PMT的大小和Light Spread

文献:A count-rate model for PET scanners using pixelated  Anger-logic detectors with different scintillators
S Surti and J S Karp

In a large continuous scintillator, the spreading of the scintillation photons depends on the thickness of the lightguide as well as the scintillator.

scintillator受激后产生的光子会在scintillator和lightguide里面扩散, 扩散的范围取决于它们的厚度.

However, the scintillator thickness also determines the sensitivity of the detector which is fixed by the scanner requirements. As a result, there is not enough control over reducing the scintillation light spread within the detector by using thinner crystals without compromising the detector sensitivity.

scintillator的厚度决定了detector的sensitivity, 所以不能只靠减少crystal的厚度来降低scintillation light spread

Depending upon the light spread within the detector which increases with the scintillator thickness, large PMTs are needed to achieve optimal sampling of scintillation light and thus attain good spatial resolution. Using many small PMTs degrades spatial resolution due to the Poisson noise in each PMT signal, while using fewer small PMTs will lead to a reduced sensitivity of the positioning algorithm to detect small changes in interaction position.

However, increased light spread and the use of large PMTs lead to increased pulse pileup or deadtime in the detectors at high count rates. Thus, in order to achieve high sensitivity and good spatial resolution in a continuous Anger-logic detector, there is a drawback due to increased pulse pileup at high count rates.

NaI(Tl)-based PET scanners like the C-PET (Philips Medical Systems) use large 1 inch thick NaI(Tl) crystals coupled to an array of 63 mm diameter PMTs via a lightguide. The crystal and lightguide thickness are well optimized to the PMT size to derive the best achievable spatial resolution and count-rate performance from these detectors.

下面是他们的G-PET(与A-PET及Mosaic接近)的设计:

Each of the GSO crystals is optically isolated from its neighbours by using a high reflectivity material such as PTFE which will restrict the scintillation photons from entering adjacent crystals. The only place where these photons can spread is in the lightguide. Essentially, with such a design we can control the light spread through the lightguide without changing the detector sensitivity. The lightguide thickness can now be adjusted to vary the light spread within the detector and thus match it to a given PMT size , thus providing the flexibility to achieve minimal pulse pileup for any PMT size.