The Rise of the Coronary CT Angiogram

Stress tests are a thing of the past. Make way for coronary CT angiography. Wide-eyed medical students might gather around a treadmill in 20 years to observe a patient doing a traditional stress test as a rarity to identify valvular disease, detect arrhythmias and gauge functional capacity, but not as a means to diagnose occlusive coronary disease.

In the not too distant future, I can envision a patient with low- or intermediate-risk pretest probability for coronary disease coming into the clinic with some potential angina chest pains  and I will not initially get a stress test or invasive coronary angiogram to see if occlusive coronary stenosis is the cause. Really!? This is what we have been doing for 50 years!! Instead, a coronary CT angiogram will be done.

Clinicians may already be able to do this, depending on where they practice. However, in the United States, it remains a bit challenging to get the test approved by insurance companies. Hopefully, the end of this roadblock is in sight.

Yes, indeed coronary computed tomographic angiography, known as coronary CTA, is without question the future of coronary imaging. All the concerns about radiation exposure, using beta-blockers to slow the heart rate for better temporal resolution are out the door.

Computed tomography technology really is advancing rapidly. Let me show you what the future holds.

What Is a Coronary CT Angiogram (Coronary CTA)?

Coronary CT angiography a.k.a. coronary CTA or cardiac CT  is a relatively new way to image the coronary arteries. This exam is not only quick and now readily available, but quite accurate. Kind of like a typical CT scan, an 18-gauge IV is required, contrast is injected and CT images are created.

So, what is the difference between this and a normal CT scan? Well, the heart moves (creating motion artifact), and the coronary vessels are quite small (making it tough to see them well). Despite this, the new scanners are so much more advanced than what we typically use today.

Just look at what we can see:

CTA-Example1

Why Is Coronary CTA So Important?

An interesting article published in The New England Journal of Medicine by Patel MR, et al.1 looked at the percent of invasive coronary angiograms that are normal. In almost 400,000 patients here, only 37.6% of patients had occlusive coronary disease! That means 62.4% did not have coronary stenosis causing symptoms, yet went through an invasive procedure with potential risks and complications to determine this! We absolutely need to improve this number. Invasive coronary angiography is the gold standard but, of course, has some risks. We must try our best to not send patients with normal coronary arteries for such an invasive test.

This is where coronary CTA steps in. As technology advances, and we can do these scans quickly with low radiation exposure, every single patient with potential angina or an abnormal stress test will get a coronary CTA prior to going for invasive coronary angiography. If normal, invasive testing will not be necessary. It makes a lot of sense and then we should just skip the stress test altogether, as the false positive rate is still significant, unlike that of a good-quality coronary CTA.

On the other end of the spectrum, if a patient has an abnormal stress test or potential angina, and left main disease is found or a total occlusion is seen, doing a risky invasive test like coronary angiography might not be needed. I have heard stories of cardiac surgeons, at some centers in Europe that are good at coronary CTA, taking patients directly to coronary artery bypass grafting (CABG) with only the coronary CTA data without invasive coronary angiography!

How Has Coronary CTA Improved So Dramatically?

CTA-Example2Currently, getting a good-quality coronary CTA is not so easy. It requires some technical expertise and practice. In addition to obtaining a good test, we can’t just focus on making the correct diagnosis patient safety is priority. We must give the absolute least amount of radiation possible during the exam. We are quickly finding out how important this is to patient safety in the long term. So, what has changed so dramatically to help improve quality and radiation safety here?

Coronary CTA Quality:

To get a good scan, there are three factors to consider.

  1. Spatial resolution: We need to be able to see those small 2 mm coronary vessels. The new scanners, up to 512-slice with improved detectors, can see the tiniest of vessels.
  2. Temporal resolution: The images must be acquired quickly, in between heartbeats, to limit cardiac motion. With most current scanners, beta-blockers are given to patients in order to keep the heart rate about 65 bpm or less to get a good picture. But the newest scanners can image the entire body from head to toe the so-called “pan-man-scan”  in just 2 seconds with astounding image quality! This means for cardiac CTA, no beta-blockers will be required, thus eliminating the need to have a nurse present during a scan.
  3. Contrast resolution: We need to be able to get good-quality pictures that aren’t blurry so we can measure the degree of stenosis present. The engineers have come a long way in improving this.

Coronary CTA Radiation Exposure:

Raise your hand if you want to get fried with radiation for your coronary CTA? No takers? Unfortunately, there are still to this day many centers where the radiation exposure for a coronary CTA is around 25 to 30 milli-Sieverts (mSv). For those not too familiar with this, that is quite a high dose! A nuclear stress test is usually around 20 to 25 mSv, a coronary angiogram around 7 mSv, and a chest X-ray around 0.1 mSv of radiation.

It has been recognized that this is a big problem, and it is crazy how much radiation exposure can be reduced for coronary CTA now and in the near future thanks to new techniques and technology. The best scans can now obtain an image of pristine quality with only 0.2 mSv of radiation, which is equivalent to only two chest X-rays!

There are quite a few complex ways this radiation dose reduction has been achieved, which we do not need to detail here. However, lets note that they include: prospective gating (predicting the next heartbeat, then scanning between, instead of scanning throughout the whole cardiac cycle); lowering tube voltage, then using “iterative reconstruction” to fill in the gaps; and the ability to obtain all images in one heartbeat (even with faster heart rates) with faster scanners.

Pros and Cons of Coronary CTA – Still Not A Perfect Test

Pros-Cons-TransparentSounds like a perfect test to get in just about every way, right? Well, there are some limitations to getting a high-quality study, and a little bit of risk still exists. The pros are pretty clear, but what about the cons? We talked about the radiation issue, which has improved; but what else limits this test?

Pros:

  1. High diagnostic accuracy: Quality has markedly improved, and coronary CTA may become new gold standard.
  2. Low radiation exposure: New scanners can easily achieve less than 1.0 mSv of radiation exposure.
  3. Efficient: Once beta-blockers are no longer needed, as is the case with most of the new scanners, this test will take just a few minutes.
  4. Non-cardiac findings: Think the patient’s symptoms are cardiac, so you ordered a coronary CTA? Quite often, a pulmonary embolism, lung mass, aortic dissection or other non-cardiac findings can be detected here but would not be seen on a stress test.

Cons:

  1. Radiation: Regardless of the low dose, radiation is radiation is radiation.
  2. Patient size: The morbidly obese patient has issues with contrast resolution. Penetration is limited, and we can get some significant image degradation, making hard to see the mid- to distal-coronary vessels. With the new scanners, this issue is markedly lessened, but still there to some degree.
  3. Iodinated contrast: How about that contrast? We still need to use iodinated IV contrast, which can of course cause severe allergic reactions at times and may also be nephrotoxic.
  4. Intervention: I always tell patients that one limitation to coronary CTA is simply that we can’t fix a coronary stenosis with a CT scan. If there is a severe proximal left anterior descending stenosis, it is still necessary to send the patient to invasive coronary angiography, where intervention can be done.
  5. Reimbursement: A big issue in the United States is simply obtaining health care coverage for the test. Currently, insurance companies will only approve a study after physician-to-physician direct discussion, if there is potential angina symptoms and an already performed stress test with equivocal results (did not answer the clinical question). Unfortunately, there is a big difference between the actual appropriateness criteria for ordering a coronary CTA and what you can actually get approved through insurance.
  6. Coronary calcium: We previously worried about coronary calcium and still do a bit. Where I perform scans now, we do a calcium score first; f the value is greater than 800, we do not perform the full coronary CTA, as artifact from the calcium reduces accuracy. New technology can subtract out the interference that calcium causes, so this won’t be a as much of a problem in the future.

Evidence to Support Coronary CTA

statins-transparentIt sounds like such a good thing. But how many times have things seemed good in theory, then never panned out in the real world? For any new thing in medicine whether a new drug, device or diagnostic test we always want to be able to show that patient outcomes are improved. This is done best by showing a mortality reduction. Here is where the problem lies in gaining more support for coronary CTA.

It has been shown that diagnosis is improved, efficiency is increased, cost is significantly reduced, and the images are pretty cool looking; but we have yet to show better patient outcomes with coronary CTA. However, coronary CTA has never been compared directly with stress testing to see which is better.

Let me briefly outline what I am talking about here, with a quick look at a few trials.

CT-STAT Trial2: Compared coronary CTA to rest-stress nuclear stress imaging to evaluate patients with low-risk chest pain in the ER. Coronary CTA was more efficient and cost effective.

ACRIN-PA Trial3: Compared coronary CTA to traditional approach in patients with low- to intermediate-risk chest pain in the ER. Coronary CTA had a high negative predictive value and, if negative, the patient can safely be discharged from the ER.

ROMICAT Trial4: Coronary CTA accurately identified patients with chest pain with normal coronary arteries who can be safely discharged from the ER.

PROMISE Trial5: Randomized symptomatic patients with suspected CAD to coronary CTA vs. traditional stress testing to evaluate chest pains. Results pending.

CONFIRM Registry6: Analyzed predictive value of coronary CTA to validate an optimized prognostic score. Results pending.

Other Interesting Applications – The “Triple Rule-Out” Scan

The use of a coronary CTA in the ER is quite cool. The “triple rule-out” scan to diagnose either occlusive coronary disease, pulmonary embolism and/or aortic dissection is coming to an ER near you!

Some other things that can be done with this technology include detecting left atrial appendage thrombi in atrial fibrillation patients, cardiac function and valve assessment, myocardial viability, perfusion imaging and fractional flow reserve (FFR) measurements with CT.

How about the portable coronary CTA? Not too far away! A 32-slice scanner that can be wheeled to the ICU is already in use! It won’t be long until coronary CTAs can be done on these scanners.

PortableCT

Coronary CTA – The New Gold Standard?

Absolutely! Coronary CT angiography will, in my opinion, be the new gold standard for coronary imaging in the future. Quick, non-invasive, low radiation exposure and absolutely spectacular image quality will make this a test used almost universally in any patient with suspected clinical coronary disease.

This is quite an exciting test that will change the way we diagnose coronary disease and significantly alter our approach to patient care. As coronary CT angiography technology and research progresses, I will be sure to keep you informed! The Society of Cardiovascular Computed Tomography will serve as a good resource to learn more.

by Steven Lome, DO, RVT

References:
1. Patel MR, et al. Low Diagnostic Yield of Elective Coronary Angiography. N Engl J Med. 2010;doi:10.1056/NEJMoa0907272.
2. Goldstein JA, et al. The Coronary Computed Tomographic Angiography for Systematic Triage of Acute Chest Pain Patients to Treatment (CT-STAT Trial) Trial. J Am Coll Cardiol. 2011;doi:10.1016/j.jacc.2011.03.068.
3. Litt HI, et al. CT Angiography for Safe Discharge of Patients with Possible Acute Coronary Syndromes. N Engl J Med. 2012;doi:10.1056/NEJMoa1201163.
4. Hoffman U, et al. Coronary Computed Tomographic Angiography For Early Triage of Patients with Acute Chest Pain - The Rule Out Myocardial Infarction Using Computer Assisted Tomography (ROMICAT) Trial. J Am Coll Cardiol. 2009;doi:10.1016/j.jacc.2009.01.052.
5. PROMISE Trial.
6. CONFIRM Registry.
Cardiac Computed Tomography Writing Group. ACCF/SCCT/ACR/AHA/ASE/ASNC/NASCI/SCAI/SCMR 2010 Appropriate Use Criteria for Cardiac Computed Tomography. Circulation. 2010;doi:10.1161/CIR.0b013e3181fcae66.