Akiyoshi Ohtsuka

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.

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%.

Assessment of Radiological Image Quality by Redundancy: Subject Contrast

The relationship between the subject contrast and the total image performance of a radiograph can be assessed by a single number. In this study, the radiological image quality containing object information in two screen-film systems was assessed by the redundancy, C(Y). The quantity C(Y) in the LTII-QS system has a lower value at low subject contrast but a higher value at high subject contrast, compared with the LHII-RX system. The value of C(Y) is affected more by the radiographic mottle of the system at low subject contrast and more by the MTF of the system at high subject contrast.

Applications of Uncertainty Analysis to Visual Evaluation of Density in Radiographs

Uncertainty analysis, developed as a method of absolute judgment in psychology, is applied to a method of radiographic image evaluation with perceptual fluctuations and to an examination of visual evaluation of density in radiographs. Subjects are composed of three groups of four neurosurgeons, four radiologic technologists and four nonprofessionals. By using a five-category rating scale, each observer is directed to classify 255 radiographs randomly presented without feedback. Characteristics of each observer and each group can be shown quantitatively by calculated information values. It is also described that bivariate uncertainty analysis or entropy method can be used to calculate the degree of agreement of evaluation.

Density-exposure conversion curve and microblackness characteristic in the radiological domain

The authors obtained two market-available microdensitometers having different characteristics (with negligible flare and with high-level flare) and measured the respective characteristic curves and microblackness characteristic (MBC) curves. They discuss the relationship of the curves. Furthermore, they measured a series of line spread functions (LSF) of a radiographic screen-film system by the use of the characteristic curves and the MBC curves obtained, and then examined the density dependence of the LSF.

Improvement in Radiographic Image Quality by Reduction in Scattered X-Rays: A New Concept in Improving Image Quality

A new concept which has been successful in improving the radiographic image quality using high-speed screen-film (SF) systems is presented. It is shown theoretically and experimentally that the image contrast and sharpness in low frequency can be improved by the reduction in scattered X-rays without increasing the patient exposure. Two radiographic imaging systems with the same sensitivity: High-speed S-F systems combined with high-ratio antiscatter grids (the Low-Scatter Systems) and medium-speed S-F systems combined with low-ratio antiscatter grids (the High-Scatter Systems); were used in this study. The radiographic contrast of the Low-Scatter Systems can be increased by a factor of 1.3-1.6 and the response can also be increased in low frequency below 1 cycle/nm compared to the High-Scatter Systems. This new method has the advantage of having no use for special devices like the moving slit assembly.