1.  Field of the Invention
The invention relates to imaging of dental and periodontal tissue.  More particularly, the invention relates to a method for detection of caries and periodontal disease by an optical imaging technique that is non-invasive and uses non-ionizing radiation. 
2.  Description of Related Art
Dental caries, caused primarily by bacterial action on sugars, are a common disease that can be easily treated if detected early.  If undetected and untreated, caries may progress through the outer enamel layer of a tooth into the softer dentin so far as to require extraction of the tooth or to cause inflammation of periodontal tissue surrounding the tooth.  The standard methods for detecting caries in teeth are by visual inspection or by the use dental x-rays.  Both methods are unreliable for the detection of small caries (&lt;1 mm) or caries between teeth.  In addition, dental x-rays subject the patient to ionizing radiation, a known mutagen. 
Non-ionizing radiation has long been used for imaging the internal structures of soft tissue, and has shown promise in such applications as mammography, neonatal brain scanning and imaging of some tumors.  Generally, however, it is unsatisfactory for tissue imaging because scattering of the lower-energy non-ionizing radiation by the tissue severely compromises the resolution of the image.  Although resolution of optical images may be improved by the use of polarizing filters or phase conjugated mirrors, or by collimation of the incident and transmitted beams to reduce interference from scattered radiation, x-ray images still produce superior resolution.  Optical imaging with non-ionizing radiation requires sophisticated techniques such as photon time-of-flight range gating to achieve image resolution comparable to x-ray techniques. 
Several imaging techniques for hard tissue such as teeth use non-ionizing radiation.  Caries may be detected using visible luminescence, based on the discovery that in certain regions of the visible spectrum the intensity of the luminescence for carious and noncarious dental tissue is nearly the same, while in other regions the luminescence intensity increases substantially in the presence of caries.  The method involves illuminating a tooth at two wavelengths, one where the intensity is similar, and another where the intensity increases in the presence of caries, then comparing the intensity of the visible luminescent radiation at the two wavelengths.  Some preliminary attempts have been made to image teeth with transillumination techniques using non-ionizing radiation, with collimation of the incident and transmitted beams to reduce interference from scattered light.  The method presently requires 1.5 hours to image one tooth with 1.5 mm.sup.2 resolution, and so is unsuitable for clinical use. 
A need exists for a new technique for detecting the presence of carious tissue in teeth that eliminates the exposure to ionizing radiation, yet offers the resolution and rapid imaging capability of x-ray techniques.  In a portion of the visible and near infrared spectrum, light can be transmitted through or reflected from internal structures in hard tissue, such as a tooth.  Also, in the wavelength region between 500 nm and 1400 nm carious dental tissue is much more strongly absorbing than healthy dental tissue, and conversely that optical radiation is more strongly transmitted through or reflected from healthy dental tissue.  The present invention uses these differential transmission, reflection, and absorption characteristics of healthy and carious dental tissue to create a shadowgraph of structures, such as caries, in a tooth, using nonionizing radiation.  The present invention also improves the spatial resolution of an imaging system for detection of caries.  Resolution is achieved by photon time-of-flight range-gating techniques coupled with optical heterodyne detection.  These techniques are well-known and widely used in applications that require fast imaging and superior image resolution.  The present invention also offers an image acquisition time that will allow its use in a clinical setting. 
Periodontal disease is the major cause of tooth loss.  Plaque is formed by bacterial action on food, and causes inflammation of the epithelial and connective tissue around a tooth.  Untreated, such an inflammation can result in loss of the connective tissue and bone that anchor the tooth, and ultimately loss of the tooth itself.  Present methods of diagnosing and monitoring a periodontal disease such as gingivitis (inflammation of the gum) rely primarily on the use of a mechanical probe to determine the epithelial attachment point and the depth of the periodontal pocket.  Several types of mechanical probes are in use, including a probe with a calibrated flexible tip, allowing the examiner to probe the periodontal pocket without penetrating the wall of the pocket and to follow the contours of the dental tissue.  Calibrations on the probe show the depth that the probe penetrates between the tooth and gum, and indicate the progress of periodontal disease.  Another periodontal probe instrument includes a pressure sensor that activates a signal processing apparatus to simultaneously measure probe pressure, periodontal pocket depth, and periodontal attachment point. 
An ultrasound probe has been revealed as a noninvasive method of recording the depth of the periodontal pocket between a tooth and a gum by comparing the delay time of ultrasonic pulses reflected at the top surface of the gum and pulses reflected at the bone surface below the periodontal pocket.  Ultrasound, however, is not suitable for early detection of gingivitis because the technique measures the alveolar bone level rather than the point of epithelial tissue attachment.  Gingivitis in the early stages is characterized by changes in the epithelial attachment level and the formation of a pocket between the tooth and epithelium, rather than loss of bone. 
The present invention fills a need for an accurate method of diagnosing and monitoring the periodontal disease known as gingivitis.  The methods currently in use that depend on a measure of probe penetration have several significant sources of error, because probe penetration may vary with insertion force, tissue inflammation, and diameter of the probe tip.  In the present invention, detection and monitoring of gingivitis is accomplished by measuring the reflection of optical radiation from dental and periodontal tissue boundaries.  Light is both reflected from and transmitted through the interface between tissues with even slightly different indices of refraction.  Light reflected from an interface can be detected and, with interferometry, can be used to accurately determine the location of the tissue boundary.  The present invention allows precise measurements to be taken of the gap between the epithelial tissue and tooth, the depth of the periodontal pocket, the point of epithelial attachment, and the position of the cemento-enamel junction.  With these measurements, a two- or three-dimensional optical map may be created of the periodontal pocket, permitting an earlier and more accurate diagnosis of gingivitis.  Besides eliminating measurement errors caused by variability of examiner probing methods, diagnosis and monitoring using the present invention is more comfortable for the patient.  In addition, the optical images or maps produced may be maintained with patient records, and a series of optical images taken over time may be compared to monitor the progress of gingivitis and assess treatment methods.