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Fetal heart rate (fHR) monitoring using the Doppler ultrasound (US) is a standard clinical practice for assessing fetal well-being before and during labor. For continuous fHR measurements, the US transducer is positioned on the maternal abdomen using a flexible belt. Due to fetal movement, the relative fetal heart location (fHL) with respect to the US transducer can change, leading to frequent periods of signal loss hampering the clinical assessment of fetal health. Consequently, the clinical staff has to repeatedly reposition the US transducer--a cumbersome task affecting clinical workflow. We propose a method to estimate the fHL during fHR monitoring to support clinicians in efficiently repositioning the US transducer. Unlike typical US transducers, which do not provide any information on the spatial fHL, we exploit the fact that multiple transducer elements are present in the array aperture of the US transducer. We developed a novel model that relates the measured Doppler power in the individual transducer elements to the fHL and use it within the probabilistic framework of an extended Kalman filter (EKF). The performance of the EKF algorithm was evaluated in simulations and in in vitro experiments using a dedicated setup of a beating fetal heart. Both simulations and in vitro experiments showed that the fHL can be determined with an accuracy of 4 mm. Furthermore, we demonstrate that when the fetal heart is drifting out of the US beam, the EKF algorithm accurately estimates the fHL up to a radial distance of $34$ mm.

Laser Doppler vibrometry (LDV) and camerabased vibrocardiography imaging (cVCGI) systems can sense cardiac-related displacements of the skin. This allows that carotid artery (CA) or jugular vein (JV) wall movements are acquired, non-obtrusively, at the neck and used for assessing cardiovascular health. However, skin-neck measurements are invalid if the CA and JV pulsations overlap. The concern is plausible since these vessels are anatomically close to one another until the carotid sinus. In this paper, we build on ultrasonographic (US) insights to verify whether trunk posture and skin-site variability within the neck influence cVCGI outcomes. Using ultrasound (US), we recorded the wall movements of the CA and JV in 4 subjects (ages, 28-41 yrs) in the supine, recumbent and seated positions at sites in the vicinity of the common CA. Skin-displacement waveforms were subsequently recorded by cVCGI and compared with US recordings. Our results show that CA displacements are dominant at the upper neck in the seated-to-recumbent positions whereas JV pulsations are best probed in recumbent-to-supine positions at the lower neck. These insights help to recognize the possible value of cVCGI in early-stage diagnosis or ambulatory monitoring.

Fetal well-being is commonly assessed by monitoring the fetal heart rate (fHR). In clinical practice, the de facto standard technology for fHR monitoring is based on the Doppler ultrasound (US). Continuous monitoring of the fHR before and during labor is performed using a US transducer fixed on the maternal abdomen. The continuous fHR monitoring, together with simultaneous monitoring of the uterine activity, is referred to as cardiotocography (CTG). In contrast, for intermittent measurements of the fHR, a handheld Doppler US transducer is typically used. In this article, the technology of Doppler US for continuous fHR monitoring and intermittent fHR measurements is described, with emphasis on fHR monitoring for CTG. Special attention is dedicated to the measurement environment, which includes the clinical setting in which fHR monitoring is commonly performed. In addition, to understand the signal content of acquired Doppler US signals, the anatomy and physiology of the fetal heart and the surrounding maternal abdomen are described. The challenges encountered in these measurements have led to different technological strategies, which are presented and critically discussed, with a focus on the US transducer geometry, Doppler signal processing, and fHR extraction methods.

Fetal heart rate (fHR) monitoring using Doppler Ultrasound (US) is a standard method to assess fetal health before and during labor. Typically, an US transducer is positioned on the maternal abdomen and directed towards the fetal heart. Due to fetal movement or displacement of the transducer, the relative fetal heart location (fHL) with respect to the US transducer can change, leading to frequent periods of signal loss. Consequently, frequent repositioning of the US transducer is required, which is a cumbersome task affecting clinical workflow. In this research, a new flexible US transducer array is proposed which allows for measuring the fHR independently of the fHL. In addition, a method for dynamic adaptation of the transmission power of this array is introduced with the aim of reducing the total acoustic dose transmitted to the fetus and the associated power consumption, which is an important requirement for application in an ambulatory setting. The method is evaluated using an in-vitro setup of a beating chicken heart. We demonstrate that the signal quality of the Doppler signal acquired with the proposed method is comparable to that of a standard, clinical US transducer. At the same time, our transducer array is able to measure the fHR for varying fHL while only using 50% of the total transmission power of standard, clinical US transducers.

The increased use of assisted reproductive technology and increasing maternal age have led to a significant increase of twin births in Western countries. Twin pregnancies are associated with increased risk for fetal distress and, therefore, monitoring fetal health is crucial. Fetal well-being is typically assessed by measuring the fetal heart rate (fHR) using a Doppler ultrasound (US) transducer. In twin pregnancies, two independent US transducers need to be manually positioned on the maternal abdomen, which requires skill and experience. Common clinical problems are: 1) both hearts lie in the same sample volume (SV) of the same transducer, leading to erroneous fHR registration, 2) the heart locations change due to fetal movement hence moving out of the sample volume, 3) registration of the maternal heart rate and 4) both US transducers aim at the same heart and measure the same fHR. In this study, we propose twin-fHR measurements using a flexible US transducer matrix. The array aperture allows measuring the Doppler power from multiple SVs simultaneously to localize both fetal hearts in 3D. Furthermore, automated Doppler signal clustering enables unambiguous determination of the fHR of both fetuses.

Electronic fetal heart-rate (fHR) monitoring is standard clinical practice to assess the fetal condition and to detect deviations from the normal fHR pattern. Typically, an ultrasound (US) transducer, operating in a pulsed-wave Doppler mode, measures the periodic motion and the resulting flow of the fetal heart, from which the fHR can be derived. However, as the fetus moves antepartum or during labour through the birth canal, the fetal-heart location (fHL) changes continuously. Consequently, the clinical staff needs to manually track the fHL and reposition the US transducer accordingly. In this research, a new flexible sensor matrix with multiple elements is designed; it allows measuring the fHR over a large range of possible fHLs. Further, a method for dynamic combination of the receiving channels is presented. This new method, compared to a standard combining technique, improves the signal-to-noise ratio (SNR) of the Doppler signal and, therefore, leads to more accurate fHR estimation.