Photoacoustic Imaging for Medical Diagnostics

Abstract

Photoacoustic imaging has the potential to provide real-time, non-invasive diagnosis of numerous prevalent diseases, due to the technology’s unique ability to visualize molecular changes deep within living tissue with spatial resolution comparable to ultrasound. Photoacoustic imaging is a hybrid imaging technique that combines the contrast capabilities and spectral sensitivities of optical imaging with the resolution and tissue penetration capabilities of ultrasound. During the photoacoustic imaging process, materials absorb light energy, and convert the light to heat via non-radiative relaxation. When materials heat, they expand in size due to their thermoelastic properties, which generates a pressure wave. These pressure waves can propagate through the surrounding environment to be detected at the surface. This effect is familiar to everyone who has experienced a summer thunderstorm—lightning rapidly heats the air, resulting in the air expanding and generating audible thunder. In general, the heating which induces the expansion of the material (e.g., the thermoacoustic effect) could be caused by many forms of energy transfer, but the term “photoacoustic” specifies the conversion of light into heat, resulting in the generation of characteristic sound waves. The photoacoustic effect was first discovered by Alexander Graham Bell in 1880.1 His experiments deduced that an intermittent bright light could heat optically absorbing materials, causing expansion of the material in a way that generated audible vibrational waves. Bell demonstrated that darker fibers produced louder sounds than lighter fibers, a principle which is consistent with the general photoacoustic relationship in use today—the amplitude of the generated photoacoustic signal is proportional to the amount of absorbed light. Bell also showed, by separating white light with a prism, certain color combinations of light and fibers could generate a louder sound. Today, multiwavelength photoacoustic imaging uses this same principle, changing the wavelength of the light and correlating the amplitude of the photoacoustic response to the absorption spectra of the materials being imaged. A modern application of the photoacoustic effect is the generation of medical images of biological chromophores typically present in tissue, which can absorb light energy resulting in the generation of photoacoustic transients. The photoacoustic pressure waves can be received by ultrasound transducers at the external surface of the tissue, making photoacoustic imaging a non-invasive, non-ionizing medical imaging method capable of resolution similar to ultrasound, at significant tissue depth. Photoacoustic medical imaging was first proposed in the mid-1990s,2,3 and initial reports of using the photoacoustic effect to image live animals were published nine years later.4 Today, many in vivo demonstrations of photoacoustic imaging of biomedical applications relevant to medical diagnostics exist, including cancer,5,6 brain vasculature and function,7–9 cardiovascular,10 and tissue engineering scaffolds,11,12 prompting translational advances in clinical photoacoustic imaging.13 While existing medical imaging methods, including ultrasound, are capable of producing remarkable images of what lies beneath our skin, most of these imaging methods provide contrast between anatomical features within tissue—for example, the difference in acoustic impedance between soft tissue and a tumor provide contrast within an ultrasound image. Though the anatomy is critical to understanding the image, in many diseases the anatomy alone cannot be used to indicate a particular diagnosis conclusively. Instead, the physiological and biochemical properties of the system influence the disease progression, and therefore the prognosis of the patient. Functional imaging capabilities are required to provide physiological information, while biochemical information can be provided by molecular imaging. In comparison to ultrasound, photoacoustic imaging provides improved capabilities for functional and molecular imaging. For example, the blood oxygen saturation, an important functional property relevant to many disease processes, can be assessed using photoacoustics.7 Photoacoustic imaging can also provide molecular information through the use of a probe or tracer, which can be used to generate the needed contrast to produce an image.14 Because of the potential to perform real-time, non-invasive in vivo functional and molecular imaging, photoacoustic imaging is increasingly being applied as both a clinical and preclinical method aimed at improving medical diagnostics.