Model of the point spread function of monolithic scintillator PET detectors for perpendicular incidence.

Item does not contain fulltextPURPOSE: Previously, we demonstrated the potential of positron emission tomography detectors consisting of monolithic scintillation crystals read out by arrays of solid-state light sensors. We reported detector spatial resolutions of 1.1-1.3 mm full width at half maximum (FWHM) with no degradation for angles of incidence up to 30 degrees, energy resolutions of approximately 11% FWHM, and timing resolutions of approximately 2 ns FWHM, using monolithic LYSO:Ce3+ crystals coupled to avalanche photodiode (APD) arrays. Here we develop, validate, and demonstrate a simple model of the detector point spread function (PSF) of such monolithic scintillator detectors. METHODS: A PSF model was developed that essentially consists of two convolved components, one accounting for the spatial distribution of the energy deposited by annihilation photons within the crystal, and the other for the influences of statistical signal fluctuations and electronic noise. The model was validated through comparison with spatial resolution measurements on a detector consisting of an LYSO:Ce3+ crystal read out by two APD arrays. RESULTS: The model is shown to describe the measured detector spatial response well at the noise levels found in the experiments. In addition, it is demonstrated how the model can be used to correct the measured spatial response for the influence of the finite diameter of the annihilation photon beam used in the experiments, thus obtaining an estimate of the intrinsic detector PSF. CONCLUSIONS: Despite its simplicity, the proposed model is an accurate tool for analyzing the detector PSF of monolithic scintillator detectors and can be used to estimate the intrinsic detector PSF from the measured one.1 april 201

Optical simulation of monolithic scintillator detectors using GATE/GEANT4

Much research is being conducted on position-sensitive scintillation detectors for medical imaging, particularly for emission tomography. Monte Carlo simulations play an essential role in many of these research activities. As the scintillation process, the transport of scintillation photons through the crystal(s), and the conversion of these photons into electronic signals each have a major influence on the detector performance; all of these processes may need to be incorporated in the model to obtain accurate results. In this work the optical and scintillation models of the GEANT4 simulation toolkit are validated by comparing simulations and measurements on monolithic scintillator detectors for high-resolution positron emission tomography (PET). We have furthermore made the GEANT4 optical models available within the user-friendly GATE simulation platform (as of version 3.0). It is shown how the necessary optical input parameters can be determined with sufficient accuracy. The results show that the optical physics models of GATE/GEANT4 enable accurate prediction of the spatial and energy resolution of monolithic scintillator PET detectors.RRR/Radiation, Radionuclides and ReactorsApplied Science

Monolithic scintillator PET detectors with intrinsic depth-of-interaction correction

We developed positron emission tomography (PET) detectors based on monolithic scintillation crystals and position-sensitive light sensors. Intrinsic depth-of-interaction (DOI) correction is achieved by deriving the entry points of annihilation photons on the front surface of the crystal from the light sensor signals. Here we characterize the next generation of these detectors, consisting of a 20 mm thick rectangular or trapezoidal LYSO:Ce crystal read out on the front and the back (double-sided readout, DSR) by Hamamatsu S8550SPL avalanche photodiode (APD) arrays optimized for DSR. The full width at half maximum (FWHM) of the detector point-spread function (PSF) obtained with a rectangular crystal at normal incidence equals ~1.05 mm at the detector centre, after correction for the ~0.9 mm diameter test beam of annihilation photons. Resolution losses of several tenths of a mm occur near the crystal edges. Furthermore, trapezoidal crystals perform almost equally well as rectangular ones, while improving system sensitivity. Due to the highly accurate DOI correction of all detectors, the spatial resolution remains essentially constant for angles of incidence of up to at least 30°. Energy resolutions of ~11% FWHM are measured, with a fraction of events of up to 75% in the full-energy peak. The coincidence timing resolution is estimated to be 2.8 ns FWHM. The good spatial, energy and timing resolutions, together with the excellent DOI correction and high detection efficiency of our detectors, are expected to facilitate high and uniform PET system resolution.RRR/Radiation, Radionuclides and ReactorsApplied Science

RAS testing in metastatic colorectal cancer : Excellent reproducibility amongst 17 Dutch pathology centers

In 2013 the European Medicine Agency (EMA) restricted the indication for anti- EGFR targeted therapy to metastatic colorectal cancer (mCRC) with a wild-type RAS gene, increasing the need for reliable RAS mutation testing. We evaluated the completeness and reproducibility of RAS-testing in the Netherlands. From 17 laboratories, tumor DNA of the first 10 CRC cases tested in 2014 in routine clinical practice was re-tested by a reference laboratory using a custom next generation sequencing panel. In total, 171 CRC cases were re-evaluated for hotspot mutations in KRAS, NRAS and BRAF. Most laboratories had introduced complete RAS-testing (65%) and BRAF-testing (71%) by January 2014. The most employed method for all hotspot regions was Sanger sequencing (range 35.7 - 49.2%). The reference laboratory detected all mutations that had been found in the participating laboratories (n = 92), plus 10 additional mutations. This concerned three RAS and seven BRAF mutations that were missed due to incomplete testing of the participating laboratory. Overall, the concordance of tests performed by both the reference and participating laboratory was 100% (163/163; κ-static 1.0) for RAS and 100% (144/144; κ-static 1.0) for BRAF. Our study shows that RAS and BRAF mutations can be reproducibly assessed using a variety of testing methods