Fig. 1.1
Bedside color echocardiogram showing massive mitral regurgitation (blue arrow)
Transthoracic echocardiography does have limitations. Ultrasound is user-dependent, so it is the least reproducible cross-sectional modality for imaging the heart. Air in the lung surrounding at least three sides of the heart scatters the beam and makes it very difficult to evaluate intrapulmonary vasculature and sometimes the aortic arch. The size of the thymic gland, which decreases with age, can significantly limit the imaging window for the heart and great vessels. Transesophageal echocardiography can avoid most of these imaging obstacles, but the scattered beam from air in the lung remains difficult, and the patient must be sedated or under anesthesia, so the noninvasive advantage is lost.
1.2 Cardiac MRI
Cardiac MRI is the most comprehensive modality for imaging the heart. Cardiac MRI can give clear anatomic evaluation and can assess myocardial perfusion, with reproducible functional evaluation of the muscle and valves of the heart (Fig. 1.2) . Velocity-encoded, phase-contrast imaging is an MRI sequence that can give accurate velocity and volume of flow across the outflow tracts and provide calculated Qp/Qs ratios , as well as evaluate stenosis, regurgitation, and velocities of vascular structures. Patients undergoing MRI receive no radiation. Patients with pacemakers historically have not been able to undergo MRI, but new MRI-compatible pacemakers are now available and are being used to overcome this limitation. Coronary artery imaging has improved significantly with 3T MRI, with improved signal-to-noise and contrast-to-noise ratios.
Fig. 1.2
Four-chamber view from a cine gradient echo (GRE) cardiac MRI in an 8-year-old with myocarditis shows the mitral (M) and tricuspid (T) valves with a moderate pericardial effusion (Eff) and lower lobe consolidation. Real-time cine showed decreased left ventricular function
The limitations of MRI mainly surround the length of time it takes to image a patient. With increasing technology, the scan times have increased so that a typical comprehensive cardiac MRI may take as long as 1–2 h. For children, anesthesia times become an issue, especially for the more critically ill patients. Claustrophobia is still a problem for older children, although some distraction methods with headphones and video display systems have reduced the number of failed exams .
1.3 Cardiac CT Angiography
Cardiac CT angiography (CTA ) produces images with the highest spatial resolution of all three cross-sectional imaging modalities. Therefore it has the best chance of visualizing the coronary arteries, aortopulmonary collaterals, and pulmonary arteries in infants, in whom these vessels may be as small as 1–2 mm in diameter (Fig. 1.3 ). Functional evaluation and cardiac perfusion imaging is possible with gated CT exams although this information comes with the cost of increased radiation to the patient. Cardiac CT also provides the best evaluation of the airway and lungs, which may be helpful. A short anesthesia time is another advantage, as a typical scan takes seconds to perform.
