Radiation Protection Dosimetry Advance Access published April 1, 2015 Radiation Protection Dosimetry (2015), pp. 1–4

doi:10.1093/rpd/ncv095

DESCRIPTION AND BENEFITS OF DYNAMIC COLLIMATION IN DIGITAL BREAST TOMOSYNTHESIS Y. Popova*, G. Hersemeule, R. Klausz and H. Souchay Digital Guidance Solutions, GE Healthcare, 283 rue de la Minie`re, Buc 78530, France *Corresponding author: [email protected]

INTRODUCTION Among the radiation safety measures in X-ray imaging, restricting the X-ray beam to an area as close as possible to the sensitive part of the image receptor is one of the most systematic. Examples can be found in IEC standards ‘X-ray equipment shall be designed to limit the radiation field not contributing to the formation of the image’(1). In mammography, the same rule has been kept with specific modifications. In particular, the role of the sides of the X-ray beam has been differentiated. For the side corresponding to the chest wall, it is accepted that the beam extends out of the breast support in order to ensure full coverage of the anatomy. On the three other sides, the beam may or must extend out of the image receptor. This rule has been introduced for screen-film mammography to ensure ‘full-film blackening’, that is, avoid that bright bands on the film edges could disturb the review of the films(2) on a view-box. In general, the extra coverage is less critical than that in other radiographic modalities since, in most cases, the breast is completely inside the X-ray beam with a raw-beam margin between the breast border and the image receptor edges. However, for some angulations, some parts of the patient’s anatomy may be exposed, such as axillary area or infra-mammary area. The congruence of the X-ray field with the detector active area is checked in most mammography quality control (QC) procedures and regulations(3 – 6). Screen or non-screen films have been the standard tool for these measurements for a long period. However, the evolution of mammography from film to digital progressively reduces the availability of processing machines in the mammography departments; more and more to perform this test film has to be replaced with chromogenic films such as Gafchromic(7).

Currently, no regulations or recommendations address digital breast tomosynthesis (dBT) collimation design or check. The current draft in progress of EUREF only describes measuring the ‘alignment between X-ray field and reconstructed tomosynthesis image at chest wall edge of the Bucky’(8). The current draft amendment of IEC standard for dBT simply excludes dBT from the design rules for the ‘correspondence between x-ray field and effective image reception area’(9). The only remaining patient protection against unnecessary radiation in dBT is therefore the primary protect4) which shall be respected in any case. ive shielding(3, , In the present study, the dynamic dBT collimation approach and methods to check the position of the lateral edges of the X-ray field are described. Problem statement dBT consists in acquiring a set of projection X-ray images for different positions of the X-ray source with respect to the detector, then reconstructing a digital representation of the breast volume. Collimation in mammography can be achieved by different devices. Most sophisticated ones are made of four rectilinear blades, the movement of which can be controlled independently. The four blades are referred to as: † Front blade, whose X-ray projection into the image plane is adjacent to the chest-wall side of the detector; † Left and right blades, which project adjacent to, respectively, the left and right sides of the detector; † Rear blade, projecting on the rear side of the detector, opposite to the chest-wall side. Usually in mammography, the X-ray source is positioned above the mid-point of the first chest-wall side

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X-ray field to image receptor active area alignment is usually tested in mammographic QC. In digital breast tomosynthesis (dBT), the source moves during the acquisition, generating a displacement of the X-ray beam edges relative to the detector, in or out of the detector active area. To minimise unnecessary radiation while maximising the useful field of view, a solution consisting in adjusting the collimation with the source rotation was implemented on the GE SenoClaire dBT system. This solution is described and tested using three different methods based on: (1) images from the detector, (2) a non-screen film and (3) a semiconductor tool providing the X-ray intensity profile. Method 1 demonstrated a maximum positioning error of 0.3 mm. Method 2 was found non-applicable; Method 3 provided measurements within 1.5 mm. Dynamic collimation enables maintaining an X-ray field to detector congruence comparable with 2D. Measuring the position of the X-ray field edges using a dedicated tool makes routine QC possible.

Y. POPOVA ET AL.

Figure 1. X-ray field displacement with source movement. S is the X-ray source at 08 angulation; S0 is the angulated X-ray source; FOVL and FOVR are the FOV left and right edges and the positions of the X-ray beam edges corresponding to S; R is the pivot; FOV0L and FOV0R are the positions of the X-ray beam corresponding to S0.

Another approach is to keep a constant collimation opening over the entire dBT acquisition, but adjusted to maximise the useful 3D-FOV. Using this geometry, the X-ray beam exceeds the image reception area by up to 37 mm and therefore further increases the risk to irradiate non-breast areas of the patient. A more sophisticated solution consists in using the capability of the collimator to adjust independently the position of the blades to maintain the X-ray beam edges in the same position relative to the detector for each angular position of the source. This approach is designated as ‘dynamic collimation’. In this study, the operation of the dynamic collimation implemented on the GE SenoClaire system is verified, first using manufacturer-specific means to establish a baseline, then investigating methods usable for routine quality check. MATERIALS AND METHODS dBT system The geometrical properties of the GE SenoClaire system are the following. The source to image distance (SID) at 08 is 660 mm. The detector active area size is 240 `  307 mm. The pivot is 40 mm above the detector image plane. The dBT acquisition (sweep) consists in nine discrete (step and shoot) projections over 258. Start position is +12.58, with 3.1258 angular increments between projections. Measurements First, the dynamic collimation operation was checked using the image receptor itself. A small FOV (19`  23 cm) dBT acquisition was used, but images corresponding to the entire X-ray detector were retrieved for each projection. This way the collimator blades were visible in the detector images and their positions measured directly by plotting their profiles. The blade position is taken at 30 % of the signal level in the image. It should be noted that this method is not accessible to the final user. The second method is derived from the current practice in 2D. A non-screen Agfa Structurix D7 DW film was positioned on the breast support and its middle aligned with the light edge for the 08 angulation. A radiopaque object was placed on the film (see Figure 2). A standard dBT acquisition was performed in large FOV (24`  31 cm) with Molybdenum track, Molybdenum filter, 28 kV and 10 mAs per projection (90 mAs total exposure). These techniques factors were adjusted to provide a range of optical densities compatible with the film digitiser used. After exposure, the film was developed and then digitised with a flat-bed scanner (Epson V700). The profile of the border of the FOV was plotted, and its width measured on the digitised image. Using the radiopaque object

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detector active line. In dBT, the source rotates in the plane orthogonal to the detector and contains this first active line. The collimator’s front blade edge is tangent to this plane, and its projection will therefore remain parallel to the detector’s chest-wall line for all positions of the X-ray source. As the source moves during the dBT acquisition, the projections of the three other blades edges move along with the X-ray source and the X-ray beam moves with respect to the detector (see Figure 1). If the edges of the X-ray beam are adjusted as for 2D imaging, that is, slightly outside the image receptor for the ‘08 angle’(6), they may move significantly relative to the active edges of the detector during the tube rotation. On the lateral side corresponding to the direction of the tube rotation, the X-ray field edge comes closer to the centre, and there is a risk to miss some part of the breast; on the opposite side, the X-ray field edge will get more distant from the detector centre, with an increased risk to hit some unintended areas of the patient body. Therefore, precautions must be taken in the adjustment of the collimation. Several approaches exist, and they were simulated for the GE SenoClaire dBT system (GE Healthcare, Chalfont, UK). One of them is to keep the collimation constant during the entire dBT acquisition and adjusted as for 2D. In this configuration, the blades would project from 13 mm inside to 15 mm outside the image reception area and either reduce the corresponding field of view (FOV) or increase the risk to irradiate the patient unnecessarily. It should also be noticed that to be nominal, the reconstruction of the tomographic planes must be performed using all the projections. If the information is truncated on the edges, the corresponding volume will be degraded.

DYNAMIC COLLIMATION IN dBT

image on both detector and digitised film image the X-ray edge profile position and size were scaled and projected into the detector plane. The third method involves a Quart Nonius(10) tool (Quart, Zorneding, DE), which consists in a 4-cmlong radio-sensitive array of photo-diodes aligned with a visible ruler on the cover. The detector is connected to a computer through a USB cable and driven with dedicated software. When an exposure is detected, the software acquires and plots the profile of the X-ray intensity detected by the sensor array. The commercial version of the software was designed for single exposures and after reset by user the device reverts to the idle state, waiting for another exposure. This mode was not adapted to the multiple exposures dBT. The Quart team offered to provide a modified version of the software enabling to detect and save the X-ray intensity profiles for multiple successive exposures, which corresponded well to the entire dBT sequence with one X-ray profile per dBT exposure. These measurements were done using a standard dBT acquisition, except technique factors adjusted to Molybdenum track, Molybdenum filter, 28 kV as for the film and 20 mAs per projection (180 mAs total exposure), to get an acceptable signal on the tool. The tool was positioned on the breast support together with a radiopaque ruler placed in the sensors plane and aligned with the zero position of the tool (Figure 3). This 0 position was also aligned with the edge of the light localiser at 08 to acquire the full X-ray edge profile for all angular positions of the tomographic sweep.

Figure 3. External detector device measurement set-up. The dashed line represents the light field edge, (a) the breast support, (b) the external detector device and (c) the radiopaque ruler.

Figure 4. Plot of blade profiles. Each curve corresponds to the profile of the right blade for each of the nine dBT projections(1 – 9). The 0 mm position is the theoretical FOV edge.

The film showed an X-ray edge variation of 4 mm, from 2.2 mm inside to 1.8 mm outside the FOV. Assessing the position of the FOV edges per projection was not possible using this method. The dedicated tool indicated a 1.5-mm maximum deviation versus the real blade positions as measured in detector images.

RESULTS The method using the detector images for blade detection demonstrated a distance of 0.3 mm between the extreme blade positions, with an average position at 2.4 mm out of the detector active area (Figure 4).

DISCUSSION When accessible, the direct measurement of the position of the X-ray field in the image provided by the detector is obviously the most accurate. Its precision

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Figure 2. Digitised image of the dBT exposed film, showing the blurred edge of the X-ray field and the radiopaque object.

Y. POPOVA ET AL.

ACKNOWLEDGEMENTS The authors thank the Quart company, and in particular Hugo de las Heras Gala for his kind support and Johanna Rohde for the development of the Nonius software extension for dBT.

FUNDING Figure 5. Collimation assessment with film for different configurations.

CONCLUSION The purpose of this study was to present the dynamic collimation approach for dBT and propose a method of verifying the effective dBT collimation. Dynamic collimation is achieved by an independent control of the collimator blades. It enables to maintain an X-ray field to detector congruence comparable with 2D. Measuring the position of the X-ray field edges using an external solid-state tool provided improved accuracy in conditions close to a routine QC.

REFERENCES 1. International Electrotechnical Commission. Medical electrical equipment – Part 1– 3: general requirements for basic safety and essential performance – collateral standard: radiation protection in diagnostic X-ray equipment IEC 60601-1-3 Edition 2.0 2008-01. International Electrotechnical Commission. 2. Whitman, G. J. and Haygood, T. M. Digital Mammography: A Practical Approach. Cambridge University Press. 77–78 (2012). 3. Medical Devices; performance standard for diagnostic xray systems; amendment, federal register, vol. 64, no. 127 (Friday 2 July 1999). Department of Health And Human Services, Food and Drug Administration. 21 CFR Part 1020 Rules and Regulations, (35924). Office of the Federal Register, United States National Archives and Records Administration. 4. International Electrotechnical Commission. Medical electrical equipment – Part 2–45: particular requirements for the basic safety and essential performance of mammographic X-ray equipment and mammographic stereotactic devices. IEC 60601-2-45 Edition 3.0 201102. International Electrotechnical Commission. 5. Commissioning and routine testing of full field digital mammography systems. NHSBSP Equipment Report 0604, Version 3. NHS Cancer Screening Programmes (2009). 6. International Atomic Energy Agency. Quality assurance programme for digital mammography. IAEA Human Health series No. 17. International Atomic Energy Agency (2011). 7. A mammograph collimation assessment using Gafchromic XR-M film. Ashland Document Library. 8. Protocol for the quality control of the physical and technical aspects of digital breast tomosynthesis systems. Draft version 0.15. European Reference Organisation for Quality Assured Breast Screening and Diagnostic Services (2014). 9. International Electrotechnical Commission. Medical electrical equipment – Part 2–45: particular requirements for the basic safety and essential performance of mammographic X-ray equipment and mammographic stereotactic devices, draft amendment 1 (in progress). 10. De las Heras, H., Mair, K., Coll Segarra, D., Schoefer, F., Blanck, O. and Szeglin, S. QUART Nonius – quality control in radiation therapy without film. Med. Phys. 40, 218 (2013).

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is only limited by the sharpness of the edge and the pixel pitch of the detector. However, it may not be available for routine controls by users and is not possible for the large FOV when the X-ray field can be larger than the detector. The film method showed unexpected results. If the film cannot be positioned in the image plane, the user needs to know both the position of the plane where the measurement is done and the position of the source for each X-ray edge appearing on the film. If these two conditions are not respected, the same image can be obtained for different geometrical configurations (see Figure 5). The cases (a) and (b) would result in the same image on the film, but only (b) demonstrates the expected result. In addition, in the case of a fixed collimation with the film in the pivot plane, the image can present an edge as sharp as in 2D [case (c)]. Therefore, using a film is not adapted to demonstrate the position of collimator blades in dBT. Using an external solid-state sensor measuring tool, it was possible to check the position of the X-ray edge per projection in step-and-shoot dBT with an accuracy of 1.5 mm. Compared with the usual tolerance of 2 % of the SID, that is, 13 mm in this case, this accuracy is sufficient for dBT collimation assessment. It should be noted that for the three methods, the measurement was made easier by the sharp edges of the X-ray field consequence of the step-and-shoot operation of the dBT system tested. These results cannot be transposed to continuous movement dBT systems without further study.

This study was made internally during the development of the GE Healthcare SenoClaire digital breast tomosynthesis equipment.