Preliminary Results of a Depth-of-Interaction based PET System with Dual Ended Readout for Combined Breast PET/CT Imaging

Objectives: Dedicated PET and CT imaging of the breast in cancer patients can have an advantage due to better spatial resolution obtained in both modalities. We propose the development of a dedicated breast PET imager that utilizes PET detectors which provide depth-of-interaction (DOI) information to reduce the spatial resolution degradation at off-center locations in the trans-axial field of view (FOV). This PET system will finally be combined with a CT to provide sequential acquisition of PET and CT data of the pendent breast. We report on the preliminary imaging results of an initial single-ring prototype built using DOI PET detectors.

Methods: We have developed a modular PET detector based on 14x14 arrays of 1.5x1.5x20 mm³ Lutetium Yttrium Orthosilicate (LYSO) crystals, unpolished and separated by a diffuse reflector film. The module employs a dual ended readout using a Position Sensitive Photo-Multiplier-Tube (PSPMT, R8900, Hamamatsu) for crystal encoding and energy detection on the backside (away from the center) and a compact Avalanche Photo Diode (APD, 21x21 mm², Radiation Monitoring Devices) detector (2 mm thick) on the front side (close to the center) to obtain energy information. The DOI information is calculated as the ratio of the total energy detected by either photo-detector. In previous characterization experiments of this module achieved a DOI resolution of 1.8 mm and energy resolution of 18% in a bench-top setup. To readout all detectors of a complete PET ring we utilize a commercial PET electronics suite (Siemens Molecular Imaging) developed for PMT-based block detectors (4 channels per detector). We have modified the firmware of the PET electronics suite to use the analog inputs of two blocks detectors (4 + 4 channels) to decode both position and DOI information as required for our PET detector. Our prototype system consists of 24 block detectors arranged in a single ring of 200 mm inner diameter covering a transaxial FOV of 150 mm and an axial FOV of 22 mm. APDs were cooled below room temperature for low noise operation. We have measured the DOI resolution performance using the implemented firmware processing and have assessed the system’s point spread function as full width at half maximum (FWHM) of a reconstructed Na²² point source (20 µCi, with artificial background during reconstruction) at the radial center and at 2.5 and 5 cm off-center. Images were reconstructed using a listmode based maximum likelihood expectation maximization algorithm.

Results: We have successfully modified the PET electronics to readout the DOI PET block detectors. Using the modified firmware we obtained a DOI resolution of 3.1 mm and energy resolution of 24%. Acquired PET images of the point source at the center position revealed a FWHM of the measured point spread function of 1.4 mm without using DOI and 1.3 mm with DOI information. At off center positions of 2.5 cm and 5.0 cm the FWHM dropped to 2.1 and 2.8 mm without DOI but remained at 1.6 mm and 1.9 mm when using the DOI information.

Conclusion: Our results show the use of a DOI-capable detector can significantly reduce the effect of image blur for radially off-center objects. Using a commercial PET electronics is possible but resulted in some loss of performance due to the limited accuracy of the FPGA implementation. Research Support: This work was supported by grant NCI R01 CA129561, by the Rusch Fund for Nuclear Medicine and by the UC Davis Department of Radiology. $$graphic_76E1D3F7-7AE2-4A8C-B4A2-A58DC8A8DCE8$$