Imaging with diffuse photon density waves

Maureen A. O'Leary and Arjun G. Yodh

 

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Abstract

Diffusing photons can be used to probe and characterize optically thick turbid samples such as paints, foams and human tissue. In this work, we present experiments which illustrate the properties of diffuse photon density waves. Our observations demonstrate the manipulation of these waves by adjustment of the photon diffusion coeficients of adjacent media. The waves are imaged, and are shown to obey simple relations such as Snell's Law.

Next we present images of heterogeneous turbid media derived from measurements of diffuse photon density waves. These images are the first experimental reconstructions based on frequency-domain optical tomography. We demonstrate images of both absorbing and scattering homogeneities, and show that this method is sensitive to the optical properties of a heterogeneity. The algorithm employs a differential measurement scheme which reduces the effect of errors resulting from incorrect estimations of the background optical properties.

In addition to imaging absorption and scattering changes, we are also able to image the lifetime and concentration profile of heterogeneous uorescent media.

Summary

We have demonstrated that diffusive light can be used to image absorption, scattering and fluorescence lifetime and concentration in thick, turbid media. We hope we have convinced the reader that this work has important medical applications. There is a great deal of work to be done if DPDW imaging is to become an accepted medical tool. First and foremost, we need many more studies which accurately measure the optical changes which accompany changes in the physiological state of tissues. The intrinsic hetereogeneity of the body makes it dificult to measure in vivo the absorption and scattering coeficients. When tissue samples are studied in vitro, the blood and oxygen supply is cut off, thus changing the tissue environment. It is not clear whether or not in vitro measurements accurately re ect the optical properties of the sample in vivo.

We believe that DPDW imaging should be used to complement other imaging modalities. For example, a DPDW probe could be easily attached to an ultrasound, x-ray, or MRI imaging device as we saw in section 4.12. When the data from these two probes are used simultaneously. we can derive accurate structural information as well as quantitative measurements of the optical properties.

DPDW imaging can also be used for low resolution breast tumor screening. There are several academic and industrial research groups performing clinical evaluations of optical mammography systems [13, 81, 82]. These studies are particularly exciting because optical mammography is inexpensive, and has no known adverse side effects. The preliminary results of these studies are encouraging, but a great deal of work still needs to be done.