Swept Source Optical Coherence Tomography (OCT) Method and System

Abstract

A method and apparatus are provided for a swept source optical coherence tomography (OCT) system utilizing a fast scanning mechanism in the sample arm and a slowly swept light source. The position data is collected rapidly while the wavelength of the source is swept slowly. The system reduces the sweep speed requirements of the light source enabling higher power, greater imaging range, and linear sweeps of the source frequency. The OCT components (or most of them) may be implemented within a hand held imaging probe. In operation, a triangulation scan may be used to orient the imaging probe with respect to a fixed coordinate system; preferably, OCT data captured by the device is then transformed to that same orientation with respect to the fixed coordinate system to improve the scanning results. [0001]This application is based on and claims priority from Ser. No. 61/170,886, filed Apr. 20, 2009, Ser. No. 61/172,513, filed Apr. 24, 2009, and Ser. No. 61/173,714, filed Apr. 29, 2009. BACKGROUND [0002]Optical coherence tomography (OCT) is a depth-resolved imaging modality. Reflections of light returning from within the tissue are used to create tomograms of the tissue microstructure. [0003]A known OCT system is described in U.S. Pat. No. 7,355,721. The OCT imaging system described there includes a light source, an optical coupler or beam splitter, a reference arm, a projector, and a sensor. The OCT imaging system also may be coupled to a processor. [0004]An alternative OCT technique uses a swept source. In one known implementation, the wavelength or frequency of a laser is swept over a range supported by the laser's gain medium. This form of OCT is called swept source OCT or optical coherence domain reflectometry (OCDR). [0005]A standard swept source OCT system is illustrated in FIG. 1. The source 100 emits light within the visible and infrared regions of the electromagnetic spectrum. At any instant of time, a laser with a narrowband of wavelengths is emitted from the light source. The source uses a spectral filter within the cavity to control the laser wavelength. The range of emitted wavelengths is dependent on a gain medium of the source. An exemplary swept source emits a laser with an instantaneous line width of 0.1 nm that is swept from 1250 to 1350 nm. [0006]A circulator 102 directs light from the swept source to a 2.times.2 coupler 104. The 2.times.2 coupler splits and directs a portion of the light to the reference and sample arms (106, 108) of a Michelson interferometer. The reference arm 106 provides a fixed optical path length. The sample arm 108 has approximately the same optical path length as the reference arm 106. The sample arm includes optical and scanning elements necessary to focus and position the beam into tissue. Light reflecting from the two arms are combined at the 2.times.2 coupler. The two beams containing the interfering signals are sent to a dual balanced detector 110. The data is then acquired and processed using a computer (not shown). This processing may include re-sampling of the waveform and Fourier transformation. [0007]In the prior art, the sweep rate of the light source governs the image acquisition rate of the OCT system. Each sweep of the source corresponds to one column, that is, an axial scan, through the tissue. A slower lateral scan within the sample arm is used to collect multiple axial scans for a 2D image. This process is illustrated in FIG. 2. At one position of a scan mirror 200, a data acquisition system (not shown) acquires the interference signal as a function of the wavelength (or frequency) emitted by the swept source. The process occurs for each subsequent position of the scan mirror until enough axial scans are recorded for a 2D image. [0008]As shown in FIG. 3, the swept source OCT system produces a two dimensional matrix 300 with wavelength represented in the 1.sup.st dimension and lateral position in the 2.sup.nd dimension. In this system of the prior art, this matrix 300 is populated by acquiring each column 302 sequentially. A computer is often used to resample and Fourier transform each column of the data matrix. [0009]In the prior art OCT system, a greater sweep rate is required from the swept laser to image with greater speed. Sweeping a laser at a fast rate, however, often comes with negative consequences. When the sweep rate increases, the light travels less through the gain medium resulting in a decrease in the optical power emitted by the source. The instantaneous line width of the source can be increased to provide more optical power, but this reduces the useful imaging range of the OCT instrument. [0010]The subject matter of this disclosure addresses these and other deficiencies of the prior art. BRIEF SUMMARY [0011]A method and apparatus are provided for a swept source optical coherence tomography (OCT) system utilizing a fast scanning mechanism in the sample arm and a slowly swept light source. The position data is collected rapidly while the wavelength of the source is swept slowly. The system reduces the sweep speed requirements of the light source, enabling higher power, greater imaging range, and linear sweeps of the source frequency. [0012]According to another feature of the disclosed subject matter, the sample, reference, and detection arms of an OCT interferometer are incorporated into an imaging probe, preferably using freespace optical elements. [0013]According to still another feature, preferably a triangulation scan is used to orient the imaging probe device with respect to a fixed coordinate system, and OCT data is transformed to that same orientation with respect to the fixed coordinate system. This approach enables a dental imaging system to provide 3D digitizing using OCT with triangulation-based registration of multiple scans, preferably from a hand held device. [0014]Other features and advantages of the invention will be apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional features and advantages be included within this description, be within the scope of the invention, and be protected by the claims.