Optical Coherence Tomography

Abstract:-:- 

 

This paper explains the state of the art optical coherence tomogra¬phy as an efficient diagnostic imaging tool for biomedical applications. It reviews the basic theory and modes of operation together with its applications and limitations. It also examines the various kinds of in¬struments, which are employed in the whole apparatus, in addition to a discussion on hardware and software methods to combat the sources of error.

Optical coherence tomography is a recent technology that has gained momentum in the medical and research fields. Its use and manipulation of light to view live tissues within the body in real time is a suited to a wide variety of fields in the medical profession. Unlike most other medical imaging techniques, optical coherence tomography has better resolution, depth in penetration, and quality. Although optical coherence tomography was initially geared towards ophthalmology, it had now expanded to many other fields in medicine. Through new technological advancements, the OCT had been increasingly improving its quality and resolution of imaging.  

Introduction

Optical Coherence Tomography (OCT) is a ten-year old imaging technique which has attracted the attention of scientists and engineers greatly. This is mainly due to the fact that it has been the first diagnostic imaging technique which has extensively employed the coherent properties of light.

Types of OCT

There are also other types of OCT which are also utilize light to produce imaging of tissue however they tend to include or vary the components of the system to provide more emphasis and extract more information from scans based on something specific from the sample. One of these ideas is the Doppler OCT which looks from for frequency shifts in the interference patterns which would show moving objects in the sample such as blood cells. This is particularly interesting to ophthalmologists since variations in blood flow can be causes of blindness, including diabetic retinapothy and macular degeneration. In addition to Doppler OCT, researchers are also looking into polarization which would measure the polarization of returning light and interference fringes since this might be a way to image damage to tissue such as nerve fibers, skin, and other connective tissues.

Applications of OCT

The numerous applications that OCT can be used for is remarkable. OCT has the potential to be used for a great number of medical fields and applications however, cancer and heart disease are two of the most pressing and promising areas of application The imaging from the optical coherence tomography has the potential to improve the current cardiovascular therapies and procedures such as stenting and balloon angioplasty by means of providing vascular images in real time to guide stent placement and balloon inflation. Since OCT has the capability of clearly identifying plaques in the blood stream thus being able to differentiate between stable plaques and unstable plaques, which are probably responsible for up to seventy percent of all heart attacks, it aids those that suffer from cardiac problems. In addition to helping those with cardiac problems, OCT can also be valuable others. 

Speed

As important as the resolution is the speed of an OCT system. The frame rate is basically determined by the speed with which the path-length could be swept, in order to obtain a complete cross correlation function.

As a result, major effort has been put into faster sweep of the time of flight difference between the reference and the sample arms. Several methods have been used that a few of them will be introduced here .

In early OCT systems the path-length in the reference arm was changed by using a moving mirror or a galvanometer. This method usually results in acquisition rate of less than 2 frames per second.

The second generation systems employed fiber stretching or piezoelectric crystals to modify the path-length of the reference arm. The limitations of this technique are polarization mode dispersion, hysteresis, crystal break¬down, and usually very high voltages.

A new technique has also been introduced recently by the employment of grating-based phase control delay. The Fourier transform is generated on the grating upon incidence of the reference beam. The scattered wave is then directed to a linear wavelength dependent phase ramp, and since linear phase ramp in the frequency domain stands for group delay in time domain, when the signal is incident on the grating for the second time, the inverse Fourier transform is generated. In this way the group delay can be swept by tilting the mirror at different angles. This method provides frame rates between 4 and 8. Moreover, it provides two other advantages, i.e., the group delay can be swept independently from the phase delay, and the group velocity dispersion could be varied without the employment of an extra prism.

Conclusions

Optical coherence tomography (OCT) has been emerged a novel diagnostic tool for bio medical applications, especially in situations where conventional imaging methods are either hazardous or of little valuable information. The continuing success of OCT depends on the design and fabrication of cost effective as well as portable ultra wide-band light sources in order to pro¬vide better resolution.