[Dynamic magnetic resonance tomography in pediatric cardiology: broadening of diagnostic possibilities by flow-rephasing gradient-echo sequences]

Abstract

Based on the phase difference method as described by Nayler et al., we developed a gradient-echo sequence, which refocuses flow-related phase shifts. With regard to the higher peak velocity and higher acceleration in infants, we reduced the echo time (TE) to 5 ms. This is effective in rephasing the flow signals even for faster heart rates. Phase shifts are further minimized by reducing the voxel size. The slice thickness down to 2 mm also improves anatomic resolution. A repetition time (TR) of 15 ms provides high temporal resolution for dynamic studies. Modification of the flow-rephasing gradient-echo sequence allows blood flow measurements in the great arteries and the calculation of blood flow volume to assess left and right ventricular stroke volume. This can also be achieved by calculating the ventricular volume from contiguous slices of the whole heart; however, it results in excessive measuring times. Compared with conventional spin-echo techniques, anatomical analysis is improved. The size of atrial septal defects (ASD) can be measured more exactly. Small atrial or ventricular septal defects (VSD), which are not detectable on spin-echo images, are demonstrated on the gradient-echo images and semi-quantitative estimation of the pressure in the right and left ventricle can be made. Pulmonary arteries and veins are clearly differentiated from bronchial structures and are traced to subsegmental levels. Without ECG-gating, contiguous slices of interesting parts of the cardiovascular system can be obtained and successfully used for secondary angiographic reconstructions.