Comparison of four depth-encoding PET detector modules with wavelength shifting (WLS) and optical fiber read-out

Abstract

We propose detectors for a laboratory positron emission tomography scanner specific for mouse imaging that utilizes fewer detectors and channels of electronics compared with existing designs. The detectors are based on lutetium oxyorthosilicate arrays, read out by orthogonal optical fibers placed on the top and bottom of the arrays. Depth of interaction (DOI) information is obtained from the ratio of the signals at either end of the array. Four different detector modules were evaluated, using different reflector materials and two types of optical fibers (wavelength shifting (WLS) fibers and clear optical fibers). The modules were compared in terms of flood histograms, energy resolution, DOI resolution and timing resolution. Energy resolution for single crystals at one irradiation depth was around 65% full-width half-maximum (FWHM). A DOI resolution of approximately 6 mm was obtained for the modules. Timing resolution was in the range of 5.1-7.8 ns. An array assembled in the laboratory and coupled with WLS fibers had the best DOI resolution; the same array with clear fibers had the best timing resolution and a commercially manufactured array and coupled with WLS fibers had the best energy resolution.

Figure 1

Schematic of detector approach using WLS fibers to read out scintillator arrays. The scintillation light of LSO (shown in blue) has an emission peak at 420 nm while the re-emitted photons from the wavelength shifting fibers (shown in green) is peaks 494 nm.

Figure 2

(a) Use of WRF to make a grid for a 4 by 4 LSO array: (1) white reflective film was cut into small pieces; (2) combining six such pieces and an outer shell, a grid was made; (3) The extra length of reflector works as a reflector and optical isolator between adjacent fibers. (b) Photograph of the three arrays being tested. From left to right: 5 by 5 array manufactured by SICCAS with ESR reflectors; 5 by 5 array manufactured by SICCAS with WRF reflectors; homemade 4 by 4 array with WRF and two layers of Teflon tape as inter-crystal reflector. (c) Photograph of a complete detector module (LAB+WLS) made of a 4×4 LSO array with WLS fibers on both ends. It also shows an individual LSO element (1.5×1.5×20mm³, unpolished crystal) and an individual WLS fiber (1.5mm square cross-section).

Figure 3

(a) Geometry for a single crystal coupled to a single square cross-section fiber. (b) Light collection efficiency of two different types of fibers coupled to an LSO crystal.

Figure 4

Experimental set-up for electronic collimation of source at different depths.

Figure 5

Data acquisition scheme for single mode and coincidence mode. (Dotted lines and boxes are for timing measurements)

Figure 6

Left three columns: 1-D and 2-D flood histograms from different detector configurations. Right two columns: profiles from 1-D flood histograms.

Figure 7

Flood histograms of the LAB+WLS module irradiated at different depths. (a), (b) 1-D flood histograms from each end, and (c) 2-D histograms calculated with signals from both ends.

Figure 8

Photopeak amplitudes measured at different depths for three modules

Figure 9

DOI ratio histograms for three detector modules. Curves with different color are histograms with different irradiation depths (from 2 mm to 18 mm with 1mm per step).

Figure 10

Measurements for three modules (a) peak positions of DOI histograms; (b) DOI resolutions at different depths