The motivation of this research was to detect and quantify stress level in the phonation signal. This signal is recorded in parallel during scanning in the magnetic resonance imager (MRI) for calculation of the 3D model of the human vocal tract. An examined person is exposed by vibration and noise originated from the gradient system of the running MRI device. The mental stress can effectively be identified by the heart rate (HR), the blood pressure, or other parameters using the photo-plethysmography (PPG) signal. The amplitude of the picked-up PPG signal is usually not constant and it can often be partially disturbed or degraded. Therefore, the sensed raw PPG signal must be smoothed before the HR determination. The filtering as well as the HR determination procedures work in real-time, so the implemented algorithms must be simple, but robust and stable. For proper and safe function in the low magnetic field environment of the MRI device, the PPG sensor must be composed of a non-ferromagnetic material including the power supply part.
The paper describes design, realization, testing, and first practical measurements with two developed wearable PPG sensors working in reflectance mode with real-time Bluetooth data transfer to an external recording device enabling post-processing and storage of PPG signals. Two described realizations differ in the type of a control unit based on the Arduino platform and the used Bluetooth (BT) communication module (working in BT 2.0/BT4.0 BLE standards). In the frame of the performed practical experiments, the basic functions of the developed PPG sensors as well as the built control application were stepwise tested. The auxiliary measurement consists of real-time sensing, transmission, and storage operations (including signal filtering and HR value determination) – in the normal laboratory conditions. Comparative measurements with the oximeter device for calibration of determined HR values were also performed. The main experiment with the PPG signal sensing inside the running MRI device has shown that the PPG signal must be filtered, all PPG sensor parts must be shielded to avoid strong radiofrequency disturbance, and the communication baud rate must be decreased in comparison with the measurement in the standard condition.