DOI Self-Calibration Method Based on Intrinsic Radiation for Positron Emission Tomography

Depth-of-interaction (DOI) decoding can reduce localization errors in the Line-of-response (LOR), thereby improving positron emission tomography (PET) image quality, especially in high-resolution PET, long-axial PET, and small-aperture PET systems. Current PET detectors are capable of achieving high-resolution DOI decoding, but most methods rely on collimation calibration for each detector. Calibrating all detectors is labor-intensive and complicates subsequent system maintenance. We aim to achieve DOI collimation for PET detectors based on the intrinsic radiation of 176Lu, which allows us to omit the complex step of calibrating DOI for all detectors. This approach allows DOI calibration to be performed automatically during each system operation, simplifying system maintenance. The pixelated LYSO detector in this study consists of LYSO pixels with dimensions of 2 × 2 × 20 mm3. The detector is arranged in an 8×10 array configuration. We obtained the relationship between intrinsic radiation and DOI through Monte Carlo simulations, and the relationship between the MPPC signal ratio (the relative proportion of light detected by different MPPCs from the same crystal, which changes with DOI) and intrinsic radiation through experiments. This allowed us to establish a conversion function between DOI and the MPPC signal ratio. We verified the DOI resolution through collimation testing and demonstrated the improvement in image quality of the DOI self-calibration based on intrinsic radiation method using the Derenzo phantom experiment. We validated the DOI resolution of our method through mechanical collimation experiments at depths of 4 mm, 10 mm, and 16 mm, achieving an average DOI resolution full width at half maximum (FWHM) of 5.2 mm and a Mean Absolute Error (MAE) of 3.6 mm. The images reconstructed with DOI correction obtained through self-collimation demonstrated significantly improved signal-to-noise ratio and spatial resolution compared to images without DOI correction. Specifically, the spatial resolution at an offset of 45 mm improved from 3.0 mm to 2.1 mm. The DOI self-calibration method based on intrinsic radiation provides a practical and efficient solution for calibrating PET detectors without the need to add complex hardware. It should be noted that this method is only applicable to PET systems with detectors employing LSO or LYSO, and therefore this method is based on spontaneous emission of 176Lu. This method has the potential to improve the image quality of PET systems and can be readily integrated into existing detector designs.