Pediatric imagers adjust sequences to suit patients

November 1, 2004

Heart disease in children is highly variable and often so complex that it completely distorts traditional imaging landmarks. A young girl with scimitar syndrome had such severe hypoplasia of the right lung that her heart was actually sitting in the right chest and her right pulmonary artery was nearly nonexistent. The blood supply to the right lung came off the descending aorta down in the belly, said Dr. Margaret Samyn, an associate professor of pediatric cardiology and radiology at the University of Florida, Gainesville. In such cases, pediatric imagers often have to tinker with standard MR protocols for best results, even during imaging.

A number of standard initial sequences help set the stage for optimal quality images. Axial T1 sequences through the whole chest and the first superior half of the liver give straight coronal, axial, and sagittal views, define the location of the basic structures of the heart, and allow planning of other imaging sequences. Two-chamber views visualize the left atrium and left ventricle, and four-chamber views depict the entire heart in an almost axial orientation. A short-axis stack cine white blood technique determines ventricular volumes and ejection fraction, even if there is only a single ventricle, Samyn said.

A non-breath-hold imaging protocol for evaluating children with congenital heart disease described by University of Iowa researchers begins with a black blood electrocardiograph-gated double inversion recovery fast spin-echo scan with a TR between 1100 and 1900 and TE of 42 msec, 5 to 8-mm slice thickness, 256 x 160 matrix, and 18 to 38-cm field-of-view. White blood imaging using cine gradient-echo applies the same slice thickness and FOV but alters the TR/TE/flip angle to 10/5.7/15¼ and the matrix to 256 x 128.

Other sequences may be added to explore specific clinical questions. Two-D cine fast low-angle-shot imaging clearly depicted the position of large atrial septal occluders in relation to adjacent cardiac veins and valves in a study of 26 pediatric patients conducted at the Hospital Sainte-Justine at the University of Montreal (Radiographics 2003;23:S51-S58).

A novel real-time flow sequence quantified pulmonary and aortic blood flow and left-to-right shunt in less than 10 seconds in a study of 14 children with congenital heart disease. Radiologists at the Clinic for Congenital Heart Disease and Institute for MRI at Ruhr University in Bochum, Germany, determined cardiac left-to-right shunts and pulmonary and aortic flow rates by nontriggered free-breathing real-time phase contrast MR, which combined single-shot echo-planar imaging with sensitivity encoding (Circulation 2004;109[16]:1987-1993).

Obtaining optimal images in children, in general, relies on careful review of the patient's history, previous echocardiography and catheterization data, and consultation with the referring physician to frame the underlying clinical question. It then relies on standard and customized sequences and hands-on decision making at the scanner.

"Congenital heart disease is very complicated, so a pediatric cardiologist and/or pediatric radiologist needs to be at the scanner during the scanning to identify the anatomy and instruct technologist how to change the angles of the planes," Samyn said.