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The blood sample is applied to a reagent strip that has specific, carefully controlled optical properties.
Enzymatic reagents embedded in the strip react with the blood sample and change color in proportion to the amount of glucose in the sample. A small optical reader incorporating visible and infrared LEDs and a solid-state detector performs a two-color reflectance measurement on the reagent strip. The ratio of the reflected powers is used to calculate glucose concentration. It is battery powered and can easily fit into a pocket or purse.
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This type of portable, easy-to-use, diagnostic instrument has revolutionized the monitoring of glucose levels in diabetics. Careful monitoring of glucose levels significantly reduces the onset of complications that can lead to retinal disease and blindness or to kidney disease and failure.
The major cause of discomfort to the user in these systems is the need for constant lancing of the finger to provide a fresh blood sample. The inconvenience and discomfort of this procedure can lead to poor patient compliance, resulting in inadequate monitoring.
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Hopefully, the next generation of instruments will provide comparable accuracy in a totally noninvasive manner, eliminating the need for a blood sample. Several dozen groups are working on a variety of approaches to develop this type of instrument. Most of these approaches are based on in vivo, noninvasive, spectroscopic measurements of glucose via its absorption properties in the near IR or other modifications of optical properties that track glucose levels. There are many blood components with interfering absorption spectra that complicate making these types of noninvasive measurements.
No FDA-approved methods for noninvasive measuring of glucose currently exist. Moreover, to be of greatest benefit a noninvasive instrument must supply the convenience, portability, and affordability of the current method. Development of this type of external, noninvasive glucose monitors is hindered by our limited understanding of the in vivo spectroscopy of blood components. Moreover, this type of instrument may not be possible using existing technology. The theophylline concentration in a blood sample is measured by using an optical scattering technique that combines immunochemistry with the light-scattering properties of latex beads.
Several of the optical elements in this instrument are integrated into an inexpensive, disposable, injection-molded plastic cartridge. This instrument allows measurement of the theophylline level in less than 3 minutes. Previously, blood samples had to be sent out to be analyzed at a blood chemistry laboratory, causing delays of up to several hours.
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Initial studies of tumor detection demonstrated that fluorescence-based techniques using either exogenous marker dyes or endogenous natural fluorophores could be used to mark gross, visually detectable tumors. Subsequent work has emphasized the ability to determine whether small, visually undetectable lesions can be identified by spectroscopic techniques.
Such optical methods might help guide conventional tissue biopsy to the most suspicious regions or might in some cases alleviate the need for a biopsy; the terms "optically guided biopsy" and "optical biopsy" have been used to describe this approach generically. There are numerous variations on the optical biopsy concept.
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Different spectroscopic techniques such as fluorescence, reflectance, and Raman scattering have been employed. These techniques have been used both to make measurements at single points with an optical fiber and to obtain images using either conventional or intensified CCD video cameras. Although the exact implementation of these concepts varies, the basic idea is always to find a spectral signature of the abnormal tissue that differentiates it from normal tissue and to develop algorithms for utilizing these signatures.
This approach has been used with laser-induced fluorescence studies of a number of organs, including the colon, bladder, and cervix. Although a number of encouraging results have been obtained, large-scale in vivo studies are generally needed before these approaches gain clinical acceptance.
Such studies have already been performed in the lungs using autofluorescence imaging and in the bladder using fluorescence imaging of a marker dye. A major engineering challenge will be to make optical systems that yield new and accurate information but are inexpensive enough to be accepted even in today's cost-conscious health care environment.
Another approach to optical biopsies uses the fluorescence lifetime of a molecule, rather than its spectrum, as a source of information. The lifetime is the time for which a fluorescent molecule emits light after a rapid optical excitation pulse. This has the advantage that compounds whose fluorescence spectra overlap can be monitored by using differences in.
In addition, lifetime changes can be used to monitor processes, such as binding of a fluorophore to tumor tissue, that cannot as easily be detected spectrally. As in fluorescence spectroscopy, both point measurements and imaging of lifetimes have been demonstrated. In addition, lifetime measurements have been obtained using both time-domain and frequency-domain techniques, discussed in more detail below.
X-ray mammography is the standard screening technique for breast cancer.
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However mammograms require highly-trained radiologists for interpretation, and even at its best, mammography fails to detect a significant number of breast cancers, especially in younger women. An optically based mammography system could complement the existing technology if it were able to find the cancers that x-ray mammography misses.
Today, a number of optical techniques aimed at this goal are being explored see Figure 2. The use of light to create images of the interior of tissue is an attractive idea whose roots can be traced to studies of tissue transillumination in the s. However, these early studies failed to overcome the effect of tissue opacity.
Whereas some materials are opaque because they strongly absorb visible light, others such as tissue may be opaque because photons traveling within these media are highly scattered. A small number of photons travel straight through such substances and can be used to make shadowgraphs of internal structures in a manner similar to x rays. However most of the light is transported through these materials in a process similar to heat diffusion Gratton and Fishkin, ; Yodh and Chance, The optical mammogram was obtained using two laser diodes operating at and nm in a frequency-domain imaging system.
Courtesy of S. Fantini, University of Illinois at Urbana-Champaign. In the biophysics and medical communities there are extensive research efforts to overcome the effects of scattering and use diffusing photons to view body function and structure. These efforts are based on the existence of a spectral window between and nm, in which photon transport within tissue is dominated by scattering rather than absorption.
Thus, to a very good approximation, near-infrared photons diffuse through human tissues and can be used for a variety of biomedical applications. In a typical measurement, the researcher uses an optical fiber to inject near-infrared photons into tissue or a tissue-like medium and a second optical fiber to detect photons at other locations. Microscopically, the injected photons experience thousands of elastic scattering events while traveling from one fiber to the other. Occasionally, the photons are absorbed in this process and are undetected.
Microscopically, individual photons undergo a "random walk" within the medium, but collectively, a spherical wave of photon density is produced and propagates outward from the source. Typically, quantities such as the photon energy density within the sample are measured to verify light transport models.
The patterns of light energy density or photon density waves are distorted as they traverse scattering media. Recent experiments and simulations using short-pulse time-domain , amplitude-modulated frequency-domain , and cw sources have utilized these distortions for spectroscopy and imaging of deep tissues. It is feasible to use these waves as probes of biological samples whose extent is of the order of 1 cm, or about transport mean free path lengths. Since tissues are often quite heterogeneous, it is natural to contemplate making images with the diffusive waves.
A simple example of the utility of imaging is the early localization of a head injury that causes brain bleeding or hematomas. Here prototype devices already detect the presence of small brain bleeds at the limit of detection by x-ray computed tomography. Such sensitivities in detecting hematomas suggest it may be possible to localize small blood vessel expansion aneurysms , which must be detected at levels below 1 cm to avoid danger of rupture. The medical utility of near-infrared spectroscopy and imaging approaches ultimately depends on whether the tissue has enough optical contrast to differentiate normal from abnormal tissue or body function.
Spectroscopy is useful for the measurement of time-dependent variations in the absorption and scattering of large tissue volumes, such as might occur following a head injury. Imaging is important when a. Here images enable experts to identify the site and extent of the trauma and to differentiate it from background tissue. The sources of image contrast when using a light probe are different from those of other imaging techniques such as those based on x rays, magnetic resonance, skin temperature measurements thermography , and ultrasound.
Spectroscopic information is available as a result of the intrinsic absorption of tissue or as a result of the absorption of contrast agents optically absorbing chemicals that may be introduced into the body.
The fluorescence spectra and lifetimes of some fluorescent dyes, which may be intrinsic or extrinsic, are sensitive to the local environments within tissue and may be useful for marking tumors. Variation in light scattering affords a novel source of contrast that is demonstrably related to intracellular organelles such as mitochondria.