Determination of the resolution of a digital system for panoramic radiography based on CCD technology.

OBJECTIVES: To determine Line Spread Functions (LSFs) and Modulation Transfer Functions (MTFs) for a digital system for panoramic radiography: the Dimax I (Planmeca Oy, Helsinki, Finland) based on Charge-Coupled Device (CCD) technology. STUDY DESIGN: A test object was specially designed having a gold foil positioned vertically. Images of the gold foil created edge functions that were used to determine LSFs and MTFs. The design of the test object made it possible to move the gold foil forward and backward relative to the central plane of the image layer by means of a micrometer screw. The experiment was carried out for different object depths in 5 different regions: the anterior, the canine, the premolar, the molar, and the TMJ regions. LSFs and MTFs were calculated using specially designed software. RESULTS: The results are presented graphically. LSFs and MTFs for the central plane were essentially the same for all regions. The MTFs for different object depths in the 5 investigated regions exhibited typical characteristics of MTFs for panoramic radiography with the exception for the functions for the molar region. CONCLUSIONS: The present findings indicate that the resolution of the Dimax I CCD system is comparable to that of film-based panoramic radiography.

Analysis of sensitivity and specificity of a new digital subtraction system: an in vitro study.

OBJECTIVE: The purpose of this study was to compare a new digital subtraction system with conventional radiograph images for the detection of periapical and periodontal bone lesions. STUDY DESIGN: Periapical and periodontal bone lesions were simulated with cortical bone chips of varying sizes placed on a human dry mandible. Radiographic film images were acquired from varying projections and were subsequently digitized, registered, and subtracted. Four clinicians evaluated the subtracted images, and sensitivity and specificity were calculated. RESULTS: The mean sensitivity and specificity of the Diagnostic Subtraction Radiography system for detecting bone lesions of all sizes with varying projection geometry were 87.90% and 85.23%, respectively. The corresponding results for conventional radiograph images were 47.54% and 97.38%. The difference in sensitivity was statistically significant, whereas the difference in specificity was not. CONCLUSIONS: These results indicate that, even when radiographs are taken from disparate projection geometries, the Diagnostic Subtraction Radiography system is capable of excellent discrimination between healthy and disease states in this in vitro model.

X-ray spectra estimation using attenuation measurements from 25 kVp to 18 MV.

Attenuation measurements for primary x-ray spectra from 25 kVp to 18 MV were made using aluminum filters for all energies except for orthovoltage where copper filters were used. An iterative perturbation method, which utilized these measurements, was employed to derive the apparent x-ray spectrum. An initial spectrum or pre-spectrum was used to start the process. Each energy value of the pre-spectrum was perturbed positively and negatively, and an attenuation curve was calculated using the perturbed values. The value of x-rays in the given energy bin was chosen to minimize the difference between the measured and calculated transmission curves. The goal was to derive the minimum difference between the measured transmission curve and the calculated transmission curve using the derived x-ray spectrum. The method was found to yield useful information concerning the lower photon energy and the actual operating potential versus the nominal potential. Mammographic, diagnostic, orthovoltage, and megavoltage x-ray spectra up to 18 MV nominal were derived using this method. The method was validated using attenuation curves from published literature. The method was also validated using attenuation curves calculated from published spectra. The attenuation curves were then used to derive the x-ray spectra.

Absolute measures of image quality for the Sens-A-Ray direct digital intraoral radiography system.

To study the noise characteristics of the Sens-A-Ray (Regam Medical Systems AB, Sundsvall, Sweden) system, 20 radiographs were obtained at each of three different exposure levels at 70 and 90 kVp with a homogeneous x-ray field. Exposures were measured with an ionization chamber. Noise power spectra were calculated over three areas within each radiograph, and ensemble averages were subsequently found from 60 data files at each exposure level. Noise equivalent quanta were calculated with the noise power spectra and modulation transfer function data from previous studies. Finally, the detective quantum efficiency was calculated by dividing the noise equivalent quanta by the estimated incident photon fluence at the different exposures. The system has a maximum detective quantum efficiency of approximately 0.030 at 70 kVp and 0.025 at 90 kVp. A broad maximum exists at approximately 2 cycles/mm, indicating that the signal-to-noise ratio is most favorable at this spatial frequency.

A simple method for measuring MTF in direct digital intraoral radiography. Technical note.

Conventional methods for measuring the modulation transfer function require the exposure of a narrow slit or straight edge followed by computationally intense data analysis. An alternative method applicable to digital systems uses the standard deviation of gray levels within different frequency regions of a bar pattern. To verify that this method gave reliable results, measurements performed for two systems for intraoral radiography with the use of this method have been compared with modulation transfer function values calculated from Fourier transformations of line spread functions measured for the two systems. The almost identical results confirm the consistency of both sets of modulation transfer function measurements and demonstrate the utility of the bar pattern method for performing rapid measurements.

Color coding of radiographic changes over time by means of image addition.

Differences between sequential radiographs may be displayed in color if the individual radiographs are transformed into monochromatic images and then added. Information in regions where the radiographs are identical is retained whereas differences are emphasized by the color coding that comes about in a quantitative manner from the gray level values in the sequence of radiographs. By using the three additive primary colors, red, blue, and green, two or three radiographs from a sequence may be added. Every possible state of a bone disease, progression, regression, or any combination, will produce a different and specific color code. Different development cycles are described, and the color coding that appears when color image addition is performed is analyzed. The color addition technique should constitute a useful substitute or alternative to subtraction.

Resolution as defined by line spread and modulation transfer functions for four digital intraoral radiographic systems.

Line spread functions for four commercially available systems for direct digital intraoral radiography were determined from images of a slit of negligible width. From the fitted line spread functions presampling modulation transfer functions were calculated. The four systems were the Sens-A-Ray (Regam Medical System AB, Sundsvall, Sweden), the VIXA/Visualix (Gendex, Chicago Ill.), the RVG (Trophy Radiologic, Paris, France), and the Flash Dent (Villa Sistemi Medicale srd, Buccinasco, Italy). Digital intraoral radiography is in a state of rapid development, and detectors as well as computer hardware and software are continually modified and improved resulting in successively changing system parameters. As this occurs the present work provides a method that may be used to determine comparable data on future systems.

6-bit and 8-bit digital radiography for detecting simulated periodontal lesions.

The purpose of this study was to compare the diagnostic performance of a digital radiography system that uses 6- and 8-bit displays with conventional D-speed film for the detection of simulated periodontal bone lesions. Eleven human hemimandibles were used as specimens. Simulated lesions were created at the buccal cortical plate in the marginal bone area with the use of a round bur 1.4 mm in diameter. Lesions were created in a defined sequence to preclude visual cues as to the depth of the lesions. Lesion size progressed in 0.5 mm increments. At each stage the mandibles were imaged with a Sens-A-Ray system (REGAM Medical Systems AB, Sundsvall, Sweden) and D-speed film. Exposure parameters for each specimen/receptor combination were standardized by either the mean optical density or mean gray value at the approximal crestal bone area. Film images and digital images displayed with 64 and 256 gray levels were presented to six observers for evaluation. Observers were ask to rate their confidence as to the presence or absence of a lesion using a 5-point confidence scale. A total of 96 lesion sites and 96 control sites were presented to the observers. Receiver operating characteristic curves were generated for each system. The area under the curve was used as the index of diagnostic accuracy. The mean receiver operating characteristic areas for 6-bit and 8-bit displays and D-speed film were 0.746 +/- 0.043, 0.717 +/- 0.056 and 0.742 +/- 0.059, respectively. Analysis of variance was used to compare the means. No statistical difference was found between any of the three image displays (p > 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Clinical comparison of two panoramic modalities and posterior bite-wing radiography in the detection of proximal dental caries.

A clinical study was designed to examine whether the Philips OrthOralix SD (Gendex Dental Systems, Monza, Italy) orthogonal panoramic projection could improve diagnostic accuracy over standard projections in the detection of proximal surface caries. Thirty-five sets of radiographs that demonstrated optimal image characteristics were selected. Using a five-point scale, 18 viewers evaluated whether specified lesions were present or absent. Viewer data was then compared with a consensus radiographic assessment of the state of the proximal areas. Receiver operating characteristic curves were generated with the use of a maximum-likelihood method of fit. The area under the receiver operating characteristic curve was used as the index of diagnostic accuracy. The mean receiver operating characteristic areas for orthogonal and standard projection panoramic and bite-wing radiography to detect the presence of proximal dental caries were 0.68 +/- 0.03, 0.69 +/- 0.03, and 0.79 +/- 0.03 respectively. Critical ratio analysis was used to compare the means for all possible pairings of imaging modalities. In overall performance, conventional bite-wing radiographs gave a significantly greater diagnostic yield for proximal caries than the Philips OrthOralix SD orthogonal or standard panoramic modalities (p > 0.05). The orthogonal projection did not improve diagnostic accuracy in the detection of proximal carious lesions compared with the standard projection.

Radiographic interproximal angulations: implications for rotational panoramic radiography.

Axial radiographs were taken on 160 subjects at the screening clinic of the Dental School, University of Texas Health Science Center at San Antonio. Intermeatal and midsagittal lines, together with coordinate axes and polynomial curves that represent the average dental arch form, were overlaid on each radiograph. Coordinate references for interproximal tangents at 3154 contacts along the average dental arch form were digitized and angulations between the arch form and midsagittal plane calculated. Interproximal angulations at the average arch form were found to vary greatly but the average fluctuated around 90 degrees over the length of the arch. Optimal beam angulations were shown to be considerably different from central ray angulations of current panoramic machines, and although most machines demonstrated favorable interproximal beam angulation in the anterior region, discrepancy in the premolar region ranged from 15 degrees to over 40 degrees. Overall the OrthOralix SD (Gendex Dental Systems, S.r.l., Monza, Italy) orthogonal projection and PM 2002 CC (Planmeca Inc. Helsinki, Finland) deviate least from optimal interproximal angulation over most of the dental arch. However, the Planmeca gives a better angulation in the canine/premolar region, whereas the OP5/10 (Palomex Instrumentarium Corp., Helsinki, Finland) is comparable with the OrthOralix SD in the molar region.

Layer thickness in panoramic radiography as defined by different noise-equivalent passbands.

The layer thickness in rotational panoramic radiography is presented with the use of the concept of the noise-equivalent passband. Conventionally, the layer thickness has been calculated only from one-dimensional data in the horizontal or rotational dimension of panoramic radiography. In the present study, results from calculations using both one- and two-dimensional data are presented. When the vertical dimension is included in the calculations, the layer is wider than when calculated from data in the horizontal dimension only. It is pointed out that the wider layer that follows from the introduction of the vertical dimension may be the most reliable measure of layer thickness in panoramic radiography.

Basic technical properties of a system for direct acquisition of digital intraoral radiographs.

The Sens-A-Ray system for direct digital intraoral radiography may be used with any computer compatible with an IBM PC/AT. The system relies on a charge-coupled device designed for direct conversion of x-ray energy to an electronic signal. It is the first such device for direct acquisition of radiographs. Technical properties of charge-coupled device detectors when exposed to radiation energies in the range of x-rays used in dental radiography have been studied. Even in the absence of light or x-radiation there is a spontaneous generation of charge within a charge-coupled device detector that gives rise to a background signal, a dark current. It was found that the dark current is a linear function of exposure time. The dose response of the charge-coupled device detector was determined at nominal kilovoltages that range from 50 to 90 kVp. The dose response was shown to be a linear function of exposure. The functions for all kVp settings were practically identical. The charge-coupled device detector is more sensitive to x-radiation than conventional dental films and, consequently, its exposure range is more narrow. The signal-to-noise ratio was calculated from the digital radiographs used for the dose response test. The ratio is above 10 for exposures higher than about 2 microC/kg. The line spread function was determined from test radiographs of a 10 microns wide slit in a test object of 1.5 mm thick tantalum. After curve fitting, the line spread function could be expressed as the sum of a Gaussian and an exponential function. Presampling modulation transfer functions valid at the detector plane and at an object plane were calculated from fitted data on the line spread function. It is concluded that the Sens-A-Ray system has such technical properties that it may replace conventional film-based systems.

Dimensional reproduction in direct digital rotational panoramic radiography.

A linear x-ray detector array can be used to perform rotational panoramic radiography provided a technique for data acquisition is used that emulates the dimensional reproduction of a conventional rotational panoramic radiographic system. This may be accomplished with the use of a variable integration time throughout the excursion. The required interval depends upon the scanning geometry, the selected image plane, and the size of pixels used in the digital image. In a prototype device developed around an Orthopantomograph Model OP10 with 0.225 mm square pixels, the integration times range from about 8 milliseconds to 28 milliseconds. An experimental test was performed by radiographing a number of steel spheres positioned at different object depths. Within the limits of experimental error, there was satisfactory agreement between the measured and theoretical magnification.

Digital subtraction temporomandibular joint tomography.

Receiver operating characteristics analysis was performed to demonstrate differences in diagnostic performance among conventional tomograms, digitized tomograms, and subtraction tomograms. Digital subtraction tomography was found to be the best imaging modality for detecting artificially created lesions in the two selected temporomandibular joint locations. There was a statistically significant difference in diagnostic performance between conventional tomograms and subtraction tomograms for the detection of temporomandibular joint bony lesions. There was also a statistically significant difference in diagnostic performance between digitized tomograms and subtraction tomograms in the detection of these bony lesions. There was no statistically significant difference in diagnostic performance between conventional tomograms and digitized tomograms for the detection of temporomandibular joint bony lesions. The significance level was set at p = 0.05. Results of the analyses of variance showed that with digital subtraction tomograms, observer reliabilities were higher than with conventional and digitized tomograms.

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.

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.

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.

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.

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.

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.