Walter Rütten

Assessment of the spatial resolution of PET scanners using a Geant4-based Monte Carlo tool

In this paper, the spatial resolution of PET scanners is assessed by means of a Geant4-based Monte Carlo tool. To obtain a high level of accuracy, all relevant contributions like /spl beta//sup +/ range and gamma noncollinearity, crystal size and material, intercrystal scatter, and scanner geometry are implemented in detail. For a system based on Anger logic, the scintillator light yield, the photon statistics and the PMT characteristics are also taken into account. In the simulation, Monte Carlo and analytical algorithms share a common setup, which allows for the necessary computation speed to generate clinical count statistics without compromising accuracy. In the first part, the major contributions to the spatial resolution are analyzed separately. The impact on the total system resolution is then illustrated by investigating three scanner designs with different gantry sizes, crystal sizes, crystal materials and /spl beta//sup +/ tracers.

Exact modeling of analog pulses for PET detector modules

The quality of PET images depends on the position, energy and time resolution of the gamma detector, which usually consists of scintillation crystals coupled to an array of photomultiplier tubes (PMTs). We have developed a simulation tool which models the conversion of scintillation photons into PMT waveforms, thereby bridging the gap between Monte Carlo simulations of the gamma interaction and pure electronics simulations of the data acquisition system. In our model, we track the scintillation photons individually from the crystal through the light guide to the photocathode of the PMT. The PMT characteristics are treated on a single photoelectron basis, incorporating pulse height distribution and single-photon response as well as spatially dependent quantum efficiency, transit time and transit time spread. From the PMT waveforms, we derive the position, energy and time resolution. The time resolution depends on the scintillator material, the PMT properties and the time-stamping method. By separating the different contributions, the limiting factors in the performance of a detector can be identified, facilitating its optimization. A comparison with measured data is included.

Rate-dependence of the key performance parameters in an Anger logic based PET detector

The image quality of a PET scanner depends strongly on the spatial, energy, and time resolution of the detector. These parameters are usually specified for the case of low count rates. At higher rates, signals from subsequent events can overlap, and this pile-up deteriorates the resolution. In an Anger based detector, the high encoding ratio of scintillator pixels to photo multiplier tubes (PMTs) facilitates pile-up. The probability of pile-up depends on the detector geometry, the scintillator material, and the trigger scheme, and it increases with the count rate. In this paper, the key performance parameters of a whole-body PET detector are investigated as a function of the rate. Experimentally, we use scintillator arrays which can be coupled to PMT arrays with PMTs of different size, but otherwise similar characteristics. First results are presented which show-that signal pile-up leads to only small performance deterioration in typical /sup 18/F oncology studies, but has to be considered when using short lived tracers, e.g. based on /sup 11/C or /sup 15/O.

Impact of the light detection chain on the NEC in a full-body PET scanner

In an ideal PET scanner, the count rate behavior depends on the true sensitivity, the scatter fraction, the singles rate and the coincidence timing window. However, a detailed system analysis shows that due to signal pile-up, the design of the light detection chain (light output, light spread, light detection) also has a strong impact on the resolution and performance of the system. In this paper, the dependence between the light detector size and the count rate behavior is analyzed, and the results are illustrated in terms of the noise equivalent count rate (NEC). Using a suite of system simulation tools, the performance of a PET scanner is modeled down to the level of single optical photons and photo electrons, allowing the count rate behavior to be derived from first principles. Hereby, scintillator materials with different decay times (LYSO, LaBr3) are considered. The results show that for activity levels encountered with short half-life tracers like 11C and 15O, the system NEC can differ by a factor of more than two for systems with identical scintillator geometries, but different light detector sizes

Dynamic X-Ray Imaging System based on an all-solid-state Detector

New digital detector systems based on all-solid-state large area elec-tronics offer a number of advantages for the user, like no image distortions, flat and light weight housing, no veiling glare, and large dynamic range. On the other hand, they require a dedicated image preprocessing to exploit their full image quality. In this paper we address design concepts, performance charac-terization and resulting image preprocessing aspects of our experimental de-tector system. The system comprises the detector frontend and a realtime image preprocessing unit with an interface to a commercial digital video system. It is intended for clinical evaluation of this new technology.