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Computed imaging: how physics connects external measurements to internal structure

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Date and time Oct 02, 2008
from 04:00 PM to 05:00 PM
Location McLennan Physics (MP) 102
Host Daniel James

Paul Scott Carney

Abstract:

isam_vs_histology_colloq.jpgMethods of computed imaging have historically provided new levels of insight and utility when coupled with established instrumentation.  Examples include the growth of X-ray projections into  modern computed tomography (CT), nuclear magnetic resonance spectroscopy into magnetic resonance imaging (MRI), seismography into seismographic tomography, and radar ranging into synthetic aperture radar imaging (SAR).  In this talk, I will discuss the role of physics in these problems and will then address two recently emergent inverse problems in modern microscopy in near-field microscopy and optical coherence tomography.

Near-field optics provides a means to observe the electromagnetic field intensity in close proximity to a scattering or radiating sample.  Modalities such as near-field scanning optical microscopy (NSOM) and photon scanning tunneling microscopy (PSTM) accomplish these measurements by placing a small probe close to the object (in the "near-zone") and then precisely controlling the position.  There are a number of problems associated with the interpretation of near-field images.  When the probe is slightly displaced from the surface of the object, the image quality degrades dramatically.  The quantitative connection between the measurements and the optical properties of the sample is unknown.  To resolve these problems it is desirable to solve the inverse scattering problem (ISP) for near-field optics.  I will present the theoretical analysis of the near-field ISP and experimental realization for thin samples.

Optical coherence tomography (OCT) has provided an alternative to physical sectioning and histology that allows for imaging of living samples and even in vivo examination of cell structure and dynamics.  Applications range from monitoring the development of engineered tissues to the diagnosis of malignancies.  The sectional imaging of OCT is achieved by direct visualization of raw data obtained in focused optical range finding.  As a result, there is, in the OCT community, a widely held belief that there exists a trade-off between transverse resolution and the thickness of the volume that may be imaged with a fixed focal plane.  The extreme manifestation of this effect may be seen in optical coherence microscopy (OCM) where a single plane is imaged using a highly focused beam to achieve micron scale resolution, but no sectioning is possible because of the defocus away from this plane.  I will show that solution of the inverse scattering problem eliminates the supposed trade-off between resolution and depth of imaging is eliminated.   I will present the theoretical analysis, numerical simulations and experimental results for samples including a tadpole, a human tumor, and a titanium dioxide particle suspension.

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