Patient-independent, MHD-robust R-peak detection for retrospective gating in cardiac MRI imaging.

Objective.In cardiovascular magnetic resonance imaging, synchronization of image acquisition with heart motion (calledgating) is performed by detecting R-peaks in electrocardiogram (ECG) signals. Effective gating is challenging with 3T and 7T scanners, due to severe distortion of ECG signals caused by magnetohydrodynamic effects associated with intense magnetic fields. This work proposes an efficient retrospective gating strategy that requires no prior training outside the scanner and investigates the optimal number of leads in the ECG acquisition set.Approach.The proposed method was developed on a data set of 12-lead ECG signals acquired within 3T and 7T scanners. Independent component analysis is employed to effectively separate components related with cardiac activity from those associated to noise. Subsequently, an automatic selection process identifies the components best suited for accurate R-peak detection, based on heart rate estimation metrics and frequency content quality indexes.Main results.The proposed method is robust to different B0 field strengths, as evidenced by R-peak detection errors of 2.4 ± 3.1 ms and 10.6 ± 15.4 ms for data acquired with 3T and 7T scanners, respectively. Its effectiveness was verified with various subject orientations, showcasing applicability in diverse clinical scenarios. The work reveals that ECG leads can be limited in number to three, or at most five for 7T field strengths, without significant degradation in R-peak detection accuracy.Significance.The approach requires no preliminary ECG acquisition for R-peak detector training, reducing overall examination time. The gating process is designed to be adaptable, completely blind and independent of patient characteristics, allowing wide and rapid deployment in clinical practice. The potential to employ a significantly limited set of leads enhances patient comfort.

Comparison and Integration of Voltage and Charge Amplifiers for Capacitive ECG Measurements.

OBJECTIVE: Sensing with capacitive electrodes is of interest for long-term, comfortable bio-potential measurements (e.g., ECG). However, due to the small body-to-electrode capacitance (Ce), the design of the associated front-end amplifier remains a challenge. Both voltage amplifiers (VA) and charge amplifiers (CA) can be employed. While basic comparisons of both typologies were done before, this paper extends the comparison to their responses to artifacts (caused by motion or interference). Further, a VA-CA-switchable amplifier is proposed, allowing to adapt the amplifier type to different situations, and enabling to estimate the body-to-electrode capacitance Ce in a passive way. METHODS: A VA-CA switchable amplifier was implemented in a 180 nm CMOS process. The responses to artifacts for VA and CA were studied by modelling, simulations and experiments using the custom IC. The proposed Ce estimation method was validated by electrical tests and in-vivo tests. RESULTS: VAs are less affected by Ce variation artifacts, while CAs recover faster from triboelectricity artifacts. In a VA, these two artifacts are multiplicative and get modulated if they occur simultaneously, but in a CA they remain independent. CONCLUSION: The combined VA-CA amplifier has the potential for optimal amplifier selection according to the properties of the recorded signal, the value of Ce and the actual presence of artifacts. Moreover, it can estimate Ce without extra hardware. SIGNIFICANCE: The proposed VA-CA switchable structure is superior to an individual VA or CA, thanks its adaptability to signal quality and artifacts, and it provides extra information on the body-to-electrode interface quality (Ce).

Automatic Optimization of Multichannel Electrode Configurations for Robust Fetal Heart Rate Detection by Blind Source Separation.

OBJECTIVE: Fetal heart rate (fHR) evaluation is fundamental to guarantee timely medical intervention in case of pregnancy complications. Due to the limitations of traditional cardiotocography, multichannel electrophysiological recording was proposed as a viable alternative, which requires Blind Source Separation (BSS) techniques. Yet effective and reliable separation of the fetal ECG remains challenging due to multiple noise sources and the effects of varying fetal position. In this work, we demonstrate that the adopted electrode configuration plays a key role in the effectiveness of BSS and propose guidelines for optimal electrode positioning. Moreover, a model is proposed to automatically predict the most suited configuration for accurate BSS-based fHR estimation with a minimal number of leads, to facilitate practical implementation. METHODS: We compared fHR estimation accuracy with different electrode configurations on in-silico data, identifying the optimal configuration for a recent BSS method. Based on features extracted from raw signals, we proposed a support vector regression model to automatically identify the best electrode configuration in terms of fHR estimation accuracy and to dynamically adjust it to varying fetal presentation. Evaluation was performed on real and synthetic data. RESULTS: Guidelines for the optimal electrode configuration are proposed by using 4 leads. Prediction of configuration quality shows 80.9% accuracy; the optimal configurat- ion is recognized in 92.2% of the subjects. CONCLUSION: The proposed method successfully predicts the quality of the configurations, demonstrating the impact of the electrode configuration on the BSS performance. SIGNIFICANCE: The method holds potential for long-term fetal monitoring, by dynamically choosing the optimal configuration.

A Prototype System With Custom-Designed RX ICs for Contrast-Enhanced Ultrasound Imaging.

This work presents a prototype system based on a multichannel receiving (RX) integrated circuit (IC) for contrast-enhanced ultrasound (CEUS) imaging. The RX IC is implemented in a 40-nm low-voltage CMOS technology and is designed to interface to a capacitive micromachined ultrasonic transducer array. To enable a direct connection of the RX electronics to the transducer, an analog multiplexer with on-chip protection circuitry is developed. Stress tests confirm the reliability of this arrangement when combined with a high-voltage pulser. The RX IC is equipped with a highly programmable bandpass filter to capture harmonic signals from ultrasound contrast agents (UCAs) while suppressing fundamental components. In order to examine the impact of analog front-end (AFE) bandpass filtering, in vitro acoustic experiments are performed with UCAs. A spatial resolution analysis suggests that the AFE bandpass filtering combined with a pulse inversion (PI) technique can improve the lateral resolution by 38% or 9% compared to the original full-bandwidth approach or a stand-alone PI approach, respectively, while the impact on axial resolution is negligible. A phantom study shows that compared to digital bandpass filtering, the AFE bandpass filtering enables better use of the dynamic range of the RX electronics, resulting in better generalized contrast-to-noise ratio from 0.44/0.53 to 0.57/0.68 without or with PI.

An RX AFE With Programmable BP Filter and Digitization for Ultrasound Harmonic Imaging.

This paper presents a front-end integrated circuit for ultrasound (US) harmonic imaging, interfacing to a one-dimensional capacitive micromachined ultrasonic transducer (CMUT). It contains a complete ultrasound receiving chain, from analog front-end (AFE) to gigabit/s data link. A two-stage self-biased inverter-based transimpedance amplifier (TIA) is proposed in this work to improve tradeoffs between power, noise, and linearity at the first stage. To improve harmonic imaging performance, the design is further equipped with a 4[Formula: see text]-order highly programmable bandpass filter, which has a tunable bandwidth from 2 MHz to 15 MHz. An 8 b 80 MS/s SAR ADC digitizes the signal, which is further encoded and serialized into an LVDS data link, enabling a reduction in the number of output cables for future systems with multiple ADCs. The design is realized in a 40 nm CMOS technology. Electrical measurements show it consumes 2.9 mW for the AFE and 2.1 mW for the ADC and digital blocks. Its overall dynamic range varies from 61 dB to 69 dB, depending on the reception bandwidth. The imaging capability of this design is further demonstrated in a US transmission and reception imaging system. The acoustic measurements prove successful ultrasound harmonic acquisition, where the on-chip bandpass filter can improve the lateral resolution by more than 30%.

Dedicated Algorithm for Unobtrusive Fetal Heart Rate Monitoring Using Multiple Dry Electrodes.

Multi-channel measurements from the maternal abdomen acquired by means of dry electrodes can be employed to promote long-term monitoring of fetal heart rate (fHR). The signals acquired with this type of electrode have a lower signal-to-noise ratio and different artifacts compared to signals acquired with conventional wet electrodes. Therefore, starting from the benchmark algorithm with the best performance for fHR estimation proposed by Varanini et al., we propose a new method specifically designed to remove artifacts typical of dry-electrode recordings. To test the algorithm, experimental textile electrodes were employed that produce artifacts typical of dry and capacitive electrodes. The proposed solution is based on a hybrid (hardware and software) pre-processing step designed specifically to remove the disturbing component typical of signals acquired with these electrodes (triboelectricity artifacts and amplitude modulations). The following main processing steps consist of the removal of the maternal ECG by blind source separation, the enhancement of the fetal ECG and identification of the fetal QRS complexes. Main processing is designed to be robust to the high-amplitude motion artifacts that corrupt the acquisition. The obtained denoising system was compared with the benchmark algorithm both on semi-simulated and on real data. The performance, quantified by means of sensitivity, F1-score and root-mean-square error metrics, outperforms the performance obtained with the original method available in the literature. This result proves that the design of a dedicated processing system based on the signal characteristics is necessary for reliable and accurate estimation of the fHR using dry, textile electrodes.