# Simulation of Light Tissue Interaction

- Institute: HOT - Hannoversches Zentrum für optische Technologien, Hannover
- Principle Investigators: Dr. Merve Wollweber, Prof. Dr. Bernhard Roth
- Researcher: Dr. Oliver Melchert

In subproject 1 the focus is on various numerical aspects of optoacoustics in the context of computational biophotonics. Our aim is to perform numerical experiments and to devise measurement protocols that facilitate a better understanding of the features of optoacoustic (OA) signals that result from melanin enriched absorbing structures within tissue.

In summary, optoacoustics is a two-part phenomenon consisting of

**Part I:** *Optical absorption* of laser beams inducing photothermal heating of tissue

**Part II:** *Acoustic emission* of ultrasound waves due to thermoelastic expansion and stress field relaxation

Whereas the optical absorption is assumed to occur instantaneously, the acoustic propagation of sound waves is a comparatively slow process that occurs on a microsecond timescale. In this regard, note that typical propagation distances are on the order of centimeters and that the propagation of acoustic waves in soft tissue (i.e. elastic solids) occurs with a speed of 1400-1600 meters per second. Hence, in subproject 1 the challenge is to combine the absorption of laser light by tissue and the acoustic propagation as a multitimescale problem. Since both underlying phenomena occur on vastly different timescales we can adopt a divide-and-conquer strategy, i.e. decouple the optical absorption problem from the acoustic propagation problem and solve both subproblems independently. Our research activity is centered around three main topics:

**Topic 1: The direct OA problem**

**Topic 2: The inverse OA problem**

# Further activities

**Student project: **We host an ongoing student-project, focused on the solution of the radiative transfer equation (RTE) for benchmarking and verification testing of Monte Carlo RTE solver.

** Bachelor thesis:** We host an ongoing bachelor thesis, focused on the numerical prediction of a piezoelectric transducer response to laser generated stress waves in the acoustic near field.

# Developed Software

**LightTransportMC**- A simple Monte Carlo model of photon transport and fluence rate estimation in semi-infinite homogeneous absorbing and scattering media**SONOS**- A fast Poisson integral solver for layered homogeneous media more...**INVERT**- Optoacoustic inversion via Volterra kernel reconstruction more...**PCPI**- A python module for optoacoustic signal generation via Polar Convolution and

Poisson Integral evaluation**dFBT-FJ**- Efficient polar convolution based on the discrete Fourier-Bessel transform

for application in computational biophotonics more...**LEPM-1DFD**- One-dimensional finite-difference code for piecewise homogeneous,

linear elastic and piezoelectric materials- in collaboration with TP2 :
**BPPA-2DFD**- Algorithms and benchmark instances used for beam propagation in the

paraxial approximation via 2D finite-difference time domain methods - in collaboration with TP4:
**FMAS**- Forward model for the analytic signal in ultrashort pulse propagation

# Publications

## Journals

**M. Suar, M. Rahlves, E. Reithmeier, B. Roth (2018):**
Numerical Investigations on Polymer-Based Bent Couplers, JOSA B 35(8), 1896-1904

DOI: 10.1364/JOSAB.35.001896

**O. Melchert, E. Blumenröther, M. Wollweber, and B. Roth (2018):**
Numerical prediction and measurement of optoacoustic signals generated in PVA-H tissue phantoms, Eur. Phys. J. D 72 (2018) 19

DOI: 10.1140/epjd/e2017-80578-6

**O. Melchert, M. Wollweber, and B. Roth (2018):**
An efficient procedure for custom beam-profile convolution in polar coordinates: testing, benchmarking and application to biophotonics, Biomedical Physics & Engineering Express

DOI: 10.1088/2057-1976/aaa51a

**O. Melchert, M. Wollweber, B. Roth (2018):**
Optoacoustic inversion via convolution kernel reconstruction in the paraxial approximation and beyond, Photoacoustics (accepted for publication)

**J. Stritzel, O. Melchert, M. Wollweber, and B. Roth (2017):**
Effective one-dimensional approach to the source reconstruction problem of three-dimensional inverse optoacoustics, Phys. Rev. E 96 (2017) 033308

DOI: 10.1103/PhysRevE.96.033308

**E. Blumenröther, O. Melchert, M. Wollweber, B. Roth (2016):**
Detection, numerical simulation and approximate inversion of optoacoustic signals generated in multi-layered PVA hydrogel based tissue phantoms, Photoacoustics 4, 125

DOI: 10.1016/j.pacs.2016.10.002

## Proceedings

**M. Suar, M. Rahlves, E. Reithmeier, B. Roth (2018):**
Simulation of straight and bent self-written waveguides in photopolymer
mixture using phenomenological and diffusion models, SPIE Optical Systems Design

DOI: 10.1117/12.2312507

**O. Melchert,E. Blumenröther,M. Wollweber,B. Roth (2018):**
Numerical prediction and approximate inversion of optoacoustic signals in tissue phantoms, SPIE Optical Systems Design

DOI: 10.1117/12.2314840

**O. Melchert, U. Morgner, B. Roth, I. Babushkin, A. Demircan (2018):**
Accurate propagation of ultrashort pulses in nonlinear waveguides using propagation models for the analytic signal, SPIE Optical Systems Design

DOI: 10.1117/12.2313255

**O. Melchert, E. Blumenröther, M. Rahlves, M. Wollweber, B. Roth (2016):**
Irradiation source profile dependence of optoacoustic signals generated in layered absorbers, DGaO Proceedings P040, ISSN: 1614-8436

**O. Melchert, J. Strizel, M. Rahlves, M. Wollweber, B. Roth (2016):**
Diffraction rewind: inversion of signals to initial stress profiles
using the 3D optoacoustic Volterra integral, DGaO Proceedings C004, ISSN: 1614-8436

## Books

**O. Melchert (2018):**
Networks – A brief introduction using a paradigmatic combinatorial optimization problem, Modern Computational Science 18 - Energy of the future (Lecture Notes from the 6th International Summer School, Oldenburg, September 3 - 14, 2018)

ISBN: 9783814223704