Simulation study of spatial resolution and sensitivity for the tapered depth of interaction PET detectors for small animal imaging

Abstract

Improvements to current small animal PET scanners can be made by improving the sensitivity and the spatial resolution of the scanner. In the past, efforts have been made to minimize the crystal dimensions in the axial and transaxial directions to improve the spatial resolution and to increase the crystal length to improve the sensitivity of the scanner. We have designed tapered PET detectors with the purpose of reducing the gaps between detector modules and optimizing the sensitivity of a future-generation small animal PET scanner. In this work, we investigate spatial resolution and sensitivity of a scanner based on tapered detector elements using Monte Carlo simulations. For tapered detector elements more scintillation material is used per detector resulting in a higher sensitivity of the scanner. However, since the detector elements are not uniform in size, degradation in spatial resolution is also expected. To investigate characteristics of tapered PET detectors, the spatial resolution and sensitivity of a one-ring scanner were simulated for a system based on traditional cuboid detectors and a scanner based on tapered detectors. Additionally, the effect of depth of interaction (DOI) resolution on the spatial resolution for the traditional and tapered detectors was evaluated. All simulations were performed using the Monte Carlo simulation package GATE. Using the tapered arrays, a 64% improvement in the sensitivity across the field of view was found compared with traditional detectors for the same ring diameter. The level of DOI encoding was found to be the dominating factor in determining the radial spatial resolution and not the detector shape. For all levels of DOI encoding, no significant difference was found for the spatial resolution when comparing the tapered and the cuboid detectors. Detectors employing the tapered crystal design along with excellent DOI resolution will lead to PET scanners with higher sensitivity and uniform spatial resolution across the field of view.

Figure 1

Left: Scanner based on cuboid LSO elements. Right: Scanner based on tapered LSO elements.

Figure 2

A photograph of a tapered LSO array next to a cuboid LSO array (left) and an illustration of the dimensions of the simulated tapered LSO array.

Figure 3

The sensitivity of both scanner designs as a function of changing lower level discriminator settings.

Figure 4

The number of coincidence counts for a line source positioned across the transverse field of view.

Figure 5

The radial and tangential spatial resolution as a function of source position for varying DOI resolutions.

Figure 6

Reconstructed images of the point source at a radial offset of 10 mm for the cuboid arrays (left), the tapered arrays (right) and varying DOI resolutions.