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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.