Characterization of Multimodal Preclinical Imaging Systems

Abstract

Background A setup of four compatible imaging systems for small animal from Bruker has been delivered to Sahlgrenska Academy at University of Gothenburg and Sahlgrenska University Hospital, and forms the basis of SBIC (Sahlgrenska Bioimaging Center). The aim of the study was to characterize the performance of the positron emission tomography (PET)system and computed tomography (CT)system at SBIC as well as the dose calibrator and two radiation protection instruments, but also to analyse two methods of attenuation correction using either data from CT or magnetic resonance imaging (MRI). Method The response of the dose calibrator was characterized for geometric factors and activity dependence using 99mTc, 177Lu, 131I, 125I and 18F sources. The PET-system was characterized by measuring the sensitivity, correction factors, linearity, highest and lowest measurable activity, scatter fraction, spatial resolution, uniformity and recovery coefficients using various phantoms filled with 18F-FDG (fluorodeoxyglucose). To characterize the CT-system three protocols, two reconstruction algorithms, uniformity, spatial resolution and noise were measured using two phantoms. CT- and MRI-based attenuation correction of PET images of a tumour bearing mouse injected with 68Ga-DOTATOC was analysed by comparing the activity concentrations in various tissues. Two radiation protection instruments, Berthold LB 122 and 124, were characterized by examining the linearity and minimum detectable activity for two phantoms containing 131I. Results Using the dose calibrator, a linearity was identified for all radionuclides, the suggested working region was 2-8 cm, larger uncertainties were found for smaller volumes, and a radionuclide dependence was identified. The sensitivity of the PET-system varied with activity, however, a linear correlation was identified with enlarged uncertainties for lower activities. Best uniformity and resolution were achieved from the image without attenuation correction. Highest and lowest measurable activity was 41 MBq and 0.1 MBq, respectively. The CT-system achieved the lowest variation of Hounsfield units (HU) when using the high-resolution protocol, but the resolution was similar for the high-resolution and the generalpurpose protocols. The CT-attenuation correction of PET images resulted in higher activity concentration than MRI-attenuation correction. A linear response was identified for both radiation protection instruments for each measurement setup. The minimum detectable activity was estimated to 35 kBq at 10 cm distance using Berthold LB 122. Conclusions The dose calibrator is appropriate to use under the suggested working conditions. Essay/Thesis: 30 hp Program and/or course: Medical physics Level: Second Cycle Term/year: Fall 2025 Supervisor: Eva Forssell-Aronsson, Hana Bakr, Johan Spetz, Lukas Lundholm, Mikael Montelius Examiner: Magnus Båth Keywords: Preclinical imaging, Multimodal, Characterization, small animal PET, micro-CT, MRI attenuation correction, Bruker, Molecubes 3 The sensitivity of the PET-system, calculated in the software PMOD, was activity dependent. The system had a resolution of at least down to 2 mm and the measurable 18F-FDG activity range was 0.1- 37 MBq. For the CT-system, the Feldkamp, Davis and Kress (FDK) algorithm with helical acquisition was less good and the image space reconstruction algorithm (ISRA) should be usen when quantification is needed. The MRI-based attenuation correction method evaluated underestimated the activity concentration compared with CT-based attenuation correction and needs to be improved. Altogether, the PET- and CT-systems were well characterized, but further studies on phantoms and animals would be interesting and give more information. The two radiation protection instruments worked well.