Followings are educational materials about the working principle of PAI and OCT :
What is PHOTOACOUSTIC IMAGING and how it works?
Photoacoustic imaging (PAI), also called optoacoustic imaging, is a three-dimensional (3-D) imaging modality that works based on the photoacoustic (PA) effect. The sample (light absorbent) to be imaged is optically excited, leading to a transient temperature rise, resulting in a thermoelastic expansion of the absorber followed by emission of acoustic waves. The absorber could be endogenous such as hemoglobin (oxy- or deoxy-), myoglobin (oxy- or deoxy-), melanin, lipid, Bilirubin, water, or an exogenous contrast agent such as dyes. The absorption spectrum of some of the endogenous and exogenous absorbers are shown in Figure 1.
The emitted acoustic waves from the absorber are detected by ultrasound (US) transducers. The transducer signals are then given to an image reconstruction algorithm to generate the absorption map of the tissue. The PAI process steps are illustrated in Figure 2.
Due to strong optical scattering in biological tissues, pure optical imaging modalities have a shallow imaging depth. The transport mean free path (i.e., the mean distance after which a photon’s direction becomes random) in biological tissues is around 1 mm. Acoustic waves experience far less tissue scattering, thus they propagate a greater distance. Although ultrasound imaging can image deep in biological tissues with a high spatial resolution, its acoustic contrast is incapable of providing certain physiological parameters. In PAI, there is no restriction for photons, thus optical excitation can travel far beyond the diffusion limit and still generate acoustic waves. The sensitivity of PAI in deep tissues is orders of magnitude higher than that of pure optical imaging modalities; the highest penetration depth reported in PAI is ~12 cm. PAI is an ideal modality for measuring or monitoring tissue physiological parameters by imaging the concentration of tissue chromophores, which are changed during the course of a disease. PAI has been evaluated in preclinical and recently in clinical applications for disease detection and monitoring purposes. For instance, it has been used to study human skin abnormalities, brain disease detection, human breast tumor detection, retina disease diagnosis, and atherosclerosis evaluation of vessel walls.
PAI can be implemented in tomography or microscopy configurations with variations in system design. Among all configurations and variations of PAI, linear-array based PAI is one of the most commonly used due to its straightforward setup, easy use, and simple clinical translatability. In addition, since linear-array ultrasound imaging systems have been well-established as clinical tools, slightly modifying them to develop more capable tools, i.e., US/PA imaging systems, is not far from reality.
To learn more read through our review articles:
1. A. Fatima et al., “Review of cost reduction methods in photoacoustic computed tomography”, Photoacoustics 15, 100137 (2019)
2. R. Manwar et al., “Overview of Ultrasound Detection Technologies for Photoacoustic Imaging”, Micromachines 11 (7), 692 (2020)
What is OPTICAL COHERENCE TOMOGRAPHY and how it works?
Optical coherence tomography (OCT) is an optical imaging modality that uses low-coherence light to produce high-resolution volumetric images of the microstructures in materials and biological samples in real-time. OCT images are cross-section morphology maps of the tissue. Depending on the configuration of the OCT, the image resolution is between 0.5 to 15 µm which is one to two orders of magnitude higher than conventional ultrasound; the penetration depth, which is limited by optical attenuation from tissue scattering and absorption, is around 2 mm in biological tissues; and the lateral scanning range can be between 1 and 10 mm. OCT imaging can be performed in situ and in real time. OCT was initially applied for imaging in the eye. Recently, OCT’s application in a wide range of medical specialties such as dermatology, gastroenterology, urology, gynecology, surgery, neurosurgery, developmental biology and rheumatology has been successfully demonstrated. OCT is of interest because it allows repeated imaging of the sample without any need for sample preparation. We explain the working principle of OCT in a time-domain OCT system setup (see Figure 3).
Figure 3 shows a schematic diagram of a time-domain OCT system. The beam splitter splits the light from the broadband source into two paths: (i) the reference arm, and (ii) sample arm. There is a mirror in the reference arm that is used for axial scanning within the sample. In the sample arm, the light is focused into a sample. The light reflected back from the reference mirror and the backscattered light from the sample are combined in the beam splitter and detected by the photodetector. With the OCT broadband optical source (i.e., with a wide range of wavelengths), a constructive interference signal is generated only when the reflected light from the reference and sample arms have travelled the same optical distance.
The axial scanning within the tissue is equivalent to the movement of the mirror, making a longer or shorter optical path. Transverse scanning of the sample (for volumetric imaging) is achieved through the rotation of a sample arm galvonometer mirror.
To learn more read through our review articles:
1. S. O'Leary et al., “OCT image atlas of healthy skin on sun‐exposed areas”, Skin Research & Technology 24 (4), 570-586 (2018)
2. S. Adabi et al, “An overview of methods to mitigate artifacts in optical coherence tomography imaging of the skin” Skin Research and Technology 24 (2), 265-273 (2018)