Characteristics of G4 and G8 Systems
| Characteristic | G4 | G8 |
| Crystal material | BGO | BGO |
| Crystal size | 1.75 × 1.75 × 7.2 mm | 1.75 × 1.75 × 7.2 mm |
| Crystal pitch | 1.83 mm | 1.83 mm |
| Crystal array | 24 × 26 crystals/PMT | 26 × 26 crystals/PMT |
| Light guide | 1-mm-thick glass layer | Tapered, pixelated light guide |
| PMT | Hamamatsu H8500 MAPMT | Hamamatsu H8500 MAPMT |
| Number of detector modules | 4 | 8 |
| Number of crystals per module | 1,248 | 676 |
| Number of crystals in total | 4,992 | 5,408 |
| Number of analog-to-digital converters | 16 | 32 |
| Signal-processing system | VHS-ADC (Lyrtech) | PicoDigitizer (Nutaq) |
| Anatomic reference approach | X-ray projection and optical photographic images | CT |