1. Point Detector Aperture
Photo-Thermal Coherent Confocal Microscope Mark Andrews, Sean Sullivan, Matthew Bouchard, Alex Nieva, Purnima Ratilal, and Charles A. DiMarzio Optical Science Laboratory, Northeastern University, Boston, MA. "This work was supported in part by CenSSIS, the Center for Subsurface Sensing and Imaging Systems, under the Engineering Research Centers Program of the National Science Foundation (Award Number EEC )."
ABSTRACT
Confocal microscopy has been shown to be useful in imaging skin slightly below the junction of the dermis and epidermis. However, the depth of imaging is a significant limitation. We present a novel concept designed both to improve the depth of penetration and to increase the information content of images obtained with a reflectance confocal microscope. Using an approach similar to optoacoustics, we plan to explore the use of laser heating to generate tissue expansion, which will be measured by the microscope. The microscope will incorporate a pulsed heating laser along the same optical path as the imaging laser in order to generate localized heating. This will result in periodic thermal expansion and contraction at the focus. Optical Quadrature detection is used to measure the phase of the scattered light, and Doppler techniques will be employed to quantify the thermal expansion. For the purposes of imaging, two lasers of different wavelengths will be needed to resolve phase ambiguities in the expansion measurement. The motion resulting from thermal expansion will provide additional discrimination against multiply scattered light. It will also provide a measurement of a mechanical parameter, the coefficient of thermal expansion, which may aid in the characterization of different types of tissue.
CONCEPT OF PHOTOTHERMAL MICROSCOPE
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A heating laser directed along same path of imaging laser creates thermal expansion at the focus in the sample. (shown on left)
Heating laser is pulsed to create periodic expansion and contraction
A computer simulation of this is shown on the right, where a 1s pulse is seen as an expansion to either side of the focus
Thermal expansion will be measured with a photothermal microscope. 3
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PHOTOTHERMAL COHERENT CONFOCAL MICROSCOPE LAYOUT
SIGNIFICANCE AND RELATION TO CenSSIS
Confocal
Validating TestBEDs R1 R2
Fundamental Science R3 S1 S4 S5 S3 S2
Bio-Med
Enviro- Civil
Optical Quadrature
Photo-thermal
GOALS
Improve upon performance of reflectance confocal microscope:
Filter out the out of focus scattered light
Increase depth of penetration
Increase information content of image by quantifying the coefficient of thermal expansion
Better skin images
The Photothermal Coherent Confocal Microscope will provide us with images at increased depths for use in skin imaging.
REFERENCES
[1] Nieva, Alex, Matthew Bouchard, Charles A. DiMarzio, “Opto-acoustic Signal Detection with a Coherent Confocal Microscope Setup,” Proc. SPIE, Vol Presented at Photonics West in San Jose, CA, Jan 05. Publication expected, July 05.
[2] Nieva, Luis A., and Charles A. DiMarzio, Ultrasound Assisted Confocal Microscopy, NU Disclosure NU--667XX. April 2004.
[3] D. O. Hogenboom, and C. A. DiMarzio, “Quadrature Detection of a Doppler Signal”, Applied Optics, 37(13), page 2569, 1998.
[4] J. B. Pawley, ed., “Handbook of Biological Confocal Microscopy”, 3rd ed. (Plenum, New York, 1996).
[5] A. F. Fercher, W. Drexler, C. K. Hitzenberger, and T. Lasser, “Optical coherence tomography-principles and applications”, Rep. Prog. Phys. 66 (2003)
Point Source of Light .
Beamsplitter
Objective Lens
Focal plane
The above microscope layout incorporates the heating laser (bottom right) with two imaging lasers (bottom left), using dichroic mirrors. The three combined beams are focused onto the sample, shown in pink.
The back-scattered light from the sample returns along the same path until it is separated from the incoming beam at the polarizing beam splitter (Custom PBS, center), and directed towards the Q and I detectors (top).
Using Optical Quadrature Detection, the phase information can be extracted by mixing the reference beam with the backscattered light at the polarizing beam splitter, and then detecting each polarization component at the I and Q detectors. This is done for both imaging wavelengths.
Using Doppler techniques, the thermal expansion at the sample can then be measured using the phase changes for both imaging wavelengths.
Point Detector Aperture
Out of focus scatter
Above, some out of focus light is scattered back along the same incident path, ends up going through the pinhole and to the detector. This contributes to signal clutter.
Confocal Microscopy provides resolutions down to 1μm but has limited depth penetration due to out of focus scattering [2]. Beyond 100m depth, signal to clutter ratio becomes too small.
Photothermal Microscope provides a solution to filter this out
PI CONTACT INFORMATION
Prof. Charles A. DiMarzio
Northeastern University
Phone:
ACKNOWLEDGEMENTS
Thanks to Prof. Ronald Roy, Prof. Todd Murray and Lei Sui for providing the phantoms and equipment for the ultrasonic transducers.