E. Cochran

Timing performance of the silicon PET insert probe.

Simulation indicates that PET image could be improved by upgrading a conventional ring with a probe placed close to the imaged object. In this paper, timing issues related to a PET probe using high-resistivity silicon as a detector material are addressed. The final probe will consist of several (four to eight) 1-mm thick layers of silicon detectors, segmented into 1 x 1 mm(2) pads, each pad equivalent to an independent p + nn+ diode. A proper matching of events in silicon with events of the external ring can be achieved with a good timing resolution. To estimate the timing performance, measurements were performed on a simplified model probe, consisting of a single 1-mm thick detector with 256 square pads (1.4 mm side), coupled with two VATAGP7s, application-specific integrated circuits. The detector material and electronics are the same that will be used for the final probe. The model was exposed to 511 keV annihilation photons from an (22)Na source, and a scintillator (LYSO)-PMT assembly was used as a timing reference. Results were compared with the simulation, consisting of four parts: (i) GEANT4 implemented realistic tracking of electrons excited by annihilation photon interactions in silicon, (ii) calculation of propagation of secondary ionisation (electron-hole pairs) in the sensor, (iii) estimation of the shape of the current pulse induced on surface electrodes and (iv) simulation of the first electronics stage. A very good agreement between the simulation and the measurements were found. Both indicate reliable performance of the final probe at timing windows down to 20 ns.

Performance of the MADEIRA PET probe prototype

This paper reports the characterization of a detector module, the building block to be used for the MADEIRA PET probe prototype. The prototype will be used in synchronization with a conventional PET ring, amplifying the basic image with a subset of events with high spatial resolution. For image improvement, the crucial parameters are the spatial and timing resolution of the probe, while the energy resolution can be used in event classification. The final prototype is made of high-resistivity silicon detectors, 1 mm thick with 1040 square pads with a size of 1 mm. The performance was characterized on an evaluation module featuring the same electronics and sensor material, save for sensor geometry where a sensor with 256 square pads with a side of 1.4 mm was used. The pads were read out with VATAGP7, a Gamma Medica-Ideas designed application specific integrated circuit (ASIC). The ASIC provides a logic trigger signal and an analog output for every out of 128 input channels. The measurement of the timing resolution was performed, using a positron source and a timing reference detector. A similar measurement was performed with a test pulse, ie. a charge injection into the ASIC, to study inherent resolution of the firststage electronics. We calibrated the analog output of the ASIC using standard gamma sources (241Am), and we determined the overall energy resolution. The comparators were calibrated by a test pulse, looking at the rate of triggers versus threshold. The gain variation was compensated with internal 3-bit DACs. The evaluation module was successfully characterized, exhibiting energy resolution of 1.5 keV FWHM after gain alignment. The spread of the comparator levels can be decreased below 1 keV. The timing performance allows timing windows of 20 ns to be used. Based on the performance of the evaluation module we conclude that the final prototype can be used as a PET insert probe.

A study of the effects of strong magnetic fields on the image resolution of PET scanners

Very high resolution images can be achieved in small animal PET systems utilizing solid state silicon pad detectors. In such systems with sub-millimeter intrinsic resolutions, the range of the positron is becoming the dominant contribution to image blur. The size of the positron range effect depends on the initial positron energy and hence the radioactive tracer used. For higher energy positron emitters, such as 68Ga and 94mTc which are gaining importance in small animal studies, the width of the annihilation point distribution dominates the spatial resolution. This positron range effect can be reduced by embedding the field of view of the PET scanner in a strong magnetic field. In order to confirm this effect experimentally we have developed a high resolution PET instrument based on silicon pad detectors that can operate in a 7 T magnetic field. In this paper we present the preliminary results of a study of the effects of magnetic fields up to 7 T on PET image resolution for 22Na and 68Ga point sources.

An FPGA based DAQ system for the readout of Madeira PET probe

Madeira project aims to significantly improve three-dimensional (3D) nuclear medicine imaging technologies via a compact photon-sensitive probe interfaced to an external conventional PET ring and placed close to the region of interest. The probe consists of several modules densely packed. Each module is made of two high-resistivity silicon detectors of 1 mm thickness and 1040 square pixels of 1 mm2. The detectors are placed back-to-back at a distance of 0.8 mm. The pads are read out with the VATAGP7 chip, a Gamma Medica — Ideas designed application specific integrated circuit (ASIC). A FPGA based DAQ system has been designed and developed at IFIC-Valencia to read out the probe. The system consists of several DAQ boards working in parallel which control the acquisition process. The DAQ board has trigger and coincidence capabilities to be used in coincidence with a conventional PET scanner. This work describes the characteristics of the system and its architecture, and also the future activities.

Very High Resolution Small Animal PET in Strong Magnetic Fields

Very high resolution images can be achieved in small animal PET systems utilizing solid state silicon detectors (Compton PET). In such systems with sub-millimeter intrinsic resolutions, the range of the positron is the largest contribution to image blur. The size of the positron range effect depends on the initial positron energy and hence the radioactive tracer used. For higher energy positron emitters, such as 124I and 94mTc which are gaining importance in small animal studies, the variation of the annihilation point dominates the spatial resolution. It has been suggested that this positron range effect can be reduced by embedding the PET field of view in a strong magnetic field. Conventional PET systems using scintillators and photomultiplier tubes require extensive modifications to operate in magnet fields; however, our silicon detector based system can operate in magnetic fields with minimal modifications. In this paper we present a progress report of embedding our small animal PET test-bench in magnetic fields up to 7 Tesla.

Report on the MADEIRA PET probe

PET probes are showing a lot of promise in extending performance of the conventional PET ring. The underlying idea is to supplement basic PET data with information collected in the finely segmented probe placed close to the region of interest. The benefit is two fold: a) data collected near the object are less prone to errors related to scattering and acolinearity and b) the object itself is magnified in the proximity focus. The principle would be beneficial to clinical applications where spatial resolution below the current limit is required in a narrow field of view. The probe should therefore have excellent spatial resolution, should be compact and robust and should be able to handle large count rates of the clinical environments. Based on those we decided to explore devices with high-resistivity silicon as the sensitive material. They provide high spatial resolution, are compact and robust, and can handle the foreseen rates. We constructed a prototype, based on 1 mm thick silicon wafers, cut into 40 by 26 mm2 detectors further segmented into 1 × 1 mm2 square pads, effectively providing 1 mm3 sensitive voxels. For a module, two such detectors were placed in a back-to-back arrangement, providing filling factor in excess of 70 %. Stacking multiple modules is foreseen to compensate for low stopping power of silicon. The sensors are read out by 128 channel VATAGP7, GM-Ideas sourced application sensitive integrated circuit. Each module requires 16 chips, placed on 4 custom made PCB boards (hybrids) which are read independently. The modules were characterized and will be placed in a test PET ring. A simple point sources and phantoms will be imaged to confirm the predicted benefits.

DAQ++: A C++ Data Acquisition Software Framework

This paper describes DAQ++, a C++ based framework for developing data acquisition software. The design of DAQ++ is fully object oriented (OO) and provides a hierarchy of objects that allow a full control of the acquisition system and, also, on-line monitoring and storage of data. Being an OO system, it is easy to extend the DAQ system over several machines using a CORBA based architecture which is also provided by DAQ++. Finally, the paper describes a DAQ software system, VMEDAQ, developed with DAQ++ for the data acquisition in the CIMA collaboration. It provides a GUI which controls the objects based on the DAQ++ hierarchy. VMEDAQ has been built in a modular way which allows to add dynamically user defined data producers or DAQ++ Modules and, even, data receivers or RunManagers that allow to define different data acquisition modes like, for instance, parameter scanning.

DAQ system for the readout of silicon pixel detectors based on VataGP7 front-end ASIC

High resistivity silicon is a competitive material for photon detectors. Its excellent spatial and energy resolution make them attractive to be used as second detector in medical physics applications. A stack of several detector layers would significantly improve the low stopping power of a single detector. This however, increases considerably the number of channels to be read-out. A Data acquisition system (DAQ) has been developed at IFIC - Valencia to cope with a stack of such detectors. The system is based on the 128 channels VATAGP7 application specific integrated circuit (ASIC) from Gamma Medica - Ideas. A dedicated electronic board (MADDAQ) is in charge of the ASIC control and the administration of the whole readout process. Collected data is processed and sent to a computer trough Ethernet connection. Several MADDAQ boards can work in parallel independently or in coincidence mode. Measurements with the system are presented and characterization of the system is described.

Spectroscopy study of imaging devices based on silicon Pixel Array Detector coupled to VATAGP7 read-out chips

Spectroscopic and timing response studies have been conducted on a detector module consisting of a silicon Pixel Array Detector bonded on two VATAGP7 read-out chips manufactured by Gamma-Medica Ideas using laboratory gamma sources and the internal calibration facilities (the calibration system of the read-out chips). The performed tests have proven that the chips have (i) non-linear calibration curves which can be approximated by power functions, (ii) capability to measure the energy of photons with energy resolution better than 2 keV (exact range and resolution depend on experimental setup), (iii) the internal calibration facility which provides 6 out of 16 available internal calibration charges within our region of interest (spanning the Compton edge of 511 keV photons). The peaks induced by the internal calibration facility are suitable for a fit of the calibration curves. However, they are not suitable for measurements of equivalent noise charge because their full width at half maximum varies with their amplitude. These facts indicate that

A demonstrator for a new axial PET concept

In PET imaging, improving sensitivity while maintaining very good spatial resolution is crucial. To achieve this goal, we propose a novel concept of PET scanner, with axially arranged crystals, providing a high sensitivity and a 3D reconstruction of the gamma interaction point. The trans-axial coordinate is given by the crystal hit, while the z coordinate is reconstructed by the weighted distribution of light escaping the crystal and entering into an array of Wave Length Shifting (WLS) strips interleaving the crystal layers. This novel configuration allows full identification of Compton interactions in the crystals that can be included in image reconstruction thus enhancing the sensitivity. We present preliminary results obtained by a small prototype consisting of 4×4 crystals with orthogonally interleaved WLS strips. Experimental data are compared to simulated data.