Optical coherence tomography

Abstract

A technique called optical coherence tomography (OCT) has been developed for noninvasive cross-sectional imaging in biological systems. OCT uses low-coherence interferometry to produce a two-dimensional image of optical scattering from internal tissue microstructures in a way that is analogous to ultrasonic pulse-echo imaging. OCT has longitudinal and lateral spatial resolutions of a few micrometers and can detect reflected signals as small as approximately 10(-10) of the incident optical power. Tomographic imaging is demonstrated in vitro in the peripapillary area of the retina and in the coronary artery, two clinically relevant examples that are representative of transparent and turbid media, respectively.

Fig. 1

Schematic of the OCT scanner. The SLD output is coupled into a single mode fiber and split at the 50/50 coupler into sample and reference arms. Reflections from the two arms are combined at the coupler and detected by the photodiode. Longitudinal scanning is performed by translating the reference mirror with a stepper motor stage at 1.6 mm s⁻¹, generating a 3.8-kHz Doppler shift. The piezoelectric transducer (PZT) in the sample arm further provides 21.2-kHz phase modulation to the interferometric signal. Interferometric modulation of the output intensity is detected by the photodetector when the reference and sample arm delays are nearly matched. The detector output is demodulated at the sum modulation frequency of 25 kHz to produce the envelope of the interferometric signal, which is then digitized (AD) and stored on computer. A series of longitudinal scans are performed. The lateral beam position is translated after each longitudinal scan.

Fig. 2

Optical coherence tomograph of human retina and optic disk in vitro (A) and histologic section of the same specimen (B). Eye bank specimens were kept at 4°C and measured within 24 hours after death. (A) Cornea and lens were removed before OCT scanning and the OCT beam was delivered through the vitreous medium and focused on the retina. The tomographic image corresponds to a section of the retina and optic disk along the papillomacular axis. The retina temporal to the disk is on the left. Identifiable structures are, from top to bottom, vitreous, retina (RNFL, red; subjacent retina, yellow to light blue), subretinai fluid (SRF), retinal pigment epithelium (RPE), and choroid and sclera. The RNFL thickness varies between 70 and 90 µm, increasing toward the optic disk. The overall retinal thickness is 220 µm. Blood vessels (BV) in the optic disk appear as characteristic dark spots. The nasal retina appears on the far right. The sampled pixel size is 3.8 (vertical) by 20 (horizontal) µm. Interpolation berween pixels was performed to improve image readability. The color scale spans 4 × 10⁻¹⁰ (black) to 10⁻⁶ (white) of the incident power. (B) Stevenol’s blue–stained plastic section. The RNFL and overall retinal thickness closely match those of the tomograph; the SRF is much smaller than in the tomograph because of dehydration during histologic processing. Vitreous (V), retina (R), sclera (S), blood vessel (B), SRF (F). Bar = 300 µm.

Fig. 3

Optical coherence tomographs of human coronary artery (A) and a histologic section of the same specimen (B). Artery specimens from a 68-year-old woman who died of myocardial infarction were obtained after autopsy and imaged before fixation. (A) The dissected specimen was maintained in saline, and the OCT beam was incident from the intimal surface. Fatty-calcified plaque (right three-quarters of image) backs carters and shadows the subjacent structures more strongly than fibroatheramatous plaque and normal arterial wall (left quarter). The sampled pixel size is 3.8 (vertical) by 25 (horizontal) µm. Interpolation between pixels was performed to improve image readability. The color scale spans 4 × 10⁻¹⁰ (black) to 4 × 10⁻⁵ (white) of the incident power. (B) Toluidine blue—stained plastic section. Lumen (L), fatty-calcified atherosclerotic plaque (C), fibroatheromatous plaque (A), media (M). Bar = 200 µm.