- Email: [email protected]
RisøScan—a new dosimetry software
ARTICLE IN PRESS
Radiation Physics and Chemistry 71 (2004) 359–362
RisøScan—a new dosimetry software Jakob Helt-Hansen*, Arne Miller Radiation Research Department, Risø National Laboratory, P.O. Box 49, Roskilde DK-4000, Denmark
Abstract RisøScan is a software package that is used for analysis of images of visibly coloured dosimeter films. The image is created by scanning the dosimeter film on a flatbed scanner. RisøScan is based on LabViews, and it is useful for analysis of dose distributions and depth dose curves. Measurement reproducibility is maintained in the analysis of dosimeter films, and measurement uncertainties (Type A) in the order of 3–5% (1 s.d.) are obtained depending on the scanner and the dosimeter film used. r 2004 Elsevier Ltd. All rights reserved. Keywords: Dosimetry; Dosimeters; Film; Scanner; Scanning; Software
1. Introduction Measurement of dose distribution in radiation processing is used in installation qualification, operational qualification and performance qualification. In particular, for electron-beam irradiation thin film dosimeters are used for these purposes, and most of these (GAF, FWT, Risø B3) colour visibly when irradiated (ISO/ ASTM, 2002a). These coloured dosimeter films can be scanned on an optical flatbed scanner, and an image file created with information about the degree of colouration. A high spatial resolution of the scanned image can be obtained, but a compromise between file size and resolution has to be obtained. The use of flatbed scanners for the measurement of dosimeter films has been proposed earlier, both for radiation processing dosimetry (Miller et al., 2000) and for medical dosimetry (Stevens et al., 1996; Cai et al., 2003); and a software package for analysis for scanned images (Scanalizer) was made by AECL (Atomic Energy of Canada, Ltd.). RisøScan version 1.0 builds on these
*Corresponding author. Tel.: +45-4677-4908; fax: +454677-4959. E-mail address: [email protected] (J. Helt-Hansen).
earlier works. The software package is made using LabViews version 6.1 from National Instruments.
2. Colour balance The measurements described in this paper are made with Risø B3 dosimeters. The scanner used was a HP 5400C. It scans in three colours; red, green and blue. The signal content in each colour channel depends on the absorption of the irradiated dosimeter film, and for Risø B3 only the green channel contains the information, while the two other channels only add to signal noise. RisøScan allows choosing a fraction of each colour channel, and in this case 100% green and 0% red and blue are chosen. Other dosimeter films may require a different RGB balance.
3. Calibration Calibration is obtained by measurement of dosimeter films irradiated to known doses. Each dosimeter is measured over its entire area, thereby effectively providing many individual measurements. The greyscale value and the corresponding dose are entered into a table, and displayed by RisøScan as a graph. A fit is
0969-806X/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2004.03.033
ARTICLE IN PRESS J. Helt-Hansen, A. Miller / Radiation Physics and Chemistry 71 (2004) 359–362
360
150
180
100
Ref. C. (50 kGy)
160
50 0 0
20
40
60
Dose, kGy
Response value
Response value
200
Risø B3 calibration
200
140 Ref. B. (25 kGy)
Residual, %
120
Risø B3 calibration, residual
3 2 1 0 -1 -2 -3 -4
100
Ref. A. (12 kGy)
80 0
0
20
40
60
Dose, kGy
Fig. 1. Calibration function and residual for Risø B3 film dosimeters using HP Scanjet 5400C flatbed scanner.
made based on a polynomial function, and the user selects the order of the polynomial based on analysis of the residuals, which are also displayed, see Fig. 1. (Sharpe and Miller, 1999). The selected calibration is stored along with information of the scanner settings used during recording of the image of the dosimeter films for the calibration.
4. Reference An office flatbed scanner is not an optical instrument, and even if it possible to use the same nominal settings of scanner for each recording of an image, then it is possible that, for example, heating of the lamp might lead to differences in the recorded images. As a control measure to limit these effects, a reference is recorded with each recording of a dosimeter. We have chosen as a reference Risø B3 dosimeters irradiated to 12, 25, and 50 kGy that are laminated for protection against humidity and other influences. The Risø B3 was chosen because it is coloured in the same way as the dosimeters to be measured. The reference is recorded during the measurement of the dosimeters for calibration, and the response values are stored together with the data of calibration. When measurements of unknown dosimeters are made, the reference is measured again, and the responses of the three reference dosimeters are compared with the
50
100 150 Scan number
200
250
Fig. 2. Stability of reference measured with HP Scanjet 5400C scanner. A reference colour chart consisting of three irradiated B3 dosimeters has been recorded for every scan. The graph shows the response value for each of the three reference dosimeters over a period of 17 months (243 scans).
values obtained during calibration. Measured differences are used to correct the doses measured by the unknown dosimeter. During more than one year’s test use at Risø of RisøScan the observed differences between the initial response values of the reference and the ones recorded during measurements have not exceeded 72% (see Fig. 2) and the standard deviation was less then 1% (1 s.d.). This represents the combined variation of reference and scanner. No drift trend was observed within this uncertainty. Measurement of the reference also serves to detect if scanner parameters, different from the ones used during the calibration, have wrongly been selected for measurement, and it serves to verify that RisøScan has functioned as intended during the measurement.
5. Measurement When a measurement of an unknown dosimeter film is made, the relevant calibration and its associated reference must be chosen, and corrections of the doses must be made if needed in accordance with the measured reference values. Dose profiles for the irradiated dosimeter films can be obtained along an X- and a Yaxis using the ‘‘Surface profile’’ option. The axis can be placed anywhere on the scanned image, or it can be placed automatically by RisøScan at the maximum or
ARTICLE IN PRESS J. Helt-Hansen, A. Miller / Radiation Physics and Chemistry 71 (2004) 359–362
361
Fig. 3. Measurement example showing a dose profile of a beam spot.
the X- and Y-axis. This option is useful for identification and characterization of dose gradients (see Fig. 4).
6. Energy determination
Fig. 4. 3-D dose measurement example of a beam spot.
the minimum dose value for the area selected for measurement (see Fig. 3). The spatial resolution of dose measurement is initially set at the width of one pixel. Wider widths can be chosen in order to reduce signal noise, but at the expense of measurement resolution. This option has therefore to be used with care in order not to invalidate the dose measurement. The measured dose value, its standard deviation (if more than one pixel width is chosen) and its location are recorded. The ‘‘3D Surface Profile’’ option presents a graph with dose at the Z-axis as a function of the position at
The depth–dose distribution in a solid is often used for determination of electron beam energy (ICRU, 1984). The measurement of the depth–dose curve can be made with a dosimeter film placed between two wedges of a solid, for example aluminium (ISO/ASTM, 2002b). The analysis of this dosimeter film can be made with the ‘‘Depth profile’’ option of RisøScan. Choosing this option requires that RisøScan is informed of the angle of the wedge. It is further necessary to be able to identify on the scanned image the starting point of the depth–dose curve. A straight line is fitted to the descending slope of the depth–dose curve, and its extrapolation is used to define the extrapolated depth. The 50% depth is also defined. These depths are used to calculate the electron beam energies by user-defined equations, usually taken from ICRU or ISO/ASTM (see Fig. 5). An option allows automatic definition of the depths and calculation of the energies.
7. Software validation Formal software validation has not yet been completed, but a validation tool is presently being evaluated.
ARTICLE IN PRESS 362
J. Helt-Hansen, A. Miller / Radiation Physics and Chemistry 71 (2004) 359–362
visibly, so that the coloured dosimeters can be recorded on a flatbed scanner. The use of simultaneous recording and measurement of a reference ensures the reproducibility of the dose measurement.
References
Fig. 5. Energy measurement using a depth–dose profile from an aluminium wedge. 50% depth is 1.29 cm and extrapolated range 1.68 cm. Using equations from ISO/ASTM (2002b) these depths correspond to an average energy of 8.01 MeV and the most probable energy of 8.75 MeV.
It is based on the measurement of computer-generated images, where the measurement results are known and tested against calculation by Excels. It is envisaged that running RisøScan with these images as input files will assure the user that RisøScan functions as intended.
8. Conclusion RisøScan is a useful tool for measurement of dose and dose distribution using thin dosimeter films that colour
Cai, Z., Pan, X., Hunting, D., Cloutier, P., Lemay, R., Sanche, L., 2003. Dosimetry of ultrasoft X-rays (1.5 keV Alka) using radiochromatic films and colour scanners. Phys. Med. Biol. 48, 4111–4124. ICRU, 1984. ICRU Report 35 (1984). Radiation dosimetry: electron beams with energies between 1 and 50 MeV. International Commission on Radiation Units and Measurements. 7910 Woodmont Avenue, Bethesda, MD 20814, USA. ISO/ASTM, 2000a. ISO/ASTM 51649:2002(E). Standard practice for dosimetry in an electron beam facility for radiation processing at energies between 300 keV and 25 MeV. ASTM International. 100 Barr Harbor Drive, P.O. Box C700, West Conshohocken, PA 19428-2959, USA. ISO/ASTM, 2000b. ISO/ASTM 51275:2002(E). Standard practice for use of a radiochromic film dosimetry system. ASTM International. 100 Barr Harbor Drive, P.O. Box C700, West Conshohocken, PA 19428-2959, USA. Miller, A., Hargittai, P., Kovacs, A., 2000. A PC based thin film dosimeter system. Radiat. Phys. Chem. 57, 679–685. Sharpe, P., Miller, A., 1999. Guidelines for the calibration of dosimeters for use in radiation processing. NPL Report CIRM 29. National Physical Laboratory, Teddington, TW11 0LW, UK. Stevens, M.A., Turner, J.R., Hugtenburg, R.P., Butler, P.H., 1996. High-resolution dosimetry using radiochromic film and a document scanner. Phys. Med. Biol. 41, 2357–2365.
Radiation Physics and Chemistry 71 (2004) 359–362
RisøScan—a new dosimetry software Jakob Helt-Hansen*, Arne Miller Radiation Research Department, Risø National Laboratory, P.O. Box 49, Roskilde DK-4000, Denmark
Abstract RisøScan is a software package that is used for analysis of images of visibly coloured dosimeter films. The image is created by scanning the dosimeter film on a flatbed scanner. RisøScan is based on LabViews, and it is useful for analysis of dose distributions and depth dose curves. Measurement reproducibility is maintained in the analysis of dosimeter films, and measurement uncertainties (Type A) in the order of 3–5% (1 s.d.) are obtained depending on the scanner and the dosimeter film used. r 2004 Elsevier Ltd. All rights reserved. Keywords: Dosimetry; Dosimeters; Film; Scanner; Scanning; Software
1. Introduction Measurement of dose distribution in radiation processing is used in installation qualification, operational qualification and performance qualification. In particular, for electron-beam irradiation thin film dosimeters are used for these purposes, and most of these (GAF, FWT, Risø B3) colour visibly when irradiated (ISO/ ASTM, 2002a). These coloured dosimeter films can be scanned on an optical flatbed scanner, and an image file created with information about the degree of colouration. A high spatial resolution of the scanned image can be obtained, but a compromise between file size and resolution has to be obtained. The use of flatbed scanners for the measurement of dosimeter films has been proposed earlier, both for radiation processing dosimetry (Miller et al., 2000) and for medical dosimetry (Stevens et al., 1996; Cai et al., 2003); and a software package for analysis for scanned images (Scanalizer) was made by AECL (Atomic Energy of Canada, Ltd.). RisøScan version 1.0 builds on these
*Corresponding author. Tel.: +45-4677-4908; fax: +454677-4959. E-mail address: [email protected] (J. Helt-Hansen).
earlier works. The software package is made using LabViews version 6.1 from National Instruments.
2. Colour balance The measurements described in this paper are made with Risø B3 dosimeters. The scanner used was a HP 5400C. It scans in three colours; red, green and blue. The signal content in each colour channel depends on the absorption of the irradiated dosimeter film, and for Risø B3 only the green channel contains the information, while the two other channels only add to signal noise. RisøScan allows choosing a fraction of each colour channel, and in this case 100% green and 0% red and blue are chosen. Other dosimeter films may require a different RGB balance.
3. Calibration Calibration is obtained by measurement of dosimeter films irradiated to known doses. Each dosimeter is measured over its entire area, thereby effectively providing many individual measurements. The greyscale value and the corresponding dose are entered into a table, and displayed by RisøScan as a graph. A fit is
0969-806X/$ - see front matter r 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2004.03.033
ARTICLE IN PRESS J. Helt-Hansen, A. Miller / Radiation Physics and Chemistry 71 (2004) 359–362
360
150
180
100
Ref. C. (50 kGy)
160
50 0 0
20
40
60
Dose, kGy
Response value
Response value
200
Risø B3 calibration
200
140 Ref. B. (25 kGy)
Residual, %
120
Risø B3 calibration, residual
3 2 1 0 -1 -2 -3 -4
100
Ref. A. (12 kGy)
80 0
0
20
40
60
Dose, kGy
Fig. 1. Calibration function and residual for Risø B3 film dosimeters using HP Scanjet 5400C flatbed scanner.
made based on a polynomial function, and the user selects the order of the polynomial based on analysis of the residuals, which are also displayed, see Fig. 1. (Sharpe and Miller, 1999). The selected calibration is stored along with information of the scanner settings used during recording of the image of the dosimeter films for the calibration.
4. Reference An office flatbed scanner is not an optical instrument, and even if it possible to use the same nominal settings of scanner for each recording of an image, then it is possible that, for example, heating of the lamp might lead to differences in the recorded images. As a control measure to limit these effects, a reference is recorded with each recording of a dosimeter. We have chosen as a reference Risø B3 dosimeters irradiated to 12, 25, and 50 kGy that are laminated for protection against humidity and other influences. The Risø B3 was chosen because it is coloured in the same way as the dosimeters to be measured. The reference is recorded during the measurement of the dosimeters for calibration, and the response values are stored together with the data of calibration. When measurements of unknown dosimeters are made, the reference is measured again, and the responses of the three reference dosimeters are compared with the
50
100 150 Scan number
200
250
Fig. 2. Stability of reference measured with HP Scanjet 5400C scanner. A reference colour chart consisting of three irradiated B3 dosimeters has been recorded for every scan. The graph shows the response value for each of the three reference dosimeters over a period of 17 months (243 scans).
values obtained during calibration. Measured differences are used to correct the doses measured by the unknown dosimeter. During more than one year’s test use at Risø of RisøScan the observed differences between the initial response values of the reference and the ones recorded during measurements have not exceeded 72% (see Fig. 2) and the standard deviation was less then 1% (1 s.d.). This represents the combined variation of reference and scanner. No drift trend was observed within this uncertainty. Measurement of the reference also serves to detect if scanner parameters, different from the ones used during the calibration, have wrongly been selected for measurement, and it serves to verify that RisøScan has functioned as intended during the measurement.
5. Measurement When a measurement of an unknown dosimeter film is made, the relevant calibration and its associated reference must be chosen, and corrections of the doses must be made if needed in accordance with the measured reference values. Dose profiles for the irradiated dosimeter films can be obtained along an X- and a Yaxis using the ‘‘Surface profile’’ option. The axis can be placed anywhere on the scanned image, or it can be placed automatically by RisøScan at the maximum or
ARTICLE IN PRESS J. Helt-Hansen, A. Miller / Radiation Physics and Chemistry 71 (2004) 359–362
361
Fig. 3. Measurement example showing a dose profile of a beam spot.
the X- and Y-axis. This option is useful for identification and characterization of dose gradients (see Fig. 4).
6. Energy determination
Fig. 4. 3-D dose measurement example of a beam spot.
the minimum dose value for the area selected for measurement (see Fig. 3). The spatial resolution of dose measurement is initially set at the width of one pixel. Wider widths can be chosen in order to reduce signal noise, but at the expense of measurement resolution. This option has therefore to be used with care in order not to invalidate the dose measurement. The measured dose value, its standard deviation (if more than one pixel width is chosen) and its location are recorded. The ‘‘3D Surface Profile’’ option presents a graph with dose at the Z-axis as a function of the position at
The depth–dose distribution in a solid is often used for determination of electron beam energy (ICRU, 1984). The measurement of the depth–dose curve can be made with a dosimeter film placed between two wedges of a solid, for example aluminium (ISO/ASTM, 2002b). The analysis of this dosimeter film can be made with the ‘‘Depth profile’’ option of RisøScan. Choosing this option requires that RisøScan is informed of the angle of the wedge. It is further necessary to be able to identify on the scanned image the starting point of the depth–dose curve. A straight line is fitted to the descending slope of the depth–dose curve, and its extrapolation is used to define the extrapolated depth. The 50% depth is also defined. These depths are used to calculate the electron beam energies by user-defined equations, usually taken from ICRU or ISO/ASTM (see Fig. 5). An option allows automatic definition of the depths and calculation of the energies.
7. Software validation Formal software validation has not yet been completed, but a validation tool is presently being evaluated.
ARTICLE IN PRESS 362
J. Helt-Hansen, A. Miller / Radiation Physics and Chemistry 71 (2004) 359–362
visibly, so that the coloured dosimeters can be recorded on a flatbed scanner. The use of simultaneous recording and measurement of a reference ensures the reproducibility of the dose measurement.
References
Fig. 5. Energy measurement using a depth–dose profile from an aluminium wedge. 50% depth is 1.29 cm and extrapolated range 1.68 cm. Using equations from ISO/ASTM (2002b) these depths correspond to an average energy of 8.01 MeV and the most probable energy of 8.75 MeV.
It is based on the measurement of computer-generated images, where the measurement results are known and tested against calculation by Excels. It is envisaged that running RisøScan with these images as input files will assure the user that RisøScan functions as intended.
8. Conclusion RisøScan is a useful tool for measurement of dose and dose distribution using thin dosimeter films that colour
Cai, Z., Pan, X., Hunting, D., Cloutier, P., Lemay, R., Sanche, L., 2003. Dosimetry of ultrasoft X-rays (1.5 keV Alka) using radiochromatic films and colour scanners. Phys. Med. Biol. 48, 4111–4124. ICRU, 1984. ICRU Report 35 (1984). Radiation dosimetry: electron beams with energies between 1 and 50 MeV. International Commission on Radiation Units and Measurements. 7910 Woodmont Avenue, Bethesda, MD 20814, USA. ISO/ASTM, 2000a. ISO/ASTM 51649:2002(E). Standard practice for dosimetry in an electron beam facility for radiation processing at energies between 300 keV and 25 MeV. ASTM International. 100 Barr Harbor Drive, P.O. Box C700, West Conshohocken, PA 19428-2959, USA. ISO/ASTM, 2000b. ISO/ASTM 51275:2002(E). Standard practice for use of a radiochromic film dosimetry system. ASTM International. 100 Barr Harbor Drive, P.O. Box C700, West Conshohocken, PA 19428-2959, USA. Miller, A., Hargittai, P., Kovacs, A., 2000. A PC based thin film dosimeter system. Radiat. Phys. Chem. 57, 679–685. Sharpe, P., Miller, A., 1999. Guidelines for the calibration of dosimeters for use in radiation processing. NPL Report CIRM 29. National Physical Laboratory, Teddington, TW11 0LW, UK. Stevens, M.A., Turner, J.R., Hugtenburg, R.P., Butler, P.H., 1996. High-resolution dosimetry using radiochromic film and a document scanner. Phys. Med. Biol. 41, 2357–2365.