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Biomedical subjects

M Wakoh

Publications and source records attributed to M Wakoh.

30 records · Page 2Linked to original sources
PubMed1993

Diagnostic image quality and dose reduction using niobium filtration for cephalometric radiography.

The usefulness of added filtration is contingent upon the maintenance of high image quality while effectively reducing the radiation dose. High-quality cephalometric radiographs, defined objectively by a panel of orthodontists, were obtained of a tissue-equivalent phantom both without and with 30 microns and 50 microns supplemental niobium filtration. Skin entrance doses in the temporomandibular joint region were compared at diagnostically acceptable exposures where there was no significant difference in radiographic image quality. Dose reductions ranged from 17% to 31% with 30 microns niobium filtration, and 17% to 39% with 50 microns. However, as kVp and HVL were increased to maintain the same level of perceived image quality, image contrast decreased. Contrast loss with 30 microns and 50 microns niobium filtration averaged 8% and 22% respectively compared with standard aluminium filtration. It would appear that additional niobium filtration can be used to reduce radiation dose without significant loss of image quality.

Analysis of Variance↗
PubMed1993

Morphometric analysis of image distortion with rotational panoramic radiography.

We studied the image distortion of spherical objects that occurred during rotational panoramic radiography. We measured the magnification of height, width, perimeter and area, and calculated the circularity index to quantify the distortion. Spherical objects of different diameters were projected on the film. A complex relationship was found between the position and the horizontal magnification for each anatomical region. The object's size influenced the horizontal magnification to a considerable extent. The greatest changes in circularity occurred at the midline. This morphological distortion was caused by non-symmetrical projection during the scanning of these spherical objects, which resulted from the continuous changes in the scanning velocity and the position of the x-ray beam. This result suggests a complex distortion of the inner structures of macroscopic objects.

Humans↗
PubMed1992

Comparison of spatial resolution between tomographic images and ghosts in rotational panoramic radiography: the effect of the first slit width.

Rotational panoramic radiography projects the object placed within the focal trough as tomographic images with minimal distortion. However, even if an object is within the focal trough, the sharpness of the image is affected by the position of the object, the difference in the relative speed of the X-ray source against the film and the difference in the direction of rotation. In this study, spatial resolutions of ghosts and tomographic images were compared. Ghosts, as well as tomographic images, were obtained from the object placed in the focal trough. The effects of the width of the first slit, a major parameter affecting image quality, were the primary concern in this study. With regard to ghosts, the influence of the position of object, in terms of central, buccal and lingual regions the focal trough, was also analyzed. The target of analysis was the mandibular ramus region. An Orthopantomograph OP5, a conventional panoramic apparatus, was used. An X-ray test chart, X-ray beams, and H-D curve were prepared for calculation of the Modulation Transfer Function (MTF), and the slit method using the Fourier transforms was applied. When the width of the first slit was changed displacement of MTF curves was most remarkable for the superimposed tomographic image which was scanned 10 mm posterior to the X-axis connecting the right and left lateral rotation centers. For reduction of the spatial resolution of ghosts, it was found effective to use a wider slit and place the object at a position lingual to the central plane.

Artifacts↗
PubMed1991

Redundant shadows in rotational panoramic radiographs. Estimating the imaging positions of objects by mathematical analysis.

The sources of real, ghost, and double images in rotational panoramic radiographs generated by a machine with a continuous movement of the center of rotation are described. No assumptions are made with regard to the trajectory of the center of rotation; rather, an empirical approximation scheme is used to establish a regression from which measured positions of reference objects can be predicted.

Artifacts↗
PubMed1991

Mathematical approach to horizontal and vertical magnification factors in rotational panoramic radiography--with attention to redundant shadows.

Rotational panoramic radiographs consist of two different images, the tomographic image and redundant shadows. Many authors have reported magnification factors for the tomographic image which include the term "real image", as proposed by McDavid et al. in 1983. However, a mathematical formula has never been proposed for ghost images, which redundant shadows projected only when the object is located between the center of rotation and the X-ray source. The purpose of this study was to propose a new mathematical approach for calculating the horizontal and the vertical magnification factors for ghost images. Using this approach, we investigated the effect of variations in the position of the object, its length, and its placement angle with respect to the X-ray beam. Since the object was assumed to be small, an approximated expression for film speed was integrated to determine the length of the object on the film in the horizontal dimension. The approximated and the geometric expressions proposed by Wakoh in 1988 were used to calculate the vertical dimension. Differences between the scanning and the film speeds were assumed to affect the horizontal magnification factors of the ghost image. In addition, differences in the setting position, length, and angle of the object affected the calculated values. Vertical magnification factors were considerably influenced by the Z coordinate of the position, even though the length and angle remained constant.

Artifacts↗
PubMed1990

Performance evaluation of X-ray computed tomography "TCT-700S".

The performance of the CT scanner "TCT-700S" (Toshiba Medical), including noise, spatial and contrast resolution, and exposure, was evaluated. Noise increased as the slice thickness decreased. No matter what the slice thickness, noise was great when the product of tube current and scan speed (described by mAs in the following sentences) was small. Furthermore, the short scan speed caused considerable large noise. Reduction of the mAs value caused remarkable increase in noise when the slice thickness was 1 mm or 2 mm. At combinations of slice thicknesses of 1 mm or 2 mm and 100 mAs, noise was 1-1.5%. When slice thicknesses were measured as the spatial resolution, in the case of a slice thickness of 1 mm, the measurements were 1.6 to 1.8 mm. When high and low contrast resolutions were measured, maximum diameters for the recognition were large as slice thickness became thinner. Although average exposure promised the improvement in the contrast resolution, great exposure increases failed to contribute to such improvement. These performances depended on the exposure. In the case of multiple scanning, average exposures in the field were uniform. However, at a minimum scan speed of 1.8 sec, a three fold discrepancy occurred in exposures observed reciprocally from the left and right sides. Average exposures were also almost the same with slice thicknesses of 10 mm, 5 mm and 2 mm, but with a slice thickness of 1 mm, average exposure was twice as great. This measurement indicates inadequate collimation in the case of a slice thickness of 1 mm. These average exposures were proportional to the mAs value. The radial-exposure profile depended on the position in the columnar object. Although similar average exposures were observed, high peak values were observed on the upper side, but exposure tended to spread at the bottom in the center. This means that the primary x-ray amounts were great in the peripheral region and scattered x-ray amounts were great at the center.

Evaluation Studies as Topic↗
PubMed1990

Digital gray-level transformation for the reduction of redundant shadows in rotational panoramic radiography.

In rotational panoramic radiography, the tomographic motion in combination with a slit scanning method make an image layer wide. But some objects outside the image layer are hard to blur and they result to redundant shadows. The gray-level transformation, as one form of digital image processing for the reduction of these redundant shadows, was evaluated. Two methods, the gamma transformation and the histogram flattening method, were examined. A drum scanner was used as the image scanner. The panoramic image on the x-ray film was turned into an 8 bit digital image on the image memory, which had the size of a 512 x 480 matrix. In rotational panoramic radiography, there are low density and contrast regions where redundant shadows of the cervical vertebrae and the mandibular ramus are superimposed on the tomographic image. The histogram of the gray-level was suppressed for the lower gray-levels. The stretching of this gray-level distribution was effective in the reducing redundant shadows. When processed by gamma transformation, the smaller gamma coefficient below 1.0 clarified the tomographic image, and when processed by the histogram flattening method, the setting of the level (L) value around 64-128 effectively reduced the redundant shadows. However, the effectiveness of both gray-level transformations was greatest in restricted cases in which the area where redundant shadows were superimposed on the tomographic image was comparatively large.

Artifacts↗
PubMed1990

[Unsharpness of redundant shadows of the mandibular ramus region in panoramic tomography. The influence by the first slit width].

In panoramic tomography, redundant shadows of the mandibular ramus overlapping with tomographic images on the opposite side prevent radiological diagnosis. Redundant shadows differ from tomographic images in sharpness, film contrast, and imaging quality. In addition to mismatching of film velocities and X-ray beams, familiar causes of such unsharpness, still another source must be taken into consideration: the influence of varying slit widths. In the past, research has been concentrated on real images defined by McDavid et al.; that is, tomographic images and tomographic blurring images. The present study, however, qualitatively evaluates the unsharpness of redundant shadows of the mandibular ramus, especially with reference to the effects of first-slit width. At the same time, comparisons are made with tomographic images overlapping with redundant shadows of the mandibular ramus region. An orthopantomograph OP5 was employed as the panoramic X-ray apparatus. In the first step of the evaluation, X-ray beam alignments were examined in the horizontal dimension. To evaluate unsharpness of the mandibular ramus, we used regression analysis by the least-squares method and the Nitka method. Horizontal beam alignments and image layers as calculated by McDavid et al. (1985) were applied in setting the aluminum plate. Conclusions 1. In the case of redundant shadows of the mandibular ramus, unsharpness of the posterior margin and the condylar process depends on first-slit width, which is a major influence on imaging quality. 2. Intensity of edge and film contrast exert about equal influences on unsharpness of redundant shadows of the mandibular ramus. Both factors were influenced by the set-up position of the subject. 3. Film contrast exerted a greater influence than edge intensity on unsharpness of overlapped tomographic images. 4. The wider the first-slit width, the greater the indistinct area of redundant shadows of the mandibular ramus and overlapped images. But, for the purposes of diagnosis, maximum slit width is probably between 3 and 5 mm.

Artifacts↗
PubMed1989

[Performance evaluation of X-ray computed tomography "TCT-700 S". 2].

UNLABELLED: The performance of the rotate/rotate CT scanner TCT-700 S (Toshiba) was evaluated on the basis of the item entitled "User's initial acceptance tests (phase II)" set forth in "Standard of performance evaluation for x-ray computed tomography (second recommendation)" proposed by Takenaka et al. (1982). This second report in the series describes measurements of exposure and the relation between exposure and such other performance aspects as noise and contrast resolution. The exposure-performance recommendation proposes measurements of average exposure in the scanning field and a radial exposure profile. RESULTS: 1. In the case of multiple scanning, average exposures in the field were uniform because all surfaces of the perpendicular object were oriented toward the incidence of the x-ray beam. At a minimum scanning speed of 1.8 second, however, a discrepancy of 3 times occurred in exposures observed reciprocally from the left and right sides of the object. This phenomenon indicates that only one-half of the object surface is oriented toward the x-ray beam and that the x-ray tube and detectors rotate in mutually opposite directions. In the case of multiple scannings, when slice thicknesses were 10 mm, 5 mm and 2 mm, average exposures were almost uniform. But with a slice thickness of 1 mm, average exposure was twice as great. This measurement indicates inadequate collimation in the case of a 1 mm slice thickness. These average exposures were roughly proportional to the mAs (product of tube current and scan speed) in the range between a minimum of 100 mAs and a maximum of 1200 mAs. 2. Noise increased in proportion to decrease of average exposure.(ABSTRACT TRUNCATED AT 250 WORDS)

Radiation Dosage↗
PubMed1989

[Performance evaluation of the X-ray computed tomography "TCT-700 S" (Part 1)].

The performance of the Rotate/Rotate CT scanner "TCT-700 S" (Toshiba) was evaluated on the basis of the item entitled "User's initial acceptance tests (phase II)," set forth in "Standard of performance evaluation for X-ray computed tomography (second recommendation)" proposed by Takenaka et al. (1982). This report describes measurements for noise, contrast scale, spatial resolution, and high- and low-contrast resolution. Recommended phantoms were used to test performance. Results 1. Noise edepended on slice thickness and mAs. It increased as slice thickness decreased. No matter what the slice thickness, noise was great when mAs settings were small. Furthermore, short scan time caused considerable noise for each slice thickness. Reduction of mAs setting caused remarkable increased in noise when slice thickness were 1 mm or 2 mm Under routine examination, when thickness was 5 mm, at 120 kV, with an mAs setting of 440, scan field S(diameter 240 mm), and convolution filter FC-2, noise was 0.31%. When slice thickness was 10 mm at 120kv, 800mAs, with a scan field S (diameter 240 mm), and convolution filter FC-2, contrast scale was 1.84 x 10(-4) (cm-1/CT number). 2. When slice thickness was 5 mm, at 120 kV, with a mAs setting of 1200, scan field S(diameter 240 mm), and a convolution filter of FC-2, spatial resolution was 0.5LP/mm. 3. When slice thickness was 5 mm, at 120kV, with a mAs setting of 1200, scan field S (diameter 240 mm), and convolution filter of FC-2, minimum high-contrast resolution was 0.5 mm.(ABSTRACT TRUNCATED AT 250 WORDS)

Evaluation Studies as Topic↗