Photoacoustic Imaging

Marius Richter, winter semester 2024/25 

Photoacoustic imaging (PAI) or optoacoustic imaging is a non-destructive testing method that utilizes the photoacoustic effect to create structural and functional images of a material. The target material is excited with a pulsed laser. Light absorption of the target material locally raises the temperature. This temperature rise leads ultimately to the emission of an ultrasound (US) wave (cf. Laser-based generation of ultrasound) that is processed similar to the Ultrasonic Pulse-Echo Method. ^[1]

This non-destructive testing method has high potential in the field of medical imaging, for instance the imaging of Breast and Skin cancer, neuroimaging, microvascular imaging and molecular imaging. ^[6] In the industrial context this method is deployed for the generation of high resolution images of for example metal surfaces for crack detection or corrosion monitoring.^[8]

Fundamentals

The PAI process begins with the illumination of a tissue with a pulsed laser. This laser-light propagates through the material and gets absorbed by molecules depending on their absorption coefficient and the wavelength of the laser. The absorption of light leads to a local temperature increase, that increases the local pressure. The local increase in pressure results in the formation of an US-wave that propagates through the material. This holds true if the excitation is in thermal and stress confinement. Thermal confined means, that the heat energy does not diffuse out of the heat-affected zone during the time of excitation with the laser pulse. Stress confined means, that the stress does not propagates out of the heat-affected zone during excitation with the laser pulse. The US- waves are acquired with piezo-electric US-Transducers or an optical method of ultrasound detection at the surface of the tissue and is used to calculate an image of the structure inside of the tissue. ^[3][6]

Absorption based contrast

Because different molecules absorb light depending on the wavelength of the light PAI can achieve a contrast depending on the absorption characteristic of the Target. The Excitation Wavelength can be chosen to match a Wavelength at which certain endogenous or exogenous molecules or compounds have a high absorption coefficient with respect to the surrounding tissue.^[1]

Where endogenous refers to molecules or compounds that are produced inside of an organism and exogenous to the opposite. ^[7]

One example for an endogenous contrast often used in PAI is hemoglobin as it enables imaging of the vascular network or even the oxygen saturation in the vascular network as the extinction coefficient of oxygenated and deoxygenated hemoglobin differ from each other depending on the wavelength as shown in Figure 1. ^[1][2]

Spatial Resolution

For imaging methods like US-imaging and PAI the achievable spatial resolution is an important characteristic. The spatial resolution can be further subdivided into the lateral- and axial resolution.

The axial resolution defines the capability to distinguish different elements in the direction of propagation of the US-wave. Lateral resolution defines the capability to distinguish different elements perpendicular to the direction of propagation of the US-wave. The axial- and lateral resolution are shown in Figure 2.^[5]

Photoacoustic imaging systems

PAI can be subdivided into photoacoustic computed tomography (PACT) and photoacoustic microscopy (PAM) as shown in Figure 3. PAM can be further subdivided into optical resolution PAM (OR-PAM) and acoustic resolution PAM (AR-PAM). PACT uses wide-field optical illumination and parallel acoustic detection which enables it to achieve penetration depths of several centimeters. PAM uses a confocal optical illumination and acoustic detection which reduces the possible penetration depth but increases the possible axial-and lateral resolution.^[2] In Figure 4 a possible configuration for PACT, OR-PAM and AR-PAM is shown.

PACT

As depicted in Figure 4 PACT usually includes a wide field optical illumination and acoustic detection by a piezo-electric transducer or transducer array or an optical method of US detection. In medical applications the exciting light is typically in the near-infrared (NIR) region where it can penetrate deep into the tissue. The acoustic waves, detected at the material boundary, are spatially resolved to form a three-dimensional depiction of the inner structure of the tissue. One Basic approach for reconstruction is to calculate the distance from every transducer element to the Target using the speed of sound in the material and the time the wave travels to reach the transducer element. Having multiple transducer elements, or measurements from the same transducer at several points, makes it possible to merge those distance information to a back-projection image. This image consists of an overlay of all spherical surfaces that have the distance from the transducer Element to the Target as their radius. Other methods of image reconstruction are the “Time-reversal method”, that consists of the inverse calculation of the US-wave propagation in the material or the “filtered backprojection”.^[6]

Used scanner types in PACT with piezo-electric transducers are spherical, cylindrical and planar scanners. They comprise of an array of transducers that are placed on a spherical, cylindrical or planar surface.^[6] Single element transducers are also possible if the PA signal is collected at several different locations.^[3] Optical US detection methods have the advantage that they can be designed to have a transparent sensor head that does not interfere with the excitation laser and that they can detect PA signals in a broad frequency-band.^[6]

A relatively cheap and easy option is to use a commercially available US-Scanner and integrate fiber bundles that transport the light to the material. Drawbacks of this method are the reduced lateral resolution because of the planar detection geometry commonly used in commercial US-Scanners, difficult illumination because of the geometry of the scanner and the fact that those Scanners are usually not broadbanded enough to detect the PA signal sufficiently. In Figure 5 one possible configuration using a commercially available US-Scanner with a linear transducer array and wide-field illumination is shown. ^[6]

PAM

In PAM either the light source or the acoustic detector is focused to one spot.  This increases the possible lateral resolution, but the penetration depth is reduced when compared with PACT. Systems with a focused light source are called OR-PAM and the ones with a focused acoustic detection AR-PAM. OR-PAM offers a significantly higher lateral resolution than AR-PAM but the maximal penetration depth is reduced to 1 mm.^[6]

The image in PAM is not retrieved by reconstruction like in PACT but through raster-scanning of the focus point (either optical or acoustic focus point) across the material. In AR-PAM the excitation light is usually also weakly focused as depicted in Figure 4. This reduces the needed excitation light energy.^[6]

To increase the sensitivity of the system the focused acoustic detection and light source are often aligned coaxially.^[2]

Applications

As PAI depends on a certain transparency of the material that is used, most common use-cases are in pre-clinical applications, where the semi-transparent properties and the multitude of different chromophores (light absorbing molecules) of tissue offer perfect conditions and possibilities. Nevertheless, PAI is also investigated for industrial applications as it can achieve a high spatial resolution and offers absorption-based contrast.^[8]

In the pre-clinical context PAI is used to provide imaging of small animals. Especially for imaging of the vascular system, and blood oxygenation. In the clinical context PAI offers many possibilities like imaging of Breast and Skin cancer and Cardiovascular imaging.^[6]

Many clinical studies have been done in the field of PAI and in 2021 the first PAI system for Breast imaging got approved by the Food and Drug Administration (FDA).^[1]

Using metal as the target material PAI can be used for crack detection, corrosion monitoring and even for imaging of lithium protrusions in Lithium metal batteries. Another application is the use of PAI in the detection of defects at varying depths of silicon wafers. Further use cases are the detection of delaminated regions in ceramic coatings, and the visualization of underdrawings in paintings.^[8]

Abbreviations

Literature

[1] Park, J., Choi, S., Knieling, F. et al.: Clinical translation of photoacoustic imaging. Nat Rev Bioeng (2024). https://doi.org/10.1038/s44222-024-00240-y

[2] Rong, Q., Humayun, L., Yao, J.: Photoacoustic Microscopy. In: Xia, W. (eds) Biomedical Photoacoustics, Springer, Cham (2024), https://doi.org/10.1007/978-3-031-61411-8_1 pp. 5-15

[3] Periyasamy, V., Gisi, K., Pramanik, M.: Principles and Applications of Photoacoustic Computed Tomography. In: Xia, W. (eds) Biomedical Photoacoustics. Springer, Cham (2024). https://doi.org/10.1007/978-3-031-61411-8_2 pp. 77-83

[4] Oh, D., Kim, C., Park, B.: Photoacoustic Imaging and Applications with Reversibly Switchable Contrast Agents. In: Xia, W. (eds) Biomedical Photoacoustics. Springer, Cham (2024), https://doi.org/10.1007/978-3-031-61411-8_6 pp. 183

[5] Joseph A. Sebastian, Eric M. Strohm, Jérôme Baranger, Olivier Villemain, Michael C. Kolios, Craig A. Simmons: Assessing engineered tissues and biomaterials using ultrasound imaging: In vitro and in vivo applications, Biomaterials Volume 296, (2023), https://doi.org/10.1016/j.biomaterials.2023.122054

[6] Beard P.: Biomedical photoacoustic imaging. Interface focus, (2011),https://doi.org/10.1098/rsfs.2011.0028, pp.602-631

[7] https://www.cancer.gov/publications/dictionaries/cancer-terms/def/endogenous retrieved 18.01.2025 19:26

[8] Chen, SL., Tian, C.: Recent developments in photoacoustic imaging and sensing for nondestructive testing and evaluation. Vis. Comput. Ind. Biomed. Art 4, 6, (2021), https://doi.org/10.1186/s42492-021-00073-1

[9] Tang, Y., Qian, X., Lee, D. J., Zhou, Q., & Yao, J.: From Light to Sound: Photoacoustic and Ultrasound Imaging in Fundamental Research of Alzheimer's Disease. OBM neurobiology, 4(2), (2020), https://doi.org/10.21926/obm.neurobiol.2002056