Activity 6.1 - hybrid semiconductor detectors for digital autoradiography and
small animal imaging scintigraphy
Coordinator: Prof. Paolo Russo
Collaborators: G. Mettivier, M.C. Montesi
Our Medical Physics group at this Department is participating to the international
collaboration Medipix2 (CERN, INFN and a number of Universities and Research
Institutes in Europe). This collaboration is developing hybrid semiconductor
radiation detectors (Si, GaAs, CdTe) with VLSI electronics (Medipix2 circuit) for
single photon counting imaging applications in biology and medicine and physics.
The hybrid pixel detectors used are Medipix2 read-out chips, bump-bonded to a
300 (m thick Si pixel detector or to a 1 mm thick CdTe. The matrix array is a
256(256 square of cells of 55 (m by side, each containing a charge preamplifier, a
double threshold discriminator and a 13 bit counter. The sensitive area is 14(14 mm2.
The chip is read out serially by a dedicated electronic interface connected to a PCI
digital I/O board.
In the Laboratory of Medical Physics, using such an imaging system, we have
performed autoradiographic studies with beta-emitting tracers used in biomedical
applications (i.e., H-3, C-14), in addition to small animal imaging scintigraphic
studies. As regards our small animal imaging studies, we have assembled a SPECT
system prototype (MediSPECT), and we have used Tc-99m and I-125 as a
radiotracers and either Si (0.3 mm thick) or CdTe (1 mm thick) pixel detectors, for in
vivo mouse imaging studies. In particular we have investigated the performance and
the feasibility of coupling coded aperture optics (as collimating system) to a Si or a
CdTe pixel detector read out by a Medipix2 chip. The coded aperture mask (no-two-
holes-touching MURA 62x62) has been designed to allow a transmission fraction of
0.03 in its opaque parts. For 125I photons, this value requires the use of 75 (m thick
tungsten, in which it is possible to etch holes with a minimum nominal width of
70 (m (Tecomet, Woburn, MA).
Laboratory calibration tests with a 109Cd 22-keV source and pilot in vivo mouse
thyroid imaging tests with 125I have shown a system resolution of about 110 m.
On behalf of all the Medipix collaboration, our Medical Physics group has concluded
the development of the Medisoft software (version 4.1) for the readout of four
Medipix2 chip (quad detector) via the MUROS2 serial interface. This imaging
detector is a multi chip module of a “large” area 28(28 mm2 Silicon pixel (512x512
pixel) sensor. This Medisoft 4.1 is now used by all partners of the European
Medipix2 Collaboration. Preliminary tests on quad detector have been carried out.
Program for 2005
During 2005 our Medical Physics group will implement the Medisoft software
(version 4.1) for the readout of single and multichip Medipix2 hybrid semiconductor
radiation detectors with functions that will make possible to acquire 3D images for
Computed Tomography and Single Photon Emission Computed Tomography. In
particular, for SPECT images, functions for reconstructing images acquired with
coded-mask optics will be implemented.
The quad detector (Si sensor, 512x512 pixel) will be tested for Computed
Tomography with a parallel read out which has been expected to reach an acquisition
rate of 20 frames per second.