Nuclear Instruments and Methods in Physics Research A 576 (2007) 204–208 Preliminary characterization of a single photon counting detection system for CT application N. Belcari, M.G. Bisogni, C. Carpentieri, A. Del Guerra, P. Delogu, D. Panetta à , M. Quattrocchi, V. Rosso, A. Stefanini Department of Physics, University of Pisa and INFN, Sezione di Pisa, Pisa, Italy Available online 6 February 2007 Abstract The aim of this work is to evaluate the capability of a single photon counting acquisition system based on the Medipix2 read-out chip for Computed Tomography (CT) applications in Small Animal Imaging. We used a micro-focus X-ray source with a W anode. The detection system is based on the Medipix2 read-out chip, bump-bonded to a 1 mm thick silicon pixel detector. The read-out chip geometry is a matrix of 256  256 cells, 55 mm  55 mm each. This system in planar radiography shows a good detection efficiency (about 70%) at the anode voltage of 30 kV and a good spatial resolution (MTF ¼ 10% @ 16.8 lp/mm). Starting from these planar performances we have characterized the system for the tomography applications with phantoms. We will present the results obtained as a function of magnification with two different background medium compositions. The effect of the reconstruction algorithm on image quality will be also discussed. r 2007 Elsevier B.V. All rights reserved. Keywords: Single photon counting; Medipix2; MicroCT; Spatial resolution 1. Introduction Computed Tomography (CT) is a very important tool for non-invasive 3D imaging of opaque objects. Dedicated CT scanners for inspection of small specimens with micrometric resolution (MicroCT) have been developed, finding applications in biomedical research [1] and in non- destructive testing (NDT) of materials and components [2]. In our laboratory, we have assembled a MicroCT scanner prototype for small animal imaging [3], based on a CCD+CsI:Tl detector. Recently, we have modified this prototype by substituting the CCD+CsI:Tl detector with a single photon counting detection system; the main goal of this study is to characterize the new prototype in terms of spatial resolution for various geometric configurations and to make a preliminary comparison of the performances of the two detection systems in CT. 2. Materials and methods The main components of the MicroCT prototype are: a micro-focus X-ray from Hamamatsu with tungsten anode, a peak voltage up to 60 kV and a maximum power of 10 W; a rotator/translator stage for the motion of the specimen; a 1 mm thick silicon pixel detector produced in collaboration with ITC-irst [4], operating at 240 V and bump-bonded to the Medipix2 chip. The detector is a matrix of 256  256 square cells, with 55 mm side. Each of the electronic cells has a low noise preamplifier, two pulse height discrimina- tors and a 13 bit pseudo-random counter [5]. The readout system is able to set both low- and high-energy thresholds, allowing the selection of an energy window in the X-ray output spectrum. The aim of this work is to assess the spatial resolution capabilities, in different geometrical conditions, of a MicroCT scanner prototype based on the photon counting detection system described above. To do this, we used a simple phantom composed by a hollow plastic cylinder of 3 cm diameter and 1 mm wall thickness, in which we placed a tungsten wire of 20 mm of diameter. The thin wire allows ARTICLE IN PRESS www.elsevier.com/locate/nima 0168-9002/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2007.01.153 à Corresponding author. Tel.: +39 050 2214 334; fax: +39 050 2214 333. E-mail address: daniele.panetta@pi.infn.it (D. Panetta).