B. Frisch

Time of flight positron emission tomography towards 100ps resolution with L(Y)SO: an experimental and theoretical analysis

Scintillation crystals have a wide range of applications in detectors for high energy and medical physics. They are recquired to have not only good energy resolution, but also excellent time resolution. In medical applications, L(Y)SO crystals are commonly used for time of flight positron emission tomography (TOF-PET). This study aims at determining the experimental and theoretical limits of timing using L(Y)SO based scintillators coupled to silicon photomultipliers (SiPMs). Measurements are based on the time-over-threshold method in a coincidence setup utilizing the ultra-fast amplifier-discriminator NINO and a fast oscilloscope. Using a 2 × 2 × 3 mm3 LSO:Ce codoped 0.4% Ca crystal coupled to a commercially available SiPM (Hamamatsu S10931-050P MPPC), we achieve a coincidence time resolution (CTR) of 108±5ps FWHM measured at E=511keV. We determine the influence of the data acquisition system to 27±2ps FWHM and thus negligible as compared to the CTR. This shows that L(Y)SO scintillators coupled to SiPM photodetectors are capable of achieving very good time resolution close to the desired 100ps FWHM for TOF-PET systems. To fully understand the measured values, we developed a simulation tool in MATLAB that incorporates the timing properties of the photodetector, the scintillation properties of the crystal and the light transfer within the crystal simulated by SLITRANI. The simulations are compared with measured data in order to determine their predictive power. Finally we use this model to discuss the influence of several important parameters on the time resolution like scintillation rise- and fall time and light yield, as well as single photon time resolution (SPTR) and the detection efficiency of the SiPM. In addition we find the influence of photon travel time spread in the crystal not negligible on the CTR, even for the used 2 × 2 × 3 mm3 geometry.

EndoTOFPET-US: a novel multimodal tool for endoscopy and positron emission tomography

The EndoTOFPET-US project aims to develop a multimodal detector to foster the development of new biomarkers for prostate and pancreatic tumors. The detector will consist of two main components: an external plate, and a PET extension to an endoscopic ultrasound probe. The external plate is an array of LYSO crystals read out by silicon photomultipliers (SiPM) coupled to an Application Specific Integrated Circuit (ASIC). The internal probe will be an highly integrated and miniaturized detector made of LYSO crystals read out by a fully digital SiPM featuring photosensor elements and digital readout in the same chip. The position and orientation of the two detectors will be tracked with respect to the patient to allow the fusion of the metabolic image from the PET and the anatomic image from the ultrasound probe in the time frame of the medical procedure. The fused information can guide further interventions of the organ, such as biopsy or in vivo confocal microscopy.

Development of an Anthropomorphic Breast Phantom for Combined PET, B-Mode Ultrasound and Elastographic Imaging

Combining the advantages of different imaging modalities leads to improved clinical results. For example, ultrasound provides good real-time structural information without any radiation and PET provides sensitive functional information. For the ongoing ClearPEM-Sonic project combining ultrasound and PET for breast imaging, we developed a dual-modality PET/Ultrasound (US) phantom. The phantom reproduces the acoustic and elastic properties of human breast tissue and allows labeling the different tissues in the phantom with different concentrations of FDG. The phantom was imaged with a whole-body PET/CT and with the Supersonic Imagine Aixplorer system. This system allows both B-mode US and shear wave elastographic imaging. US elastography is a new imaging method for displaying the tissue elasticity distribution. It was shown to be useful in breast imaging. We also tested the phantom with static elastography. A 6D magnetic positioning system allows fusing the images obtained with the two modalities. ClearPEM-Sonic is a project of the Crystal Clear Collaboration and the European Centre for Research on Medical Imaging (CERIMED).

A Systematic Study to Optimize SiPM Photo-Detectors for Highest Time Resolution in PET

We report on a systematic study of time resolution made with three different commercial silicon photomultipliers (SiPMs) (Hamamatsu MPPC S10931-025P, S10931-050P, and S10931-100P) and two LSO scintillating crystals. This study aimed to determine the optimum detector conditions for highest time resolution in a prospective time-of-flight positron emission tomography (TOF-PET) system. Measurements were based on the time over threshold method in a coincidence setup using the ultrafast amplifier-discriminator NINO and a fast oscilloscope. Our tests with the three SiPMs of the same area but of different SPAD sizes and fill factors led to best results with the Hamamatsu type of 50×50×μm2 single-pixel size. For this type of SiPM and under realistic geometrical PET scanner conditions, i.e., with 2×2×10×mm3 LSO crystals, a coincidence time resolution of 220 ±4 ps FWHM could be achieved. The results are interpreted in terms of SiPM photon detection efficiency (PDE), dark noise, and photon yield.

Development of ClearPEM-Sonic — A multimodal positron emission mammograph and ultrasound scanner

Breast cancer is one of the most frequent causes of premature death amongst women. To enhance the chances for complete recovery, the tumor shall be detected at an early stage. It is therefore necessary to develop efficient diagnostic means with high spatial resolution. In this focus, we developed a positron emission tomograph coupled to an ultrasound elastography system, dedicated to mammography: the ClearPEM-Sonic.

Development of breast anthropomorphic phantoms for combined PET-ultrasound elastography imaging

A phantom has been developed for PET/US breast imaging. The phantom reproduces the acoustic and elastic characteristics of human breast tissue, and the different tissues in the phantom can be labeled with 18F-FDG. The phantom was imaged with whole body PET/CT and also with the shear wave elastography with Supersonic Imagine Aixplorer system. We also test the phantom for other elastography methods such as static elastography. A 6D magnetic positioning system is used for image matching and fusion. This phantom is developed for ClearPEM/US multimodal breast imaging.

Development of ClearPEM-Sonic, a multimodal mammography system for PET and Ultrasound

ClearPEM-Sonic is an innovative imaging device specifically developed for breast cancer. The possibility to work in PEM-Ultrasound multimodality allows to obtain metabolic and morphological information increasing the specificity of the exam. The ClearPEM detector is developed to maximize the sensitivity and the spatial resolution as compared to Whole-Body PET scanners. It is coupled with a 3D ultrasound system, the SuperSonic Imagine Aixplorer that improves the specificity of the exam by providing a tissue elasticity map. This work describes the ClearPEM-Sonic project focusing on the technological developments it has required, the technical merits (and limits) and the first multimodal images acquired on a dedicated phantom. It finally presents selected clinical case studies that confirm the value of PEM information.

LuAG material for dual readout calorimetry at future high energy physics accelerators

One of the main challenges for detectors at future high energy collider experiments is high precision measurement of hadrons and jet energy and momentum. One possibility to enable such measurement is the particle flow approach (PFA) that requires a complex highly segmented calorimeter system to identify and to track all particles in a jet. An alternative so-called dual readout approach consists of simultaneously recording, in an active medium, scintillation light that is proportional to total energy deposit and Cerenkov light that is proportional to the electromagnetic part only, thus allowing extracting the electromagnetic fraction of the total shower energy on an event by event basis. A promising candidate to be used for dual readout calorimeter is Lutetium Aluminium Garnet Lu3Al5O12 (LuAG). Its high density, relatively short radiation length and interaction length provide sufficient stopping power necessary for hadronic calorimetry at high energy colliders, the fundamental absorption edge at 177nm combined with a very large optical transmittance window and a high refraction index (n=1.84) make LuAG an excellent Cerenkov radiator. Moreover cerium doped LuAG (LuAG:Ce3+) has good scintillating performances in terms of light yield and decay time due to the efficient radiative recombination 5d-4f from Ce3+ ion. LuAG can be produced either in bulk crystal or as fiber crystal allowing to assess its performance in two different approaches for dual readout calorimetry. As bulk crystals Cerium doped LuAG can be used in the method based on the separation of scintillation and Cerenkov signal produced in homogeneous detector blocks. In form of fibers it can be used in the method using so-called meta-crystals consisting of both Ce doped and undoped LuAG crystal fibers. The undoped fibers behave as Cerenkov radiator while Ce-doped fibers behave as scintillators. This talk reviews the properties of LuAG material in view of its use as scintillator both for bulk and fiber scintillators.

Comparison of Spectral and Scintillation Properties of LuAP:Ce and LuAP:Ce,Sc Single Crystals

Scintillation properties of LuAP:Ce and LuAP:Ce,Sc crystal series grown by the Bridgman method were studied under excitation by γ-rays from a 137Cs source. Both series were prepared using the same quality of starting oxides and demonstrated comparable optical quality in terms of underlying absorption at 260 nm, slope of the optical edge and transmission in the range of emission. The light yield in the present series of LuAP:Ce crystals measured in 0.2 cm × 0.2 cm × 0.8 cm pixels increases linearly with the Ce concentration reaching at 0.58 at.% 6448±322 ph/MeV and 9911±496 ph/MeV in the long and in the short directions respectively (the light yield ratio is 65%) and shows no sign of light saturation. The energy resolution is found to depend, among other factors, on the uniformity of Ce concentration within the pixels and is improved to 7.1±0.4% (l=0.2 cm), 9.5±0.5% (l=0.8 cm). Intentional co-doping with Sc3+ ions was tested and resulted in increase of the Ce distribution coefficient from 0.17 in LuAP:Ce to about 0.3 in LuAP:Ce,Sc. This enabled to increase the concentration of Ce in LuAP:Ce,Sc crystals up to 0.7 at.%, while conserving high optical quality. In contrast to LuAP:Ce, the light yield in LuAP:Ce,Sc crystals does not increase with Ce concentration, the photo peak being gradually suppressed. The involved mechanisms are discussed basing on the results of measurements of the unit cell volumes, Ce concentration uniformity, x-ray rocking spectra, absorption spectra of pure and variously doped LuAP crystals, and emission spectra under different excitations.

Comparison of spectral and scintillation properties of LuAP:Ce and LuAP:Ce, Sc single crystals

Scintillation properties of LuAP:Ce and LuAP:Ce,Sc crystal series were studied under excitation by gamma-rays from a 137Cs source. Both series demonstrated comparable optical quality in terms of underlying absorption at 260 nm, slope of the optical edge and transmission in the range of emission. The light yield of LuAP:Ce crystals measured in 0.2 cm times 0.2 cm times 0.8 cm pixels increases linearly with the Ce concentration reaching at 0.58 at. % 6448 plusmn 322 ph/MeV and 9911 plusmn 496 ph/MeV in the long and in the short directions respectively (the light yield ratio is 65%) and shows no sign of light saturation. The energy resolution is found to depend, among other factors, on the uniformity of Ce concentration within the pixels and is improved to 7.1 plusmn 0.4% (I = 0.2 cm), 9.5 plusmn 0.5% (I = 0.8 cm). Intentional co-doping with Sc + ions was tested and resulted in increase of the Ce distribution coefficient to about 0.3. This enabled to increase the concentration of Ce in LuAP:Ce,Sc crystals up to 0.7 at. %, while conserving high optical quality. In contrast to LuAP:Ce, the light yield in LuAP:Ce,Sc crystals does not increase with Ce concentration, the photo peak being gradually suppressed. The involved mechanisms are discussed basing on measurements of the unit cell volumes, Ce concentration uniformity, x-ray rocking spectra, absorption spectra of pure and variously doped LuAP crystals, and emission spectra under different excitations.