Advances in Cardiac CT & MRI

Journal Editors Highlight Significant Progress in Cardiovascular Imaging

(Left) Patrick M. Colletti, MD, assistant editor of the American Journal of Roentgenology and professor of radiology, medicine, biokinesiology, pharmacology and pharmaceutical sciences at the University of Southern California in Los Angeles. (Right) Uwe Joseph Schoepf, MD, associate editor of Radiology and professor of radiology and cardiology at the Medical University of South Carolina in Charleston

Heart disease has been the leading cause of death in the United States for the past 80 years and is a major cause of disability, according to the Centers for Disease Control and Prevention. An estimated 61 million Americans have coronary artery disease; therefore, accurate cardiovascular imaging is a critical component for diagnosis and treatment. Two of diagnostic imaging’s most powerful modalities—CT and MR—have seen rapid growth in utilization for cardiovascular indications.

The past year has seen great strides in cardiac CT and MR technology available for cardiovascular clinicians to assist their efforts in diagnosing heart disease. A pair of imaging’s most widely read clinical journals, Radiology and the American Journal of Roentgenology (AJR), have published a number of compelling scientific studies this year exploring the cardiac capabilities of these modalities.

“Significant events have happened over the past year on a variety of different levels,” notes Uwe Joseph Schoepf, MD, associate editor of Radiology and professor of radiology and cardiology at the Medical University of South Carolina in Charleston.

Technology advances, particularly in cardiac CT, have led the way as detectors have grown from 64-slice to 128-slice to 256- and 320-slice deployments, Schoepf says; in addition, the clinical introduction of dual-source capabilities has expanded the scientific possibilities for cardiovascular CT.

“On the near horizon for cardiac CT is volume imaging with multiple arrays of detectors; that’s coming and that’s coming fast,” says Patrick M. Colletti, MD, assistant editor of AJR as well as professor of radiology, medicine, biokinesiology, pharmacology and pharmaceutical sciences at the University of Southern California in Los Angeles.

“Dual-energy CT will help us do tissue characterization for myocardium, particularly for plaque,” Colletti says. “It may be able to help us eliminate some of the artifact from calcified plaque to help us evaluate the non-calcified plaque adjacent to calcified plaque.”

Advances in cardiac MR imaging this year have been devoted to further refinement and focus of the modality in areas in which it has demonstrated strong clinical capabilities; namely, imaging of the myocardium, Schoepf says.

 “We’re seeing a further refinement of the dividing line between cardiac CT and cardiac MRI,” he notes. “These have been two very different and very complementary modalities; CT has been used predominantly for looking at the coronary arteries and cardiac MRI has been used predominantly for looking at the myocardium.”

A study from Radiology (April 2008) demonstrates that usage of the modalities can successfully overlap for some cardiac indications; particularly for patients who present with contraindications for CT or MR imaging.

A team of researchers from Massachusetts General Hospital in Boston prospectively compared the use of 64-slice CT and cardiac MRI for the early assessment of myocardial enhancement and infarct size after acute reperfused myocardial infarction.

They found that early hypoenhancement was recognized on all CT and MR images; however, delayed hyperenhancement was better observed with cardiac MR than with cardiac CT.

“Our results show delayed-enhancement imaging of myocardial CT is possible and can be considered for imaging of infarct size in patient with contraindications to MR imaging,” the authors wrote.

A pair of articles in AJR (December 2007) explored the targeted use of MR angiography (MRA) after 64-slice CT angiography (CTA) in assessing the severity of focal calcific coronary lesions.

An international team from the United States and China found that coronary MRA has higher image quality for coronary segments with nodal calcification than for coronary segments with diffuse calcification. In addition, their results showed that coronary MRA has better diagnostic performance than coronary CTA for the detection of significant stenosis in patients with high calcium scores.

“The results open the door for future studies about the use of MRA for targeted site-to-site comparison studies after coronary CTA stumbles onto a blind spot caused by a high focal concentration of calcium,” wrote Andre J. Duerinckx, MD, in a commentary on the study.

In March, the AJR published results of a study comparing CT and intravascular ultrasound (IVUS) for the identification and quantification of coronary atherosclerotic plaques. The study, conducted in Beijing, found that 64-slice CT has a good ability to identify and quantify these plaques, compared with IVUS.

“While the reliable differentiation of the composition of individual noncalcified plaques by CT is limited, it may be used to help risk-stratify and monitor patients with suspected or known coronary atherosclerotic disease,” Colletti says, commenting on the study.

The utilization of CT for cardiac imaging has received a great deal of attention from the lay press this year, Schoepf observes. Much of the attention has been focused on the radiation dose received by patients during these procedures.

“We still need to do everything possible to keep the radiation dose as low as reasonably achievable in these patients,” Schoepf says.

A pair of articles published this year in Radiology offered clinicians the results of studies that examined dose-lowering techniques for cardiac CT.

A team of physicians from Fairfax Radiological Consultants in Fairfax, Va., retrospectively compared image quality, radiation dose and blood vessel assessability for CTA obtained with a prospectively gated transverse (PGT) CT technique and a retrospectively gated helical (RGH) CT technique in the March issue of Radiology.

The PGT method takes advantage of the large 40-mm volume coverage available with a 64-detector CT scanner, which enables complete coverage of the heart in three or four incremental 40-mm acquisitions.

Two mixed plaques (arrowheads) and calcification (arrow) in left anterior descending (LAD) artery are visible on MDCT angiography maximum-intensity-projection (MIP) image. AO = aorta. MR angiography MIP image shows significant stenoses (arrowheads) at sites of two mixed plaques shown in A and mild stenosis (arrow) at site of calcification shown in A. Image and caption courtesy of the American Roentgen Ray Society.

“By using this technique, the table is stationary during image acquisition and then moves to the next location for another scan that is initiated by the subsequent cardiac cycle,” the authors wrote. “The result is very little overlap between the scans, substantial reduction in radiation dose, and more robust and adaptive ECG gating.”

The researchers found that the use of PGT, compared with RGH, decreased the effective radiation dose to the patient during a CTA procedure by 83 percent. In addition, they reported that the PGT technique improved image quality.

A study published by Radiology in August from Hiroshima, Japan, demonstrated that prospective CTA can reduce radiation dose (79 percent) below that of retrospective CTA with dose modulation, while maintaining image quality and the ability to assess luminal obstructions in patients with heart rates of less than 75 beats per minute.

“We’re starting to see a departure in the journals from the feasibility studies that have marked the field of cardiac imaging,” Schoepf says. “We’re looking into evidence-based medicine, cost-effectiveness, and the prognostic value of cardiac imaging studies.”

A recent study (August 2008) from St. Vincent’s University Hospital Elm Park in Dublin, Ireland, applied an evidence-based practice (EBP) model to cardiac CT. The team investigated currently published cardiac CT literature and evaluated the technical and diagnostic performance of the modality compared with invasive coronary angiography.

“EBP principles applied to the literature involving cardiac CT show the current generation of cardiac CT to have a high sensitivity and specificity with satisfactorily narrow confidence intervals for the detection of hemodynamically significant coronary stenosis in patients with low and intermediate pretest probabilities,” the authors wrote.