An approach for three-dimensional visualization using high-resolution MRI of the temporomandibular joint

OBJECTIVES

To visualize the temporomandibular joint (TMJ) and the surrounding tissues in detail utilizing high-resolution MR images for the diagnosis of soft- and hard-tissue abnormalities. Clinically routine MR slices are processed by tissue segmentation and three-dimensional (3D) reconstruction and viewed with visualization software.

MATERIALS AND METHODS

A 1.5 T MRI system was used. The double-echo procedure for taking oblique sagittal images was applied to obtain both proton density-weighted (PDW) and T2 weighted (T2W) images simultaneously, with separate examinations in both open and closed mouth positions. Diagnosis of the abnormality in the placement and morphology of articular discs and the joint effusion status is usually performed using multiple MRI slices. Clinically routine continuous MR slices were processed by segmentation, reconstruction and visualization algorithms, and the mandibular condyle, fossa, articular disc and other intra-articular tissues were visualized on the 3D and two-dimensional (2D)-3D fusion images.

RESULTS

In a clinical case, the anterior disc displacement without reduction, with mouth open and closed, was clearly depicted in the 3D images. Also 2D-3D superposed images with changeable tissue transparency successfully depicted the stereoscopic TMJ morphology in three dimensions.

DISCUSSION AND CONCLUSION

High-resolution PDW- and T2W MR images could be processed by tissue segmentation and 3D-reconstruction procedures, and the resultant images showed the anatomical details in an easily recognizable way. By the simultaneous visualization of both bony surfaces and soft tissues, disc displacement and deformity can be recognized in a 3D context. The additional superposition of the 3D visualization with the original 2D MR slices allows for a combination with conventional diagnostics.

Two cases of schwannoma with marked cystic changes

We report two cases of schwannoma displaying marked cystic changes; one in the temporalis muscle and one in the submandibular space. The first patient, a 44-year-old male, presented after complaining of a swelling rapidly increasing in size in the left temporal region. Computed tomography (CT) indicated a low-density area surrounded by soft tissue. Magnetic resonance imaging (MRI) revealed signal hypointensity on T1 weighted imaging and strong signal hyperintensity on T2 weighted imaging. The extirpated tumour specimen measured 58 mm x 58 mm x 30 mm. Histopathological examination identified schwannoma, comprising spindle cell proliferation in a palisading pattern with obvious cystic changes. The second case involved a 46-year-old female who presented with swelling of the right submandibular region. Panoramic radiography and lateral oblique mandible projection, which were used together with conventional sialography of the submandibular gland, revealed the so-called "ball in hand" appearance of the submandibular gland, and contrast-enhanced CT identified a lesion of 30 mm diameter with a well-defined annular margin and homogeneous low-density near the tumour centre. Benign pleomorphic adenoma was suspected, but histopathological examination identified schwannoma, predominantly comprising Antoni B type tissue.

Three-dimensional volumetric visualization of arteriovenous malformation of the maxilla

The appearance of an arteriovenous malformation is described from images derived from spiral CT and reconstructed in three dimensions using computer graphics. The existence and shape of a vascular malformation is demonstrated using a vascular protocol. This three-dimensional CT technique is an important adjunct to diagnosis using CT, MRI and conventional radiography.

Digital imaging modalities for dental practice

The introduction of the computed tomograph in the 1970s revolutionized medical diagnosis by initiating the transition from analogue to digital imaging. During this period, more specialized equipment for image processing was developed, such as cathode-ray tubes for image display, special sensors for image acquisition, and storage devices for image archiving. Digital imaging systems designed exclusively for use in dentistry were developed in the latter half of the 1980s. Some are now being clinically applied under conditions of close scrutiny to determine diagnostic accuracy, image quality, and radiation exposure to patients. This article reviews the enabling technologies of digital systems used in dentistry, and focuses upon intraoral digital imaging systems, concepts for digital image acquisition, and variations in radiation dose and their effects on diagnostic accuracy of caries detection.

Sensitometric properties of Agfa Dentus OrthoLux, Agfa Dentus ST8G, and Kodak Ektavision panoramic radiographic film

OBJECTIVES

The purpose of this study was to compare the sensitometric properties of and visualization of anatomical structures with Agfa OrthoLux green-sensitive panoramic radiographic film, Agfa ST8G green sensitive panoramic radiographic film, and Kodak Ektavision green-sensitive panoramic radiographic film used in combination with an Agfa Ortho Regular 400 imaging screen, Kodak Ektavision imaging screen, and Kodak Lanex Regular imaging screen.

METHODS

The density response and resolution of panoramic radiographic film/intensifying screen combinations was evaluated by means of Hunter and Driffield curves, modulation transfer functions, and noise-equivalent number of quanta. Image clarity of selected anatomical structures was rated independently by 6 oral and maxillofacial radiologists.

RESULTS

The ISO speed for the Agfa OrthoLux panoramic radiographic film combinations was the fastest, and the ISO speed for the Kodak Ektavision green-sensitive panoramic radiographic film combinations was the slowest. The average gradient for the Agfa ST8G systems was relatively steep in comparison with those for the other film/screen combinations. The modulation transfer functions for the Kodak Ektavision film were higher than those for the other films, irrespective of the screen combination used, and those for Agfa OrthoLux film were slightly higher than those for Agfa ST8G film. The noise-equivalent number of quanta for the Agfa ST8G film/screen combinations was lower than those for the other film/screen combinations. The noise-equivalent number of quanta for the Kodak Ektavision film/screen combinations was well within the high-frequency range, whereas Agfa OrthoLux combined with either the Kodak Ektavision imaging screen or the Kodak Lanex Regular imaging screen produced a noise-equivalent number of quanta similar to those of the Kodak Ektavision film/screen combinations in the low-frequency range. Agfa OrthoLux was perceived to provide clearer images of the selected anatomical details than Agfa ST8G, and the Agfa OrthoLux/Agfa Ortho Regular 400 combination was not significantly different from the Kodak Ektavision/Kodak Lanex Regular combination in terms of perceived image quality.

CONCLUSION

Agfa OrthoLux is an improvement over Agfa ST8G in film speed, spatial resolution, granularity, and perceived diagnostic image quality. The Agfa OrthoLux/Agfa Ortho Regular 400 combination did not exceed the Kodak Ektavision film/Kodak Ektavision imaging screen combination in resolution, granularity, or perceived image quality.

Contrast-enhanced conventional CT in patients after surgery for malignant tumors: evaluation of the optimal method of the administration of the contrast medium

Patients after ablative surgery for malignant tumors require computed tomography (CT) examination of a wide area on the head and neck to follow-up for recurrence and lymph metastasis. The aim of this study was to determine a more effective method for the infusion of the contrast medium into post-operative patients undergoing conventional CT, based on the relationship between the method of administering the contrast medium and the contrast-enhancing effect in the internal jugular vein. First eleven images were selected from the existing contrast-enhanced and plain CT images in a manner such that the CT values of the internal jugular vein were distributed evenly in a range of 50-180. Seven experienced observers evaluated the contrast-enhancing effect of each image set at a window value of 40 and window widths of 120, 200, and 280. Secondly, the CT values of the right internal jugular vein were measured in a total of 10 CT images from the thyroid to maxillary sinus level from each of 60 post-operative patients. The injection needles and contrast-enhancing techniques used in the 60 patients were drip infusion using an 18G injection needle in 20, drip infusion using a 21G injection needle with bolus intravenous injection immediately before scanning in 20, and drip infusion using a 23G injection needle with bolus intravenous injection immediately before scanning in 20. A CT value of 100 or above, preferably 120 or above, in the internal jugular vein was needed for the contrast-enhancing effect of a CT image to be judged as clinically significant. Our results found that, when a conventional CT was used in patients after surgery for malignant tumors, drip infusion using a 21G or 23G injection needle should be combined with bolus injections immediately before the beginning of scanning, and at the glottis or submandibular gland level during the scanning. A sufficient contrast-enhancing effect can also be obtained by drip infusion using an 18G injection needle without bolus injection.

Computed tomography of emphysema following tooth extraction

Subcutaneous and tissue space emphysema following surgical extraction of mandibular third molars is a rare and serious complication of dental surgery. Recognition of mediastinal emphysema following surgical extraction is difficult because there are no absolute clinical symptoms and signs. We present two cases of emphysema following extraction of a lower third molar and discuss the contribution of CT to the early recognition of the presence and spatial migration of air and to clinical management.

Suitability of the general-purpose graphic printer as an image output device for digital dental x-ray images

OBJECTIVES

The purpose of this study was to evaluate the suitability of the general-purpose graphic printer as an image output device for digital dental x-ray images.

METHODS

The image quality as obtained by a thermal printer and by a dye sublimation printer was investigated. A grid pattern image was used to check parallelism and verticality of lines in each hard copy. A step-wedge image was printed with each printer, and the optical density, gradient, and root mean square granularity were compared. Depiction ability was also compared by using test images, including small signals.

RESULTS

All of the lines were parallel and vertical on hard copies of both printers. The dye sublimation printer showed better results on optical density, gradient, root mean square granularity, and depiction ability.

CONCLUSION

The dye sublimation printer produces images of the depiction ability comparable to the cathode ray tube display and seems suitable as an image output device for digital dental x-ray images.

Assessing the image quality of a CCD-based digital intraoral radiography system: application of perceptibility curve test

The perceptibility curve test is a method for evaluating the psychophysical properties of radiographic systems. The concept of the perceptibility curve is based on the minimum perceptible exposure differences that are recorded by a particular imaging system. The perceptibility curve test was applied to a Charge-Coupled Device (CCD)-based digital intraoral radiography system in this study. A test object was made of a square aluminum block, 28 mm in both height and length and 10 mm in thickness. The test object had ten contrast details in the form of round holes with diameters of 1.5 mm. The depths of the holes ranged from 0.05 mm to 0.5 mm in steps of 0.05 +/- 0.01 mm. An X-ray unit was used that operated in the range of 60 to 90 kVp. Test radiographs were shown to observers in random order. Six observers were asked to rate them regarding the number of spots representing holes that they could perceive. The smallest perceptible difference in gray level on the computer monitor, a 15-inch cathode-ray tube (CRT), was then found. The minimum difference in gray level between a perceptible detail and the background was determined. Dose response functions were employed to determine exposures corresponding to these gray levels. For each case, delta logE as a function of the number of object details was calculated. Finally the reciprocal of all values of (delta logE)min were calculated. The number of details which the observers perceived decreased linearly with decreasing delta logE values. The maximum number of details that observers perceived was eight or nine. The perceptibility curves showed that their peaks shifted slightly with a change of the kVp but that their shapes were similar at different kVp settings.

Dependency of dose response of five charge-coupled device-based digital intra-oral radiographic systems on tube voltage

OBJECTIVE

To compare the dependency of dose response of five CCD-based digital intra-oral radiographic systems on tube voltage.

METHODS

Characteristic curves for Sens-A-Ray (old and new generations; Regam Medical Systems, Sundsvall, Sweden), Computed Dental Radiography (Schick Technologies, Long Island City, NY, USA), CompuRay (Yoshida, Tokyo, Japan), and VIXA (Dentsply/Gendex, Des Plaines, IL, USA) were generated between 60 and 90 kVp at intervals of 10 kVp and corrected by subtraction of dark current. Their sensitivities were compared.

RESULTS

The sensitivity of the old generation Sens-A-Ray decreased with increase in tube voltage whereas in the other systems it increased.

CONCLUSION

In the newer systems using a rare earth intensifying screen as a scintillator, the sensitivity increases with increase in tube voltage, thus reducing the radiation dose to the patient.

A sensitometric comparison of Kodak Ektavision and Fuji Super HR-S panoramic radiographic films

OBJECTIVES

The purpose of this study was to compare the combination of Ektavision film and Ektavision intensifying screen with the combination Super HR-S panoramic radiographic film and HR-4 intensifying screen with respect to sensitometric properties and the visualization of anatomic structures.

METHODS

The density response and resolution of both panoramic radiographic film/intensifying screen combinations were evaluated through use of the generated characteristic curves and the modulation transfer function. The diagnostic clarity was evaluated by five oral and maxillofacial radiologists who used a rating scale to evaluate the image clarity of selected anatomic structures.

RESULTS

The film speed and radiographic contrast, respectively, were 1.3 and 1.2 times higher for Ektavision than for Super HR-S. The image resolution with Ektavision was similar to that with Super HR-S. Ektavision was perceived to provide clearer images of the selected anatomic details than did Super HR-S.

CONCLUSION

With respect to the clarity of anatomic structures, Ektavision film, with its higher speed and higher contrast, proved superior to Super HR-S film.

Sensitometric effects of varying the intensifying screens used with Agfa Dentus ST8G and RP6 panoramic radiographic films

OBJECTIVES

To compare the sensitometric effects and information yield of varying the intensifying screens used with both Dentus ST8G and RP6 Agfa Gevaert, Dormagen, Germany panoramic radiographic films.

MATERIAL AND METHODS

Four screen-film combinations were employed for each of the two film types. The screens used were blue fluorescing PX-III (Kasei Optonix, Tokyo, Japan) and Special (Siemens AG, Bensheim, Germany), as well as green fluorescing Lanex Regular (Eastman Kodak, Rochester, NY, USA) and Trimax T16 (3M, Mineapolis, Minnesota, USA). The density response for each screen-film combination was evaluated using the characteristic curves generated. Information yield, as determined by the radiographic detection of defects in an aluminium test object, was evaluated by nine observers.

RESULTS

The characteristic curves for ST8G were different when green and blue fluorescing screens were used; however, those for RP6 varied little irrespective of the choice of intensifying screens. Observers were able to perceive defects at significantly lower radiation exposures for ST8G combined with green fluorescing screens compared with blue emitting screens. RP6 with all screen combinations provided similar image detail perceptibility at comparable exposures with ST8G with green-fluorescing screens.

CONCLUSIONS

RP6 is suitable for use with either the spectrally matched blue emitting screens or green-emitting screens. ST8G radiographic film should always be matched to rare earth screens.

Evaluation of XD/A Plus and ST8G films for cephalometric radiography with Grenex G8 and BH-III screens

OBJECTIVES

Sensitometric properties, clinical image quality, and patient dose requirements are important considerations when selecting film for cephalometrics. Two recently released films, XD/A Plus and ST 8G green sensitive films, were studied.

METHODS

The films were each combined with Grenex G8 (Fuji Medical) green-fluorescing matched and BH-III (Kasei Optonix) blue-fluorescing mismatched intensifying screens. The density response and resolution for each screen-film combination were evaluated by use of the characteristic curve and modulation transfer function. The kilovoltage settings providing clinically acceptable images were assessed individually by 12 observers. Clinically acceptable images for each combination were also compared, and the skin entrance doses in the temporomandibular joint region were determined.

RESULTS

The average contrast at the most effective density range was found to be slightly higher for the BH-III group than for the G8 group. The modulation transfer function for the BH-III group was inferior to that for the G8 screens. There were no significant differences in diagnostically acceptable image quality among the four combinations; nevertheless the BH-III screen group required two to three times more exposure than the G8 screen group.

CONCLUSIONS

XD/A Plus and ST8G films provide acceptable image detail for cephalometrics. To minimize the patient dose they should be used with green-emitting screens.

Perceptibility of defects in an aluminum test object: a comparison of the RVG-S and first generation VIXA systems with and without added niobium filtration

OBJECTIVE

To compare the first generation VIXA (Gendex, Milan, Italy) and the RVG-S (Trophy Radiologie, Vincennes, France) for detection of defects in an aluminium test object at various exposures both with and without added niobium filtration.

METHODS

Images of a 7 mm aluminium test object with defects ranging from 0.1 to 1.5 mm with standardized projection geometry. Seven dentists acted as observers. Perceptibility curves were developed for both sensors under the various filtration conditions and exposures.

RESULTS

At optimum exposures both the VIXA and the RVG-S permitted the same number of defects to be observed. The optimum exposures were similar for the two systems, but the dynamic range was greater for the RVG-S. Addition of niobium filtration did not appreciably alter the radiation dose required to perceive a given number of defects using either system.

CONCLUSIONS

The two systems performed equally at optimum exposure when the dose is approximately the same irrespective of the presence or absence of niobium filtration. Beyond the optimal level, the RVG-S outperformed the VIXA system.

A dual sensitivity screen system for TMJ image enhancement in cephalometric radiography: sensitometric evaluation

OBJECTIVES

An in vitro evaluation of the image quality of the TMJ OrthoCeph Slimline Cassette System (TOSCS).

METHODS

The density response, resolution and clarity of the system were evaluated by use of the characteristic curve, modulation transfer function and root mean square quantum mottle. Relative sharpness was evaluated qualitatively by five observers.

RESULTS

Increased density with Trimax 12 gave almost the same contrast as that with Trimax 8 in the diagnostic range, whereas MTF and RMS for Trimax 12 were inferior.

CONCLUSIONS

The dual-sensitivity screen system should lead to improved radiographic contrast of the TMJ region in cephalometric radiography. The reduction in sharpness is probably clinically insignificant.

Observer performance in the assessment of contract enhanced CT: effects of contrast medium and grey scale window on detection of defects in a simulated peripheral rim border

The image contrast of X-ray CT systems depends on the properties of the imaging system, the number of grey scales used in the digital sampling process, and the addition or otherwise of a contrast medium. The effect of contrast medium is often emphasized in the peripheral rim of pathologic masses. In this study, a special hexagonal test object made of acrylic phosphate and a cylindrical container were employed. When the contrast medium IOPAMIRON 300 was used to fill the space between the test object and its cylindrical container, a contrast-enhanced peripheral rim with or without signal of defect was observed. Visual perceptibility was examined from the signals resulting from varying contrast by use of contrast medium and adjusting the grey scale by image processing. Threshold contrast influenced by window width was recorded in each image contrast, different ranges of window width was recorded in each instance using statistical analyses. While window width was inversely proportional to image contrast, different ranges of window width and image contrast provided better visual perceptibility for each concentration of the contrast medium. The range of window width for obtaining the best visual perceptibility was determined for a contrast-enhanced image.

Objective and subjective assessments of Kodak Ektaspeed plus new dental X-ray film: a comparison with other conventional X-ray films

A new dental X-ray film called EKTASPEED Plus has been released by the Eastman Kodak Company. Its sensitometric properties, inclusive of modulation transfer function (MTF) and Wiener spectrum (WS) and the visibility of radiographic details, were compared with those of other conventional dental X-ray films, EKTASPEED EP-22 (Eastman Kodak Co.) and Ultra-Speed DF-57 (Eastman Kodak Co.). The base plus fog density of EKTASPEED Plus was between those for EKTASPEED and Ultra-Speed. Relative speed of EKTASPEED Plus was slightly faster than EKTASPEED, and about two times faster than that of Ultra-Speed. The contrast of EKTASPEED plus was close to that of Ultra-Speed. Only minor differences existed between the film types studied when the sharpness and the granularity were compared. The MTF and WS also revealed no substantial differences between the films. Perceptibility testing showed minor differences between the three films, with EKTASPEED Plus consistently revealing more defects at lower exposures irrespective of whether a Burger's phantom or aluminum test block was used.

Comparing the sensitometric properties of dental X-ray films

This article describes a study that compared the sensitometric properties and information yields of four dental X-ray films: Eastman Kodak Ultra-Speed DF-57 and Ektaspeed EP-21, Flow DV-58 and Agfa Gevaert Dentus M2 Comfort. Dentus M2 Comfort gave the greatest contrast, a speed between the conventional D- and E-speed film groups and an exposure latitude wider than Ultra-Speed but less than DV-58. A significant difference in detail was found with low exposures, Dentus M2 Comfort and Ektaspeed outperforming the other two film types.

Quantitative assessment of image quality using niobium filtration for cephalometric radiography

The effect of added niobium filtration on image quality in cephalometric radiography was quantitatively evaluated using the Modulation Transfer Function (MTF). A conventional full-wave cephalometric X-ray generator (Quint Sectograph) and a constant potential, high frequency X-ray generator (Orthophos CD) were compared with respect to the effects on the MTF of the added filtration. There was a slight reduction in image quality with both machines when adding niobium filtration.

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.

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.

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.

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.

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.

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.

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.

[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.

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

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)

[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)