- Email: [email protected]
The development of a pseudo-3D imaging system (tomosynthesis) for security screening of passenger baggage
Nuclear Instruments and Methods in Physics Research A 652 (2011) 108–111
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
The development of a pseudo-3D imaging system (tomosynthesis) for security screening of passenger baggage C.B. Reid a,, M.M. Betcke b, D. Chana c, R.D. Speller a a
Department of Medical Physics and Bioengineering, University College London, London WC1E 6BT, United Kingdom Department of Computer Science, University College London, London WC1E 6BT, United Kingdom c Department for Transport, London SW1E 6DT, United Kingdom b
a r t i c l e in f o
a b s t r a c t
Available online 27 August 2010
This paper describes a study investigating the potential of tomosynthesis as a post check-in baggage scanning system. A laboratory system has been constructed consisting of a moveable source and detector, arranged around a mini 901 bend conveyor system, from which multiple projection images can be collected. Simulation code has been developed to allow the optimum source and detector positions to be determined. Reconstruction methods are being developed to modify the Shift-And-Add (SAA) algorithm to accommodate the non-typical imaging geometry. & 2010 Elsevier B.V. All rights reserved.
Keywords: Tomosynthesis Airport security Limited angle tomography Image reconstruction
1. Introduction 2D projection radiography is the most widely implemented method of baggage security screening for the detection of explosives and illicit materials at airports, however, there are two principle limitations in this method. Firstly, projections of individual items within baggage are super-imposed creating flattened images making it difficult to discriminate objects. Secondly, variations in X-ray absorption properties within the baggage can distort image information. These effects can lead to loss of important information and, where anomalies are flagged by operators, an increase in the baggage handling time as bags are manually searched or screened with more complex and costly systems. It is proposed that these limitations can be resolved using digital X-ray tomosynthesis, a pseudo-3D imaging technique, as the principle imaging method for passenger baggage. As checked-in baggage moves through an airport baggage transport system its direction of motion may change several times. If X-ray sources and detectors were positioned at the points of direction change, a sufficient number of projection images could be taken to enable on-belt tomosynthesis (ObT) of the moving baggage. The images may be reconstructed to create focussed 2D slice images of an arbitrary number of planes which can be combined to create a pseudo-3D image of the object. The ObT system could be retro fitted into existing airport conveyor system layout, creating a cost effective 3D primary imaging method for all checked-in passenger baggage.
Corresponding author.
E-mail address: [email protected] (C.B. Reid). 0168-9002/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2010.08.081
The post check-in baggage transport system at two London airports were evaluated to establish baggage motion, and thus the feasibility, of an ObT system. At both airports potential ObT positions were identified varying from 401 to 1801, with 901 being the most common. The movement of baggage was smooth and steady through the full rotation of the conveyor bends, highlighting the suitability of the baggage motion for the implementation of an ObT system. To further investigate this, we have developed a laboratory baggage transport system, simulation code and reconstructed to establish the feasibility of an ObT system. These are described in the following sections. 2. Laboratory system The laboratory system, shown in Fig. 1(a), consists of a conveyor system (Transnorm, Ltd.), a tungsten target X-ray source (X-Tek, Ltd.) and two PaxScan detectors (Varian, Ltd.). The source and detector were arranged around the conveyor belts, as shown in Fig. 1(b), where the position of the source can be varied. The driven conveyor belts consist of a 1 m long section leading into a 901 bend section. The control systems for the conveyor drives were installed outside the X-ray lab to enable remote control of the system. The speed of operation of the system is variable, with a minimum speed of 1.2 m/min. A video system has been installed in the set-up to enable accurate location of the baggage as it moves around the conveyor. This is to enable successful image reconstruction of the images by registering, during the image reconstruction routines, the 2D radiographs to optical images of the baggage movements, enabling the precise relative positions between the baggage, sources and detectors to be known.
C.B. Reid et al. / Nuclear Instruments and Methods in Physics Research A 652 (2011) 108–111
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Fig. 1. Images showing the (a) schematic and (b) photo of the set-up of the laboratory baggage conveyor system.
Fig. 2. Figure showing (a) the phantom schematic, (b) the baggage orientation and imaging geometry, (c) the reconstructed image of the plane indicated by a solid black line in (b), where a section of both aluminium plates should be in focus and (d) the reconstructed image of the plane indicated by a dashed black line in (b), where a section of only one aluminium plate and a section of the large spherical absorber should be in focus.
3. Reconstruction methods Typically in tomosynthesis, slice images of common planes through an object are generated from the summation of a set of shifted projection images acquired at different orientations around the object. This is referred to as the Shift-And-Add (SAA) reconstruction, and takes into consideration that the projection of objects at different heights above the detector will be dependant on the relative heights of the objects above the imaging plane [1]. A SAA reconstruction of images of a simple baggage phantom (two pieces of aluminium sheet: 62 mm 58 mm 1 mm and 84 mm 79 mm 1 mm, and two spherical perspex absorbers: 30 mm and 19 mm diameter held in a soft fabric bag and arranged as shown in Fig. 2(a)) was performed. The phantom was imaged in the geometry illustrated in Fig. 2(b), where the X-ray source and detector are locked together and rotate about a fixed central point on opposite sides of a circle. Eleven images were collected over a 901 range with a separation of 91 between images.
Seven planes, indicated by the dashed lines in Fig. 2(b), were reconstructed following previously outlined procedures [2]. For brevity only two of these planes are shown here, in Fig. 2(c) and (d). The effect of blurring is apparent in these images, where the image of the flat absorber is superimposed over each of the images creating some distortion. Nevertheless, it is evident that the SAA image reconstruction of tomosynthesis images is capable of bringing the absorbing objects within the phantom into focus. The movement of the source and detector with relation to the reconstruction planes within the baggage is complex. As the baggage moves with respect to the source and detectors, as opposed to the more typical arrangement where the source and detectors move with respect to the imaged object, no equivalent parallel planes through the baggage exist. Equivalent parallel planes are achieved by transforming the position of the source and detectors relative to the baggage, for each detector projection following which it is possible to Shift-And-Add the images. This is discussed further in Section 4. Existing transforms are used to compensate for the variation in magnification of object structures with projection angle [2,1].
4. Computer simulation In order to determine the optimal source and detector positions for the ObT system, we have developed a numerical simulation software package to study the optimality criteria of the system. These criteria include, for example, the range of effective projection angles (i.e. angles from which the baggage is seen), the completeness of the views, the uniformity of the sampling of the effective projection angle range, the uniformity of the magnification of the object and the effect of image resolution. These criteria were included in the simulation software and can be varied such that an optimal design of an ObT can be determined. In addition to the optimality criteria, the design of the ObT system is limited by constraints such as the feasibility and compactness of the scanner design which must also be taken into consideration when determining an optimal system set-up. A top view of the simulated ObT system is shown in Fig. 3 which shows a system comprising a 901 turn, a number of stationary sources and one stationary flat detector perpendicular to the belt planes. The presence of multiple sources enables a number of different source positions from which projections can be taken. The conveyor belt is rotating in the xy-plane around a point (the origin of the global coordinate system (x, y, z)), resulting in the 3D axis of rotation being the z-axis.
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C.B. Reid et al. / Nuclear Instruments and Methods in Physics Research A 652 (2011) 108–111
Fig. 3. Top view of the baggage tomosynthesis setup.
The simulation has been formulated in terms of a Volume of Interest (VOI), which has been chosen to be a cuboid large enough such that any size baggage of interest can be inscribed into it. The positioning of the VOI reflects the fact that the bags rest on the conveyor belt. Baggage moving around the 901 turn corresponds to the 901 rotation of the VOI around the z-axis, therefore the motion of the baggage can be adequately described through the rotation of the VOI around the origin. While the baggage is transported on the conveyor belt a number of X-ray views are acquired at different rotation angles fp of the VOI, giving VOIp, p ¼ 1, . . . ,P. The admissible range of angles corresponds to those angles for which the intersection of the VOI with the field of view of the source and detector (FOV) is nonempty. The latter depends solely on the relative position of the source and detector and hence it changes with the source position. As suggested in Section 3, in an ObT system the movement of the source and detector with relation to the movement of the baggage is complex. If we consider that the relative orientation of VOIp to the source and detector, after the VOIs rotation by fp around the z-axis, is equivalent to the rotation on the source and detector by fp around the z-axis while keeping the VOI stationary, we are able to choose a ‘reference position’ for the VOI, VOIref, and perform a relative source and detector rotation with respect to this reference position. To efficiently implement the simulation, and the reconstruction methods in the future, it is necessary to express the scanning process as seen from the perspective of the VOIref, i.e. to calculate the source and active detector positions with respect to the local coordinate system associated with VOIref. This will enable equivalence between the orientation of the baggage at each projected X-ray view and result in a transformation of the position of the source and detector relative to the baggage for each different projection. Fig. 4 shows two views of the ObT system with a single source at its optimal position. The top two images show the movement of the baggage around the bend and its corresponding projection onto the detector at two different rotation angles of the VOI while the bottom two images show their equivalents in the effective geometry, i.e. as seen from the VOIref. From these images we can see the effect of the corregistration of the baggage locations on the source and detector locations, which must be accounted for in future image reconstruction methods, as mentioned in Section 3. After the source and the detector positions have been calculated for the discrete set of angles fp , p ¼ 1, . . . ,P, we compile the list of the
Fig. 4. 2nd and 4th out of 9 ObT views for the optimal single source position for the 901 turn and their equivalents in the effective geometry i.e. as seen from the VOIref. Due to the source position the system is symmetric w.r.t. x-axis.
corresponding source–detector pairs. This list in then input into an Open MP parallelised C implementation of the Siddon algorithm to obtain simulated data. As a result of the development of this software, some rudimentary conclusions have been drawn. As the source approaches the centre of rotation the views taken from the source become increasingly alike, hence they contain little additional information and are not useful for tomosynthesis. Translating the source parallel to x-axis changes the source detector distance (and the size of the projection on the detector). As the source gets closer to the VOI (and to the detector), the projections get magnified while the opposite happens if the source is shifted away from the object. The growing and shrinking of the occupied detector area affects the resolution proportionally, i.e. the larger the projection area the higher the resolution. The relative proximity of the source and the detector is at the least limited by the size of the VOI, hence restricting the achievable magnification. An excessive magnification will result in incomplete data as the projection grows too large for the detector. On the other hand, as the source nears the detector, the FOV shrinks, thus reducing the admissible range of VOI rotation angles. The range of the effective view angles, i.e. angles from which VOI is seen is increased by placing the centre of the VOI as close as possible to the centre of rotation ensuring the maximal possible rotation of the VOI in the FOV of the scanner.
5. Conclusions The potential of ObT as a post check-in baggage scanning system is being determined. Despite the reconstructed images being noisy, the potential of the technique is apparent. The images are heavily affected by blurring because they contain images from every plane of the imaged object; one plane in focus and all the others smeared on top. De-blurring routines, implemented on top of the SAA image reconstruction method, have been developed to remove artifacts and improve the reconstructed image quality [3]. Iterative and algebraic image reconstruction techniques, which have shown the promise of generating clearer reconstructed images, are alternate image reconstruction methods to the SAA
C.B. Reid et al. / Nuclear Instruments and Methods in Physics Research A 652 (2011) 108–111
routine. We would aim to investigate both approaches to improving image quality and implement the most suitable to the ObT application. The simulation of projection images will enable a thorough theoretical study of the optimal projection angle range and number of projections. The range of tomographic angles and the number of views used dictates the reconstructed slice thickness and, therefore, 3D image accuracy [4]. We also aim to establish guidelines directing acceptable image quality as well as advanced methods of image analysis, to fully utilise the complex images that an ObT system will generate.
Acknowledgement We thank David Szotten for making his Open MP parallelised ray-tracing code available. This project is funded under the
111
Innovative Research Call in Explosives and Weapons Detection (2007), a cross-government programme sponsored by a number of government departments and agencies under the CONTEST strategy. M. Betcke is supported by the Engineering and Physical Sciences Grant EP/H02865X/1.
References [1] L.T. Niklason, B.T. Christian, L.E. Niklason, D.B. Kopans, D.E. Castleberry, B.H. Ong, C.E. Landberg, P.J. Slanetz, A.A. Giardino, R. Moore, D. Albagli, M.C. DeJule, P.F. Fitzgerald, D.F. Fobare, B.W. Giambattista, R.F. Kwasnick, J. Liu, S.J. Lubowski, G.E. Possin, J.F. Richotte, C.Y. Wei, R.F. Wirth, Radiology 205 (2) (1997) 399. [2] Z. Kolitsi, G. Panayiotakis, V. Anastassopoulos, A. Scodras, N. Pallikarakis, Medical Physics 19 (4) (1992) 1045. [3] J.T. Dobbins III, D.J. Godfrey, Physics in Medicine and Biology 48 (2003) R65. [4] G.M. Stevens, R.L. Birdwell, C.F. Beaulieu, D.M. Ikeda, N.J. Pelc, Radiology 228 (2) (2003) 569.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
The development of a pseudo-3D imaging system (tomosynthesis) for security screening of passenger baggage C.B. Reid a,, M.M. Betcke b, D. Chana c, R.D. Speller a a
Department of Medical Physics and Bioengineering, University College London, London WC1E 6BT, United Kingdom Department of Computer Science, University College London, London WC1E 6BT, United Kingdom c Department for Transport, London SW1E 6DT, United Kingdom b
a r t i c l e in f o
a b s t r a c t
Available online 27 August 2010
This paper describes a study investigating the potential of tomosynthesis as a post check-in baggage scanning system. A laboratory system has been constructed consisting of a moveable source and detector, arranged around a mini 901 bend conveyor system, from which multiple projection images can be collected. Simulation code has been developed to allow the optimum source and detector positions to be determined. Reconstruction methods are being developed to modify the Shift-And-Add (SAA) algorithm to accommodate the non-typical imaging geometry. & 2010 Elsevier B.V. All rights reserved.
Keywords: Tomosynthesis Airport security Limited angle tomography Image reconstruction
1. Introduction 2D projection radiography is the most widely implemented method of baggage security screening for the detection of explosives and illicit materials at airports, however, there are two principle limitations in this method. Firstly, projections of individual items within baggage are super-imposed creating flattened images making it difficult to discriminate objects. Secondly, variations in X-ray absorption properties within the baggage can distort image information. These effects can lead to loss of important information and, where anomalies are flagged by operators, an increase in the baggage handling time as bags are manually searched or screened with more complex and costly systems. It is proposed that these limitations can be resolved using digital X-ray tomosynthesis, a pseudo-3D imaging technique, as the principle imaging method for passenger baggage. As checked-in baggage moves through an airport baggage transport system its direction of motion may change several times. If X-ray sources and detectors were positioned at the points of direction change, a sufficient number of projection images could be taken to enable on-belt tomosynthesis (ObT) of the moving baggage. The images may be reconstructed to create focussed 2D slice images of an arbitrary number of planes which can be combined to create a pseudo-3D image of the object. The ObT system could be retro fitted into existing airport conveyor system layout, creating a cost effective 3D primary imaging method for all checked-in passenger baggage.
Corresponding author.
E-mail address: [email protected] (C.B. Reid). 0168-9002/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2010.08.081
The post check-in baggage transport system at two London airports were evaluated to establish baggage motion, and thus the feasibility, of an ObT system. At both airports potential ObT positions were identified varying from 401 to 1801, with 901 being the most common. The movement of baggage was smooth and steady through the full rotation of the conveyor bends, highlighting the suitability of the baggage motion for the implementation of an ObT system. To further investigate this, we have developed a laboratory baggage transport system, simulation code and reconstructed to establish the feasibility of an ObT system. These are described in the following sections. 2. Laboratory system The laboratory system, shown in Fig. 1(a), consists of a conveyor system (Transnorm, Ltd.), a tungsten target X-ray source (X-Tek, Ltd.) and two PaxScan detectors (Varian, Ltd.). The source and detector were arranged around the conveyor belts, as shown in Fig. 1(b), where the position of the source can be varied. The driven conveyor belts consist of a 1 m long section leading into a 901 bend section. The control systems for the conveyor drives were installed outside the X-ray lab to enable remote control of the system. The speed of operation of the system is variable, with a minimum speed of 1.2 m/min. A video system has been installed in the set-up to enable accurate location of the baggage as it moves around the conveyor. This is to enable successful image reconstruction of the images by registering, during the image reconstruction routines, the 2D radiographs to optical images of the baggage movements, enabling the precise relative positions between the baggage, sources and detectors to be known.
C.B. Reid et al. / Nuclear Instruments and Methods in Physics Research A 652 (2011) 108–111
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Fig. 1. Images showing the (a) schematic and (b) photo of the set-up of the laboratory baggage conveyor system.
Fig. 2. Figure showing (a) the phantom schematic, (b) the baggage orientation and imaging geometry, (c) the reconstructed image of the plane indicated by a solid black line in (b), where a section of both aluminium plates should be in focus and (d) the reconstructed image of the plane indicated by a dashed black line in (b), where a section of only one aluminium plate and a section of the large spherical absorber should be in focus.
3. Reconstruction methods Typically in tomosynthesis, slice images of common planes through an object are generated from the summation of a set of shifted projection images acquired at different orientations around the object. This is referred to as the Shift-And-Add (SAA) reconstruction, and takes into consideration that the projection of objects at different heights above the detector will be dependant on the relative heights of the objects above the imaging plane [1]. A SAA reconstruction of images of a simple baggage phantom (two pieces of aluminium sheet: 62 mm 58 mm 1 mm and 84 mm 79 mm 1 mm, and two spherical perspex absorbers: 30 mm and 19 mm diameter held in a soft fabric bag and arranged as shown in Fig. 2(a)) was performed. The phantom was imaged in the geometry illustrated in Fig. 2(b), where the X-ray source and detector are locked together and rotate about a fixed central point on opposite sides of a circle. Eleven images were collected over a 901 range with a separation of 91 between images.
Seven planes, indicated by the dashed lines in Fig. 2(b), were reconstructed following previously outlined procedures [2]. For brevity only two of these planes are shown here, in Fig. 2(c) and (d). The effect of blurring is apparent in these images, where the image of the flat absorber is superimposed over each of the images creating some distortion. Nevertheless, it is evident that the SAA image reconstruction of tomosynthesis images is capable of bringing the absorbing objects within the phantom into focus. The movement of the source and detector with relation to the reconstruction planes within the baggage is complex. As the baggage moves with respect to the source and detectors, as opposed to the more typical arrangement where the source and detectors move with respect to the imaged object, no equivalent parallel planes through the baggage exist. Equivalent parallel planes are achieved by transforming the position of the source and detectors relative to the baggage, for each detector projection following which it is possible to Shift-And-Add the images. This is discussed further in Section 4. Existing transforms are used to compensate for the variation in magnification of object structures with projection angle [2,1].
4. Computer simulation In order to determine the optimal source and detector positions for the ObT system, we have developed a numerical simulation software package to study the optimality criteria of the system. These criteria include, for example, the range of effective projection angles (i.e. angles from which the baggage is seen), the completeness of the views, the uniformity of the sampling of the effective projection angle range, the uniformity of the magnification of the object and the effect of image resolution. These criteria were included in the simulation software and can be varied such that an optimal design of an ObT can be determined. In addition to the optimality criteria, the design of the ObT system is limited by constraints such as the feasibility and compactness of the scanner design which must also be taken into consideration when determining an optimal system set-up. A top view of the simulated ObT system is shown in Fig. 3 which shows a system comprising a 901 turn, a number of stationary sources and one stationary flat detector perpendicular to the belt planes. The presence of multiple sources enables a number of different source positions from which projections can be taken. The conveyor belt is rotating in the xy-plane around a point (the origin of the global coordinate system (x, y, z)), resulting in the 3D axis of rotation being the z-axis.
110
C.B. Reid et al. / Nuclear Instruments and Methods in Physics Research A 652 (2011) 108–111
Fig. 3. Top view of the baggage tomosynthesis setup.
The simulation has been formulated in terms of a Volume of Interest (VOI), which has been chosen to be a cuboid large enough such that any size baggage of interest can be inscribed into it. The positioning of the VOI reflects the fact that the bags rest on the conveyor belt. Baggage moving around the 901 turn corresponds to the 901 rotation of the VOI around the z-axis, therefore the motion of the baggage can be adequately described through the rotation of the VOI around the origin. While the baggage is transported on the conveyor belt a number of X-ray views are acquired at different rotation angles fp of the VOI, giving VOIp, p ¼ 1, . . . ,P. The admissible range of angles corresponds to those angles for which the intersection of the VOI with the field of view of the source and detector (FOV) is nonempty. The latter depends solely on the relative position of the source and detector and hence it changes with the source position. As suggested in Section 3, in an ObT system the movement of the source and detector with relation to the movement of the baggage is complex. If we consider that the relative orientation of VOIp to the source and detector, after the VOIs rotation by fp around the z-axis, is equivalent to the rotation on the source and detector by fp around the z-axis while keeping the VOI stationary, we are able to choose a ‘reference position’ for the VOI, VOIref, and perform a relative source and detector rotation with respect to this reference position. To efficiently implement the simulation, and the reconstruction methods in the future, it is necessary to express the scanning process as seen from the perspective of the VOIref, i.e. to calculate the source and active detector positions with respect to the local coordinate system associated with VOIref. This will enable equivalence between the orientation of the baggage at each projected X-ray view and result in a transformation of the position of the source and detector relative to the baggage for each different projection. Fig. 4 shows two views of the ObT system with a single source at its optimal position. The top two images show the movement of the baggage around the bend and its corresponding projection onto the detector at two different rotation angles of the VOI while the bottom two images show their equivalents in the effective geometry, i.e. as seen from the VOIref. From these images we can see the effect of the corregistration of the baggage locations on the source and detector locations, which must be accounted for in future image reconstruction methods, as mentioned in Section 3. After the source and the detector positions have been calculated for the discrete set of angles fp , p ¼ 1, . . . ,P, we compile the list of the
Fig. 4. 2nd and 4th out of 9 ObT views for the optimal single source position for the 901 turn and their equivalents in the effective geometry i.e. as seen from the VOIref. Due to the source position the system is symmetric w.r.t. x-axis.
corresponding source–detector pairs. This list in then input into an Open MP parallelised C implementation of the Siddon algorithm to obtain simulated data. As a result of the development of this software, some rudimentary conclusions have been drawn. As the source approaches the centre of rotation the views taken from the source become increasingly alike, hence they contain little additional information and are not useful for tomosynthesis. Translating the source parallel to x-axis changes the source detector distance (and the size of the projection on the detector). As the source gets closer to the VOI (and to the detector), the projections get magnified while the opposite happens if the source is shifted away from the object. The growing and shrinking of the occupied detector area affects the resolution proportionally, i.e. the larger the projection area the higher the resolution. The relative proximity of the source and the detector is at the least limited by the size of the VOI, hence restricting the achievable magnification. An excessive magnification will result in incomplete data as the projection grows too large for the detector. On the other hand, as the source nears the detector, the FOV shrinks, thus reducing the admissible range of VOI rotation angles. The range of the effective view angles, i.e. angles from which VOI is seen is increased by placing the centre of the VOI as close as possible to the centre of rotation ensuring the maximal possible rotation of the VOI in the FOV of the scanner.
5. Conclusions The potential of ObT as a post check-in baggage scanning system is being determined. Despite the reconstructed images being noisy, the potential of the technique is apparent. The images are heavily affected by blurring because they contain images from every plane of the imaged object; one plane in focus and all the others smeared on top. De-blurring routines, implemented on top of the SAA image reconstruction method, have been developed to remove artifacts and improve the reconstructed image quality [3]. Iterative and algebraic image reconstruction techniques, which have shown the promise of generating clearer reconstructed images, are alternate image reconstruction methods to the SAA
C.B. Reid et al. / Nuclear Instruments and Methods in Physics Research A 652 (2011) 108–111
routine. We would aim to investigate both approaches to improving image quality and implement the most suitable to the ObT application. The simulation of projection images will enable a thorough theoretical study of the optimal projection angle range and number of projections. The range of tomographic angles and the number of views used dictates the reconstructed slice thickness and, therefore, 3D image accuracy [4]. We also aim to establish guidelines directing acceptable image quality as well as advanced methods of image analysis, to fully utilise the complex images that an ObT system will generate.
Acknowledgement We thank David Szotten for making his Open MP parallelised ray-tracing code available. This project is funded under the
111
Innovative Research Call in Explosives and Weapons Detection (2007), a cross-government programme sponsored by a number of government departments and agencies under the CONTEST strategy. M. Betcke is supported by the Engineering and Physical Sciences Grant EP/H02865X/1.
References [1] L.T. Niklason, B.T. Christian, L.E. Niklason, D.B. Kopans, D.E. Castleberry, B.H. Ong, C.E. Landberg, P.J. Slanetz, A.A. Giardino, R. Moore, D. Albagli, M.C. DeJule, P.F. Fitzgerald, D.F. Fobare, B.W. Giambattista, R.F. Kwasnick, J. Liu, S.J. Lubowski, G.E. Possin, J.F. Richotte, C.Y. Wei, R.F. Wirth, Radiology 205 (2) (1997) 399. [2] Z. Kolitsi, G. Panayiotakis, V. Anastassopoulos, A. Scodras, N. Pallikarakis, Medical Physics 19 (4) (1992) 1045. [3] J.T. Dobbins III, D.J. Godfrey, Physics in Medicine and Biology 48 (2003) R65. [4] G.M. Stevens, R.L. Birdwell, C.F. Beaulieu, D.M. Ikeda, N.J. Pelc, Radiology 228 (2) (2003) 569.