Practical use of the Modified Bronnikov Algorithm in micro-CT M. Boone a,∗ , Y. De Witte a , M. Dierick a , J. Van den Bulcke b , J. Vlassenbroeck a , L. Van Hoorebeke a a Ghent University, Subatomic and Radiation Physics Proeftuinstraat 86 9000 Gent b Ghent University, Department of Forest and water management Coupure Links 653 9000 Gent Abstract Through the years, the resolution of X-ray Computed Tomography (CT) systems has increased rapidly, in particular for the newer micro- and nano-CT systems. With this increasing resolution, the limits of absorption contrast CT are being reached. At the same time, a new type of contrast becomes visible: phase-contrast. Mainly for low absorbing objects such as insects and wood, phase-contrast can lead to a new type of CT reconstruction using the Modified Bronnikov Algorithm (MBA)[1]. Despite it’s theoretical limitation to pure phase objects, the algorithm has some clear advantages with respect to filtered back-projection (FBP). The MBA is therefore commonly used at the Centre for X-ray Tomography of the Ghent University (UGCT) to obtain additional information for optimal scanning results. Full article can be found at http://dx.doi.org/10.1016/j.nimb.2009.01.129 Key words: Phase contrast, Modified Bronnikov Algorithm, Phase propagation imaging, micro-CT PACS: 42.30.Wb, 81.70.Tx 1. Introduction Since the development of X-ray computed tomography (CT) by Hounsfield in 1972, the basic principle has not changed. A sample is placed between an X-ray source and a detector and rotated relative to the source-detector system. X-rays are attenuated in the sample, giving rise to a projection image. By taking images at different angles, the linear attenuation coefficient μ can be calculated by the standard filtered back projection algorithm (FBP) in each volume element of the object(voxel), starting from the Lambert-Beer law I/I 0 =  X−raypath e −μ dL (1) where I 0 is the incident X-ray intensity and I the trans- mitted intensity.[2] For low-density objects such as wood, μ is rather low. For increasing resolution, thus when samples become very small, this leads to insufficient absorption contrast for a good reconstruction. At the same time, a new type of contrast becomes visible for this kind of samples: phase contrast. Caused by a phase shift of the propagating wave, a typical edge enhancement effect is visible in the images. Although this results in seemingly sharpened radiographs, it induces * Corresponding author. Tel. +32 9264 6628 Fax +32 9264 6697 Email addresses: Matthieu.Boone@UGent.be (M. Boone) URL: http://www.ugct.ugent.be (M. Boone) erroneous values and artefacts in the CT reconstruction. Different techniques are available to exploit this effect. For x-ray tubes the two most popular are grating interferom- etry [3, 4] and phase propagation imaging[5, 6]. Other techniques are available at synchrotron radiation setups, but considering the high operating cost and low accessi- bility, these do not serve the purpose of easy and frequent usage. Phase propagation imaging results in mixed phase and absorption images. A reconstruction algorithm to recon- struct only the phase component of these images, the Mod- ified Bronnikov Algorithm (MBA), will be explained in this article and tested on several different samples. The advan- tages and disadvantages of the algorithm will be shown on the basis of these examples. 2. Theoretical background The interactions of light with a medium can be de- scribed by its refractive index n(λ): n(λ)=1 − δ(λ)+ iβ(λ) (2) where λ is the wavelength. The imaginary part iβ is the ex- tinction coefficient, responsible for absorption of the wave in the medium. The real part 1 − δ denotes the ratio of the phase velocity of the wave through vacuum and the phase velocity in the medium. For visible light, this δ is between 0 and −1 for most materials, meaning the phase velocity is smaller in a medium than it is in vacuum, which causes refraction, a deviation of the propagation direction of the Preprint submitted to NUCLEAR INSTRUMENTS AND METHODS IN PHYSICS RESEARCH SECTION B November 9, 2009