Richard L. Webber

A digital subtraction technique for dental radiography

The relative accessibility and stability of the teeth facilitate generation of geometrically identical radiographs over a period of time. When such radiographs are digitized with the aid of a TV camera coupled to an analog to digital converter and a computer, it is possible to conveniently produce subtraction images of high diagnostic value. A digital subtraction technique for longitudinal dental radiography is described; this technique should be applicable to diagnostic situations characterized by a need to detect small changes occurring over a period of time in areas where substantial amounts of structured noise limit their detectability.

Fractal dimension from radiographs of peridental alveolar bone: A possible diagnostic indicator of osteoporosis

The purpose of this study was to investigate whether a radiographic estimate of osseous fractal dimension is useful in the characterization of structural changes in alveolar bone. Ten dry mandibular bone segments were radiographed from three controlled projection angles (-5, 0, +5 degrees), before and after acid-induced partial decalcification. Fractal dimension was estimated by regression analysis of power spectra computed by Fourier transform of selected regions of interest in digitized images of the radiographs. Repeated-measures ANOVA showed that fractal dimension so determined varied over anatomic locations (p less than .01), but increased after acid-induced demineralization (p less than .0005), irrespective of the radiographic projection angles (p greater than .99). In vivo fractal dimension was computed from randomly selected intraoral radiographs of six premenopausal (ages, 32.8 +/- 3.9) and six postmenopausal (ages, 62.5 +/- 4.1) women. A significantly (p less than .01) higher fractal dimension was observed in the older group.

Comparison of tomosynthesis methods used with digital mammography

RATIONALE AND OBJECTIVES The authors performed this study to investigate the potential applicability of tomosynthesis to digital mammography. Four methods of tomosynthesis-tuned aperture computed tomography (TACT)-backprojection, TACT-iterative restoration, iterative reconstruction with expectation maximization, and Bayesian smoothing-were compared to planar mammography and analyzed in terms of their contrast-detail characteristics. Specific comparisons between the tomosynthesis methods were not attempted in this study. MATERIALS AND METHODS A full-field, amorphous, silicon-based, flat-panel digital mammographic system was used to obtain planar and tomosynthesis projection images. A composite tomosynthesis phantom with a centrally located contrast-detail insert was used as the object of interest. The total exposure for multiple views with tomosynthesis was always equal to or less than that for the planar technique. Algorithms were used to reconstruct the object from the acquired projections. RESULTS Threshold contrast characteristics with all tomosynthesis reconstruction methods were significantly better than those with planar mammography, even when planar mammography was performed at more than twice the exposure level. Reduction of out-of-plane structural components was observed in all the tomosynthesis methods analyzed. CONCLUSION The contrast-detail trends of all the tomosynthesis methods analyzed in this study were better than those of planar mammography. Further optimization of the algorithms could lead to better image reconstruction, which would improve visualization of valuable diagnostic information.

Computer Correction of Projective Distortions in Dental Radiographs

Distortions in radiographic images caused by changes in shape and position of the film relative to the x-ray beam can result in registration artifacts which interfere with interpretation of subtracted images. A warping transformation is described which maps known points of reference in one image into homologous points in another. The corrected image pair can then be superimposed and subtracted with significantly-reduced differences in registration to facilitate the detection of localized changes of diagnostic interest.

Restoration of Digital Multiplane Tomosynthesis by a Constrained Iteration Method

Tomosynthetic reconstructions suffer from the disadvantage that blurred images of object detail lying outside the plane of interest are superimposed over the desired image of structures in the tomosynthetic plane. It is proposed to selectively reduce these undesired superimpositions by a constrained iterative restoration method, suitably generalized to permit simultaneous deconvolution of multiple planes. Sufficient conditions are derived ensuring the convergence of the iterations to the exact solution in the absence of noise and constraints. Although in practice the restoration process must be left incomplete because of inescapable noise and quantization artifacts, the experimental results demonstrate that for reasons of stability the convergence conditions derived for the noise-free, unconstrained case should be satisfied. In order to establish a basis for a formal stopping criterion of the iteration procedure, the buildup of noise in the sequence of iterative restorations arising from white noise in the original radiographs is investigated theoretically and experimentally. This results in the derivation of an approximation to the limiting noise variance in the reconstructions which is verified experimentally.

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.

Future trends in dental radiology

Direct digital dental radiographic systems offer the potential to radically change the way dentists diagnose and treat dental pathoses. They offer instantaneous availability of radiographs, markedly lower patient radiation exposure, and the elimination of developing chemicals and developing equipment. The storage of dental radiographs as digital data permits their transmittal over phone lines facilitating phone consultations and may someday allow expedited authorization of treatment plans by dental insurance companies. With the use of digital subtraction radiology the dental practitioner will be able to diagnose periodontal disease progression and dental caries progression long before current techniques can detect a change. With tuned aperture computed tomography, the owner of a filmless digital system can make tomographic radiographs that allow the visualization of slices through areas of interest without having to buy additional hardware. Computer-aided diagnosis will facilitate the detection of proximal dental caries and osteoporosis, and may someday allow automated tracing of cephalometric radiographs.

An in vivo comparison of diagnostic information obtained from tuned-aperture computed tomography and conventional dental radiographic imaging modalities

OBJECTIVE The purpose of this study was to compare diagnostic information obtained by means of 3-dimensional tuned-aperture computed tomography (TACT) and by means of conventional radiography of patients requiring surgery. STUDY DESIGN TACT produced digital images that yielded a series of tomographic slices viewed interactively. Controls were conventional periapical and/or panoramic radiographs. Each of 4 independent dentists performed 2 tasks, one requiring an estimation of confidence in their clinical assessments of the patient and the other requiring an estimation of the resulting diagnostic potential for altering associated treatment options. Data were analyzed through use of the nonparametric Mann-Whitney U Wilcoxon rank sum W test. RESULTS A statistically significant difference for both tasks was observed (2-sided; P<.001). CONCLUSIONS TACT displays were more diagnostically informative and had more impact on potential treatment options than did conventional radiographs.

Computerized tomosynthesis of dental tissues

A digital method that produces an arbitrary number of tomographic slices from a finite number of dental radiographs is described. The relation between the slice thickness of the tomograms, the number of radiographs required, and the tomographic angle is determined on the basis of existing theory. In order to test the method, a series of radiographs of a radiographic phantom were taken and subsequently digitized to facilitate computer processing. From eight radiographs with an angular disparity of 4.5 degrees, tomograms with a slice thickness of about 3 mm, were produced. These not only indicate the relative position of the dental tissues in space but also show clinical relationships that are not visible in the original radiographs.

Comparison of film, direct digital, and tuned-aperture computed tomography images to identify the location of crestal defects around endosseous titanium implants

This study compared diagnostic performance obtained from two-dimensional and three-dimensional x-ray images. The latter were produced with a new tomosynthetic method based on aperture theory called tuned-aperture computed tomography. Seven human cadaver mandibular segments containing a total of 20 endosseous implants with a small randomly positioned alveolar crestal defect at each implant site were imaged in two dimensions with periapical film and with a charge-coupled digital detector, and digitally with the same detector in three dimensions with tuned-aperture computed tomography and subtracted tuned-aperture computed tomography techniques. Seven trained dentists viewed randomized displays of all modalities. Outcomes of the diagnostic task of identifying the locations of crestal defects were quantified with accuracy, confidence, and time performance measures. Analyses of variance demonstrated that differences between either three-dimensional technique and either two-dimensional modality were significant for all measures (p < 0.001). These findings suggest that clinically applied TACT methods hold promise as an improvement over the status quo.