M.C. Montesi

Preliminary test of Medisoft 4: control software for the Medipix2 read-out chip

We present here preliminary tests of Medisoft 4, a software procedure for the control and the readout of the single photon counting radiation imaging systems based on the Medipix2 chip (256/spl times/256 pixels, 55 /spl mu/m pitch). The system has been developed in the framework of the Medipix2 collaboration. This hardware and software system is the successor of the system based on the Medipix1 photon counting chip (64/spl times/64 pixels, 170 /spl mu/m pitch). Following the Medipix system evolution, Medisoft 4 allows the user to access the new implemented features such as the higher resolution, the faster data communication rates, the daisy-chain multichip mode, the energy windowed acquisition, the continuous acquisition mode, the double signal polarity (holes and electrons), etc. The readout of the data from the Medipix2 chip is presently via the chip serial bus through a MUROS2 interface board and a commercial input/output board, but a parallel readout via a is also foreseen. The current version (Medisoft 4.0), here the subject of preliminary tests connected via MUROS2 to a Medipix2 chip not bump-bonded to any detector , reads out only single chips and features a reduced set of functionalities. Future versions will read up to eight chips in daisy chain and provide full system performance, including high frame rate acquisitions and spectroscopic imaging. After a description of the Medipix2 chip, its readout interfaces and the Medisoft 4 software architecture, we show the results of preliminary software tests on serial communication protocol and speed with MUROS2 and Medipix2, analogue test input, internal digital analog converters calibration and threshold uniformity.

First images of a digital autoradiography system based on a Medipix2 hybrid silicon pixel detector

We present the first images of beta autoradiography obtained with the high-resolution hybrid pixel detector consisting of the Medipix2 single photon counting read-out chip bump-bonded to a 300 microm thick silicon pixel detector. This room temperature system has 256 x 256 square pixels of 55 microm pitch (total sensitive area of 14 x 14 mm2), with a double threshold discriminator and a 13-bit counter in each pixel. It is read out via a dedicated electronic interface and control software, also developed in the framework of the European Medipix2 Collaboration. Digital beta autoradiograms of 14C microscale standard strips (containing separate bands of increasing specific activity in the range 0.0038-32.9 kBq g(-1)) indicate system linearity down to a total background noise of 1.8 x 10(-3) counts mm(-2) s(-1). The minimum detectable activity is estimated to be 0.012 Bq for 36,000 s exposure and 0.023 Bq for 10,800 s exposure. The measured minimum detection threshold is less than 1600 electrons (equivalent to about 6 keV Si). This real-time system for beta autoradiography offers lower pixel pitch and higher sensitive area than the previous Medipix1-based system. It has a 14C sensitivity better than that of micro channel plate based systems, which, however, shows higher spatial resolution and sensitive area.

High-Resolution

We report on tests of a radionuclide imaging system for in vivo investigations in small animals with low-energy photons as from 125I (27-35 keV). Imaging optics features a high-resolution coded aperture mask and a fine pitch hybrid pixel detector (silicon 300-mum or 700-mum thick, or CdTe 1 mm thick) of the Medipix2 series (55 mum pitch, 256 x 256 pixels). The coded aperture had 480 70-mum holes in 100-mum-thick tungsten. Laboratory tests with a 109Cd 22 keV source and a microfocus X-ray tube (35 kVp, Mo anode) show a system resolution of about 110 mum at magnification m = 2.12 and a sensitivity improvement of 30:1 as compared to a 300-mum pinhole collimator. The field of view also depends on magnification: in the experiments presented, it varied from 6 mm (m = 2.12) to 21 mm (m = 0.66). 125I in vivo mouse thyroid imaging with the 70 mum coded aperture, a 300 mum pinhole and a 100 mum parallel hole collimator was also performed to obtain a qualitative comparison. This low energy, semiconductor-based, compact gamma-ray imaging system can be used as a gamma-ray sub-millimeter resolution imager for energies below about 35 keV and it is the basic imaging unit of a small animal Single Photon Emission Computed Tomography system (MediSPECT) built at University of Napoli Federico II and Istituto Nazionale Fisica Nucleare (INFN), Napoli.

^{125}

We present the design and preliminary imaging evaluation of an X-ray cone-beam breast computed tomography (CBBCT) apparatus, assembled for laboratory test of various optimization techniques for clinical CBBCT. The prototype is characterized by the computer-control of X-ray tube (35–80 kVp, 0.25 mA), CsI:Tl flat panel detector (12×12 cm2 area, 50 μm pitch), translation and rotation stages with eight degrees of freedom, fan-beam or cone-beam FDK backprojection. First tomographic images, obtained by rotating 14-cm diameter PMMA breast phantoms are presented.

I Small Animal Imaging With a Coded Aperture and a Hybrid Pixel Detector

We report on tests of a combined fluorescence and radionuclide planar imaging system for in vivo investigation on small animals. Combined images of anaesthetized mice bearing a surface solid tumor are presented. The fluorescent marker is a hematoporphyrin compound laser-excited with green light and imaged in the red fluorescence emission with a standard monochrome charge coupled device (CCD) camera. The gamma-ray (/sup 99m/Tc) pinhole imaging system uses a semiconductor pixel detector obtained by hybridizing a Silicon (300-/spl mu/m thick) or a CdTe (1-mm thick) pixel detector to the Medipix2 (55-/spl mu/m pitch) readout integrated circuit for single photon counting. The acquisition of combined images of the tumor area (fluorescence: animal top view; radionuclide: bottom view) shows that the tumor area can be imaged in a few minutes, with a few millimeter resolution (1-mm pinhole diameter), radioactively (/sup 99m/Tc MIBI, 74 MBq), and with the optical system. Combined imaging revealed also a different uptake of the two types of tumors studied (one grown from anaplastic human thyroid carcinoma-derived cells, the other from human papillary carcinoma-derived cells). Future progress will be toward a more compact optical setup and the use of a thicker CdTe detector.

X-ray Cone-Beam Breast Computed Tomography: Phantom Studies

A new digital imaging system for beta and gamma autoradiography has been realized. The technological advancement of this imaging system is represented by the second-generation Medipix2 chip (a single photon/particle counting chip), developed at CERN in the framework of the Medipix2 European collaboration, as a successor of the Medipix1 chip. Medipix2 is realized in 0.25 /spl mu/m CMOS technology; it has a sensitive area of 14/spl times/14 mm/sup 2/ and a pixel pitch of 55 /spl mu/m, it has been bump-bonded to a 300 /spl mu/m thick silicon pixel detector. A dedicated electronic interface and a complete image acquisition and processing software allow to operate this hybrid pixel detector for digital autoradiography with acquisition time as long as several days. In a preliminary test of this imaging system for digital autoradiography we obtained a detection threshold of 1600 electrons (equivalent to about 6 keV in Si), a noise of 10/sup -3/ cps/mm/sup 2/, a minimum detectable activity of 0.32 Bq in 14 h for /sup 3/H and 0.012 Bq in 10 h for /sup 14/C. A preliminary spatial resolution test for /sup 14/C gives a FWHM resolution of 30 /spl mu/m. Real time images of /sup 3/H, /sup 14/C, /sup 125/I autoradiographic microscales are also presented.

Experimental study on in vivo optical and radionuclide imaging in small animals

The measurement of the direction of WIMP-induced nuclear recoils is a compelling but technologically challenging strategy to provide an unambiguous signature of the detection of Galactic dark matter. Most directional detectors aim to reconstruct the dark-matter-induced nuclear recoil tracks, either in gas or solid targets. The main challenge with directional detection is the need for high spatial resolution over large volumes, which puts strong requirements on the readout technologies. In this paper we review the various detector readout technologies used by directional detectors. In particular, we summarize the challenges, advantages and drawbacks of each approach, and discuss future prospects for these technologies.

Digital autoradiography with a Medipix2 hybrid silicon pixel detector

We report on tests of a room-temperature particle counting silicon pixel detector of the Medipix2 series as the detector unit of a positron autoradiography (AR) system, for samples labelled with (18)F-FDG radiopharmaceutical used in PET studies. The silicon detector (1.98 cm(2) sensitive area, 300 microm thick) has high intrinsic resolution (55 microm pitch) and works by counting all hits in a pixel above a certain energy threshold. The present work extends the detector characterization with (18)F-FDG of a previous paper. We analysed the systems linearity, dynamic range, sensitivity, background count rate, noise, and its imaging performance on biological samples. Tests have been performed in the laboratory with (18)F-FDG drops (37-37 000 Bq initial activity) and ex vivo in a rat injected with 88.8 MBq of (18)F-FDG. Particles interacting in the detector volume produced a hit in a cluster of pixels whose mean size was 4.3 pixels/event at 11 keV threshold and 2.2 pixels/event at 37 keV threshold. Results show a sensitivity for beta(+) of 0.377 cps Bq(-1), a dynamic range of at least five orders of magnitude and a lower detection limit of 0.0015 Bq mm(-2). Real-time (18)F-FDG positron AR images have been obtained in 500-1000 s exposure time of thin (10-20 microm) slices of a rat brain and compared with 20 h film autoradiography of adjacent slices. The analysis of the image contrast and signal-to-noise ratio in a rat brain slice indicated that Poisson noise-limited imaging can be approached in short (e.g. 100 s) exposures, with approximately 100 Bq slice activity, and that the silicon pixel detector produced a higher image quality than film-based AR.

Readout technologies for directional WIMP Dark Matter detection

We report on tests of a radionuclide imaging system for in vivo investigations in small animals with low energy gamma-rays as from /sup 125/I (27-35 keV). The system imaging optics features a high resolution coded aperture mask and a fine pitch silicon hybrid pixel detector of the Medipix2 series (55 /spl mu/m pitch). The coded aperture (no-two-holes-touching MURA 62/spl times/62) had 70 /spl mu/m holes in 75 /spl mu/m tungsten, and was used in a 2:1 magnification for a field of view of about 7 mm. Laboratory tests with a /sup 109/Cd 22-keV source and in vivo mouse thyroid imaging tests with /sup 125/I show a system resolution of about 110 /spl mu/m. This low energy, semiconductor-based, compact gamma camera is the basic imaging unit of a small animal single photon emission computed tomography system with deep sub-millimeter resolution.

18F-FDG positron autoradiography with a particle counting silicon pixel detector

Nowadays there is compelling evidence for the existence of dark matter in the Universe. A general consensus has been expressed on the need for a directional sensitive detector to confirm, with a complementary approach, the candidates found in conventional searches and to finally extend their sensitivity beyond the limit of neutrino-induced background. We propose here the use of a detector based on nuclear emulsions to measure the direction of WIMP-induced nuclear recoils. The production of nuclear emulsion films with nanometric grains is established. Several measurement campaigns have demonstrated the capability of detecting sub-micrometric tracks left by low energy ions in such emulsion films. Innovative analysis technologies with fully automated optical microscopes have made it possible to achieve the track reconstruction for path lengths down to one hundred nanometers and there are good prospects to further exceed this limit. The detector concept we propose foresees the use of a bulk of nuclear emulsion films surrounded by a shield from environmental radioactivity, to be placed on an equatorial telescope in order to cancel out the effect of the Earth rotation, thus keeping the detector at a fixed orientation toward the expected direction of galactic WIMPs. We report the schedule and cost estimate for a one-kilogram mass pilot experiment, aiming at delivering the first results on the time scale of six years.