FDA Scholar Program: Blood-Mimicking Phantoms for Assessing Oximetry Performance of Photoacoustic Imaging Systems

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NON-TECHNICAL SUMMARYUltrasound is a powerful tool to image diseases including cancer, orthopedic disorders, and heart function. One limitation of ultrasound is that it suffers from low contrast (contrast is the difference in signal intensity between the region of interest and the background tissue). Therefore, there is a large body of research into a special kind of ultrasound known as photoacoustic imaging. Photoacoustic imaging uses light to generate sound only in the area of interest—this increases the contrast. Unfortunately, photoacoustic ultrasound is not yet approved for widespread use in people. This might be partially due to a lack of devices and methods to validate and standardize the novel imaging equipment needed for photoacoustic imaging. Therefore, this work will create specialized plastic objects with optical and acoustic properties that mimic human tissue with different amounts of tissue oxygenation. These objects can be used to calibrate and standardize photoacoustic imaging equipment. This proposal combines expertise from academia and the Food and Drug Administration to identify materials that have similar optical and acoustic properties as human tissue. We will then add dyes that have absorption spectra similar to hemoglobin. The resulting test objects will improve knowledge of how to best create photoacoustic imaging instrumentation that measures tissue oxygenation and might also streamline regulatory approval of this equipment. In turn, this will increase patient access to this important imaging technique to ultimately advance the health and quality of life of US taxpayers. TECHNICAL SUMMARYPhotoacoustic imaging provides deep tissue imaging similar to ultrasound but with enhanced optical contrast and additional functional and molecular imaging capabilities. However, no standardized performance test methods or phantoms exist for photoacoustic imaging system evaluation unlike mature techniques such as computed tomography. The fundamental limitation—and scientific problem to be studied here—is a lack of materials to simultaneously simulate tissue properties over a broad range of optical wavelengths and acoustic frequencies. This leaves investigators, instrument manufacturers, and regulatory agencies without clear strategies to evaluate device safety and effectiveness. Our prior work with the Food and Drug Administration (FDA) built stable, biologically relevant imaging phantoms with well-characterized optical absorption/scattering coefficients, acoustic impedance, etc. that broadly simulate tissue over a wide range of optical wavelengths and acoustic frequencies. We will now integrate chromophores to simulate tissue oxygenation (SO2) over a range of oxygen saturation/perfusion values. Objective 1 of this research will develop phantoms that simulate blood oxygen-dependent photoacoustic spectra. Combinations of dyes will be selected to develop tunable formulations that reproduce blood-like multispectral photoacoustic signals at sets of discrete optical wavelengths commonly used for photoacoustic oximetry. Photoacoustic-derived SO2 measurements will be compared against ground truth values as well as against photoacoustic measurements in bovine blood with variable SO2. Ground truth SO2 of bovine blood will be measured by oximetry. Objective 2 will use the phantoms to establish quantitative oximetry test methods. These methods will be performed on three different photoacoustic systems located at UCSD and FDA. Phantoms with different background optical properties and containing blood-mimicking inclusions at different depths and mimicked SO2 levels will be used for parametric study of photoacoustic device oximetry performance. Improvements in the SO2 measurement accuracy of our three photoacoustic systems using various fluence correction algorithms will be quantified to determine device sensitivity to tissue properties and morphology.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.