An optical coherence tomography (OCT) imager uses low coherence light to image a region of interest (ROI) of a target material and provide a 3D image of the ROI. For many applications, the target materials are biological tissues, and the OCT imagers in their various configurations are operable to acquire 3D images of tissue ROIs that extend to depths of up to a few millimeters below the tissue surfaces. By way of example, OCT imagers are used to acquire 3D images of ROIs in the retina, the walls of blood vessels, and arterial occlusions.
An OCT imager typically comprises a light source, such as a superluminescent diode (SLED) or halogen lamp filtered with a monochromator, which transmits a low coherence beam of light having a relatively short coherence length. The coherence length generally ranges from a few micrometers to about a few millimeters. The beam is split to provide an imaging light beam and a reference light beam. The imaging light beam is directed to illuminate an ROI of a target material. The reference light beam is directed to illuminate a reflector.
Light from the imaging beam that is reflected by material in the ROI is combined with light from the reference beam that is reflected by the reflector to generate an interference pattern on a photosensor, which images the interference pattern. The inference pattern results from phase differences at the photosensor between the reference light and imaging light reflected by material at different locations in the ROI. The phase differences are caused by differences in path lengths from the light source to the photosensor between reference light that reaches the photosensor after reflection from the reflector and imaging light that reaches the photosensor after reflection from the different locations in the ROI. For convenience of presentation, locations of material in an ROI imaged by an OCT imager are referenced by x, y, and z-coordinates of a Cartesian coordinate system for which the z-axis is parallel to a direction of propagation in the ROI of light in the imaging beam.
Intensity of an interference pattern imaged by the photosensor that is generated by light from a narrow band of wavelengths in the imaging and reference beams represent values of a Fourier cosine integral over the z-coordinate for distribution of material in the ROI evaluated at the wavelengths in the wavelength band. Imaged interference patterns, hereinafter also referred to as an “interferograms”, generated by light from different narrow wavelength bands are inverse Fourier transformed to provide a 3D image of the material in the ROI that is illuminated by the imaging beam.
For some OCT imagers, the imaging and reference beams are small cross section “pencil” beams. A Fourier interference pattern generated by the imaging and pencil beams provides information for an image of material in the ROI as a function of the z-coordinate for a small cross section “pencil shaped” region of the ROI at fixed x and y coordinates. To provide a 3D image of material in the ROI as a function of x and y coordinates as well as the z-coordinate, the imaging and reference pencil beams are scanned to provide z-axis Fourier interference patterns for each of a plurality of x and y coordinates.
“Full field OCT” imagers on the other hand use relatively large cross section imaging and reference beams to acquire 3D images of an ROI. A full field OCT imager images the imaging and reference beams on a photosensor, such as a CCD or CMOS photosensor, comprising a plurality of pixels to acquire interferograms for an ROI. Each of the interferograms provides values for a Fourier transform of material in the ROI as a function of a relatively large range of x and y coordinates, and therefore for a relatively large transverse cross section of the ROI. (A transverse cross section is a cross section in a plane perpendicular to the z-axis.) Full field OCT imagers are therefore operable to relatively rapidly acquire 3D images of a relatively large volume ROI in a target material.
The volume of the ROI is limited along the z-axis by the smaller of a coherence length of the imaging and reference beams, attenuation length of imaging light in the material of the ROI, and depth of field (DOF) of an optical system in the OCT that images imaging and reference light on the OCT photosensor. Depth of field is often the limiting factor for the z-axis dimension of the ROI and is typically determined by a tradeoff between depth of field and lateral image resolution, that is image resolution, “δxy”, along the x,y-axes. If ΔZdof represents magnitude of the depth of field, then assuming Gaussian optics, a constraint between lateral image resolution δxy and ΔZdof may be expressed asδxy=[λΔZdof/π]1/2,  1)from which it is seen that as depth of field increases, lateral resolution decreases. (Lateral resolution improves as δxy decreases and degrades as δxy increases. δx is an inverse measure of resolving power).