A High Spatial Resolution CT Scanner for Small Animal Imaging

A micro-CT system prototype has been built at INFN in Pisa. This system will be later integrated with a micro-(S)PET scanner. Its main components are: a microfocus X-ray source with a peak voltage of up to 60 kV, a power of 10W and a focal spot size of 30 micrometers; a CCD coupled to columnar-grown CsI(Tl) scintillator for the detection of the X-rays, 6.144mm high by 147.456mm long, subdivided into 128 by 3076 square pixels, each with a size of 48 micrometers x 48 micrometers; stepping motors for the phantom roto-translation; a PCI acquisition board. Dedicated electronic boards were built for the control and the read-out of the CCD and a program in Labview was developed for the data acquisition. Reconstruction algorithms were implemented for fan-beam (2-D reconstruction) and cone-beam configuration (3-D reconstruction). The use of a CCD as X-ray detector together with an X-ray source, that operates in continuous mode only, has introduced a specific noise problem: this noise generates “ghosts” in the planar images, and therefore also in the tomographic images. This problem was overcome by developing a specific “double acquisition” technique. Planar images of a bar pattern (0.05 mm-thick lead layers, from 0.25 up to 10 line pairs per mm) were acquired to evaluate the X-ray detector performance: the CTF curve was extracted from the data, obtaining a value of 10% at 5 line pairs per millimeter and about 3% at 10 line pairs per millimeter. Tomographic acquisitions of a test phantom were performed: this phantom consists of a Plexiglas cylinder, 3 cm of diameter, with holes from 3 mm down to 0.6 mm of diameter, filled with different materials. The contrast resolution was extracted from the reconstructed images: a value of 6 % (in water) for a cubic voxel size of 80 micrometers was obtained. At present, we are working to improve the acquisition speed of the system and to optimise the acquisition parameters (such as the X-ray source spectrum).