Tomohiro Okano

Exposure Geometry And Film Contrast Differences As Bases For Incomplete Cancellation Of Irrelevant Structures In Dental Subtraction Radiography

Subtraction radiography for longitudinal studies requires both reproducible imaging geometry and film contrast characteristics to permit perfect alignment of the radiographs, and achieve good cancellation of diagnostically irrelevant background structures. The standard deviation (SD) of gray levels about the mean in a subtraction image was used as a relative measure of the residual structured noise. In order to estimate the effects of improperly standardized radiographs on SD in the subtraction image, both the imaging angle and film exposure time were systematically varied. The results showed that SD changed linear for small misalignment angles of the central beam, the variance attributable to this error source reaching about the same magnitude as the variance due to anatomical differences for angulation errors within ±20. The SD increase due to large film contrast disparity could be partly reverted for angulation errors within this bound by using a quadratic transformation which matched the first two moments of the gray level distributions in the two parent radiographs. Therefore, in order to use some of the retrospective data obtained under less stringent standardizations for subtraction imagery, it appears possible to adjust for differences in film contrast, and, perhaps, correct for geometric misalignment by a separate algorithm.

Statistical contrast enhancement of subtraction images for radiographic caries diagnosis

The effects of the nonlinear contrast-enhancement technique are examined in terms of diagnostic performance obtainable from subtracted dental radiographs for a variety of simulated exposures. Conventional bitewing radiographs of patients known to be free of caries were digitized by means of a computer interfaced with a conventional television camera. The resulting images were duplicated and stored in the computer. Radiolucencies similar in appearance to interproximal caries were simulated analytically in one set of the images. Reference images were superimposed spatially and subtracted from their counterparts containing the induced interproximal lesions after simulating the effects of quantum limited exposure on both sets. This was done for each separate image element independently by replacing original gray levels in each image with levels determined by a Poisson random deviate. The resulting difference images were contrast enhanced by a method which first smooths out local variations in gray level and then reassigns gray-level values in a way determined by the observed second-order spatial statistics. To aid in localization, these images were then again subtracted from the original noise-degraded pictures without lesions, rendering images similar to conventional radiographs but contrast enhanced. Observer performance by means of these enhanced images was compared with that produced from unenhanced-lesion-containing controls. The results suggest that enhancement increases the certainty with which diagnosis can be made and, further, that diagnostic accuracy can be improved in severely degraded images which simulate the effects of reduced levels of exposure.

Limitation of panoramic radiography in diagnosing adenomatoid odontogenic tumors

Adenomatoid odontogenic tumor is frequently not diagnosed preoperatively even when radiographs are available. This study evaluates radiographic findings with special emphasis on the characteristic radiopacities of adenomatoid odontogenic tumor. In correlating radiologic findings with pathologic findings, differences in diagnostic accuracy between intraoral periapical and panoramic radiographs were found. The intraoral periapical radiograph allows perception of the radiopacities in adenomatoid odontogenic tumor as discrete foci having a flocculent pattern within radiolucency even with minimal calcified deposits. Panoramic radiography was often unable to demonstrate radiopacities in adenomatoid odontogenic tumor when the calcification was minimal. The result indicates that intraoral radiographs may be essential for correct radiographic interpretation of an adenomatoid odontogenic tumor in the presence of minimal quantities of calcified deposits.

Digital subtraction of radiograph in evaluating alveolar bone changes after initial periodontal therapy

The purpose of this study was to determine the effect of initial periodontal therapy on crestal bone remineralization with the use of a digital subtraction method for quantitative analysis. Fifteen crestal sites randomly selected from five patients were examined at the initial visit, after completion of oral hygiene instruction, and 1, 3, and 6 months after scaling and root planing with standardized radiographs. The radiographs were digitized by means of a drum-scanning microdensitometer with a sampling aperture of 50 microns producing 256 gray levels and corrected for difference of contrast between image pairs. Results were as follows: (1) statistically significant changes were observed even 1 month after the therapy and (2) the extent of changes was quantitatively expressed and monotonically increased with time. These results indicate that subtraction radiography may be a reproducible and quantitative method for the evaluation of periodontal therapy.

Diagnostic accuracy on detection of proximal enamel lesions in nonscreen radiographic performance.

The effect of differences in image contrast and mean density associated with Kodak Ektaspeed and Ultraspeed films was examined in association with systematic variations in exposure geometry and beam characteristics. Performance was expressed in terms of diagnostic accuracy obtainable by eight dentists who attempted to detect a known distribution of induced proximal enamel lesions in a radiographic phantom. Differences associated with the two types of film and x-ray beam quality were found to have small but statistically significant effects on diagnostic performance. The other factors of film density and projection geometry influenced diagnostic accuracy much more. The largest source of variation was observed among the individual dentists serving as observers.

Image quality assessment of three cone beam CT machines using the SEDENTEXCT CT phantom

OBJECTIVES The SEDENTEXCT Project proposed quality assurance (QA) methods and introduced a QA image quality phantom. A new prototype was recently introduced that may be improved according to previous reports. The purpose of this study is to evaluate image quality in various protocols of three cone beam CT (CBCT) machines using the proposed QA phantom. METHODS Using three CBCT machines, nine image quality parameters, including image homogeneity (noise), uniformity, geometrical distortion, pixel intensity value, contrast resolution, spatial resolution [line pair (LP) chart, point spread function (PSF) and modulation transfer function (MTF)] and metal artefacts, were evaluated using a QA phantom proposed by SEDENTEXCT. Exposure parameters, slice thickness and field of view position changed variously, and the number of total protocols was 22. RESULTS Many protocols showed a uniform gray value distribution except in the minimum slice thickness image acquired using 3D Accuitomo 80 (Morita, Kyoto, Japan) and Veraviewepocs 3Df (Morita). Noise levels differed among the protocols. There was no geometric distortion, and the pixel intensity values were correlated with the CT value. Low contrast resolution differed among the protocols, but high contrast resolution performed well in all. Many protocols showed that the maximum line pair was larger than 1 LP mm(-1) but smaller than 3 LP mm(-1). PSF and MTF did not correlate well with the pixel size. The measured metal artefact areas varied for each device. CONCLUSIONS We studied the image quality of three CBCT machines using the SEDENTEXCT phantom. Image quality varied with exposure protocols and machines.

Changes in Cortical Bone Mineralization in the Developing Mandible: A Three‐Dimensional Quantitative Computed Tomography Study

Quantitative computed tomography (QCT) was completed in 34 subjects between the ages of 9 and 33 years with symmetrical mandibles in order to investigate the three‐dimensional cortical bone mineral density (BMD) distribution in the mandible. The number and distribution of the pixels were determined at three levels: (1) representing the entire mandibular bone; (2) the cortical bone at 60% above the baseline defined as the segmentation level (around 1050 mg/cm3) and representative of only cortical bone; and (3) the highest mineralized cortical bone (>1250 mg/cm3). The geometrical distribution of the highest mineralized areas was evaluated by three‐dimensional reconstruction of the images. The total number of pixels for the entire mandible increased significantly at each time point represented at four increasing ages groups (9–11 years of age, 12–14 years of age, 15–17 years of age, and >18 years of age). The male and female subjects had a similar total number of pixels for the entire mandible before the age of 11, but the male subjects showed a significantly larger total number of mandibular pixels after that age. Comparison of the number of pixels for pure cortical bone (60% segmentation level) and the highest mineralized cortical bone indicated a significant increase with maturation with the greatest change occurring between the 13‐year and 16‐year age groups. However, the ratio of cortical bone/total bone increased at a more rapid rate in the male subjects and reached a plateau by the 16‐year age group, showing distinct differences in mineralization of the mandible between the sexes.

The effect of surrounding conditions on pixel value of cone beam computed tomography

OBJECTIVE The purpose of this study was to evaluate the reliability of pixel value in CBCT, especially with regard to the effect of surrounding objects that are presented outside the field of view (FOV). MATERIALS AND METHODS This experiment used the GE Hi-Speed QXi, a multidetector helical computed tomography (MDCT) scanner, and the 3D Accuitomo FPD 8, a cone beam computed tomography (CBCT) scanner. Two types of phantoms were used, both of which contained Lipiodol Ultra Fluid (Lipiodol UF). The type A phantom was a target phantom for pixel value measurement while type B was used for the surrounding environment. For CBCT, the type A phantom was placed in a water bath, and 4 types of surrounding environmental conditions were created: (1) no other phantom present, (2) phantom type B also within the FOV, (3) half of phantom type B within the FOV, (4) phantom type B entirely outside the FOV but within the path of x-rays aimed at phantom A. RESULTS In MDCT, pixel value (CT number) showed an almost linear correlation with the concentration of Lipiodol UF. In CBCT, on the other hand, pixel value was not linearly correlated with Lipiodol UF concentration. The position of the type B phantom affected pixel values in images of the type A phantom. CONCLUSIONS Pixel value in CBCT may be affected by various conditions such as beam hardening and surrounding materials, and is therefore not reliable. Caution is essential when pixel values in CBCT are used to estimate bone density at potential implant sites.

Effects of exposure reduction on the accuracy of an intraoral photostimulable-phosphor imaging system in detecting incipient proximal caries

Digital imaging systems for use in intraoral radiography are becoming common in clinical practice, although its physical properties such as resolution and available dynamic range may be inferior to conventional film systems. The purpose of this study was to determine the diagnostic accuracy in detecting incipient proximal caries using a digital system with storage phosphor plate (Digora®) in comparison with a conventional intraoral film (Kodak Ektaspeed Plus®). Thirty-one extracted upper premolars were selected. Of the 62 surfaces, 33 had proximal caries with discoloration or incipient lesions, and the rest showed no evidence of caries verified on strict inspection using fiber-optic transillumination. All teeth were radiographed following the paralleling technique (60 kV, 40 cm focus-to-film distance). Exposure was adjusted to Ektaspeed Plus film, while reduced to 1/2, 1/4, and 1/8 on Digora®. The films and digital images were evaluated by three observers, using a confidence rating for the presence of caries. On Digora®, the observers could control contrast and brightness. The results showed that Digora® had better performance in detecting caries than the film, even when the exposure was used for the film, although there were no statistically significant differences. The results suggest that the storage phosphor plate system may be used clinically to diagnose incipient proximal caries, while reducing the exposure to the patient.

Composition of the blood supply in the sublingual and submandibular spaces and its relationship to the lateral lingual foramen of the mandible

OBJECTIVE The purpose of this research was to contribute to minimizing arterial disruption during mandibular surgical procedures by clarifying the course of the arteries supplying the sublingual and submental regions. STUDY DESIGN Heads of 75 human cadavers were dissected to identify the arterial supply of the sublingual and submental regions. Computed tomography scans were performed to visualize the lateral lingual foramina of the mandibles. RESULTS The facial artery was found to contribute to the composition of almost half of the sublingual arteries studied. Furthermore, it was demonstrated that the arterial supply to an incisor tooth is often from the submental artery. CONCLUSIONS Our data provide important information for coping with bleeding or hematoma occurring during surgical procedures in the mandibular region.