PHOTOACOUSTIC TOMOGRAPHY Ultrasonically Breaking through the Optical Diffusion Limit

[seminar class]

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Motivation for In Vivo Optical Imaging (Partial List)
• Safety — Non-ionizing radiation: photon energy is ~2 eV
• Physics — Molecular structure and composition of tissue
• Optics — High intrinsic contrast. E.g., absorption probes
 Oxy-hemoglobin & deoxy-hemoglobin
 Melanin
 Lipid
 Water
 DNA & RNA
• Physiology — Endogenous contrast for functional imaging
 Concentration of hemoglobin (angiogenesis)
 Oxygen saturation of hemoglobin (hyper-metabolism)
 Cell nuclei
 Blood flow (Doppler)
• Biology — Exogenous contrast for molecular imaging
http://oilab.seas.wustl.edu -- 4
o ogy oge ous co t ast o o ecu a ag g
 Biomarkers (Integrin, VEGF, HER2, etc.)
 Reporter genes
Fundamental Challenges in High-resolution
Optical Imaging: Diffraction and Diffusion
• Diffraction (wave phenomenon)
 Limits the spatial resolution of ballistic imaging (planar, confocal,
& two-photon microscopy, optical-coherence tomography).
 Has been overcome for super-resolution imaging (PALM/STORM).
Photoacoustic effect:
1. Light absorption
2. Temperature rise
[A G B ll O th d ti d
3. Thermoelastic expansion
4. Acoustic emission