Biomedical Optics

Eye Surgery: Excimer Laser Surgery and LASIK?

(The following is extracted from a commercial site, and is trying to sell the procedure. It is essentially accurate.)

Excimer Laser Surgery is an outpatient treatment that uses a cool beam of light to gently reshape the surface of the eye - the cornea - thereby improving vision. LASIK is the most advanced vision correction procedure available today. Doctors have been routinely treating refractive vision errors with incisions for over 25 years. Today's technologically advanced Excimer laser is now in its 2nd decade of use for treating nearsightedness, farsightedness and astigmatism. To date, medical doctors around the world have performed over 1.5 million procedures. Dr. Miller's own statistics show that 100% of his patients achieve "driving vision" or better without glasses.

 "LASIK" is an acronym for LASer In-situ Keratomileusis, which simply means, "to shape the cornea from within using a laser." The laser is guided by an advanced computer and is able to reshape the cornea precisely. Its "cool" or non-thermal light beam gently pulses to remove a microscopic amount of tissue, correcting vision by reshaping the cornea (outer window of the eye). This allows light rays to focus more precisely, producing a sharper image on the retina, thereby eliminating or reducing refractive errors and the need for glasses or contacts.

Where and when was the Excimer laser developed?

It was developed by IBM in 1979 to etch circuits in computer microchips. In the early 1980's researchers found the Excimer laser to have medical applications as well. The first human procedures were done in Berlin in 1987. Over the past eleven years more than 1,500,000 procedures have been performed.

Photodynamic Therapy Light Dosimetry

Photodynamic therapy (PDT) is a cancer treatment that has been used experimentally for various types of malignancies including head and neck, bronchial, gastric, skin, lung, bladder and esophageal carcinoma. PDT with Photofrin(r) has recently received FDA approval in the US for treatment of esophageal cancer.

PDT involves the interaction of a photosensitizing dye and red light, neither of which have any effect alone. Twenty-four hours after intravenous administration, the dye is cleared from normal tissue and localized in tumor tissue. Red laser light (chosen for optimal penetration) is applied to the tumor site. The red light is absorbed by the dye and raises the dye to an excited singlet state. The excited dye transfers energy to molecular oxygen in the tumor, resulting in highly reactive singlet oxygen that oxidizes biological targets such as cell membranes and organelles. The dye has a finite excited lifetime so the oxygen must be available near the excited dye molecule. The singlet oxygen also has a short lifetime and must be created near the desired target. PDT requires dual dosimetry for the drug dose and the light dose to ensure effective treatment of the entire tumor. It has been shown that a threshold light absorption is necessary to cause irreversible tumor necrosis. The light dose applied to the front surface must be sufficient to allow a threshold absorption at the deepest part of the tumor. Light penetration through the skin depends on tissue pigmentation, dye absorption and light scattering.

A material is optically characterized at the wavelengths of interest by microscopic absorption and scattering coefficients,k and s, and the anisotropy, g, or average cosine of the scattering angle. The light flux in a highly scattering medium can be calculated with the photon diffusion equation which is derived from radiative transfer theory. The distribution of light throughout the material as well as the diffuse reflectance and transmittance, R and T, of a material can be determined from k, s and g with mathematical tissue optics theories or with a Monte Carlo computer simulation. In order to determine the microscopic coefficients, the "inverse method" is employed: R and T are measured for an optically thin sample and k and s(1-g) are mathematically fitted to the measurements with optical theory.

The propagation of light through tissue has been successfully modeled with Monte Carlo simulation. Photon "bundles" are injected into the tissue and allowed to move throughout the specified dimensions with scattering and absorbing events occurring until the bundle is attenuated. Light that emerges from the front or rear surface is counted as reflection or transmission. Absorbed light is counted in a two or three-dimensional array to allow for flux density output. Monte Carlo Scattering Program V1.0 is the program I wrote that allows one to see light transport in three dimensions through a user defined tissue sample.

Optical Coherence Tomography for Dental Applications

Researchers at the Lawrence Livermore National Laboratory are developing a new optical technique for non-invasive imaging of biological tissue. Optical Coherence Tomography (OCT) generates high-resolution (<20 micron) cross-sectional images of tissue, without the need for tissue biopsy. The images are taken using near-infrared light, avoiding the dangers associated with ionising radiation, as with x-ray images.

Near-infrared light penetrates deeply into tissue, making it useful for imaging of internal structure. The majority of the light, however, is highly scattered as it penetrates into the tissue. These scattered photons dominate in most imaging applications, leading to blurred images. By using a white light Michelson interferometer as a gate, OCT detects only the unscattered "ballistic" photons and is thus able to generate high resolution images. In addition, heterodyning techniques are used to detect very low levels of reflected light from the tissue (10-10 reflectivity). OCT is most useful for imaging relatively accessible regions of the body such as skin, internal body cavities, and arteries.

The architecture of the OCT system.

The OCT system is based on a single mode fiber-optic Michelson white light interferometer. High resolution cross-sectional imaging is achieved by focusing light from an optical low coherence source on the biological tissue using a hand-held scanner and measuring the intensities of the backscattered reflections as a function of their axial and transverse positions in the tissue. The light is scanned axially through the tissue by varying the reference arm pathlength. Intensity modulation associated with interference between light from the sample and reference arm reflections (heterodyning) occurs only when the optical pathlengths of the two arms are matched to within the coherence length of the source. The intensity of backscattered light is given by the amplitude of this heterodyned signal and plotted as a function of axial position in the sample, generating one scan. Translating the sample arm transversely generates a series of these scans which are combined to create a two dimensional plot or cross-sectional image of backscattered intensity as a function of transverse and axial position in the tissue.

Applications

Periodontal disease diagnosis: Periodontal diseases are plaque-induced disorders that result in loss of connective tissue attachment and resorption of alveolar bone. An important aspect of periodontal disease assessment is determining the location of the soft tissue attachment to the tooth surface. Currently, mechanical or pressure sensitive probes are used to assess periodontal conditions. These probes can be painful for the patient and have several sources of error resulting from variations in insertion force, inflammatory status of tissue, and anatomical tooth contours. OCT is not sensitive to these errors and thus should be a more reproducible and reliable method for determining attachment level. Moreover, directly imaging tooth and soft tissue structures and contour in vivo may provide information that would allow diagnosis of periodontal diseases before attachment loss occurs.

Detection of caries: Dental caries are a common disease that can be easily treated if detected early enough. If undetected and untreated, caries may progress through the outer enamel layer of a tooth into the softer dentin, requiring extraction of the tooth or causing inflammation of the periodontal tissue surrounding the tooth.

The standard methods for detecting caries in teeth are by visual inspection or by the use of dental x-rays. Both methods are unreliable for the detection of small caries. In addition, dental x-rays subject the patient to ionizing radiation, a known mutagen. OCT imaging offers a safe, noninvasive alternative for locating potential and actual sites of caries incursion and therefore improves early disease detection and treatment.

Restoration placement/evaluation: Dental restorations are used to provide a barrier restricting oral fluids and bacteria from entering through the tooth into the systemic system as a result of dental decay or trauma. An inadequate seal can result in a loss of tooth structure, infection, and dissemination of bacteria. The most commonly used methods for evaluating the seal and structural integrity of restorations is visual and tactile examination. OCT has the advantage over these methods of visualizing structural and marginal restoration defects before significant leakage occurs, minimizing tooth loss and decreasing the number of unnecessary replacement restorations.

Endoscopy

Endoscopes are essentially tools which allow us to see inside small spaces. An architect can use an endoscope to look into a scale model of a design. Endoscopes provide an insight which may approach an image of reality, with the possibility to look around, to test different proposals and to change design options with relative ease.

The endoscope can be used for the presentation of designs, as a design tool and possibly as an instrument in perception research. Optical endoscopes consist of a lenstube providing a virtual image of an object. The place and direction of the endoscope (the point of view) determine what virtual image the endoscope-user perceives. The object of study is generally an existing or designed environment. New and developing techniques bring with them a specific terminology, a technical 'jargon' which in time may, or may not be absorbed into language. Computer jargon has introduced concepts like: 'virtual reality', 'artificial environments', 'interaction', 'full immersive worlds'1, and even 'cyberspace'. The boundary between scientific reality and futuristic wishful thinking is not always clear. Steuer (1992)2 defines virtual reality as follows: "Virtual reality is a remote or artificially constructed environment in which one feels a sense of presence, as a result of using a communication medium."

"Researchers using endoscopes tend traditionally to be more interested in optics and in technical aspects that can be linked to movie making than in computer technology. However the optical and digital techniques are more and more frequently used side by side. From the point of view of Steuers definition, endoscopy may be considered as a kind of virtual reality, in fact, endoscopy can give us a very good video- or photographic-image of a 'virtual world' (=a scale model) with 'real time' possibilities to look around and to 'interact' and in the same way it can be useful to view new spatial software products as 'digital' endoscopes. Optical endoscopy and the upcoming computer applications do have a number of aspects in common, the techniques of these 'optical' and 'digital' endoscopes can be compared.

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Page last modified: November 26, 2002