Optimum exposure ranges for computed dental radiography

The Effects on Absorbed Dose Distribution in Intraoral X-ray Imaging When Using Tube Voltages of 60 and 70 kV for Bitewing Imaging

Objectives

Efforts are made in radiographic examinations to obtain the best image quality with the lowest possible absorbed dose to the patient. In dental radiography, the absorbed dose to patients is very low, but exposures are relatively frequent. It has been suggested that frequent low-dose exposures can pose a risk for development of future cancer. It has previously been reported that there was no significant difference in the diagnostic accuracy of approximal carious lesions in radiographs obtained using tube voltages of 60 and 70 kV. The aim of this study was, therefore, to evaluate the patient dose resulting from exposures at these tube voltages to obtain intraoral bitewing radiographs.

Material and Methods

The absorbed dose distributions resulting from two bitewing exposures were measured at tube voltages of 60 and 70 kV using Gafchromic^® film and an anatomical head phantom. The dose was measured in the occlusal plane, and ± 50 mm cranially and caudally to evaluate the amount of scattered radiation. The same entrance dose to the phantom was used. The absorbed dose was expressed as the ratio of the maximal doses, the mean doses and the integral doses at tube voltages of 70 and 60 kV.

Results

The patient receives approximately 40 - 50% higher (mean and integral) absorbed dose when a tube voltage of 70 kV is used.

Conclusions

The results of this study clearly indicate that 60 kV should be used for dental intraoral radiographic examinations for approximal caries detection.

Effect of two X-ray tube voltages on detection of approximal caries in digital radiographs. An in vitro study

This study evaluated the effect of two different tube voltages on clinicians' ability to diagnose approximal carious lesions in digital radiographs. One hundred extracted teeth were radiographed twice at two voltage settings, 60 and 70 kV, using a standardized procedure. Seven observers evaluated the radiographs on a standard color monitor pre-calibrated according to DICOM part 14. Evaluations were made at ambient light levels below 50 lx. All observations were analyzed with receiver operating characteristic curves. A histological examination of the teeth served as the criterion standard. A paired t test compared the effects of the two voltages. The significance level was set to p < 0.05. Weighted kappa statistics estimated intra-observer agreement. No significant difference in accuracy of approximal carious lesion diagnosis was found between the two voltage settings. But five observers rated dentin lesions on radiographs exposed at 70 kV better than on radiographs exposed at 60 kV. Intra-observer agreement differed from fair to moderate. There was no significant difference in accuracy of approximal carious lesion diagnosis between digital radiographs exposed with 60 or 70 kV.

The influence of tube potential on periodontal bone level measurements and subjective image quality using a digital photostimulable storage phosphor sensor

Objectives

The purpose of the present study was to determine the measurement accuracy and subjective image quality for periodontal disease diagnosis when using two X-ray tube voltages with a digital photostimulable storage phosphor sensor.

Material and Methods

A digital photostimulable storage phosphor (PSP) sensor (Vistascan) and a multipulse X-ray generator (Prostyle Intra) with two tube voltages were used in this study. The front, premolar and molar region of two adult human cadaver skulls jaws were imaged using the X-ray tube at 63 kV and 70 kV, both at 8 mA and decreasing exposure times (160 ms, 120 ms and 80 ms). A standardized exposure protocol containing waxed occlusal keys and an aiming device ensured proper and reproducible beam alignment. Three observers assessed the digital radiographs for 31 selected periodontal bone loss sites. Radiographic measurements were compared to physical measurements (Standard). Subjective ratings of lamina dura, crater defect and furcation involvement visibility, contrast perception and bone quality were also performed.

Results

Multiple regression equation of the variables kV and exposure time demonstrated no significant difference for the periodontal bone level measurements (P > 0.05). In 90.3% and 96.7% of the measurements for 70 kV and 63 kV respectively, deviation was within 1 mm. The subjective ratings produced similar findings in terms of image quality for both tube voltages and the three exposure times.

Conclusions

The results of the present study revealed that tube voltages of 63 kV and 70 kV provided similar accuracy and image quality for periodontal disease diagnosis.

Development of a novel digital subtraction technique for detecting subtle changes in jawbone density

The objective of this paper is to develop a novel digital subtraction technique for serial intra-oral radiography, which would allow the detection of subtle variations in grey values. Digital images of the maxilla of a dried human skull and of a fresh pig mandible were acquired using intra-oral photostimulable phosphor plates (Digora FMX, Soredex, Helsinki) with an aluminium calibration stepwedge incorporated in the filmholder. Exposures were made with an X-ray tube for intra-oral radiography (Prostyle Intra, Planmeca, Helsinki). During pilot testing, parameter settings were adapted to reach an optimal contrast. Exposures were repeated within a 1-week interval to determine the test-retest reliability of the development. After in vitro and in vivo testing, the exposure technique and software development were used to evaluate its applicability in a pilot clinical case. Although parameter settings remained stable during the in vitro studies, the clinical exposures yielded non-linear digital images, thus, not readily suitable for data acquisition and comparison of the regions of interest. To allow further analysis, image processing was carried out using self-developed software for semi-automated linearisation and optimised contrast normalisation. This processing significantly increased the precise quantisation of jawbone density and the assessment of subtle bone density changes in arbitrarily selected regions of interest of in vivo exposures. The clinical applicability of the technique is demonstrated in a pilot case. It was demonstrated that minute densitometric deviations could be detected. The present technique and image processing may allow the quantification of jawbone density.

Effect of beam energy and filtration on the signal-to-noise ratio of the Dexis intraoral X-ray detector

OBJECTIVE

To evaluate the signal-to-noise ratio (SNR) of the Dexis charge-coupled device (CCD) intraoral X-ray detector, with special reference to the influence of beam energy and filtration.

MATERIAL AND METHODS

Digital radiographic images were made of a nine-step dental aluminium step wedge using a Dexis intraoral detector. The X-ray generator was a GE 1000 operating at 10 mA with 2.7 mm aluminium filtration. The peak tube voltage settings applied were 50 kVp, 60 kVp, 70 kVp, 80 kVp and 90 kVp. Various exposure times were used at each beam energy and the exposures (in micro C kg(-1)) were determined in each case using a 3 cl beryllium-windowed ionization chamber. SNR was defined using the surrogate measure of mean pixel value (surrogate for signal) divided by the standard deviation of the pixel value (surrogate for noise). SNRs were measured at separate regions of interest (ROIs) at the centre and at both sides of the detector for each aluminium step as well as without the step.

RESULTS

For beam energies of 50 kVp and 60 kVp, the estimated SNR improved both with increased exposure and with increased filtration (thickness of the aluminium step), and image saturation did not occur within the exposure time range permitted by the Dexis software. At 70 kVp and above, the SNR was optimized in the middle of the exposure range: 4 micro C kg(-1) with 290:1 at the ninth step (13.5 mm Al) for 70 kVp; 3.5 micro C kg(-1) with 240:1 at the ninth step (13.5 mm Al) for 80 kVp; 3.5 micro C kg(-1) with 160:1 at the ninth step (13.5 mm Al) for 90 kVp. Saturation did occur at these beam energies within the exposure time range permitted by the Dexis software.

CONCLUSION

For the Dexis intraoral radiographic imaging system, estimated SNR improved both with higher filtration and with lower kVp. The Dexis detector was capable of generating acceptable images of the step wedge at a wide range of kVp settings.

Image quality in digital radiographic systems

The aim of the present study was to evaluate the image quality of four direct digital radiographic systems. Radiographs were made of the maxillary central incisor and mandibular left molar regions of a dry skull, and an aluminum step-wedge. The X-ray generator operated at 10 mA. 60 and 70 kVp, and images were acquired with 3, 5, 8, 12, 24 and 48 exposure pulses. Six well-trained observers classified the images by means of scores from 1 to 3. Collected data were submitted to nonparametric statistical analysis using Fisher's exact test. Statistical analysis showed significant differences (p < 0.01) in image quality with the four systems. Based on the results, it was possible to conclude that: 1) all of the digital systems presented good performance in producing acceptable images for diagnosis, if the exposures of the step-wedge and the maxillary central incisor region were made at 5 pulses, as well as at 8 pulses for the mandibular left molar region, selecting 60 or 70kVp; 2) higher percentages of acceptable images were obtained with the administration of lower radiation doses in CCD-sensors (charge-coupled device); 3) the Storage Phosphor systems produced acceptable images at a large range of exposure settings, that included low, intermediate and high radiation doses.

[Clinical and comparative study of the image quality of 3 digital radiographic systems, E-speed film and digitalized film]]

The aim of this study was to evaluate, subjectively, the image quality of two CCD digital systems, a digital storage phosphor system, a digitized film and an E-speed film. Five objects were radiographed with 50, 60 and 70 kVp, with exposure times of 0.08, 0.13, 0.2, 0.4 and 0.8 s. In order to maximize the fidelity of the results, the dental X-ray unit employed was submitted to an evaluation so that the relation between exposure time and dose could be established. A good reproducibility was obtained. Six evaluators analyzed 375 images, using a scoring scale which ranged from 0 to 4. The results revealed different responses, indicating that the storage phosphor system had the best performance, in the various exposures employed. However, it was possible to conclude that all systems studied, except for the Sens-A-Ray, can offer images in ideal conditions for the diagnosis, as long as their latitudes are respected.

Assessing the effectiveness of direct digital radiography barrier sheaths and finger cots

BACKGROUND

Effective cross-contamination prevention is critical for direct digital radiography, or DDR, sensors, which are not sterilizable; however, current manufacturers' recommendations for standard precautions are limited to the use of plastic barrier sheaths, which are commonly known to tear or leak. The authors sought to determine the incidence of digital radiography barrier-sheath leakage, with and without additional latex finger cot protection, as measured by a water pressure test.

METHODS

Four hundred plastic barrier sheaths were randomly assigned to four groups based on intraoral radiograph positioning device use and supplemental barrier protection with a latex finger cot. Sheaths were carefully placed to cover DDR sensors for a single intraoral use, gently removed from the sensors and tested for leakage through a water pressure technique.

RESULTS

Perforations occurred in 44 to 51 percent of plastic sheaths after a single radiographic exposure. However, only up to 6 percent of the plastic sheaths that were covered by a latex finger cot leaked during the water pressure test.

CONCLUSIONS

At least 44 percent of the plastic barrier sheaths leaked after a single intraoral radiographic exposure. Use of a latex finger cot over the plastic sheath significantly reduced leakage to no more than 6 percent.

CLINICAL IMPLICATIONS

Latex finger cots used in conjunction with the standard plastic sheaths that cover DDR sensors may more effectively prevent cross-contamination than do plastic sheaths alone. Dentists who use DDR sensors during highly invasive dental procedures such as dental implant surgery are encouraged to consider supplemental barrier protection for these delicate, expensive and nonsterilizable sensors to prevent patient cross-contamination.

Assessing the image quality of a CCD-based digital intraoral radiography system: application of perceptibility curve test

The perceptibility curve test is a method for evaluating the psychophysical properties of radiographic systems. The concept of the perceptibility curve is based on the minimum perceptible exposure differences that are recorded by a particular imaging system. The perceptibility curve test was applied to a Charge-Coupled Device (CCD)-based digital intraoral radiography system in this study. A test object was made of a square aluminum block, 28 mm in both height and length and 10 mm in thickness. The test object had ten contrast details in the form of round holes with diameters of 1.5 mm. The depths of the holes ranged from 0.05 mm to 0.5 mm in steps of 0.05 +/- 0.01 mm. An X-ray unit was used that operated in the range of 60 to 90 kVp. Test radiographs were shown to observers in random order. Six observers were asked to rate them regarding the number of spots representing holes that they could perceive. The smallest perceptible difference in gray level on the computer monitor, a 15-inch cathode-ray tube (CRT), was then found. The minimum difference in gray level between a perceptible detail and the background was determined. Dose response functions were employed to determine exposures corresponding to these gray levels. For each case, delta logE as a function of the number of object details was calculated. Finally the reciprocal of all values of (delta logE)min were calculated. The number of details which the observers perceived decreased linearly with decreasing delta logE values. The maximum number of details that observers perceived was eight or nine. The perceptibility curves showed that their peaks shifted slightly with a change of the kVp but that their shapes were similar at different kVp settings.

Assessments of the physical performance of 2 generations of 2 direct digital intraoral sensors

OBJECTIVE

To evaluate the performance of 4 intraoral direct digital sensors regarding their fundamental physical characteristics. The sensors are made by Schick Technologies Inc (Long Island City, NY) and Gendex Dental Systems (Milan, Italy).

STUDY DESIGN

The sensors were exposed by using a Prostyle dental x-ray machine (Planmeca Oy, Helsinki, Finland) operating at 50 kV, 8 mA and various exposure times. Three test phantoms were used: a homogeneous 10-mm thick aluminum block, an aluminum block with a pattern of holes of varying sizes, and a resolving power target. Digital images were transferred as 8 bit TIFF files and analyzed by using a personal computer.

RESULTS AND CONCLUSIONS

Some improvements could be observed in the physical performance of the new generation of direct digital radiographic sensors when compared with the earlier generation. Smaller pixels and higher quantum efficiency have improved sensor performance.

Technical report. Processing to achieve high-contrast images with computed dental radiography

OBJECTIVES

To determine the effects of equalization on image contrast and signal-to-noise ratio (SNR) with Computed Dental Radiography (CDR).

METHODS

Pixel value distributions were measured for various tissue thicknesses of a dental quality assurance jaw phantom and the average pixel values and SNR calculated.

RESULTS

Equalization resulted in enhanced image contrast of underexposed images by effecting pixel value adjustment with little change in SNR. Overexposed images demonstrated minimal difference in contrast or SNR when equalization was applied.

CONCLUSION

The equalize function of CDR is a convenient method to provide high-contrast images from underexposed images without detriment to the SNR. As it does not compensate for overexposure, the function will not lead to inappropriate overdosage of the patient.