Dental Radiography Radiation dosage of a dental imaging system George Soh* / Fun-Chee Loh^i^* / Yea-Hwe Ch ong* This study compared the radiation dosage necessary for obtaining clinically acceptable images with a new dental imaging system (radiovisiography) to that required with a conventional-film radiographie technique. Results showed that radiovisiography required significantly less radiation exposure, only 22.3% of that used in the conventional technique. Problems, such as lack offlexibilityand the limited size ofthe x-ray-sensitive surface, are associated with radiovisiography; however, the system effectively complements conventional-film radiography. (Qu\n(essence Int 1993:24:189-191.) Introduction Dental radiographs have been an important diagnostic :ool for dental practitioners, Bitewing radiographs Dlay an efñcacious role in the detection of interproxi- Tial caries' and in examination of the quality of amal- gam restorations," Intraoral radiographs also contrib- Jte to the detection of obscure intraosseous lesions Df the face and jaws/' Dentists today use radiographs OT routine screening of dental diseases, for checking if progressive stages in root canal therapy, and for •nonitoring of healing after dental treatment. The benefits of dental radiography can only be as- iessed against the risk of radiation exposure. The ra- diation that is of concern is the ionizing radiation that onizes atoms and molecules in body tissues. Previous ;tudies suggest that leukemia and other forms of :ancer occur more frequently in persons exposed to greater amounts of radiation,''" The findings of those studies were based on examination of records related to human exposure to ionizing radiation. Such findings highlighted the vulnerability of the human body to radiation exposure. In dentiil radiography, an image of clinical diagnos- tic quality should be obtained with minimal radiation dosage to the patient. Although dental radiographie film offers a reliable method of reeording images intra- orally, attempts have been made to produce high-qual- ity images of dental structures and yet reduce radia- tion exposure compared to that resulting from conven- tional film techniques,''The latest alternative to intra- oral film radiography is radiovisiography (RVG), a dig- itai imaging system that utilizes an intraoral sensor in place of the radiographie film to produce instant im- ages on a monitor screen, Radiovisiography not only produces instant images on a screen without the need for film processing, it supposedly requires only 20% of the radiation needed for conventional radiography. The aim of this study was to investigate the amount of radiation required by RVG to produce images of human dentition for comparison with the correspond- ing conventional film technique. Senior Lecturer, Deparcmetil of Preventive Dentistry, National University of Singapore. Faculty of Dentistry, National Uni- versity Hospital. 5 Lower Kent Ridge Road. Singapore 0511, Senior Lecturer, Department of Oral and Masillofaeial Surgery. National University of Singapore, Senior Lecturer, Department of Restorative Dentistry, National University of Singapore, Method and materials The study evaluated the first model of the RVG sys- tem (Trophy Radiologie, Cedex), which was recently introduced. The system consists of four components: an x-ray tube and generator with electronic timer, an intraoral sensor, an image processor and display unit, and a thermal printer. The 7ü-kV{p) x-ray unit is capable iuintessence International Volume 24, Number 3/1993 189
Transcript
Dental Radiography
Radiation dosage of a dental imaging systemGeorge Soh* / Fun-Chee Loh i * / Yea-Hwe Chong*
This study compared the radiation dosage necessary for obtaining clinically acceptableimages with a new dental imaging system (radiovisiography) to that required with aconventional-film radiographie technique. Results showed that radiovisiography requiredsignificantly less radiation exposure, only 22.3% of that used in the conventionaltechnique. Problems, such as lack of flexibility and the limited size ofthe x-ray-sensitivesurface, are associated with radiovisiography; however, the system effectively complementsconventional-film radiography. (Qu\n(essence Int 1993:24:189-191.)
Introduction
Dental radiographs have been an important diagnostic:ool for dental practitioners, Bitewing radiographsDlay an efñcacious role in the detection of interproxi-Tial caries' and in examination of the quality of amal-gam restorations," Intraoral radiographs also contrib-Jte to the detection of obscure intraosseous lesionsDf the face and jaws/' Dentists today use radiographsOT routine screening of dental diseases, for checkingif progressive stages in root canal therapy, and for•nonitoring of healing after dental treatment.
The benefits of dental radiography can only be as-iessed against the risk of radiation exposure. The ra-diation that is of concern is the ionizing radiation thatonizes atoms and molecules in body tissues. Previous;tudies suggest that leukemia and other forms of:ancer occur more frequently in persons exposed togreater amounts of radiation,''" The findings of thosestudies were based on examination of records related
to human exposure to ionizing radiation. Such findingshighlighted the vulnerability of the human body toradiation exposure.
In dentiil radiography, an image of clinical diagnos-tic quality should be obtained with minimal radiationdosage to the patient. Although dental radiographiefilm offers a reliable method of reeording images intra-orally, attempts have been made to produce high-qual-ity images of dental structures and yet reduce radia-tion exposure compared to that resulting from conven-tional film techniques,''The latest alternative to intra-oral film radiography is radiovisiography (RVG), a dig-itai imaging system that utilizes an intraoral sensor inplace of the radiographie film to produce instant im-ages on a monitor screen, Radiovisiography not onlyproduces instant images on a screen without the needfor film processing, it supposedly requires only 20% ofthe radiation needed for conventional radiography.
The aim of this study was to investigate the amountof radiation required by RVG to produce images ofhuman dentition for comparison with the correspond-ing conventional film technique.
Senior Lecturer, Deparcmetil of Preventive Dentistry, NationalUniversity of Singapore. Faculty of Dentistry, National Uni-versity Hospital. 5 Lower Kent Ridge Road. Singapore 0511,Senior Lecturer, Department of Oral and MasillofaeialSurgery. National University of Singapore,Senior Lecturer, Department of Restorative Dentistry, NationalUniversity of Singapore,
Method and materials
The study evaluated the first model of the RVG sys-tem (Trophy Radiologie, Cedex), which was recentlyintroduced. The system consists of four components:an x-ray tube and generator with electronic timer, anintraoral sensor, an image processor and display unit,and a thermal printer. The 7ü-kV{p) x-ray unit is capable
iuintessence International Volume 24, Number 3/1993 189
Dental Radiography
Table 1 Mean radiation dosage for RVG and eonven-tional radiography (27 exposures)
RVGConventional
Radiation dosage
Mean'"
47.7213.7
( m re ill)
SD
19.06fi,5
Difference in the two menn values is sliiiisticiiliy signilicLinL atP< .llfJOl (paircd-samplo/test).
of providing exposures required for eonventionalradiography as well as reduced exposures for RVGoperation. The sensor consists of an exchangeablescintillating screen, optical fibers, and a miniaturecharged-couple device imaging system enclosed in a4 X 2-cm plastic casing. The processor unit digitizes theanalog signal from the intraoral sensor and the outputis reflected on a display screen that allows enhancementofthe image.
A dried human skull with a complete permanentdentition was used in the study. Ten periapieal imagesof the mandibular left second molar were first takenwith RVG to test the reliability of the thermolumines-cent dosimetry (TLD) method of measuring radiationdosage. Tlie rmo I um i ne scent dosimetry is an establishedmethod of monitoring radiation exposure in healthcare personnel who operate radiographie machines.'The process was repeated and the mean dosage usedfor generating each of the two sets of images was com-pared with Student's i test.
Clinically acceptable periapical images of each toothin the dried skull, except for the third molars, wereproduced with both RVG and conventional radio-graphy. The TLD chip was placed on or about thesame spot on the buceal surface of the tooth, and thedistance of the cone lo the tooth surface was maintainedfor each of the radiographie methods. The conventionalradiographs were taken with the recommefided F-speeddental film (Kodak Dental Film, Eastman Kodak).
Radiation dosage was measured with thermolumi-nescent dosimetry. The TLD chip was annealed for1 hour at 400°C and then exposed tcj a drying oven for2 hours at 100°C before it was used. The radiationdosage absorbed by the chip was read by the UniversalToledo TLD ehip reader (Vintén Instruments) and ex-pressed as millirems (mrem). The radiation dosages
used to produce the required images of each tooth byeach of the two methods were compared with thepaired-sample ( test.
Re,sults
Evaluation of the TLD method of estimating radiationdosage showed that the mean dosages for the two setsof radiographs did not differ significantly (( = LÜ1;df= 18).
Radiovisiography required significantly less radia-tion Ihan did the conventional radiography to producecomparatively similar images (t = 12.8; df = 27; P <,0001). The mean amount of radiation used by RVGwas only 22.3% of that of the conventional method.
Discussion
Although dental radiographs play an indispensablerole in the detection of dental diseases, the risk ofradiation exposure remains an important concern. Im-ages of adequate diagnostic value must be obtainedwith minimal radiation dosage to patients. To reducethe dangers of radiation from dental radiography, leadshields are being used to protect vital parts of thebody. In particular, the use of a thyroid shield hasbeen found to be effective in minimizing radiation dur-ing intraoral and extraoral radiography.*^ Additionalmeans of reducing radiation dosage include the use offiltration devices,' high-speed dental x-ray films,'" andimaging systems.
Various forms of imaging systems have been assessedfor applications in dentistry. Dental xeroradiographywas one of the earlier imaging systems evaluated," andradiovisiography is the latest.'"'^ One distinct advan-tage of any imaging system over conventional filmradiography has been the lower radiation exposurenecessary to produce clinically acceptable images.One study reported that a xeroradiographic system issuperior for obtaining images of both hard and soft tis-sues with only one third the radiation exposure of coi-responding conventional film teehniques.'' For theRVG system, the radiation required is less than onequarter of eonventional radiography, as reported in aprevious study" and confirmed in this study.
Although the possible clinieal appheations of theRVG fall beyond the scope of this study, previousstudies have made some observations,'^'-' GenerallyRVG has been found to he capable of generating clini-cally acceptable images of sufficient diagnostic quality.
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Denta! Radiography
lowever, the resolution and latitude seem to he in-;nor to ihose of conventional dentai film—an observa-on shared by the investigators of this study as well,lowever, becLiuse images can be obtained without theeed for further processing, cost savings are lealizednd problems associated with film processing arevoided. For example, variations in the quality of imagesn radiographs can result from the use of incompatibleIms, ^ fluctuations in the kilovoltage, changes inle temperature of processing solutions,'" and duplica-on conditions,'' '"Because of the rapidity of us image processing, the
l\'G system would be most suitable when a numberf radiographs need to be taken at the same site with-ut the need lor high tesohition, such as for root canalrocedures,'" The RVG system can be connected to aersonal computer through a special interface card. Ank and management software facilitates storage and;trieval of images and their transmittal across phonenes-
One problem associated with the RVG system is theick of flexibility and the hmited size of the x-ray-;nsitive surface of the intraoral sensor. The lack ofexibility makes adaptation to the posterior region ofle mouth difficult, while the limited size of x-ray-însitivc surface reduces the number of teeth that cane captured in one frame. The limited size of the sensor3uld also mean a greater number of RVG imagesould be needed to screen the same area than woulde required with conventional intraoral radiographs.Tie end result would negate the reduction in radiation^posure. The maximum number of frames that can beisplayed at any one time on the monitor screen ismited to four, thus preventing a simultaneous displayf a series of complete-mouth radiographie images.
However, the RVG system is a useful complement> conventional-film radiography, and system modifica-ons to address its existing limitations may make RVGmore clinically effective substitute for conventional
idiography as a diagnostic tool.
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