Optical Coherence Tomography (OCT)

INTRODUCTION

Optical Coherence Tomography (OCT) is a noninvasive imaging technique for obtaining high-resolution retinal cross-sectional images. To aid in the early detection and diagnosis of retinal disorders and ailments, the layers within the retina can be separated and retinal thickness can be quantified.

The ophthalmologist can observe each of the retina’s different layers using OCT. The ophthalmologist will be able to map and assess their thickness as a result of this. These metrics aid in the diagnosing process. They also offer medical advice for glaucoma and retinal problems. Age-related macular degeneration (AMD) and diabetic eye disease are examples of retinal illnesses.

For the examination and treatment of most retinal diseases, OCT testing has become the gold standard. OCT measures the thickness of the retina using light beams. This test does not include any radiation or X-rays, and an OCT scan is not painful or unpleasant.

An OCT scan may be performed for a variety of purposes, including monitoring the progression of the disease, verifying or discounting suspected retinal edema, or comparing OCT results to other data to assess the efficacy of the current drug regimen.

Optical Coherence is a term that refers to the ability of light to Tomography is similar to ultrasound in that it uses light instead of sound to obtain better, sharper resolution.

What happens during OCT?

The ophthalmologist may or may not use dilating eye drops to prepare you for an OCT exam. These drops dilate the pupil, making it simpler to look at your retina.

OCT Procedure

  1. Time-domain optical coherence tomography- An optical light source and an interferometer are used in a TD-OCT system. A reference beam is created using a reference mirror and an optical splitter. Light is conveyed from the splitter to and from the object of interest, as well as up to a processing unit, via a microscope interface optics. The interference of light between the reference beam and the beam back from the item of interest is performed by the processing unit, which also processes and analyses the interference signal. By reducing the length of the reference path from the length of the object path, the optical path difference (OPD) is calculated. The OPD can be used to figure out which of an object’s layers can be photographed.

The inclusion of a transversal scanner to a low coherence interferometer to sweep the beam over the object laterally led to the invention of OCT. Taking adjacent scans of the object yields a cross-section image, commonly known as axial TD-OCT. En face OCT is another way to accomplish TD-OCT, which includes getting one-dimensional reflectivity scans (T-scans), with the final image consisting of several T-scans.

  1. Spectral-domain optical coherence tomography- The spectral interrogation of the spectrum at the interferometer output is referred to as SD-OCT. There are two ways to accomplish this. The first method includes combining a broadband optical source with a spectrometer-based processing unit. The second method employs a swept-source (SS) and a photodetector in the processing system. The charges on the array are read in the spectrometer in SB-OCT or by tuning the frequency of the laser in SS-OCT, rather than scanning the OPD as in TD-OCT.

SB-OCT is built on the optical spectrum demodulation output of a low coherence interferometer. The modulation period is proportional to the OPD in the interferometer, thus the spectrum comprises peaks and troughs. Because of this relationship, the larger the OPD, the more peaks in the spectrum there are. The camera used to capture the image must have pixels small enough to sample the spectrum’s peaks and troughs. The cameras used in SB-OCT operate at a frequency of 20-70 kHz.

When tuning the SS with SS-OCT, the laser line must be thinner than the spectral distance between adjacent peaks in order for the photodetector to detect it. This enables the photo-detected signal to take the channeled spectrum’s structure. The periodicity of the spectrum is translated into peaks of varying frequency in respect to the OPD, resulting in an A-scan. After that, the A-scan is used to create an image.

 Uses OCT

OCT can be used to diagnose a variety of eye disorders, including:

  • hole in the macular
  • puckering of the macular
  • macular edema 
  • Macular degeneration caused by old age
  • glaucoma
  • central serous retinopathy is a kind of retinopathy that affects the central
  • Diabetic retinopathy is a type of retinopathy caused by diabetes
  • traction on the vitreous

OCT and cancer

Melanoma and non-melanoma cancer are the two types of skin cancer. The ability of OCT to visualize various forms of skin cancer has been determined in numerous research. OCT pictures of malignant melanoma in the lower epidermis revealed irregular formations that matched histopathology.

Oral cancer is commonly treated with a combination of surgery and radiotherapy, with squamous cell carcinoma accounting for 90% of cases. Because of the difficult position and the importance of the surrounding key structures, full tumor eradication is critical. In ten research, optical coherence tomography (OCT) was used to assess its utility.

CT and flexible bronchoscopy are frequently used to diagnose lung cancer, which is the most common cancer in men worldwide. Bronchoscopy, on the other hand, is insensitive, especially in the early stages of cancer. OCT was used in five studies to aid in the visibility of lung cancer during bronchoscopy and after surgery on resected specimens.

In breast cancer patients, OCT was employed to detect tumors as well as sentinel lymph nodes. When compared to histology, a variety of particular criteria were used and showed good diagnostic accuracy in margin evaluation. For margin evaluation, a handheld OCT camera that could be used intraoperatively was also used, with encouraging results. This suggests that, with more training and development, OCT could be utilized as an additional tool for tumor detection during surgery. FF-OCT was able to discriminate malignant lymph node invasion from benign lymph node invasion with good sensitivity and specificity when it came to lymph node identification.

OCT is a well-known diagnostic tool that is increasingly being used in preclinical cancer research. OCT provides imaging capabilities that cover certain gaps left by conventional intravital techniques, and as a result, it may become a standard tool in the biological laboratory. Of course, OCT’s anatomical contrast would be enhanced by labeled methods’ molecular knowledge. In preclinical situations, multimodality imaging should be used. The expansion of both the research and commercial industries supporting OCT will continue to generate improvements in performance and widen access to the apparatus, driving adoption even further.