PubMed Health⌕ Search

Biomedical subjects

W D McDavid

Publications and source records attributed to W D McDavid.

At least 19 recordsLinked to original sources
PubMed1992

Preliminary evaluation of a digital system for rotational panoramic radiography.

A prototype system for direct digital panoramic radiography has been evaluated with respect to density, contrast, magnification, distortion, resolution, and overall image quality. Density and contrast depend on detector calibration and may be modified by the display system or by digital processing of the captured image. Variation of magnification in the horizontal and vertical dimensions gives rise to distortion phenomena that are identical to those encountered in film-based systems. Resolution in the vertical dimension is determined by the pitch of the detector elements. In the horizontal dimension, resolution is limited by the effective width of the detector elements. To evaluate the clinical acceptability of the images, radiologists and general practice residents were asked to assess the perceptibility of important radiographic landmarks in film-based and digital images of both a radiographic phantom and a patient. The digital system performed on a par with film in the representation of normal morphologic structures of the clinical human subject whereas more differences were apparent in the phantom images. The general practice residents consistently rated the digital images higher than their radiologist counterparts did. No consistent trends were found to indicate any inherent deficiencies of the digital system in the depiction of any one area. The results indicate the promise of direct digital acquisition as a method of panoramic imaging.

Evaluation Studies as Topic↗
PubMed1992

Absorbed dose determination for tomographic implant site assessment techniques.

A set of data to compare the absorbed dose delivered by tomographic implant site assessment techniques was generated. Absorbed doses were measured in fourteen anatomic sites from (1) computed tomography scans and (2) a series of tomographic cuts performed on a linear tomography unit. The doses to the thyroid gland, the active bone marrow, the brain, the salivary glands, and the eyes were determined with the use of a tissue-equivalent phantom with lithium fluoride thermoluminescent dosimeters at the appropriate locations.

Absorption↗
PubMed1992

Direct digital radiography for the detection of periodontal bone lesions.

In this study the diagnostic accuracy of D-speed and E-speed film in the detection of simulated periodontal bone lesions was compared with that of an electronic direct digital image receptor. Lesions of increasing depth were created in 11 human hemimandibles at the buccal cortical plate in the interproximal marginal bone area by means of 1.4 mm diameter round bursa. Specimens were imaged at each lesion stage with the use of all three receptors. Nine viewers used a 5-point rating scale to evaluate whether lesions were present or absent in the resulting images. Receiver operating characteristic curves were generated, and maximum-likelihood curve areas were calculated. The area under the curve was used as the index of diagnostic accuracy. The mean receiver operating characteristic areas for D-speed film, E-speed film, and the direct digital system were 0.745 +/- 0.038, 0.740 +/- 0.038, and 0.741 +/- 0.037, respectively. Critical ratio analysis was used to compare the means. No statistical difference was found between any of the three image receptors (p > 0.05) for the detection of simulated periodontal lesions 1.0 to 3.0 mm in depth, which suggested that the digital system performed comparably with conventional film systems.

Alveolar Bone Loss↗
PubMed1992

Direct digital extraoral radiography of the head and neck with a solid-state linear x-ray detector.

A narrow fan beam of x-rays intercepted by a linear array detector was used to acquire transmission data for a radiographic phantom moved across the beam. The digital data were displayed as images representing a variety of extraoral views of the head and neck. Projections investigated include the straight lateral view, two frontal projections, and a half-axial projection. The digital images appear to provide adequate contrast and resolution for common diagnostic tasks. In addition, the use of a scanning linear detector reduces the amount of scatter, which increases contrast relative to images made with an area detector. The system appears to provide a versatile and convenient means for the acquisition of extraoral views relevant to dental practice while it eliminates the logistical difficulties and errors associated with film processing.

Analog-Digital Conversion↗
PubMed1992

CT reconstruction algorithm for a dental panoramic x-ray unit.

A variable Jacobian and weighted backprojection algorithm, used for medical CT, was adapted to perform CT reconstructions on data obtained with a dental panoramic x-ray unit. A detector array, fitted to the unit for the purpose of acquiring digital panoramic radiographs, was used to collect the data. Compensations were made for the incomplete (230 degrees) rotation of the panoramic x-ray unit, the non-fixed centre of rotation, the irregular rotation of the x-ray target and detector, and the resulting variances in magnification. The algorithm was tested on mathematically simulated phantoms and on acquired data. Reconstruction of simulated data proved the success of the algorithm. Real data reconstructions showed some defects as a result of inaccuracies in quantifying the experimental panoramic device.

Algorithms↗
PubMed1992

Digital subtraction radiography in artificial recurrent caries detection.

The diagnostic accuracy of digital subtraction radiography in detection of artificial recurrent caries lesions was assessed in this project. The use of digital subtraction radiography has been shown to markedly increase the accuracy of the detection of destruction in the periodontal bone, but the method has not been evaluated in secondary caries detection. Defects of three different sizes, simulating recurrent caries, were sequentially prepared in the interproximal cavity preparation margins of 28 teeth. Two composite restorative materials with different radiographic densities were used as posterior restorations, and a radiograph was obtained of each defect size and restorative material. The radiographs were digitized and subtracted from the reference images, and the conventional radiographs and the subtraction images were evaluated by seven observers. The data were analysed with ROC statistics. Subtraction radiography was found to be superior to conventional radiography in recurrent caries detection, mainly by reducing the false-positive diagnoses. The radiopacity of the restorative material had a significant effect on accuracy with conventional but not with subtraction radiography.

Analysis of Variance↗
PubMed1992

A comparison of conventional intra-oral radiography and computer imaging techniques for the detection of proximal surface dental caries.

The detection of proximal surface caries by the visual interpretation of bitewing radiographs is known to be relatively inaccurate. The present study was designed to examine whether computer image processing could improve the diagnostic accuracy. A computer-aided, software-driven, TV-based system was used to digitize conventional radiographs and digitally process the images using histogram equalization and grey-scale inversion to enhance the images. The computer-enhanced images were compared with conventional intra-oral radiographs for the detection of proximal surface caries using receiver operating characteristic analysis. The results indicate that the digital image processing techniques used did not improve the diagnostic accuracy of dental radiographs. No significant difference in diagnostic accuracy could be detected between the non-enhanced digital images and conventional film-based images for the detection of proximal surface caries.

Dental Caries↗
PubMed1992

Design and implementation of an image management and communications system (IMACS) for dentomaxillofacial radiology.

An Image Management and Communications System for digital dentomaxillofacial radiology is under development at the Department of Dental Diagnostic Science at the University of Texas Health Science Center at San Antonio, Texas, USA. In its final stage, the system will provide a method of integrating different direct digital image acquisition modalities such as intraoral, panoramic and extraoral radiography. A review of the design criteria necessary for establishing such a system for clinical dentistry is discussed and the first phase of its implementation described.

Computer Communication Networks↗
PubMed1991

Electronic system for digital acquisition of rotational panoramic radiographs.

A prototype system for digital panoramic imaging of the maxillofacial complex has been developed. In this system x-ray film is replaced by an electronic sensor that delivers the image information to a computer for storage in digital format. The images, which are similar to conventional panoramic radiographs, are displayed on a high-resolution video monitor and may be stored on optical disk for future use. Hard-copy output is also available. The present prototype system has been installed on an Orthopantomograph model OP10 panoramic x-ray machine is programmed for operation with this machine, but in principle the system can be installed on any such device. The system may be incorporated into the design of future panoramic x-ray systems or may be used to retrofit panoramic x-ray systems now using photographic film to record the radiographic image. Greater sensitivity of electronic sensors should make possible a reduction of x-ray dose to the patient, compared with film-based systems.

Analog-Digital Conversion↗
PubMed1990

Nonrotational scanning in panoramic radiography.

A mathematical analysis of orthoradiography, nonrotational scanning in panoramic radiography, and rotational panoramic radiography shows that they may all be described by the same set of equations. The other two techniques may be regarded as special cases of rotational panoramic radiography.

Humans↗
PubMed1990

Reversed layer position in rotational panoramic radiography.

In rotational panoramic radiography, the image layer is generally positioned between the rotation center of the beam and the film. It is also possible to create a layer that is positioned between the rotation center and the x-ray source. The reversed layer position is useful clinically and gives rise to image properties that are somewhat different from the conventional geometry.

Humans↗
PubMed1990

Applicability of simultaneous rotational panoramic radiography.

The position of the image layer in simultaneous multilayer rotational panoramic radiography using films placed parallel in one cassette was calculated for curved and flat cassettes. Parallel films do not result in parallel layers. The use of a flat cassette results in minor shifts in layer position compared with a curved cassette. The position of the film required to produce parallel curved layers was calculated. Using curved cassettes, extraordinary modifications of the film position would be needed for the anterior region. The use of flat cassettes appears to be practically impossible. Routine use of this technique is therefore unlikely.

Computer Simulation↗
PubMed1989

Spatial resolution requirements for digitizing dental radiographs.

A study was performed to determine the appropriate spatial resolution for digitizing and transmitting dental radiographs with the KODAK EKTASCAN, a computer-based digital enhancement and transmission system. Periapical, bitewing, and panoramic radiographs were digitized in three formats representing varying spatial resolution parameters. Eight viewers used a 5-point rating scale to evaluate the detectability of periapical pathosis on the periapical images, of proximal surface caries on the bitewing images, and of various bony abnormalities on the panoramic images. Receiver operating characteristic curves were generated, and the results of the periapical, bitewing and panoramic experiments were presented as trapezoidal and maximum likelihood receiver operating characteristic curve areas. The results of this study indicate that digital images of dental radiographs provide adequate diagnostic accuracy for evaluating the presence of periapical pathosis, proximal surface caries, and specified bony abnormalities. The digitization parameters established for the KODAK EKTASCAN provide a guide for digitizing dental radiographs on other commercially available digital image-processing systems.

Computers↗
PubMed1989

Comparison of image layer location among panoramic machines of the same manufacturer.

The central plane of the image layer was located in several panoramic machines of the same manufacturer by making radiographs of a spherical test object placed in different positions. A round image indicated that the test object was at the central plane. The inconsistency of the location of the central plane in machines tested indicates the need for establishing a quality assurance test to verify image-layer location before patient exposure.

Calibration↗
PubMed1989

Image layers in the Zonarc.

The Zonarc (Palomex Oy, Helsinki, Finland) uses the principles of rotational panoramic radiography to image various structures of the head and neck. System parameters obtained from the manufacturer have been used to derive data on the imaging characteristics of the Zonarc when using each of the following programmes: MT, DENT, TMJ, EAR, CV and LAT. The beam path along with the shape, position and relative thickness of the layer has been calculated for each programme, together with the resolution and magnification associated with the different layers.

Humans↗
PubMed1989

Standard forms of dentition and mandible for applications in rotational panoramic radiography.

Mathematical expressions describing the average form and size of the dentition and the mandible are presented. These expressions should be of value in applications of panoramic radiography when reference to an average standard jaw form is of interest. Data were collected from axial radiographs of 35 males and 35 females of three ethnic groups: Mexican-Americans, black Americans and American and Scandinavian Caucasians. Curves were traced on the axial radiographs representing the dentition and the mandible and points along these curves digitized. Mathematical expressions were established using advanced algorithms for orthogonal polynomial curve fitting, i.e. perpendicular distances to the curved dentition and mandible were minimized rather than distances parallel to the y-axis in an arbitrarily chosen coordinate system. The standard deviations around the polynomials defining the average curves are demonstrated and expressions for calculating the continuously varying standard deviations are given.

Dentition↗
PubMed1989

Layer thickness in rotational panoramic radiography: some specific aspects.

It is customary to define the thickness of the image layer in rotational panoramic radiography along the central ray of the X-ray beam. This results in calculated values which overestimate its thickness, especially in regions where the central ray deviates markedly from orthogonality. A supplementary definition is proposed which considers layer thickness in a direction perpendicular to the central plane of the image layer. This provides values which may be more easily related to the thickness of the object being radiographed. A mathematical approximation for calculations involving the supplementary definition is described and illustrated with numerical examples.

Radiography, Panoramic↗