1Faculty of Engineering Gunma University, 2Department of Obstetrics and Gynecology Teikyo University
Fetal heart rate, Phase modulation noise, Index for measurement accuracy
Fetal heart rate is recognized as a useful index for evaluating fetal health. Though fetal heart rate monitoring devices based on ultrasonic pulsed Doppler technology are widely used in medical diagnosis today, they can not adequately measure fetal heart rate until the middle stage of pregnancy, and no suitable device is available. We thus developed a new method that is based on measurement of the small internal displacement of the fetal heart during the early stages of pregnancy. This method uses ultrasonic pulsed Doppler technology, first, to measure fetal-wall movement quantitatively, and then to determine the small internal displacement of the fetal heart. Observed displacement approximates the form of a sine wave, the fundamental frequency of which corresponds to fetal heart rate. Fetal heart rate is estimated by applying an instantaneous frequency estimation algorithm to the data. Here, estimation accuracy of the fetal heart rate measured by this method is evaluated under existence of noises that modulate phase of displacement. Phase-modulated noise is generated, especially when the monitoring point of the internal displacement deviates from the target inside the fetal body. Estimation error is evaluated numerically for various noise levels, and an index that can estimate accuracy of measurement is derived. This index is defined as the difference between two estimated heart-rate values that are obtained from slightly different reference frequencies in the fetal heart rate estimation algorithm. The usefulness of these analyses is demonstrated by experimental results.