Photoacoustic or optoacoustic imaging is arguably the most exciting biomedical imaging technique of the decade. The method has captured the attention and imagination of applied physicists, applied mathematicians, biomedical engineers, and clinical imaging specialists, often with an affinity for biomedical optics and/or ultrasound, the two fields brought together in the method.
The technique typically uses short-pulsed electromagnetic radiation as probing energy, while detecting ultrasound generated by photon absorption and thermoelastic expansion. When short pulses of light are used, the thermal expansion results in a mechanical wave being generated originating at the locations where optical absorption took place. The mechanical wave has frequency components in the ultrasound regime, and can be detected at the boundary of the tissue using ultrasound detectors. From this point on the ultrasound source positions can be located using methods well known in ultrasound imaging.
Figure 1: Pulsed photoacoustic effect. (a) Scattering sample containing optical absorbers is illuminated with short laser pulses. (b) Absorption of the light energy leads to thermoelastic expansion and subsequent generation of propagating elastic waves that are recorded using detector/s placed outside the sample. (From Manohar & Razansky (2016) Adv Optics Photon., Vol. 8, pp. 586-617)
This is in contrast to purely optical imaging, where multiple scattering randomizes photon direction, thus deteriorating resolution and contrast when these photons are detected. The particular strength of photoacoustics in relation to imaging applications arises from the fact that the detected energy is not light, but is ultrasound, which generally undergoes considerably less scattering and attenuation in tissue compared with light. The consequence is that the spatial distribution of optical absorption can be ascertained deep in tissue with better resolution and greater penetration depths than achievable with other optical imaging methods.
To summarize: The mechanism of pulsed photoacoustic signal generation consists of the following steps:
1) light is selectively absorbed in higher absorbing regions, when the investigated volume is exposed to pulsed laser radiation;
2) fast non-radiative relaxation of excited states takes place with thermalization of absorbed optical energy;
3) the resulting local thermal expansion produces pressure transients.
The stress waves have frequencies in the ultrasound (US) range and propagate to the tissue boundary with low scattering and finite velocity. Using a plurality of US detectors, the signals can be detected and the origin of the photoacoustic (PA) sources localized. Thus, while detection of light, would have resulted in washed-out detail due to scattering, detection of US provides high spatial resolution. The PA method combines the rich spectroscopic contrast arising from the use of light as excitation, with the high resolutions arising from low-scattered US propagation and detection.
The primary contrast mechanism explored in biomedical photoacoustics, namely, optical absorption, possesses information regarding the presence of tissue components such as hemoglobin, oxy-hemoglobin, melanin, bilirubin, lipids, and water. Strong optical absorption by the first two biochromes allows the visualization of blood vessels, maintaining sub-millimeter resolutions at depths of 5 cm and beyond within highly scattering living tissues. Further, these and other biochromophores have specific spectral signatures that allow them to be distinguished from each other within an integrated absorption signal. Their relative presence carries rich information about function and/or pathological status of tissue being examined, since this presence is carefully choreographed in healthy tissue.
Photoacoustic breast imaging
In the group we have pioneered work in what we call PAMmography (Photoacoustic Mammography) is being done in collaboration with radiologists at the Medisch Spectrum Twente (MST) in Enschede and Oldenzaal, and at the Ziekenhuisgroep Twente (ZGT) in Hengelo and Almelo.
The work has seen the development of a 2nd Generation instrument the Photoacoustic Mammoscope 2 (Figure 2) developed in collaboration with the UT start-up, PA Imaging BV.
Figure 2: The PAM 2 system with (a) the bed on which a woman lies prone, (b) a foot rest, and (c) the breast aperture with (d) a schematic of the shape and position of the imaging tank below the bed, (e) the power supply and cooling unit, (f) laser head (behind the panel), (g) DAQ unit, and (h) step stool. (From Schoustra et al (2019) J. Biomed Opt., 24(12), 121909)
Figure 3 Color-coded images of the left breast of a healthy volunteer, acquired using PAM2, illuminated with 755 nm in three orientations (a) coronal, (c) sagittal, and (d) transverse. The zoomed subsection (b) shows the nipple features. (From Schoustra et al (2019) J. Biomed Opt., 24(12), 121909)
The PAMMOTH project and 3rd Generation Imager
A highly sophisticated 3rd Generation Photoacoustic Mammoscope, has been developed with the EU H2020 consortium project PAMMOTH, as discussed in this video.
Figure 4 gives information regarding the partners.
The current status is that the instrument has been installed in the Medisch Spectrum Twente. The patient studies which are conducted by the University of Twente with the Medisch Spectrum Twente, were to commence in 2020 have been delayed due to the restrictions to manage the COVID-19 pandemic. Early in 2021, we expect to commence the measurements.
We need highly creative, industrious and passionate students to help us move the field further. The backrounds required are in Biomedical Engineering, Applied Physics, Technical Medicine, and Computer Science. If you are interested in making an important contribution to this area, contact Prof. Dr.Srirang Manohar (firstname.lastname@example.org)
Spatially Compounded Plane Wave Imaging using a Laser-Induced Ultrasound Source
D Thompson, D Gasteau, S Manohar
Photoacoustics, 100154 (2020)
Semi-anthropomorphic photoacoustic breast phantom
M Dantuma, R van Dommelen, S Manohar
Biomedical optics express 10 (11), 59211 (2019)
A semi-anthropomorphic breast phantom with tunable blood oxygenation levels for use in quantitative photoacoustics
M Dantuma, JO Julia, S Manohar
TENCON 2019-2019 IEEE Region 10 Conference (TENCON), 114-117( 2019)
Photoacoustic Imaging Assisted Radiofrequency Ablation: Illumination Strategies and Prospects
KJ Francis, E Rascevska, S Manohar
TENCON 2019-2019 IEEE Region 10 Conference (TENCON), 118-122 (2019)
Twente Photoacoustic Mammoscope 2: system overview and three-dimensional vascular network images in healthy breasts
SM Schoustra, D Piras, R Huijink, TJPM Op't Root, L Alink, WM Kobold, ...
Journal of biomedical optics 24 (12), 121909 (2019)
Photoacoustic imaging in percutaneous radiofrequency ablation: device guidance and ablation visualization
KJ Francis, S Manohar
Physics in Medicine & Biology 64 (18), 184001 (2019)
Laser-induced ultrasound transmitters for 3D photoacoustic and ultrasound tomography
D Gasteau, D Thompson, S Manohar
Opto-Acoustic Methods and Applications in Biophotonics IV 11077, 1107716 (2019)
Current and future trends in photoacoustic breast imaging
S Manohar, M Dantuma
Photoacoustics 7 (2019)
Photoacoustic assisted device guidance and thermal lesion imaging for radiofrequency ablation
KJ Francis, S Manohar
European Conference on Biomedical Optics, 11077_40 (2019)
Annular illumination photoacoustic probe for needle guidance in medical interventions
E Rascevska, KJ Francis, S Manohar
European Conference on Biomedical Optics, 11077_20 (2019)
Monitoring radiofrequency ablation of biological tissue using broadband time-resolved diffuse optical spectroscopy
P Lanka, KJ Francis, H Kruit, SKV Sekar, A Farina, R Cubeddu, ...
European Conference on Biomedical Optics, 11074_94 (2019)
A Partially-Learned Algorithm for Joint Photo-acoustic Reconstruction and Segmentation
YE Boink, S Manohar, C Brune
IEEE transactions on medical imaging 39 (1), 129-139 (2019)
Robustness of a partially learned photoacoustic reconstruction algorithm
YE Boink, C Brune, S Manohar
Photons Plus Ultrasound: Imaging and Sensing 2019 10878, 108781D (2019)
Plane wave and synthetic transmit aperture echography using laser-induced ultrasound (Conference Presentation)
D Thompson, L Demi, E Kruit, D Gasteau, M Olsman, S Manohar
Photons Plus Ultrasound: Imaging and Sensing 2019 10878, 108781A (2019)
International Photoacoustic Standardisation Consortium (IPASC): overview
S Bohndiek, J Brunker, J Gröhl, L Hacker, J Joseph, WC Vogt, P Armanetti, ...
Photons Plus Ultrasound: Imaging and Sensing 2019 10878, 108781N (2019)
Breast tumor appearances in photoacoustic tomography from fine 3D optical-acoustic simulations (Conference Presentation)
M Dantuma, F Lucka, J Jaros, B Teeby, B Cox, S Manohar
Photons Plus Ultrasound: Imaging and Sensing 2019 10878, 108781I (2019)
A 3D semi-anthropomorphic photoacoustic breast phantom
M Dantuma, RC Van Dommelen, S Manohar
Photons Plus Ultrasound: Imaging and Sensing 2019 10878, 108781P (2019)
The Twente Photoacoustic Mammoscope 2: 3D vascular network visualization
SM Schoustra, R Huijink, L Alink, TJPM op't Root, D Sprünken, D Piras, ...
Photons Plus Ultrasound: Imaging and Sensing 2019 10878, 1087813 3
Sensitivity of a partially learned model‐based reconstruction algorithm
YE Boink, SA Van Gils, S Manohar, C Brune
PAMM 18 (1), e201800222 1
A framework for directional and higher-order reconstruction in photoacoustic tomography
YE Boink, MJ Lagerwerf, W Steenbergen, SA van Gils, S Manohar, ...
Physics in Medicine & Biology 63 (4), 045018 13