Toward Characterization of Diseased Vascular Structures Using Noncontact Photoacoustic and Laser-Ultrasound Imaging: A Phantom Study

Visualizing and characterizing atherosclerotic plaques is important in determining the vulnerability of a plaque to rupture. To evaluate rupture risk, several compositional factors should be evaluated, including inflammation, the presence and size of lipid pools, thickness of the fibrous cap, and calcification. Currently, a need exists for an imaging modality that can detect each of these factors in a safe, noninvasive manner with high resolution and contrast at clinically relevant depths. Photoacoustic imaging is a growing field that has the potential to improve plaque diagnosis. Spectroscopic methods have shown promise toward detection of constituents of plaque with unique optical absorption spectra, such as lipids; however, detection of molecules with indistinct spectra, such as calcium, is not readily achieved using this approach. The acoustic properties of calcium, in contrast, are different than soft tissue, which causes calcification to scatter acoustic waves. In this work, a method of evaluating both spectroscopic and acoustic properties of vascular structures is presented by combining photoacoustic (PA) and laser-ultrasound (LU) techniques. These methods could inform treatment decisions and improve patient outcomes by detecting the smallest plaque and calcification deposits in the early stages of disease. When the disease has progressed to a later stage, such that surgical intervention is required, PA and LU imaging could inform surgical decisions to ensure that appropriate precautions are taken and the least-risk procedure is chosen. Experiments on tissue phantoms were conducted with healthy and diseased artery surrogates embedded. All imaging was accomplished using noncontact, noninvasive PA and LU imaging. Through implementation of geophysical image processing techniques, improved image resolution and a method of comparing contrast among optical absorbers and acoustic scatterers was demonstrated. Absorbing structures on the order of 1.5 mm were clearly identified, and acoustic scattering by stiff structures with a wall thickness of 233.5 µm was detected at a depth of 18 mm. The results demonstrate that dual photoacoustic and laser-ultrasound imaging has the potential to characterize multiple constituents of atherosclerotic plaque in a safe, noninvasive manner