Sammendrag
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 degrees (0.26 mm) and elevational resolution of 0.5 degrees (0.44 mm).
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