Know All About : Optical coherence tomography (OCT)

Optical coherence tomography (OCT) is a fundamentally new type of optical imaging modality. OCT performs high-resolution, cross-sectional tomographic imaging of the internal microstructure in materials and biologic systems by measuring backscattered or backreflected light. OCT images are two-dimensional data sets which represent the optical backscattering in a cross-sectional plane through the tissue. Image resolutions of 1 to 15 µm can be achieved one to two orders of magnitude higher than conventional ultrasound. Imaging can be performed in situ and in real time. The unique features of this technology enable a broad range of research and clinical applications.

In general, there are three types of clinical scenarios where we believe that OCT could have important applications: 1) where conventional excisional biopsy is hazardous or impossible, 2) where conventional biopsy has an unacceptably high false negative rate because of sampling errors, and 3) For guidance of surgical interventional procedures. In this manuscript, we review the fundamental concepts of OCT technology and discuss potential applications to biomedical research and clinical medicine.

OCT performs imaging by measuring the echo time delay of reflected light using low-coherence interferometry. The system is based on a Michelson type interferometer. Reflections or backscattering from the object being imaged are correlated with light which travels a reference path.

Cross-sectional images are constructed by performing measurements of the echo time delay of light at different transverse positions. The result is a two-dimensional data set which represents the backscattering in a cross-sectional plane of the tissue. This data can be displayed as a gray scale or false color image.

Biomedical Imaging Using Optical Coherence Tomography

Several features of OCT suggest that it will be an important technology for biomedical imaging.

OCT can image with axial resolutions of 1 to 15 µm, one to two orders of magnitude higher than conventional ultrasound. This resolution approaches that of histopathology, allowing architectural morphology and some cellular features to be resolved. Unlike ultrasound, imaging can be performed directly through air without requiring direct contact with the tissue or a transducing medium.

Imaging can be performed in situ, without the need to excise a specimen. This enables imaging of structures in which biopsy would be hazardous or impossible. It also allows better coverage, reducing the sampling errors associated with excisional biopsy.

Imaging can be performed in real time, without the need to process a specimen as in conventional biopsy and histopathology. This allows pathology to be monitored on screen and stored on high-resolution video tape. Real-time imaging can enable real-time diagnosis, and coupling this information with surgery, it can enable surgical guidance.

OCT is fiber optically based and can be interfaced to a wide range of instruments including catheters, endoscopes, laparoscopes, and surgical probes. This enables imaging of organ systems inside the body.

Finally, OCT is compact and portable, an important consideration for a clinically viable device.

 

Uses

1. Ophthalmic Imaging :

The OCT image provides a cross-sectional view of the retina with unprecedented high resolution and allows detailed structures to be differentiated. Although the retina is almost transparent and has extremely low optical backscattering, the high sensitivity of OCT imaging allows extremely weak backscattering features such as the vitreal-retinal junction to be visualized. The retinal pigment epithelium and choroid, which is highly vascular, are visible as highly scattering structures in the OCT image. The retinal nerve fiber layer is visible as a scattering layer originating from the optic disk and becoming thinner approaching the fovea. The total retinal thickness as well as the retinal nerve fiber layer thickness can be measured.

Clinical studies have been performed to investigate the feasibility of using OCT for the diagnosis and monitoring of retinal diseases such as glaucoma, macular edema, macular hole, central serous chorioretinopathy, age related macular degeneration, epiretinal membranes, optic disc pits, and choroidal tumors.

2. Imaging in Nontransparent Tissues:

One class of applications where OCT could be especially powerful is where conventional excisional biopsy is hazardous or impossible. For example, in ophthalmology, biopsy of the retina is impossible and OCT can provide high-resolution images of pathology that cannot be obtained using any other technique. Another scenario where biopsy is not possible is imaging of atherosclerotic plaque morphology in the coronary arteries.

3. Optical Biopsy and Detecting Early Neoplastic Changes:

The diagnostic indicators of early neoplastic changes include accelerated rate of growth, mass growth, local invasion, lack of differentiation, anaplasia and metastasis. The evaluation of structural and cellular features of these types is necessary for the correct identification and grading of neoplasias. Changes in architectural morphology and glandular organization are relatively easy to identify because they fall within the resolution limits of most standard resolution OCT systems.

4. Catheter and Endoscopic OCT Imaging :

The catheter/endoscope OCT system enables the acquisition of in vivo images of internal organ systems. In vivo imaging of the pulmonary, gastrointestinal, and urinary tracts as well as arterial imaging have been demonstrated in New Zealand White rabbits .

5. Guiding Surgical Intervention: Optical instruments such as surgical microscopes are routinely used to magnify tissue to prevent iatrogenic injury and to guide delicate surgical techniques. OCT can be easily integrated with surgical microscopes. Hand-held OCT surgical probes and laparoscopes have also been demonstrated.

Summary

OCT can perform a type of optical biopsy, the micron-scale imaging of tissue morphology in situ and in real time. Image information is available immediately without the need for excision and histologic processing of a specimen. The development of high-resolution and high-speed OCT technology as well as OCT compatible catheter/endoscopes and other delivery systems represent enabling steps for many future OCT imaging clinical applications. More research remains to be done and numerous clinical studies must be performed to determine in which clinical situations OCT can play a role. However, the unique capabilities of OCT imaging suggest that it has the potential to have a significant impact on the diagnosis.