Junji Morishita

Basic imaging properties of a computed radiographic system with photostimulable phosphors

We measured the characteristic curve, modulation transfer function (MTF), and the Wiener spectrum of a commercially available computed radiographic (CR) system with photostimulable phosphor plate (imaging plate, IP). The characteristic curve (system response) obtained by an inverse-square x-ray sensitometry showed a wide dynamic range (order of 10(3) in maximum). The slit technique was employed to determine the MTFs, such as IP MTF, presampling MTF including the unsharpness of the detector (IP) and the blurring effect of the sampling aperture, and laser-printer MTF. It was found that the MTF of the standard type of IP was comparable to that of medium-speed screen/film systems. The noticeable degradation of resolution in our CR system, however, occurred at the stage of image data sampling: the presampling MTF was inferior to the IP MTF due to the effect of the scattering and resultant spreading of the incidence laser beam and the emitted luminescence. The noise was characterized by means of digital Wiener spectrum using uniformly exposed noise data. Exposure ranges could be separated into different sections depending upon the noise sources, such as quantum mottle at low exposure and system structure noise at high exposure.

Comparison of two methods for accurate measurement of modulation transfer functions of screen‐film systems

The modulation transfer function (MTF) of a screen-film system can be measured by two methods, i.e., a slit method with Fourier transform on the line spread function and a square-wave response function (SWRF) method. However, it is still uncertain whether MTFs obtained by the two methods are identical. In this study, MTFs of relatively sharp and unsharp screen-film systems were measured by using the two methods. The slit method provided slightly greater MTF for the relatively sharp system than the SWRF method. However, MTFs of the unsharp system obtained with the two methods were comparable. Generally, the slit method tends to provide reliable results for unsharp systems, whereas the SWRF method is favorable for sharp systems. Accuracy and consistency of these measurements were examined by comparison of experimental and theoretical edge responses derived from the measured MTFs. However, the difference in edge responses obtained by the two methods was relatively small compared with the variation of the measured edge responses, and thus results were considered inconclusive as to whether either of the methods can provide more accurate MTFs. International interlaboratory comparison indicated that the variation in the measured MTFs at six different institutions was relatively large for both methods. However, the MTFs of two screen-film systems measured by the slit method appear to agree with those by the SWRF method within the variation expected from the interlaboratory comparison.

Computer‐aided diagnosis for interstitial infiltrates in chest radiographs: Optical‐density dependence of texture measures

We have been developing a computerized scheme for automated detection and characterization of interstitial infiltrates based on the Fourier transform of lung texture. To improve the performance of the scheme, which was developed using digitized screen-film radiographs, optical-density dependence of both the gradient of the film used and the system noise associated with the laser scanner were investigated. Two hundred chest radiographs, including 100 abnormal cases with interstitial infiltrates, were digitized using a laser scanner. The root-mean-square (RMS) variations and the first moments of the power spectra, which correspond to the magnitude and coarseness of lung texture, were determined by Fourier transform of lung textures in numerous regions of interest (ROIs). The RMS variation was dependent upon the average optical density in the ROI, though no obvious trend existed for the first moment of the power spectrum. Dependence of the RMS variations on optical density was corrected for using the gradient curve of the film. Also, system noise associated with the laser scanner was corrected. Results indicated that the specificity was improved from 81% (without correction) to 89% (with corrections), without any loss of sensitivity (90%). Thus, the correspondence between the computer output and consensus interpretation of radiologists was improved with the new scheme compared to the previous one. This improved computerized scheme may be useful to radiologists in detecting interstitial infiltrates in chest radiographs.

Evaluation of edge effect due to phase contrast imaging for mammography

It is well-known that the edge effect produced by phase contrast imaging results in the edge enhancement of x-ray images and thereby sharpens those images. It has recently been reported that phase contrast imaging using practical x-ray tubes with small focal spots has improved image sharpness as observed in the phase contrast imaging with x-ray from synchrotron radiation or micro-focus x-ray tubes. In this study, we conducted the phase contrast imaging of a plastic fiber and plant seeds using a customized mammography equipment with a 0.1 mm focal spot, and the improvement of image sharpness was evaluated in terms of spatial frequency response of the images. We observed that the image contrast of the plastic fiber was increased by edge enhancement, and, as predicted elsewhere, spectral analysis revealed that as the spatial frequencies of the x-ray images increased, so did the sharpness gained through phase contrast imaging. Thus, phase contrast imaging using a practical molybdenum anode tube with a 0.1 mm-focal spot would benefit mammography, in which the morphological detectability of small species such as microcalcifications is of great concern. And detectability of tumor-surrounded glandular tissues in dense breast would be also improved by the phase contrast imaging.

Resolution Properties Of A Computed Radiographic System

The analysis of spatial-resolution properties in terms of the modulation transfer function (MTF) has been presented in a computed radiographic (CR) system (FCR-101) with the photostimulable-phosphor plate (imaging plate, IP). The newly devised method of determining the presampling MTF which includes the x-ray detector (IP) unsharpness and the unsharpness of the sampling aperture is described in which an image of a slightly-angulated lead slit relative to a horizontal or vertical direction is employed. The IP MTFs as an analog MTF in the system, the presampling MTFs for different types of IPs, different sampling distances, different versions of IPs, simultaneous multisection tomography and magnification radiography, and the laser-printer MTFs as display MTF are measured and shown. The effective sampling aperture MTFs calculated indicate that the noticeable degradation of resolution occurs at the stage of image data sampling. The usefulness of the magnification technique for mammography and bone radiography is demonstrated. It is shown that both of the digital MTF and the overall MTF are difficult to use for general purpose due to the aliasing artifacts. The effect of glare on the contrast is also characterized by lead-disk method. The glare fraction is found to be approximately 6.5%.

Effect of test patterns on measurement of the luminance of LCD devices by use of a telescopic-type luminance meter

Quality assurance of electronic display devices is important for maintaining reliable soft-copy image interpretations. This paper presents effects of test patterns on measurement of the luminance of liquid-crystal display (LCD) devices by use of a telescopic-type luminance meter. The luminance for different types of test patterns having different backgrounds and measurement areas was measured and compared with the results obtained with AAPM task group-18 (TG-18) LN test patterns. The luminance measured for the test patterns with a black background was lower than that measured for TG-18 LN test patterns due to the light emitted from the outside the focused area of the telescopic-type luminance meter. Also, the luminance obtained with smaller measurement areas indicated lower luminance. These tendencies were particularly obvious at low luminance. The luminance of the LCD device by use of a telescopic-type luminance meter should be measured with test patterns that have a black background and a smaller measurement area than that for the TG-18 LN test patterns.

Evaluation of an asymmetric screen‐film system for chest radiography

To evaluate the potential utility of an asymmetric screen‐film system for chest radiography, its image quality and detail visibility compared with a conventional screen‐film system are investigated. The basic imaging properties were evaluated by measuring Hurter and Driffield curves, resolution properties, and noise Wiener spectra. The visibility of simulated anatomical and pathological details in radiographs of a chest phantom and normal anatomy in chest radiographs of patients were evaluated subjectively. The dynamic range of each system is comparable, though the asymmetric screen‐film system can provide an advantage over the conventional system due to a relative dose reduction of approximately 35% and higher resolution properties at high optical densities. The noise level of the asymmetric screen‐film system is slightly greater at low optical densities and much greater at high optical densities. However, the visibility of lung details with the asymmetric screen‐film system is slightly superior to the conventional screen‐film system despite the increase in noise. Mediastinal and retrodiaphragmatic details are similar, though marginally superior with the asymmetric screen‐film system. It is concluded that the asymmetric screen‐film system provided slightly superior image quality to the conventional screen‐film system for chest radiography, provided the average lung density is maintained at a higher level than is customary with conventional systems.

Investigation of optimum anti-scatter grid selection for digital radiography: physical imaging properties and detectability of low-contrast signals

Our aim in this study was to evaluate the effect of the grid variations on the imaging performance for a computed radiographic system under identical exposure condition. Digital radiographies using a 20-cm Lucite phantom were performed without grid and with grid ratios of 5:1, 8:1, 10:1, 12:1, and 14:1. The scatter fraction, the incident dose to the image receptor, the Wiener spectrum (WS), and the noise-equivalent quanta (NEQ) were measured. Visibility of low-contrast signals was evaluated using a contrast-detail phantom. The scatter fractions decreased considerably with an increase in the grid ratio. On the other hand, the WSs were increased (the noise property deteriorated) as the grid ratio increased due to a decreased incident dose to the image receptor under the identical exposure condition. The NEQs were improved as the grid ratio increased. The high grid ratios provided higher low-contrast detectability compared to the low grid ratios. Our results indicated that the removal of scattered radiation was very effective in improvement of the NEQ in the digital system under the identical exposure condition.

Comparison of viewing angle and observer performances in different types of liquid-crystal display monitors

It is known that the performance of liquid-crystal display (LCD) monitors, such as the luminance and contrast ratio, is dependent on the viewing angle. Our purpose in this study was to compare the angular performance and the effect on observer performance of different types of LCD monitors. The luminance performance and contrast ratio as a function of viewing angle (−60° to 60°) in each direction for two types of LCD monitors, namely, a general-purpose LCD monitor and one especially designed for medical use, were measured in this study. Furthermore, the observer performance at various viewing angles in the horizontal direction for a medical-grade LCD monitor was investigated by eight observers based on a contrast-detail diagram. The two types of LCD monitors showed notable variations in luminance and contrast ratio as a function of the viewing angle. Acceptable viewing angles in terms of the contrast ratio were much smaller in each direction than those for nominal viewing angles in the specifications provided by the manufacturers, and those for the medical-grade LCD monitor in the horizontal and vertical directions were broader than those of the general-purpose LCD monitor. There was no significant difference in observer performance between 0° and 40°. On the other hand, our results showed a statistically significant difference in observer performance between 0° and 60°.

Preliminary Evaluation of a Phase Contrast Imaging with Digital Mammography

X-ray beams irradiated from an x-ray tube with a point source or a sufficiently small focal spot produce edge-enhanced images in the boundary of an object due to the effect of phase shift of x-rays. This technique is called phase contrast imaging. A digital phase contrast imaging system based on a photostimulable phosphor plate (imaging plate) designed for mammography has been developed for clinical use recently and now commercially available. In this study, the digital phase contrast images of an acrylic plate and plant seeds were acquired without any increase of incident dose to detector when compared to conventional contact digital imaging. Improvement of image edge sharpness was evaluated in terms of spatial edge response and spectral analysis of the images. In addition, the improvement of the sharpness of the image was also evaluated in clinical mammograms. Our results indicated that higher image sharpness in the boundary of the object was observed. The power spectrum of the digital phase contrast image was found to be higher than that of the digital contact image at wide spatial frequency region. In conclusion, the commercially available phase contrast imaging system can provide breast images with details that are not available in conventional mammograms. The digital phase contrast imaging would be useful to detect diseases, especially microcalcifications, in mammograms without any increase of exposure dose.