Cardiologists disagree about the most effective modality for cardiac imaging
At the 16th Annual Nurse and Technologist Symposium at the Transcatheter Cardiovascular Therapeutics (TCT) conference in Washington, D.C., this week, three cardiologists made the case for different modalities, including CT, MR and other unique invasive imaging modalities, in the lecture series “Non-invasive Imaging in an Invasive World.”

Michael Poon, MD, associate clinical professor of cardiology and emergency medicine at the Mount Sinai Medical Center in New York City, made the case for CT angiography (CTA). He highlighted four reasons:
•    spatial and temporal resolution, for accurate coronary anatomic diagnosis;
•    radiation exposure, which is comparable to other contemporary imaging modalities;
•    diagnostic accuracy and short-term prognosis, which he said is “good’ in CT and has a “longer warranty;” and
•    vessel-wall biology, which is important for behavior modification.

With regard to spatial and temporal resolution, Poon reported that the diagnostic angiogram has the highest spatial resolution at 0.2 mm, but the 64-slice multidetector CTA is the closest with 0.4 mm. Poon said the Centers for Medicare & Medicaid Services (CMS) view CTA as a “gatekeeper for invasive test.” He supported the assertion with a quote from the CMS cardiac imaging guidelines: “Assessment of suspected congenital anomalies of coronary circulation of vessels.” Poon said that the appropriate criteria should be for patients with pre-test probability of coronary artery disease (CAD) and an equivocal stress test.

To accentuate that CT is not inferior to stress tests for diagnostic accuracy and short-term prognoses, Poon cited from a study in the Feb. 27 issue of the Journal of American College of Cardiology (JACC), which stated that for radiation exposure, cardiac CTA exposes patients with between 5 to 12 mSv, while nuclear imaging subjects patients to between 6 and 9 mSv.

Poon also said dual-source CT can decrease radiation exposure.

He suggested that doctors not wait until symptoms present themselves; he believes that characterization of plaque is enough of an indication to use CT for potential cardiac problems. The calcified or obstructed plaque stage is too late.

Poon said that vessel-wall biology is the future of CT. In screening for atherosclerosis, a clinician must perform lots of blood tests, or go right to imaging.

Edward Martin, MD, an invasive/noninvasive cardiologist at the Oklahoma Heart Institute in Tulsa, Okla., made the case for MR angiography (MRA).

Among the advantages of cardiac MR, Martin listed: high spatial, temporal and contrast resolution; obtains images in virtually any plane, including the whole chest; large fields of view; no ionizing radiation; and non-invasive.

Martin said that the MRA can look at heart as well as blood vessels. Overall, diagnostic accuracy is comparable to the catheter accuracy, according to Martin. He also said that it is the “best imaging modality for looking at the right ventricle.” The MRA contrast agents are not nephrotoxic, which Martin highlighted as another advantage.

Finally, Martin said that it is safe to image stents, even one day after placement.

Robert Schwartz, MD, of the Minneapolis Heart Institute, made a case for invasive procedures using imaging modalities, including optical coherence tomography (OCT) and in-vivo MRI. 

In order to stress the importance of invasive procedure imaging, Schwartz quoted from findings at last year’s TCT: “In the absence of plaque ruptures, plaques only rarely progress to severe narrowing.”

As a result, intra-plaque hemorrhage and plaque ruptures are responsible for progressive luminal narrowing, according to Schwartz. He said that the vulnerable sections can be detected focally.

Schwartz highlighted OCT because the tests are principally anatomic; the pictures have high resolution; and it is invasive imaging, which is not limited and can be used to interrogate large vessels; and Schwartz said that it has no proven tissue characterization.

Schwartz suggested that in-vivo MRI is adaptive for human coronary arteries, but he said that intravascular MRI has poor resolution, long acquisition time, difficulty in characterizing large vessels and has limited tissue characterization. Proven tissue characterization is another advantage of invasive procedures using imaging.

Todd Chitwood, manager of the Oregon Cardiology Diagnostic Center in Eugene, Ore., said CTA can complement other modalities, but the disadvantages of CTA are its contrast load and radiation, administration of sedation and beta-blockers and atherosclerosis.

He said that CTA has the additional limitations of cardiac motion, respiratory motion, vessel size and distal segments.

For cardiac MR (CMR), Chitwood said its advantages are differentiations; no radiation; excellent predictor viability; it is excellent with velocities in pulmonary arteries; and it detects masses well. The disadvantages of using CMR include the magnetic environment produced by the modality, as well as some poor and complex imaging.

In conclusion, Chitwood said that both CMR and CTA “provide supplements to conventional cardiac imaging.”