MRI—A Revolution in Cardiac Diagnostic Imaging
from Practice Update, Fall 2004
This is a four chamber view of the heart in a patient with mitral valve prolapse.
2a. Right ventricle,2b. Left ventricle,2c. Right atrium,2d. Left atrium, 3a. Prolapsing mitral valve with a
visible regurgitant jet
Pediatric cardiology, the study of congenital heart disease, is a young subspecialty that has evolved remarkably in its short history. Progress over the past century has been driven by the ability of pediatric cardiologists to accurately identify and characterize complex heart lesions. Cardiac catheterization became a reality only 60 years ago and echocardiography, subsequently, emerged in the 1970s. Both are now essential tools for cardiac diagnosis and management in any pediatric cardiology program. Cardiac magnetic resonance imaging (MRI) is the newest tool available to physicians managing patients with congenital heart disease and will revolutionize the field yet again.
Cardiac MRI is a noninvasive imaging modality that provides anatomic and functional analysis of congenital heart disease. Although MRI technology is not new, imaging the beating heart was not possible until the past decade with faster and more powerful magnets and the development of special software. By “gating” the heart (timing the MRI to the heart cycle) the images obtained by cardiac MRI are crisp, unlike those that can be obtained by echocardiography or catheterization. And, unlike catheterization or computed tomography (CT), MRI does not involve the use of ionizing radiation.
For assessment of anatomy, echocardiography has been the imaging modality of choice. Portability, quality of imaging and the non-invasive nature of echocardiography will continue to ensure its place as an essential imaging tool in pediatric cardiology. However, in patients who have undergone multiple surgical procedures resulting in chest wall scar tissue, or in patients who are older and bigger, ultrasound waves do not penetrate as well and image quality can be significantly reduced. This poses a great problem in the growing population of patients who are surviving well into adulthood and continue to require routine surveillance. Without adequate image quality, clinicians frequently resort to invasive catheterization, simply to image the heart and surrounding structures. Cardiac MRI obviates this need, providing exquisite, non-invasive imaging of cardiac and vascular anatomy.
Hemodynamic evaluation is the greatest strength of cardiac MRI. Already, MRI is the gold standard for assessment of ventricular systolic function of both the left and right ventricles. Other diagnostic tests do a fair job of assessing left ventricular function when there is good image quality, but there is always considerable doubt as to the accuracy of these same tests when they are used to assess the right ventricle. This is unacceptable in patients with congenital heart disease who frequently rely on the right ventricle as a systemic pump, or in those patients with pulmonary hypertension, in whom right ventricular failure is an ominous sign. More accurate assessment of ventricular function, alone, is justification for cardiac MRI.
Flow quantification is a novel technique that provides additional hemodynamic information. Historically, flow analysis has been used to determine the presence and degree of shunting of blood between the right and left sides of the heart. This requires the use of certain assumptions, and is performed with the patient under some degree of sedation. Cardiac MRI measures blood flow through blood vessels and valves directly and non-invasively. No longer do patients require a diagnostic catheterization to determine the degree of shunting from a ventricular or atrial septal defect. MRI can quantify the amount of regurgitation present through a valve. Not possible with any other modality, amount of regurgitation is an important piece of information for a cardiologist when deciding the timing of valve replacement, especially in patients with tetralogy of Fallot.
Ischemic heart disease, though certainly more prevalent in adults, is also an unfortunate reality for many young patients with congenital heart disease. Kawasaki disease-induced aneurysms, coronary insufficiency after arterial switch, or anomalous coronary artery reimplantation, and post-transplant coronary artery disease are but a few disease processes that can result in impaired coronary artery perfusion and damaged myocardium. Pharmacologic stress MRI studies with dobutamine or adenosine are invaluable for assessing ventricular function and coronary perfusion. Contrast-enhanced studies target scar tissue to identify ventricular muscle that is scarred and is no longer viable.
This is a contrast-enhanced
MRangiogram of the aortic
arch in anadult patient with
coarctation ofthe aorta.
Cardiac MRI is an important adjunct to catheterization and echocardiography in a successful pediatric cardiology program. With The Children’s Hospital’s commitment to build a reputation as one of the most successful heart centers in the country, MRI will be an integral resource for our cardiologists, surgeons and researchers. The clinical program began in August, with a successful collaboration between the departments of Cardiology and Radiology. In the six weeks since the program’s inception, there have already been some patients in whom echocardiography has been unable to delineate vascular andcardiac structures. Instead of scheduling an invasive catheterization, MRI has provided the necessary information for surgical management.
The research arm of the program is equally exciting. Plans are underway to study fluid dynamics utilizing flow imaging, MRI sequence design and molecular imaging. Clinical studies will focus on the clinical strengths of The Children’s Hospital Heart Institute including the transplant and pulmonary hypertension programs. With Fitzsimons on the horizon, the next goal in the program is to join cardiac catheterization with MRI in a hybrid laboratory which will allow catheter-based interventions to be performed with less or even no radiation exposure to patients.
The future of cardiac imaging involves cardiac MRI. With one of only a few programs in the country, The Children’s Hospital is demonstrating its commitment to being on the forefront of patient care and research by embracing new medical technology.
