Introduction
Diagnostic methods are essential in medical science to determine the underlying pathology. Over the years, different diagnostic methods have been introduced to study tissue characteristics. One such method is optical elastography.
Elastography is a non-invasive imaging method to study the biomechanical behavior of tissues. Optical elastography techniques use optical sources to evaluate tissue characteristics. This method uses two-and three-dimensional images of the tissues at a depth of 1 to 2 mm to map and evaluate mechanical properties.
What Is the History?
The word elastography was first used in 1979. Subsequently, ultrasonography and magnetic resonance imaging was developed as elastography method. But these techniques are useful for images at the macroscopic level. After that atomic force microscopy method was developed to study microscopic images, this method uses a sub-nano Newton force on the cell membrane to produce deformity. But this method is only useful for cell mechanics and its biology. Developing a technique to bridge the gap between macroscopic and sub-microscopic levels was necessary.
Elastographic techniques based on optic principles are useful to meet these demands. These techniques are laser speckle imaging, multiphoton microscopy, confocal Brillouin microscopy, and optical coherence tomography (OCT).
Among these four techniques, laser speckle imaging and optical coherence tomography are widely used in medicine to study different human body tissues.
In the 1990s, Fercher and Briers first demonstrated the application of laser speckle contrast imaging (LSCI). On the other hand, the concept of optical coherence tomography was first proposed by Huang et al. in 1991 and developed by Schmitt in 1998.
What Is Laser Speckle Contrast Imaging?
In this method, laser light is focused on the targeted biological tissues. The presence of interference in the tissue causes backscattering of the light and generates speckle patterns. This speckle pattern generates a bright and dark spot caused by different optical paths of the scattering light. Static and dynamic speckles are two types of images generated in this method. Static speckle image is formed by nonmoving objects, whereas dynamic speckle is formed by moving objects, and images change over time. But the moving objects cause blurring in the speckle pattern. The application of three methods eliminated this problem.
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Use of multi-exposure speckle imaging.
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Correction of velocity distribution by application of Lorentzian distribution.
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Application of adhesive opaque patch for correcting the movement of the artifacts.
What Is the Use of Laser Speckle Contrast Imaging?
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In Rheumatology, an assessment of perfusion is done in systemic sclerosis patients.
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Easy early detection and staging of arthritis can be done.
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Healing patterns and assessment of types of burns can be made using this method.
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Different vascular malformations, skin ulcers, and port wine stains can be evaluated using this technique. This method helps identify the blood perfusion rate, flow rate, and nature of the vascular pattern.
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It is extensively used in ophthalmology to assess various diseases such as glaucoma, retinopathy, and macular degeneration. Blue component argon or diode laser is a light source to study choroidal circulation retinal and optical nerve head.
What Is Optical Coherence Tomography?
In this method, in-depth cross-sectional analysis of the tissue is done through quasi-static compression by reflected light. This method is similar to the ultrasound method, but unlike ultrasound, this uses light as a source. The light source used in this method is superluminescent diodes, ultrafast lasers, and matured supercontinuum. The light originating from the source travels much faster than ultrasound and is reflected by the targeted organ. The delay in the reflected light is measured through an interferometer.
Different Types of Optical Coherence Tomography (OCT)
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In time-domain OCT (TD-OCT), the reference length was modulated for each depth scan, and the intensity of the combined light was recorded at the sensor, giving the reflectance profile of the sample. In this technique, two types of images can be generated one with the depth profile of the object, which is known as amplitude scan or A-scan, and another is B-scan which generates a cross-sectional image.
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Fourier domain OCT (FD-OCT, also frequency domain OCT) is the second-generation technology that uses low-coherence interferometry.
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Spectrometer-based FD-OCT ( SD-OCT) uses a broadband-based light source.
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Swept-source OCT (SS-OCT) is the most advanced version of the tomography method. In this technique, two light sources are used. One is broadband, and another is narrowband, and the light source repeatedly sweeps between narrowband and broadband.
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Apart from all these types, optical coherence tomography has been added to different types of other technologies.
OCT Angiography
Blood vessels show much more variation and backscattering than other tissues. For this reason, the B-scan of the area is taken repetitively, and a sophisticated mathematical model is used to eliminate variations and artifacts.
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OCT with visible light (Vis-OCT) - This method uses the visible light source of the wavelength range from 560 to 1310 nm.
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Polarization-sensitive OCT (PS-OCT) - This is one type of swept-source OCT where an additional change in the polarization standard of backscattered light is evaluated.
What Is the Use Of Optical Coherence Tomography?
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Corneal structure, characteristics, and pathology can be assessed. It is very helpful to access keratoconus.
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Spectral analysis of the corneal elastic wave by quantifying the wave phase helps assess corneal biomechanical properties.
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Assessment of vascular wall plaque formation can be evaluated by this method. OCT angiography can be used to understand neovascularizations, blood perfusion, and blood flow rate.
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Age-related muscle degradation and pathologic degradation of structure can be evaluated through spectrometer-based FD-OCT.
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Delayed choroidal vascular filling in diabetic retinopathy can be detected through spectrometer-based FD-OCT.
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Soft tissue Intraocular tumors such as choroidal melanomas and nevi can be detected through the measurement of tissue stiffness by this method.
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Tissue birefringence can be detected through polarization-sensitive OCT (PS-OCT).
What Is the Advantage?
There is a marked difference between optical coherence elastography (or optical elastography) and traditional elastography techniques.
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The spatial resolution of images is 1–10 μm in the case of optical elastography, which is much higher than ultrasonography and other elastography methods.
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The two-dimensional image accusation speed is much higher than conventional ultrasound elastography or magnetic resonance elastography.
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The optical elastography technique can displace the sub-nano level, making them more sensitive to microscopic tissue changes. But then, such accuracy is not observed in traditional elastography techniques.
Conclusion
Diagnostic modalities are an essential tool for clinical practice. Optical elastography methods diagnose tissue conditions, disease risk factors, and prognosis. In the future, more advancement is needed for further development of these technologies.
