Applied Physics

Mokhtar Chmeissani

The focus of the applied physics research at IFAE is to develop sensor technologies with applications in medical imaging, high-energy physics and other scientific or industrial fields by exploting the valuable knowledge available at IFAE and fostering collaborations with other research centres in Catalonia like the Centro Nacional de Microelectronica (CNM), medical centers like Hospital Parc Taulí, or companies like Multiscan Technologies.

Introduction

In 2018, the medical imaging/applied physics group at IFAE was engaged in 4 projects: Exploiting the potential use the VIP PET as Compton-PET Scanner , the production of 6cm x 6cm pixel camera for the 3D-Biopsy-Tomosynthesis project , stimulating the rat retina with electric signals as part of THEIA project, and participating in the design of 12 bits ADC for Timepix4 ASIC.

The VIP-PET

The high spatial and energy resolutions that VIP-PET detector module provides, open the way to construct a large Field Of View (FOV) Compton Gamma Camera at room temperature. If one combines the PET design with Compton Camera to form Compton-PET scanner, one can obtain a novel device that provide PET images without image reconstruction. In another word, the VIP Compton-PET scanner can provide a real time map for PET isotopes concentrations in the object being diagnosed. This is possible when the isotope emits an extra gamma photon along with the e+ , such as in 44Sc where an additional gamma of 1157 keV is emitted. Preliminary tests, using 3mm point source of 22Na, shows an image of the source with FWHM of 3mm without the use of any image reconstruction algorithm. The spatial resolution is equivalent of TOF of 10 ps

Figure 2: Histogram of the intersection of the LOR with the Compton Cone.

The 3D-BON project

Project funded by RIS3CAT that aims at developing a novel 3D Tomosynthesis breast biopsy system in collaboration with IFAE, Centro Nacional de Microelectronica (CNM-IMB), Hospital Parc Tauli, IDNEO Technologies S.L., and VENTURA Medical Technologies. In 2018 IFAE team concluded the prototype hardware of the full camera as shown in figure 2. To reduce the dead area because of the back-end of the sensor chip, we opt of designing the motherboard with 4 long baby board (bb) in a form of ladder shape, with each bb carries 4 sensors. In this approach the top view show one solid area of 16 sensors. The camera sensor dissipate about 30W and to extract such amount of heat we decided to use cold plate under the bb, made via 3D metal printing device.

Figure 3: Motherboard with 4 bb mounted, each hold 4 sensors. Side view of the MB and the stacking of the 4 bb in the form of ladder is clear.

Figure 4: Motherboard with 4 bb mounted, each hold 4 sensors. Top view of the MB and one see the 16 sensors forming one single sensors with minimum dead area between adjacent sensors due to the guard ring.

The THEIA project

The THEIA (Toward the implementation of a multi-electrodes array for retinal prosthesis ) project, started in 2017 and it is carried out by IFAE, ICFO, ICN2, and Barraquer Ophthalmology Center. It is a multidisciplinary R&D project (Instrumentation, Graphene, Optics, and Ophthalmology) aims to develop a novel retinal implant based on graphene electrodes for better stimulus of the retinal ganglion cells (RGCs). The project was funded again by Barcelona Institute of Science and Technology (BIST) for the year 2018. IFAE has developed an elaborated electronics card and the corresponding GUI, see figure 3, to send programmable current pulse of specific shape and amplitude to a specific electrode on which the retina of the rat is placed. With the RGCs in rat retina transduces genetically with encoded calcium indicator GCaMP5G, fluorescence light can be detected when local RGCs get stimulated with electric pulse as one can see in figure 4. This technique is important to understand how to optimize the coupling between the electrodes of the retinal implant prosthesis and the surface of retina, and at the same time to optimize the shape of the electric pulse to be able to induce the stimulus in the RGCs with the minimum charge injection.

Figure 5: Electronics (hardware+ control) to stimulate the ganglion cells via an array of graphene electrodes

Future is plan is to convert the design from discrete component to an ASIC of 3mm x 4mm based on 65nm CMOS process technology. Two important feature of this ASIC, it will be powered and programmed wirelessly. And excite independently 960 positions on the retina. A patent application has been filed about this topic.

Figure 6: One region of the rat retina in contact with the graphene electrode. Background fluoresces emitted by the RGCs without any electric stimulation pulse

Figure 7: One region of the rat retina in contact with the graphene electrode. Same region is shown after being stimulated with electric pulses

The Medipix4 project

Medipix4 is the 4th generation of the of Medipix photon counting ASIC. IFAE, as member of the Medipix Collaboration, is participating in the design of Timepix4 chip, which is a variant ASIC derived from Medipix4. The chip will be fabricated with a 65 nm CMOS process technology during 2019. IFAE role in the design is a 12-bits incremental sigma-delta Analog to Digital Converter (ADC). It will be used to monitor 32 different internal voltages of the chip which will be multiplexed to the input of the ADC. The integration of an ADC inside the chip will allow to reduce the number of I/O pads as well as to provide a direct digital measure. Compared to the other studied solutions, sigma delta converters offer better performances in terms of resolution, area and power consumption. In addition, most of the signal processing is done in the digital domain, so it is more robust to mismatching during fabrication and no calibration is needed. The main features of the ADC are summarized in the table shown in figure 5. The conversion rate is of 1 kSample/s and consumes 8 µW at an oversampling rate of 250 kHz. The total area occupied by the ADC is small, only 210 x 100 µm2 as shown in figure 4.

Figure 8: Table with the main features of the ADC and its layout.

Final note, In October 2018, IFAE submitted 9 proposals to “ATTRACT Call of Proposals for breakthrough ideas”.