A Cone-Shaped Phantom for Assessment of Small Animal PET Scatter Fraction and Count Rate Performance

Abstract

PurposePositron emission tomography (PET) image quality deteriorates as the object size increases owing to increased detection of scattered and random events. The characterization of the scatter component in small animal PET imaging has received little attention owing to the small scatter fraction (SF) when imaging rodents. The purpose of this study is first to design and fabricate a cone-shaped phantom which can be used for measurement of object size-dependent SF and noise equivalent count rates (NECR), and second, to assess these parameters for two small animal PET scanners as function of radial offset, object size and lower energy threshold (LET).MethodsThe X-PET™ and LabPET-8™ scanners were modeled as realistically as possible using GATE Monte Carlo simulation platform. The simulation models were validated against experimental measurements in terms of sensitivity, SF and NECR. The dedicated phantom was fabricated in-house using high-density polyethylene. The optimized dimensions of the cone-shaped phantom are 158 mm (length), 20 mm (minimum diameter), 70 mm (maximum diameter) and taper angle of 9°.ResultsThe relative difference between simulated and experimental results for the LabPET-8™ scanner varied between 0.7% and 10% except for a few results where it was below 16%. Depending on the radial offset from the center of the central axial field-of-view (3–6 cm diameter), the SF for the cone-shaped phantom varied from 26.3% to 18.2%, 18.6 to 13.1% and 10.1 to 7.6% for the X-PET™, whereas it varied from 34.4% to 26.9%, 19.1 to 17.0% and 9.1 to 7.3% for the LabPET-8™, for LETs of 250, 350 and 425 keV, respectively. The SF increases as the radial offset decreases, LET decreases and object size increases. The SF is higher for the LabPET-8™ compared with the X-PET™ scanner. The NECR increases as the radial offset increases and object size decreases. The maximum NECR was obtained at a LET of 350 keV for the LabPET-8™ and 250 keV for the X-PET™. High correlation coefficients for SF and NECR were observed between the cone-shaped phantom and an equivalent volume cylindrical phantom for the three considered axial fields of view.ConclusionsA single cone-shaped phantom enables the assessment of the impact of three factors, namely radial offset, LET and object size on PET SF and count rate estimates. This phantom is more realistic owing to the non-uniform shape of rodents’ bodies compared to cylindrical uniform phantoms and seems to be well suited for evaluation of object size-dependent SF and NECR.

Scatter Characterization and Correction for Simultaneous Multiple Small-Animal PET Imaging

“…timing window of 10 ns and lower energy threshold of 250, 350 and 425 keV (the upper threshold was set to 650 keV). As expected, the SF decreased with increasing the LET [8].…”

“…As such, it is considered as a useful tool to assist in the development and evaluation of scatter correction techniques. An experimentally validated Monte Carlo simulation model of the LabPET™-8 scanner was used in this work [8]. The LabPET™-8 scanner is modelled as realistically as possible in terms of geometry, physics of radiation transport and signal processing using the Geant4 Application for Tomographic Emission (GATE) Monte Carlo simulation toolkit [26].…”

“…Attenuation is the loss of true events due to scatter and absorption which impacts the measured data and results in inaccurate quantification of reconstructed PET images, whereas scatter degrades image contrast and affects both relative and absolute PET quantification. The scatter magnitude and spatial distribution depend mainly on the size and density of the object and scanner-related parameters such as design geometry, detector energy resolution and acquisition energy window [8]. In clinical PET imaging, it is well established that scatter correction is essential for accurate quantification of tracer uptake since the scatter fraction (SF) varies between 30 and 35 % in brain imaging and 50 and 60 % in whole-body imaging in three-dimensional acquisition mode [9,10].…”

Guide to Plant-PET Imaging Using 11CO2

“…Such considerations led to the development of the noise equivalent count rate (NECR) as a metric of PET system performance ( Strother et al, 1990 ; Yang and Peng, 2015 ). Conventionally, NECR is measured for clinical PET systems by scanning the cylindrical NEMA phantom (20 cm diameter × 70 cm long) but it can be measured for laboratory animal PET systems as well by making use of a cone-shaped phantom ( NEMA, 2012 ; Prasad and Zaidi, 2012 ). However, the most challenging image degrading factor with regard to imaging plants is related to the positron range.…”

Development of computational small animal models and their applications in preclinical imaging and therapy research

“…These are commonly combined with simulation tools accounting for all physical aspects involved in the image acquisition process and characteristics of the imaging system to generate a simulated dataset that closely mimic clinical and experimental studies. The known features of computational models and simulated datasets provide precise information enabling to evaluate the impact of physical degrading factors inherent to the imaging process, [131][132][133][134][135] assess different design concepts and performance of medical imaging systems, [136][137][138][139][140][141][142][143][144][145][146][147][148][149][150] and advance the development and validation of new image segmentation, [151][152][153][154][155] registration, 156-161 reconstruction, [162][163][164][165][166][167] and processing techniques. [168][169][170][171][172][173][174][175] Likewise, the Digimouse and MOBY models served as optically heterogeneous virtual subjects for light propagation calculations to assess the impact of various parameters involved in optical molecular imaging techniques [176]…”