Ralf Dorscheid

Performance evaluation of a prototype Positron Emission Tomography scanner using Digital Photon Counters (DPC)

We show performance results of a prototype Positron Emission Tomography scanner based on digital SiPMs, or Digital Photon Counters (DPC), developed by the Philips Digital Photon Counting unit. The scalability of the DPC technology is demonstrated by an excellent system coincidence timing resolution of 266 ps FWHM and an energy resolution of 10.7 % FWHM. Even while using 4 mm × 4 mm × 22 mm LYSO crystals, the spatial resolution is close to 2.4 mm. Although not optimized yet, the image homogeneity is 5.8 %. We show that the system performance is maintained even at highest count rates encountered in PET scans.

Two-dimensional anti-scatter grids for computed tomography detectors

The use of two-dimensional, focused, anti-scatter-grids (ASGs) in computed tomography is one essential solution to reduce the scatter radiation for large area detectors. A detailed analysis of the requirements and related image quality aspects lead to the specification of the two-dimensional focused geometry of the X-ray absorbing grids. Scatter simulations indicated trade-off conditions and provided estimations for the expected scatter reduction performance. Different production technologies for focused two-dimensional structures have been evaluated. The presented technology of Tomo Lithographic Molding (TomoTM) shows good fulfilment of the specifications. TomoTM is a synthesis of lithographic micromachining, precision stack lamination, molding, and casting processes with application-specific material systems. Geometry, material properties, and scatter performance have been investigated. Different analysis methods will be presented and results of the investigations demonstrate the performance capability of this two-dimensional grid technology. Material composition of the tungsten-polymer composite, homogeneity of wall thickness, and precision of the focusing have the biggest influence on the X-ray behavior. Dynamic forces on the anti-scatter-grid during CT operations should not lead to dynamic shadowing or intensity modulation on the active pixel area. Simulations of the wall deformation have been done to estimate the maximum position deviation. Prototype two-dimensional ASGs have been characterized and show promising results.

PhenoPET: A dedicated PET scanner for plant research based on digital SiPMs (DPCs)

In the framework of the German Plant Phenotyping Network (DPPN) we developed a novel PET scanner for imaging plants and crops. The observation of the carbon transport within the plant becomes possible by using 11CO2 as PET tracer. The use of the rather short living isotope C-11 asks for a scanner with high dynamic range. That means fast timing and high data rates are important features which let us choose the Philips Digital Photon Counter (DPC) as photo detector. Due to the fast photo detectors and the special crystal matrix arrangement the system will allow measurements with rather high activities. We could measure a coincidence resolution time of ~ 250 ps FWHM between two detector elements. This opens the opportunity to employ time-of-flight information for the first time on a PET scanner of this size. This paper presents very first results from a prototype single-ring system with a FOV of 18 cm diameter and 6.5 cm axial height.

Read-out electronics for digital silicon photomultiplier modules

This work has its focus on the development of fast read-out electronics for digital silicon photomultipliers (dSiPM -called Digital Photon Counter (DPC) by Philips).

CMOS compatible through wafer interconnects for medical imaging detectors

Modern medical imaging systems like computed tomography (CT) require advanced technologies for the imaging sensor and processing electronics as well as for the packaging technologies to build an integrated sensor-system. As the size of the overall detector increased within the last years, new solutions for the realization of these large area detectors are required especially for advanced systems with integrated detector electronics. As the pixel size is about 1.1 times 1.4 mm2 the overall size of the detector, with about 60,000 pixels, is in the range of 76,000 mm2. Several advanced concepts - realized in standard CMOS technology - for active pixel arrays with charge-integration, high dynamic range current amplifier and in-pixel sigma-delta-modulator have been investigated. For the usage in large area detectors new packaging concepts have to be developed as a four-side-buttable (tile) structure can only be realized with a backside connection of the chip. In our development the through wafer interconnects (TWI) do not necessarily show up in the front side as metal signal layers could be used for signal routing on top of the TWI. From CT application the geometric and the electric specifications for the TWI have been derived. The optical sensitive front-side of the chip that is attached to a scintillator crystal is not influenced by the processing of the TWI. The basic idea for the CMOS-compatible TWI technology is the design of interconnecting conductive trench geometries in the wafer prior to the CMOS processing.

Through Wafer Interconnects - A Technology not only for Medical Applications

The foremost driver for the development of fully CMOS compatible Through Wafer Interconnects (TWIs) is the need of very large photodiode arrays for detectors, e.g. in computed tomography applications. The front to back-side contact allows the four-side buttable chip placement of the already large chips (20mm × 22mm 2 ). The TWI technology allows an interconnection for chips up to 280μm thickness. This technique does not require any via opening at the font side, thus enabling a metal signal routing on the active side, on top of the interconnection. The application specific optical sensitive front-side of the chip is fully accessible. The production process is separated into three main steps. The first step is the implementation of the special TWI geometry into the CMOS substrate. Depending on the electrical and geometrical requirements of the circuit, different TWI structures are built with deep trenches (up to 280μm), which are passivated and filled with doped poly-silicon. The technologies used in this process, such as DRIE-etching, oxidation and low pressure CVD, are standard CMOS compatible processes. The use of poly-silicon prevents from achieving very low resistivity interconnections but allows the use of all CMOS process steps for an imager production (no temperature limitation – compared to other TWI process flows). The second step is the standard CMOS processing on the substrate already including the TWIs. The third step is a low temperature back-side process starting with wafer thinning down to 280μm or less to open the implemented TWI structure from the back-side. The thickness may be selected depending on the target application. A modified under ball metallization (UBM) process, which could include also re-routing of signals on the back-side, concludes the process flow until the solder ball placement, or similar bond connections. The special process flow opens a variety of applications which benefit from the full CMOS compatible processing and the accessible front-side.

PhenoPET — results from the plant scanner

Within the German Plant Phenotyping Network (DPPN), we developed a novel PET scanner based on Philips Digital Photon Counters (DPCs, or dSiPMs = digital Silicon Photomultipliers). The scanner is dedicated for plant research and provides functional information on carbon transport within the plant. To this end the detector ring is oriented horizontally. It provides a Field-of-View of 18 cm dia. and 20 cm in height. The read-out electronics cluster hits from different photodetector pixels when they originate from the same scintillation event. These single events are written via USB 3.0 with up to 300 MB/s to the computer system. Crystal identification, energy discrimination and coincidence detection is realized in software. The spatial resolution in the center Field-of-View (CFOV) could be estimated to approx. 1.6 mm from measurements of a dedicated hot rod phantom. Preliminary sensitivity measurements result in a peak sensitivity of 4.04% (ΔE = 250-750 keV) in the CFOV and a Coincidence Resolving Time of 298 ps could be achieved.