cardiac ct & perfusion

7/29/2019 Cardiac CT & Perfusion

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Cardiac CT protocol identifies perfusion defects, stenoses

Boston, MA - A novel cardiac computed-tomography (CT) protocol that combines stress and rest

myocardial perfusion imaging with coronary CT angiography in a single test can identify perfusion

defects with diagnostic accuracy comparable to single-photon-emission computed tomography

(SPECT), a new study has shown [1].

The combined protocol, performed by Dr Ron Blankstein (Massachusetts General Hospital, Boston,

MA) and colleagues, is still considered investigational but suggests that cardiac CT has a potential

role in myocardial perfusion imaging for the detection of myocardial ischemia as well as providing

information on coronary stenoses.

"Although future studies are needed to further define the diagnostic accuracy of combining CTangiography and CT perfusion, in patients already undergoing cardiac CT for evaluation of coronary

anatomy, rest perfusion defects, when present, can be very helpful in identifying areas of infarcted

myocardium," write the authors in the September 15, 2009 issue of the Journal of the American

College of Cardiology.

In total, 33 patients who had a nuclear stress test and invasive angiography were included in the

study. The protocol consisted of a contrast-enhanced scan during adenosine infusion, done to

visualize the coronary arteries and identify coronary artery stenoses; a second contrast-enhancedscan acquired at rest to identify perfusion defects; and a third, delayed to visualize late

enhancement.

On a per-vessel basis, CT perfusion had a sensitivity of 79% and a specificity of 80% for the detection

of stenosis >50% compared with a sensitivity of 67% and specificity of 83% for SPECT. The CT

myocardial perfusion assessment at stress had high sensitivity, 93%, and 74% specificity, to identify

coronary artery stenoses >50% associated with a SPECT perfusion abnormality. Sensitivity for the

detection of very high-grade stenoses was 96%, which was higher than for CT angiography alone.

In an editorial accompanying the published study [2], Dr Stephan Achenbach (University of Erlangen,

Germany) said the study protocol was comprehensive and approached what some experts are calling

"one-stop-shop" imaging. Still, he notes that before CT myocardial perfusion imaging is backed by

data "even remotely as strong as for single-photon-emission CT, many giant steps will still need to be

taken."

Achenbach adds that although there was high sensitivity for stress CT myocardial perfusion,

specificity was a bit disappointing at approximately 75%. "Specificity is a problem of coronary CT


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angiography to begin with, and it seems that adding another test with limited specificity may be a bit

problematic," he writes. "We will need to better understand the true accuracy of stress CT

myocardial perfusion before it can be used clinically."

By Catherine Carrington

For anyone keeping an eye on advances in cardiac CT, dont blink. Once dominated by coronary

calcium screening and weighed down by controversy, cardiac CT has gotten a second wind, racing

past technological obstacles and impressing former skeptics with its clinical promise.

Early research suggests that CT could become the preferred tool for noninvasive angiography,

differentiate soft atherosclerotic plaque from its less vulnerable calcified form, and perhaps add a

new dimension to myocardial perfusion imaging. Cardiac CT is being cheered on by both radiologistsand referring clinicians.

Ive never seen a year in cardiac MR as Ive seen in cardiac CT, said Dr. Richard D. White, head of

cardiovascular imaging at the Cleveland Clinic. Its taken off very, very rapidly.

Electron-beam technology clearly is responsible for showing what CT can do when it images fast

enough to stop cardiac motion. But credit for the enthusiasm that propels the field today lies withmultidetector spiral CT, along with three-dimensional reconstruction technology, according to Dr.

Lawrence M. Boxt, cardiovascular imaging chief at Beth Israel Medical Center in New York City.

It allowed radiologists who were doingconventional CT to start doing cardiac CT. They didnt have

to buy a machine just for one organ, he said. With very fast scanners and the new 3 -D

reconstruction technology for handling stacks and stacks of data in a convenient manner, people

started seeing the coronary arteries-and started thinking about going after them.

While researchers are most excited about their progress in CT coronary angiography and intrigued by

the possibility of perfusion imaging, other applications of cardiac CT have already become part of

routine clinical practice. In some cases, CT is taking work away from established forms of cardiac

imaging.

MR, for example, is being elbowed aside (see sidebar) as CT takes over the evaluation of large

aneurysms of the thoracic aorta, particularly when stent-graft therapy is likely. Imaging to determine


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whether chest pain results from an abnormal pericardium is another example of an exam MR is

ceding to CT, as is the initial evaluation of arrhythmogenic right ventricular dysplasia.

Its now commonplace for us to first screen for arrhythmogenic right ventricular dysplasia with CT,rather than MR, because we can quickly detect the likelihood of significant muscle disease of the

right heart, White said. The first line of diagnostic workup in our institution is becoming CT. Then

we go to MR when needed and do a more tailored examination.

CT may also provide a noninvasive alternative to intravascular ultrasound in evaluating patients for

transplant vasculopathy, and it is ideal for guiding certain electrophysiologic procedures, such as

catheter ablation of atrial fibrillation. Often the source of the arrhythmia can be found at the opening

of the pulmonary vein. CT can help determine which patients are good candidates for ablation by

defining the size of that vessel and the pattern of its side branches. Providing guidance during

electrophysiologic procedures is also a role CT could claim in the future, White said.

Coronary CTA: No Joke

When talk turns to noninvasive coronary angiography, CT is increasingly the subject matter. Many

imagers say they have all but given up on MRA of the coronaries.

I dont do coronary MRs anymore. Its just too time-consuming, and I cant get a straight answer,

Boxt said.

Instead, he is one of several researchers fueling a flurry of studies into coronary CTA. Boxt expected

to begin a study in October comparing CTA with conventional angiography in patients already

scheduled for the cath lab. Even before that, he and his colleagues were performing coronary CTA

under certain circumstances, such as the evaluation of coronary artery anomalies or low-likelihood

stenoses. Boxt said he once considered cardiac CT a joke, but he has been impressed by the results.

The pictures are just spectacular. When you electrocardiograph-gate, you see everything: the entire

course of the right coronary artery, the left main becoming the circumflex, the anterior descending,

and side branches, he said. Were seeing incredible detail.

Dr. Tom Brady, director of the cardiac imaging program at Massachusetts General Hospital, is a little

more reserved in his praise of coronary CTA. At press time, he and his colleagues had compared the

results of coronary CTA and conventional angiography in about 30 patients with known or suspected


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coronary artery disease. Early results suggested CTAs overall sensitivity for coronary artery stenoses

was in the range of 70%: better in the proximal portions of the coronary arteries and worse in the

distal segments. The right coronary artery can also present a challenge, because it moves out of the

plane of acquisition as the heart beats.

CTA is coming along nicely, but it still needs more work. We need to decrease the temporal

resolution of the acquisition and improve a couple of other technical parameters before its going to

give us a great study every time from the coronary ostia all the way down to the apex. But Im very

bullish on it, Brady said.

Not everyone is convinced of a clinical role for coronary CTA. Dr. William Stanford, a professor of

chest and cardiovascular imaging at the University of Iowa, believes that a patient who has a high

score on coronary calcium screening, for example, should probably have a nuclear stress test to look

for perfusion defects caused by flow-limiting stenoses.

That individual probably ought to go to cath, not only to define the anatomy, but also because you

can do balloon angioplasty at the same time. Im having trouble finding where CT angiography

though its talked about a lothas a big clinical use, he said.

White holds the opposite view. Even if it takes several years for CTA to fully overcome its limitations,

its potential value remains high, he said. Just being able to tell clinicians that proximal arterial

segments are clear may be enough to eliminate unnecessary conventional angiography in many

cases, saving the patient from an invasive procedure and reducing healthcare costs.

We dont necessarily have to shoot for the stars to have an impact, White said.

Soft Plaque

The proper role of coronary calcium screening in determining the risk of heart disease has been

controversial and remains hotly debated. The subject is the center of a technological tug-of-war

between electron-beam and multidetector technology. Just as studies increasingly supported its

value as a cardiovascular risk factor, the attention of clinicians and researchers shifted to the

identification of soft plaque.

Many researchers are observing what they believe to be soft plaque on CTA images. Since soft plaque

does not show up on conventional angiography and is more likely to be unstable than calcified


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plaque, this finding has sparked intense interest. So far, CTA cant reliably determine which soft

plaques are stable and which are likely to rupture and cause a heart attack, but research is moving in

that direction.

Detecting segmental enlargement of the coronary artery and the presence of soft plaque is a pretty

ominous sign, and were able to pick up on that even now, White said. I think we can get a hint of a

less-than-desirable situation-one that hasnt presented with symptoms yet-and maybe use this to

monitor therapies directed at plaque progression.

Perfusion Imaging

Mention myocardial perfusion imaging and CT in the same sentence, and the typical response is a

blend of interest and skepticism. Dr. Ting-Yim Lee plans to turn skeptics into believers.

CT perfusion imaging is here already. We can calculate blood flow maps in an ischemic model, and

where you expect the ischemia to be, its there, said Lee, a Ph.D. researcher at the John P. Robarts

Research Institute and the Lawson Health Research Institute, both in London, Ontario. The challenge

facing us is to prove to the world that it really works.

Lee has developed a method to quantify myocardial blood flow and distribution volume usingcontrast-enhanced multidetector CT. The results, displayed in pseudocolor maps, show perfusion

defects and reveal the presence of infarcted tissue. But they also take advantage of CTs spatial

resolution to suggest whether the infarction is transmural or extends only partway through the

myocardium. Thats something PET, perfusion imagings gold standard, cant do.

Im very excited about this, Lee said. Were using an ordinary CT scanner, we are injecting contrast

using standard techniques that CT techs use day in and day out, and the time of scanning is less than

30 seconds. And out of that you get all this information.

So far, Lee and his colleagues have studied dogs with experimentally induced ischemia, but they

anticipate beginning studies in human heart patients next year.

The perfusion imaging protocol teams a four-slice multidetector CT scanner, ECG gating, and

retrospective reconstruction of projection data selected from the end diastolic phase of the heart

cycle, when the heart is nearly motionless. Perfusion studies are done following an intravenous

injection of contrast. CT tracks the rate at which contrast passes through the aorta into the


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myocardial capillary network and then through various regions of the myocardium. From these two

pieces of data, separate software that Lee has developed and licensed to GE Medical Systems-known

as CT Perfusion 2-calculates blood flow, blood volume, mean transit time, and leakage of contrast

from the capillaries into the myocardial interstitial space, and then creates a pseudocolor perfusion

map.

For determining myocardial distribution volume, CT scanning is done first without contrast, then

again after a continuous 30 to 60-minute infusion. Baseline images are subtracted from contrast-

enhanced, steady-state images. An above-normal distribution volume would indicate the breakdown

of myocardial cell membranes and leakage of contrast into the intracellular space, a sign of

myocardial infarction. CTs spatial resolution is high enough to show whether the increased

distribution volume-and, hence, the infarct-extends through the myocardial wall.

Lees next step will be to validate his blood flow measurements against those determined with

radiolabeled microspheres that have a diameter of about 15 micron-just large enough to pass

through the coronary arteries and lodge in the myocardial capillaries. Assuming these animal studies

go well, Lee plans to validate his technique in humans using PET as the quantitative gold standard.

White and his colleagues have had some success with CT perfusion imaging, detecting a few cases of

myocardial infarction from perfusion defects observed while conducting contrast-enhanced CT of

suspected aortic dissection. Still, he is convinced that CT perfusion imaging must overcome severalobstacles before it can be accepted clinically. Lees technique requires slowing the heart rate to 60 to

80 bpm by administering medications like beta blockers, something that White prefers to avoid in

sick patients. In addition, improvements in contrast agents that would enable them to pass less

quickly through the coronary circulation would be helpful, he said.

Youcant overlook some of the limitations of CT, including its speed. Its not so fast that you can

necessarily appreciate a first-pass effect, which is what is needed, given the agents at hand, White

said.

The imaging industry is advancing quickly to give researchers increasingly sophisticated tools,

developing scanners capable of acquiring eight to 16 simultaneous slices of imaging data. In what

could be an even bigger technological leap, volume CT systems are under development.

Within the next five years were going to see the next generation of CT going the extra length,

White said. I think were really seeing a new CT.


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Ms. Carrington is a freelance medical writer in Vallejo, CA.

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