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Advances in CMOS (complementary metal oxide semiconductor) imaging sensor (CIS) module technology are shrinking pixel size, allowing more pixels to fit into a smaller footprint than ever before. CIS technology is used in many products, from smartphones for camera and face recognition to tablets and gaming consoles, as well as applications in automotive, security, and many more. The technology has also made its way into medical device industry, especially in smaller flexible video endoscopes such as laryngoscopes, broncho-scopes, arthroscopes, cystoscopes, ure-terorenoscopes, and hysteroscopes. The next frontier is the cardiovascular world, where CIS technology is poised to enable the delivery of direct and real-time color images.

The adoption of CIS module technology in medical applications has been driven by the advent of minimally invasive procedures. Minimally invasive devices need to be smaller than traditional surgical instruments, and miniaturized CIS modules are a great match for their development. The miniaturization of CIS modules has been led by smart-phone requirements. By comparison with charge-coupled devices (CCDs), the need for lower power consumption, smaller pixel size, and lower cost with volume production drove wide acceptance of CIS module technology. Do you remember picture quality taken by your old smartphones? Picture quality today is much nicer, while the camera size remains same or is even smaller. In 2005, pixel size was about 3 μm; today it is less than 1 μm, which means that roughly 9 times more pixels can be accommodated in the same CMOS chip footprint.

Fig. 1 - Endoscopy: A roadmap for small-diameter videoscopes. (Credit: Solid State Medical Imaging Report, Yole Développment, 2017).

Reducing pixel size sounds great, but there is a trade-off between spatial resolution and light sensitivity. In general, a smaller pixel has lower sensitivity so it needs more light to produce a good image. Back side illumination (BSI) technology successfully improved CIS module sensitivity and made it possible to have a great image with smaller pixels. Newly developed image sensor packaging such as through-silicon via (TSV) technology has been dedicated to minimizing the footprint needed for CIS modules. In addition, advances in microassembly of microelectronics devices has also contributed to CIS miniaturization. Tiny parts such as optical lenses at <1 mm, CIS chips, thin cables, and narrow pitch and lower profile connectors have been assembled by using superfine microsoldering technology with high reliability.

The miniaturization of CIS modules is a key reason that this technology is ideal for medical devices, especially for smaller flexible video endoscopes. CIS modules for medical use are also designed and manufactured according to ISO 13485, including meeting the requirements for biocompatibility and sterilization.

CIS Meets Cardiovascular

A CMOS image sensor module is ideal for cardiovascular endoscopes.

According to the World Health Organization, ischemic heart disease and stroke caused 15 million deaths in 2015. The two diseases were the biggest killers in the world, and so finding ways to address them is critical.1 Great diagnostic modalities such as intravascular ultrasound (IVUS), x-ray, echocardiography, computed tomography (CT), and magnetic resonance imaging (MRI) are saving thousands of lives every day, but in general, these modalities produce only black and white images. Direct vision with a catheter, or a visualized catheter, can provide information that other modalities can't, including color, luminal coronary surface, and thrombosis conditions. Color conveys a great deal of information that can help physicians identify the most appropriate treatment method. In conjunction with other modalities, a visualized catheter in cardiovascular diagnosis and treatment offers many benefits. This article explores three examples: pulmonary vein (PV) isolation for atrial fibrillation (AFib), coronary stent placement, and 3D percutaneous endoscopy.

PV Isolation for AFib. Approximately 2.8 million people in the United States are living with AFib today. According to the Cleveland Clinic, the success rate for single ablation procedure is 75-80 percent, but approximately 20-30 percent of patients need a second PV isolation.2 A physician's experience often influences operation time and success rate, but the procedure generally takes 4-5 hours.

PV isolation of AFib is a well-established therapy with radiofrequency (RF) ablation. The ablated area turns to white so that the treated area can be identified easily. PV isolation requires special expertise of physicians who manipulate a mapping catheter and an ablation catheter at the same time under fluoroscopy. For this procedure, a real-time color image provided from an ablation catheter would eliminate the need for a mapping catheter. It would simplify the procedure for physicians, decreasing the average operation time. A shorter surgery means less exposure to x-ray for the patient as well as reduced cost for the procedure.

Another benefit of a visualized catheter might be a decreased learning curve for physicians, which may also improve the success rate of this procedure. Improving the success rate, of course, also improves the quality of life for patients.

Coronary Stent Placement. A coronary stent is a tube-shaped device placed in the coronary arteries to keep the blocked arteries open in the treatment of coronary heart disease. In coronary artery disease, plaque narrows the inner walls of the arteries, which restricts blood flow to the heart, starving it of oxygen. The plaque has several color tones, ranging from white to yellow.

The white plaque has a thicker fibrous cap with a large lipid core and is considered stable plaque. The yellow plaque has a thinner fibrous cap and is considered unstable plaque. Unstable plaque means a higher risk of rupture, and the rupture increases the risk of thrombosis. By observing the plaque color, the level of risk for the rupture can be identified, and preventive actions can be taken. This distinction makes the color information is very important.

A key to a long-lasting stent is the optimal placement of the stent within the artery. Stents come in a variety of diameters and lengths, and choosing right size is critical for the procedure. Stent diameters vary from 2 to 5 mm in 0.25-mm increments. CIS-enabled direct visualization shows a clear edge of the inner wall of coronary arteries and can help easily determine the correct diameter of the stent. By contrast, in other diagnostic modalities, such as IVUS, the image of the artery wall edge is fuzzy and not sharp enough to provide an accurate measurement for the stent. Thus, direct and real-time color imaging can greatly improve intra-vascular operations.

3D Percutaneous Endoscopy. For many years, all coronary artery surgeries were done via an open heart procedure, and surgical scars were large and noticeable. Thanks to advances in medical device technology, heart disease today is treated percutaneously and requires only a few small incisions in the chest. Endoscope technology has advanced from 2D to 3D, so it is much easier to determine the depth from the surgical tool to the organs, making surgical procedures easier to perform.

A 3D endoscope includes several relay lenses inside. Two sets of lenses are necessary to make 3D image. For that reason, 3D endoscopes are rigid and heavy, with a larger diameter (e.g., 10 mm), but they provide excellent image quality. However, lenses can be damaged if the scope is dropped from surgical tray. In addition, periodic refurbishment is often required to maintain good image quality. Overall maintenance costs are high, and not all hospitals can afford proper maintenance.

Simply replacing relay lenses with miniaturized CIS modules would decrease the diameter of the 3D endoscope to less than half of the diameter of traditional endoscopes. CIS technology would also enable the scopes to be flexible and much lighter than those currently available. A 3D flexible video endoscope would provide cardiac surgeons with direct and real-time color images.

One potential difficulty is how to deliver such an endoscope into a coronary artery. A robotics assistant system could be the answer. Such a system could manipulate and deliver the medical device smoothly and accurately to the desired area. Combining a 3D visualized catheter with a robotics assistant system would not only enhance the accuracy of delivery, but would also enable the cardiac surgeon to gather more data. Software technology, including artificial intelligence (AI), could accelerate development of a 3D flexible video endoscope to next level.

More data input via AI would improve the endoscope's output and support a physician's treatment decision. Narrow and difficult-to-reach areas found in cardiovascular need accurate measurements. Diagnostics with direct and real-time color imaging capabilities could revolutionize these procedures.

Summary

The demand for direct visualization already exists in the cardiovascular market as is evidenced by fiber optics-based angioscopy. Advances in CIS module technology are now bringing even better image quality and smaller diameter scopes to the cardiovascular market. Combining visualized catheters with other diagnostic modalities should provide physicians with more and better information about a patient's condition, leading to faster, more accurate procedures and thus improving the quality of life for patients.

CIS miniaturization will continue to bring more possibilities to medical device design. With currently available CIS technology, the diameter of a flexible video endoscope is approximately 1.3 mm (4 Fr) with fiber optic illumination. Direct and real-time color imaging is already being commercialized in many minimally invasive devices for other medical areas, and it will bring many benefits to cardiovascular world as well.

This article was written by Shingo Ishii, Senior Manager, Medical Business Development, for Fujikura America, Sunnyvale, CA. For more information, Click Here.