Few resources exist to educate cardiac sonographers regarding proper techniques and specific issues to consider when performing pediatric coronary artery imaging. The main objective of this report is to review the echocardiographic techniques used to image the coronary arteries of children when screening for anomalous origin of the coronary arteries, as well as coronary involvement in the setting of Kawasaki disease. The authors discuss the physics and instrumentation for developing optimal coronary artery imaging system settings and present the commonly used anatomic and echocardiographic views. Use of the correct settings and a thorough understanding of the associated ultrasound physics are crucial for obtaining quality images. With this report, the authors provide guidance to sonographers and a resource for pediatric echocardiography laboratories to help ensure high-quality echocardiographic imaging of the coronary arteries.
Highlights
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High-quality diagnostic imaging of the coronary arteries in children is challenging.
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Echocardiographic instrumentation should be optimized for coronary artery imaging.
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Thorough understanding of coronary anatomy will reduce imaging time.
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More comprehensive coronary imaging is required for patients with Kawasaki disease.
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Echocardiographic views and corresponding computed tomographic images are provided.
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Understanding Coronary Artery Anatomy
Accurate imaging and diagnosis rely on a familiarity with cardiac anatomy, specifically the normal arrangement of the coronary arteries. Random searching is not usually a successful strategy when imaging the coronary arteries, and in the interest of timeliness, which is especially important in the pediatric setting, knowing where normal coronary arteries should be will reduce the time needed to complete an examination.
The standard echocardiographic views will not provide all of the information required to rule out potential abnormalities, and nonstandard echocardiographic views are routinely used to identify coronary origins and to follow the course of the vessels around the heart. Sonographers should thus have a solid understanding of normal coronary artery branching and course over the surface of the heart. Heart models, pathologic specimens, anatomy textbooks, cardiac angiograms, or cardiac computed tomographic three-dimensional reconstructions are helpful resources that can give context to what a sonographer is trying to capture with two-dimensional (2D) echocardiography. Once normal anatomy is mastered, additional study of abnormal arrangements will prepare the sonographer for atypical presentations. Taking the time to understand this anatomy in advance will reduce the amount of time spent and level of frustration when performing the echocardiographic examination.
Coronary arteries have corresponding coronary veins that, because of their often larger diameters, tend to be more prominent (especially in the posterior interventricular sulcus). The coronary veins may also be affected in Kawasaki disease, which can make them even more conspicuous. Assessment of vessel flow is helpful in differentiating the arteries from the veins. The direction of coronary artery flow is away from the aorta and base of the heart (predominantly during diastole), whereas coronary vein flow is toward the coronary sinus and base of the heart (predominantly during systole). A quick check of vessel flow using color Doppler is prudent before acquiring images that may result in inaccurate diagnoses ( Figure 1 ). Coronary vein walls are thinner than coronary artery walls, which can also be helpful in differentiating the two structures.
The following sections provide strategies to obtain quality diagnostic echocardiographic images and a brief overview of the potential anomalous and aberrant coronary artery findings during a pediatric echocardiographic study. It is important to remember that coronary artery imaging takes time and skill, and not every study will produce all of the images presented in this report.
Understanding Coronary Artery Anatomy
Accurate imaging and diagnosis rely on a familiarity with cardiac anatomy, specifically the normal arrangement of the coronary arteries. Random searching is not usually a successful strategy when imaging the coronary arteries, and in the interest of timeliness, which is especially important in the pediatric setting, knowing where normal coronary arteries should be will reduce the time needed to complete an examination.
The standard echocardiographic views will not provide all of the information required to rule out potential abnormalities, and nonstandard echocardiographic views are routinely used to identify coronary origins and to follow the course of the vessels around the heart. Sonographers should thus have a solid understanding of normal coronary artery branching and course over the surface of the heart. Heart models, pathologic specimens, anatomy textbooks, cardiac angiograms, or cardiac computed tomographic three-dimensional reconstructions are helpful resources that can give context to what a sonographer is trying to capture with two-dimensional (2D) echocardiography. Once normal anatomy is mastered, additional study of abnormal arrangements will prepare the sonographer for atypical presentations. Taking the time to understand this anatomy in advance will reduce the amount of time spent and level of frustration when performing the echocardiographic examination.
Coronary arteries have corresponding coronary veins that, because of their often larger diameters, tend to be more prominent (especially in the posterior interventricular sulcus). The coronary veins may also be affected in Kawasaki disease, which can make them even more conspicuous. Assessment of vessel flow is helpful in differentiating the arteries from the veins. The direction of coronary artery flow is away from the aorta and base of the heart (predominantly during diastole), whereas coronary vein flow is toward the coronary sinus and base of the heart (predominantly during systole). A quick check of vessel flow using color Doppler is prudent before acquiring images that may result in inaccurate diagnoses ( Figure 1 ). Coronary vein walls are thinner than coronary artery walls, which can also be helpful in differentiating the two structures.
The following sections provide strategies to obtain quality diagnostic echocardiographic images and a brief overview of the potential anomalous and aberrant coronary artery findings during a pediatric echocardiographic study. It is important to remember that coronary artery imaging takes time and skill, and not every study will produce all of the images presented in this report.
Optimizing System Settings for Pediatric Coronary Artery Imaging
When imaging the coronary arteries in children, it is important that sonographers apply their knowledge regarding ultrasound physics and system optimization. In 2D imaging, the best resolution is obtained when the coronary vessel is perpendicular to the beam (axial resolution), while color and spectral Doppler quantification of flow velocities are most accurate when the beam is parallel to flow. Sonographers should continually strive to optimize images by referencing these basic principles.
Imaging of the coronary arteries should be performed with the highest frequencies possible relative to patient size. The use of higher frequencies (i.e., 8, 10, or 12 MHz) provides superior grayscale resolution and should be readily accessible during coronary imaging. Depending on the imaging system, sonographers may need to change transducers multiple times during a study to obtain the best images. High-frequency transducers should not be reserved for neonates and small children, as they are also useful in larger patients when imaging in the near field.
Frame rate is an important factor in all ultrasound imaging, especially for imaging coronary arteries. Because the coronary vessels are small, superficial structures, and heart rates in children are typically higher than in adults, temporal resolution is a priority. Reducing depth and sector size has the greatest impact on improving frame rate. To obtain detailed information, sonographers should also zoom in on the structure or increase the magnification. However, enlarging images that have not been properly optimized does not improve their resolution and should be avoided. The focal zone length should be adjusted and placed appropriately. Beam-width artifacts can be minimized by proper placement of the focal zone at the site of interest.
Harmonics and compression should be used sensibly, and both of these imaging adjustments are particularly important in the setting of Kawasaki disease. Sonographers should exercise sound judgment on the basis of an understanding of the benefits and pitfalls of harmonics and increased compression with regard to imaging and resolution. Harmonics degrade axial resolution, particularly in the near field, and may give 2D structures a “thickened” appearance. Consequently, the coronary artery wall may appear falsely abnormal. Bright, echogenic walls may be seen in the acute phase of Kawasaki disease, but this finding can be exaggerated with the use of harmonics. On the other hand, harmonics may be helpful in clarifying coronary artery origins in larger patients or those with technically challenging acoustic windows. Excessive compression may actually filter out low-level sound waves that might otherwise identify a subtle abnormality such as a thrombus within the vessel lumen. Alternately, when measuring the internal lumen of coronary arteries in Kawasaki disease, gain can be judiciously lowered and compression increased to achieve a more black-and-white image to help define the intima-lumen interface in better detail.
Color Doppler settings should also be carefully optimized, as the flow in coronary arteries is subtle and often difficult to capture. A Nyquist limit of 30 cm/sec may be used as the starting point for color Doppler interrogation; however, some situations may require a lower scale. The actual value of the Nyquist limit is not as important as the quality of the Doppler signal. A systematic approach of gradually lowering the limit until flow is detected while avoiding indistinguishable artifacts can help achieve diagnostic accuracy. Sonographers should use their judgment, remembering that a lower scale with a suboptimal angle of insonation may increase the potential for artifact. Noise in the color flow electronics used to produce the color Doppler signals can appear as artificial “flow” in hypoechoic areas, and clutter from tissue motion can obscure the true underlying blood flow. These artifacts are often mistaken for coronary artery blood flow, offering yet another reason why the angle of the ultrasound beam should be as parallel to flow as possible ( Figure 1 C). This may require acquisition of a 2D image in one view and a color Doppler image in an alternative view. Higher frequency transducers may not be sensitive enough to demonstrate color Doppler flow in coronary imaging, and attempts should be made to use lower frequency transducers if this information is the focus of the examination. Vascular studies use transducers with slightly lower frequencies, as echoes returning from blood flow are weaker than those from anatomic imaging. The same principles apply to echocardiography, and further support the use of multiple transducers to achieve the highest quality images. Small adjustments to persistence may also be helpful if color flow imaging is difficult to obtain. Persistence averages individual frames, which reduces noise and results in a smoother image. Flow detection in the right coronary artery (RCA) can be challenging to demonstrate using the standard parasternal short-axis (PSAX) view in healthy patients due to the perpendicular angle of insonation. Flow may appear more prominent if there is pathophysiology present that increases flow velocity.
Image acquisition should include moving clips followed by still images. Still images alone do not provide adequate diagnostic information and may bias the interpreting physician into making a false diagnosis. Moving images provide a contextual reference in relation to other cardiac structures and capture additional information that might be overlooked when concentrating on specific anatomy. The reading physician does not have the benefit of hands-on scanning and thus relies on the sonographer to provide as much information as possible so that an accurate diagnosis can be made.
An electrocardiographic tracing is necessary to correctly assess coronary artery blood flow. Left coronary artery flow occurs predominantly in diastole, while RCA flow occurs in both diastole and systole. Comparing the color Doppler flow to the electrocardiographic tracing will confirm timing of flow in the coronary arteries. Coronary artery flow reversal in any part of the cardiac cycle is abnormal and should increase suspicion of a potentially serious anomaly, necessitating a thorough investigation of the anatomy.
Coronary arteries are very small superficial structures that may be difficult to image in any patient. The technical issues are further confounded by the fact that children, especially those in the painful, acute phase of Kawasaki disease, may not tolerate lying still for extended periods of time. If concerns exist regarding the coronary artery anatomy for any reason, sedation may be required to obtain the best diagnostic images. If the coronary arteries are not adequately visualized with echocardiography, other imaging modalities (i.e., computed tomography, magnetic resonance imaging, or cardiac catheterization) may need to be considered.
Anomalous or Aberrant Coronary Arteries
Although the incidence of anomalous or aberrant coronary arteries is rare, 0.6% to 1.3% as reported in several series, the risks associated with this diagnosis can be serious. Congenital coronary anomalies are the second most common underlying cause of sudden cardiac death or arrest in athletes (15%–25% of cases), with hypertrophic cardiomyopathy being the most common. Exertional chest pain, syncope, and exercise-induced arrhythmias in children may be caused by coronary artery abnormalities. Echocardiography is a low-risk procedure that has been proven successful in diagnosing these anomalies. The coronary abnormalities that are most often seen are anomalous origin of the left coronary artery from the pulmonary artery (ALCAPA), aberrant origin of the coronary artery from the incorrect sinus of Valsalva, single coronary artery, and coronary artery fistula ( Figure 2 ). Aberrant coronary arteries that have interarterial, intramural courses within the wall of the aorta are of great concern. Intramural segments can be difficult to visualize and easy to miss without a careful assessment of coronary origins, and confirmation of an intramural segment may be possible only by direct inspection. The course of the proximal coronary segment is usually perpendicular to the aortic sinus, and unusual orientations should be highly suspect. Acute takeoffs of either coronary artery from the opposite coronary sinus of Valsalva with an intramural course and ostial valve–like ridge have also been associated with sudden death ( Figure 2 C). Although the significance of an intramyocardial course of the left coronary artery is somewhat controversial, the risk for sudden death with this anomaly is considered low ( Figure 2 E).
