickness, we calculated the mean energy and the mean energy separation

抄録全体を表示

Scatter rejection effects of the air-gap method and angular distributions of the scattered radiation were analyzed by using primary and scattered x-ray spectra obtained by computer simulation techniques and experiments. We have carried out the simulation and the measurement for a 70 kV tube voltage, an object of 20-cm acrylic resin, and 2-cmφ and 3-cmφ fields. The objectdetector distance was varied from 1 to 50 cm, The Monte Carlo method was applied in the simulations. A planar Ge detector was used in the experiments. Both results of spectra coincided qualitatively. And the results show quantitively that scatter fractions do not decrease prominently by the air-gap method for the 30-cmφ field, but they decrease noticeably for the 2-cmφ field. For the 30-cmφ field we have obtained angular dependence relations of scatter spectra, mean energies and exposures by attaching beam stoppers of various diameters to the exit surface of the acrylic resin. It was confirmed that the larger scatter angle yielded the lower mean energy and the lower exposure. These phenomena are caused by the Compton effect.

抄録全体を表示

A method for generating repetitional pulsed x-rays using a universal x-ray generator having a cold cathode radiation tube is described. This generator consisted of the following major components: a high speed power supply, a repetitional high voltage pulse generator, a turbo molecular pump, and an oil-cooled x-ray tube. The pulse height became higher according to increases in the condenser charging voltages, and the widths were about 5Ons. The repetitional rate was primarily determined by the current capacity of the power supply and the time constant (τ=CR) of the pulser circuit, and its value was than 20Hz.
The x-ray quality primarily became hard by increasing the charging voltage and by inserting metal filters. The effective focal spot size primarily varied according to the diameter of the anode tip, and its size was less than 3.0mm in diameter. Pulsed x-ray fluoroscopy was performed by using an image intensifier system for medical use.

抄録全体を表示

An arithmetic coding is well known as a powerful and an interesting data compression techniques from information theoretic viewpoints. For the practical use, some fundamental problems such as an error propagation and an application to multi-valued images are remained to be settled. In order to apply this technique for the areas of multi-valued images, a simplified and a feasible method have to be developed.
In this paper, by introducing a bit plane decomposition and by employing the gray code representation instead of natural binary representation, for the image pixel, we present an effective and an executable procedures for medical X-ray images.

抄録全体を表示