Factors Affecting In-line Phase Contrast Imaging with a Laboratory Microfocus X-ray Source

An in-line method of phase contrast imaging is investigated using a polychromatic laboratorybased microfocus X-ray source. The effect of source size is considered, and by introducing water into the imaging path the influence of water on the phase contrast is determined. The results confirm that good phase contrast images can be achieved using a polychromatic laboratory-based X-ray source, and demonstrate the importance of transverse coherence length. Phase contrast images are achieved with samples containing water, however beam hardening has a detrimental effect on phase contrast. INTRODUCTION Conventional X-ray imaging relies on absorption contrast, which is excellent for many applications but has shortcomings when attempting to image materials of low or similar density [1]. Here, phase contrast imaging comes into its own because it relies not on the complex term of the material refractive index (the absorption component) but the real term representing phase [2]. The phase of an X-ray beam is altered on passing through a material according to the following: ( ) dz z ∫ ∞ − ⎟ ⎠ ⎞ ⎜ ⎝ ⎛ − = 0 2 δ λ π φ (1) [3] where z is the sample thickness and δ is the complex component of the refractive index, by β α i n + − = 1 (2) [4] Much of the X-ray phase contrast imaging work published to date has been performed using monochromatic synchrotron radiation. Due to the high intensities available across a wide range of wavelengths, monochromated synchrotron radiation is commonly used for phase contrast imaging [5]. Realistically, however, synchrotron radiation cannot be used for the majority of potential commercial applications of phase contrast imaging. Thus further development of phase contrast imaging techniques for practical application ultimately depends on the suitability of laboratory X-ray sources. In practice, low dispersion monochromatic X-ray radiation such as that obtained at synchrotron sources is not achievable by laboratory-based microfocus X-ray sources. Wilkins et al have already determined that a laboratory X-ray source can provide good phase contrast images with its natural polychromatic spectrum [6]. Another example of polychromatic X-ray phase contrast imaging uses a scorpion sting, shown in Figure 1. While an absorption image of the scorpion sting shows up minimal detail, the phase contrast image retains far more information. This includes a crisp outline of the sting, clear edge detail within the poison sac and tail joint, and the narrow shaft through which poison is injected into the scorpion’s prey. This 31 Copyright ©JCPDS-International Centre for Diffraction Data 2006 ISSN 1097-0002