## 3 Ultra-High Spatial Resolution (<2 mm) Clinical PET Systems
### Brain-Dedicated PET Systems  
For the sake of comparison, we briefly describe the ECAT high-resolution research tomograph (HRRT), a well-known, commercially available brain-dedicated PET system from CTI PET Systems Inc. [58] that for about two decades served as the highest resolution commercially available clinical PET system. This system incorporates a phoswich design, implemented with dual layers of 8 \(\times\) 8 lutetium-oxy-orthosilicate (LSO) scintillation crystal arrays, each with an individual crystal size of 2.1 \(\times\) 2.1 \(\times\) 7.5 mm\({}^{3}\)[51]. Spatial resolutions of less than 2.4 mm and 2.8 mm were reported at the center and 10 cm off-center, respectively. The transaxial plane's spatial resolutions were relatively uniform compared to those along the axial direction due to the DOI information enabled by the phoswich design.  
Over the years, several research groups have proposed unique designs for brain-dedicated PET systems to enhance PET image quality beyond the ECAT HRRT. One such system is a brain-dedicated scanner developed by Hamamatsu Photonics K.K., capable of producing images with a spatial resolution of less than 2.1 mm [52]. This system employs a four-layer DOI detector module. Each layer is composed of five 32 \(\times\) 32 LYSO scintillation crystal arrays with a 1.2 mm pitch, arranged together to form the detection area. Four different crystal layer thicknesses are used depending on the layer's location relative to the front entrance face to uniformly distribute count rates across each layer. A 0.25 mm diameter \({}^{22}\)Na point radiation source was used to evaluate reconstructed spatial resolutions according to NEMA NU 4-2008 standards for small animal PET. A spatial resolution of 1.8 mm to 2.1 mm, reconstructed with 2-D FBP, was reported throughout the entire FOV.  
The MINDVIEW (Multimodal Imaging of Neurological Disorder) PET insert utilized monolithic scintillation crystals to improve spatial resolution [53]. This MR-compatible PET system consists of 50 \(\times\) 50 \(\times\) 20 mm\({}^{3}\) monolithic crystal blocks. Black paint was applied to the lateral sides of the crystal arrays, and the front surface (50 \(\times\) 50 mm\({}^{2}\)) was treated with retroreflector (RR) layers. Initial results reported FWHM spatial resolutions ranging from 1.7 mm to 3.5 mm throughout the FOV using iterative algorithms with a 1 mm \({}^{22}\)Na point source. Using the FBP algorithm, spatial resolutions were measured with a 0.25 mm diameter \({}^{22}\)Na point source at various reconstruction voxel sizes (1 mm, 0.5 mm, and 0.25 mm), reporting resolutions of 1.5 mm to 4.9 mm at different positions in the FOV. Additionally, the system successfully resolved 2.5 mm rods in reconstructed images of a [PERSON] phantom.  
The NeuroEXPLORER (NX) PET/CT system [57], developed collaboratively by Yale, UC Davis, and United Imaging Healthcare, features high sensitivity and high resolution with a LYSO-based U-shaped light-sharing detector design that provides DOI information. With a 49.5 cm axial field of view, its spatial resolution ranges from 1.63 mm to 3.05 mm, measured using a 0.25 mm \({}^{22}\)Na point source and FBP reconstruction. Sensitivity was evaluated using a NEMA PET sensitivity phantom, yielding 46-47.6 cps/kBq. The system employs a 3D time-of-flight (TOF)-DOI-PSF OSEM algorithm for image reconstruction, offering enhanced imaging for both phantoms and human studies. The NX holds promise for advancing neuroimaging, particularly for detailed cortical and subcortical analyses.