Herman T. van Dam

A novel, SiPM-array-based, monolithic scintillator detector for PET

Silicon photomultipliers (SiPMs) are of great interest to positron emission tomography (PET), as they enable new detector geometries, for e.g., depth-of-interaction (DOI) determination, are MR compatible, and offer faster response and higher gain than other solid-state photosensors such as avalanche photodiodes. Here we present a novel detector design with DOI correction, in which a position-sensitive SiPM array is used to read out a monolithic scintillator. Initial characterization of a prototype detector consisting of a 4 x 4 SiPM array coupled to either the front or back surface of a 13.2 mm x 13.2 mm x 10 mm LYSO:Ce(3+) crystal shows that front-side readout results in significantly better performance than conventional back-side readout. Spatial resolutions <1.6 mm full-width-at-half-maximum (FWHM) were measured at the detector centre in response to an approximately 0.54 mm FWHM diameter test beam. Hardly any resolution losses were observed at angles of incidence up to 45 degrees , demonstrating excellent DOI correction. About 14% FWHM energy resolution was obtained. The timing resolution, measured in coincidence with a BaF(2) detector, equals 960 ps FWHM.

LaBr3 : Ce and SiPMs for time-of-flight PET: achieving 100 ps coincidence resolving time

The use of time-of-flight (TOF) information in positron emission tomography (PET) enables significant improvement in image noise properties and, therefore, lesion detection. Silicon photomultipliers (SiPMs) are solid-state photosensors that have several advantages over photomultiplier tubes (PMTs). SiPMs are small, essentially transparent to 511 keV gamma rays and insensitive to magnetic fields. This enables novel detector designs aimed at e.g. compactness, high resolution, depth-of-interaction (DOI) correction and MRI compatibility. The goal of the present work is to study the timing performance of SiPMs in combination with LaBr(3):Ce(5%), a relatively new scintillator with promising characteristics for TOF-PET. Measurements were performed with two, bare, 3 mm x 3 mm x 5 mm LaBr(3):Ce(5%) crystals, each coupled to a 3 mm x 3 mm SiPM. Using a (22)Na point source placed at various positions in between the two detectors, a coincidence resolving time (CRT) of approximately 100 ps FWHM for 511 keV annihilation photon pairs was achieved, corresponding to a TOF positioning resolution of approximately 15 mm FWHM. At the same time, pulse height spectra with well-resolved full-energy peaks were obtained. To our knowledge this is the best CRT reported for SiPM-based scintillation detectors to date. It is concluded that SiPM-based scintillation detectors can provide timing resolutions at least as good as detectors based on PMTs.

The lower bound on the timing resolution of scintillation detectors

The timing performance of scintillation detectors is ultimately limited by photon counting statistics. In fact, photon counting statistics form a dominant contribution to the overall timing resolution of many state-of-the-art detectors. A common approach to investigate this contribution is to calculate the variance in the registration times of individual scintillation photons within the photosensor. However, in general the single-photon variance is not equal to the intrinsic limit on the timing resolution, since in principle one can make use of the timing information carried by all photons detected. In this work, the Cramér-Rao lower bound on the timing resolution of a scintillation detector, based on the information contained in the full set of registered photons, is calculated. The results appear to be in good agreement with trends observed in the literature. Furthermore, it is shown that the timestamp obtained from any single scintillation photon never yields the optimum timing resolution for realistic scintillation detectors. Yet, it appears that the intrinsic timing resolution limit can be approached closely by making use of the timestamps from a relatively small number of photons emitted during the initial part of the scintillation pulse.

Sub-200 ps CRT in monolithic scintillator PET detectors using digital SiPM arrays and maximum likelihood interaction time estimation

Digital silicon photomultiplier (dSiPM) arrays have favorable characteristics for application in monolithic scintillator detectors for time-of-flight positron emission tomography (PET). To fully exploit these benefits, a maximum likelihood interaction time estimation (MLITE) method was developed to derive the time of interaction from the multiple time stamps obtained per scintillation event. MLITE was compared to several deterministic methods. Timing measurements were performed with monolithic scintillator detectors based on novel dSiPM arrays and LSO:Ce,0.2%Ca crystals of 16 × 16 × 10 mm(3), 16 × 16 × 20 mm(3), 24 × 24 × 10 mm(3), and 24 × 24 × 20 mm(3). The best coincidence resolving times (CRTs) for pairs of identical detectors were obtained with MLITE and measured 157 ps, 185 ps, 161 ps, and 184 ps full-width-at-half-maximum (FWHM), respectively. For comparison, a small reference detector, consisting of a 3 × 3 × 5 mm(3) LSO:Ce,0.2%Ca crystal coupled to a single pixel of a dSiPM array, was measured to have a CRT as low as 120 ps FWHM. The results of this work indicate that the influence of the optical transport of the scintillation photons on the timing performance of monolithic scintillator detectors can at least partially be corrected for by utilizing the information contained in the spatio-temporal distribution of the collection of time stamps registered per scintillation event.

First characterization of a digital SiPM based time-of-flight PET detector with 1 mm spatial resolution

Monolithic scintillator detectors can offer a combination of spatial resolution, energy resolution, timing performance, depth-of-interaction information, and detection efficiency that make this type of detector a promising candidate for application in clinical, time-of-flight (TOF) positron emission tomography (PET). In such detectors the scintillation light is distributed over a relatively large number of photosensor pixels and the light intensity per pixel can be relatively low. Therefore, monolithic scintillator detectors are expected to benefit from the low readout noise offered by a novel photosensor called the digital silicon photomultiplier (dSiPM). Here, we present a first experimental characterization of a TOF PET detector comprising a 24 × 24 × 10 mm(3) LSO:Ce,0.2%Ca scintillator read out by a dSiPM array (DPC-6400-44-22) developed by Philips Digital Photon Counting. A spatial resolution of ~1 mm full-width-at-half-maximum (FWHM) averaged over the entire crystal was obtained (varying from just below 1 mm FWHM in the detector center to ~1.2 mm FWHM close to the edges). Furthermore, the bias in the position estimation at the crystal edges that is typically found in monolithic scintillators is well below 1 mm even in the corners of the crystal.

A practical method for depth of interaction determination in monolithic scintillator PET detectors

Several new methods for determining the depth of interaction (DOI) of annihilation photons in monolithic scintillator detectors with single-sided, multi-pixel readout are investigated. The aim is to develop a DOI decoding method that allows for practical implementation in a positron emission tomography system. Specifically, calibration data, obtained with perpendicularly incident gamma photons only, are being used. Furthermore, neither detector modifications nor a priori knowledge of the light transport and/or signal variances is required. For this purpose, a clustering approach is utilized in combination with different parameters correlated with the DOI, such as the degree of similarity to a set of reference light distributions, the measured intensity on the sensor pixel(s) closest to the interaction position and the peak intensity of the measured light distribution. The proposed methods were tested experimentally on a detector comprised of a 20 mm × 20 mm × 12 mm polished LYSO:Ce crystal coupled to a 4 × 4 multi-anode photomultiplier. The method based on the linearly interpolated measured intensities on the sensor pixels closest to the estimated (x, y)-coordinate outperformed the other methods, yielding DOI resolutions between ∼1 and ∼4.5 mm FWHM depending on the DOI, the (x, y) resolution and the amount of reference data used.

Ultra precise timing with SiPM-based TOF PET scintillation detectors

The combination of SiPMs with fast and bright scintillators, such as LaBr3:Ce, seems very promising for application in time-of-flight (TOF) PET. We therefore conducted a series of experiments with the goal of obtaining the best possible timing resolution with SiPM-based scintillation detectors in order to establish a bench mark for future experiments with different detector designs. The detectors employed in our measurements consisted of two SiPMs (Hamamatsu MPPC-S10362-33-050C), which were directly coupled to small scintillation crystals, viz. LaBr3:Ce and LYSO. An excellent coincidence resolving time (CRT) for 22Na 511 annihilation photons of 99.5 ps ± 0.6 ps FWHM could be achieved at the optimized electronics and digitizer settings with two LaBr3:5%Ce crystals. A CRT of 171.5 ps ± 0.8 ps FWHM was obtained with L(Y)SO crystals. These results compare well to the predictions of a statistical model which was developed to describe the timing performance of SiPM based scintillation detectors.

Monolithic LaBr3:Ce crystals on silicon photomultiplier arrays for time-of-flight positron emission tomography

Positron emission tomography detectors based on monolithic scintillation crystals exhibit good spatial and energy resolution, intrinsically provide depth-of-interaction information, have high γ-photon capture efficiency, and may reduce the manufacturing costs compared to pixelated crystal arrays. Here, we present the characterization of a detector consisting of a 18.0 mm×16.2 mm×10.0 mm monolithic LaBr₃:5%Ce scintillator directly coupled to a 4×4 array of silicon photomultipliers. An energy resolution of 6.4% full-width-at-half-maximum (FWHM) was obtained. The point-spread-function (PSF) was determined for different regions of the detector. The full-width-at-half-maximum (FWHM) of the PSF was measured to be <1.5 mm at the center of the detector and <1.7 mm averaged over the entire crystal. Both values are not corrected for the ~0.6 mm FWHM test beam diameter. Furthermore, the influence of edge effects was investigated. We found that near the edges of the detector the spatial resolution degrades to 2.2 mm (FWHM), and a bias in the position estimates, up to 1.5 mm, was observed. Moreover, the coincidence resolving time for two identical detectors in coincidence was measured to be as small as ~198 ps FWHM.

Optimization of digital time pickoff methods for LaBr 3 -SiPM TOF-PET detectors

The relatively new inorganic scintillator LaBr3:Ce is well suited for time-of-flight positron emission tomography (TOF-PET), since it has short scintillation decay time, high light yield and very good energy resolution. Silicon photomultipliers (SiPMs) show low noise, high gain and small transit-time jitter, and are thus well suited for fast timing applications. The work presented here focuses on the timing performance of bare LaBr3:Ce(5%) crystals coupled to commercially available SiPMs. First, relatively small crystals coupled to single SiPMs were used to study the intrinsic timing resolution of such detectors. Special attention was paid to the optimization of digital signal processing (DSP) time-pickoff methods. A coincidence timing resolution of 101 ps FWHM was achieved. Next, a monolithic LaBr3:Ce crystal was coupled to a 4×4 SiPM array and an intial timing performance characterization was performed. A single detector timing resolution of 225 ps FWHM was achieved. The timing delay induced by the scintillation photon transport was virtually constant over the depth-of-interaction (DOI) range of the detector.

The statistical distribution of the number of counted scintillation photons in digital silicon photomultipliers: model and validation

In the design and application of scintillation detectors based on silicon photomultipliers (SiPMs), e.g. in positron emission tomography imaging, it is important to understand and quantify the non-proportionality of the SiPM response due to saturation, crosstalk and dark counts. A new type of SiPM, the so-called digital silicon photomultiplier (dSiPM), has recently been introduced. Here, we develop a model of the probability distribution of the number of fired microcells, i.e. the number of counted scintillation photons, in response to a given amount of energy deposited in a scintillator optically coupled to a dSiPM. Based on physical and functional principles, the model elucidates the statistical behavior of dSiPMs. The model takes into account the photon detection efficiency of the detector; the light yield, excess variance and time profile of the scintillator; and the crosstalk probability, dark count rate, integration time and the number of microcells of the dSiPM. Furthermore, relations for the expectation value and the variance of the number of fired cells are deduced. These relations are applied in the experimental validation of the model using a dSiPM coupled to a LSO:Ce,Ca scintillator. Finally, we propose an accurate method for the correction of energy spectra measured with dSiPM-based scintillation detectors.