Morphological Evaluation of Congenital Heart Disease
Depiction of the anatomy of simple and complex forms of congenital heart disease constitutes the most common clinical indication for magnetic resonance imaging (MRI) of the heart at many centers in the United States.
The most frequently applied noninvasive technique for the assessment for congenital heart disease is echocar- diography. Consequently, MRI is used in situations in which the information provided by echocardiography is incomplete or cannot be obtained. MRI using the ECG- gated spin-echo technique has been shown to have high diagnostic accuracy for demonstrating the morphologic aspects of many forms of congenital heart disease, in- cluding both simple and complex lesions. Due to the ready availability of echocardiography and its greater fa- miliarity, the major uses of MRI in the evaluation of con- genital heart disease are aimed at specific indications in which the information is either unique or supplementary to that obtained with echocardiography.
The current indications for the use of MRI for the mor- phologic diagnosis of congenital heart disease include:
evaluation of coarctation of the aorta, aortic arch anom- alies and other vascular rings, pulmonary venous con- nections and pulmonary venous obstructions, the assess- ment of pulmonary arterial dimensions and sites of ob- struction or atresia, the evaluation of complex cyanotic anomalies including various types of single ventricle (uni-ventricular connections), and postoperative follow- up of congenital heart disease. Perhaps the most frequent use of MRI is, and in the future will be, for monitoring the morphology of cyanotic congenital heart disease and, to some extent, evaluating the success of complex opera- tions used in its treatment. MRI also permits precise as- sessment of the morphology of the supracardiac as well as intracardiac aspects affected by these complex opera- tions. Moreover, it has also accurately demonstrated residual obstructions of the central or branch pulmonary arteries after operations involving the pulmonary arteries.
This is particularly the case for surgery to correct tetral- ogy of Fallot, pulmonary atresia, and truncus arteriosus.
For the evaluation of coarctation of the aorta, MRI has been effective in demonstrating the site, extent and sever-
ity of the coarctation, and the status of the aortic arch. In the evaluation of vascular rings, sagittal and transverse tomograms demonstrate the severity of airway compres- sion as well as the precise anatomy of the vascular anom- aly. For complex lesions, such as single ventricle, the to- mographic nature of MRI allows depiction of the atri- oventricular and arterioventricular connections, which is necessary in defining the anatomy of the lesions.
Likewise, MRI has been effective in defining the anato- my of surgical procedures used in the correction of these complex cyanotic congenital heart lesions. A number of studies have compared the normal anatomy with compli- cations following use of the Rastelli procedure, Fontan procedure, Jatene procedure, Norwood procedure, and other operations involving the great vessel and cardiac levels. Most of these operations influence the size of stenoses involving the pulmonary arteries. Recent studies have demonstrated that MRI is more effective than echocardiography in detecting abnormalities in the right and left pulmonary arteries occurring residual to or as a complication of previous operative procedures.
Functional Evaluation of Congenital Heart Disease
Depiction of the morphology of congenital heart disease is the most important aspect in the diagnosis of the many forms of this disease. However, the functional aspects of congenital heart disease are also important and in some cases are critical to patient management. A number of new magnetic resonance techniques have been devised or already applied to the assessment of several functional aspects of congenital heart disease. Evaluation of the vol- umes and functions of both ventricles has been achieved using cine-MRI and more recently, breath-hold cine- MRI. Compared to echocardiography and other imaging techniques, MRI has the advantage of providing better quantification of right ventricular volume and function.
Many congenital heart lesions, especially cyanotic le- sions, frequently afflict and alter the dimensions and shape of the right ventricle. Consequently, a three-di- mensional imaging technique that provides direct mea- surements of right ventricular volume and mass is im-
IDKD 2007
Diagnosis of Congenital Heart Disease I
C.B. Higgins
Department of Radiology, University of California San Francisco, San Francisco, CA, USA 158_159_Higgins 2-03-2007 10:32 Pagina 158
portant in the management of a number of types of con- genital heart disease. The determination of adequate vol- ume and function of two ventricles may be important in surgical consideration and in the decision as to whether to perform a bi-ventricular or uni-ventricular reconstruc- tion in complex congenital lesions.
The measurement of blood flow in the great vessels and cardiac chambers may be extremely important in the assessment of congenital heart disease. The velocity-en- coded cine-MRI technique can provide measurements of blood flow in the ascending aorta and pulmonary artery simultaneously. This technique has been used to measure the difference between pulmonary arterial flow and aor- tic flow in order to calculate the volume of left-to-right shunts and the pulmonary to systemic flow ratio (Q
p/Q
s) of shunts. The Q
p/Q
sderived from velocity-encoded cine- MRI close correlates with the same measurement derived from oximetric data obtained during cardiac catheteriza- tion in patients with atrial septal defects. Moreover, this technique has been used to separately measure flow in the right and left pulmonary arteries, and has shown that the sum of flow measured in the right and left pulmonary artery is equivalent to the flow measured in the main pul- monary artery in normal individuals. This technique has been used to quantify the distribution of blood flow in the right and left pulmonary arteries in patients with congen- ital lesions, in whom blood flow to the two lungs is gross- ly unequal as a consequence of stenoses within the branches of the right or left pulmonary artery, after con- struction of systemic to pulmonary shunts, or in atresia or hypoplasia of one of the hilar pulmonary arteries.
Velocity-encoded cine-MRI has also been used to esti- mate the peak velocity across stenoses in order to calcu- late the gradient across them. This information has been useful in the assessment of congenital heart disease as it allows the gradient across pulmonary valvular stenoses, pulmonary arterial stenoses, and Rastelli conduits to be estimated. Several recent studies have also shown a cor- relation between estimates of the gradients across valvu- lar stenoses and those across coarctation of the aorta with measurements made at cardiac catheterization.
Velocity-encoded cine-MRI has recently been applied to determine the volume of the collateral circulation in coarctation of the aorta. The rationale in this application is to demonstrate greater flow in the distal descending aorta than in the proximal descending aorta. The increase in flow from the proximal to the distal aorta is presumed to occur as a result of retrograde flow in the intercostal arteries and other branches of the descending aorta. In normal volunteers, the total flow in the distal part of the descending aorta was slightly decreased compared with the flow in the proximal part of the descending aorta be- cause of normal antegrade flow through the intercostal arteries. However, in patients with hemodynamically sig- nificant coarctation (>20 mm gradient), velocity-encod-
Diagnosis of Congenital Heart Disease I 159
ed cine-MRI measurements demonstrated a substantial increase in the distal aorta compared to the proximal part of the descending aorta. The increase in flow was pre- sumably due to the retrograde flow in branches of the de- scending aorta. In that study, there was a significant lin- ear relationship between the volume of collateral flow measured by velocity-encoded cine-MRI and the mor- phologic severity of the stenosis, as measured by the per- cent reduction in luminal diameter of the aorta at the site of the coarctation. It thus seems likely that this technique can provide a new method for defining the hemodynam- ic significance of a coarctation.
The Future of MRI in Congenital Heart Disease:
Catheterless Cardiac Catheterization
The multifaceted morphologic and functional informa- tion provided by MRI suggests that this technique has the potential to replace cardiac catheterization while provid- ing the same information. In recent years, the volume of cardiac catheterization has declined considerably due to the capability of echocardiography for evaluating mor- phology and function in congenital heart disease. The combination of echocardiography and MRI should elim- inate a substantial amount of cardiac catheterizations. In the future, the major use of cardiac catheterization in con- genital heart disease may be to apply interventional ther- apeutic procedures.
Under optimal circumstances of application, MRI can supply most of the information for which cardiac catheterization is currently used. This information in- cludes morphologic, pressure, and oximetric data in order to define the pressure gradient across stenoses, the vol- ume of shunts, and the presence and severity of pul- monary arterial hypertension. Currently, the morphology of congenital heart disease is defined by ECG-gated, spin-echo, and cine-MRI with a precision that rivals and usually surpasses that provided by catheterization.
Quantitative function of both ventricles can be provided by cine-MRI, and the accuracy and reproducibility of this approach should be better than that obtained with cardiac angiography because of the three-dimensional imaging.
As discussed above, velocity-encoded cine-MRI can be used to estimate the gradients across valves and great ar- teries and to calculate the volume of left-to-right shunts.
However, the reliability of this method for estimating gra- dients across stenoses under a variety of pathologic cir- cumstances has not yet been established. Moreover, it is now theoretically possible to estimate pulmonary arterial pressure by examining the jet of tricuspid regurgitation usually present in patients with pulmonary arterial hy- pertension. However, no studies have documented the ca- pability of MRI to provide an estimate of pulmonary ar- terial pressure.
158_159_Higgins 2-03-2007 10:32 Pagina 159
