For Computed Radiography (CR) systems that use a columnar phosphor plate (CPP) and a powder phosphor plate (PPP), we designed the systems to obtain the best image quality. To determine the optimum phosphor layer thickness for each phosphor plate, the relationship between the intensity and spatial spread of photo-stimulated luminescence (PSL), and the phosphor layer thickness of the phosphor plate is quantitatively clarified. Next, to determine the stimulation light intensity, we measured PSL, modulation transfer function (MTF) and detective quantum efficiency (DQE) by varying the stimulation light intensity, using the determined optimum phosphor layer thickness. We also investigated the noise components of each phosphor plate. Results show that, compared to the PPP, the CPP is more favorable in allowing thicker phosphor layer without reduction in MTF. As the result of the relationship between the layer thickness and the PSL, noise analysis, it was confirmed that the CPP could detect PSL in the deep region of the phosphor layer without reducing the intensity of PSL. This suggests that in comparison to the PPP, the CPP can make efficient use of X-ray information, thereby promising to enhance image quality and to reduce exposure dose.
In X-ray-to-light conversion digital radiography, we compared the image quality of a system in which photodetection is done from the X-ray incident surface (hereafter referred to as a front exposure system) and a system in which photodetection is done from the back side opposite the X-ray incident surface (hereafter referred to as a back exposure system). Modulation transfer function (MTF) and detective quantum efficiency (DQE) measurements were performed using the method IEC prescribes. Both MTF and DQE were higher with the front exposure system than with the back exposure system, with the former delivering better image quality. This difference can be accounted for by differences in the distribution of absorbed X-ray doses in the phosphor layer, the readout efficiency, which varies as a function of depth in the phosphor layer, and depth-dependent blurs of light. Furthermore, we determined changes in image quality incurred by varying the quality of X-rays, the thickness of the phosphor layer and the crystal structure of phosphors. The advantage of the front exposure system becomes more pronounced with decreasing X-ray tube voltage, increasing phosphor layer thickness, and the use of phosphors in powder form.
The concept proposed by EUREF which determines the AGD between upper limit corresponding to the acceptable level
of AGD and lower limit corresponding to lower limit of image quality was applied to CR Mammography, and the
resulting object thickness tracking and tube voltage tracking were determined.
EUREF specifies threshold contrast visibility for a 5 cm of PMMA. In accordance with this definition, the lower limit of
CNR for a 5 cm of PMMA was determined by measuring the CNR at the lower limit of AGD where threshold contrast
visibility was just acceptable. The obtained lower limit of CNR was then multiplied by the object thickness
compensation factor to estimate the lower limits of CNR for all object thicknesses. AGDs are now determined for each
lower limit of CNR to obtain thickness tracking and tube voltage tracking characteristics. Although the limited range of
examined target/filter combinations and tube voltages should be taken into consideration, these tracking characteristics
for constant CNR differ in their profiles from those of a screen-film system with AEC for providing constant optical
density. Among our findings, we found that a lower AGD is achieved while maintaining CNR for a thick object when
using a combination of target/filter and tube voltage that generates higher X-ray energy compared to the combination
given in the EUREF's typical spectra per PMMA thickness (Mo/Rh 32 kV at PMMA 4cm and Rh/Rh 28 kV at PMMA
6 cm, for example).
We also found that the thickness tracking characteristics for constant S value behaves similarly to constant CNR under
certain conditions of the target/filter combination and tube voltage.
We analyzed the relationship between the noise components and detectability of the various Imaging Plates to find the essential factors for improving Imaging Plate.
CR system noise is classified into quantum noise and fixed noise. The dominant component of fixed noise is the structural noise of IP. The contribution of the structural noise was relatively high at higher exposure and at higher spatial frequency. For example the ratio of structural noise of early type of IP is 55% in case of 2 mR (5.16 × 10-7 C/kg) exposure at 2 cycles/mm spatial frequency. On the other hand, the ratio of the latest type is about 20%. This
improvement leads to 2.3 times NEQ combined with improvement of quantum noise. Threshold depth of the 0.5 mm diameter hole of CD diagram was improved from 1.4 mm to 0.8 mm according to the improvement of NEQ from 50,000 to 100,000 /mm2 at the related spatial frequencies. Amount of improvement of threshold contrast was influenced especially by the NEQ at relatively high spatial frequencies. So improvement of the structural noise, which
is dominant at higher frequency, is important as well as quantum noise.
We performed an image quality simulation for the line scan system, which realizes a compact and high-speed Computed Radiography (CR) system.
The line scan system uses a line light source and a linear CCD sensor. In this system, the emitted light must be efficiently focused onto the CCD sensor to detect the emitted light as much as possible. To realize the effective light detection, we analyzed the spread of the light in the photostimulable phosphor layer. We also estimated the image quality based on X-ray absorption, the amount of emitted light, light collecting efficiency and electric noise. It clarified the image quality is affected strongly by such factors as the spread of the PSL, the size of photo diodes of the CCD sensor and electric noise.
We developed a high resolution Imaging Plate (IP) with a transparent support and a reading system that can detect emissions from dual sides of the IP to improve the image quality of the CR mammography. And we proposed an addition method using the filtering processing in real space in order to achieve optimum addition in all the spatial frequencies. The image quality of the system was evaluated by NEQ and CD- MAM phantom. Furthermore we separated the noise component that consisted of X-ray photon noise, light photon noise, and the structural noise of the IP utilizing laser power dependency of the amount of emissions and the Wiener Spectrum. By using the above reading system and the addition method, NEQ of the system was improved by 40% - 50% compared to the latest CR mammography system. We confirmed by use of CD-MAM that the detectability of the image in this reading system was remarkably improved. The noise analysis showed that the ratios of the light photon noise at 1c/mm of the front side image and the back side image were about 15% and 40%, respectively, and according as the spatial frequency became higher the ratio of the light photon noise increased.
This paper describes an approach to further improving the image quality of computed radiography (CR) by use of a both- side reading method and an Imaging Plate (IP) with a transparent support. The most important factor for determining the image quality in an X-ray imaging system is X-ray utilization efficiency. Accordingly, we have proposed an imaging plate that consists of a transparent support and a thicker photostimulable phosphor layer, and a reading method that permits detection of emissions from both sides of the IP, so that an improvement in image quality can be achieved by adding image data derived from both sides of the IP. We have also found that the optimal addition ratio is dependent upon spatial frequency, and proposed a spatial filter-based addition process method for achieving optimal addition over the entire spatial frequency. The system based on the proposed reading method and addition method provided 30 - 40 % increase in detective quantum efficiency (DQE), as compared to single- side reading method. Furthermore, it has been confirmed that the DQE of the addition image is dependent upon the correlation between the data of the two images detected from both sides of the imaging plate. The reading method proposed in this paper can provide increased DQE, so that enhancement in diagnostic performance can be anticipated.
The computed radiography (CR) system consists of three processes: reading, image processing, and display. Image noise from the reading process consists of quantum noise and fixed noise. Quantum noise is dependent on exposure but fixed noise is independent of it. Quantum noise can be divided into light photon noise and x-ray photon noise. The former is inversely proportional to the light detection efficiency and the latter is independent of it. We separated the noise components of the Fuji computed radiography (FCR) 7000 system. At a spatial frequency of 1 cycle/mm, the ratio of x ray photon noise to light photon noise to fixed noise was about 8:1:1 at 1 mR(2.58 X 10-7 C/kg). In the new CR system (FCR9000), we decreased x-ray photon noise and fixed noise. As a result, the FCR9000 system yields DQE of approximately 1.4 times higher at a spatial frequency of 0.5 cycle/mm and approximately 1.2 times higher at 1.0 cycles/mm at 1 mR (2.58 X 10-7 C/kg).
Computed radiography (CR) utilizing imaging plates (IPs), which consist of a photostimulable BaFX:Eu2+(X equals Cl,Br) phosphor, has found wide clinical use. In the present paper, a photostimulable SrFBr:Eu2+ (SFB) phosphor, as well as an IP employing this phosphor (SFB-IP), are experimentally prepared. A test apparatus for reading the SFB-IP is constructed, and the imaging characteristics are evaluated. The x ray imaging characteristics of the SFB-IP are different from those of the BFB-IP (the IP which consists of BaFBr:Eu2+). The difference is due to their x-ray energy absorption coefficients. Under the mammographic exposure conditions, the measurements of noise equivalent quanta as well as the visual evaluation of phantom images reveal that the SFB-IP and BFB-IP have the same quality level of imaging in spite of the PSL intensity of SFB being lower than that of BFB. However, for practical use, many problems are left on the SFB-IP and its reading apparatus.
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