N. Sabba

An integrated PET-SPECT small animal imager: preliminary results

The authors have successfully built and characterised a small animal PET based on 4 rotating detectors with a spatial resolution <2 mm over its field of view and a sensitivity of 640 cps//spl mu/Ci at the centre. The scanner is based on four matrices of 400 YAP:Ce finger crystals (2/spl times/2/spl times/30 mm/sup 3/ each) coupled to Position Sensitive PhotoMultipliers (Hamamatsu R2486-06.) The authors have now applied two high resolution collimators to two opposite detectors, hence realising an integrated PET-SPECT scanner for small animals. The collimators are made of lead with 20 mm long, 0.6 mm hexagonal holes with 0.15 mm septa. The read-out and data acquisition system are handled by NIM-CAMAC standard electronics. The Field Of View (FOV) of the tomograph has a diameter of 4 cm and an axial length of 4 cm in both PET and SPECT configuration which is appropriate for mice and rat studies.

YAP-(S)PET small animal scanner: Quantitative results

The University of Ferrara YAP-(S)PET scanner is currently being employed in small animal SPECT studies, with /sup 99m/Tc labeled radiotracers, with the goal of obtaining quantitative activity measurements from region of interest (ROI) analysis of reconstructed images of rats. The measurements will be compared directly with traditional ex vivo measurement of activity with gamma counters. To achieve this goal, the scanner was calibrated relative to a standard Isodose dose calibrator which was calibrated with various certified activity sources. The calibration factor K was defined as the ratio of the activity measured with the Isodose (MBq) and the image count rate (cps). We find K = 0.1346 /spl plusmn/ 0.0008 MBq/cps, with good linearity over a wide range of activities (9-88 MBq). With the calibrated scanner we compared results of activity measurements from images of whole-rat heart acquisitions versus excised hearts. We found good agreement between the two measurements.

High Z and medium Z scintillators in ultra high resolution small animal PET

As small animal PET scanners are continuously improving in their performances, one is lead to the question of how far can spatial resolution go. In this paper we address the limiting effects to spatial resolution and whether the photoelectric interaction, and therefore high Z materials, outperform medium Z scintillators. In particular, with a Monte Carlo simulation, we compare the ultimate performances, in spatial resolution, of three scintillators: BGO, NaI(Tl) and YAP:Ce. BGO is the PET scintillator which has the highest photofraction whereas YAP:Ce has the lowest. NaI(Tl), instead is a relatively high Z but low density scintillator. There are three principle contributions to the degradation of spatial resolution: multiple Compton scattering electron range after a gamma interaction and K-shell fluorescence emission. We present the results of simulations of crystals with different thicknesses, with and without K-shell fluorescence emission and electron transport. We conclude that the effect of multiple scattering, electron range and fluorescence emission to the spatial resolution are smaller for low Z, high density materials like YAP:Ce. The fraction of misplaced events, defined here as F = N/sub Wrong//N/sub Tot/, is F/sub 0.5mm/ = 52% for BGO in the case of 0.5 mm binning, increasing to F/sub 0.1mm/ = 80% for the 0.1 mm binning. In the case of YAP:Ce, the scatter fractions are respectively F/sub 0.5mm/ = 27% and F/sub 0.1mm/ = 44%. We conclude that for ultra high resolution PET detectors, medium Z scintillators, such as YAP:Ce, may outperform high Z materials.

Latest achievements in PET techniques

Positron emission tomography (PET) has moved from a distinguished research tool in physiology, cardiology and neurology to become a major tool for clinical investigation in oncology, in cardiac applications and in neurological disorders. Much of the PET accomplishments is due to the remarkable improvements in the last 10 years both in hardware and software aspects. Nowadays a similar effort is made by many research groups towards the construction of dedicated PET apparatus in new emerging fields such as molecular medicine, gene therapy, breast cancer imaging and combined modalities. This paper reports on some recent results we have obtained in small animal imaging and positron emission mammography, based on the use of advanced technology in the field of scintillators and photodetectors, such as Position-Sensitive Detectors coupled to crystal matrices, combined use of scintillating fibers and Hybrid-Photo-Diodes readout, and Hamamatsu flat panels. New ideas and future developments are discussed.

A dedicated system for breast cancer study with combined SPECT–CT modalities

A prototype ofa combined CT–SPECT tomograph f or breast cancer study has been developed and evaluated. It allows to perform scintimammography and X-ray CT in the same geometrical conditions. The CT system is based on a quasi-monochromatic beam tuned at 28 keV and an array ofultra f ast ceramic scintillators coupled to photodiodes whilst the SPECT system is based on two scintillator matrices coupled to position sensitive photomultipliers. CT and SPECT sinograms ofa test phantom were recorded and reconstruted with both modalities. Image f ofCT and SPECT images was then performed. The developed CT–SPECT prototype is able to detect a region of interest of 1 cm 3 ,

Monte Carlo study and experimental measurements of breast tumor detectability with the YAP-PEM prototype

A prototype for positron emission mammography is under development within a collaboration of the Italian Universities of Pisa, Ferrara, Bologna and Roma. The device is composed of two stationary detection heads, each with an active area of 6 cm /spl times/ 6 cm, made of 30/spl times/30 YAP:Ce finger crystals of 2 mm /spl times/ 2 mm /spl times/ 30 mm. The EGSnrc Monte Carlo code has been used to perform a complete simulation of this camera. We have used a fast three-dimensional iterative algorithm (30 s per iteration on a PC-Pentium III 800 MHz processor) for image reconstruction. The performed study indicates that tumors of 5 mm diameter, i.e., 0.065 cm/sup 3/ volume, with 37 kBq/cm/sup 3/ (1 /spl mu/Ci/cm/sup 3/) specific activity embedded in a breast active phantom, are detectable in 10 minutes for a 10:1 tumor/background ratio with an 8.7 Signal-to-Noise Ratio value. Experimental measurements with the small animal tomograph YAP-PET have validated the Monte Carlo predictions.

Measurement of photoelectron yield from scintillating fibres coupled to a YAP:Ce matrix

Abstract In applications where high spatial resolutions in the determination of the position of an interaction are necessary crystal matrices are often used. The readout of such matrices can be difficult especially if these have a large number of pixels (>400). One possible way to solve the difficulty is by reading out the row and the column by coupling wavelength shifting fibres to the front and back of the matrix. Measurements have been carried out on the photoelectron yield of one fibre coupled to a 5×5 YAP:Ce matrix so as to prove the principle. The fibre was read out using a single pixel HPD. The results of these measurements will be presented.

Calibration and Performance of the Fully Engineered YAP-(S)PET Scanner for Small Rodents

Functional imaging of small animals,such as mice and rats,using high-performance positron emission tomography (PET)and single-photon emission tomography (SPECT), is becoming a valuable tool for studying animal models of human disease. The possibility to use either PET or SPECT allows the scientist to exploit the advantages of both techniques, e.g., the wide range of easily accessible single-photon radiotracers for SPECT and the exquisite performance of PET. The combination of PET and SPECT techniques for small-animal studies could offer the unique possibility of developing new and interesting protocols for the investigation of many biological phenomena more effectively than with PET or SPECT modality alone.

A triple modality device for simultaneous small animal CT and PET-SPECT imaging

Multimodality simultaneous functional and morphological imaging is being pursued by several research groups. In this line of research the high resolution YAP-(S)PET small animal integrated PET-SPECT imaging system, constructed by our group of medical physics at the University of Ferrara, is being upgraded with a computed tomography (CT). In this way it will be possible to perform in vivo molecular and genomic imaging studies on small animals (such as mice and rats) and at the same time obtain morphological information necessary for attenuation correction and in order to localize more accurately the region under investigation. We have take simultaneous PET-CT and SPECT-CT images of phantoms obtained with a single scanner.

Sampling considerations for high resolution small animal SPECT

Using the small animal hybrid PET/SPECT scanner YAP-(S)PET we demonstrate how we can improve both image quality and spatial resolution in SPECT modality, by acquiring data with sampling steps smaller than the detector intrinsic sampling pitch. Due to the planar configuration of our pixellated detector we can easily perform this by shifting detector heads from the central position of an arbitrarily small step tangential to the field of view. This acquisition technique, together with the knowledge of the detector response function, allows us to determine the best sampling step as a trade-off between image quality, resolution, and acquisition time. In the real case, the more one samples the FOV with a finer step, the more one reduces aliasing (better image quality). Furthermore one also records higher frequency components in the image spectrum (better spatial resolution). To demonstrate how this technique can enhance the image, images of capillaries and phantoms will be presented. In particular the trend of resolution versus sampling step is reported. In this paper we present two different kinds of results: first we reconstruct images, acquired with different sampling modalities, with the same spatial frequency cut-off; in this way we show how the decreasing of the sampling step enhances image quality for a fixed resolution. Second we reconstruct images by recovering all possible spatial frequencies so as to enhance spatial resolution. All images are deconvolved for the frequency response (transfer function) of our system by implementing deconvolution in a standard filtered back projection (FBP) algorithm. The results show that it is possible to approach the 2 mm resolution limit imposed by the crystal; in fact we can recover resolution down to 2.2 mm (/spl plusmn/3.5%).