What have we done with photoacoustic tomography?
Multispectral optoacoustic tomography
Multi-spectral optoacoustic tomography (MSOT), also known as functional photoacoustic tomography (fPAT), is an imaging technology that generates high-resolution optical images in scattering media, including biological tissues. MSOT illuminates tissue with light of transient energy, typically light pulses lasting 1-100 nanoseconds. The tissue absorbs the light pulses, and as a result undergoes thermo-elastic expansion, a phenomenon known as the optoacoustic or photoacoustic effect. This expansion gives rise to ultrasound waves (photoechoes) that are detected and formed into an image. Image formation can be done by means of hardware (e.g. acoustic focusing or optical focusing) or computed tomography (mathematical image formation). Unlike other types of optoacoustic imaging, MSOT involves illuminating the sample with multiple wavelengths, allowing it to detect ultrasound waves emitted by different photoabsorbing molecules in the tissue, whether endogenous (oxygenated and deoxygenated hemoglobin, melanin) or exogenous (imaging probes, nanoparticles). Computational techniques such as spectral unmixing deconvolute the ultrasound waves emitted by these different absorbers, allowing each emitter to be visualized separately in the target tissue. In this way, MSOT can allow visualization of hemoglobin concentration and tissue oxygenation or hypoxia. Unlike other optical imaging methods, MSOT is unaffected by photon scattering and thus can provide high-resolution optical images deep inside biological tissues.
Super-resolution photoacoustic imaging
The key idea is that the sample is pumped with high levels of energy such that all of its electrons are saturated to the excitated state. In this saturated state, the additional energy will be utilized in different ways such as nonlinear thermal expansion. From these pressure waves, images corresponding to the absorption of different molecules can be reconstructed. In Figure 2a and 2b, two 100 nm gold nanoparticles are resolved with super resolution enhanced photoacoustics and verified by atomic force microscopy, a different modality that can break the diffraction limit resolution since it does not rely on light.
Photoacoustic microscopy
Following the initial pressure rise, a photoacoustic wave propagates at the speed of sound within the medium and can be detected with an ultrasound transducer.
Photoacoustic imaging
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.
Photoacoustic spectroscopy
Photoacoustic spectroscopy also has many military applications. One such application is the detection toxic chemical agents. The sensitivity of photoacoustic spectroscopy makes it an ideal analysis technique for detecting trace chemicals associated with chemical attacks.
Photoacoustic microscopy
Visible light absorbers (λ = 400 to 700 nm) include oxyhemoglobin, deoxyhemoglobin, melanin, and cytochrome c. Visible light photoacoustic microscopy is particularly useful in determining hemoglobin concentration and oxygen saturation due to the difference in absorption profiles of oxyhemoglobin and deoxyhemoglobin. Real-time analysis can then be used to determine blood flow speed and oxygen metabolism rate. In addition, photoacoustic microscopy is capable of early melanoma detection due to the high concentration of melanin found in skin cancer cells.
Stephen A. Boppart
Boppart started the Biophotonics Imaging Laboratory at UIUC in 2000, an interdisciplinary research group working at the intersection of engineering, medicine, and biology. The research focuses on the development of biophotonics for translational medicine in primary care and oncology. Building upon the development of OCT in his graduate studies, Boppart expanded OCT to new application areas. OCT was used to image the tympanic membrane (eardrum) to determine the presence of biofilm, a complicating factor in ear infections. He also developed OCT for intraoperative breast cancer imaging to determine the status of the resection margin.
Photoacoustic spectroscopy
Recently, the photoacoustic approach has been utilized to quantitatively measure macromolecules, such as proteins. The photoacoustic immunoassay labels and detects target proteins using nanoparticles that can generate strong acoustic signals. The photoacoustics-based protein analysis has also been applied for point-of-care testings.
Multispectral optoacoustic tomography
Optoacoustic. This term denotes the combination of "optical" (Greek, oπτικός) and "acoustic" (Greek, ακουστικός) energy (or components) in a single modality, which distinguishes optoacoustic imaging from optical imaging. Photoecho denotes the combination of light (Greek, Φως ) and sound ( Ήχος ) or reflection of sound Hχώ ). The term photoacoustic is also widely used, and it denotes the generation of acoustic energy by light. "Photoecho" and "photoacoustic" are to optoacoustics what photon is to optics: optical methods rely on photons, whereas optoacoustic methods rely on photoechoes or photoacoustic responses.
Super-resolution photoacoustic imaging
Super resolution techniques break this limit by generally using nonlinear techniques such as saturations, switchable quenching, or multiphoton absorption. Photoacoustics directly benefits from these techniques as the excitation can be held to a tighter spot and hence detect acoustic waves to a tighter spot, ultimately improving resolution.
Focal plane tomography
This is the most basic form of conventional tomography. The X-ray tube moved from point "A" to point "B" above the patient, while the detector (such as cassette holder or "bucky") moves simultaneously under the patient from point "B" to point "A". The fulcrum, or pivot point, is set to the area of interest. In this manner, the points above and below the focal plane are blurred out, just as the background is blurred when panning a camera during exposure. Rarely used, and has largely been replaced by computed tomography (CT).
Immunoassay
The photoacoustic immunoassay measures low-frequency acoustic signals generated by metal nanoparticle tags. Illuminated by a modulated light at a plasmon resonance wavelength, the nanoparticles generate strong acoustic signal, which can be measured using a microphone. The photoacoustic immunoassay can be applied to lateral flow tests, which use colloidal nanoparticles.
Multispectral optoacoustic tomography
MSOT detects photoechoes, i.e. ultrasound waves generated by thermo-elastic expansion of a sample (e.g. tissue) after absorption of transient electromagnetic energy. Typically, the sample is illuminated with light pulses in the nanosecond range, although intensity-modulated light can also be used. At least some of the electromagnetic energy absorbed by the sample is converted to heat; the resulting temperature rise, on the order of milli-Kelvins, leads to thermo-elastic expansion of the sample. This creates a pressure wave in the form of a broadband ultrasound wave. The ultrasound waves emitted by the sample are detected by transducers positioned near the sample, usually at multiple positions around it. The amplitude of the pressure wave provides information about the local absorption and propagation of energy in the sample, while the time interval between the illumination pulse and arrival of the ultrasound wave at the detector provides information about the distance between the detector and photoecho source. Optoacoustic data collected over time and at multiple positions around the sample are processed using tomographic reconstruction to produce images of the distribution of photoabsorbers in the sample. Data collected after illumination at single wavelengths allow imaging of the distribution of photoabsorbers that share similar absorption characteristics at the given wavelength. Data collected after illumination with multiple wavelengths allow specific distinction of photoabsorbers with different optical absorption spectra, such as oxy- and deoxy-hemoglobin, myoglobin, melanin or exogenous photoabsorbers. The wavelengths of light used to illuminate samples in MSOT are selected based on the absorption characteristics of the target photoabsorbers. To resolve the individual photoabsorbers, images obtained at multiple wavelengths must be further processed using subtraction or spectral unmixing techniques. Background in images can be reduced by exploiting differences in time (baseline subtraction) and in absorption spectra of the various photoabsorbers (spectral unmixing).
Super-resolution photoacoustic imaging
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Thermoacoustic imaging
Sound, which propagates as a pressure wave, can be induced in virtually any material, including biologic tissue, whenever time-varying electromagnetic energy is absorbed. The stimulating radiation that induces these thermally generated acoustic waves may lie anywhere in the electromagnetic spectrum, from high-energy ionizing particles to low-energy radio waves. The term "photoacoustic" (see photoacoustic imaging in biomedicine) applies to this phenomenon when the stimulating radiation is optical, while "thermoacoustic" is the more general term and refers to "all" radiating sources, including optical.
Photoacoustic microscopy
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Atomic force acoustic microscopy
In the AFAM setup the sample is coupled to a piezoelectric transducer. This emits longitudinal acoustic waves into the sample, causing out-of-plane vibrations in the sample's surface. The vibrations are transmitted into the cantilever via the sensor tip. The cantilever vibrations are measured by a 4-section photo-diode and evaluated by a lock-in amplifier. This setup can be used either to acquire cantilever vibration spectra or to take acoustic images. The latter are maps of cantilever amplitudes on a fixed excitation frequency near the resonance. A contact-mode topography image is acquired simultaneously with the acoustic one.
Édouard-Léon Scott de Martinville
To collect sound, the phonautograph used a horn attached to a diaphragm which vibrated a stiff bristle which inscribed an image on a lamp black coated, hand-cranked cylinder. Scott built several devices with the help of acoustic instrument maker Rudolph Koenig. Unlike Thomas Edison's later invention of 1877, the phonograph, the phonautograph created only visual images of the sound and did not have the ability to play back its recordings. Scott de Martinville's device was used only for scientific investigations of sound waves.
Thermoacoustic imaging
Thermoacoustic imaging was originally proposed by Theodore Bowen in 1981 as a strategy for studying the absorption properties of human tissue using virtually any kind of electromagnetic radiation. But Alexander Graham Bell first reported the physical principle upon which thermoacoustic imaging is based a century earlier. He observed that audible sound could be created by illuminating an intermittent beam of sunlight onto a rubber sheet. Shortly after Bowen's work was published, other researchers proposed methodology for thermoacoustic imaging using microwaves. In 1994 researchers used an infrared laser to produce the first thermoacoustic images of near-infrared optical absorption in a "tissue-mimicking" phantom, albeit in two dimensions (2D). In 1995 other researchers formulated a general reconstruction algorithm by which 2D thermoacoustic images could be "computed" from their "projections," i.e. thermoacoustic computed tomography (TCT). By 1998 researchers at Indiana University Medical Center extended TCT to 3D and employed pulsed microwaves to produce the first fully three-dimensional (3D) thermoacoustic images of biologic tissue [an excised lamb kidney (Fig. 1)]. The following year they created the first fully 3D thermoacoustic images of cancer in the human breast, again using pulsed microwaves (Fig. 2). Since that time, thermoacoustic imaging has gained widespread popularity in research institutions worldwide. As of 2008, three companies were developing commercial thermoacoustic imaging systems – Seno Medical, Endra, Inc. and OptoSonics, Inc.
Photoacoustic microscopy
The goal of photoacoustic microscopy is to find the local pressure rise formula_1, which can be used to calculate the absorption coefficient formula_2 according to the formula:
Ocean acoustic tomography
One of the intriguing aspects of tomography is that it exploits the fact that acoustic signals travel along a set of generally stable ray paths. From a single transmitted acoustic signal, this set of rays gives rise to multiple arrivals at the receiver, the travel time of each arrival corresponding to a particular ray path. The earliest arrivals correspond to the deeper-traveling rays, since these rays travel where sound speed is greatest. The ray paths are easily calculated using computers ("ray tracing"), and each ray path can generally be identified with a particular travel time. The multiple travel times measure the sound speed averaged over each of the multiple acoustic paths. These measurements make it possible to infer aspects of the structure of temperature or current variations as a function of depth. The solution for sound speed, hence temperature, from the acoustic travel times is an inverse problem.
Unmanned Aerial Photogrammetric Survey
Unmanned Aerial Photogrammetric Survey is the use of Unmanned Aerial Vehicles (UAVs) to take photos for use in photogrammetry, the science of making measurements from photographs. Instruments manufactured for UAVs could be mounted on unmanned flying platforms of various sizes and types, such as octocopters. These machines are suitable for the full geodetic survey of a study site by creating a point cloud of measurements of nearly homogenous quality and accuracy. These detailed point clouds (of various types of data) could be used in line with orthophotos etc. to obtain a complex data system representing the study site.
Digital subtraction angiography
DSA is done less and less routinely in imaging departments. It is being replaced by computed tomography angiography (CTA), which can produce 3D images through a test which is less invasive and stressful for the patient, and magnetic resonance angiography (MRA), which avoids X-rays and nephrotoxic contrast agents.
Multispectral optoacoustic tomography
Because of its ability to provide spatial and spectral resolution in real time on multiple scales, optoacoustic imaging in general and MSOT in particular are likely to play an important role in clinical imaging and management of cancer, cardiovascular disease and inflammation. MSOT presents numerous advantages over other radiology modalities because of its ability to resolve oxygenated and deoxygenated hemoglobin, its compatibility with a broad array of exogenous contrast agents, its scalability and its ability to image rapidly even below the tissue surface.
Multispectral optoacoustic tomography
The possibility of applying optoacoustics to the microscopic regime has been suggested. This involves scanning focused light on the tissue surface. The imaging depth (typically <1 mm) and quality of the resulting image are limited by optical diffraction and scattering, not by ultrasound diffraction. In other words, optoacoustic microscopy suffers from the same limitations as conventional optical microscopy. Together, however, the two microscopies can provide more information than either on its own.
Microscopy
Ultrasonic force microscopy (UFM) has been developed in order to improve the details and image contrast on "flat" areas of interest where AFM images are limited in contrast. The combination of AFM-UFM allows a near field acoustic microscopic image to be generated. The AFM tip is used to detect the ultrasonic waves and overcomes the limitation of wavelength that occurs in acoustic microscopy. By using the elastic changes under the AFM tip, an image of much greater detail than the AFM topography can be generated.
Photoacoustic microscopy
Near-Infrared absorbers (λ = 700 to 1400 nm) include water, lipids, and glucose. Photoacoustic determination of blood glucose levels can be used for treating diabetes, while studying lipid concentrations within blood vessels is important for monitoring the progression of atherosclerosis. It is still feasible to quantify and compare deoxyhemoglobin and hemoglobin concentrations at this wavelength, trading deeper tissue penetration for lower absorption.
Multispectral optoacoustic tomography
Optoacoustic imaging in general and MSOT in particular may address a number of challenges for surgical procedures by providing real-time visualization below the tissue surface. In particular, optoacoustic imaging can provide immediate information on the perfusion status of tissues based on analysis of hemoglobin dynamics and oxygenation. This may, for example, detect areas at high risk of anastomotic leakage under ischemic conditions in the colon or esophagus, allowing preventive measures to be taken.
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 flash ("pulsed light"). The photoacoustic effect is quantified by measuring the formed sound (pressure changes) with appropriate detectors, such as microphones or piezoelectric sensors. The time variation of the electric output (current or voltage) from these detectors is the photoacoustic signal. These measurements are useful to determine certain properties of the studied sample. For example, in photoacoustic spectroscopy, the photoacoustic signal is used to obtain the actual absorption of light in either opaque or transparent objects. It is useful for substances in extremely low concentrations, because very strong pulses of light from a laser can be used to increase sensitivity and very narrow wavelengths can be used for specificity. Furthermore, photoacoustic measurements serve as a valuable research tool in the study of the heat evolved in photochemical reactions (see: photochemistry), particularly in the study of photosynthesis.
Photoconductive atomic force microscopy
In more recent studies, pc-AFM has been employed to gather information regarding the photoactive regions from the use of quantum dots. Because if their relative ease of use, along with size-tunable excitation attributes, quantum dots have commonly been applied as sensitizers in optoelectronic devices. The authors have studied the photoresponse of sub-surface foundations such as buried indium arsenide (InAs) quantum dots through the implementation of pc-AFM. Through the use of pc-AFM, information regarding quantum dot size, as well as the dispersion of quantum dots within the device, can be recorded in a non-destructive manner. This information can then be used to display local variances in photoactivity relating to heterogeneities within the film morphology.
Photoacoustic microscopy
At depths greater than 1 mm and up to 3 mm, acoustic-resolution photoacoustic microscopy (AR-PAM) is more useful due to greater optical scattering. Acoustic scattering is much weaker beyond the optical diffusion limit, making AR-PAM more practical as it provides higher lateral resolution at these depths. The lateral resolution of AR-PAM is determined by the formula:
Photoacoustic effect
The discovery of the photoacoustic effect dates back to 1880, when Alexander Graham Bell was experimenting with long-distance sound transmission. Through his invention, called "photophone", he transmitted vocal signals by reflecting sun-light from a moving mirror to a selenium solar cell receiver. As a byproduct of this investigation, he observed that sound waves were produced directly from a solid sample when exposed to beam of sunlight that was rapidly interrupted with a rotating slotted wheel. He noticed that the resulting acoustic signal was dependent on the type of the material and correctly reasoned that the effect was caused by the absorbed light energy, which subsequently heats the sample. Later Bell showed that materials exposed to the non-visible (ultra-violet and infra-red) portions of the solar spectrum can also produce sounds and invented a device, which he called "spectrophone", to apply this effect for spectral identification of materials. Bell himself and later John Tyndall and Wilhelm Röntgen extended these experiments, demonstrating the same effect in liquids and gases. However, the results were too crude, dependent on ear detection, and this technique was soon abandoned. The application of the photoacoustic effect had to wait until the development of sensitive sensors and intense light sources. In 1938 Mark Leonidovitch Veingerov revived the interest in the photoacoustic effect, being able to use it in order to measure very small carbon dioxide concentration in nitrogen gas (as low as 0.2% in volume). Since then research and applications grew faster and wider, acquiring several fold more detection sensitivity.
Photoacoustic imaging
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.
Photoconductive atomic force microscopy
Until recently, microscopy methods used in the characterization of these OPVs consisted of atomic force microscopy (AFM), transmission electron microscopy (TEM) and scanning transmission X-ray microscopy (STXM). These methods are very useful in the identification of the local morphology on the film surface, but lack the ability to provide fundamental information regarding local photocurrent generation and ultimately on the device performance. To obtain information which links the electrical and optical properties, the use of electrical scanning probe microscopy (SPM) is an active area of research. Electrostatic force microscopy (EFM) and scanning Kelvin probe microscopy (SKPM) have been utilized in the studies of electron injection and charge trapping effects, while scanning tunneling microscopy (STM) and conductive atomic force microscopy (c-AFM) have been used to investigate electron transport properties within these organic semiconductors. 
Conductive AFM has been widely used in characterizing the local electric properties in both photovoltaic fullerene blends and organic films, but no reports have shown the use of c-AFM to display the distribution of photocurrents in organic thin films. The most recent variation of SPM devices include (tr-EFM) and photoconductive AFM (pc-AFM) . Both these techniques are capable of obtaining information regarding photo-induced charging rates with nano-scale resolution. The advantage of pc-AFM over tr-ERM is present in the maximum obtainable resolution by each method. pc-AFM can map photocurrent distributions with approximately 20 nm resolution, whereas tr-EFM was only able to obtain between 50–100 nm resolution at this time. Another important factor to note is although the tr-EFM is capable of characterizing thin films within organic solar cells, it is unable to provide the needed information regarding the capacitance gradient nor the surface potential of the thin film.
The origin of PC-AFM is due to the work performed by Gerd Binnig and Heinrich Rohrer on STM for which they were awarded the Nobel Prize in physics in 1986. They fabricated an instrument called scanning tunneling microscope (STM) and demonstrated that STM provides surface topography on the atomic scale. This microscopy technique yielded resolutions which were nearly equal to scanning electron microscopy (SEM).
Multispectral optoacoustic tomography
Optoacoustic imaging in general, and MSOT in particular, have been applied to various analyses of animal models, including imaging of organs, pathology, functional processes and bio-distribution. This range of applications demonstrates the flexibility of MSOT, which reflects the range of contrast agents available. Practically every molecule that absorbs light and converts it to a pressure wave has the potential to be detected with optoacoustics. Contrast agents absorbing light in the near-IR are particularly attractive, because they enable imaging at greater depth.
Optical tomography
Optical tomography is a form of computed tomography that creates a digital volumetric model of an object by reconstructing images made from light transmitted and scattered through an object. Optical tomography is used mostly in medical imaging research. Optical tomography in industry is used as a sensor of thickness and internal structure of semiconductors.
Optical tomography relies on the object under study being at least partially light-transmitting or translucent, so it works best on soft tissue, such as breast and brain tissue.
Photoacoustic microscopy
Photoacoustic microscopy achieves greater penetration than conventional microscopy due to ultrasonic detection. As a result, axial resolution is defined acoustically and is determined by the formula:
Holotomography
The principle of HT is very similar to X-ray computed tomography (CT) or CT scan. CT scan measures multiple 2-D X-ray images of a human body at various illumination angles, and a 3-D tomogram (X-ray absorptivity) is then retrieved via the inverse scattering theory. Both the X-ray CT and laser HT shares the same governing equation – Helmholtz equation, the wave equation for a monochromatic wavelength. HT is also known as optical diffraction tomography.
Focus assessed transthoracic echocardiography
Focus assessed transthoracic echocardiography (or FATE) is a type of transthoracic echocardiogram, or sonogram of the heart, often performed by non-cardiologist. The protocol has been used since 1989 and has four projections; subcostal four-chamber, apical four-chamber, parasternal long axis and parasternal short axis.
Photon scanning microscopy
PSTM can be combined with both ESTM and AFM in order to simultaneously record optical, conductive, and topological information of a sample. This experimental apparatus, published by Iwata et al., allows the characterization of semiconductors such as photovoltaics, as well as other photo-conductive materials. The experimental configuration utilizes a cantilever consisting of a bent optical fiber sharpened to a tip diameter of less than 100 nm, coated with an ITO layer, and a thin Au layer. Hence, the fiber probe acts as the AFM cantilever for force sensing, is optically conductive to record optical data, and electrically conductive to record current from the sample. The signals from the three detection methods are recorded simultaneously and independently in order to separate topographical, optical, and electrical information from the signals..
Photoacoustic effect
While the photothermal mechanism is universal, there could exist additional other mechanisms, superimposed on the photothermal mechanism, which may contribute significantly to the photoacoustic signal. These mechanisms are generally related to photophysical processes and photochemical reactions following light absorption: (1) change in the material balance of the sample and/or the gaseous phase around the sample; (2) change in the molecular organization, which results in molecular volume changes. Most prominent examples for these two kinds of mechanisms are in photosynthesis
Photoacoustic effect
Although much of the literature on the subject is concerned with just one mechanism, there are actually several different mechanisms that produce the photoacoustic effect. The primary universal mechanism is "photothermal", based on the heating effect of the light and the consequent expansion of the light-absorbing material. In detail, the photothermal mechanism consists of the following stages: The main physical picture, in this case, envisions the original temperature pulsations as origins of propagating temperature waves ("thermal waves"), which travel in the condensed phase, ultimately reaching the surrounding gaseous phase. The resulting temperature pulsations in the gaseous phase are the prime cause of the pressure changes there. The amplitude of the traveling thermal wave decreases strongly (exponentially) along its propagation direction, but if its propagation distance in the condensed phase is not too long, its amplitude near the gaseous phase is sufficient to create detectable pressure changes. This property of the thermal wave confers unique features to the detection of light absorption by the photoacoustic method. The temperature and pressure changes involved are minute, compared to everyday scale – typical order of magnitude for the temperature changes, using ordinary light intensities, is about micro to milli-degrees and for the resulting pressure changes is about nano to micro-bars.
Photoacoustic 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 (HbO) and deoxygenated hemoglobin (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" for tumor angiogenesis monitoring, blood oxygenation mapping, functional brain imaging, skin melanoma detection, methemoglobin measuring, etc.
Focal plane tomography
This was achieved using a more advanced X-ray apparatus that allows for more sophisticated and continuous movements of the X-ray tube and film. With this technique, a number of complex synchronous geometrical movements could be programmed, such as hypocycloidic, circular, figure 8, and elliptical. Philips Medical Systems for example produced one such device called the 'Polytome'. This pluridirectional unit was still in use into the 1990s, as its resulting images for small or difficult physiology, such as the inner ear, was still difficult to image with CTs at that time. As the resolution of CT scanners got better, this procedure was taken over by CT.
Photoacoustic spectroscopy
A photoacoustic spectrum of a sample can be recorded by measuring the sound at different wavelengths of the light. This spectrum can be used to identify the absorbing components of the sample. The photoacoustic effect can be used to study solids, liquids and gases.
Optical coherence tomography
Within the range of noninvasive three-dimensional imaging techniques that have been introduced to the medical research community, OCT as an echo technique is similar to ultrasound imaging. Other medical imaging techniques such as computerized axial tomography, magnetic resonance imaging, or positron emission tomography do not use the echo-location principle.
Earthscope
Tomography is a method of producing a three-dimensional image of the internal structures of a solid object (such as the human body or the earth) by the observation and recording of differences in the effects on the passage of energy waves impinging on those structures. The waves of energy are P-waves generated by earthquakes and are recording the wave velocities. The high quality data that is being collected by the permanent seismic stations of USArray and the Advanced National Seismic System (ANSS) will allow the creation of high resolution seismic imaging of the Earth’s interior below the United States. Seismic tomography helps constrain mantle velocity structure and aids in the understanding of chemical and geodynamic processes that are at work. With the use of the data collected by USArray and global travel-time data, a global tomography model of P-wave velocity heterogeneity in the mantle can be created. The range and resolution of this technique will allow investigation into the suite of problems that are of concern in the North American mantle lithosphere, including the nature of the major tectonic features. This method gives evidence for differences in thickness and the velocity anomaly of the mantle lithosphere between the stable center of the continent and the more active western North America. This data is vital for the understanding of local lithosphere evolution, and when combined with additional global data, will allow the mantle to be imaged beyond the current extent of USArray.
Willi A. Kalender
The development and introduction of volumetric spiral computed tomography was a highlight of his work; the world’s first clinical spiral CT studies were presented at RSNA 1989 [3] and were highly welcomed. The combination of continuous data acquisition with slip-ring-based data transmission and continuous table translation led to considerable reduction of examination times, to an essential reduction of motion artefacts and image quality significantly improved by providing isotropic spatial resolution as a decisive feature.
The development of a number of new and advanced applications such as angio-CT [4] and heart phase-specific cardiac imaging [6] followed and became a big success.
Other highly important fields of W. Kalender’s research have been radiation protection and the development of intelligent and very efficient dose reduction approaches such as tube current modulation [7] and the optimization of the choice of x-ray spectra [8]. They allow reducing the patient dose by an order of magnitude in many cases without impairing image quality. An overview of all measures he introduced is given in [9].
Photoconductive atomic force microscopy
Modifications of this early work have been implemented to perform AFM analysis on both conducting and non-conducting materials. Conductive atomic force microscopy (c-AFM) is one such modification technique. The c-AFM technique operates by measuring fluctuations in current from the biased tip and sample while simultaneously measuring changes in the topographical features. In all techniques of AFM, two modes of operation can be used: contact mode and non-contact mode. In c-AFM resonant contact mode is used to obtain topographical from current that is measured between the biased AFM tip and the sample surface. In this type of operation, the current is measured in the small space between the tip and the sample surface. This quantification is based on the relationship between the current traveling through the sample and layer thickness. In the previous equation, A is the effective emission area at the injecting electrode, q is the electron charge, h is planck’s constant, "m" / "m" =0.5, which is the effective mass of an electron in the conduction band of a sample, "d" is the sample thickness and "Φ" is the barrier height. The symbol, "β", the field enhancement factor, accounts for the non-planar, geometry of the tip used.
Stress wave tomography
Acoustic or stress wave tomography is a non-destructive measurement method for the visualization of the structural integrity of a solid object. It is being used to test the preservation of wood or concrete, for example. The term "acoustic tomography" refers to the perceptible sounds that are caused by the mechanical impulses used for measuring. The term "stress wave tomography" describes the measurement method more accurately.
Photoacoustic microscopy
where formula_18 is the central wavelength of the photoacoustic wave and formula_19 is the numerical aperture of the ultrasound transducer. Higher lateral resolution can therefore be achieved by increasing the center frequency of the ultrasound transducer and tighter acoustic focusing. AR-PAM systems can typically achieve a lateral resolution of 15 to 50 μm.
CT scan
Photon counting computed tomography is a CT technique currently under development. Typical CT scanners use energy integrating detectors; photons are measured as a voltage on a capacitor which is proportional to the x-rays detected. However, this technique is susceptible to noise and other factors which can affect the linearity of the voltage to x-ray intensity relationship. Photon counting detectors (PCDs) are still affected by noise but it does not change the measured counts of photons. PCDs have several potential advantages including improving signal (and contrast) to noise ratios, reducing doses, improving spatial resolution and, through use of several energies, distinguishing multiple contrast agents. PCDs have only recently become feasible in CT scanners due to improvements in detector technologies that can cope with the volume and rate of data required. As of February 2016 photon counting CT is in use at three sites. Some early research has found the dose reduction potential of photon counting CT for breast imaging to be very promising.
Super-resolution photoacoustic imaging
Super-resolution photoacoustic imaging is a set of techniques used to enhance spatial resolution in photoacoustic imaging. Specifically, these techniques primarily break the optical diffraction limit of the photoacoustic imaging system. It can be achieved in a variety of mechanisms, such as blind structured illumination, multi-speckle illumination, or photo-imprint photoacoustic microscopy in Figure 1.
History of computed tomography
Before computed tompography, tomographic images could be made by radiography by focal plane tomography, representing a single slice of the body on radiographic film. This method was proposed by the Italian radiologist Alessandro Vallebona in the early 1900s. The idea is based on simple principles of projective geometry: moving synchronously and in opposite directions the X-ray tube and the film, which are connected together by a rod whose pivot point is the focus; the image created by the points on the focal plane appears sharper, while the images of the other points annihilate as noise. This is only marginally effective, as blurring occurs in only the "x" plane. This method of acquiring tomographic images using only mechanical techniques advanced through the mid-twentieth century, steadily producing sharper images, and with a greater ability to vary the thickness of the cross-section being examined. This was achieved through the introduction of more complex, pluridirectional devices that can move in more than one plane and perform more effective blurring. However, despite the increasing sophistication of focal plane tomography, it remained ineffective at producing images of soft tissues. With the increasing power and availability of computers in the 1960s, research began into practical computational techniques for creating tomographic images, leading to the development of computed tomography (CT).
Multispectral optoacoustic tomography
Tomography. This term denotes images formed by combining raw measurements from multiple points around the specimen in a mathematical inversion scheme. This process is analogous to x-ray computed tomography, except that tomographic mathematical models describe light and sound propagation in tissues.
"Fig. 1: Operational capabilities of MSOT. Hybrid image showing an optical micrograph of part of a mouse kidney cross-section (gray), overlaid with the distribution of an exogenous fluorescent agent imaged using MSOT (right). Distribution of oxy-hemoglobin (red) and deoxy-hemoglobin (blue) in the tumor, imaged using MSOT (left)."
Ultrasound
Medical sonography (ultrasonography) is an ultrasound-based diagnostic medical imaging technique used to visualize muscles, tendons, and many internal organs to capture their size, structure and any pathological lesions with real time tomographic images. Ultrasound has been used by radiologists and sonographers to image the human body for at least 50 years and has become a widely used diagnostic tool. The technology is relatively inexpensive and portable, especially when compared with other techniques, such as magnetic resonance imaging (MRI) and computed tomography (CT). Ultrasound is also used to visualize fetuses during routine and emergency prenatal care. Such diagnostic applications used during pregnancy are referred to as obstetric sonography. As currently applied in the medical field, properly performed ultrasound poses no known risks to the patient. Sonography does not use ionizing radiation, and the power levels used for imaging are too low to cause adverse heating or pressure effects in tissue.
Although the long-term effects due to ultrasound exposure at diagnostic intensity are still unknown, currently most doctors feel that the benefits to patients outweigh the risks. The ALARA (As Low As Reasonably Achievable) principle has been advocated for an ultrasound examination – that is, keeping the scanning time and power settings as low as possible but consistent with diagnostic imaging – and that by that principle nonmedical uses, which by definition are not necessary, are actively discouraged.
Medical ultrasound
In molecular ultrasonography, the technique of acoustic radiation force (also used for shear wave elastography) is applied in order to literally push the targeted microbubbles towards microvessels wall; firstly demonstrated by Dr Paul Dayton in 1999. This allows maximization of binding to the malignant tumor; the targeted microbubbles being in more direct contact with cancerous biomolecules expressed at the inner surface of tumoral microvessels. At the stage of scientific preclinical research, the technique of acoustic radiation force was implemented as a prototype in clinical ultrasound systems and validated "in vivo" in 2D and 3D imaging modes.
Photoacoustic microscopy
One of the major benefits of photoacoustic microscopy is the simplicity of image reconstruction. A laser pulse excites tissue in the axial direction and the resulting photoacoustic waves are detected by an ultrasound transducer. The transducer then converts the mechanical energy into a voltage signal that can be read by an analog-to-digital converter for post-processing. A one-dimensional image, known as an A-line, is formed as a result of each laser pulse. Hilbert transform of an A-line reveals depth-encoded information. A 3D photoacoustic image can then be formed by combining multiple A-lines produced by 2D raster scanning.
Bruce J. Tromberg
Tromberg and others in his laboratory have developed new methods for 
broadband diffuse optical spectroscopy (DOS) including diffuse optical spectroscopic imaging (DOSI); 
spatial frequency domain imaging (SFDI); and a type of optical tomography combining techniques for 
second harmonic generation tomography (SHG) and 
two-photon excited fluorescence (TPEF).
Photoacoustic imaging
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Nuclear medicine
More recent developments in nuclear medicine include the invention of the first positron emission tomography scanner (PET). The concept of emission and transmission tomography, later developed into single photon emission computed tomography (SPECT), was introduced by David E. Kuhl and Roy Edwards in the late 1950s. Their work led to the design and construction of several tomographic instruments at the University of Pennsylvania. Tomographic imaging techniques were further developed at the Washington University School of Medicine. These innovations led to fusion imaging with SPECT and CT by Bruce Hasegawa from University of California San Francisco (UCSF), and the first PET/CT prototype by D. W. Townsend from University of Pittsburgh in 1998.
Holotomography
Holotomography (HT) is a laser technique to measure three-dimensional refractive index (RI) tomogram of a microscopic sample such as biological cells and tissues. Because the RI can serve as an intrinsic imaging contrast for transparent or phase objects, measurements of RI tomograms can provide label-free quantitative imaging of microscopic phase objects. In order to measure 3-D RI tomogram of samples, HT employs the principle of holographic imaging and inverse scattering. Typically, multiple 2D holographic images of a sample are measured at various illumination angles, employing the principle of interferometric imaging. Then, a 3D RI tomogram of the sample is reconstructed from these multiple 2D holographic images by inversely solving light scattering in the sample.
Photoacoustic microscopy
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Photoacoustic microscopy
Photoacoustic microscopy has a wide range of applications in the biomedical field. Due to its ability to image a variety of molecules based on optical wavelength, photoacoustic microscopy can be used to gain functional information about the body noninvasively. Blood flow dynamics and oxygen metabolic rates can be measured and correlated to studies of atherosclerosis or tumor proliferation. Exogenous agents can be used to bind to cancerous tissue, enhancing image contrast and aiding in surgical removal. On the same note, photoacoustic microscopy is useful in early cancer diagnosis due to the difference in optical absorption properties compared to healthy tissue.
Phonautograph
Nearly 150 years after they had been recorded, promising specimens of Scott de Martinville's phonautograms, stored among his papers in France's patent office and at the Académie des Sciences, were located by American audio historians. High-quality images of them were obtained. In 2008, the team played back the recordings as sound for the first time. Modern computer-based image processing methods were used to accomplish the playback. The first results were obtained by using a specialized system developed for optically playing recordings on more conventional media which were too fragile or damaged to be played by traditional means. Later, generally available image-editing and image-to-sound conversion software, requiring only a high-quality scan of the phonautogram and an ordinary personal computer, were found to be sufficient for this application.
Super-resolution photoacoustic imaging
This particular biomedical imaging modality is a combination of optical imaging, and ultrasound imaging. In other words, a photoacoustic (PA) image can be viewed as an ultrasound image in which its contrast depends on the optical properties, such as optical resolution of biomolecules like hemoglobin, water, melanin, lipids, and collagen. The advantages of photoacoustic imaging are that it gives higher specificity than conventional ultrasound imaging and greater penetration depth than conventional ballistic optical imaging modalities. Photoacoustic imaging works by irradiating the target with a short-pulsed laser, or alternatively an intensity-modulated laser. The target absorbs this optical energy, which is converted into heat; in most cases the amount of heat is a fraction of approximately (1 - fluorescence quantum yield). Heat is further converted into a pressure rise via thermoelastic expansion, and the pressure rise behaves as an ultrasonic wave, the wave of which is called a “photoacoustic wave” that propagates throughout the surroundings of the target. The photoacoustic wave is then detected by ultrasonic transducers, and these signals are used by an image processor and computer to reconstruct an image of the target.
Super-resolution microscopy
Stochastic optical reconstruction microscopy (STORM), photo activated localization microscopy (PALM) and fluorescence photo-activation localization microscopy (FPALM) are super-resolution imaging techniques that utilize sequential activation and time-resolved localization of photoswitchable fluorophores to create high resolution images. During imaging, only an optically resolvable subset of fluorophores is activated to a fluorescent state at any given moment, such that the position of each fluorophore can be determined with high precision by finding the centroid position of the single-molecule images of particular fluorophore. The fluorophore is subsequently deactivated, and another subset is activated and imaged. Iteration of this process allows numerous fluorophores to be localized and a super-resolution image to be constructed from the image data. These three methods were published independently during a short period of time and their principle is identical. STORM was originally described using Cy5 and Cy3 dyes attached to nucleic acids or proteins, while PALM and FPALM was described using photoswitchable fluorescent proteins. In principle any photoswitchable fluorophore can be used, and STORM has been demonstrated with a variety of different probes and labeling strategies. Using stochastic photoswitching of single fluorophores, such as Cy5, STORM can be performed with a single red laser excitation source. The red laser both switches the Cy5 fluorophore to a dark state by formation of an adduct and subsequently returns the molecule to the fluorescent state. Many other dyes have been also used for STORM. In addition to single fluorophores, dye-pairs consisting of an activator fluorophore (such as Alexa 405, Cy2, and Cy3) and a photoswitchable reporter dye (such as Cy5, Alexa 647, Cy5.5, and Cy7) can be used for STORM. In this scheme, the activator fluorophore, when excited near its absorption maximum, serves to reactivate the photoswitchable dye to the fluorescent state. Multicolor imaging has been performed by using different activation wavelengths to distinguish dye-pairs based on the activator fluorophore used or using spectrally distinct photoswitchable fluorophores either with or without activator fluorophores. Photoswitchable fluorescent proteins can be used as well. Highly specific labeling of biological structures with photoswitchable probes has been achieved with antibody staining, direct conjugation of proteins, and genetic encoding.
STORM has also been extended to three-dimensional imaging using optical astigmatism, in which the elliptical shape of the point spread function encodes the x, y, and z positions for samples up to several micrometers thick, and has been demonstrated in living cells. To date, the spatial resolution achieved by this technique is ~20 nm in the lateral dimensions and ~50 nm in the axial dimension and the temporal resolution is as fast as 0.1–0.33s.
Conductive atomic force microscopy
Conductive atomic force microscopy (C-AFM) or current sensing atomic force microscopy (CS-AFM) is a mode in atomic force microscopy (AFM) that simultaneously measures the topography of a material and the electric current flow at the contact point of the tip with the surface of the sample. The topography is measured by detecting the deflection of the cantilever using an optical system (laser + photodiode), while the current is detected using a current-to-voltage preamplifier. The fact that the CAFM uses two different detection systems (optical for the topography and preamplifier for the current) is a strong advantage compared to scanning tunneling microscopy (STM). Basically, in STM the topography picture is constructed based on the current flowing between the tip and the sample (the distance can be calculated depending on the current). Therefore, when a portion of a sample is scanned with an STM, it is not possible to discern if the current fluctutations are related to a change in the topography (due to surface roughness) or to a change in the sample conductivity (due to intrinsic inhomogeneities).
Super-resolution photoacoustic imaging
where "m" = 1...,M. formula_14 is the PSF of the photoacoustic imaging system in discrete form, formula_15 is the pointwise multiplication, formula_16 is the discrete deconvolution step, and ε is the noise in the data.
Super-resolution photoacoustic imaging
Photoacoustics relies on optical excitation of targets and detection of acoustic emission. Since the frequencies of electromagnetic waves (light) are significantly higher than that of acoustic waves, the optical excitation generally sets the absolute resolution. This absolute resolution is well known in optics and is called the diffraction limit of light. This diffraction limit shown below represents the minimum distance that can be resolved between two objects (or similarly the minimum separation distance between two objects excitated by a laser).
Graphene nanoribbon
Hybrid imaging modalities, such as photoacoustic (PA) tomography (PAT) and thermoacoustic (TA) tomography (TAT) have been developed for bioimaging applications. PAT/TAT combines advantages of pure ultrasound and pure optical imaging/radio frequency (RF), providing good spatial resolution, great penetration depth and high soft-tissue contrast. GNR synthesized by unzipping single- and multi-walled carbon nanotubes have been reported as contrast agents for photoacoustic and thermoacoustic imaging and tomography.
History of sound recording
In the spring of 1877 another inventor, Charles Cros, suggested that the process could be reversed by using photoengraving to convert the traced line into a groove that would guide the stylus, causing the original stylus vibrations to be recreated, passed on to the linked diaphragm, and sent back into the air as sound. An inventor from America soon eclipsed this idea, and it was not until 1887 that yet another inventor, Emile Berliner, actually photoengraved a phonautograph recording into metal and played it back.
Photoacoustic imaging
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 formula_9). 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.
Photoacoustic imaging
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:
Multispectral optoacoustic tomography
A large amount of data must be collected and processed for truly three-dimensional imaging, necessitating a large detector array, long scanning times, and heavy computational burden. To reduce these requirements, the three-dimensional problem is often simplified to a quasi-two-dimensional problem by using focused ultrasound detectors to limit ultrasound detection to a two-dimensional plane in the illuminated volume. The result is a series of two-dimensional, cross-sectional images, which can be collected in real time and can show quite high in-plane resolution if detector elements are packed at high density around the image plane. Translating the detector along the third dimension then allows volumetric scanning.
"Fig. 2: Volumetric optoacoustic imaging and comparison with conventional ultrasound imaging. Maximal-intensity projections (MIP) of whole-body ultrasound (right) and optoacoustic (middle) image stacks of a living mouse. Histology cross-sections of the tissue shown on the left."
Super-resolution photoacoustic imaging
where λ is the excitation wavelength, and "n" is the medium refractive index. The important property of the axial resolution is its independence from the frequency of the photoacoustic waves. Therefore, lower frequency ultrasound waves can be used for deeper detection.
Photoacoustic effect
Recently, the photoacoustic approach has been utilized to quantitatively measure macromolecules, such as proteins. The photoacoustic immunoassay labels and detects target proteins using nanoparticles that can generate strong acoustic signals. The photoacoustics-based protein analysis has also been applied for point-of-care testings.
Positron emission tomography
Analytical techniques, much like the reconstruction of computed tomography (CT) and single-photon emission computed tomography (SPECT) data, are commonly used, although the data set collected in PET is much poorer than CT, so reconstruction techniques are more difficult. Coincidence events can be grouped into projection images, called sinograms. The sinograms are sorted by the angle of each view and tilt (for 3D images). The sinogram images are analogous to the projections captured by computed tomography (CT) scanners, and can be reconstructed in a similar way. The statistics of data thereby obtained are much worse than those obtained through transmission tomography. A normal PET data set has millions of counts for the whole acquisition, while the CT can reach a few billion counts. This contributes to PET images appearing "noisier" than CT. Two major sources of noise in PET are scatter (a detected pair of photons, at least one of which was deflected from its original path by interaction with matter in the field of view, leading to the pair being assigned to an incorrect LOR) and random events (photons originating from two different annihilation events but incorrectly recorded as a coincidence pair because their arrival at their respective detectors occurred within a coincidence timing window).
Medical imaging
Photoacoustic imaging is a recently developed hybrid biomedical imaging modality based on the photoacoustic effect. It combines the advantages of optical absorption contrast with an ultrasonic spatial resolution for deep imaging in (optical) diffusive or quasi-diffusive regime. Recent studies have shown that photoacoustic imaging can be used in vivo for tumor angiogenesis monitoring, blood oxygenation mapping, functional brain imaging, and skin melanoma detection, etc.
Cantilever enhanced photoacoustic spectroscopy
Cantilever enhanced photoacoustic spectroscopy enables the detection of small amount of trace gases which is vital in many applications. Photoacoustic spectroscopy is one of the most sensitive optical detection schemes. It is based on detecting a gas specific acoustic wave generated that originates from the absorption of light in the medium. The sensitivity of the traditional membrane microphones is limited by electrical noise and the nonlinearity of the displacement of the mechanical sensor at high optical power levels. Conventional membrane microphones can be replaced with optically measured micromechanical cantilevers to enhance sensitivity.
Photoacoustic imaging
Photoacoustic imaging (optoacoustic imaging) is a biomedical imaging modality based on the photoacoustic effect. In photoacoustic imaging, non-ionizing laser pulses are delivered into biological tissues (when radio frequency pulses are used, the technology is referred to as thermoacoustic imaging). Some of the delivered energy will be absorbed and converted into heat, leading to transient thermoelastic expansion and thus wideband (i.e. MHz) ultrasonic emission. The generated ultrasonic waves are detected by ultrasonic transducers and then analyzed to produce images. It is known that optical absorption is closely associated with physiological properties, such as hemoglobin concentration and oxygen saturation. 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.
Tomography
Tomography is imaging by sections or sectioning, through the use of any kind of penetrating wave. The method is used in radiology, archaeology, biology, atmospheric science, geophysics, oceanography, plasma physics, materials science, astrophysics, quantum information, and other areas of science. The word "tomography" is derived from Ancient Greek τόμος "tomos", "slice, section" and γράφω "graphō", "to write" (see also Etymology). A device used in tomography is called a tomograph, while the image produced is a tomogram.
Ronald N. Bracewell
Imaging in astronomy led to participation in development of computer assisted x-ray tomography, where commercial scanners reconstruct tomographic images using the algorithm developed by Bracewell for radioastronomical reconstruction from fan-beam scans. This corpus of work has been recognized by the Institute of Medicine, an award by the University of Sydney, and the Heinrich Hertz medal. Service on the founding editorial board of the "Journal for Computer-Assisted Tomography", to which he also contributed publications, and on the scientific advisory boards of medical instrumentation companies maintained Bracewell's interest in medical imaging, which became an important part of his regular graduate lectures on imaging, and forms an important part of his 1995 text on imaging.
Photoactivated localization microscopy
Photo-activated localization microscopy (PALM or FPALM)
and stochastic optical reconstruction microscopy (STORM) are widefield (as opposed to point scanning techniques such as laser scanning confocal microscopy) fluorescence microscopy imaging methods that allow obtaining images with a resolution beyond the diffraction limit. The methods were proposed in 2006 in the wake of a general emergence of optical super-resolution microscopy methods, and were featured as Methods of the Year for 2008 by the "Nature Methods" journal.
The development of PALM as a targeted biophysical imaging method was largely prompted by the discovery of new species and the engineering of mutants of fluorescent proteins displaying a controllable photochromism, such as photo-activatable GFP. However, the concomitant development of STORM, sharing the same fundamental principle, originally made use of paired cyanine dyes.
One molecule of the pair (called activator), when excited near its absorption maximum, serves to reactivate the other molecule (called reporter) to the fluorescent state.
Photoacoustic microscopy
Fluorescent proteins have been developed for fluorescence microscopy imaging and are unique in that they can be genetically encoded and therefore do not need to be delivered into the body. Using photoacoustic microscopy, fluorescent proteins can be visualized at depths beyond the limit of typical microscopy methods. Frequency-dependent acoustic attenuation in tissue and dampening of higher frequencies limits the bandwidth of light propagation through deeper regions in tissue. Fluorescent proteins act as light source at the target region, bypassing the limitation of optical attenuation. However, the effectiveness of fluorescent proteins is limited by low fluence changes, as the light diffusion equation predicts lower than 5% increase.
Active pixel sensor
Fossum now performs research on the Quanta Image Sensor (QIS) technology. The QIS is a revolutionary change in the way we collect images in a camera that is being invented at Dartmouth. In the QIS, the goal is to count every photon that strikes the image sensor, and to provide resolution of 1 billion or more specialized photoelements (called jots) per sensor, and to read out jot bit planes hundreds or thousands of times per second resulting in terabits/sec of data.
Image-guided surgery
Image-guided surgery was originally developed for treatment of brain tumors using stereotactic surgery and radiosurgery that are guided by computed tomography (CT), magnetic resonance imaging (MRI) and positron emission tomography (PET) using a technology known as the N-localizer.
Photoacoustic microscopy
where formula_12 is the speed of sound in the medium and formula_13 is the photoacoustic signal bandwidth. The axial resolution of the system can be improved by using a wider bandwidth ultrasound transducer as long as the bandwidth matches that of the photoacoustic signal.
The lateral resolution of photoacoustic microscopy depends on the optical and acoustic foci of the system. Optical-resolution photoacoustic microscopy (OR-PAM) uses a tighter optical focus than acoustic focus, while acoustic-resolution photoacoustic microscopy (AR-PAM) uses a tighter acoustic focus than optical focus.
Multispectral optoacoustic tomography
Ultrasound detectors have been developed that collect bandwidths of 10-200 MHz or wider, which allows unprecedented mesoscopy at tissue depths of 0.1–1 cm with resolution that can exceed 10 microns even at depths of several millimeters. Illumination light is typically unfocused and selected from the visible and near-IR regions of the spectrum. Images are generated using computed tomography. Such mesoscopy can analyze morphology and biological processes such as inflammation in greater detail than macroscopy, revealing, for example, microvasculature networks in skin and epithelial tissues or the microenvironment within a tumor. Regions of interest are approximately 50 mm, and resolution of 5-30 microns is typical.
Photoacoustic spectroscopy
Photoacoustic spectroscopy has become a powerful technique to study concentrations of gases at the part per billion or even part per trillion levels. Modern photoacoustic detectors still rely on the same principles as Bell’s apparatus; however, to increase the sensitivity, several modifications have been made.
The use of intense lasers instead of the sun to illuminate the sample since the intensity of the generated sound is proportional to the light intensity; this technique is referred to as laser photoacoustic spectroscopy (LPAS). The ear has been replaced by sensitive microphones. The microphone signals are further amplified and detected using lock-in amplifiers. By enclosing the gaseous sample in a cylindrical chamber, the sound signal is amplified by tuning the modulation frequency to an acoustic resonance of the sample cell.
Atomic force acoustic microscopy
Atomic force acoustic microscopy (AFAM) is a type of scanning probe microscopy (SPM). It is a combination of acoustics and atomic force microscopy. The principal difference between AFAM and other forms of SPM is the addition of a transducer at the bottom of the sample which induces longitudinal out-of-plane vibrations in the specimen. These vibrations are sensed by a cantilever and tip called a probe. The figure shown here is the clear schematic of AFAM principle here B is the magnified version of the tip and sample placed on the transducer and tip having some optical coating generally gold coating to reflect the laser light on to the photodiode.
Tomography
Focal plane tomography was developed in the 1930s by the radiologist Alessandro Vallebona, and proved useful in reducing the problem of superimposition of structures in projectional radiography. In a 1953 article in the medical journal "Chest", B. Pollak of the Fort William Sanatorium described the use of planography, another term for tomography. Focal plane tomography remained the conventional form of tomography until being largely replaced by mainly computed tomography the late-1970s. Focal plane tomography uses the fact that the focal plane appears sharper, while structures in other planes appear blurred. By moving an X-ray source and the film in opposite directions during the exposure, and modifying the direction and extent of the movement, operators can select different focal planes which contain the structures of interest.
Physical acoustics
Physical acoustics is the area of acoustics and physics that studies interactions of acoustic waves with a gaseous, liquid or solid medium on macro- and micro-levels. This relates to the interaction of sound with thermal waves in crystals (phonons), with light (photons), with electrons in metals and semiconductors (acousto-electric phenomena), with magnetic excitations in ferromagnetic crystals (magnons), etc. Some recently developed experimental techniques include photo-acoustics, acoustic microscopy and acoustic emission. A long-standing interest is in acoustic and ultrasonic wave propagation and scattering in inhomogeneous materials, including composite materials and biological tissues.
Stethoscope
Because the sounds are transmitted electronically, an electronic stethoscope can be a wireless device, can be a recording device, and can provide noise reduction, signal enhancement, and both visual and audio output. Around 2001, Stethographics introduced PC-based software which enabled a phonocardiograph, graphic representation of cardiologic and pulmonologic sounds to be generated, and interpreted according to related algorithms. All of these features are helpful for purposes of telemedicine (remote diagnosis) and teaching.
Photoacoustic microscopy
Within the ultraviolet light range (λ = 180 to 400 nm), the primary absorber in the body is DNA and RNA. By using ultraviolet photoacoustic microscopy, DNA and RNA can be imaged in the cell nuclei without the use of fluorescence labeling. Since cancer is associated with DNA replication failure, UV photoacoustic microscopy has the potential to be used for early cancer detection.
Photoacoustic microscopy
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Photoacoustic imaging
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.
Operation of computed tomography
Tomography is a technology that uses a tomographic optical system to obtain virtual 'slices' (a tomographic image) of specific cross section of a scanned object, allowing the user to see inside the object without cutting. There are several types of tomographic optical system including the parallel beam irradiation optical system. Parallel beam irradiation optical system may be the easiest and most practical example of a tomographic optical system therefore, in this article, explanation of "How to obtain the Tomographic image" will be based on "the parallel beam irradiation optical system". The resolution in tomography is typically described by the Crowther criterion.
Super-resolution photoacoustic imaging
Figure 3: Before and after super resolution techniques applied to photoacoustic phase microscopy.
Multispectral optoacoustic tomography
Optoacoustic (photoacoustic) imaging is intrinsically a three-dimensional imaging method, since photoechoes (optoacoustic waves) propagate in all three spatial dimensions. Optimal tomographic imaging is therefore achieved by recording time-resolved pressure waves along a closed surface volumetrically surrounding the target tissue. Typically, three-dimensional imaging systems achieve this by scanning a single ultrasound sensor around the sample, or by using one-dimensional or two-dimensional ultrasound sensor arrays to parallelize detection.
Photoacoustic imaging
where formula_9 is the initial photoacoustic pressure.