IEEE Photonics Society

Boston Photonics Society Chapter

Boston Chapter of the IEEE Photonics Society

Imaging Workshop PDF

Wednesday, April 6, 13, 20, 28*, May 4, 2011, 7:00–9:30 PM (* Thursday April 28th)
Located at MIT Lincoln Laboratory – 244 Wood Street, Lexington, MA, 02420, USA

April 20, 2011
7 PM

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Photoacoustic Tomography: Ultrasonically Breaking through the Optical Diffusion Limit Slides

Prof. Lihong V. Wang, Washington University, St. Louis, MO


Prof. Lihong V. Wang, Washington University, St. Louis, MO

Abstract:  We develop photoacoustic imaging technologies for in vivo early-cancer detection and functional or molecular imaging by physically combining non-ionizing electromagnetic and ultrasonic waves.  Unlike ionizing x-ray radiation, non-ionizing electromagnetic waves—such as optical and radio waves—pose no health hazard and reveal new contrast mechanisms.  Unfortunately, electromagnetic waves in the non-ionizing spectral region do not penetrate biological tissue in straight paths as x-rays do.  Consequently, high-resolution tomography based on non-ionizing electromagnetic waves alone—such as confocal microscopy, two-photon microscopy, and optical coherence tomography—is limited to superficial imaging within approximately one optical transport mean free path (~1 mm in the skin) of the surface of scattering biological tissue.  Ultrasonic imaging, on the contrary, provides good image resolution but has strong speckle artifacts as well as poor contrast in early-stage tumors.  Ultrasound-mediated imaging modalities that combine electromagnetic and ultrasonic waves can synergistically overcome the above limitations.  The hybrid modalities provide relatively deep penetration at high ultrasonic resolution and yield speckle-free images with high electromagnetic contrast.

In photoacoustic computed tomography, a pulsed broad laser beam illuminates the biological tissue to generate a small but rapid temperature rise, which leads to emission of ultrasonic waves due to thermoelastic expansion. The short-wavelength pulsed ultrasonic waves are then detected by unfocused ultrasonic transducers. High-resolution tomographic images of optical contrast are then formed through image reconstruction. Endogenous optical contrast can be used to quantify the concentration of total hemoglobin, the oxygen saturation of hemoglobin, and the concentration of melanin. Melanoma and other tumors have been imaged in vivo. Exogenous optical contrast can be used to provide molecular imaging and reporter gene imaging.

In photoacoustic microscopy, a pulsed laser beam is focused into the biological tissue to generate ultrasonic waves, which are then detected with a focused ultrasonic transducer to form a depth resolved 1D image. Raster scanning yields 3D high-resolution tomographic images. Super-depths beyond the optical diffusion limit have been reached with high spatial resolution.

Thermoacoustic tomography is similar to photoacoustic tomography except that low-energy microwave pulses, instead of laser pulses, are used.  Although long-wavelength microwaves diffract rapidly, the short-wavelength microwave-induced ultrasonic waves provide high spatial resolution, which breaks through the microwave diffraction limit. Microwave contrast measures the concentrations of water and ions.

The annual conference on this topic has been doubling in size approximately every three years since 2003 and has become the largest in SPIE’s Photonics West as of 2009.

Recommended Reading:

Photoacoustic Imaging and Spectroscopy, L.V. Wang

Biomedical Optics: Principles and Imaging, L. V. Wang and H. I. Wu


Biography:  Prior to his arrival at Washington University in St. Louis in 2006, Professor Wang was the Royce E. Wisenbaker II Endowed Professor of Biomedical Engineering and Electrical Engineering at Texas A&M University. He also worked as a postdoctoral fellow and later as an assistant professor at the University of Texas MD Anderson Cancer Center from 1991-1996.

Professor Wang is chair of the International Biomedical Optics Society and has authored more than 200 peer-reviewed journal articles. He received the NIH FIRST award, National Science Foundation CAREER award, and Outstanding Young Scientist Award sponsored by Johnson & Johnson Medical and the Houston Society for Engineering in Medicine and Biology. He is a fellow of the AIMBE, IEEE, OSA, and SPIE. Professor Wang was appointed as the Editor-in-Chief of the Journal of Biomedical Optics.

Professor Wang authored one of the first textbooks in biophotonics entitled Biomedical Optics: Principles and Imaging.


For more information on the technical content of the workshop, contact either:
1) Farhad Hakimi (, Imaging Workshop Committee Co-Chair
2) William Nelson (, Imaging Workshop Committee Co-Chair
3) Reuel Swint (, Imaging Workshop Committee Co-Chair
4) Robert Stephenson (, Boston Photonics Society Chair