## 4 The 1 mm Resolution Clinical PET System  
To further enhance reconstructed spatial resolution, it is essential to (1) reduce the crystal element width and (2) improve the DOI resolution while maintaining adequate crystal element length to promote high sensitivity. To address these challenges, we have been developing a prototype 1 mm resolution clinical PET system (see Figure 4) [38; 39; 50; 61; 62; 63; 64; 65; 66; 67; 68; 69; 70; 71; 72], which aims to achieve sub-millimeter spatial resolutions with exceptionally high 511-keV photon sensitivity. Figure 4: The 1 mm\({}^{3}\) resolution clinical PET system [61]. **(a)** CAD drawing of an 8-layer “edge-on” 3D position-sensitive dual LYSO-PSAPD module section from one of the panels. **(b)** One of the two detector panels housing 32 layers, each consisting of 16 dual LYSO-PSAPD modules. **(c)** The same detector panel showing both front-end and back-end electronics. **(d)** CAD depiction of the system scanning the head/neck region of a patient. The patient-adjustable dual-panel geometry optimizes 511-keV photon sensitivity, achieving 1 mm spatial resolution with good SNR in reconstructed images. The system features two patient-adjustable panels to maximize sensitivity and enhance spatial resolution for visualization and quantification of smaller lesions. This dual-panel system is mounted on a custom-built mobile gantry capable of rotating the detector panels along three orthogonal axes to meet clinical requirements (Figure 5). Each basic detector unit, termed a dual LYSO position-sensitive avalanche photodiode (dual-LYSO-PSAPD) detector (Figure 6), embodies a 3D position-sensitive scintillation detector design concept that enables the measurement of photon energies, interaction locations, and arrival times in three dimensions (Figure 7) [65]. The dual-LYSO-PSAPD consists of two \(8\times 8\) arrays with individual crystal element dimensions of \(0.9\) mm \(\times\) 0.9 mm \(\times\) 1.0 mm, each coupled to a PSAPD, which is a high-gain, position-sensitive, large-area avalanche photodiode [73]. Each detector layer is arranged in a novel \"edge-on\" orientation with respect to incoming photons. Figure 5: A 3D rendering of the 1 mm resolution clinical PET system positioned for a breast scan (**top left**) and a head-and-neck scan (**top right**). The system is mounted on a patient-adjustable, movable gantry, allowing it to scan both breast cancer and head-and-neck cancer patients in various orientations (**bottom left**, **bottom right**). Figure 6: A dual-LYSO-PSAPD detector, where each of the two \(8\times 8\) LYSO crystal arrays is coupled to a PSAPD. Each panel houses multiple stackable detector layers, each comprising 16 dual-LYSO- PSAPD detectors (Figure 7) [50]. The current prototype system has 32 detector layers, with the final system planned to have 48 layers per panel. The PSAPDs in the system are biased by a programmable high-voltage distribution circuit [69], and the output signals from the dual-LYSO-PSAPD detectors are read out by RENA-3 ASIC chips [74] after analog preprocessing [66]. The RENA-3 outputs are then processed through FPGA-based backend boards, which send the event data to a master data-acquisition (DAQ) computer via Ethernet connections (Figure 8) [75]. The overall configuration of the system is summarized in Table 3, and the system architecture is comprehensively discussed in [PERSON] et al. [50,71,72]. The reconstructed spatial resolution, a primary focus of this section, is also detailed by [PERSON] et al. [50]. A micro-Derenzo hot-rod phantom (Data Spectrum Corporation, Durham, NC) was scanned at the center of the FOV, and a separate scan was conducted with the same phantom placed at the edge of the FOV (Figure 9). All rods in the phantom, including those with a 1.2 mm diameter, were clearly visible when positioned at the center.