Photoacoustic imaging or optoacoustic imaging is a biomedical imaging modality based on the
photoacoustic effect
The photoacoustic effect or optoacoustic effect is the formation of sound waves following light absorption in a material sample. In order to obtain this effect the light intensity must vary, either periodically (''modulated light'') or as a single ...
. Non-ionizing
laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The fi ...
pulses are delivered into biological tissues and part of the energy will be absorbed and converted into heat, leading to transient thermoelastic expansion and thus wideband (i.e. MHz)
ultrasonic
Ultrasound is sound waves with frequencies higher than the upper audible limit of human hearing. Ultrasound is not different from "normal" (audible) sound in its physical properties, except that humans cannot hear it. This limit varies fr ...
emission. The generated ultrasonic waves are detected by
ultrasonic transducer
Ultrasonic transducers and ultrasonic sensors are devices that generate or sense ultrasound energy. They can be divided into three broad categories: transmitters, receivers and transceivers. Transmitters convert electrical signals into ultrasoun ...
s and then analyzed to produce images. It is known that optical absorption is closely associated with physiological properties, such as
hemoglobin
Hemoglobin (haemoglobin BrE) (from the Greek word αἷμα, ''haîma'' 'blood' + Latin ''globus'' 'ball, sphere' + ''-in'') (), abbreviated Hb or Hgb, is the iron-containing oxygen-transport metalloprotein present in red blood cells (erythrocyt ...
concentration and
oxygen saturation
Oxygen saturation (symbol SO2) is a relative measure of the concentration of oxygen that is dissolved or carried in a given medium as a proportion of the maximal concentration that can be dissolved in that medium at the given temperature. It ca ...
.
As a result, the magnitude of the ultrasonic emission (i.e. photoacoustic signal), which is proportional to the local energy deposition, reveals physiologically specific optical absorption contrast. 2D or 3D images of the targeted areas can then be formed.
Biomedical imaging
![HbAbs v3](https://upload.wikimedia.org/wikipedia/commons/c/c0/HbAbs_v3.png)
The optical absorption in biological tissues can be due to
endogenous molecules such as hemoglobin or
melanin
Melanin (; from el, μέλας, melas, black, dark) is a broad term for a group of natural pigments found in most organisms. Eumelanin is produced through a multistage chemical process known as melanogenesis, where the oxidation of the amino ...
, or exogenously delivered contrast agents. As an example, Fig. 2 shows the optical absorption spectra of
oxygenated hemoglobin (HbO
2) and
deoxygenated hemoglobin
Hemoglobin (haemoglobin BrE) (from the Greek word αἷμα, ''haîma'' 'blood' + Latin ''globus'' 'ball, sphere' + ''-in'') (), abbreviated Hb or Hgb, is the iron-containing oxygen-transport metalloprotein present in red blood cells (erythrocyt ...
(Hb) in the visible and near infrared region. 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
Studies that are ''in vivo'' (Latin for "within the living"; often not italicized in English) are those in which the effects of various biological entities are tested on whole, living organisms or cells, usually animals, including humans, and ...
'' for tumor
angiogenesis monitoring,
blood oxygenation
Pulse oximetry is a noninvasive method for monitoring a person's oxygen saturation. Peripheral oxygen saturation (SpO2) readings are typically within 2% accuracy (within 4% accuracy in 95% of cases) of the more accurate (and invasive) reading ...
mapping, functional brain imaging, skin
melanoma detection,
methemoglobin
Methemoglobin (British: methaemoglobin) (pronounced "met-hemoglobin") is a hemoglobin ''in the form of metalloprotein'', in which the iron in the heme group is in the Fe3+ (ferric) state, not the Fe2+ (ferrous) of normal hemoglobin. Sometimes, it i ...
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.
]
Photoacoustic computed tomography
General equation
Given the heating function , the generation and propagation of photoacoustic wave pressure in an acoustically homogeneous inviscid medium is governed by
:
where is the speed of sound in medium, is the thermal expansion coefficient, and 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.
The forward solution of Eq. (1) is given by
:
In stress confinement, which occurs when the laser pulsewidth is much shorter than the stress relaxation time, Eq. (2) can be further derived as
:
where is the initial photoacoustic pressure.
Universal reconstruction algorithm
In a PAT system, the acoustic pressure
Sound pressure or acoustic pressure is the local pressure deviation from the ambient (average or equilibrium) atmospheric pressure, caused by a sound wave. In air, sound pressure can be measured using a microphone, and in water with a hydrophone ...
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 ). A representative method applied for PAT reconstruction is known as the universal backprojection algorithm. This method is suitable for three imaging geometries: planar, spherical, and cylindrical surfaces.
The universal back projection formula is
where is the solid angle subtended by the entire surface with respect to the reconstruction point inside , and
Simple system
A simple PAT/TAT/OAT system is shown in the left part of Fig. 3. 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.
Biomedical applications
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:
Brain lesion detection
Soft tissues with different optical absorption properties in the brain can be clearly identified by PAT.
Hemodynamics monitoring
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. Thus, the relative total concentration of hemoglobin (HbT) and the hemoglobin oxygen saturation
Oxygen saturation (symbol SO2) is a relative measure of the concentration of oxygen that is dissolved or carried in a given medium as a proportion of the maximal concentration that can be dissolved in that medium at the given temperature. It ca ...
(sO2) can be derived. Therefore, cerebral hemodynamic changes associated with brain function can be successfully detected with PAT.
Breast cancer diagnosis
By utilizing low scattered microwave for excitation, TAT is capable of penetrating thick (several cm) biological tissues with less than mm spatial resolution. Since cancerous tissue and normal tissue have about the same responses to radio frequency radiation, TAT has limited potential in early breast cancer diagnosis.
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.
Other applications
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
Painting is the practice of applying paint, pigment, color or other medium to a solid surface (called the "matrix" or "support"). The medium is commonly applied to the base with a brush, but other implements, such as knives, sponges, and ai ...
. 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.
Advances in photoacoustic imaging
Photoacoustic imaging has seen recent advances through the integration of deep learning principles and compressed sensing. For more information on the deep learning applications in photoacoustic imaging, see Deep learning in photoacoustic imaging.
See also
* Multispectral optoacoustic tomography
* Photoacoustic microscopy
* Deep learning in photoacoustic imaging
*Photoacoustic effect
The photoacoustic effect or optoacoustic effect is the formation of sound waves following light absorption in a material sample. In order to obtain this effect the light intensity must vary, either periodically (''modulated light'') or as a single ...
References
External links
Recent advances in application of acoustic, acousto-optic and photoacoustic methods in biology and medicine
{{DEFAULTSORT:Photoacoustic Imaging In Biomedicine
Medical imaging