Angle-resolved low-coherence interferometry (a/LCI) is an emerging^[when?] biomedical imaging technology which uses the properties of scattered light to measure the average size of cell structures, including cell nuclei. The technology shows promise as a clinical tool for in situ detection of dysplastic, or precancerous tissue.

A/LCI combines low-coherence interferometry with angle-resolved scattering to solve the inverse problem of determining scatterer geometry based on far field diffraction patterns. Similar to optical coherence domain reflectometry (OCDR) and optical coherence tomography (OCT), a/LCI uses a broadband light source in an interferometry scheme in order to achieve optical sectioning with a depth resolution set by the coherence length of the source. Angle-resolved scattering measurements capture light as a function of the scattering angle, and invert the angles to deduce the average size of the scattering objects via a computational light scattering model such as Mie theory, which predicts angles based on the size of the scattering sphere. Combining these techniques allows construction of a system that can measure average scatter size at various depths within a tissue sample.

At present the most significant medical application of the technology is determining the state of tissue health based on measurements of average cell nuclei size. It has been found that as tissue changes from normal to cancerous, the average cell nuclei size increases.^[1] Several recent studies^[2] have shown that via cell nuclei measurements, a/LCI can detect the presence of low- and high-grade dysplasia with 91% sensitivity and distinguish between normal and dysplastic with 97% specificity.

Since 2000, light scattering systems have been used for biomedical applications such as the study of cellular morphology^[3] as well as the diagnosis of dysplasia.^[4] Variations in scattering distributions as a function of angle or wavelength have been used to deduce information regarding the size of cells and subcellular objects such as nuclei and organelles. These size measurements can then be used diagnostically to detect tissue changes—including neoplastic changes (those leading to cancer).

Light scattering spectroscopy has been used to detect dysplasia in the colon, bladder, cervix, and esophagus of human patients.^[2] Light scattering has also been used to detect Barrett's esophagus, a metaplastic condition with a high probability of leading to dysplasia.^[5]

However, in contrast with a/LCI, these techniques all rely on total intensity based measurements, which lack the ability to provide results as a function of depth in the tissue.

The Fourier-domain version of the a/LCI system uses a superluminescent diode (SLD) with a fiber-coupled output as the light source. A fiber splitter separates the signal path at 90% intensity and the reference path at 10%.

The light from the SLD passes through an optical isolator and subsequently a polarization controller. It has been shown that control of light polarization is important for maximizing optical signal and comparing angular scattering with the Mie scattering model.^[10] A polarization-maintaining fiber is used to carry the illumination light to the sample. A second polarization controller is similarly used to control the polarization of the light passing through the reference path.

The output of the fiber on the right is collimated using lens L1 and illuminates the tissue. But because the delivery fiber is offset from the optical axis of the lens, the beam is delivered to the sample at an oblique angle. Backscattered light is then collimated by the same lens and collected by the fiber bundle. The fibers are one focal length from the lens, and the sample is one focal length on the other side. This configuration captures light from the maximum range of angles and minimizes light noise due to specular reflections.

At the distal end of the fiber bundle, light from each fiber is imaged onto the spectrometer. Light from the sample and reference arms are mixed by a beamsplitting cube (BS), and are incident on the entrance slit of an imaging spectrometer. Data from the imaging spectrometer are transferred to a computer via USB interface for signal processing and display of results. The computer also provides control of the imaging spectrometer.

The a/LCI system has recently been enhanced to allow operation in a clinical setting with the addition of a handheld wand.^[11] By carefully controlling the polarization in the delivery fiber, using polarization-maintaining fibers and inline polarizers, the new system allows manipulation of the handheld wand without signal degradation due to birefringence effects. In addition, the new system employed an anti-reflection coated ball lens in the probe tip, which reduces reflections that otherwise limit the depth range of the system.

The portable system uses a 2 ft by 2 ft optical breadboard as the base, with the source, fiber optic components, lens, beamsplitter, and imaging spectrometer mounted to the breadboard. An aluminum cover protects the optics. A fiber probe with a handheld probe enables easy access to tissue samples for testing. On the left side sits a white sample platform, where tissue is placed for testing. The handheld probe is used by the operator to select specific sites on the tissue from which a/LCI readings are acquired.

• Applied spectroscopy
• Coherence length
• Fourier transform
• Optical interferometry
• Optical coherence tomography