Dynamic 3D ultrasound imaging of the fetal heart

Dynamic 3D ultrasound imaging of the fetal heart is difficult due to the absence of ECG signal for synchronization between loops. In this work we introduce a technique for image synchronization based on acquisition of tissue Doppler signals dedicated to compute a sync-signal. We have applied this method to dynamic 3D reconstructions of the heart of 9 fetuses aged 19 to 24 weeks. The sync-signal is generated by peak velocity detection of the TVI data. Once it is available, we can use a standard technique for reconstruction; in our case the EchoPAC3D software from GE Medical Systems. The basis for one reconstruction is a 2D sector scan, taken trans-abdominally and slowly tilted in free-hand, so that we cover the entire fetal heart over approximately 40 cardiac cycles. The total angle of the sweep is estimated by recording a separate loop through the centre of the heart, in the elevation direction of the sweep. We show an example with 1950 frames over 50 cardiac cycles, recorded with a frame rate of 96 and hence a time span of 20 seconds. The reconstruction consists of 31 volumes, each assumed to have 50 equidistantly tilted frames. Furthermore, every frame consists of 91 beams with 583 samples each. A typical good position of the heart is about 5 cm from the probe, which gives us a radial resolution of 0.15 mm, azimuthal resolution of 0.3° ∼ 0.26 mm and elevational resolution of 0.5° ∼ 0.44 mm.