15/19/2011, Reid McCargar

Photo-acoustic phase conjugation for biomedical imaging

Reid McCargar, Research Assistant

25/19/2011, Reid McCargar

Outline

• Brief overview – current diagnostic imaging technologies and the motivation for new technologies

• Wavefront aberration by scattering

• Dynamic holography and phase conjugation

• The acousto-optic effect

• Synthetic guide star acousto-optic tomography

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Project relevance

Near-IR illumination used to image vasculature Visible light is of limited use in

modern diagnostic imaging mostly due to high scattering

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Current imaging technologies

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Why change anything?

• Not all features of medical interest have adequate contrast when viewed with currently-available technologies.

• Many of the imaging technologies involve ionizing radiation, require chemical contrast agents, or have prohibitive cost.

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Why not use optical wavelengths?

With 2-way propagation, imaging may be possible down to a centimeter or so.

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Wavefront aberration by scattering

On passing through a turbulent, or highly-scattering medium, a coherent wavefront becomes tattered.

If the shape of the wavefront can be duplicated by an appropriately-shaped mirror, the wavefront can be refocused in backward travel.

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Wavefront sensing

• Problem with recreating the shape of a tattered wave: optical detectors are phase-insensitive.

Locally regarded as plane-waves, a wavefront shape can be detected by focal position shifts

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Implementation

GS

CCDWS

CPU

Historically used for sharpening astronomical images, but can you imagine the implementation in diagnostic imaging?

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Not good – lasers inside people

GS

CC

DW

S

CPU

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Dynamic holography

E1

E2 = k E1 *E4 = k E3 *

E3

Material with transmittance changes due to interference of E1 and E3

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Acousto-optic frequency shifting

Ultrasound transducer

Transmitted zero-order-beam

1st -order diffracted beam, up-shifted in frequency by facoustic

Acoustic wave propagation

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Synthetic guide star

Ultrasound transducer

LaserPhotodetector identifies frequency-shifted photons as having passed through ultrasound focus

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Putting the pieces together – Xu, et. al

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A less bewildering look

AO AO

US

CCD

R (f0) R* (f0 )

fs=f0-F

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Results

Tissue analog imaged to 5 mm with better resolution than ultrasound focus

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Conclusions

• It is possible to image tissue with visible light to perhaps several centimeters.

• Applications of this new technology are mostly untested.

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References

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Questions?

Thank you