NON-TECHNICAL ABSTRACT: Ultrasound 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 known optical and acoustic properties suitable for calibrating and standardizing photoacoustic imaging equipment. This proposal combines expertise from academia and the Food and Drug Administration to identify materials that have optical and acoustic properties similar to human tissue. We will create test objects and methods that will be useful to instrument manufacturers and physicians. The resulting test objects will improve knowledge of how to best create photoacoustic imaging instrumentation 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 national health and quality of life.
Photoacoustic 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. This project with the Food and Drug Administration (FDA) builds 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. Objective 1 of this research develops phantoms with biologically relevant heterogeneous morphologies and light scattering artifacts during photoacoustic imaging. Phantom material optical and acoustic properties are rigorously characterized using spectrophotometry and acoustic pulse-transmission equipment available at FDA. The imaging phantoms contain multiple layers of background material with different optical and acoustic properties designed to mimic natural tissue (e.g. breast fat and glandular tissue or muscle and fat layers) as well as target inclusions. The inclusions have regular shapes (cylinders, spheres) of varying sizes as well as image-derived, tortuous vessel-mimicking structures. Objective 2 uses these phantoms to establish test methods that evaluate the impact of fluence artifacts on device performance in three different imaging systems. System image uniformity and out-of-plane sensitivity are evaluated during mechanical scanning of probes over realistic vessel-mimicking inclusions known to produce volumetric images. Sets of phantoms containing inclusions mimicking blood absorption at several oxygen saturation levels-but with different background optical properties and layer morphologies-serve in parametric studies of device performance in the face of spectral coloring artifacts. The measurement accuracy of our three photoacoustic imaging systems is quantified to determine device robustness using various fluence correction methods (diffusion theory, Monte Carlo simulations). The outcome is a well-validated tissue-mimicking phantom to support device developers and inform regulatory decision-making including use as a potential Food and Drug Administration Medical Device Development Tool.
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.
