Features July 2019 Issue

Sound Waves Can Help Doctors See Inside Your Coronary Arteries

Intravascular ultrasound (IVUS) reveals details about the amount and quality of plaque. This information helps physicians provide better care.

if you haven't heard about an imaging modality known as intravascular ultrasound (IVUS), keep your ears open. IVUS is being used with increasing frequency when cardiologists need more information about the size or quality of a blockage in the coronary arteries. IVUS is not a substitute for angiography, the test that takes moving X-rays during a cardiac catheterization. Rather, IVUS adds to the information gathered in this standard procedure.

"Angiography is a two-dimensional imaging technique, which has limitations. It tells us whether a coronary artery is narrowed or blocked, but it cannot reveal the true extent of the blockage, the size of the lumen or vessel dimensions, nor whether the plaque is soft, hard or mixed. It also does not show the degree of calcium around the circumference of the artery. IVUS can do all this," says Cleveland Clinic interventional cardiologist Rishi Puri, MD, PhD.

How IVUS Works

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The IVUS image above shows a healthy artery. The lumen (area through which blood flows) is wide open. The inner layer (intima) is thin and smooth. In the diseased vessel, the intima has become thick with plaque, narrowing the lumen and restricting blood flow.

IVUS employs a tiny ultrasound transducer on the tip of a probe that bounces sound waves off the inside walls of the coronary arteries. The probe is delivered over a regular guide wire used for angioplasty and stenting. The signals are sent back to a computer, which translates the images and displays them on a monitor. The images show the artery walls, the central channel (lumen) through which blood flows and the presence or absence of plaque.

Hard and soft plaques reflect sound waves differently. The resulting images enable a cardiologist to differentiate between plaque that is stable and plaque that is more likely to rupture and cause a heart attack.

When IVUS Is Most Useful

IVUS shines when the cardiologist is unsure whether a patient's angiogram tells the entire story.

"IVUS is particularly valuable when we're trying to reconcile how much an artery is narrowed and how plaque is distributed along its length," says Dr. Puri. "It also tells us which type of plaque is present-which may tell us that we need to change our stenting technique.

"Most importantly, it reveals the true size of the coronary artery we want to stent. This helps us optimize the size of the balloons and stents we will use forthe procedure," Dr. Puri explains.

If angioplasty and stenting are recommended, the information gleaned from IVUS can help cardiologists choose the right-size stent. After the stent has been inserted, IVUS can be used again to verify that the stent has been fully dilated, opening the artery as wide as possible.

Popularity Rising

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IVUS is not utilized with angiography in every patient with coronary artery disease, because not all coronary interventions require IVUS guidance. But Dr. Puri feels that IVUS could be used more often.

"IVUS really should be used when stenting the left main coronary artery, when the true size of the vessel to be stented is unknown or when plaque has clogged a stent," he says. "In these cases, there is solid evidence that when IVUS is used there will be fewer stent-related issues, such as stent re-narrowing or heart attacks."

IVUS in Research

IVUS is a valuable asset in clinical trials of novel drugs for treating coronary artery disease (CAD). Dr.Puri explains why:

"In such trials, we want to see whether a novel compound halts the progression of CAD, causes it to regress or has no effect. When a patient enrolls in the trial, we use IVUS to measure the volume of plaque in their coronary arteries. We then randomize the patient to take the study drug orplacebo.

"Periodically, patients come back to be re-evaluated with IVUS. If there is less plaque in the treatment group than in the placebo group, it tells us the new molecule is likely to work when tested on a larger scale. At this point, we have the confidence to proceed with its development."

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