Application of the draft EUREF protocol for Quality Control of digital

BHPA 2014 abstract submission form Preceding information Page I. Author information Name First Name Institution Name Institution Address e-‐mail Marshall Nicholas Medical Physics, UZ Leuven Radiology, UZ Gasthuisberg, Herestraat 49, 3000 Leuven [email protected] II. Abstract information Preferred type of presentation Oral High level poster Please select one or more categories Y N N Radiotherapy Radiology Nuclear Medicine Medical Imaging Dosimetry and radiobiology Professional matters Other Please list up to five keywords Mammography, Digital Breast Tomosynthesis Eligible for young physicist award (< 28 yrs) N Y N Y N N N Submit to submission.bhpa2014-‐[email protected] before November 15th 2013 Application of the draft EUREF protocol for Quality Control of digital breast tomosynthesis (DBT) systems N.W. Marshall1, H. Bosmans1 1
Medical Physics, UZ Leuven, Campus Gasthuisberg, Herestraat 49, 3000 Leuven I. Introduction and purpose Digital breast tomosynthesis (DBT) is a technique in which a series of planes through the breast are reconstructed parallel to the detector, from a set of projection images acquired over a limited angular range. The DBT systems currently available and in clinical use are able to perform standard planar (2D) mammography with/without an antiscatter grid and DBT acquisitions. Guidance on testing DBT systems is available in the form of a draft protocol from the EUREF group and hence the aim of this work was to apply this protocol to three different DBT systems. II. Material and methods The three systems assessed were the Siemens Inspiration with tomosynthesis option, a Hologic Selenia Dimensions and a GE Essential with the additional Mammo Tomo Device (MTD) required for DBT. Draft version 0.10 of the EUREF protocol was applied in which tests are divided into the following sections: X-‐ray generation, Automatic Exposure Control (AEC) system, image receptor, image quality of the reconstructed image and dosimetry. The protocol specifies a 0° DBT mode in which projection images can be acquired without the x-‐ray tube moving along the angular arc – this is available on the Hologic and GE systems but is not available on the Siemens unit. The protocol also requires access to both the projection data (as ‘FOR PROCESSING’ DICOM images) and the reconstructed planes, also as a set of DICOM images. Currently, the Siemens and GE systems provide the projections and reconstructed planes while the Hologic just provides the reconstructed planes – proprietary software (‘mview.exe’) from Hologic is required in order to unpack the projection images. An RTI Barracuda MPD was used to measure tube voltage accuracy and x-‐ray tube output. The X-‐ray generator measurements for the Hologic and GE units were acquired using 0° DBT mode as this avoids the influence of the dosemeter angular acceptance window on the measurements. Given the lack of the 0° mode for the Siemens, the x-‐ray tube and generator was tested in 2D planar mode – this is possible as the Siemens uses the same target/filter (T/F) (W/Rh) combination for DBT and planar acquisitions. The AEC was tested using standard 10 mm thick plates, covering a total thickness ranging from 20 mm to 70 mm; spacers were used to set the equivalent breast thickness. A 0.2 mm thick Al square of size 10 x 10 mm was used as the contrasting object to assess signal difference to noise ratio (SDNR). The acquisition factors from the AEC test were then used to calculate mean glandular dose (MGD) using DBT correction factors given in the protocol. Image receptor response was measured from the projection images; pixel value and variance were taken from the first projection image in a given sequence. Detector brightness uniformity was calculated from the first, central and last projections. System projection modulation transfer function (MTF) was measured from the central projection, using a standard edge test object supported 40 mm above the breast table. Stability of the DBT reconstruction was assessed using CDMAM, imaged with 20 mm PMMA above and below 1 the CDMAM test object; CDCOM was used to score these images. Reconstructed in-‐plane (x,y) sharpness was measured with a 25 µm diameter tungsten wire while depth (in the z direction) resolution was measured with a 0.5 mm diameter Aluminium sphere set within a 40 mm thick PMMA test object. III. Results and discussion Between 60 and 180 minutes were needed to acquire the images for the protocol, depending the scan acquisition rate, number of projection images and reconstruction speed of the DBT unit. Transfer of the images to hard disk for data processing and calculation also required considerable time; between 22 GB and 65 GB of image data were generated for a typical protocol test. Some examples of the data to be presented are discussed here. The AEC SDNR measured in the planes showed some unusual trends (e.g. constant SDNR) with PMMA thickness. This may be related to the fact that no linearization is possible for data taken from the planes. Change of relative SDNR (measured in the projections) with PMMA thickness followed trends similar to those for 2D planar mammography. Detector response function gradient was higher in DBT mode for all three systems, consistent with an increase in detector gain. The z-‐resolution test using the 0.5 mm sphere gave different profiles for the three systems, however only small differences in the FWHM were found, despite the difference in scan angles. Projection MTF measured 40 mm above the table in the tube travel direction showed strong differences between systems (Fig 1a); the GE system employs step and shoot acquisition and had the highest projection MTF. These differences were reflected in the in-‐plane DBT MTF measure with 25 µm wire (Fig 1b). Fig 1 MTF in the tube travel direction measured 40 mm above the table a) projection MTF measured using standard edge b) in-‐plane MTF measured using 25 µm diameter W-‐wire IV. Conclusions The number and frequency of tests should be revised to give a protocol that is easier to implement practically for QC. While the tests give a good overview of DBT technical parameters, the main drawback for the protocol is the lack of a DBT image quality test. References European Reference Organisation for Quality Assured Breast Screening and Diagnostic Services (EUREF) ’Protocol for the Quality Control of the Physical and Technical Aspects of Digital Breast Tomosynthesis Systems’ Draft version 0.10 2

Download Report