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Dipartimento di Fisica - Politecnico di Milano

Design of next generation time-domain diffuse optical instruments

In the last decade, light has been considered a powerful tool to non-invasively investigate tissues (e.g. optical mammography, functional brain imaging, muscle oxygenation, etc.). The time-domain (TD) technique relies on the injection of short pulses of light and on the collection of the temporal distribution of the re-emitted photons. The use of TD technique has the advantages of intrinsically encode in time the mean depth reached by photons and to allow the retrieval of both absorption and reduced scattering coefficients (connected to tissue constituents’ concentration and microstructure, respectively).
In the last years, new solid-state detectors with large photons collection area (Silicon Photomultipliers, SiPM) have been proposed for diffuse optical measurements. With respect to other “classical” single-photon detectors (e.g. photomultipliers tube) they are rugged, inexpensive and can be hosted in a single chip, thus possibly realizing a detector that can be directly used in contact with the tissue under investigation, maximizing in this way the light harvesting capability.
Recently, it has been demonstrated that, to reach the ultimate performance of time-domain diffuse optics, there is the need for a dense and large grid of measurements points that can be enabled with fast transition (<500 ps). In this way, time-domain technique will be able to provide information about structures/organs down to a depth of 5-6 cm, thus allowing to optically probe the lung, heart or even the brain cortex in utero.
However, at the moment the area of suitable SiPMs is limited to 1 mm2 and they can not be enabled in ultra-fast regime, thus preventing the so called “short source-detector distance” approach.

The GAP lab mission is the development of new technologies and strategies to build the next generation of diffuse optics instruments. In this contest, 2 theses are proposed (their schematic description can also be downloaded by clicking on the link at the bottom of the page).


To boost the performances of TD-instruments, approaching the ultimate limits of the technique, a large area detector is needed. For this reason, we are currently studying the feasibility of a 9 mm2 area SiPM detector, featuring an increase of nearly a factor of 10 in light harvesting. This allows to increase the number of photons detected, thus permitting to enhance the collection of the few photons arriving from deeper layers of the tissue.
However, with classical approach, the signal should be limited both for practical (i.e. maximum count rate sustained by the timing electronics) and theoretical reasons (i.e. single-photon statistics), thus setting a limit to the achievable performances. The former limitation has been recently overcome thanks to the new timing board that can sustain up to 40 Mcps.
With this work we want to explore the possibility to work at high count-rate, overcoming the single-photon statistics. Additionally, the use of high count-rate allows to decrease the integration time (< 50 ms), thus allowing the possibility to see fast dynamics. For example, we may be able to see a single event during brain activation (e.g. single movement of the finger). Moreover, due to the larger light harvesting, we do expect an improvement in the signal to noise ratio thus allowing to probe also deep organs (such as lung or heart), opening a new way to non-invasively image inside the body.
Additionally, due to the low cost of the detector, many 9 mm2 SiPM detectors can be arranged and used to have a fast real-time tomography system, capable of detecting fast dynamics and give 3D reconstructions of also deep structures.
On the experimental side, the student is expected to actively participate in the building and testing of the new instrument with the scope to understand improvements and limitations with respect to the state-of-the-art systems. Once this evaluation is done, the instrument will be tested for both phantom and in-vivo  measurements, possibly also in a tomographic configuration.



As already stated, diffuse optics is expected to reach its ultimate performance when a dense grid of sources and large area fast-gated detectors will be available. To make a step toward the new generation of diffuse optics system is under development a so called “smart optode” which will host in a small case (1 x 1 x 4 cm3) all the components (multi-wavelength lasers, large area fast-gated detector and electronics to reconstruction the time-of-flight distribution).

The design of this smart optode is one of the main achievement of the SOLUS project (an European project, see for details), whose aim is to develop a new multimodal imaging system (featuring diffuse optics, ultrasound and share waves acquisitions) which can classify breast lesions detected by mammography screening in a non-invasive manner, thus significantly improving the ability to differentiate between benign and malignant tumors.
Within this framework, the proposed thesis work aims firstly to explore the performance of single miniaturized elements of the smart optode (lasers, detectors and timing electronics) and to assess the improvements with the state-of-the-art bulky components.
After this phase (which is supposed to last about 2 months), the complete optode will be ready to test. A first series of measurements on phantom and ex-vivo will be carried on to understand the potentialities of this device (e.g. maximum penetration depth, capability to properly recover the optical properties of the sample, etc). Then, the smart optode will be tested in several applications to explore the breakthrough in the diffuse optics provided by this technology.
The student is expected to take part to the experimental activity, performing measurements, analyzing data achieved and supporting in the interpretation of the results. Measurement sessions in collaboration with other partner of the project (industries, university and hospital) are also expected, following the requirements of the SOLUS project.



The proposed theses are meant for engineering students (Physics, Biomedical, Electronics..) who are interested in the applications of photonics. The main requirements are: motivation, commitment and group work.
Students are supposed to have basic knowledge of physics, math and optics. During the thesis work, students are expected to work with software for measurements automation (e.g. LabWindows), numerical simulations and data analysis (Matlab), as well as to write reports (e.g. Office and Origin)

Duration, venue:
Theses are supposed to last about 8-10 months with full time work. It is suggested not to have more than 2 or 3 exams to be taken to better focus on the experimental work.
The activity will be carried on in the Physics Department labs (Leonardo)

Student’s role and skill acquired:
In the first period, the student will be trained so as to acquire the skills (both on the experimental and on the software side) needed for carrying on the work. After training, the student will contribute to the lab activity more and more independently, following a road map planned with lab’s responsibles.
The proposed work is experimental and will follow the project carried on in the GAP lab, thus introducing the students to the research activity.
At the end of the thesis period, the student will have acquired skills about the development and exploitation of new prototypes of photonics instruments. He/she will have experience in the assessment of the single components and/or instrument performances, as well as their validation in operative conditions (both on phantom and on in-vivo).
Additionally, due to the collaboration with other partners (both from academy and industry), the student will experience and learn team work and how to manage collaborations with other institutions.
Both soft and hard skills will be useful for a possible PhD.

Positions available: 2-3

• Dr. Laura Di Sieno (tel. +39 02 2399 6597; email:
• Prof. Alberto Dalla Mora (tel. +39 02 2399 6108; email:

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