M. Chmeissani

LISA Pathfinder: mission and status

LISA Pathfinder, the second of the European Space Agencys Small Missions for Advanced Research in Technology (SMART), is a dedicated technology demonstrator for the joint ESA/NASA Laser Interferometer Space Antenna (LISA) mission. The technologies required for LISA are many and extremely challenging. This coupled with the fact that some flight hardware cannot be fully tested on ground due to Earth-induced noise led to the implementation of the LISA Pathfinder mission to test the critical LISA technologies in a flight environment. LISA Pathfinder essentially mimics one arm of the LISA constellation by shrinking the 5 million kilometre armlength down to a few tens of centimetres, giving up the sensitivity to gravitational waves, but keeping the measurement technology: the distance between the two test masses is measured using a laser interferometric technique similar to one aspect of the LISA interferometry system. The scientific objective of the LISA Pathfinder mission consists then of the first in-flight test of low frequency gravitational wave detection metrology. LISA Pathfinder is due to be launched in 2013 on-board a dedicated small launch vehicle (VEGA). After a series of apogee raising manoeuvres using an expendable propulsion module, LISA Pathfinder will enter a transfer orbit towards the first Sun?Earth Lagrange point (L1). After separation from the propulsion module, the LPF spacecraft will be stabilized using the micro-Newton thrusters, entering a 500?000 km by 800?000 km Lissajous orbit around L1. Science results will be available approximately 2 months after launch.

The LISA Pathfinder Mission

In this paper, we describe the current status of the LISA Pathfinder mission, a precursor mission aimed at demonstrating key technologies for future space-based gravitational wave detectors, like LISA. Since much of the flight hardware has already been constructed and tested, we will show that performance measurements and analysis of these flight components lead to an expected performance of the LISA Pathfinder which is a significant improvement over the mission requirements, and which actually reaches the LISA requirements over the entire LISA Pathfinder measurement band.

Performance limits of a 55-/spl mu/m pixel CdTe detector

In this work, the results from simulation of a CdTe pixel detector with a pixel size of 45 mum and a pitch of 55 mum are presented. Simulation of charge sharing between neighboring pixels has been compared with experimental results obtained by the use of the Medipix2 photon counting chip by means of a unique read-out system developed for the Dear-Mama project. Electrical simulations have been performed using the commercial software package DESSIS (ISETCAD) and combined with the Monte Carlo code GEANT4 to simulate the process of interaction and subsequent charge transport of charges induced by radiation. This allowed the analysis of charge sharing between pixel elements, an important limiting factor in imaging applications. Simulation was compared with experimental results obtained by means of a 1 mm thick CdTe detector. Each of these pixel electrodes was connected to the corresponding readout pixel on the Medipix2 chip, via an indium bump-bond. The backside contact was biased at -100 V, so that the pixel electrodes were collecting electrons. The results obtained imply that using a detector 1 mm thick with a pixel size smaller than 55 mum in single photon counting mode is unrealistic, because such fine spatial resolution cannot be attained in the corresponding image due to charge sharing

Energy and coincidence time resolution measurements of CdTe detectors for PET

We report on the characterization of 2 mm thick CdTe diode detector with Schottky contacts to be employed in a novel conceptual design of PET scanner. Results at -8°C with an applied bias voltage of -1000 V/mm show a 1.2% FWHM energy resolution at 511 keV. Coincidence time resolution has been measured by triggering on the preamplifier output signal to improve the timing resolution of the detector. Results at the same bias and temperature conditions show a FWHM of 6 ns with a minimum acceptance energy of 500 keV. These results show that pixelated CdTe Schottky diode is an excellent candidate for the development of next generation nuclear medical imaging devices such as PET, Compton gamma cameras, and especially PET-MRI hybrid systems when used in a magnetic field immune configuration.

Modeling and simulation of PET scanner based on pixelated solid-state detector

Novel conceptual design of Pixel-PET scanner, based on pixelated solid-state detector, is presented. Pixel-PET simply solves most of the intrinsic limitations that are inherited in the current PET scanners, and in particular those that are based on scintillating crystals, such as low detection efficiency, low energy resolution, low spatial resolution, parallax effect, image noise from scattered photons, and non-compatibility with strong magnetic field. The simulation results followed by image reconstruction show, when compared with state-of-the-art PET scanner for head, based on scintillating crystals (LSO/LYSO), the detection efficiency increases by a factor of 2.3 and the scattered photons are reduced from 98% to 3%, thus allowing to resolve the 1.2mm rod in Derenzo-like phantom with peak-to-valley ratio of 3 to 1.

Toward VIP-PIX: A Low Noise Readout ASIC for Pixelated CdTe Gamma-Ray Detectors for Use in the Next Generation of PET Scanners

VIP-PIX will be a low noise and low power pixel readout electronics with digital output for pixelated Cadmium Telluride (CdTe) detectors. The proposed pixel will be part of a 2D pixel-array detector for various types of nuclear medicine imaging devices such as positron-emission tomography (PET) scanners, Compton gamma cameras, and positron-emission mammography (PEM) scanners. Each pixel will include a SAR ADC that provides the energy deposited with 10-bit resolution. Simultaneously, the self-triggered pixel which will be connected to a global time-to-digital converter (TDC) with 1 ns resolution will provide the events time stamp. The analog part of the readout chain and the ADC have been fabricated with TSMC 0.25 μm mixed-signal CMOS technology and characterized with an external test pulse. The power consumption of these parts is 200 μW from a 2.5 V supply. It offers 4 switchable gains from ± 10 mV/fC to ± 40 mV/fC and an input charge dynamic range of up to ± 70 fC for the minimum gain for both polarities. Based on noise measurements, the expected equivalent noise charge (ENC) is 65 e- RMS at room temperature.

Simulation of the Expected Performance of a Seamless Scanner for Brain PET Based on Highly Pixelated CdTe Detectors

The aim of this work is the evaluation of the design for a nonconventional PET scanner, the voxel imaging PET (VIP), based on pixelated room-temperature CdTe detectors yielding a true 3-D impact point with a density of 450 channels/cm3, for a total 6 336 000 channels in a seamless ring shaped volume. The system is simulated and evaluated following the prescriptions of the NEMA NU 2-2001 and the NEMA NU 4-2008 standards. Results show that the excellent energy resolution of the CdTe detectors (1.6% for 511 keV photons), together with the small voxel pitch (1 × 1 × 2 mm3), and the crack-free ring geometry, give the design the potential to overcome the current limitations of PET scanners and to approach the intrinsic image resolution limits set by physics. The VIP is expected to reach a competitive sensitivity and a superior signal purity with respect to values commonly quoted for state-of-the-art scintillating crystal PETs. The system can provide 14 cps/kBq with a scatter fraction of 3.95% and 21 cps/kBq with a scatter fraction of 0.73% according to NEMA NU 2-2001 and NEMA NU 4-2008, respectively. The calculated NEC curve has a peak value of 122 kcps at 5.3 kBq/mL for NEMA NU 2-2001 and 908 kcps at 1.6 MBq/mL for NEMA NU 4-2008. The proposed scanner can achieve an image resolution of ~ 1 mm full-width at half-maximum in all directions. The virtually noise-free data sample leads to direct positive impact on the quality of the reconstructed images. As a consequence, high-quality high-resolution images can be obtained with significantly lower number of events compared to conventional scanners. Overall, simulation results suggest the VIP scanner can be operated either at normal dose for fast scanning and high patient throughput, or at low dose to decrease the patient radioactivity exposure. The design evaluation presented in this work is driving the development and the optimization of a fully operative prototype to prove the feasibility of the VIP concept.

Pixelated CdTe detectors to overcome intrinsic limitations of crystal based positron emission mammographs.

A positron emission mammograph (PEM) is an organ dedicated positron emission tomography (PET) scanner for breast cancer detection. State-of-the-art PEMs employing scintillating crystals as detection medium can provide metabolic images of the breast with significantly higher sensitivity and specificity with respect to standard whole body PET scanners. Over the past few years, crystal PEMs have dramatically increased their importance in the diagnosis and treatment of early stage breast cancer. Nevertheless, designs based on scintillators are characterized by an intrinsic deficiency of the depth of interaction (DOI) information from relatively thick crystals constraining the size of the smallest detectable tumor. This work shows how to overcome such intrinsic limitation by substituting scintillating crystals with pixelated CdTe detectors. The proposed novel design is developed within the Voxel Imaging PET (VIP) Pathfinder project and evaluated via Monte Carlo simulation. The volumetric spatial resolution of the VIP-PEM is expected to be up to 6 times better than standard commercial devices with a point spread function of 1 mm full width at half maximum (FWHM) in all directions. Pixelated CdTe detectors can also provide an energy resolution as low as 1.5% FWHM at 511 keV for a virtually pure signal with negligible contribution from scattered events.

Characterization of CdTe detector for use in PET

CdTe diode detectors with Schottky contact have been characterized in terms of energy resolution and time of response. A resolution of 0.98% at 511keV has been achieved with a 4 mm × 4 mm × 2 mm detector at 900 V/mm and −7 °C. At the same bias and temperature conditions, two identical CdTe detectors show a coincidence time FWHM of 25 ns. Additionally, the effect of a strong magnetic field on the charge sharing has been studied for a 9-pixel array detector of 55 µm × 55 µm × 800 µm pixel size connected to MediPix2 front end electronics. No effect on the charge sharing distribution has been observed up to 4 T. These results show that CdTe Schottky diodes are excellent candidates for the development of next generation nuclear medical imaging devices such as PET, Compton gamma camera, and especially PET-MRI when used in a magnetic field immune configuration.

Modeling, simulation, and evaluation of a compton camera based on a pixelated solid-state detector

A novel Compton camera design based on pixelated solid-state detectors is proposed and evaluated via Monte Carlo simulation, using the Geant4-based Architecture for Medicine-Oriented Simulations GAMOS. For the image reconstruction, the Stochastic Origin Ensemble (SOE) method has been used. The efficiency of the reconstruction of Compton prompt events is constant up to activities of 107 Bq. The signal-to-noise ratio (SNR), i.e., the ratio between real coincidences and mis-reconstructed ones, was above 85% for photon energies ranging from 141 to 511 keV. For a 18F isotope source, a sensitivity of 12 cps/kBq has been obtained. For a 99mTc isotope source, a sensitivity of 15 cps/kBq has been obtained. Using the NEMA NU-4 2008 standard for the PSF estimation, values for the FWHM of 1.80 mm for the spatial resolution with a 18F radioactive source and 3.82 mm with a 99mTc source were obtained.