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Optical coherence tomography (OCT) and photoacoustic microscopy (PAM) are two highly useful modalities that produce microscopic, three-dimensional images noninvasively. And pairing the two provides even more comprehensive information about biological tissues.
The current strategy for doing so is to use two different light sources. But using a single light source would enable simultaneous PAM and OCT imaging—and thus correlative study of both optical scattering and absorption. Plus, the risk of light damage would be minimized by decreasing light exposure, and the resulting images would be intrinsically registered in lateral directions.
|In this experimental system for free-space optical coherence photoacoustic microscopy (OC-PAM), a dye laser provides the photons for both imaging modes.|
Now researchers at Florida International University (Miami) and Northwestern University (Evanston, IL) have realized that vision through application of a pulsed broadband light source. They call their concept optical coherence photoacoustic microscopy (OC-PAM).1 In their experimental system, an Nd:YAG laser pumped a broadband dye laser (center wavelength 580 nm, bandwidth 20 nm), which was coupled into a Michelson interferometer to form the framework for OCT. In the sample arm of the interferometer, the light was scanned and focused onto the sample and underwent scattering and absorption. The back-scattered photons from the sample were combined with the light reflected in the reference arm and detected by a spectrometer, accomplishing OCT imaging in the spectral domain. In the meantime, photoacoustic waves generated by the absorbed photons were detected by an unfocused needle ultrasonic transducer, accomplishing PAM imaging. With this setup, each laser pulse can simultaneously generate an OCT A-scan (a one-dimensional scan along the path of the beam) and a PAM A-scan.
Experiments demonstrated that the contrasts in the two modalities are complementary. But while the scientists achieved only about half the imaging depth of standard near-infrared (NIR) OCT, and only signals generated from blood vessels are observable in the PAM images, they think a more stable pulsed broadband NIR source will provide better image quality and deeper imaging depth. And a reduction in noise would enable one more potential advantage: Because the pulse width of OC-PAM is in the nanosecond range, it could allow imaging of fast dynamic processes.
1. X. Zhang, H. F. Zhang, and S. Jiao, J. Biomed. Opt., 17, 3, 030502 (2012).