Exposure in Dental Radiology: A Comparison Between Intra-oral, Panoramic and Tomographic Examinations

Transcription

1 Exposure in Dental Radiology: A Comparison Between Intra-oral, Panoramic and Tomographic Examinations S. Baechler 1, P. Monnin 1, A. Aroua 1, J.F. Valley 1, M. Perrier, P. Trueb 3, F.R. Verdun 1 1 University Institute of Applied Radiation Physics, Grand-Pré 1, CH-1007 Lausanne, Switzerland sebastien.baechler@hospvd.ch University Dental General Hospital of Lausanne, CH-1011 Lausanne, Switzerland 3 Swiss Federal Office of Public Health, CH-3003 Bern, Switzerland Abstract. The purpose of this paper is to present a comparison of doses delivered to the patients when dealing with intra-oral, panoramic and CT dental examinations. The technology progress will be discussed from a dose and an image quality point of view. DAP values were measured for two intra-oral systems using speed D and E/F dental films, and a digital system in two different acquisition modes (CCD technology). For each system the DQE was also assessed. DWP and DAP were measured on three panoramic systems of various generations of units. Finally, CTDI w and DLP were measured using a standard CT (Dentascan) and a new tomographic system (NewTom) in order to assess the dose delivered when planning dental implants. The use of an E/F system instead of the D system allowed to reduce the DAP by a factor of (respectively 56 and mgy cm ) without significant loss of image quality. A further dose reduction by a factor of four was possible with digital systems but with a significant loss of spatial resolution. The DAP measured on OPG systems showed that the oldest system (0 years old) delivered a higher dose than a more recent system (respectively 0. and 0.07 Gy cm ). Finally, DLP values of mgy cm and 55 mgy cm were obtained with respectively the standard CT acquisition and a new tomographic system designed specifically for dental radiology. The introduction of digital detectors has a significant potential in dose reduction. Nevertheless, this will impose to use intra-oral units that handle short exposure times. The new tomographic technology enables to reduce the dose delivered to the patient by a factor of when compared with Dentascan. 1 Introduction Dental radiology belongs to the most common low-dose examinations and affects predominantly young subjects. Optimization efforts are permanently carried out by introducing new technologies in the field of intra-oral digital radiology and tomography by the advent of dedicated system. Thus, comparing different products available in the market enables to evaluate the potential dose reduction using these new devices. For this purpose, D and E/F films are compared with a digital detector in order to assess the dose delivered to the patient as well as the image quality. In addition, dose measurements are performed for extra-oral examinations: panoramic (orthopantomograms - OPG), dedicated and conventional CT systems. In radiodiagnostic, monitoring the dose delivered to the patient is generally performed using dosimetric indicators, such as the entrance skin dose (ESD) and the dose area product (DAP). These values are easily measurable and enable to estimate the effective dose delivered to a «standard» patient. In addition, TLD measurements are used to determine dose distribution in the region under examination. Materials and methods.1 Intra-oral detection systems.1.1 Installation and beam quality Measurements were performed on a conventional Toshiba radiological installation. The beam quality was selected according to the ISO standard 5799:1991(E). The total filtration of the tube was 7.0 ± 0.5 mm of 99.9% pure aluminium (internal filtration of.5 mm Al + additional filtration of.5 mm Al). A high voltage of 50 kv was used so that the half-value layer (HVL) of the beam was 3.0 ± 0. 1

2 mm equivalent aluminium. For Ultra-speed and Insight films, two product tube current time values of 5, respectively 18, were selected. Similarly, two product tube current time values of 10 and.8 were used in high-resolution, respectively high-sensitivity mode of the detector based on a CCD technology..1. Detectors Each detector was characterized by the sensitivity, the Modulated Transfer Function (MTF) and the Detection Quantum Efficiency (DQE). The intra-oral detection systems used in the present study were the followings: Films Two types of films were characterized: - Kodak Ultra Speed DF 58 / batch Class D - Kodak Insight IP 1 / batch Class E or F Film responses were determined at three dose levels leading to optical density (OD) values of 0.8, 1. and.3. The optical density of 1. corresponds to a standard level of film exposure whereas optical densities of 0.8 and.3 correspond respectively to slightly under and over-exposed levels. Digital Detector The RVG Trophy system includes an indirect CCD digital detector having a sensitive area of 36 x 6.5 mm covered by a matrix of 180 x 1360 leading to a pixel size of 19.5-µm. The detector has a dynamic range of 1 bits (096 gray-levels per pixel). The software performs data compression on 8 bits as well as logarithmic image correction. The detector can be used in high-resolution (HR) or highsensitivity (HS) mode. For both acquisition modes, three dose levels leading to background gray levels of 0, 55 and 10 were characterized. These values correspond to a realistic use of the detector (50), as well as under and over-exposed levels..1.3 Dose measurements The ESD and DAP indicators were measured using a Radcal dosimeter (Radcal 3036) connected to a 11 cm 3 ionizing chamber. The conversion factors to determine the effective dose for a skull region exposure, at 70 kv and using a.5-mm thick aluminium filtration, are 0.09, 0.0 and 0.07 msv Gy -1 cm - for ante-posterior, post-anterior and lateral incidences according to Ref. [1].. Extra-oral detection systems..1 Detectors OPG Two conventional OPG installations and one digital OPG system based on the CCD technology have been included in this study: - OPG 1 : Philips Ortho-oralix - monopulse DFF 7 cm (0 years old) - OPG : Soradex Cranex dc monopulse DFF 50 cm (8 years old) - OPG 3 : Planmeca PM00cc Proline monopulse DFF 50 cm (recent installation) Tomographic system Dedicated CT system : Newtom DVT QR This installation does not only enable to take projection radiographs similar to images obtained by OPG, but also tomograms that are used for maxillo-facial surgery planning. The acquisition is performed using a "cone beam" geometry as opposed to the traditional fan beam geometry used in CT. The field dimension in the axial direction is 130 mm with an opening angle of 1. The focus-skin distance is about 30 mm and the distance between the focus and the rotation axis is about 680 mm. The detector uses a /3 CCD sensor (.7 x 7.1 cm ).

3 Conventional CT system. The 8-slice CT scanner used for the present study was the GE LightSpeed. Measurements were realized considering classical parameters for dental tomographic examinations (Dentascan)... Dose measurements The NRPB proposes the use of the dose width product (DWP) as a dose indicator when dealing with OPG examinations. In this study DWP were measured using a pencil ionization chamber connected to a Radcal dosimeter (Radcal 1515) [3]. The length of the RX beam was measured using a radiological film place in a light tight envelope. The conversion factor of 0.06 msv Gy -1 cm - recommended by the NRPB for OPG systems was used to calculate the effective dose from the DAP []. Entrance distribution was also evaluated using TLD measurements. In this case, bands of 6 TLD chips were positioned on an anthropomorphic skull phantom to determine ESD values. In the present work, dose distributions were determined for the OPG device as well as the Newtom DVT and Dentascan tomographic installations. 3 Results and discussion 3.1 Intra-oral detection systems Characteristic curves Film Characteristic curves of both Ultra-speed and Insight Kodak films are showed in Figure 1 for automatic (warm) and manual (cold) development conditions. Sensitivity and average contrast values extracted from the curves are given in Table 1. 5 Automatic development ULT Film IN Film Manual development ULT Film IN Film Optical density Exposure [µgy] FIG. 1 Characteristic curves of Ultra-speed (ULT) and Insight (IN) Kodak films. Sensitivity values for both films are in good agreement with classes, D respectively E/F, indicated by the manufacturer. Sensitivity loss due to manual development is higher than 30 % for the Insight system but is negligible for the Ultra-speed system. The Insight film is. times more sensitive than the Ultra-speed film. The average contrast of both types of films is nearly identical (.05). 3

4 Table I. Characteristic values of the Ulta-speed and Insight films. * Film Development Base + fog Dose at a net OD of 1 [µgy] Sensitivity [mgy -1 ] Average contrast ULT Manual Automatic IN Manual Automatic * Comparable results were obtained when using a standard intra-oral unit with a high voltage value of 70 kv. Digital detector Detector responses (56 gray-levels for 8 bits images) as function of the dose are showed in Figure for both high resolution and high sensitivity modes. Interpolation curves were used to linearize the images. The 6 characterization points corresponding to three levels of gray at 0, 55 and 10 are also reported on the graph. The sensitivity difference between HS and HR acquisition modes is obvious. The patient exposure is saved by a factor 5 for an image taken under standard conditions (background gray level 50 and 100) using the HS mode RVG Trophy high-resolution mode (HR) high-sensitivity mode (HS) characterization points Grey level Dose [µgy] FIG. Response curves of the RVG Trophy in high-resolution (HR) and high-sensitivity (HS) modes Sensitivity and dose indicators The sensibility was defined here as the inverse dose value at the detector entrance corresponding to a background gray level of 55 (standard clinical gray level). The DAP was calculated by multiplying the ESD and the surface area of the maximum acceptable exposure field corresponding to a diameter of 6 cm. The conversion factor used to determine the effective dose is about 0.03 msv Gy -1 cm -. The Insight film is. times more sensitive than the Ultra-speed. An image taken with the digital detector using the HR mode require a 6 times less exposure than when using an Insight Kodak film. Choosing the HS mode instead of the HR mode makes it possible to save an additional exposure factor of 5 (see Table II) Spatial resolution The Figure 3 shows the MTFs of the detectors under study. Both the 0-µm-pixel size of and the scintillation detection method restrict the resolution of the digital detector compared to the films. At 5 mm -1, the MTF of the films is still close to 90 % while the MTF of the Trophy system has already

5 dropped to about 55 % in HR mode. Using the HS mode reduces significantly the resolution in comparison with the HR mode. Table II. Sensitivity and dose area product (DAP) of intra-oral detection systems. Detection system Sensibility [mgy -1 ] Entrance Skin Dose [mgy] Dose Area Product [mgy cm ] Film Kodak Ultra-speed (D) Film Kodak Insight (E/F) RVG Trophy mode HR RVG Trophy mode HS * E=effective dose using the factor 0.03 msv Gy -1 cm -. E * µsv Film IN Film ULT Trophy mode HR Trophy mode HS 0.6 MTF Spatial Frequency [mm -1 ] FIG. 3 Comparison of MTFs for intra-oral detectors Detection quantum efficiency The figure compares the DQE of intra-oral detectors for a usual exposure level (optical density of 1. for films and gray level of 55 for digital images). Since there is no intensifying screen with intraoral dental films, the sensitivity and the DQE are lower compared to the digital detector. With comparable noise levels at low frequencies and. times higher sensitivity than the Ultra-speed film, the Insight film gives a. times better DQE (. % vs. 1%). Because of its high sensitivity, the digital detector provides a DQE definitely higher than films (15.7 % for the HR mode and 1.8 % for the HS mode). The high value obtained for the HS mode is also explained by a more effective detection process. However, due to its lower resolution, the DQE advantage of the digital detector is limited to detection at low and intermediate frequencies (up to 10 mm -1 ). For the same reasons, the DQE improvement using the HS mode compared to the HR mode of the Trophy system is restricted to low frequencies up to.5 mm -1. 5

6 1 8 6 Film ULT Film IN Trophy mode HR Trophy mode HS DQE Spatial Frequency [mm -1 ] FIG. Comparison of DQE for intra-oral dental detectors Extra-oral detection systems 3..1 OPG dose indicator The results obtained for the three OPG are summarized in Table III. The DAP obtained for the two most recent systems are about times smaller than the value of the 0 years old device, and in good agreement with the NRPB reference values. Installations Table III. DWP and DAP values as well as effective doses for the three OPG. Voltage kv Current ma Duration s DWP mgy mm Length/Width Slit (mm) DAP E ** Gy cm µsv OPG 1 (0 years old) / OPG (8 years old) / OPG 3 (recent) / Reference * 65 / /0.1 * Average value given in Ref. [] ** E = effective dose using the factor 0.06 msv Gy -1 cm OPG TLD measurements in phantom The doses distribution obtained for a normal adult when using the conventional OPG are the followings: an average dose of 0.3 mgy to the jawbone (peak dose of 0.8 mgy); a dose of about 0. mgy to the thyroid ; a dose below 0.1 mgy to the crystalline lens CT TLD measurements in phantom and CTDI w DVT Newtom 9000 and Dentascan (GE LightSpeed) To evaluate the dose delivered to the patient, the skull phantom equipped with bands of TLD was scanned using characteristic parameters for an adult. The obtained results are given in Fig. for both DVT Newtom and Dentascan. 6

7 0. mgy 0. mgy 0. mgy.1 mgy.5 mgy 3.5 mgy 1.0 mgy 1 mgy 3.9 mgy 3 mgy a).5 mgy b) 3.5 mgy FIG. 5. Dose distribution using (a) the Newtom DVT and (b) Dentascan technology To extent dose measurements for CT examinations, using CTDI w enables to compare effective doses. The Newtom DVT system yields a CTDI w value of. mgy using the 16-cm diameter phantom. For an irradiation length of about 13 cm, this examination delivers a DLP of 55 mgy cm. Using the CT conversion factors gives an effective dose of 0.13 msv. A note published by Health Physics Society of Michigan gives for this type of installation a CTDI w value of.5 mgy and an effective dose of 0.05 msv. For the Dentascan, the CTDI w was 19 mgy for the lower jawbone examination corresponding to a DLP of 106 mgy cm and an effective dose of 0.5 msv. The same measurement for the upper jawbone gave a CTDI w of 3.5 mgy, a DLP of 118 mgy cm and an effective dose of 0.7 msv. Finally, Dentascan examinations deliver a total effective dose of 0.5 msv. The Health Physics Society of Michigan gives effective dose ranging from 0.15 to 0.58 msv. The Dentascan delivers about times higher doses than the Newtom DVT. 3.3 Conclusion The Insight Kodak film is approximately twice more sensitive than the Ultra-speed film. Thus it is possible to reduce the dose by a factor while keeping similar image quality. Compared to intra-oral films, the RVG Trophy digital detector offers a much higher DQE thanks to the about 7 times higher sensitivity for a comparable level of noise in high-resolution mode. However, this advantage is relevant only if the unit enables to control short exposure time. The dose indicator used for intra-oral radiology or OPG examination is the DAP. For examinations performed using D films, the average DAP is about 0.05 Gy cm. Using digital systems such as Trophy will enable to reduce significantly the dose level in the future. Concerning the OPG, the average DAP was about 0.11 Gy cm, which corresponds approximately to the DAP of intra-oral images. For OPG examination, using digital detector do not reveal a dose reduction at all. An intraoral examination using D films yields an effective dose of about 1.7 µsv while an OPG yields about 7 µsv. Using the dedicated Newtom DVT 9000 CT system reduces the patient exposure by a factor compared to an optimized conventional CT examination (Dentascan). Indeed, the DVT system yields a DLP of 55 mgy cm that corresponds to an effective dose of 0.13 msv (0.5 msv for the conventional CT device). 7

8 References [1] NRPB Guidelines on patient dose to promote the optimisation of protection for diagnostic medical exposure. NRPB, Chilton UK, (1) (1999). [] Hart D., Jones D.G., Wall B.F., Estimation of effective dose in diagnostic radiology from entrance surface dose and dose area product measurements, NRPB, Chilton UK R6. [3] Isoadi P., Ropolo R., Measurements do pose width product in panoramic dental radiology. BJR, 76:19-131, (003). 8