Entwicklung und Charakterisierung eines dedizierten PET-Prototyps für die in-vivo Bildgebung von adulten Zebrafischen und kleinen Wassertieren

The development of a positron emission tomography (PET) prototype
dedicated for the imaging of adult zebrafish and small aquatic animals is the central
goal of the MERMAID project and the core of this dissertation. In the field of
biomedical research, nuclear imaging and PET play an important role in functional
imaging of model organisms such as mice and rats. Due to a steadily growing interest
in zebrafish as an additional model organism, there is a need for a dedicated PET
scanner that is specifically adapted to the needs of zebrafish imaging, which are not
met by current, commercially available, small animal PET scanners.
The developed MERMAID prototype consists of pixelated scintillation detectors
with one-to-one coupled silicon photomultipliers (SiPMs). This type of coupling,
together with the very small individual crystals, is intended to ensure a particularly
high spatial resolution of the prototype. The individual crystals are arranged in
matrices with 16×8 elements, which together with two SiPMs with 8×8 detector
elements each form a detector module. Two detector modules are then arranged
in a detector head at an angle of 33◦ to each other. The prototype has a total of
two detector heads with a distance of 66 mm, which rotate around the object to be
examined. Together with the necessary readout electronics and other mechanical
components, the detectors are combined in a mobile structure.
All individual components of the system were calibrated, characterized and their
interaction checked for the planned imaging. With the selected components, the
system achieves an energy resolution of 21.6% at 511 keV for 18F-FDG despite a
pronounced saturation behavior of the SiPMs. The time resolution is just below
300 ps. Using a reconstruction algorithm adapted to the system geometry, the system
achieves a high average spatial resolution of 0.74mm over all axes for point
sources. Measurements with more complex image quality phantoms based on standards
for small animal PET scanners (NEMA NU4-2008) have further demonstrated
the suitability of the system for the planned measurements, even if the field of view
along the scanner’s axis of rotation still requires a large number of measurements
for larger objects due to its length of 7 mm.
In addition to the prototype, an imaging chamber was designed for the in-vivo
imaging of adult zebrafish, which gently immobilizes the fish under a constant flow
of water and thus enables longer measurements. This chamber and initial approaches
to tracer administration have already been tested in in-vivo experiments.