Optoacoustic imaging
Encyclopedia
Optoacoustic imaging is an imaging technology
based on the photoacoustic effect, and can be used for obtaining images of structures in turbid environments. The optoacoustic technique combines the accuracy of spectroscopy
with the depth resolution of ultrasound
.
is believed to be the photophone
, a communication device invented by Bell (1880). Bell observed the emanation of audible sounds from a transparent tube containing material in suspension
, when a modulated light source is focused on the tube. Modulation of the light impinging on an absorbing substance will produce a similar modulation in temperature
via the photothermal effect.
pulse
irradiates an absorbing medium there is local absorption →local heating → local expansion. This local expansion leads to ultrasonic pressure waves that travel through the medium at the speed of sound
, and can be recorded using high frequency pressure sensors (such as piezoelectric sensor
s or optical sensors). The slow speed of sound in tissue (~1500 m/s) in comparison to the speed of light allows for the time resolved detection of these pressure waves and the determination of depth from where these pressure waves originated. By using an array of sensors the temporal delay of these incoming pressure wavefront
s can be combined into an ultrasound image.
detection equipment
, as well as equipment used for measuring blood oxygenation levels. In both cases, the change in the optical properties of blood in respect to oxygen saturation
and the strong optical contrast between hemoglobin
and surrounding tissue is utilized.
Imaging technology
Imaging technology is the application of materials and methods to create, preserve or duplicate images. This can mean several things:*Computer graphics*Microfilm and Micrographics*Visual arts**Etching**Drawing and Technical drawing**Cinema**Painting...
based on the photoacoustic effect, and can be used for obtaining images of structures in turbid environments. The optoacoustic technique combines the accuracy of spectroscopy
Spectroscopy
Spectroscopy is the study of the interaction between matter and radiated energy. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, e.g., by a prism. Later the concept was expanded greatly to comprise any interaction with radiative...
with the depth resolution of ultrasound
Ultrasound
Ultrasound is cyclic sound pressure with a frequency greater than the upper limit of human hearing. Ultrasound is thus not separated from "normal" sound based on differences in physical properties, only the fact that humans cannot hear it. Although this limit varies from person to person, it is...
.
Background
The oldest technical application of the photothermal effectPhotothermal effect
Photothermal effect is a phenomenon associated with electromagnetic radiation. It is produced by the photoexcitation of material, resulting in the production of thermal energy ....
is believed to be the photophone
Photophone
The photophone, also known as a radiophone, was invented jointly by Alexander Graham Bell and his then-assistant Charles Sumner Tainter on February 19, 1880, at Bell's 1325 'L' Street laboratory in Washington, D.C...
, a communication device invented by Bell (1880). Bell observed the emanation of audible sounds from a transparent tube containing material in suspension
Suspension (vehicle)
Suspension is the term given to the system of springs, shock absorbers and linkages that connects a vehicle to its wheels. Suspension systems serve a dual purpose — contributing to the car's roadholding/handling and braking for good active safety and driving pleasure, and keeping vehicle occupants...
, when a modulated light source is focused on the tube. Modulation of the light impinging on an absorbing substance will produce a similar modulation in temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
via the photothermal effect.
Optoacoustic wave generation
When a short laserLaser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of photons. The term "laser" originated as an acronym for Light Amplification by Stimulated Emission of Radiation...
pulse
Pulse
In medicine, one's pulse represents the tactile arterial palpation of the heartbeat by trained fingertips. The pulse may be palpated in any place that allows an artery to be compressed against a bone, such as at the neck , at the wrist , behind the knee , on the inside of the elbow , and near the...
irradiates an absorbing medium there is local absorption →local heating → local expansion. This local expansion leads to ultrasonic pressure waves that travel through the medium at the speed of sound
Speed of sound
The speed of sound is the distance travelled during a unit of time by a sound wave propagating through an elastic medium. In dry air at , the speed of sound is . This is , or about one kilometer in three seconds or approximately one mile in five seconds....
, and can be recorded using high frequency pressure sensors (such as piezoelectric sensor
Piezoelectric sensor
A piezoelectric sensor is a device that uses the piezoelectric effect to measure pressure, acceleration, strain or force by converting them to an electrical charge.-Applications:...
s or optical sensors). The slow speed of sound in tissue (~1500 m/s) in comparison to the speed of light allows for the time resolved detection of these pressure waves and the determination of depth from where these pressure waves originated. By using an array of sensors the temporal delay of these incoming pressure wavefront
Wavefront
In physics, a wavefront is the locus of points having the same phase. Since infrared, optical, x-ray and gamma-ray frequencies are so high, the temporal component of electromagnetic waves is usually ignored at these wavelengths, and it is only the phase of the spatial oscillation that is described...
s can be combined into an ultrasound image.
Proposed applications
Although the technology is still in its infancy, optoacoustic imaging is being employed in the development of various devices. Such devices include breast cancerBreast cancer
Breast cancer is cancer originating from breast tissue, most commonly from the inner lining of milk ducts or the lobules that supply the ducts with milk. Cancers originating from ducts are known as ductal carcinomas; those originating from lobules are known as lobular carcinomas...
detection equipment
Photodiode
A photodiode is a type of photodetector capable of converting light into either current or voltage, depending upon the mode of operation.The common, traditional solar cell used to generateelectric solar power is a large area photodiode....
, as well as equipment used for measuring blood oxygenation levels. In both cases, the change in the optical properties of blood in respect to oxygen saturation
Oxygen saturation
Oxygen saturation or dissolved oxygen is a relative measure of the amount of oxygen that is dissolved or carried in a given medium. It can be measured with a dissolved oxygen probe such as an oxygen sensor or an optode in liquid media, usually water.It has particular significance in medicine and...
and the strong optical contrast between hemoglobin
Hemoglobin
Hemoglobin is the iron-containing oxygen-transport metalloprotein in the red blood cells of all vertebrates, with the exception of the fish family Channichthyidae, as well as the tissues of some invertebrates...
and surrounding tissue is utilized.
External links
See also
- Reconstruction-based photoacoustic imaging in biomedicine
- photoacoustic tomographyPhotoacoustic tomographyPhotoacoustic tomography , or photoacoustic computed tomography , is a materials analysis technique based on the reconstruction of an internal photoacoustic source distribution from measurements acquired by scanning ultrasound detectors over a surface that encloses the source under...