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Intravascular Ultrasound (IVUS)

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IVUS Image
Intravascular ultrasound images
from console: cross-sectional (l)
and reconstruction (r)
courtesy Volcano Corporation
What is IVUS?
Intravascular Ultrasound (or IVUS) allows us to see a coronary artery from the inside-out. This unique point-of-view picture, generated in real time, yields information that goes beyond what is possible with routine imaging methods, such as coronary angiography, performed in the cath lab, or even non-invasive Multislice CT scans.

This cross-section view can aid in stent sizing, and in confirmation that the stent has been placed optimally, is fully expanded and hugging the vessel wall. A growing number of cardiologists feel that the new information yielded by IVUS can make a significant difference in how a patient is treated, and can provide for more accurate stent placement, reducing complications and the incidence of stent thrombosis.

How Does IVUS Work?
IVUS uses echocardiography: the same technology as the ultrasound imaging used in treadmill tests and many other medical exams. Very high frequency sound waves, called ultrasound, are emitted by a transducer. These ultrasound waves, which are beyond the range of human hearing, bounce off the various types of tissue structures in the body and the echo of these waves is then converted into a picture.

intravascular ultrasound image,
courtesy Volcano Corporation

In the case of Intravascular Ultrasound, the transducers have been miniaturized to less than four hundredths of an inch and placed on the tip of a catheter. This catheter can be slipped into the coronary arteries over the same guide wire that is used to position angioplasty balloons or stents. It becomes, in effect, a tiny camera that gives us a cross-sectional view of the artery, a view that shows distinct circular layers, using shades of gray or colors, the major ones being:

  1. the adventitia -- the outer covering of the artery;
  2. the media -- the actual wall of the artery;
  3. the intima -- a layer of endothelial and other cells that make direct contact with the blood inside the artery -- in normal arteries this layer is thin; in diseased arteries (shown here) the intima is thickened by plaques or other tissue growth, often eccentric or asymmetrical;
  4. the lumen -- the actual open channel of the artery through which the blood flows.


How is IVUS Different from Standard Angiography?
The current “gold standard” of invasive angiography shows only the lumen, as an X-ray shadow image created by the injection of contrast dye (as seen on the left).

Although angiography shows "narrowings", as well as a dynamic picture of the blood flow, it does not differentiate the other layers (or even the plaque itself, as shown in the artist's rendition on the right).

In several situations, the increased information provided by Intravascular Ultrasound literally can change the picture of the disease, and affect treatment decisions. For example, in a normal artery the intimal layer is thin -- when measured, there is little difference between the diameter of the lumen (open channel) and the diameter of the media (the arterial wall). In a "blocked" or diseased artery, the intima is thickened by plaques or other tissue growth, and the lumen diameter is reduced.

But often the plaque or tissue growth is not evenly distributed, resulting in an eccentric shaped lumen. This eccentric shape is clearly shown by intravascular ultrasound. But the X-ray angiogram only shows a "side-view" and the eccentric shape is not seen. Depending on the angle of view, this may make the artery look more blocked than it really is -- or conversely, may give a false impression that the artery is only slightly blocked and does not need to be treated. With IVUS, just a few clicks on the console measures the area of the blockage, the size of the artery and yields an accurate percentage of narrowing.

Another example is that sometimes the plaque pushes deeper into the vessel wall, giving the appearance that the lumen is not significantly blocked. Yet a significant amount of diseased plaque may exist within the arterial wall, ready to rupture and cause a cascade of events, resulting in a heart attack. This is called vulnerable plaque, and cannot be visualized using standard angiography.

When is IVUS Done?
Intravascular ultrasound is done in the catheterization laboratory in conjunction with angiography. Some cardiologists use it occasionally, in difficult cases, or to assist in the selection and sizing of stents and balloons. Others use it routinely, to confirm accurate stent placement and optimal stent deployment.

How Can IVUS Make Stenting More Accurate?
One of the causes of stent thrombosis or restenosis is poor "stent apposition" -- the stent has not been expanded to the full width of the artery, and this under-expansion creates a "pocket" which can collect platelets and other debris, causing a reblockage. Research conducted using IVUS has confirmed that one of the causes of restenosis is inadequate dilatation; that is, physicians, concerned with injuring or dissecting the artery with a balloon inflation, have tended to "play it safe" and end up under-sizing or under-inflating the balloon and stent.

With the accurate measurements of both the true diameter of the artery and the diameter of the open lumen channel provided by IVUS, the guesswork is taken out of choosing the correct size balloon and stent. Using only angiography, a cardiologist may underestimate the size of a diseased artery.

IVUS can also measure the length of the diseased area, so the precise length of the stent needed can be determined ahead of time, reducing the need for overlapping stents which are known to increase the risk of thrombosis.

Once the stent has been implanted, IVUS can clearly show the stent struts in relation to the arterial wall and plaque. If the stent has been undersized or if there is any area that needs "touching up", a larger balloon can be directed to it and expanded to fit the stent optimally.

Although IVUS was first used over 20 years ago, the current concerns over stent thrombosis and patient outcomes have spurred a new interest. The recent S.T.L.L.R. study, sponsored by Johnson & Johnson, showed that current DES deployment techniques led to some form of geographic miss in 66.5% of patients. That means two-thirds of stents are not optimally placed, which translates into negatively impacted patient outcomes, with significantly higher restenosis, thrombosis and myocardial infarction rates in patients where the stent was not placed properly. The study concluded that "a re-examination of stent placement technique including the use of IVUS is certainly warranted."

Modern IVUS systems are completely integrated into the catheterization lab and with proper training, the cardiologist can add this new imaging technology to a standard diagnostic angiogram with a minimum of impact on the patient.

In-Depth For more information visit our Intravascular Guidance Center.


Revised February 2013, Angioplasty.Org staff