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British Journal of Radiology (2003) 76, 129-131
© 2003 British Institute of Radiology
doi: 10.1259/bjr/58851704
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Short communication

Measurement of dose–width product in panoramic dental radiology

P Isoardi, PhD and R Ropolo, PhD

Department of Health Physics, Azienda Ospedaliera San Giovanni Battista di Torino, c.so Bramante 88, 10126 Torino, Italy


   Abstract
Top
 Abstract
Introduction
 Methods and materials
 Results
 Discussion
 Conclusions
 References
 
The National Radiological Protection Board has recommended the introduction of dose–width product (DWP) for the measurement of patient dose in panoramic dental radiology. The present work describes a method for measuring DWP using a pencil ionization chamber. The technique was tested on five panoramic dental units; the reproducibility of the method was 5.7%. In order to test the method, DWP was also assessed using thermoluminescent dosemeters and film. The results obtained agreed within 8.6% with those obtained using the pencil ionization chamber method. The proposed method appears to be simple and precise.


   Introduction
Top
 Abstract
 Introduction
Methods and materials
 Results
 Discussion
 Conclusions
 References
 
Diagnostic reference levels (DRLs) are widely used for the control of medical radiation exposure to a level commensurate with the clinical purpose of a medical imaging task. In principle, DRLs are applicable for standard procedures in all areas of diagnostic radiology, but they are particularly useful in those areas where a considerable reduction in collective dose can be obtained, i.e. in frequent examinations and in examinations involving more radiosensitive patients, such as children.

Since orthopantomography is often performed in paediatric cases and involves a large population, a reference dose appears to be worthwhile. Recommended image quality and reference dose criteria are provided in document EUR 16260 [1] for several standard radiographic procedures. In the case of examinations that include projections not described in document EUR 16260, such as orthopantomography, there is no consensus on methodology for dosimetry.

Several methods have been proposed for assessing patient dose in panoramic dental radiography [2–5] and different values of DRL have also been proposed [3, 6].

As part of the National Radiological Protection Board's (NRPB) dental X-ray protection services, Napier [2] reported dose–width product (DWP) as a useful tool for panoramic dental radiographic reference dose determination. In fact, DWP is well correlated with the dose–area product (DAP), and DAP can be converted to effective dose using the conversion factor estimated by Williams and Montgomery [4]. The NRPB assessment of panoramic X-ray sets measures the absorbed dose (to air) at the front side of the secondary collimator, integrated over a standard adult exposure cycle. Again, this quantity is used for its ease of measurement and does not require a patient to be present during a measurement. The dose per exposure cycle is then multiplied by the horizontal width of the beam at the receiving slot. The NRPB recommends the adoption of a reference DWP of 65 mGy mm for a standard adult panoramic radiograph. This reference value can be used as a guide to accepted clinical practice.

The proposed methods that use film and/or thermoluminescent dosemeters (TLDs) to assess DWP suffer from several disadvantages, such as set-up complexity and delay from dose measurement to the reading of TLDs or film, while DRL should be assessed by an easily measurable quantity [7].

The aims of this study were to develop an easy method for assessing DWP using a pencil ionization chamber, to compare the proposed method with that using TLDs and to assess the accuracy and reproducibility of the method.


   Methods and materials
Top
 Abstract
 Introduction
 Methods and materials
Results
 Discussion
 Conclusions
 References
 
The pencil ionization chamber is a long, thin chamber with an active length usually of 10 cm. When irradiated, the distribution of dose along the axis of the chamber cannot be measured, but an averaged dose can be obtained. Since the chamber is calibrated by irradiating its entire length in a uniform field, the calibration factor can be given in terms of dose x chamber length per coulomb (mGy cm C-1), or in terms of dose per coulomb (mGy C-1) if the chamber length is accurately specified.

The pencil chamber is usually used to measure the CT dose index (CTDI), which is designed to represent the dose per slice received by a patient in a multislice study. If the calibration factor is given in terms of mGy C-1, CTDI is defined as Go


where T is the nominal slice thickness, Da(z) is the dose profile, M is the instrument reading, F is the calibration factor to convert the instrument reading to absorbed dose, L is the chamber length (as specified by the manufacturer), and ftp is the factor to correct for any difference in temperature and pressure at the time of measurement from those prevailing when the instrument was calibrated. The limits of integration are, theoretically, from - {infty} to + {infty}, but in practice the limits can be taken to be zi±{Delta}zi, where zi is the centre point of the dose profile and {Delta}zi is a cut-off beyond which the contribution can be considered to be negligible. The value Go


is the integral of the dose profile and the same concept can be applied to measurements on panoramic dental X-ray units. In this case, P corresponds, in practice, to DWP as defined by NRPB and can be directly obtained from the chamber reading.

Measurements were performed on five panoramic dental X-ray units, listed in Table 1Go, using a medium adult setting. Measurement of dose at the secondary collimator on dental X-ray units was made using a pencil ionization chamber (chamber PC-4P; Capintec, Pittsburgh, PA and dosemeter; UNIDOS, Freiburg, Germany). In two cases the chamber was mounted in an air-equivalent jig (Figure 1Go) and in three cases the chamber was directly mounted at the secondary collimator. The ionization chamber was perpendicular to the secondary collimator. Before each measurement, test rotations were made to ensure that there was sufficient space between the jig and the headrest whilst the secondary collimator was rotated. Measurements were repeated on a few sets to test the reproducibility of results. The ionization chamber was calibrated by a calibration centre of the Italian National Calibration system.


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  		Table 1. Types of panoramic X-ray unit tested, settings used for measurements, dose–width product (DWP) directly measured by pencil chamber method (P), measured dose with thermoluminescent dosemeters (D), full-width at half maximum (FWHM) measured with film and DWP as D x FWHM

 


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  		Figure 1. Measurement set up with a pencil ionization chamber mounted on a dental panoramic unit.

 
To test the pencil chamber measurement method, DWP was also measured on units using TLDs (TLD-100; Harshaw, Solon, OH) and film (X-Omat V; Kodak, Rochester, NY). Kodak X-Omat V film is a relatively low speed film normally used for verifying orientation and for approximating patient dosage in radiation therapy procedures. The response of the film at low doses was obtained using a radiographic unit (Multidiagnost 3; Philips Medical Systems, Best, The Netherlands) using a kilovoltage and total filtration setting (70 kV, 3 mm aluminium) comparable with those of the dental units. The optical density was measured with a densitometer (Densoquick 2; Pehamed, Sulzbach, Germany) and the dose with a solid state dosemeter (MX; RTI Electronics, Goteborg, Sweden). The results show an acceptably linear dose–density response (Figure 2Go).



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  		Figure 2. Variation of optical density (OD) with dose for the Kodak X-Omat V film (Kodak, Rochester, NY).

 
The NRPB method for DWP measurement uses a film to measure absorbed dose, D, and width of the beam, characterized by the full width at half maximum (FWHM). DWP is considered equivalent to D x FWHM. The method proposed by Williams et al [4] uses a TLD array to measure absorbed dose and the FWHM of the dose profile; DWP is considered equivalent to the integrated dose over all TLDs, Dint. In this work an array of TLDs was used to measure Dmax and a film was used to measure the FWHM with more precision.

The film was exposed at the secondary collimator, digitized (ARCUS II; AGFA, Livingstone, UK) and the FWHM of the dose profile calculated using commercial software (FotoLook 95 V2.08; AGFA, Livingstone, UK). The digitization was performed with a resolution of 118 pixels per centimetre and 212 grey levels; for the dose profiles obtained (Figure 3Go) the value of FWHM was calculated by a simple macro with an uncertainty of less than 2%.



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  		Figure 3. Typical dose profile at the secondary collimator for a Philips, Best, The Netherlands, Orthoralix C Panoramic X-ray unit (code 5 in Table 1Go).

 
TLDs were 1 mm thick and were mounted in an array of four. The TLDs were irradiated with the axis of the TLD column coincident with the centre of the slot using an air equivalent jig. Readings of TLDs were undertaken using a TLD reader (Universal Toledo 654 D; Vinten Instruments, Weybridge, UK) with contemporary readings of calibration and background TLDs. The average of the readings from the TLDs was used in order to determine D. Finally DWP can be obtained as D x FWHM since D corresponds to a value of dose integrated over 1 mm. TLDs were calibrated at 70 kVp to a dose of approximately 20 mGy against a calibrated ionization chamber (chamber type 2571; NE, Beenham, UK, and dosemeter; UNIDOS, Freiburg, Germany).


   Results
Top
 Abstract
 Introduction
 Methods and materials
 Results
Discussion
 Conclusions
 References
 
The results of the measurements are shown in Table 1Go. Measurements were performed with a kilovoltage setting ranging from 64 kV to 71 kV. Values of P and DWP are compared for each unit. The maximum difference found between values obtained by the pencil beam method and those obtained by the TLD method was 8.6%.

Measurements were repeated six times on unit number 5 in order to test reproducibility. The repeated results agreed within 5.7%.

Uncertainties in the measurement of P arise from the pencil chamber calibration. A standard error of less than 5% at the 95% confidence level can be considered as typical.


   Discussion
Top
 Abstract
 Introduction
 Methods and materials
 Results
 Discussion
Conclusions
 References
 
The original NRPB method uses a film to measure absorbed dose and slit width and DWP is considered equivalent to D x FWHM. Williams et al [4] demonstrate that DWP can be obtained using a TLD array to measure the absorbed dose profile. In fact, DWP is considered equivalent to the integrated dose over all TLDs, Dint, under the hypothesis that Dint corresponds to the DWP for a square beam profile. In both techniques the dose is defined as the peak dose. The difference found [4] between the values of Dint and DWP was considered to be due to a systematic error in measurement owing to the detector size, and the significant contribution to dose from the profile tail.

A pencil chamber can be used to make a simple measurement of DWP because the electrometer reads the charge product over the entire length of the chamber; the length of the chamber is greater than the width of the beam, so the result of the measurement corresponds to the integrated dose that can be considered equivalent to the DWP for a square beam profile.

The results obtained with the pencil chamber method agree within 8.6% with those obtained by the conventional measurement of DWP; reproducibility was 5.7%, comparable with 7.2% for the conventional method (series of six measurements).

The major advantage of using a pencil chamber in order to obtain DWP is its immediacy; the method does not require use of film and/or TLDs and the chamber can be easily positioned for the measurement.


   Conclusions
Top
 Abstract
 Introduction
 Methods and materials
 Results
 Discussion
 Conclusions
References
 
DWP for panoramic dental radiology can be readily determined by measurement performed with a pencil chamber. The method appears to be simple, reliable and precise.


   Acknowledgments
 
The authors would like to thank Alfredo Izzo and Luigi Savio for their helpful assistance.

Received for publication April 8, 2002. Revision received July 31, 2002. Accepted for publication October 21, 2002.


   References
Top
 Abstract
 Introduction
 Methods and materials
 Results
 Discussion
 Conclusions
 References
 

  1. Carmichael JHE, Maccia C, Moores BM, Oestmann JW, Schibilla H, Teunen D, et al, editors. European guidelines on quality criteria for diagnostic radiographic images. EUR 16260. European Commission, 1996.
  2. Napier ID. Reference doses for dental radiography. Br Dent J 1999;186:392–6.[CrossRef][Medline]
  3. Gonzalez L, Vano E, Fernandez R. Reference doses in dental radiodiagnostic facilities. Br J Radiol 2001;74:153–6.[Abstract/Free Full Text]
  4. Williams JR, Montgomery A. Measurement of dose in panoramic dental radiology. Br J Radiol 2000;73:1002–6.[Abstract]
  5. Goldstein A. Exposure and dose in panoramic radiology. Med Phys 1998;25:1033–40.[Medline]
  6. The Institute of Physics and Engineering in Medicine. Recommended standards for the routine performance testing of diagnostic X-ray imaging systems, IPEM Report 77. York, UK: IPEM, 1997.
  7. International Commission on Radiological Protection. 1990 Recommendations of the ICRP. ICRP Publication 60. Annals of the ICRP. Oxford, UK: Pergamon Press, 1991.



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This Article
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