One stop shopping with cardiac imaging: CT or MRI?
Two radiologists debated the value of MR compared to CT for cardiac imaging. Ricardo Cury, MD, assistant professor of Harvard Medical School, director of clinical cardiac MRI at Massachusetts General Hospital in Boston, heralded the benefits of CT for cardiac imaging, while J. Paul Finn, MD, professor of radiology and medicine at University of California at Los Angeles, supported the use of MR for cardiac imaging. The session was part of the 93rd annual meeting of the Radiological Society of North America (RSNA) in Chicago.

Cury highlighted how the last seven years of multidetector cardiac CT (CCT) has seen an important evolution because temporal resolution is increasing and scan time has dropped as well.

He believes that the CT can be used for comprehensive cardiac acquisition, including the coronary arteries, left ventricular function (LVF), coronary valves, and for perfusion and viability.

To effectively image the coronary arteries with 64-slice, Cury said “you need to slow down the heart rate, as well as using beta-blockade and nitrates.”

For stenosis detection, he questioned which CT reconstruction method should be used: While the axial, myocardial perfusion reserve (MPR), maximum intensity projection
(MIP), or Curved MPR, Curved MIP all showed comparably high accuracy, 3D Volume Rendering Technique (VRT) had lowest accuracy. Cury said the “overall message is don’t rely on the VRT reconstruction for stenosis detection.”

Multidetector CT (MDCT) had a high rate of detection of stenosis compared with cardiac MR. Cury also said its negative predictive value was between 96 and 98 percent. Overall, he said that the “16-slice and 64-slice had a very high accuracy for stenosis detection, better diagnostic accuracy, which results in the greater clinical usage.”
Cury suggested that the cardiac CT should not to be used in patients with typical symptoms, mainly used to rule coronary artery events. On the other hand, he said the coronary CTA, should be used to “rule out coronary stenosis in patients with atypical chest pain and to visualize anomalous coronary arteries.”

Cury acknowledged the limitations of CCT angiography are: radiation exposure of 5 to 20 mSv; IV contrast; irregular heart rate; and no hemodynamics information is revealed. To rectify some of these limitations, Cury said that “a new technology is evolving where heart rate control, breathhold cooperation, and other current complications might no longer be impediments.”

Stent evaluated is a potential application of CCTA. Another application is the detection of coronary plaque because CCTA outperforms IVUS (92 percent to 88 percent) for plaque detection, although Cury said “its benefits are unclear for non-calcified plaque.” For acute chest pain and for possible ACS patients, CCTA has potential because he said four of six studies using a 64-slice MDCT in the ER show sensitivity above 90 percent. Finally, Cury listed cost-effectiveness as an attribute of CCTA because of its ability to detect potential risks before they need lead surgical requirements.

Cury did concede that “MRI is still the gold-standard for perfusion. The overall comparison is good, but for late-enhancement perfusion, MRI has a much higher accuracy rate.”

In his final comparison of cardiac MR to CCTA, Cury said the “speed of acquisition is better for CT, also potentially better reimbursement, but lack of radiation dose and contrast usage still make MR attractive to many physicians.”

In conclusion, Cury cited the American Heart Association (AHA) recommendations to use CT as pretest for potential coronary artery disease (CAD) events. According to Cury, CCTA will become the dominant noninvasive method to assess CAD because it can provide a comprehensive assessment of coronaries, LVF, myocardial perfusion and LV.

On the other hand, Finn said the CMR is a “very versatile and flexible modality to approach cardiac imaging.”

For instance, Finn demonstrated that with the cine MR, radiologists can acquire pieces of images in a single heartbeat. He said that several low-resolution images can acquired through this method in a single breathhold of 5 to 6 seconds, which will provide images with a good temporal resolution. Finn suggested that the trade-off between spatial resolution and temporal resolution in CT to MR is the acquisition of a good “amount of data.” He added that with the “MRI, we tend to acquire the images individually, as opposed to a reconstruction with CT.”

Finn stressed the clear advantage of cardiac MR is the evaluation of viability and myocardial perfusion, as well as the lack of gadolium contrast injection, which has lead to questionable results with viable myocardium. 

He acknowledged that coronary MR angiography (CMRA) is not currently “part of the routine work-up of CAD events.” However, Finn did say that the unresolved issues with radiation exposure and contrast injection still make CMR an appealing option.

Finn also concurred with Cury that for myocardial perfusion, CMRA is the gold-standard, “and it is becoming increasingly more popular because of its effectiveness.”

He concluded his lecture suggesting if that CCTA was able to resolve the complications due radiation exposure and contrast injection, then CT would be widely adopted. Yet, until that point, MR will continue to be employed in cardiac imaging because of its effectiveness.