Biomedical Imaging with the Medipix2 semiconductor detector at UniAndes
I. Caicedo 1*, C. Avila 1, B. Gomez 1, C. Bula 1, C. Roa 1, J. C. Sanabria 1
1 Universidad de Los Andes, Bogotá, Colombia.
The MediPix2 detector is a “High spatial, high contrast resolving CMOS pixel read-out chip working in single photon counting mode” developed at CERN in order to share the technology of the LHC accelerator experiments with other scientific areas. The High Energy Physics Group at UniAndes is a member of the Medipix3 collaboration. Its research activities are focused on developing set-ups for biomedical applications and particle tracking using the Medipix2 and Timepix detectors, and assessing the feasibility of the Medipix3 detector for future applications.
As a first step to acquire biomedical images using Medipix2, we developed a set-up using a PHYWE X-Ray Unit and a Step Motor as sample holder. Then, we learned about the equalization and features of the detector and we performed preliminary raw acquisitions. As the Beam Hardening effect arose in the radiographies, it was necessary to correct the images through a Direct Thickness Calibration. The result was the acquisition of high resolution (~ µm) images from 10 to 35 KeV, with small exposure times.
After the characterization of the set-up, we proceeded to use it to image motionless and living tissue. We also studied its feasibility to image samples bigger than its sensitive part and to use it for vascular imaging. The size of the sensitive part of the chip (1.4 cm * 1.4 cm) limits the imaging of bigger surfaces. To solve this issue, we proceeded to develop a movable support for the chip and its read-out USB interface, and to acquire images by moving it in front of the sample. On the other hand, the contrast of the images was enhanced by the Beam-Hardening Correction; this feature was a motivation to image wax phantoms and vascular-like systems at different energies, exposure times and contrast agent concentrations to explore the possibility to acquire vascular images using Medipix2.
The high granularity of the detector makes it suitable for micro-CT. We used commercial software (Octopus) to do the 3D reconstruction of the samples in the first place, and we worked on our own free reconstruction software afterwards. Medipix has a very fast response (~ hundreds of nanoseconds) and high sensibility. These features allow obtaining nearly in-vivo high resolution (55 µm * 55 µm) images. We used an exposure time of 0.1 s for each frame, and the resulting images were “animated”.
Some of the tests were validated using Monte Carlo simulations (ROSI).
Key Words: Medipix; Pixel detectors; X-ray Imaging; Beam Hardening effect; ROSI; In vivo X-ray micro-Imaging; micro-CT.