Euromedim 2006 :1st European Conference on Molecular Imaging Technology

CT WITH A CMOS FLAT PANEL DETECTOR INTEGRATED ON THE YAP-(S)PET SCANNER FOR IN VIVO SMALL ANIMALS IMAGING

11 May 2006, 15:45

15m

Palais du Pharo, Marseille

oral S7 Multimodalities Multi modality

Speaker

Dr Giovanni Di Domenico (Universita' di Ferrara & INFN-Sezione di Ferrara - Italy)

Description

Introduction During last years the University of Ferrara has pursued an interest for in-vivo molecular and genomic imaging and has developed the YAPPET scanner [1], a integrated small animal PET-SPECT small animal scanner. The spatial resolution in PET is better than 1.8 mm whereas in SPECT it is better than 3.5 mm The PET sensitivity at the center is 640cps/µCi while in SPECT mode it is 1.1 cps/µCi with 4 detectors, constant over FOV. Now we are improving this device with a computed tomography (CT) in order to obtain simultaneously functional and morphological information and consequently we can, firstly, better visualize the region of interest and, secondly, obtain information on the attenuation coefficients necessary for improving reconstruction and quantitative measurements. At present we are characterizing and testing the CT-system with phantoms. Soon we will assemble the CT detector on the PET-SPECT device and at the congress in May we will present combined PET-CT and SPECT-CT images all acquired with a single scanner. Materials and Methods Materials: The CT system employed consist of a X ray tube and a large-area 2D image sensor. The tube has a tungsten anode with a focal spot of 0.6x0.6mm2; in the present measurements we used a 1.08mm aluminium filter giving an X-ray energy of 22 keV. The detector is a dual CMOS photodiode array (RadEye Inc.) whose large 49.2mm by 49.3mm active area consist of a 1024 by 1024 matrix of 48 µm silicon photodiodes. On the detector is deposited a uniform layer of Lanex Fast scintillato (130mg/cm2 of gadolinium oxysulfide Gd2O2S) which converts the X-rays into light. The detector is optimized to detect x-rays in 10keV to 50keV range. The flat panel detector, via an adapter board, is connected to the national instruments PCI-6111E board which acquires the signals. We have developed a dedicated LabVIEW programme that controls the image acquisition. In this way it is possible, via software, to change integration time, readout closck frequency and number of acquired images. The same LabVIEW program controls the X-ray exposure and motor motion and synchronizes all operations. In this preliminary stage we rotate the object under investigation keeping the detector and X ray tube fixed. The motor is a brushless DC-servomotors. In the final configuration the detector and the X ray tube will be fixed on the YAPPET gantry and consequently will rotate around the object under investigation. Methods: The X ray tube is 38cm from the object which in turn is at 4cm from the detector. The signals, that come from the CMOS detector, are converted with a 11 bits ADC and the counts per pixel are saved into hard disk in a raw-file format. Generally we use a clock at 625kHz, in this way the reading of the whole detector lasts 0.880 seconds. We have synchronized the X ray shoot and the acquisition in order to perform a reset before acquiring an image. Every image undergoes the following corrections: 1- dark current subtraction (at our integration time, 1sec, roughly 20 counts per pixel) 2- flat field “normalization” in order to correct the non uniform sensitivity pattern. Results and first CT-images We are aiming at high contrast images rather than high CT spatial resolution and the final voxel size will be of 0.5 x 0.5 x 0.5 mm3. CT images are taken with 180 projections over 180 degree in a total of 6 minutes. Finally we compared the attenuation coefficients obtained from the reconstructed images with literature values (XCOM: Photon Cross Sections Database-NIST Standard Reference Database 8) for 22keV X-rays (the mean energy of our spectrum). We report the values: µwater(literature at 22keV)=0.65cm-1;µPE(literature at 22keV)=0.349cm-1. µwater(measured)=0.581±0.015cm-1; µPE(measured)=0.361±0.006cm-1.