Evaluation of a clinical TOF-PET detector design that achieves ⩽100 ps coincidence time resolution

Abstract

Commercially available clinical positron emission tomography (PET) detectors employ scintillation crystals that are long ([Formula: see text]20 mm length) and narrow (4-5 mm width) optically coupled on their narrow end to a photosensor. The aspect ratio of this traditional crystal rod configuration and 511 keV photon attenuation properties yield significant variances in scintillation light collection efficiency and transit time to the photodetector, due to variations in the 511 keV photon interaction depth in the crystal. These variances contribute significant to coincidence time resolution degradation. If instead, crystals are coupled to a photosensor on their long side, near-complete light collection efficiency can be achieved, and scintillation photon transit time jitter is reduced. In this work, we compare the achievable coincidence time resolution (CTR) of LGSO:Ce(0.025 mol%) crystals 3-20 mm in length when optically coupled to silicon photomultipliers (SiPMs) on either their short end or long side face. In this 'side readout' configuration, a CTR of 102 ± 2 ps FWHM was measured with [Formula: see text] mm³ crystals coupled to rows of [Formula: see text] mm² SensL-J SiPMs using leading edge time pickoff and a single timing channel. This is in contrast to a CTR of 137 ± 3 ps FWHM when the same crystals were coupled to single [Formula: see text] mm² SiPMs on their narrow ends. We further study the statistical limit on CTR using side readout via the Cramér-Rao lower bound (CRLB), with consideration given to ongoing work to further improve photosensor technologies and exploit fast phenomena to ultimately achieve 10 ps FWHM CTR. Potential design aspects of scalable front-end signal processing readout electronics using this side readout configuration are discussed. Altogether, we demonstrate that the side readout configuration offers an immediate solution for 100 ps CTR clinical PET detectors and mitigates factors prohibiting future efforts to achieve 10 ps FWHM CTR.

Figure 1

Short and long scintillation crystals with standard light readout at the end of the crystal are shown in (a) and (b). In (c), an alternative scintillation light readout configuration is depicted, where light is readout by a photosensor coupled to the side of the crystal.

Figure 2

The full temporal luminescence profile for LGSO:Ce(0.05 mol%) (fast-LGSO) is shown in (a), and magnified view of the rising edge of the temporal profile is shown in (b). A two-component decay fit yielded measured 37.2 and 11.8 ns with 90.6 and 9.4% relative intensities, respectively. The measured rise time was 8.9 ps [Gundacker 2017].

Figure 3

An experimental setup used to quantify light output of fast-LGSO crystals in “end readout” and “side readout” with a Hamamatsu R9779 PMT is shown in (a). In (b), the multi-photoelectron charge spectrum for the PMT used is shown.

Figure 4

In (a), a schematic of the circuit designed to test the achievable CTR with the side readout configuration. One of the printed and assembled test boards is shown in (b).

Figure 5

Experimental setups to measure CTR with crystals and SiPMs oriented in end (a) and side (b) readout configuration. Side readout experiments were performed using the test board shown in Figure 4(b), where a row of SiPM pixels were connected together and coupled to the long side of the crystals. An experimental setup used to measure CTR with side readout using depth-of-interaction corrected time stamps is shown in (c).

Figure 6

An illustration of the procedure used to make the depth-dependent time walk correction on timestamps for the side readout of 20 mm length crystals. “Signal₁” and “Signal₂” refer to the energy signals from the anodes of two groups of three SiPMs. Likewise with “Signal₃” and “Signal₄”.

Figure 7

Illustrations of side readout configurations used to measure the achievable CTR for “sparse” sensor readout. 20 mm length crystals were tested with with four 3×3 mm² SiPMs in the center of the crystals with (a) and without (b) ESR reflector on the “open” area at the bottom of the crystal. A similar sensor/reflector arrangement was also tested for 10 mm length crystals with two SiPMs ((c) and (d)).

Figure 8

Measured light output at 511 keV energy deposition for fast-LGSO crystals coupled to a Hamamatsu R9779 PMT on their narrow ends or long sides.

Figure 9

Measured CTR for fast-LGSO crystals with 3–20 mm lengths in end readout is shown in (a). In (b), the measured CTR with the same crystals using side readout is shown. 20 mm length crystals in side readout are denoted both “NC” for “not corrected” and “C” for “corrected” for the case without and with a time walk correction. Error bars represent a 95% confidence interval on the fit to the CTR distributions.

Figure 10

Measured CTR for 2.9×2.9×3 mm³ fast-LGSO crystals is shown as the number of 3×3 mm² SiPM pixels connected together to a single output was increased (black), with a linear fit to these data (red). Also shown are the best CTR values for 5, 10, and 20 mm length crystals with side readout arranged according to the number of SiPM pixels they were coupled to.

Figure 11

CTR distributions from validation measurements, where a Ge-68 source was displaced by known distances of 2 cm, resulting in an expected shift of CTR distributions of 133 ps.

Figure 12

Measured CTR for sparse sensor readout configurations with 10 and 20 mm length crystals are shown in (a). The terms “OB” and “CB” are used to denote open and closed bottom, where the portion of the scintillation light exit interface of the crystal not coupled to SiPMs is either left open or covered with ESR reflector. An example of the affect on the 511 keV photopeak with a 20 mm length crystal having sparse sensor readout and OB (green) and one where the bottom is covered by SiPMs.

Figure 13

Calculated CRLB for 3×3×20 mm³ fast-LGSO crystals considered 10 prompt Cherenkov photons, where SPTR and PDE are parametrically varied.

Figure 14

A traditional PET detector design with an array of long and narrow scintillation crystals coupled end-on to an array of SiPMs is shown in (a). In (b), a proposed detector design employing side readout of the same crystal array, with very thin SiPM devices mounted on a very thin flex circuit.

Figure 15

End-on view of a row of crystal elements coupled to SiPMs on their side, showing the achievable packing fraction with off-the-shelf version of the sensors used in this work (a) and a greatly improved packing fraction if the optical window could be removed in a future design (b).