Advances in Intravascular Ultrasound Technology

This illustrates Philips' IVUS system, which can be integrated with angiography and FFR to provide more information on coronary lesions.

Intravascular ultrasound (IVUS) is a primary intravascular imaging modality in the cardiac catheterization laboratory. It helps physicians develop optimal treatment plans, guide stent placement, and confirm and optimize treatment outcomes. IVUS can confirm whether a stent implanted in a blood vessel is fully apposed to the vessel wall, avoiding in-stent restenosis due to poor apposition.

Although optical coherence tomography (OCT) can provide higher-resolution intravascular imaging, OCT, based on reflected light, lacks the tissue penetration of IVUS, making it difficult to observe the deeper vessel wall and thicker plaques. OCT also has its technical limitations. During use, it requires the use of saline or contrast agent to clear blood from the target vessel segment, avoiding signal loss due to blood interference, so that the light emitted by the catheter can penetrate the tissue and reflect back.

IVUS mounts a miniature ultrasound transducer to the tip of a catheter, providing real-time high-resolution images of the vessel lumen. It can display the vessel lumen, plaque, and vessel wall, as well as the true lumen within the plaque. IVUS can measure the length of coronary lesions and the actual diameter of the blood vessel, guiding the selection of appropriate stent size and stent implantation location. Simultaneously, IVUS can further interpret the angiographically observed vascular obstruction from the morphology of the blood vessels. It provides deeper insights into ambiguous lesions, pre-treated lesions, bifurcation lesions, diffuse lesions, and multiple imaging of the left main stem lesions.

Boston Scientific was the first company to bring this technology to market, followed by Volcano (acquired by Philips), Acist, InfraredX, and Conavi, who also launched their respective products.

In recent years, Philips has been promoting the development of IVUS, including the integration of IVUS with angiography and physiological assessment tools such as fractional flow reserve (FFR) or instantaneous wave-free ratio (iFR). This fusion technology can be intuitively displayed on the screen in front of the operating table in the cardiac catheterization lab, providing physicians with more information about the lesion and its severity in the same vessel segment. It also helps physicians quickly identify culprit lesions.

Philips' IVUS system also provides a ChromaFlow function similar to Doppler ultrasound. This function can highlight blood flow information in red within the vessel lumen, clearly showing the junction of the vessel lumen and the bifurcated vessel, making the black-and-white IVUS image easier to understand.

Philips also provides a series of seven IVUS catheters with different functions for various coronary and peripheral vessels.

In 2018, Conavi Medical's Novasight system, which integrates OCT and IVUS technologies, received 510(k) clearance from the U.S. FDA. This fusion imaging system not only has the excellent image resolution and contrast of OCT, but also the detection depth of IVUS. The multimodal combination imaging of this system is better and more accurate.

Ten years ago, Infraredx (a subsidiary of Nipro) developed a near-infrared spectroscopy system (NIRS), an imaging modality for assessing the lipid burden in a vessel segment. However, most interventional cardiologists found this imaging modality difficult to understand. A few years later, the combined use of IVUS and NIRS led to a new method for intracoronary imaging and plaque component analysis.

Many interventional cardiologists quickly discovered the potential of the NIRS-IVUS system for detecting vulnerable plaques, used for precise localization of lipid-rich plaques (LRP) to closely monitor cases or preemptively implant stents to treat them before cardiovascular events occur. Both LRP and PROSPECT II[2] studies have shown that NIRS-IVUS technology can help identify high-risk characteristics of non-obstructive lesions detected by angiography that may lead to future cardiovascular events.

Dr. Ron Waksman, Associate Director of Cardiology at MedStar Heart Institute in Washington, said, "Multi-vessel NIRS technology can easily and safely assess and identify vulnerable patients and vulnerable plaques. Intravascular NIRS imaging can be used in mild stenosis or non-obstructive coronary arteries to identify potentially high-risk patients and non-culprit vessels, and should be considered for patients undergoing cardiac catheterization who may undergo PCI surgery."

Infrared's latest generation Makoto system automatically quantifies all lipid cores in the region of interest, expressed as the lipid core burden index (LCBI). By analyzing data on lesion tissue structure and composition using NIRS-IVUS technology, the effects of drug treatment on plaque size and lipid core composition are further evaluated. This technology is also being used in research to develop new preventive methods for stabilizing plaques because it can be used to assess the reversal of plaque size and composition.

Intravascular imaging was initially used primarily in coronary arteries, but now this technology has been applied in interventional procedures in peripheral arteries and veins.

In the field of peripheral artery treatment, it was not until October 2020 that a committee of 40 global interdisciplinary medical experts drafted the first consensus document on the reasonable use of IVUS in peripheral vascular disease (PVD). Currently, there is no uniform standard for the use of IVUS in PVD. The new expert consensus aims to help establish global standards and improve the quality of PVD treatment.

The new IVUS consensus and recommendations are based on clinical evidence, expert practice standards, and clinical experience, aiming to improve the quality of PVD treatment.

Dr. Eric A. Secemsky, interventional cardiologist at Beth Israel Deaconess Medical Center and assistant professor of medicine at Harvard Medical School, said, "The launch of this expert consensus is strong support for the use of IVUS in peripheral interventional procedures. Future efforts need to focus on improving the IVUS operation in clinical practice and surgical procedures, thereby improving patient outcomes."

The consensus document includes a review of the main clinical scenarios and decision-making processes for IVUS use. The document was presented at a symposium at the Vascular Interventional Advances (VIVA) meeting in 2021. In this way, experts optimized IVUS use, identified potential discrepancies, reached clinical consensus, and established cross-clinical professional standards, resulting in positive outcomes for patients.

In venous interventional therapy, a recent double-blind controlled study showed that in a large number of cases of iliac vein stent implantation, IVUS was superior to venography in determining treatment localization. This study included 155 patients who underwent treatment for chronic iliac vein occlusion.^[3]In assessing major venous stenosis, 19% of cases in the venography group failed to identify the stenosis, 68% underestimated the degree of venous stenosis and failed to accurately identify the location of venous stenosis, while IVUS was more accurate than venography in all three key indicators.

Dr. Myriam Montminy, first author from the RANE Center at St. Dominic Memorial Hospital in Jackson, Mississippi, and Fellow of the Royal College of Surgeons of Canada, explained: "In the treatment of chronic venous occlusion in the deep venous system, the correct assessment of the location, degree, and anatomy of venous stenosis is critical for optimal stent placement and successful intervention. Although venography is more readily available and less costly than IVUS, more and more studies have shown that IVUS has higher sensitivity than venography in identifying stenotic lesions in the iliac veins. Our study aimed to further improve the success rate of stent placement in chronic venous occlusion in the deep venous system by comparing the accuracy of these diagnostic techniques in assessing key parameters required to guide stent placement."

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