Technical evaluation of Philips Microdose SI digital ...

1 DRAFT Technical evaluation of Philips Microdose SI digital mammography system NHSBSP Equipment Report 1310 August 2013 Philips Microdose SI digital mammography system (needs final edit) 2 About the NHS Cancer Screening Programmes The national office of the NHS Cancer Screening Programmes is operated by Public Health England. Its role is to provide national management, coordination, and quality assurance of the three cancer screening programmes for breast, cervical, and bowel cancer. About Public Health England Public Health England s mission is to protect and improve the nation s health and to address inequalities through working with national and local government, the NHS, industry and the voluntary and community sector. PHE is an operationally autonomous executive agency of the Department of Health. Lead authors: C Strudley KC Young All of the National Coordinating Centre for the Physics of mammography , Guildford Crown copyright 2013 PHE gateway number: You may re-use this information (excluding logos) free of charge in any format or medium, under the terms of the Open Government Licence To view this licence, visit OGL or email Where we have identified any third party copyright information you will need to obtain permission from the copyright holders concerned.

2 Any enquiries regarding this publication should be sent to Sarah Sellars: Philips Microdose SI digital mammography system (needs final edit) 3 Document lnformation Title Technical evaluation of Philips Microdose SI digital mammography system Policy/document type Equipment Report 1310 Electronic publication date Version 1 Superseded publications None Review date None Author/s C Strudley KC Young Owner Comments may be sent to Ken Young Document objective (clinical/healthcare/social questions covered) To provide an evaluation of this equipment s suitability for use within the NHSBSP Population affected Women eligible for routine and higher-risk breast screening Target audience QA Radiographers, Physicists Philips Microdose SI digital mammography system (needs final edit) 4 Contents Contents 4 Acknowledgements 5 Executive Summary 5 1. Introduction 6 Testing procedures and performance standards for digital mammography 6 Objectives 6 2.

3 Methods 7 System tested 7 Output and half-value-layer (HVL) 9 Detector response 9 Dose measurement 9 Contrast to noise ratio 9 AEC performance for local dense areas 11 Noise analysis 12 Image quality measurements 13 3. Results 16 Output and HVL 16 Detector response 16 AEC performance 17 Noise measurements 21 Image quality measurements 23 Comparison with other systems 27 4. Discussion 31 5. Conclusions 32 6. Manufacturer s comments 32 7. References 33 Philips Microdose SI digital mammography system (needs final edit) 5 Acknowledgements The authors are grateful to the staff at the breast unit at Addenbrooke s Hospital, Cambridge, for their cooperation in the evaluation of the system at their site Executive Summary The purpose of the evaluation was to determine whether the Philips Microdose SI breast imaging system meets the main standards in the NHSBSP and European protocols, and to provide performance data for comparison against other products.

4 The spectral imaging capability of this model was not implemented at the time of testing but is expected to be available as an optional upgrade. The system exceeded the minimum standards in the NHSBSP and European protocols and showed an improvement in image quality compared to our previous measurements on the Microdose L30 model. This model has two collimators to enable larger breasts to be imaged. The use of the high collimator for larger breasts produced images of similar quality to those produced using the low collimator. As with earlier models the fact that one cannot give higher doses for the larger breasts limits image quality to close to the minimum rather than the achievable level for large breasts. Philips Microdose SI digital mammography system (needs final edit) 6 1. Introduction Testing procedures and performance standards for digital mammography This report is one of a series evaluating commercially available digital mammography systems on behalf of the NHS Breast Screening Programme (NHSBSP).

5 The testing methods and standards applied are mainly derived from NHSBSP Equipment Report 06041 and are referred to in this document as the NHSBSP protocol . The standards for image quality and dose are the same as those provided in the European protocol,2,3 but the latter has been followed where it provides a more detailed performance standard: for example, for the automatic exposure control (AEC) system. Objectives The purpose of these tests was to produce a report on the Microdose SI breast imaging system. In particular we wanted to compare the system s performance with the previous model the Microdose L30. The new detector system used in the SI is the L50 and differs from the L30 in that it is designed to permit spectral imaging. However this is an aspect that was not evaluated here as it is not routinely available yet. The new model also differs from the L30 in that it has two types of collimator referred to here as high and low.

6 The low collimator is similar to that used in the L30. The high collimator allows the system to image breasts with greater thicknesses than was possible with the L30 ( greater than 100mm). A key question addressed is how changing the collimator affects the Technical performance in terms of dose and image quality. Philips Microdose SI digital mammography system (needs final edit) 7 2. Methods System tested The tests were conducted at the Breast Unit at Addenbrooke s Hospital in Cambridge, on a Philips Microdose SI system as described in Table 1. The system was equipped with two AEC settings called Automatic and Smart AEC and defaulted to the Smart AEC for every exposure. The Smart AEC selects the tube voltage and a target signal-to noise (SNR) based on the measured compression height. The scan velocity is then adjusted during the scan based on the measured detector signal, in order to give the proper exposure for any breast density.

7 In Automatic mode, there is no feedback during the scan, and the scan velocity is constant and based on the expected breast density and transmission for the measured compression height. The Smart AEC is similar to that on the earlier L30 model. However on the system tested there was only one dose level available which was comparable to the higher dose level on the L30 ( C120). The Smart AEC varies the scan speed according to the attenuation of the breast being imaged. The manufacturer intends that the high collimator only be used for large breasts with a thickness above 100mm. Thus the low collimator will be used for almost all exposures. Philips Microdose SI digital mammography system (needs final edit) 8 Table 1 System Description Manufacturer Philips Model Microdose SI System serial number 800369-10 Target material Tungsten Added filtration 500 m Aluminium Detector type L50 photon counting silicon detector Detector serial number 115654-10 Pixel size 50 m (at table surface) Detector area x mm Pixel array 4915 x 5355 Pixel value offset 0 Source to detector distance 660 mm Source to table distance mm AEC modes Smart AEC, Automatic Software version P1\ (457)\ (5916)\CCS Version (5876) Philips Microdose SI digital mammography system (needs final edit) 9 Output and half-value-layer (HVL) The output and HVL were measured as described in the NHSBSP protocol, at intervals of 3 kV for each target/filter combination.

8 Detector response The attenuator used was 2 mm aluminium placed on the raised paddle. This is a suitable alternative to 45 mm polymethyl-methacrylate (PMMA) at the tube head, which is normally used in measurements following the NHSBSP protocol. Except for the different attenuator, the detector response was measured as described in the NHSBSP protocol. An ion chamber was positioned above the table, 4 cm from the chest wall edge, to determine the incident air kerma at the detector surface for a range of manually set mAs values at 32 kV. The readings were corrected to the surface of the detector using the inverse square law. No correction was made for attenuation by the table and detector cover. Images acquired at the same mAs values were saved as unprocessed files and transferred to another computer for analysis. A 10 mm square region of interest (ROI) was positioned on the midline, 4 cm from the chest wall edge of each image.

9 The average pixel value and the standard deviation of pixel values within that region were measured. The relationship between average pixel values and the detector entrance surface air kerma was determined. Dose measurement Doses were measured using the X-ray set s automatic exposure control using the automatic and Smart AEC modes to expose different thicknesses of PMMA. The PMMA blocks had an area of 18 x 24 cm. The paddle height was adjusted to be equal to the equivalent breast thickness. For convenience the aluminium square required for the CNR measurements was included with the PMMA as described in section It is expected that the measured dose will be unaffected by the presence of the aluminium in Automatic mode but may increase in Smart AEC mode. The AEC settings (Phantom, PMMA20, PMMA30, etc) were used as provided by the system, so as to give the appropriate exposures corresponding to those for breasts of the equivalent thickness.

10 While these settings ensure that the thickness is for the equivalent breast, Smart AEC is still active and the mAs is selected on the basis of transmission. Mean glandular doses (MGDs) were calculated for the equivalent breast thicknesses for all the exposures Contrast to noise ratio To measure the contrast-to-noise ratio (CNR), an aluminium square, 10 mm x 10 mm and mm thick, was placed between two 10 mm thick blocks, with one edge on the Philips Microdose SI digital mammography system (needs final edit) 10 midline, 6 cm from the chest wall edge. Additional layers of PMMA were placed on top of these to vary the total thickness. Exposures were made at each thickness using both the Smart AEC and Automatic modes The images were analysed to obtain the CNRs. Twenty small square ROIs (approximately mm x mm) were used to determine the average signal and the standard deviation in the signal within the image of the aluminium square (4 ROI) and the surrounding background (16 ROI), as shown in Figure 1.