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Optical coherence tomography ( OCT ) has made great progress in the past few years. Since the advent of OCT, ophthalmologists have used near-infrared technology to capture high-quality images of the posterior segment of the eye. Because the eye tissue is translucent, OCT can provide images that show retinal pathology, thereby diagnosing and monitoring retinal diseases such as glaucoma and macular edema. Today, many OCT-based medical applications have matured, and many new applications are in the development stage.

Figure 1. Example of an ophthalmologist using an OCT instrument

The principle of OCT imaging is similar to that of ultrasound, except that it uses reflected near-infrared light as an imaging medium instead of reflected sound waves to form an image. The near-infrared light source (generally 800 to 1300 nanometers) is split into two beams. One beam is emitted to the sample tissue, and the other is emitted to the reference reflector. When the sampling arm scans the tissue, the interferometer performs a digital signal processing algorithm on the light reflected back from the rear end of the sample tissue to achieve a depth-resolved axial scan. These scans are superimposed on each other to form a 2D or 3D image of the tissue. In general, OCT can resolve images at a tissue depth of 3 to 5 mm at an extremely high resolution of less than 10 microns.

In the first generation time domain system, the reference mirror, one of the key components of the OCT system, is a mechanical component, so it moves slowly and the image resolution is limited. The second generation OCT system replaces the mechanical reference mirror with a fixed reference mirror, and uses a spectrometer and powerful digital signal processing technologies such as fast Fourier transform (FFT), series operation and logarithmic compression to resolve the embedded depth information and combine it with the lateral scan data in real time, which can greatly shorten the imaging time and improve the image resolution.

OCT Applications in Biomedicine

Currently, OCT medical systems are mostly used in ophthalmology, but several new applications have emerged in the past few years. For example, ENT physicians and pediatricians also use OCT technology as a diagnostic tool . Generally, doctors use an otoscope to check the ear, external auditory canal, and eardrum for symptoms of redness caused by bacterial infection. OCT can improve diagnostic accuracy by imaging the epidermal and subcutaneous membranes to determine whether there is infection with pathogenic bacteria. After taking several antibiotics, OCT systems can be used to analyze whether the antibiotics are effective. If the biofilm of the infection has been removed, the patient can stop taking antibiotics.

Other emerging OCT medical applications include dental diagnostic systems and interdisciplinary surgical techniques. Dentists can use OCT imaging to identify early caries and certain gum diseases that are not visible on X-rays or visual inspection, allowing for more effective preventive procedures.

In interdisciplinary surgery, OCT can be used to analyze the presence of cancer during surgery to remove a tumor. Generally speaking, surgeons always hope to remove all cancer cells when removing tissue surrounding the tumor. The removed tumor and surrounding tissue are sent to the pathology laboratory for analysis for several weeks, and then a written report is made after the operation. Because OCT images use the same resolution in histological and pathological applications, the OCT system in the operating room allows surgeons to know exactly how much tissue to remove during surgery and how much safety margin to leave. This approach will not mistakenly remove tissue that is not infected with cancer, thus saving the cost and pain of subsequent operations. OCT technology allows doctors to see images with histological resolution in real time so that they can make better decisions when performing surgery to remove a tumor for the first time.

There will be more medical applications for OCT in the future. For example, OCT can be used with a needle biopsy to remove small tumors in the early stages. For patients with breast cancer, OCT can be used with vision and " intelligent " signal processing technology to guide the needle to the exact tumor location to identify suspected infected tissue and minimize the invasiveness of the surgery. For cardiovascular patients, OCT can be used with very small catheter stents to more accurately locate intravascular stents or check for plaque accumulation. In these types of applications, advanced digital signal processing technology can not only achieve excellent image quality, but also enable tissue classification.

Changes in OCT storage methods

When OCT technology for medical imaging was first introduced, it used the personal computer (PC) as the system platform, which was later modified by the second generation systems and will be changed by the third generation systems currently under development. Several OCT system manufacturers have or will soon adopt embedded processing platforms with single or multi-core digital signal processors (DSPs) instead of general-purpose processors (GPPs) used in PCs. DSPs can achieve higher signal processing performance per milliwatt of power consumption than traditional computing methods, which means that programmable algorithms can produce accurate results without the need for expensive power supplies and heat dissipation components. DSP-based system-on-chip (SoC) can help designers reduce system size and power consumption by allowing powerful signal processors to coexist with system application processors with appropriate interfaces for data processing, memory and storage. Using a DSP platform can reduce the actual size of the system and reduce power consumption, so battery-powered portable OCT systems will be available in the near future. Like portable ultrasound systems, portable OCT systems will help this technology become commonplace in many clinics and physicians' offices. Additionally, portable OCT systems will be an effective point-of-care diagnostic tool for medical and emergency professionals responding to natural disasters and accidents.

Figure 2. Example of TI multicore DSP development platform for medical imaging

Future medical applications

In the future trend of the next generation of OCT medical imaging technology, more powerful multi-core (DSP) will be deployed to shorten imaging time and improve image quality. Software algorithm enhancements for processing OCT images are currently under development. A technology called all-fiber polarization-sensitive optical coherence tomography system OCT (PS-OCT) can use processing algorithms to polarize light signals to produce images with higher visual contrast. High-contrast images can show small holes in caries or tiny cysts and tumors.

Another future OCT application is to examine the very small blood vessels in the eye. OCT can use Doppler imaging technology to draw blood flow charts and estimate blood flow velocity, similar to ultrasound but with higher resolution, which can diagnose diabetes and certain eye diseases at an early stage. The programmable DSP architecture can provide an accurate and scalable platform for signal processing applications, thus facilitating the development and rapid deployment of such new algorithms.

Keywords：OCT Reference address：Technological innovation promotes the development of advanced OCT imaging applications

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