photoacoustic imaging

File:HbAbs v3.png

{{main|Biomedical imaging}}

The optical absorption in biological tissues can be due to endogenous molecules such as hemoglobin or melanin, or exogenously delivered contrast agents. As an example, Fig. 2 shows the optical absorption spectra of oxygenated hemoglobin (HbO₂) and deoxygenated hemoglobin (Hb) in the visible and near infrared region.[http://omlc.ogi.edu/spectra/ Optical Properties Spectra] Since blood usually has orders of magnitude higher absorption than surrounding tissues, there is sufficient endogenous contrast for photoacoustic imaging to visualize blood vessels. Recent studies have shown that photoacoustic imaging can be used in vivo for tumor angiogenesis monitoring, blood oxygenation mapping, functional brain imaging, skin melanoma detection, methemoglobin measuring, etc.

Two types of photoacoustic imaging systems, photoacoustic/thermoacoustic computed tomography (also known as photoacoustic/thermoacoustic tomography, i.e., PAT/TAT) and photoacoustic microscopy (PAM), have been developed. A typical PAT system uses an unfocused ultrasound detector to acquire the photoacoustic signals, and the image is reconstructed by inversely solving the photoacoustic equations. A PAM system, on the other hand, uses a spherically focused ultrasound detector with 2D point-by-point scanning, and requires no reconstruction algorithm.

Given the heating function $H(\backslash vec\{r\},t)$, the generation and propagation of photoacoustic wave pressure $p(\backslash vec\{r\},t)$ in an acoustically homogeneous inviscid medium is governed by

:$\backslash nabla^2p(\backslash vec\{r\},t)-\backslash frac\{1\}\{v\_s^2\}\backslash frac\{\backslash partial^2\}\{\backslash partial\{t^2\}\}p(\backslash vec\{r\},t)=-\backslash frac\{\backslash beta\}\{C\_p\}\backslash frac\{\backslash partial\}\{\backslash partial\; t\}H(\backslash vec\{r\},t)\; \backslash qquad\; \backslash qquad\; \backslash quad\; \backslash quad\; (1),$

where $v\_s$ is the speed of sound in medium, $\backslash beta$ is the thermal expansion coefficient, and $C\_p$ is the specific heat capacity at constant pressure. Eq. (1) holds under thermal confinement to ensure that heat conduction is negligible during the laser pulse excitation. The thermal confinement occurs when the laser pulsewidth is much shorter than the thermal relaxation time.{{cite book |author1=L.H. Wang |author2=H.I. Wu |title=Biomedical Optics|publisher=Wiley |year=2007|isbn=978-0-471-74304-0}}

The forward solution of Eq. (1) is given by

:$\backslash left.p(\backslash vec\{r\},t)=\backslash frac\{\backslash beta\}\{4\; \backslash pi\; C\_p\}\; \backslash int\; \backslash frac\{d\; \backslash vec\{r\text{'}\}\}$

In stress confinement, which occurs when the laser pulsewidth is much shorter than the stress relaxation time, Eq. (2) can be further derived as

:$p(\backslash vec\{r\},t)=\backslash frac\{1\}\{4\; \backslash pi\; v\_s^2\}\; \backslash frac\{\backslash partial\}\{\backslash partial\; t\}\; \backslash left\; [\backslash frac\{1\}\{v\_s\; t\}\; \backslash int\; d\; \backslash vec\{r\text{'}\}\; p\_0(\backslash vec\{r\text{'}\})\; \backslash delta\; \backslash left\; (t-\backslash frac$

where $p\_0$ is the initial photoacoustic pressure.

In a PAT system, the acoustic pressure is detected by scanning an ultrasonic transducer over a surface that encloses the photoacoustic source. To reconstruct the internal source distribution, we need to solve the inverse problem of equation (3) (i.e. to obtain $p\_0$). A representative method applied for PAT reconstruction is known as the universal backprojection algorithm.{{cite journal |author=M. Xu|title=Universal back-projection algorithm for photoacoustic-computed tomography|journal=Physical Review E|volume=71|page=016706|year=2005|doi=10.1103/PhysRevE.71.016706 |pmid=15697763|issue=1|display-authors=etal|bibcode=2005PhRvE..71a6706X|url=https://authors.library.caltech.edu/67913/1/PhysRevE.71.016706.pdf|hdl=1969.1/180492|hdl-access=free}} This method is suitable for three imaging geometries: planar, spherical, and cylindrical surfaces.

The universal back projection formula is

{{center|1=$\backslash left.p\_0(\backslash vec\{r\})=\backslash int\_\{\backslash Omega\_0\}\; \backslash frac\{d\; \backslash Omega\_0\}\{\backslash Omega\_0\}\; \backslash left\; [2\; p(\backslash vec\{r\_0\},v\_s\; t)\; -\; 2\; v\_s\; t\; \backslash frac\{\backslash partial\; p(\backslash vec\{r\_0\},v\_s\; t)\}\{\backslash partial\; (v\_s\; t)\}\; \backslash right]\backslash right|\_\{t=|\backslash vec\{r\}\; -\; \backslash vec\{r\_0\}|/v\_s\},\backslash qquad\; \backslash quad(4),$}}

where $\backslash Omega\_0$ is the solid angle subtended by the entire surface $S\_0$ with respect to the reconstruction point $\backslash vec\{r\}$ inside $S\_0$, and

{{center|1=$d\; \backslash Omega\_0\; =\; \backslash frac\{d\; S\_0\}$

A simple PAT/TAT/OAT system is shown in the left part of Fig. 3.{{where|date=May 2017}} The laser beam is expanded and diffused to cover the whole region of interest. Photoacoustic waves are generated proportional to the distribution of optical absorption in the target, and are detected by a single scanned ultrasonic transducer. A TAT/OAT system is the same as PAT except that it uses a microwave excitation source instead of a laser. Although single-element transducers have been employed in these two systems, the detection scheme can be extended to use ultrasound arrays as well.

Intrinsic optical or microwave absorption contrast and diffraction-limited high spatial resolution of ultrasound make PAT and TAT promising imaging modalities for wide biomedical applications:

Soft tissues with different optical absorption properties in the brain can be clearly identified by PAT.{{cite journal |author=X. Wang|title=Non-invasive laser-induced photoacoustic tomography for structural and functional imaging of the brain in vivo|journal=Nature Biotechnology|volume=21|pages=803–806|year=2003|doi=10.1038/nbt839 |pmid=12808463 |issue=7|s2cid=2961096|display-authors=etal|url=https://authors.library.caltech.edu/67790/2/nbt839-S1.pdf}}

Since HbO₂ and Hb are the dominant absorbing compounds in biological tissues in the visible spectral range, multiple wavelength photoacoustic measurements can be used to reveal the relative concentration of these two chromophores.{{cite journal |author=X. Wang|title=Non-invasive imaging of hemoglobin concentration and oxygenation in the rat brain using high-resolution photoacoustic tomography|journal=Journal of Biomedical Optics|volume=11|pages=024015|year=2006|doi=10.1117/1.2192804 |pmid=16674205 |issue=2|display-authors=etal|bibcode=2006JBO....11b4015W|s2cid=9488754 |url=https://authors.library.caltech.edu/72156/1/024015_1_2006.pdf}} Thus, the relative total concentration of hemoglobin (HbT) and the hemoglobin oxygen saturation (sO₂) can be derived. Therefore, cerebral hemodynamic changes associated with brain function can be successfully detected with PAT.

By utilizing low scattered microwave for excitation, TAT is capable of penetrating thick (several cm) biological tissues with less than mm spatial resolution.{{cite journal |author=G. Ku|title=Thermoacoustic and photoacoustic tomography of thick biological tissues toward breast imaging|journal=Technology in Cancer Research and Treatment|volume=4|pages=559–566|year=2005 |issue=5 |pmid=16173826|display-authors=etal|doi=10.1177/153303460500400509|hdl=1969.1/181686|s2cid=15782118|hdl-access=free}} Since cancerous tissue and normal tissue have about the same responses to radio frequency radiation, TAT has limited potential in early breast cancer diagnosis.

{{main|Photoacoustic microscopy}}

The imaging depth of photoacoustic microscopy is mainly limited by the ultrasonic attenuation. The spatial (i.e. axial and lateral) resolutions depend on the ultrasonic transducer used. An ultrasonic transducer with high central frequency and broader bandwidth are chosen to obtain high axial resolution. The lateral resolution is determined by the focal diameter of the transducer. For instance, a 50 MHz ultrasonic transducer provides 15 micrometre axial and 45 micrometre lateral resolution with ~3 mm imaging depth.

Photoacoustic microscopy has multiple important applications in functional imaging: it can detect changes in oxygenated/deoxygenated hemoglobin in small vessels.{{Cite journal|last1=Yao|first1=Junjie|last2=Wang|first2=Lihong V.|date=2013-01-31|title=Photoacoustic microscopy|journal=Laser & Photonics Reviews|volume=7|issue=5|pages=758–778|doi=10.1002/lpor.201200060|pmid=24416085|pmc=3887369|bibcode=2013LPRv....7..758Y|issn=1863-8880}}{{Cite journal|last1=Zhang|first1=Hao F|last2=Maslov|first2=Konstantin|last3=Stoica|first3=George|last4=Wang|first4=Lihong V|date=2006-06-25|title=Functional photoacoustic microscopy for high-resolution and noninvasive in vivo imaging|journal=Nature Biotechnology|volume=24|issue=7|pages=848–851|doi=10.1038/nbt1220|pmid=16823374|s2cid=912509|issn=1087-0156|url=https://authors.library.caltech.edu/67910/2/nbt1220-S1.pdf}}

Photoacoustic imaging was introduced recently in the context of artwork diagnostics with emphasis on the uncovering of hidden features such as underdrawings or original sketch lines in paintings. Photoacoustic images, collected from miniature oil paintings on canvas, illuminated with a pulsed laser on their reverse side, revealed clearly the presence of pencil sketch lines coated over by several paint layers.{{Cite journal|last1=Tserevelakis|first1=George J.|last2=Vrouvaki|first2=Ilianna|last3=Siozos|first3=Panagiotis|last4=Melessanaki|first4=Krystallia|last5=Hatzigiannakis|first5=Kostas|last6=Fotakis|first6=Costas|last7=Zacharakis|first7=Giannis|date=2017-04-07|title=Photoacoustic imaging reveals hidden underdrawings in paintings|journal=Scientific Reports|language=En|volume=7|issue=1|pages=747|doi=10.1038/s41598-017-00873-7|pmid=28389668|pmc=5429688|issn=2045-2322|bibcode=2017NatSR...7..747T}}

Gold nanocages have been presented as a promising contrast agent for photoacoustic tomography due to their tunable optical properties.{{Cite journal |last=Skrabalak |first=Sara E. |last2=Chen |first2=Jingyi |last3=Sun |first3=Yugang |last4=Lu |first4=Xianmao |last5=Au |first5=Leslie |last6=Cobley |first6=Claire M. |last7=Xia |first7=Younan |date=2008-12-16 |title=Gold Nanocages: Synthesis, Properties, and Applications |url=https://pubs.acs.org/doi/10.1021/ar800018v |journal=Accounts of Chemical Research |volume=41 |issue=12 |pages=1587–1595 |doi=10.1021/ar800018v |issn=0001-4842 |pmc=2645935 |pmid=18570442}} Photoacoustic imaging has seen recent advances through the integration of deep learning principles and compressed sensing. For more information, see Deep learning in photoacoustic imaging.

• Multispectral optoacoustic tomography
• Photoacoustic microscopy
• Deep learning in photoacoustic imaging
• Photoacoustic effect

{{reflist}}

• [https://archive.today/20130105112043/http://www3.interscience.wiley.com/journal/116325199/abstract Recent advances in application of acoustic, acousto-optic and photoacoustic methods in biology and medicine]

{{DEFAULTSORT:Photoacoustic Imaging In Biomedicine}}

Category:Medical imaging