Cardiac MR imaging has become a mainstream clinical instrument to assess congenital heart disease. It is a cutting-edge technology that is beginning to replace fluoroscopy as the modality of choice to guide minimally invasive interventions in the heart.
The practice and the promise of cardiac MR in these roles were investigated in a plenary session at the SCMR meeting. Although echocardiography is still preferred for newborns and infants with complex congenital heart defects, MR is replacing cardiac catheterization and bronchoscopy in these patients to compensate for echo's inherent inadequacies, said Dr. Richard Slaughter, chair of medical imaging at Prince Charles Hospital in Merthyr Tydfil, U.K. MR is decidedly better than echo for evaluation of defects surrounding the heart, he said. It is preferred for measuring ventricular and valvular adequacy before surgery and for diagnosing pulmonary atresia, hyperplasia, dysplasia, and abnormalities in the pulmonary and systemic venous systems.
MR is well suited for examining aortic arch abnormalities, and it is outperforming all alternatives for investigating suspected pulmonary atresia. Slaughter presented an example of a five-day-old child whose echo study suggested pulmonary atresia, including a large vessel from the descending aorta. An axial reconstruction of the MR study further indicated that the vessel supplied both lungs, and no collateral vessels to the lungs were supplementing that blood flow. He was confident enough in the MR findings to recommend corrective surgery to reduce the risk of pulmonary hypertension without first catheterizing the patient.
"MRI is probably superior to other techniques at detecting collateral pulmonary vessels in infants whose chests are only a few centimeters wide. There is little reason to perform a pulmonary venous injection when MRI is an option," Slaughter said.
MR angiography, especially techniques that quantify flow volumes and direction, is a natural for evaluating congenital abnormalities, according to Dr. Matthias Gutberlet, a professor of cardiovascular radiology at Charite Virchow Clinic in Berlin, Germany.
Academic radiologists are especially interested in applying it to the quantification of regurgitant fraction or shunt volumes. Both classes of studies are performed along with morphological imaging and flow volume measurements to confirm the results, he said.
Because the right ventricle is situated relatively far from the body's midline, transesophageal echocardiography cannot measure right ventricular function as well as MR imaging. These measures are important when assessing surgeries for congenital heart disease because many of these procedures are designed to improve pulmonary circulation, said Dr. Albert de Roos, a professor of radiology at the University of Leiden in the Netherlands.
The right ventricle's complex geometry lends itself to MR assessment. Right ventricular wall thickening and associated hypertrophy after surgery to correct transposition of the great arteries is easily appreciated with morphologic MRI, de Roos said.
Investigators are showing interest in delayed-enhancement MR imaging to diagnose aneurysms that can appear in the body of the right ventricle after surgery to repair tetralogy of Fallot defects. The aneurysms are associated with right ventricular failure and can be seen to vary in size and shape throughout the cardiac cycle when examined with a gradient-echo MR sequence.
Cardiologists are adopting MR as the instrument of choice for monitoring the long-term patency of replacement pumonary valves. Unlike ultrasound, MR measurements for systolic ventricular function can be corrected to reflect the degree of pulmonary insufficiency affecting the contractility of the right ventricle.
CATHETERIZATION IN CHILDREN
The opportunity to reduce radiation dose gives interventional cardiovascular radiologists a huge incentive to replace fluoroscopy with real-time MR guidance, said Dr. Reza S. Razavi, a lecturer in congenital heart disease at Guy's, King's and St. Thomas' School of Medicine in London. Citing a report in The Lancet (2004;363:345-351), Razavi noted that routine diagnostic catheterization imparts a one-in-1000 lifetime chance of cancer to a five-year-old patient. Children with congenital heart disorders are often examined repeatedly with contrast-enhanced x-ray angiography, boosting their already high odds of eventually developing cancer.
"It is important for us to keep that in mind as we move forward," he said.
Ironically, while safety concerns are compelling radiologists to move ahead in developing MR-guided interventions, safety issues associated with guidewire heating are restraining them. Although the progress of MR-guided catheter-based interventions is impressive, research has been limited to large animals.
To overcome this limitation, multimodality MR/x-ray interventional suites were installed at Guy's Hospital in London and the University of California, San Francisco. The setups allow interventionalists to use MR guidance whenever possible. X-ray angiography is still generally used for guidance.
At both hospitals, a 1.5T dedicated cardiac MR scanner is positioned on one side of an operating room and a C-arm angiographic suite is located on the other. A partition embedded with radio-frequency shielding and a wide door separate the two imaging arenas. Patients are moved from one imaging system to the other on a specially designed table.
As of late January, 33 patients had been examined in Razavi's multimodality suite. Twenty-seven of those patients were 16 or younger. Most of the catheterizations addressed diagnostic questions, although a few involved therapy.
Most experimental studies employing both modalities for interventional guidance in UCSF's XMR suite were applied to animal models, according to Dr. Charles Higgins, XMR suite director.
Unlike angiography, MRI generates a representation of anatomy and real-time physiological data, such as pulmonary arterial resistance and load-dependent ventricular functional data.
"The decisions we make on the basis of these resistance calculations determine whether we operate or choose other approaches to patient management. The decision can define the difference between a good and bad outcome," Higgins said.
The XMR suite at Guy's Hospital has the potential of helping in electrophysiological studies associated with RF ablation to treat arrhythmias, Razavi said. Fiducial markers and a proprietary tracking technique enable the interventionalist to apply information gathered on the MR scanner to the actual procedure performed in the angiographic suite. A misregistration rate of about 3 mm is possible as the interventionalist views the recorded MR images on a screen adjacent to the monitor showing real-time fluoro images during the actual procedure.
This slight misregistration was apparent in images acquired before ventricular RF ablation was performed on a 15-year-old boy with sustained ventricular tachycardia, Razavi said. The imaging problem was corrected by moving the MR volume to fit exactly into the fluoro volume of the heart.
After this adjustment, the interventionalist could see electrical activation in the actual anatomy. Normal sinus beats could be differentiated from beats affected by ventricular tachycardia.
"You can actually see where the focus of the tachycardia is, and you potentially can ablate it," he said.
Additionally, Razavi and his colleagues have combined cine MR and spiral CT to study the timing of electrical activation. Elements of both modalities have been fused to produce images showing the heart's electrical and mechanical behavior simultaneously.
"This opens up the potential for us to do these electrophysiological studies to determine noninvasively with MRI where the arrhythmias are coming from," he said.