Towards a Timepix4 compact gamma camera for coded aperture nuclear medicine imaging with depth resolution.

The Timepix4 $24.7\times$ 30.0 ${mm^{2}}$ hybrid pixel detector readout ASIC ({55 $\mu m$ pitch) is under test by the Medipix4 Collaboration. A dedicated readout system is under development at University and INFN Ferrara. We have designed a compact gamma camera based on a $1 {mm}$ thick CdTe semiconductor pixel detector for nuclear medicine tasks, $e.g.$, sentinel lymph node imaging with Tc-99$m$ radiotracer and I-123 thyroid imaging. The detector will be coupled to a coded aperture collimator (modified uniformly redundant array, rank 31, antisymmetric, with $0.25 {mm}$ diameter round holes made of 3D printed $1 {mm}$ thick tungsten). The detector assembly works in minification for a variable field of view. Geant4 Monte Carlo simulations showed that for a Tc-99$m$ source, at $50 {mm}$ source-collimator distance, where the field is $123.5 \times$ 150.0 ${mm^{2}}$, the collimator sensitivity is 370 times that of a single hole in the mask and the spatial resolution is $<$ 2 ${mm}$ FWHM. Real-time coded aperture image reconstruction is performed via autocorrelation deconvolution. First tests with a silicon pixel detector bump-bonded to a Timepix4 chip show time-over-threshold and time-of-arrival capabilities with Sr-90, Am-241 and Fe-55 sources.