Reconstruction Parameter Assessment for Small Animal Imaging on a Clinical SiPM PET/CT

Introduction: Small animal imaging requires high resolution and good contrast to identify small structures and sites of disease being studies. In another submitted work, we showed the feasibility of imaging small 25 gram mice on a clinical SiPM PET/CT. Clinical systems and their associated processing and reconstruction algorithms are optimized for performance based on typical patient sizes and may have issues with reconstructing objects that are far outside of the geometric assumptions of the system. These systems are also tuned to work best within typical patient dose ranges which are 50-100x more than what typically is injected into a rodent. This study examines the impact of different reconstruction parameters on image quality and quantification related to mouse imaging on a clinical SiPM PET/CT system.

Methods: A 25 gram wild type mouse was injected with 130 microCuries of a research 18F-fluorine based tracer. The animal was sacrificed after a 40 minute uptake period and placed into the center of the field of view. A single bed acquisition was acquired for 20 minutes at both 79 minutes and 169 minutes post-injection. A series of reconstructions was performed using the manufacturer's reconstruction options of OSEM, OSEM+TOF, PSF, and PSF+TOF. As subsets are locked to 5 on our system, iterations were stepped by 2 from 3 to 21 with data processed and reconstructed using the JSRecon and e7 sino tools from Siemens Healthineers. Image panels were created to qualitatively compare reconstruction techniques and settings. Regions of interest were drawn on each image volume to assess changes in image noise and quantitative values with each reconstruction and parameter change.

Results: Reconstructed images show improvements in resolution and contrast as iterations increase with increased details even rendered at 21 iterations, although artifacts start to become more prevalent at that number. At 21 iterations for PSF+TOF, details in the structure of small 4.3 mm kidneys can be seen in the images. PSF+TOF shows a marked improvement in image quality even in the small object as compared to OSEM or OSEM+TOF. Peak to valley assessments show improvements in recovery of values with increasing iteration with this effect most pronounced for PSF+TOF reconstruction methods. The figure shows an image panel showing examples from 3, 11, and 21 iterations for each reconstruction technique used.

Conclusions: Small animal imaging is possible on clinical SiPM-based systems with increased iterations recommended to compensate for the small object size. Good object recovery is seen with higher iterations with the best compromise in image quality vs. artifacts seen by using at least 11 iterations or higher. Quantitative recovery of activity is seen to be stable across iterative techniques with all methods showing reasonable calculations of the expected on-board activity within the animal at the time of imaging even at the small volumes being resolved in the system. Small animal imaging is feasible in a clinical PET/CT system with adjustments needing to be made to compensate for the small object size with the use of PSF+TOF techniques with higher numbers of iterations.

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