Deriving Hounsfield units using grey levels in cone beam CT: a clinical application

OBJECTIVE

To present a clinical study demonstrating a method to derive Hounsfield units from grey levels in cone beam CT (CBCT).

METHODS

An acrylic intraoral reference object with aluminium, outer bone equivalent material (cortical bone), inner bone equivalent material (trabecular bone), polymethlymethacrylate and water equivalent material was used. Patients were asked if they would be willing to have an acrylic bite plate with the reference object placed in their mouth during a routine CBCT scan. There were 31 scans taken on the Asahi Alphard 3030 (Belmont Takara, Kyoto, Japan) and 30 scans taken on the Planmeca ProMax 3D (Planmeca, Helsinki, Finland) CBCT. Linear regression between the grey levels of the reference materials and their linear attenuation coefficients was performed for various photon energies. The energy with the highest regression coefficient was chosen as the effective energy. The attenuation coefficients for the five materials at the effective energy were scaled as Hounsfield units using the standard Hounsfield units equation and compared to those derived from the measured grey levels of the materials using the regression equation.

RESULTS

In general, there was a satisfactory linear relation between the grey levels and the attenuation coefficients. This made it possible to calculate Hounsfield units from the measured grey levels. Uncertainty in determining effective energies resulted in unrealistic effective energies and significant variability of calculated CT numbers. Linear regression from grey levels directly to Hounsfield units at specified energies resulted in greater consistency.

CONCLUSIONS

The clinical application of a method for deriving Hounsfield units from grey levels in CBCT was demonstrated.

Characterization and correction of cupping effect artefacts in cone beam CT

OBJECTIVE

The purpose of this study was to demonstrate and correct the cupping effect artefact that occurs owing to the presence of beam hardening and scatter radiation during image acquisition in cone beam CT (CBCT).

METHODS

A uniform aluminium cylinder (6061) was used to demonstrate the cupping effect artefact on the Planmeca Promax 3D CBCT unit (Planmeca OY, Helsinki, Finland). The cupping effect was studied using a line profile plot of the grey level values using ImageJ software (National Institutes of Health, Bethesda, MD). A hardware-based correction method using copper pre-filtration was used to address this artefact caused by beam hardening and a software-based subtraction algorithm was used to address scatter contamination.

RESULTS

The hardware-based correction used to address the effects of beam hardening suppressed the cupping effect artefact but did not eliminate it. The software-based correction used to address the effects of scatter resulted in elimination of the cupping effect artefact.

CONCLUSION

Compensating for the presence of beam hardening and scatter radiation improves grey level uniformity in CBCT.

Pre-clinical evaluation of a new dental panoramic radiographic system based on tomosynthesis method

OBJECTIVES

The purpose of the study was to compare the image generated by a classic panoramic machine equipped with a cadmium telluride sensor capable of digital tomosynthesis and special software with images produced by other popular panoramic X-ray machines using a charge-coupled device and native software for image capture.

METHODS

Panoramic images were made using a phantom of a human skull on Planmeca ProMax, Planmeca EC Proline, Kodak 8000 and PC-1000. With the last machine we used the PanoACT® software to adjust the entire arch and to adjust the image in selected regions of interest (ROIs). Ten viewers evaluated the images and provided the viewer data. ANOVA for repeated measures was used to compare the means by pairwise comparisons of means.

RESULTS

The image of the entire arch adjusted by the PanoACT® software was statistically superior to the images produced by other machines. The images generated and individually adjusted by PanoACT® were statistically superior to all other images.

CONCLUSIONS

The image generated by the cadmium telluride sensor has great potential and can be processed to create superior images to those taken with other machines. Furthermore, the ROI individual images enhanced by the PanoACT® were superior to the entire arch adjusted by the same software.

Deriving Hounsfield units using grey levels in cone beam computed tomography

OBJECTIVES

an in vitro study was performed to investigate the relationship between grey levels in dental cone beam CT (CBCT) and Hounsfield units (HU) in CBCT scanners.

METHODS

a phantom containing 8 different materials of known composition and density was imaged with 11 different dental CBCT scanners and 2 medical CT scanners. The phantom was scanned under three conditions: phantom alone and phantom in a small and large water container. The reconstructed data were exported as Digital Imaging and Communications in Medicine (DICOM) and analysed with On Demand 3D(R) by Cybermed, Seoul, Korea. The relationship between grey levels and linear attenuation coefficients was investigated.

RESULTS

it was demonstrated that a linear relationship between the grey levels and the attenuation coefficients of each of the materials exists at some "effective" energy. From the linear regression equation of the reference materials, attenuation coefficients were obtained for each of the materials and CT numbers in HU were derived using the standard equation.

CONCLUSIONS

HU can be derived from the grey levels in dental CBCT scanners using linear attenuation coefficients as an intermediate step.

Development of a new dental panoramic radiographic system based on a tomosynthesis method

The objective of this study was to develop a new practical method to reconstruct a high-quality panoramic image in which radiographers would be free from the onerous task of correctly locating the patient's jaws within the image layer of the panoramic unit. In addition, dentists would be able to freely select any panoramic plane to be reconstructed after the acquisition of the raw scan data. A high-speed data acquisition device was used with a CdTe (cadmium telluride) semiconductor detector and a sophisticated digital signal-processing technique based on tomosynthesis was developed. The system processes many vertical strip images acquired with the detector and generates a high-resolution and high-contrast image. To apply the tomosynthesis technique to the acquired strip images correctly, the actual movement of the panoramic unit was measured, including the X-ray tube and detector, in a scan using a calibration phantom and the authors generated a shift amount table needed for the shift-and-add tomosynthesis operation. The results of the experiments with a PanoACT-1000 panoramic unit, which was a PC-1000 panoramic unit fitted with a high frame rate semiconductor detector SCAN-300FPC, demonstrated the capability of a tomosynthesis technique which, when applied to the strip images of a dry skull phantom, could change the location and inclination of an imaging plane. This system allowed the extraction of an optimum-quality panoramic image regardless of irregularities in patient positioning. Moreover, the authors could freely reconstruct a fine image of an arbitrary plane with different parameters from those used in the original data acquisition to study fine anatomical details in specific locations.

Transtomography: a new tomographic scanning technique

OBJECTIVES

To introduce and test a new tomographic technique, namely transtomography, making it possible to expose tomographic images employing the narrow beam of an advanced panoramic machine.

METHODS

The working principle of the new technique is described. It combines a translational movement with a pendular movement of the beam that creates motion unsharpness analogous to that of conventional tomography. In order to verify whether the new technique works and results in tomographic images, test radiographs were exposed on phantoms.

RESULTS

The principle of transtomography works and give images with properties essentially equivalent to those of conventional tomography. With the transtomographic technique using a narrow beam, tomographic images may be exposed with an advanced panoramic machine.

CONCLUSION

Transtomography may be employed to expose tomographic images basically on the same indications as for conventional tomography.

Comparison of the psychophysical properties of various intraoral film and digital systems by means of the perceptibility curve test

OBJECTIVES

To compare psychophysical properties of two intraoral films and three digital systems using the perceptibility curve (PC) test.

MATERIALS AND METHODS

A test object was used to determine the exposures and exposure differences between the total thickness of the test object and details consisting of holes of increasing depth. The PCs for the two intraoral films, UltraSpeed and EktaSpeed Plus, were constructed employing exposure and exposure differences from dose response functions. Integrals of the PCs were calculated to obtain the psychophysical properties of the two films. Psychophysical properties of the two films were compared with those of the three digital systems published previously (CDR, Dixel and Digora).

RESULTS

The PC for the EktaSpeed Plus showed a slightly higher peak than that for the UltraSpeed. Available exposure ranges were comparable. The PC for the EktaSpeed Plus was shifted to the left of the exposure axis indicating its higher sensitivity as compared with UltraSpeed. All three digital systems had narrower but higher peaks compared with the films. The integrals for the digital systems were considerably larger than those for the two film types.

CONCLUSIONS

All the three digital systems have superior psychophysical properties compared with the two tested films.

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.

Evaluation of endodontic files in digital radiographs before and after employing three image processing algorithms

OBJECTIVES

Two digital image processing algorithms, one aimed at correction for exponential attenuation and one at correction for visual response, have been developed. The aims of the present study were to test whether digital radiographs processed with these algorithms improve determination of the length of endodontic files and whether such processed radiographs are comparable with the radiographs processed with a default image processing method employed by one commercially available digital intraoral system.

METHODS

A dried human skull embedded in an acrylic compound was used for exposing radiographs of the upper and lower premolars and molars with endodontic files (Kerr files size 10 and size 15) positioned to the full length of the roots or 1.5 mm short of it. Radiographs were then processed in three sets. In one set, the radiographs were processed to compensate for exponential attenuation and the response of the human visual system. In the second, the radiographs were processed with the same compensation but with an additional shift in grey levels so that the output luminance in dentin at root tips corresponds to the mean of the luminance range of a computer monitor. In the third, the radiographs were processed with the default processing method in the Sidexis program. Ten viewers evaluated all radiographs. Receiver operating characteristic (ROC) curves were obtained and areas under the curves were calculated.

RESULTS

For file size 10, ROC curves for processed radiographs were higher than that for originals, while for file size 15, ROC curves for processed and original radiographs were close to each other. Significant differences were found between processed and original radiographs regarding areas under ROC curves for file size 10 but not between the differently processed radiographs. For file size 15, no significant differences were found.

CONCLUSION

Radiographs processed to correct for attenuation and visual response may improve determination of the length of thin endodontic files. Such processed radiographs are comparable with the radiographs processed with the default processing method in the Sidexis program.

Perceptibility curve test for digital radiographs before and after correction for attenuation and correction for attenuation and visual response

OBJECTIVE

Two digital image processing methods, correction for X-ray attenuation and correction for attenuation and visual response, have been developed. The aim of the present study was to compare digital radiographs before and after correction for attenuation and correction for attenuation and visual response by means of a perceptibility curve test.

MATERIAL AND METHODS

Radiographs were exposed of an aluminium test object containing holes ranging from 0.03 mm to 0.30 mm with increments of 0.03 mm. Fourteen radiographs were exposed with the Dixi system (Planmeca Oy, Helsinki, Finland) and twelve radiographs were exposed with the F1 iOX system (Fimet Oy, Monninkylä, Finland) from low to high exposures covering the full exposure ranges of the systems. Radiographs obtained from the Dixi and F1 iOX systems were 12 bit and 8 bit images, respectively. Original radiographs were then processed for correction for attenuation and correction for attenuation and visual response. Thus, two series of radiographs were created. Ten viewers evaluated all the radiographs in the same random order under the same viewing conditions. The object detail having the lowest perceptible contrast was recorded for each observer. Perceptibility curves were plotted according to the mean of observer data.

RESULTS

The perceptibility curves for processed radiographs obtained with the F1 iOX system are higher than those for originals in the exposure range up to the peak, where the curves are basically the same. For radiographs exposed with the Dixi system, perceptibility curves for processed radiographs are higher than those for originals for all exposures. Perceptibility curves show that for 8 bit radiographs obtained from the F1 iOX system, the contrast threshold was increased in processed radiographs up to the peak, while for 12 bit radiographs obtained with the Dixi system, the contrast threshold was increased in processed radiographs for all exposures. When comparisons were made between radiographs corrected for attenuation and corrected for attenuation and visual response, basically no differences were found.

CONCLUSION

Radiographs processed for correction for attenuation and correction for attenuation and visual response may improve perception, especially for 12 bit originals.

Perceptibility curves for the Digora system

OBJECTIVES

To construct perceptibility curves (PCs) for given calibration settings in order to define psychophysical properties of the Digora storage phosphor system and to evaluate the effects of automatic exposure correction (AEC) on the PCs.

METHODS

The Digora system was calibrated at two exposures, 80 microC kg(-1) (high calibration) and 40 microC kg(-1) (low calibration). Since the grey levels displayed in the radiographs are adjusted by AEC, dose-response functions at high calibration were obtained using AEC on and off modes. The dose-response function at low calibration was obtained with AEC off. The PC at each experimental setting was calculated using known physical parameters of the system and the performance of the average observer used in a previous study. In addition, PCs were also constructed using transmitted radiation flux behind the test object calculated from the attenuation coefficient in order to study observer and system performance. PCs obtained under these conditions were compared.

RESULTS

The PC using calculated transmitted radiation flux behind the test object showed a wide plateau at the peak owing to AEC, while the PC obtained by a modified approach showed a higher but narrower peak. There were no differences between the two PCs using AEC on and off modes when the PCs were constructed using a modified approach. There were no differences between the two PCs obtained at high and low calibration settings or between the three PCs obtained with AEC on except for the position along the exposure axis.

CONCLUSIONS

Psychophysical properties of the Digora system may be determined if we employ registered exposures from a dose-response function with AEC off under a given calibration setting. Under these circumstances the shape and height of the PCs will be unchanged irrespective of the AEC mode.

Detection of approximal caries in digital radiographs before and after correction for attenuation and visual response. An in vitro study

OBJECTIVE

To evaluate if digital compensation for exponential attenuation and the characteristics of the human visual system improves the diagnosis of approximal caries from digital radiographs.

MATERIAL AND METHODS

Forty premolar teeth were mounted in plaster blocks. Radiographs of the teeth were exposed with the Dixi digital intraoral system employing a Prostyle Intra dental X-ray unit (Planmeca Oy, Helsinki, Finland). Thirteen radiographs were then processed to compensate for the exponential attenuation and for the characteristic of the human visual system using equations presented in the paper. Ten observers were asked to diagnose approximal caries in all radiographs. ROC analyses were performed. The teeth were subsequently sectioned for histological validation of the lesions. The areas under ROC curves of original and processed radiographs were compared and analysed using Wilcoxon's signed-ranks test.

RESULTS

There were significant diagnostic differences between the two types of radiographs (all lesions P=0.005. enamel P=0.028, and dentine P=0.050).

CONCLUSION

Digital radiographs processed to compensate for exponential attenuation and the characteristics of the human visual system significantly improves the diagnosis of approximal caries in vitro.

Correction for attenuation and visual response in digital radiography

OBJECTIVES

To derive a method to correct for the exponential attenuation adding visual linearization for digital radiography and to perform an experimental test to study effects on observer performance.

METHOD

A theoretical analysis was performed and expressions were derived to correct radiographic data for the attenuation. An experiment was performed exposing an aluminum step wedge with holes of increasing depths behind each step plus soft tissue simulation. Seven original images were created with a digital intraoral X-ray sensor and a further 14 transformed images produced. Ten observers examined the images in a randomized order reporting the number of holes seen per wedge step. The results were analysed by One Way ANOVA.

RESULTS

It was possible to correct for attenuation and the response of the human visual system to light intensities from a computer monitor. Perception was significantly improved in all recalculated radiographs that took both attenuation and the response of the eye into account (P-values ranged from <0.0001 to <0.0286).

CONCLUSIONS

The transforms may be useful for improved perception when viewing digital radiographs.

Visual linearization of the display of digital radiographs

OBJECTIVES

To derive and test a method to linearize the visual response of the display of digital radiographs so that equal steps in gray levels will be perceived as equal steps in brightness.

METHOD

A mathematical analysis was performed and expressions for visual linearization were derived. Twenty-four test images were computer generated to confirm that visual linearization may be achieved. Each image had three groups of square areas of different size placed in three rows. The left and right squares in each row were given different gray levels to simulate various contrast levels. The middle squares were initially given the same gray level value as one of the outer squares. The images were examined by ten observers who could change the gray levels of the middle squares so that the step in brightness between the middle square and the outer squares become subjectively equal. The test was performed three times employing two different monitors.

RESULTS

The experimental test confirmed that visual linearization could be achieved. Linear regression analyses gave determination coefficients of 0.9926 amd 0.9942 for monitors with gamma-values of 1.93 and 2.50. respectively. The mean data from the ten observers perfectly fitted those theoretically calculated.

CONCLUSION

Visual linearization of gray levels can be achieved but further clinical research is needed to determine if this improves diagnosis.

Comparison of five methods for the derivation of spectra for a constant potential dental X-ray unit

OBJECTIVES

To compare the diagnostic X-ray spectra derived by different methods for a constant potential dental X-ray unit.

MATERIALS AND METHODS

Five methods of deriving X-ray spectra for a constant potential dental X-ray unit were compared: measurement by spectrometer using cadmium-zinc-telluride (CZT) detector, calculation by Monte Carlo simulation, calculation by two different, semi-empirical methods and estimation from transmission data. The dental X-ray set was a Heliodent MD unit (Sirona, Charlotte, NC, USA) operable at 60 or 70 kV. A semiconductor detector was used in the spectrometer measurements and an ionization chamber dosimeter in the transmission measurements. From the five methods, photon-fluence spectra were derived. Based on the photon-fluence spectra, average energies and transmission curves in aluminum were calculated.

RESULTS

For all five methods, the average energies were within 2.4% of one another. Comparison of the transmission curves showed an average difference in the range of 1 to 6%.

CONCLUSION

All of the five methods of deriving spectra are in extremely good agreement with each other.

Conventional and predicted perceptibility curves for contrast-enhanced direct digital intraoral radiographs

OBJECTIVES

To construct Perceptibility Curves (PCs) for contrast-enhanced digital intraoral radiographs.

METHODS

Radiographs of a test object having holes of increasing depths were exposed using three digital systems, the CDR (Schick Technologies, Long Island, NY, USA), the Dixel (J Morita MFG, Kyoto, Japan) and the Sens-A-Ray (Regam Medical Systems, Sundsvall, Sweden). The radiographs were contrast-enhanced and PCs constructed in the conventional way using 10 observers. Predicted PCs were calculated and compared with observer data.

RESULTS

The PCs showed that contrast enhancement is effective for the perception of small contrast details, especially in the low exposure range. Predicted PCs demonstrated excellent agreement with observer data.

CONCLUSIONS

Contrast enhancement should be advantageous in digital radiography. The effects of contrast enhancement on PCs may be predicted without previous knowledge of observer performance.

Modification and benchmarking of MCNP for low-energy tungsten spectra

The MCNP Monte Carlo radiation transport code was modified for diagnostic medical physics applications. In particular, the modified code was thoroughly benchmarked for the production of polychromatic tungsten x-ray spectra in the 30-150 kV range. Validating the modified code for coupled electron-photon transport with benchmark spectra was supplemented with independent electron-only and photon-only transport benchmarks. Major revisions to the code included the proper treatment of characteristic K x-ray production and scoring, new impact ionization cross sections, and new bremsstrahlung cross sections. Minor revisions included updated photon cross sections, electron-electron bremsstrahlung production, and K x-ray yield. The modified MCNP code is benchmarked to electron backscatter factors, x-ray spectra production, and primary and scatter photon transport.

Measurement and validation of benchmark-quality thick-target tungsten X-ray spectra below 150 kVp

Pulse-height distributions of two constant potential X-ray tubes with fixed anode tungsten targets were measured and unfolded. The measurements employed quantitative alignment of the beam, the use of two different semiconductor detectors (high-purity germanium and cadmium-zinc-telluride), two different ion chamber systems with beam-specific calibration factors, and various filter and tube potential combinations. Monte Carlo response matrices were generated for each detector for unfolding the pulse-height distributions into spectra incident on the detectors. These response matrices were validated for the low error bars assigned to the data. A significant aspect of the validation of spectra, and a detailed characterization of the X-ray tubes, involved measuring filtered and unfiltered beams at multiple tube potentials (30-150 kVp). Full corrections to ion chamber readings were employed to convert normalized fluence spectra into absolute fluence spectra. The characterization of fixed anode pitting and its dominance over exit window plating and/or detector dead layer was determined. An Appendix of tabulated benchmark spectra with assigned error ranges was developed for future reference.

Using measured 30-150 kVp polychromatic tungsten x-ray spectra to determine ion chamber calibration factors, Nx (Gy C(-1))

Two methods for determining ion chamber calibration factors (Nx) are presented for polychromatic tungsten x-ray beams whose spectra differ from beams with known Nx. Both methods take advantage of known x-ray fluence and kerma spectral distributions. In the first method, the x-ray tube potential is unchanged and spectra of differing filtration are measured. A primary standard ion chamber with known Nx for one beam is used to calculate the x-ray fluence spectrum of a second beam. Accurate air energy absorption coefficients are applied to the x-ray fluence spectra of the second beam to calculate actual air kerma and Nx. In the second method, two beams of differing tube potential and filtration with known Nx are used to bracket a beam of unknown Nx. A heuristically derived Nx interpolation scheme based on spectral characteristics of all three beams is described. Both methods are validated. Both methods improve accuracy over the current half value layer Nx estimating technique.

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.

Physical properties of a photostimulable phosphor system for intra-oral radiography

OBJECTIVES

To determine physical properties of the Digora digital intra-oral radiographic system (Soredex Orion Corporation, Helsinki, Finland) for different calibration settings and beam energies.

METHODS

The line spread function (LSF) and the modulation transfer function (MTF) were determined from radiographs of a slit. Noise power spectra (NPS) were determined from radiographs exposed to homogeneous radiation fields at 10, 50 and 100% of the calibration exposure for three tube potentials. All calculations were performed using relative values of exposure comprised of gray level, the signal at the photomultiplier tube and the amplified signal in order to confirm agreement between these different approaches. Noise equivalent quanta (NEQ) were calculated from the one-dimensional NPSs and the MTF. Detective quantum efficiencies (DQE) were determined from the NEQs and representative values of the photon fluence. Signal-to-noise ratios (SNR) were calculated for different signal contrasts applying the NEQs.

RESULTS

The MTF of the system exhibited typical characteristics and falls to a value close to zero at the Nyquist frequency of about 7 cycles/mm. Noise as expressed by the NPS was found to be relatively low, i.e. about 10(-5) to 10(-6) mm2 depending on exposure and frequency. There was no significant difference between data obtained at different beam energies. The NEQ and hence the DQE were relatively high. DQE decreased with increased exposure. For exposures in the clinical range of the DQE reached a peak value of about 25%. SNRs are favorable.

CONCLUSION

The physical properties of the Digora intra-oral system indicate that it is suitable for digital intra-oral radiography.

Physical evaluation of a system for direct digital intra-oral radiography based on a charge-coupled device

OBJECTIVES

To determine technical properties of a direct digital intra-oral radiographic system, the Dixel(R) (J Morita Corporation, Kyoto, Japan).

METHODS

A dose response function and the dark current were calculated from two series of exposures to a homogeneous radiation field. The line spread function (LSF) and the modulation transfer function (MTF) were determined from radiographs of an edge. The noise power spectrum (NPS) was determined at three exposures from radiographs exposed to homogeneous radiation fields. Noise equivalent quanta (NEQ) were calculated from the one-dimensional NPS and the MTF. The detective quantum efficiency (DQE) was determined from the NEQ and a representative value of the photon fluence. Signal-to-noise ratios (SNR) were calculated from the NEQs and different signal contrasts.

RESULTS

The dose response function demonstrated a slight curvature. There was no effect of the dark current. NPS ranged from 10-5 - 10-7 mm2 depending on exposure and frequency. At a peak of about 2 cycles/mm the DQE is on an average about 30 per cent. SNRs are favorable.

CONCLUSION

The technical properties found in this study indicate that the Dixel(R) system is suitable for intra-oral dental radiography.

Dose response of a storage phosphor system for intraoral radiography

OBJECTIVES

To determine the dose response function for the Digora(R) (Soredex Orion Corporation, Helsinki, Finland) storage phosphor system for intra-oral radiography.

METHODS

Specially designed software provided by the manufacturer was used to register data from the pre-scanning that takes place prior to readout. These data contain information about the signal from the photomultiplier and the amplification that forms the basis for the AD conversion to 8 bit image data. Dose response functions were determined for different calibration settings and also combined into one single function.

RESULTS

The dose response function is linear irrespective of calibration. The gray levels of the final radiographs are directly proportional to the amplified signal. The Digora(R) does not fully compensate for exposure differences. Increased exposure results in slightly darker radiographs.

CONCLUSIONS

Since the gray levels of the final radiographs are directly proportional to the amplified signal, the gray levels may be used as a relative measure of the exposure in the Digora(R) system provided that they are subtracted from the maximum value of 255.

Prediction of perceptibility curves of direct digital intraoral radiographic systems

OBJECTIVES

To derive and test a method to predict Perceptibility Curves (PCs) for direct digital intraoral radiographic systems.

METHODS

A test object was used to determine the performance of an average observer with respect to the threshold contrast and internal noise of the human visual system. These data were combined with system parameters to predict PCs mathematically.

RESULTS

Data on the performance of an average observer could be defined to obtain values of the effective threshold contrast as a function of background gray level. This function combined with the gamma-value of the system predicted PCs that agreed well with PCs obtained in the conventional way.

CONCLUSION

It is possible to predict PCs from a limited number of system parameters together with predetermined data on an average observer.

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.

A simplified method to obtain perceptibility curves for direct dental digital radiography

OBJECTIVES

To derive and test a simplified method to construct Perceptibility Curves (PCs) for dental digital detectors.

METHODS

Mathematical expressions were derived to make it possible to construct PCs from viewer data obtained at two exposures, one low and one high. PCs were constructed applying these expressions and compared with data previously obtained employing the conventional method.

RESULTS

PCs constructed according to the simplified method agree extremely well with conventionally obtained data.

CONCLUSIONS

Reliable PCs may be constructed according to the simplified method.

The perceptibility curve test applied to direct digital dental radiography

OBJECTIVE

To compare the effect of an additional scintillator layer on the psychophysical properties of a CCD detector for digital dental radiography.

METHODS

Radiographs of a test object containing ten holes of increasing depth were obtained throughout the exposure range of two CCD detectors at 70 and 90 kVp. One was the original detector for the Sens-A-Ray system (Regam Medical Systems, Sundsvall, Sweden) and the other the same detector covered by a scintillator layer. Ten viewers evaluated the radiographs for the number of perceptible holes. From these data and the dose response functions for the two detectors, the minimum perceptible exposure difference was found. The reciprocal values of this parameter were plotted against the logarithm of exposure to create Perceptibility Curves (PCs).

RESULTS

The four PCs had essentially the same shape and height. There was a shift to lower exposures in the PCs for the detector covered by a scintillator.

CONCLUSIONS

The two detectors have essentially the same psychophysical properties. Since the detector covered by a scintillator layer is more sensitive, it should be preferred for clinical practice since the dose to the patient is reduced.

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.

Radiographic determination of canal length direct digital radiography versus conventional radiography

This research compared the length determination images produced by Trophy and Regam direct digital radiographic systems with conventional E-speed radiographs. Size #15 K files were placed in the canals of 19 teeth of cadaver specimens and radiographed in a geometrically standardized bench device. Three evaluators estimated the length adjustment necessary to position the file at the apex. The mean estimated adjustment lengths were compared with the true, anatomically determined adjustment lengths. Statistical analysis with analysis of variance and the Student-Newman-Keuls test indicated no significant difference between direct digital thermal print images and conventional radiographs (p > 0.10). Length adjustment estimates with conventional radiographs were significantly more accurate than Regam high-resolution computer monitor images (p < 0.01). Additionally, it was determined that length adjustment estimates were significantly more accurate when the file was placed short of the apex (p < 0.001). The clinical relevance of these differences is discussed.

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.

Digital panoramic and extraoral imaging

Intraoral, panoramic, and extraoral radiographs are the primary means of diagnosing hard-tissue disorders of the dentomaxillofacial region. These imaging methods require the use of x-ray film and subsequent chemical processing to produce diagnostic images. A goal of recent research has been the replacement of this film-based technology with computer-based devices that use electronic or storage phosphor receptors to record the x-ray image in a digital format. This article discusses some of these emerging technologies and their potential effect on the future of panoramic and extraoral radiology.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.