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Assessment of glandular dose and image quality in mammography using computerised radiography employing a polymethylmetacrilate breast simulator
Radiation Measurements 46 (2011) 2081e2085
Contents lists available at ScienceDirect
Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas
Assessment of glandular dose and image quality in mammography using computerised radiography employing a polymethylmetacrilate breast simulator Marcio Alves Oliveira*, Marcelino Vicente A. Dantas, Priscila C. Santana, Peterson L. Squair, Danielle S. Gomes, Maria S. Nogueira Cidade Universitária e Pampulha, Belo Horizonte, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 November 2010 Accepted 6 June 2011
In Brazil there are around 600 mammography equipment with CR system. Taking into account the quality of image is essential to evaluate the mean glandular dose so you can optimize the radiation protection of patients evaluated with this type of system. Therefore, this study aimed to determine the mean glandular dose and quality of image for the CR system of laboratory of radioprotection applied to mammography of the Centre of Development of Nuclear Technology (CDTN). For this, we evaluated the linearity of the detector’s response, contrast to noise ratio and signal to noise ratio, which, according to European protocol, showed results within acceptable limits. Next, evaluated the quality of image with the CDMAM Phantom and mean glandular dose to the detector Fluke Biomedical TNT 12000WD, where they presented, respectively, within the results expected by the manufacturer and the limits of acceptable and desirable by the European protocol. then, the CR system of CDTN is optimized. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Mammography Computerised radiography (CR) Quality control Dosimetry
1. Introduction Breast cancer nowadays is one of the main causes of death amongst women throughout the world WHO, 2009. Many times they are not diagnosed in an early stage; therefore, mammography arises as an important tool in the control of the disease, being able through routine tests to detect possible tumour nodules in an early stage. However, the test calls for certain attention, for dissatisfactory image quality may lead to wrong diagnosis such as false positive or false negatives, giving rise to unnecessary biopsies, undetected cancers or delays in detection. Another factor to be considered is that all tracking programmes must be justified and optimised in terms of radiological protection BRAZIL, 1998, they should also provide benefits in terms of public health, mainly with regard to the number of cancers detected and induced by mammography. Therefore, indication of benefits as well as the number of lives saved by a tracing programme must be assessed, so that the risks of induced fatal cancers can be reduced NHS Cancer Screening Programmes, 2003. In recent years, conventional film/ecran mammography is being replaced by digital procedures, therefore, this new technology calls for new methods of optimisation WHO, 2009,which includes the possibility of an increase in the dose and the effect it has in the
* Corresponding author. Tel.: þ55 31 88591348. E-mail address: [email protected] (M.A. Oliveira). 1350-4487/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2011.06.015
image processing, so that the risks induced by this new procedure can be estimated. However, when the monitoring of image quality is taken into account, it is essential to evaluate the mean glandular dose (MGD) if we wish to optimise the protection of the patients undergoing digital mammography. The present work aims at determining the mean glandular dose and the image quality in computerised radiography systems (CR) NHS Breast Screening Programmes, 2009, Del Rio et al., 2002, European Commission, 2006. 2. Materials and methods The assessment of the image and of the technical parameters in the determining the MGD was made in the laboratory of radioprotection applied to mammography of the Centre of Development of Nuclear Technology (CDTN), which holds a Siemens Mammomat 3000 Nova mammographer and a Kodak Direct View CR850 digitiser. 2.1. Quality of the CR/mammography system In order to guarantee the minimum possible integration quality between the mammography equipment and the digitiser, the following parameters were assessed: 2.1.1. Linearity of the detector’s response Using 4 PMPA plates close to the X- Ray tube output, being each 1 cm thick (Fig 1a), the Kerma in the entrance surface air (k1) was
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M.A. Oliveira et al. / Radiation Measurements 46 (2011) 2081e2085
Fig. 1. (a) Kerma’s measurement Geometry. (b) Fluke Biomedical TNT 12000WD Detector and Display.
measured with the Fluke Biomedical TNT 12000WD solid state detector (Fig 1b), for a set voltage of 28 kV, combination target/filter in Mo/Mo and loads of 4,8,16,25,32,45,63,100 and 140mAs. 2.1.2. Contrast to noise ratio (CNR) The CNR was achieved through the exposure of PMPA plates whose thickness varied from 2 to 7 cm using the automatic exposure control (AEC) The PMMA plates from 2 to 7 cm thick were positioned on the breast support and in order to produce a contrast area, an Fig. 2. position of the aluminium plate in the CNR measurement.
Fig. 3. CDMAM Phantom.
M.A. Oliveira et al. / Radiation Measurements 46 (2011) 2081e2085
2083
aluminium foil 0.2 cm thick, measuring 2 2 cm, was placed 6 cm far from the thoracic wall with one of its sides resting on the central line of the field (Fig. 2). The exposure was done in the semi- automatic mode and the image was registered as raw data. With the aid of the Image J software, two regions of interest (ROI) were drawn in the images the ROI 1 in the region outside the 0.2 mm Al foil and ROI 2 inside the Al foil region. The (MPV) mean pixel value and the standard deviation (o) in each of the ROIs was calculated. Therefore, the value of the CNR for that PMMA thickness was calculated according to the following formula European Commission, 2005; Dance, 1990:
pffiffiffi MPV1 MPV2 exp a 2exp a a CNR ¼ sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2ffi MPV1 MPV2 2 2 s1 exp s2 exp a a
(1)
where: MPV1 ¼ mean pixel value outside the Al foil. MPV2 ¼ mean pixel value inside the Al foil. s1 ¼ standard deviation outside the al foil. s2 ¼ standard deviation inside the aluminium foil. a ¼ angular coefficient of the line obtained in the detector linearity response test. Thus, the value of the CNR was calculated for all the thickness of the PMMA up to a thickness of 7 cm. The assessment of the results was made through the relative CNR (Equation 2) which relates the values of values of the CNR in different thickness with the values of standard thickness CNR e 5 cm.
CNRrel ¼
CNRn 100ð%Þ CNR5
(2)
2.1.3. Signal to noise ratio (SNR) The assessment of the SNR was made through the images obtained in the different areas of the PMMA plate in the CNR test. The results of the MPVs and of the in the ROI 1, were used to calculate the SNR for each area of the PMMA using Equation 3.
MPV
(3)
s
2.2. Quality of the image By utilising a technique of 28 kV in the semi-automatic mode, an exposure of the Contrast-Detail Phantom Artinis CDMAM type 3.4 simulator was made - The simulator has an aluminium base with gold discs of various thickness and diameters (Fig. 3). Right afterwards, in accordance with the specification of the manufacturer, the image of the CDMAM Phantom was assessed visually in a PLANAR GX5MP monitor and then automatized through the “cdcom.exe” software. 2.3. Mean glandular dose (MGD) Firstly, with the use of the Fluke Biomedical TNT 12000WD, the performance of the (Y) and the semi reducer layer were determined. Then, utilising the radiographic techniques for each one of the thickness - from 1 to 7 cm, the values of the MGD were determined according to Equation 4.
Where: CNRrel ¼ value of the relative CNR. CNRn ¼ value of the CNR of the n thickness of the PMMA. CNR5 ¼ value of the CNR for a PMMA 5 cm thick.
SNR ¼
Fig. 4. Linearization of the response of the detector in function of the linearized MPVand Ki.
Having the values of the SNR for each thickness of the PMMA plate (2e7 cm), allowed the calculation of the average of these values (SNRm). The SNR was estimated by measuring the variation of each value in relation to the average value.
MGD ¼ c$s$Y$PIT $
dref 2 d
(4)
Where C ¼ conversion coefficient of Ki for MGD Dance, 1990; Dance et al., 2000. s ¼ correction factor, factor which depends on the anode/filter combination. Y ¼ performance at 1.0 m from the focal point in mGy/mAs. PIT ¼ load in mAs. dref ¼ reference distance e 1.0 m. d ¼ focus-simulator distance.
Table 1 Values of CNR for thickness ranging from 2 to 7 cm. Thickness of the PMMA [mm]
kV target/Filter
mAs
Value of the pixel in the ROI 1 Average
s
Average
s
20 30 40 50 60 70
26 27 28 30 32 32
22.6 44.4 77.8 93.1 112 191
2311.6 2311.3 2312.5 2301.4 2302.8 2319.9
7.7 7.8 7.8 8.1 8.0 7.7
2212.0 2224.6 2229.3 2230.4 2229.0 2252.1
8.7 8.8 8.5 8.9 8.6 8.6
Mo/Mo Mo/Mo Mo/Mo Mo/Mo Mo/Mo Mo/Mo
Value of the pixel for 0.2 mm Al
CNR Measured
CNR Relative
Limit Value Relative (%)
12.16 10.46 10.20 8.36 8.89 8.34
145.4 125.2 122.0 100.0 106.3 99.7
>115 >110 >105 >100 >95 >90
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M.A. Oliveira et al. / Radiation Measurements 46 (2011) 2081e2085
Table 2 SNR values for thickness 2e7 cm. Thickness of PMMA (mm)
kV target/Filter
mAs
MPV in ROI 1
s
SNR measured
Percentage variation of SNR (%)
Limit value for the percentage variation of the SNR (%)
20 30 40 50 60 70
26 27 28 30 32 32
22.6 44.4 77.8 93.1 112 191
2311.6 2311.3 2312.5 2301.4 2302.8 2319.9
7.7 7.8 7.8 8.1 8.0 7.7
55.20 54.49 54.49 52.47 53.13 55.20
1.91 0.60 0.60 3.12 1.91 1.91
10%
Mo/Mo Mo/Mo Mo/Mo Mo/Mo Mo/Mo Mo/Mo
3.3. Signal-noise ratio (SNR)
Table 3 automatic and visual assessment of the CDMAM image. Diameter (mm) Threshold of the gold thickness (mm) Visual Assessment Automatic Assessment Acceptable Value 0.10 0.25 0.50 1.00 2.00
2.00 0.36 0.16 0.08 0.06
2.00 0.16 0.10 0.03 0.05
2.00 0.36 0.16 0.10 0.06
Table 2 the measured SNR values, percentage variation, and the acceptable limit for all exposures. All the values of the SNR percentage variation are below 10%, meaning that they conform to the limits adopted in Europe European Commission, 2005, EUREF, 2003. Taking into consideration the fact that the linearity tests of the detector‘s response, CNR and SNR are conform, one can conclude that the Automatic Exposure Control (AEC) is working under adequate calibration conditions.
3. Results and discussion 3.4. Quality of image 3.1. Linearity of the detector response Fig. 4 presents the line of tendency of the linearized MPV values obtained from images acquired from a 4 cm PMMA plate exposed to a fixed voltage of 28 kV (target filter in Mo/Mo) and loads of 4, 8, 16, 25, 32, 45, 63, 100 e 140mAs, in function of its respective Ki. Therefore, the response of the detector is linear, since the R2 value of the tendency line is greater than 0.99.
In accordance with the Phantom CDMAM manufacturer’s direction, visual and automatic assessments of the image produced by the exposure of the simulator in the semi- automatic mode at a fixed voltage of 28 kV were made. Table 3 presents the results of the assessment. All the values for both assessments were equal to or below the acceptable values, with the result that the image conforms to the manufacture’s specifications.
3.2. Contrast-noise ratio (CNR)
3.5. Mean glandular dose (MGD)
Table 1 shows, the values of measured, relative and limit CNR for all the thickness of the PMMA.
Fig. 5 presents acceptable, desired and measured limits observed at the CDTN laboratory with an uncertainty of 3.72%.
Fig. 5. Acceptable, desired and measured MGDs obtained at CDTN for thickness from 2 to 7 cm.
M.A. Oliveira et al. / Radiation Measurements 46 (2011) 2081e2085 Table 4 comparison between doses in different studies. MGD (mGy) Kodak 850 CR System (minemax) This studya Dantasb Coltinhoc NHSBSP, 2008,a NHSBSP, 2009a
2.73 0.10 2.20 0.14 (1.39e2.97) 1.74 0.52 (0.97e3.28) 2.29 2.7
a Values calculated from Ki for PMMA 5 cm thick (equivalent to a 6 cm thick breast) in laboratory. b Values calculated from Ki for PMMA 5 cm thick(equivalent to a 6 cm thick breast)in 14 different facilities Dantas et al., 2010. c Values measured with TLD in a 6 cm thick phantom 392 patients Coutinho, 2009.
Considering the uncertainty of the measurements, one can observe that the MGD values stand between the acceptable and desired limits. Table 4 presents a comparison amongst the MGD values met in this study and the doses found in other studies of the same digital systems. 4. Conclusions Taking into consideration the fact that that our CR mammographer/digitiser system obtained good results in the assessment of the linearization of the response of the detector, CNR, SNR and assessment of the image quality of the CDMAM Phantom - visual and automatized alike - one can conclude that the system is calibrated and optimised, for the MGD is below the acceptable limit
2085
and stands near the values presented in Table 4, which use the same methodology to determine the MGD for a PMMA 5 cm thick. Another important factor is that the AEC is working under adequate calibration condition, given that the linearization tests of the detector eCNR e SNR e proved conform. However, taking into consideration the quality of the image, the MGDs for all other thickness are also optimised. References BRAZIL, 1998. Radiological Protection Directives for Medical and Dental Radiodiagnostic [in Portuguese]. Coutinho, C.M.C., 2009. Evaluation of glandular dose in conventional and digital mammography system. UFRJ/COPPE, Rio de Janeiro, Brazil [in Portuguese]. Dance, R.D., 1990. Monte Carlo calculation of conversion factors for the estimation of mean glandular breast dose. Phys. Med. Biol 35, 1211e1219. Dance, R.D., Skinner, C.L., Young, K.C., Beckett, J.R., Kotre, C.J., 2000. Additional factors for the estimation of mean glandular dose using UK mammography protocol. Phys. Med. Biol. 45, 3225e3240. Dantas, M.V.A., et al., 2010. Glandular dose in mammography services with computed radiography systems in Belo Horizonte, Brazil. V Latin American Congress of Medical Physics, Cusco, Peru [in Portuguese]. Del Rio, M.C., Penco, P.M., Echevarne, J.J.M., Cabrera, R.T., 2002. Protocol quality control in digital mammography. Spanish Society of Medical Physics, 18e69. EUREF, 2003. Addendum on Digital Mammography to Chapter 3 of the European Guidelines for Quality Assurance in Mammography Screening Nijmegem, the Netherlands. European Commission, 2005. European Protocol for the Quality Control of the Physical and Technical Aspect for Mammography Screening European. European Commission, 2006. European Guidelines for Quality Assurance in Breast Cancer Screening and Diagnosis, Fourth Edition ISBN 92-79-01258-4. NHS Cancer Screening Programmes, 2003. Review of Risk in Breast Screening. No. 54. NHSBSP Publication, UK. NHS Breast Screening Programmes, 2008. Equipment Report 0803, Technical Evaluation of the GE Essential Full Field Digital Mammography Systems. NHS Breast Screening Programmes, 2009. Equipment Report 0604 Version 3, Commissioning and Routine Testing of Full Field Digital Mammography Systems. WHO, 2009. World Cancer Report, 2008. International Agency for Research on Cancer, Lyon.
Contents lists available at ScienceDirect
Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas
Assessment of glandular dose and image quality in mammography using computerised radiography employing a polymethylmetacrilate breast simulator Marcio Alves Oliveira*, Marcelino Vicente A. Dantas, Priscila C. Santana, Peterson L. Squair, Danielle S. Gomes, Maria S. Nogueira Cidade Universitária e Pampulha, Belo Horizonte, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history: Received 23 November 2010 Accepted 6 June 2011
In Brazil there are around 600 mammography equipment with CR system. Taking into account the quality of image is essential to evaluate the mean glandular dose so you can optimize the radiation protection of patients evaluated with this type of system. Therefore, this study aimed to determine the mean glandular dose and quality of image for the CR system of laboratory of radioprotection applied to mammography of the Centre of Development of Nuclear Technology (CDTN). For this, we evaluated the linearity of the detector’s response, contrast to noise ratio and signal to noise ratio, which, according to European protocol, showed results within acceptable limits. Next, evaluated the quality of image with the CDMAM Phantom and mean glandular dose to the detector Fluke Biomedical TNT 12000WD, where they presented, respectively, within the results expected by the manufacturer and the limits of acceptable and desirable by the European protocol. then, the CR system of CDTN is optimized. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Mammography Computerised radiography (CR) Quality control Dosimetry
1. Introduction Breast cancer nowadays is one of the main causes of death amongst women throughout the world WHO, 2009. Many times they are not diagnosed in an early stage; therefore, mammography arises as an important tool in the control of the disease, being able through routine tests to detect possible tumour nodules in an early stage. However, the test calls for certain attention, for dissatisfactory image quality may lead to wrong diagnosis such as false positive or false negatives, giving rise to unnecessary biopsies, undetected cancers or delays in detection. Another factor to be considered is that all tracking programmes must be justified and optimised in terms of radiological protection BRAZIL, 1998, they should also provide benefits in terms of public health, mainly with regard to the number of cancers detected and induced by mammography. Therefore, indication of benefits as well as the number of lives saved by a tracing programme must be assessed, so that the risks of induced fatal cancers can be reduced NHS Cancer Screening Programmes, 2003. In recent years, conventional film/ecran mammography is being replaced by digital procedures, therefore, this new technology calls for new methods of optimisation WHO, 2009,which includes the possibility of an increase in the dose and the effect it has in the
* Corresponding author. Tel.: þ55 31 88591348. E-mail address: [email protected] (M.A. Oliveira). 1350-4487/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2011.06.015
image processing, so that the risks induced by this new procedure can be estimated. However, when the monitoring of image quality is taken into account, it is essential to evaluate the mean glandular dose (MGD) if we wish to optimise the protection of the patients undergoing digital mammography. The present work aims at determining the mean glandular dose and the image quality in computerised radiography systems (CR) NHS Breast Screening Programmes, 2009, Del Rio et al., 2002, European Commission, 2006. 2. Materials and methods The assessment of the image and of the technical parameters in the determining the MGD was made in the laboratory of radioprotection applied to mammography of the Centre of Development of Nuclear Technology (CDTN), which holds a Siemens Mammomat 3000 Nova mammographer and a Kodak Direct View CR850 digitiser. 2.1. Quality of the CR/mammography system In order to guarantee the minimum possible integration quality between the mammography equipment and the digitiser, the following parameters were assessed: 2.1.1. Linearity of the detector’s response Using 4 PMPA plates close to the X- Ray tube output, being each 1 cm thick (Fig 1a), the Kerma in the entrance surface air (k1) was
2082
M.A. Oliveira et al. / Radiation Measurements 46 (2011) 2081e2085
Fig. 1. (a) Kerma’s measurement Geometry. (b) Fluke Biomedical TNT 12000WD Detector and Display.
measured with the Fluke Biomedical TNT 12000WD solid state detector (Fig 1b), for a set voltage of 28 kV, combination target/filter in Mo/Mo and loads of 4,8,16,25,32,45,63,100 and 140mAs. 2.1.2. Contrast to noise ratio (CNR) The CNR was achieved through the exposure of PMPA plates whose thickness varied from 2 to 7 cm using the automatic exposure control (AEC) The PMMA plates from 2 to 7 cm thick were positioned on the breast support and in order to produce a contrast area, an Fig. 2. position of the aluminium plate in the CNR measurement.
Fig. 3. CDMAM Phantom.
M.A. Oliveira et al. / Radiation Measurements 46 (2011) 2081e2085
2083
aluminium foil 0.2 cm thick, measuring 2 2 cm, was placed 6 cm far from the thoracic wall with one of its sides resting on the central line of the field (Fig. 2). The exposure was done in the semi- automatic mode and the image was registered as raw data. With the aid of the Image J software, two regions of interest (ROI) were drawn in the images the ROI 1 in the region outside the 0.2 mm Al foil and ROI 2 inside the Al foil region. The (MPV) mean pixel value and the standard deviation (o) in each of the ROIs was calculated. Therefore, the value of the CNR for that PMMA thickness was calculated according to the following formula European Commission, 2005; Dance, 1990:
pffiffiffi MPV1 MPV2 exp a 2exp a a CNR ¼ sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2ffi MPV1 MPV2 2 2 s1 exp s2 exp a a
(1)
where: MPV1 ¼ mean pixel value outside the Al foil. MPV2 ¼ mean pixel value inside the Al foil. s1 ¼ standard deviation outside the al foil. s2 ¼ standard deviation inside the aluminium foil. a ¼ angular coefficient of the line obtained in the detector linearity response test. Thus, the value of the CNR was calculated for all the thickness of the PMMA up to a thickness of 7 cm. The assessment of the results was made through the relative CNR (Equation 2) which relates the values of values of the CNR in different thickness with the values of standard thickness CNR e 5 cm.
CNRrel ¼
CNRn 100ð%Þ CNR5
(2)
2.1.3. Signal to noise ratio (SNR) The assessment of the SNR was made through the images obtained in the different areas of the PMMA plate in the CNR test. The results of the MPVs and of the in the ROI 1, were used to calculate the SNR for each area of the PMMA using Equation 3.
MPV
(3)
s
2.2. Quality of the image By utilising a technique of 28 kV in the semi-automatic mode, an exposure of the Contrast-Detail Phantom Artinis CDMAM type 3.4 simulator was made - The simulator has an aluminium base with gold discs of various thickness and diameters (Fig. 3). Right afterwards, in accordance with the specification of the manufacturer, the image of the CDMAM Phantom was assessed visually in a PLANAR GX5MP monitor and then automatized through the “cdcom.exe” software. 2.3. Mean glandular dose (MGD) Firstly, with the use of the Fluke Biomedical TNT 12000WD, the performance of the (Y) and the semi reducer layer were determined. Then, utilising the radiographic techniques for each one of the thickness - from 1 to 7 cm, the values of the MGD were determined according to Equation 4.
Where: CNRrel ¼ value of the relative CNR. CNRn ¼ value of the CNR of the n thickness of the PMMA. CNR5 ¼ value of the CNR for a PMMA 5 cm thick.
SNR ¼
Fig. 4. Linearization of the response of the detector in function of the linearized MPVand Ki.
Having the values of the SNR for each thickness of the PMMA plate (2e7 cm), allowed the calculation of the average of these values (SNRm). The SNR was estimated by measuring the variation of each value in relation to the average value.
MGD ¼ c$s$Y$PIT $
dref 2 d
(4)
Where C ¼ conversion coefficient of Ki for MGD Dance, 1990; Dance et al., 2000. s ¼ correction factor, factor which depends on the anode/filter combination. Y ¼ performance at 1.0 m from the focal point in mGy/mAs. PIT ¼ load in mAs. dref ¼ reference distance e 1.0 m. d ¼ focus-simulator distance.
Table 1 Values of CNR for thickness ranging from 2 to 7 cm. Thickness of the PMMA [mm]
kV target/Filter
mAs
Value of the pixel in the ROI 1 Average
s
Average
s
20 30 40 50 60 70
26 27 28 30 32 32
22.6 44.4 77.8 93.1 112 191
2311.6 2311.3 2312.5 2301.4 2302.8 2319.9
7.7 7.8 7.8 8.1 8.0 7.7
2212.0 2224.6 2229.3 2230.4 2229.0 2252.1
8.7 8.8 8.5 8.9 8.6 8.6
Mo/Mo Mo/Mo Mo/Mo Mo/Mo Mo/Mo Mo/Mo
Value of the pixel for 0.2 mm Al
CNR Measured
CNR Relative
Limit Value Relative (%)
12.16 10.46 10.20 8.36 8.89 8.34
145.4 125.2 122.0 100.0 106.3 99.7
>115 >110 >105 >100 >95 >90
2084
M.A. Oliveira et al. / Radiation Measurements 46 (2011) 2081e2085
Table 2 SNR values for thickness 2e7 cm. Thickness of PMMA (mm)
kV target/Filter
mAs
MPV in ROI 1
s
SNR measured
Percentage variation of SNR (%)
Limit value for the percentage variation of the SNR (%)
20 30 40 50 60 70
26 27 28 30 32 32
22.6 44.4 77.8 93.1 112 191
2311.6 2311.3 2312.5 2301.4 2302.8 2319.9
7.7 7.8 7.8 8.1 8.0 7.7
55.20 54.49 54.49 52.47 53.13 55.20
1.91 0.60 0.60 3.12 1.91 1.91
10%
Mo/Mo Mo/Mo Mo/Mo Mo/Mo Mo/Mo Mo/Mo
3.3. Signal-noise ratio (SNR)
Table 3 automatic and visual assessment of the CDMAM image. Diameter (mm) Threshold of the gold thickness (mm) Visual Assessment Automatic Assessment Acceptable Value 0.10 0.25 0.50 1.00 2.00
2.00 0.36 0.16 0.08 0.06
2.00 0.16 0.10 0.03 0.05
2.00 0.36 0.16 0.10 0.06
Table 2 the measured SNR values, percentage variation, and the acceptable limit for all exposures. All the values of the SNR percentage variation are below 10%, meaning that they conform to the limits adopted in Europe European Commission, 2005, EUREF, 2003. Taking into consideration the fact that the linearity tests of the detector‘s response, CNR and SNR are conform, one can conclude that the Automatic Exposure Control (AEC) is working under adequate calibration conditions.
3. Results and discussion 3.4. Quality of image 3.1. Linearity of the detector response Fig. 4 presents the line of tendency of the linearized MPV values obtained from images acquired from a 4 cm PMMA plate exposed to a fixed voltage of 28 kV (target filter in Mo/Mo) and loads of 4, 8, 16, 25, 32, 45, 63, 100 e 140mAs, in function of its respective Ki. Therefore, the response of the detector is linear, since the R2 value of the tendency line is greater than 0.99.
In accordance with the Phantom CDMAM manufacturer’s direction, visual and automatic assessments of the image produced by the exposure of the simulator in the semi- automatic mode at a fixed voltage of 28 kV were made. Table 3 presents the results of the assessment. All the values for both assessments were equal to or below the acceptable values, with the result that the image conforms to the manufacture’s specifications.
3.2. Contrast-noise ratio (CNR)
3.5. Mean glandular dose (MGD)
Table 1 shows, the values of measured, relative and limit CNR for all the thickness of the PMMA.
Fig. 5 presents acceptable, desired and measured limits observed at the CDTN laboratory with an uncertainty of 3.72%.
Fig. 5. Acceptable, desired and measured MGDs obtained at CDTN for thickness from 2 to 7 cm.
M.A. Oliveira et al. / Radiation Measurements 46 (2011) 2081e2085 Table 4 comparison between doses in different studies. MGD (mGy) Kodak 850 CR System (minemax) This studya Dantasb Coltinhoc NHSBSP, 2008,a NHSBSP, 2009a
2.73 0.10 2.20 0.14 (1.39e2.97) 1.74 0.52 (0.97e3.28) 2.29 2.7
a Values calculated from Ki for PMMA 5 cm thick (equivalent to a 6 cm thick breast) in laboratory. b Values calculated from Ki for PMMA 5 cm thick(equivalent to a 6 cm thick breast)in 14 different facilities Dantas et al., 2010. c Values measured with TLD in a 6 cm thick phantom 392 patients Coutinho, 2009.
Considering the uncertainty of the measurements, one can observe that the MGD values stand between the acceptable and desired limits. Table 4 presents a comparison amongst the MGD values met in this study and the doses found in other studies of the same digital systems. 4. Conclusions Taking into consideration the fact that that our CR mammographer/digitiser system obtained good results in the assessment of the linearization of the response of the detector, CNR, SNR and assessment of the image quality of the CDMAM Phantom - visual and automatized alike - one can conclude that the system is calibrated and optimised, for the MGD is below the acceptable limit
2085
and stands near the values presented in Table 4, which use the same methodology to determine the MGD for a PMMA 5 cm thick. Another important factor is that the AEC is working under adequate calibration condition, given that the linearization tests of the detector eCNR e SNR e proved conform. However, taking into consideration the quality of the image, the MGDs for all other thickness are also optimised. References BRAZIL, 1998. Radiological Protection Directives for Medical and Dental Radiodiagnostic [in Portuguese]. Coutinho, C.M.C., 2009. Evaluation of glandular dose in conventional and digital mammography system. UFRJ/COPPE, Rio de Janeiro, Brazil [in Portuguese]. Dance, R.D., 1990. Monte Carlo calculation of conversion factors for the estimation of mean glandular breast dose. Phys. Med. Biol 35, 1211e1219. Dance, R.D., Skinner, C.L., Young, K.C., Beckett, J.R., Kotre, C.J., 2000. Additional factors for the estimation of mean glandular dose using UK mammography protocol. Phys. Med. Biol. 45, 3225e3240. Dantas, M.V.A., et al., 2010. Glandular dose in mammography services with computed radiography systems in Belo Horizonte, Brazil. V Latin American Congress of Medical Physics, Cusco, Peru [in Portuguese]. Del Rio, M.C., Penco, P.M., Echevarne, J.J.M., Cabrera, R.T., 2002. Protocol quality control in digital mammography. Spanish Society of Medical Physics, 18e69. EUREF, 2003. Addendum on Digital Mammography to Chapter 3 of the European Guidelines for Quality Assurance in Mammography Screening Nijmegem, the Netherlands. European Commission, 2005. European Protocol for the Quality Control of the Physical and Technical Aspect for Mammography Screening European. European Commission, 2006. European Guidelines for Quality Assurance in Breast Cancer Screening and Diagnosis, Fourth Edition ISBN 92-79-01258-4. NHS Cancer Screening Programmes, 2003. Review of Risk in Breast Screening. No. 54. NHSBSP Publication, UK. NHS Breast Screening Programmes, 2008. Equipment Report 0803, Technical Evaluation of the GE Essential Full Field Digital Mammography Systems. NHS Breast Screening Programmes, 2009. Equipment Report 0604 Version 3, Commissioning and Routine Testing of Full Field Digital Mammography Systems. WHO, 2009. World Cancer Report, 2008. International Agency for Research on Cancer, Lyon.