ABSTRACT
Time-of-flight (TOF) capability is becoming an important capability offered in both commercial and research PET scanners. Often commercial vendors and laboratory researchers develop and utilize proprietary electronics for their devices. Consequently, it is challenging for independent research groups to develop their own TOF-PET scanners. In this investigation, we tested a prototype scanner consisting of commercially available TOF-capable modules from Hamamatsu Photonics that can be used as building blocks for PET scanners. The scanner consists of a ring of 16 modules, for a total diameter of 26.7 cm. Testing demonstrated that the scanner is capable of sustaining ~1 MHz single counting rate with a peak noise equivalent count rate (NECR) of 117.5 kHz at 75.25 MBq measured with NEMA NU-4 "rat" phantom. Spatial resolution of 2.3 mm 5 mm from the center of the scanner was measured. Energy resolution of 17.2% at 511 keV was measured. Peak sensitivity of 1.28% is reported. All the measurements were performed with energy cuts from 350 to 700 keV Finally, scanner timing resolution was found to be 462 ps. Results from testing of a prototype scanner constructed using newly released TOF-capable detector modules produced by Hamamatsu demonstrated the promise for these devices to create high performance PET system with TOF capabilities.
ABSTRACT
Meditation refers to a family of mental training practices that are designed to familiarize the practitioner with specific types of mental processes. One of the most basic forms of meditation is concentration meditation, in which sustained attention is focused on an object such as a small visual stimulus or the breath. In age-matched participants, using functional MRI, we found that activation in a network of brain regions typically involved in sustained attention showed an inverted u-shaped curve in which expert meditators (EMs) with an average of 19,000 h of practice had more activation than novices, but EMs with an average of 44,000 h had less activation. In response to distracter sounds used to probe the meditation, EMs vs. novices had less brain activation in regions related to discursive thoughts and emotions and more activation in regions related to response inhibition and attention. Correlation with hours of practice suggests possible plasticity in these mechanisms.
