Euromedim 2006 :1st European Conference on Molecular Imaging Technology

Performance evaluation of a Small Animal PET Scanner. Spatial resolution characterization using 18-F and 11-C

9 May 2006, 14:00

1h

Palais du Pharo, Marseille

poster • System performance seen by the users, dose reduction Poster Session :Simulation, Modeling, Reconstruction

Speaker

Dr Antonello Spinelli (Policlinico S. Orsola-Malpighi, Bologna, Italy)

Description

Introduction The main objective of this work was to characterize the physical performance of a small animal PET scanner (GE eXplore Vista). The detector modules of the eXplore Vista consists of an 13 x 13 array of 1.45 mm square “phoswich” elements providing depth-of-interaction information. Each element is made of two scintillator crystal layers of Lutetium Yttrium Orthosilicate (LYSO) and Gadolinium Orthosilicate (GSO) optically glued together in order to form a single 15 mm long segment. The 36 detectors blocks are arranged in two rings giving a total number of phoswich elements equal to 6084. The transverse field of view (FOV) is equal to 6 cm while the axial FOV is equal to 4.6 cm. Typically PET images of small animals are performed using 18-F and 11-C labelled tracers. Because the mean positron range for the two isotopes is respectively equal to 0.6 mm and 1.1 mm, it is interesting to investigate the effect of such difference on the scanner spatial resolution. Materials and methods In order to evaluate the performance of the eXplore Vista several parameters such as spatial resolution, sensitivity, scatter fraction (SF) and noise equivalent counts (NEC) were measured. Spatial resolution was measured by using a point source, a line source and a custom made high resolution phantom. The point source (about 0.2 mm diameter) was obtained by soaking a small piece of zeolite with a 18-F or 11-C solution. The line source was obtained by filling a small needle (internal diameter of 0.3 mm) with 18-F or 11-C isotopes. In order to evaluate the dependence of spatial resolution with respect to transaxial axis position, the line source filled with 18-F was moved along the y axis and four measurements spaced by 5 mm were acquired. Images of a custom made high resolution phantom (HRP) with rectangular arrays of holes of 2, 1.5, 1.0 and 0.5 mm with the centres spaced by 4, 3, 2, 1 mm were also acquired. The HRP was filled with a solution of about 15 MBq of 18-F or 11-C and images were acquired for 1 h. All the images were reconstructed using Fourier rebinning and filtered back projection (ramp filter). In order to measure the absolute eXplore Vista sensitivity a small 18-F point source of about 1.2 MBq was placed at the centre of the field of view (FOV). The SF measurements were carried out using a small capillary (0.8 mm internal diameter) in air and in water (3 cm diameter cylindrical phantom). The SF measurement was performed accordingly with a method similar as the one described in the NEMA NU2–2001 report. In order to estimate the NEC values a rat sized phantom and a mouse sized phantom were filled with an high amount of 18-F (about 200 MBq) and let to decay for six half life. Measurements were acquired every 15 minutes. Results All the results described in this section are referred to the 250-700 keV energy window. The transaxial full width half maximum (FWHM) with the point source at the centre of FOV are respectively equal to 1.48 mm and 1.74 mm for 18-F and 11-C. The 18-F axial resolution is equal to 1.77 mm giving a volumetric resolution of 3.82 mm^3, while for 11-C the axial resolution is 1.91mm and the resulting volumetric resolution is equal to 5.78 mm^3. The FWHM measured with the 18-F line source at four transaxial positions were equal to 1.47 mm, 1.71 mm and 2.1 mm at the centre, 1 cm and 2 cm off-axis, respectively. Analysis obtained by drawing several profiles along the HRP hot spots confirmed a worsening in spatial resolution when using 11-C instead of 18-F (for high resolution small animal scanners). The absolute eXplore Vista sensitivity was equal to 2.2% (400-700 keV energy window), 3.9% (250-700 keV) and 5.9% (100-700 keV) without dead time correction applied. For a line source, the measured SF value in air was equal to 19.3%. SF values obtained in water were equal to 26.2% and 34% for a mouse and a rat-size phantom. The NEC peak rate were respectively equal to 147 kcps at 710 kBq/ml and 83 kcps at 169 kBq/ml for a mouse and rat-size phantom. Conclusions Results show that the eXplore Vista provides high spatial resolution and at the same time good sensitivity. The value of the spatial resolution for 11-C labelled tracers was worse respect to 18-F, such loss of resolution for 11-C was also evident by looking at in vivo mouse images. The eXplore Vista has been in use at our institution for the last twelve months showing good stability and high throughput.