Improvement of depth estimation for crosshair light sharing TOF-DOI PET detector

<i>Objective.</i>We previously developed a crosshair light sharing (CLS) PET detector capable of simultaneously obtaining depth-of-interaction (DOI) and time-of-flight information. The CLS detector based on a single-ended readout acquires DOI information from the output ratio between two SiPMs by sharing light between a pair of crystal bars. However, the nonlinearity of the DOI response was induced a challenge due to the loop-shaped light path being twice as long as crystal length. In this work, we developed a new high-precision DOI estimation method for the CLS detector.<i>Approach.</i>The CLS detector uses a 2D crystal array with three layers of reflective material that forms loop structures within a pair of crystals. The dimensions of the fast lutetium-gadolinium oxyorthosilicate crystals are 1.45 × 1.45 × 15 mm³. The developed method uses the single output rather than an output ratio for DOI estimation. This approach circumvents the nonlinearity issue by using the output of transfer side for detection depths up to 6 mm, and the output of the interaction side for depths over 6 mm.<i>Main Results.</i>Results from slit irradiation measurements demonstrated that while the output ratio method provided a DOI resolution of over 4 mm, the proposed method achieved a DOI resolution of 1.6-1.9 mm. Additionally, the CLS detector achieved a coincidence resolving time (CRT) of 191 ps.<i>Significance.</i>Our developed method allows the CLS detector to simultaneously achieve a DOI resolution of less than 2 mm and the CRT of less than 200 ps.