Holographic Imaging of Tissue Elastic Properties
Changes in tissue elasticity ("stiffness") can serve as an indicator of disease. For example, tumors in breast cancer have an elasticity 3-10x greater than normal, healthy tissue. Elastography is the non-invasive method by which the elastic properties of soft tissue are measured and classified, and used to detect the presence of tumors.
In the optical coherence imaging laboratory, we have developed an Image Plane Digital Holography system for measurement of the elastic properties of soft tissue. As shown in the figure 1 below, a piezo-electric transducer is used to generate surface acoustic waves (SAWs) on breast tissue phantoms. A Mach-Zehnder interferometer, in conjunction with a high speed CMOS camera, is used to produce and capture digital interferograms (i.e. holograms) of these surface waves. A reconstruction algorithm is then applied to the digital holograms to extract the SAWs (Figure 2), and their velocities, which depend on the elastic modulus of the sample, are determined.
Figure 1. Digital holography setup for recording of surface acoustic waves in soft-tissue samples.
Furthermore, we have shown that the penetration depth of the SAWs is approximately one wavelength. Thus in samples composed of tissues with different elastic moduli at different depths, lower frequency SAWs have a speed closer to that of the deeper lying tissue, and vice versa.
Figure 2. (a) A Digital Hologram recorded by the camera (b) The corresponding SAW phase image generated by a reconstruction algorithm.
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