Phase-contrast Imaging and Tomography
Contact Dr. K.M. Pavlov
When monochromatic scalar x-ray radiation passes through an object, the resulting transmitted wavefield usually contains transverse modulations in both intensity and phase. Traditional means of x-ray imaging, such as absorption contrast radiography, are only sensitive to intensity variations. This is inconvenient for imaging soft tissues and other weakly absorbing objects, which typically have a stronger influence on the phase compared to the intensity of the transmitted x-ray wave. Accordingly, many methods of phase-contrast x-ray imaging have been developed (see Fig.1). Two such methods, namely propagation-based (PB) phase-contrast and analyser-based (AB) phase-contrast, have received much attention in recent years. The former technique uses free-space propagation in order to render phase shifts visible as intensity variations in a Fresnel diffraction pattern (see Fig.1c and Fig.2). A particular advantage of such an approach is its experimental simplicity. The latter phase-contrast technique (see Fig.1b) involves diffraction of a wavefield (which has been transmitted through a sample) by an analyser crystal in order to make use of the crystal's exquisite sensitivity to transverse phase gradients in the wavefield incident upon it.
Figure 1. Experimental setups for phase-sensitive imaging. (a) An x-ray interferometer, consisting of three perfect crystals that serve as phase-coherent beam splitters and mirrors, generates interference fringes that reflect the phase changes produced in a sample placed in one of the beam paths. (b) In diffraction-enhanced imaging, variations in the refraction of x-rays in the sample produce contrast because the intensity of the beam that is reflected by the analyzer crystal depends on the relative angle of the incident beam with respect to the Bragg angle (inset). (c) In in-line phase-contrast imaging, the detector is placed sufficiently far behind the sample that wavefront distortions generated by the sample produce interference fringes at the detector. At an appropriate object–image distance d, these fringes yield edge enhancements in the image. (from R. Fitzgerald,"Phase-sensitive X-ray Imaging" Physics Today, July 2000, 23-26 )
Figure 2. The effect of sample-to-detector propagation distance on images of a newborn rabbit pup. Images are recorded using beamline 20B2 at the SPring-8 Japanese synchrotron facility using an energy of 25 keV, an exposure time of 500 ms and a CCD x-ray camera having an effective pixel size of 12 μm. (a) Distance = 7 cm, (b) distance = 3 m, (c) enlarged area marked by rectangle in (a), and (d) enlarged area marked by rectangle in (b). (from R.A. Lewis, N. Yagi, M.J. Kitchen, M.J. Morgan, D. Paganin, K.K.W. Siu, K. Pavlov, I. Williams, K. Uesugi, M.J.Wallace, C.J. Hall, J. Whitley, S.B. Hooper (2005). Dynamic imaging of the lungs using X-ray phase contrast. Phys. Med. Biol. 50, 5031-5040. )
PhD Project 1:The subject of this project is the development of phase-contrast limited-data tomography with applications to real-time imaging of alloys solidification. This project will be conducted in close collaboration with leading groups involved in the light alloys study at the University of Osaka and University of Queensland.
PhD Project 2:The project will develop the methodology of propagation-based and analyser-based phase-contrast imaging of soft biological tissues.
References:1) D. J. Vine, D. M. Paganin, K. M. Pavlov, J. Kräußlich, O. Wehrhan, I. Uschmann, and E. Förster. Analyser-based phase contrast imaging and phase retrieval using a rotating anode X-ray source. Applied Physics Letters 91, 254110 (2007).
2) M.J. Kitchen, K.M. Pavlov, K.K.W. Siu, R.H. Menk, G. Tromba, R.A. Lewis. Analyser-based phase-contrast image reconstruction using geometrical optics. -Physics in Medicine and Biology (2007) 52, 4171 -4187.
3) D.J. Vine, D.M. Paganin, K.M. Pavlov, and S.G. Podorov. Unambiguous reconstruction of the complex amplitude reflection coefficient of a laterally homogeneous crystal using analyser-based phase-contrast imaging (2007) J. Appl. Cryst. 40, 650-657.
4) T.E. Gureyev, Ya.I. Nesterets , K.M. Pavlov and S.W. Wilkins. Computed tomography with linear shift-invariant optical systems. (2007) J. Opt. Soc. Am. A. 24(8), 2230-2241.
5) Ya.I. Nesterets, T.E. Gureyev, K.M. Pavlov, D.M. Paganin, S.W. Wilkins (2006). Combined analyser-based and propagation-based phase-contrast imaging of weak objects. Optics Communications, 259(1), 19-31.
6) R.A. Lewis, N. Yagi, M.J. Kitchen, M.J. Morgan, D. Paganin, K.K.W. Siu, K. Pavlov, I. Williams, K. Uesugi, M.J.Wallace, C.J. Hall, J. Whitley, S.B. Hooper (2005). Dynamic imaging of the lungs using X-ray phase contrast. Phys. Med. Biol. 50, 5031-5040.
7) Dahliyani Briedis, Karen KW Siu, David M Paganin, Konstantin M Pavlov and Rob A Lewis (2005). Analyser-based mammography using single-image reconstruction. Physics in Medicine and Biology, 50 3599-3611.
8) M.J. Kitchen, R.A. Lewis, N. Yagi, K. Uesugi, D. Paganin, S.B. Hooper, G. Adams, S. Jureczek, J. Singh, C.R. Christensen, A.P. Hufton, C.J. Hall, K.C. Cheung, and K.M. Pavlov. (2005) Phase contrast X-ray imaging of mice and rabbit lungs: a comparative study. British Journal of Radiology 78, 1018-1027.
9) K. K.W. Siu, M. J. Kitchen, K.M. Pavlov, John. E. Gillam, R.A. Lewis, K. Uesugi, N. Yagi. (2005) An improvement to the diffraction enhanced imaging method that permits imaging of dynamic systems. Nucl. Instr. And Meth. A 548, 169-174.
10) K. M. Pavlov, T. E. Gureyev, D. Paganin, Y. I. Nesterets, M. Kitchen, K. K.W. Siu, J. Gillam, K. Uesugi, N. Yagi, M. J. Morgan, R. A. Lewis. (2005) Unification of analyser-based and propagation-based X-ray phase-contrast imaging. Nucl. Instr. And Meth. A 548, 163-168.
11) Pavlov, K.M., Gureyev, T.E., Paganin, D., Nesterets, Ya.I., Morgan, M.J. and Lewis, R.A. (2004) Linear systems with slowly varying transfer functions and their application to X-ray phase-contrast imaging. J. Phys. D: Appl. Phys. 37, 2746-2750.
12) D. Paganin, T.E. Gureyev, K.M. Pavlov, R.A. Lewis, M. Kitchen (2004). Phase retrieval using coherent imaging systems with linear transfer functions. Optics Communications, 234, 87-105.
13) Ya.I.Nesterets, T.E.Gureyev, D.Paganin, K.M.Pavlov and S.W.Wilkins (2004) Quantitative diffraction-enhanced X-ray imaging of weak objects. J. Phys. D: Appl. Phys. 37, 1262-1274.
14) K.M. Pavlov, C.M. Kewish, J.R. Davis and M.J. Morgan (2001). A Variant on the Geometrical Optics Approximation in Diffraction Enhanced Tomography. J. Phys. D: Applied Physics 34, A168-A172.
15) C.M. Kewish, J.R. Davis, A.Y. Nikulin, N. Benci, K.M. Pavlov, M.J. Morgan, J.R. Hester (2001). Implementation of an Analyser Crystal Method for X-ray Diffraction Tomography. J. Phys. D: Applied Physics 34, 1059-1064.
16) K.M. Pavlov, C.M. Kewish, J.R. Davis, M.J. Morgan (2000): A new theoretical approach to X-ray diffraction tomography, Journal of Physics D: Applied Physics 33, 1596-1605.