Page 19 - Volume 8, Issue 4 - Winter 2012

PHOTOACOUSTIC IMAGING FOR MEDICAL DIAGNOSTICS
Carolyn L. Bayer
Department of Biomedical Engineering, The University of Texas at Austin Austin, Texas 78712
Geoffrey P. Luke
Department of Electrical and Computer Engineering, The University of Texas at Austin Austin, Texas 78712
Stanislav Y. Emelianov
Departments of Biomedical and Electrical and Computer Engineering, The University of Texas at Austin Austin, Texas 78712
“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.”
Introduction
Photoacoustic imaging has the
potential to provide real-time,
non-invasive diagnosis of numer-
ous prevalent diseases, due to the tech-
nology’s unique ability to visualize
molecular changes deep within living
tissue with spatial resolution compara-
ble to ultrasound. Photoacoustic imag-
ing is a hybrid imaging technique that
combines the contrast capabilities and
spectral sensitivities of optical imaging
with the resolution and tissue penetra-
tion capabilities of ultrasound. During
the photoacoustic imaging process,
materials absorb light energy, and con-
vert the light to heat via non-radiative
relaxation. When materials heat, they
expand in size due to their thermoelas-
tic properties, which generates a pres-
sure 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 sum- mer thunderstorm—lightning rapidly heats the air, result- ing 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 “photoa- coustic” 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 absorb- ing 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 pho- toacoustic 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 pho-
toacoustic effect is the generation of medical images of biological chro- mophores typically present in tissue, which can absorb light energy resulting in the generation of photoacoustic tran- sients. 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 tis- sue depth. Photoacoustic medical imag- ing was first proposed in the mid- 1990s,2,3 and initial reports of using the photoacoustic effect to image live ani-
mals 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 vascula- ture and function,7-9 cardiovascular,10 and tissue engineering scaffolds,11,12 prompting translational advances in clinical
13
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 influ- ence the disease progression, and therefore the prognosis of the patient. Functional imaging capabilities are required to provide physiological information, while biochemical infor- mation can be provided by molecular imaging. In compari- son to ultrasound, photoacoustic imaging provides improved capabilities for functional and molecular imaging. For exam- ple, the blood oxygen saturation, an important functional property relevant to many disease processes, can be assessed
7
using photoacoustics. Photoacoustic imaging can also pro-
vide molecular information through the use of a probe or
photoacoustic imaging.
While existing medical imaging methods, including
Photoacoustic Imaging for Medical Diagnostics 15