Multi-conjugate optimization of wave imaging: From seismology to ultrasound imaging (M/F)

Job Description

Thesis Subject

In wave imaging, we seek to characterize an unknown environment by actively probing it and then recording the waves reflected by the medium. This is, for example, the principle behind ultrasound imaging and reflection seismology. Conventional imaging methods are based on two fundamental assumptions: the homogeneity of wave velocity in the medium and a simple scattering regime. In reality, these assumptions are rarely verified, whether whether in in vivo medical ultrasound or in situ seismic exploration. Spatial variations in phase velocity distort wavefronts and can induce multiple scattering or reverberations, significantly degrading the focusing process and, ultimately, the resolution and contrast of the image. These phenomena represent fundamental limitations to the imaging of complex media. To overcome these challenges, the concept of matrix imaging has recently been developed in various fields of wave physics. It involves measuring the reflection matrix associated with an array of sensors positioned opposite the medium. Once this matrix is known, a set of operations can be applied to it in order to learn how to virtually focus on any point in the medium and thus estimate a map of its reflectivity that is faithful to reality. However, the approaches developed to date, inspired by adaptive optics in astronomy, have been based on local isoplanicity [1], i.e., a spatial invariance of aberrations that does not hold for high-order aberrations induced by reverberations [2] and multiple scattering [3].

The objective of this thesis is to develop a method for mapping the distribution of wave velocities in the medium, thereby going beyond the conventional reflectivity image (ultrasound image), which is merely qualitative and whose analysis remains highly operator-dependent. Furthermore, a precise understanding of wave velocity fluctuations will allow us to obtain a reflectivity image of much higher quality in terms of contrast and resolution. Indeed, it will be possible to correct high-order aberrations, which typically degrade image quality and have remained a blind spot in adaptive focusing methods until very recently. These ambitious goals can be achieved by combining:
- physical approaches based on the spatiotemporal correlations of wavefront distortions induced by heterogeneities in wave velocity within the medium under study [4]
- computational approaches based on the optimization of a metric such as focus quality [5]
The idea, therefore, is to combine these two approaches, which are perfectly complementary (the first being robust but less detailed, the second being precise), in order to tackle various problems in wave imaging:
- reflection imaging of seismic fault zones [6] and volcanoes [7] in seismology to obtain a tomographic map of seismic wave velocity. This information is crucial because the speed of the waves allows us to characterize the mechanical state of rocks in the subsurface and monitor the movement of magma at depth.
- quantitative ultrasound imaging to improve the diagnosis of certain diseases such as fatty liver disease [8] or breast cancer [90], conditions for which sound-speed imaging would enhance their detection and monitoring.
From a scientific standpoint, the goal will be to go beyond the two approaches mentioned above in order to address the reverberation issues that affect both ultrasound imaging [2] and seismic imaging [9]. This will require optimizing more sophisticated models (modified Born series) than those used to date (beam propagation / split-step angular-spectrum method) and bridging the gap with more computational methods such as full waveform inversion developed in seismology [10].

Your Work Environment

The thesis will be conducted at the Langevin Institute.

The Langevin Institute is a joint research unit of the CNRS affiliated with ESPCI Paris - PSL University, located in the 5th arrondissement of Paris and comprising approximately 100 members, including more than 30 researchers and faculty members. The laboratory's research focuses on numerous aspects of wave physics, ranging from optics to radio frequencies and acoustics. The thesis topic falls under the Institute's “Waves, Complexity & Information” research theme of the Langevin Institute.

Constraints and risks

N/A

Compensation and benefits

Compensation

2300 € gross monthly

Annual leave and RTT

44 jours

Remote Working practice and compensation

Pratique et indemnisation du TT

Transport

Prise en charge à 75% du coût et forfait mobilité durable jusqu’à 300€

About the offer

About the CNRS

The CNRS is a major player in fundamental research on a global scale. The CNRS is the only French organization active in all scientific fields. Its unique position as a multi-specialist allows it to bring together different disciplines to address the most important challenges of the contemporary world, in connection with the actors of change.

CNRS

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