Photoacoustic microscopy
Photoacoustic microscopy takes advantage of the local temperature rise that occurs as a result of light absorption in tissue.
Using a nanosecond pulsed laser beam, tissues undergo thermoelastic expansion, resulting in the release of a wide-band acoustic wave that can be detected using a high-frequency ultrasound transducer.
By adjusting the optical and acoustic foci, lateral resolution may be optimized for the desired imaging depth.
[3] 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.
A laser pulse excites tissue in the axial direction and the resulting photoacoustic waves are detected by an ultrasound transducer.
However, the lateral resolution is limited by the presence of side lobes, which appear at polar angles and are dependent on the width of each element.
Due to the fact that almost all molecules are capable of nonradiative relaxation, photoacoustic microscopy has the potential to image a wide range of endogenous and exogenous agents.
[3] Current research in photoacoustic microscopy takes advantage of both endogenous and exogenous contrast agents to gain functional information about the body, from blood saturation levels to cancer proliferation rate.
Endogenous contrast agents, molecules naturally occurring within the body, are useful in photoacoustic microscopy due to the fact that they may be imaged non-invasively.
[2] 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.
[5] 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.
[3] In addition, photoacoustic microscopy is capable of early melanoma detection due to the high concentration of melanin found in skin cancer cells.
[2] It is still feasible to quantify and compare deoxyhemoglobin and hemoglobin concentrations at this wavelength, trading deeper tissue penetration for lower absorption.
[6] Although endogenous contrasts agents are noninvasive and simpler to use, they are limited by their inherent behavior and concentration, making it difficult to monitor certain processes if optical absorption is weak.
Through selective binding, exogenous contrast agents are capable of targeting specific molecules of interest while also enhancing resulting images.
[3] Organic dyes, such as ICG-PEG and Evans blue, are used to enhance vasculature as well as to improve tumor imaging.
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.
[9][10] Due to a tighter optical focus, OR-PAM is more useful for imaging in the quasi-ballistic range of depths up to 1 mm.
Acoustic scattering is much weaker beyond the optical diffusion limit, making AR-PAM more practical as it provides higher lateral resolution at these depths.
[2] Higher lateral resolution can therefore be achieved by increasing the center frequency of the ultrasound transducer and tighter acoustic focusing.
By ignoring ballistic light, dark-field confocal photoacoustic microscopy reduces surface signal.
This method uses a dark-field pulsed laser and high-NA ultrasonic detection, with the fiber output end coaxially aligned with the focused ultrasound transducer.
Filtration of ballistic light relies on the altered shape of the excitation laser beam instead of an opaque disk, as used in conventional dark-field microscopy.
The general reconstruction technique is used to convert the photoacoustic signal into one A-line, and B-line images are produced by raster scanning.
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.
