A Positron Emission Mammography (PEM) sensor is an organ-dedicated Positron Emission Tomography (PET) scanner for breast screening with a restricted field of view (FOV) to achive higher detection performance in terms of both sensitivity and specificity with respect to the conventional whole-body PET scanners. Additional advantages are a lower cost and lower necessary dose. However, most state-of-the-art PEM designs are based on crystals, which have a large parallax error due to poor depth of interaction (DOI) estimation.

In the framework of the Voxel Imaging PET (VIP) Patfinder project, a novel PEM scanner based on pixelated solid-state CdTe detectors has been proposed to overcome the intrinsic limitations of crystal detectors. The VIP-PEM is based on the VIP unit detector module (figure 1 A) made of four CdTe pixelated detectors bonded to a thinned read out chip (ROC) and then mounted on a kapton printed circuit board (PCB). Each one of the four detectors has a parallelepiped shape with 2 cm x 1 cm surface and 2 mm thickness. The 2000 V high voltage is applied such to have the electric field perpendicular to the surface with resulting 1000 V/mm bias and an expected energy resolution of 1.5% for 511 keV photons at room temperature. The CdTe detectors are segmented into 1 mm x 1 mm pixels for a total of 4 x 200 channels per module. The ROC and the PCB are 50 mm thick each, and a 15 mm additional thickness is due to the conductive glue between the ROC, the PCB, and the CdTe detectors. The combined attenuation coefficient of the passive material accounts for less than 2% compared to 2 mm CdTe. To obtain the energy and time information, each channel is bonded to a microchip hosting a fully integrated front-end electronic. Finally, one side of the module hosts an electronic connector to the external bus.

Figure 1. Basic unit detector (A) and full detector (B) geometrical specifications.

The VIP modular design is extremely versatile with the possibility to build virtually any detector geometry by stacking the needed number of modules in arbitrary patterns. The resulting detector has a 3-D segmentation with a channel density of ~ 470 voxels per cm3. The positioning of the module with respect to the incident radiation, with photons entering from the opposite side with respect to the electronic connector, is a key feature introduced to maximize the stopping power and minimize the amount of passive material of the final scanner. In the proposed design, incident radiation is facing a minimum of 4 cm depth of interaction in CdTe. Following the typical coplanar design, the VIP mammograph consists of two parallel paddles, each one hosting one sliding detector head. The two heads are made of 80 modules each, arranged along two parallel lines of 40 modules for a total of 64000 channels per head.
The head section is 170 mm wide along the x-axis and 40 mm wide along the z-axis, and the two detector heads must slide axially for a complete scan of the 170 mm x 60 mm x 240 mm FOV.

Results from simulation cover the assessment of the expected counting and imaging performance of the proposed device. With a sensitivity of 2 cps/kBq and a PSF of 1 mm FWHM along the three spatial directions, the VIP-PEM has 6 times better volumetric resolution with respect to conventional scanners and can detect lesions down to 2 mm diameter with a 3:1 TNR. The results show the potential of the novel design for providing high specificity metabolic images for very small lesions and low TNR. A fully operative dual head prototype is being developed within the framework of the VIP project to confirm the simulation findings.