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Abstract. This paper proposes an alternative method for pulse oximetry processing based on time-frequency analysis. Instead of using the fast Fourier transform (FFT), the continuous wavelet transform (CWT) together with the median calculation was proposed for finding the frequency components of the normalized first derivative red and infrared photoplethysmographic signals (PPG). In order to estimate the arterial oxygen saturation (% 2 ), the ratio of each red CWT modulus to each corresponding infrared CWT modulus was computed at each frequency, and then converted to the oxygen saturation by using the saturation equation obtained from calibration. All of the oxygen saturation data were plotted on a histogram. The greatest oxygen saturation with the most occurrences was considered to represent the % 2 . The experiments were held to evaluate the performance of the proposed processing method compared with the conventional pulse oximetry (CPO) processing. The red and infrared PPGs were acquired from left and right index fingers simultaneously by using the PPG measurement system constructed in the laboratory. During measurement, the left index finger was stationary, while the right index finger was performed the finger bend to induce the motion artifact (MA) in transient and periodic manners. All detected PPGs were processed by the proposed method and the CPO processing. The % 2 obtained from different processing methods and positions were compared. Experimental results showed that the proposed method was more resistant to the MA than the conventional method.

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The ultrasonic pressure waveforms produced at the output of medical ultrasound devices must be determined and strictly regulated to ensure patient safety and to ascertain whether the ultrasound devices are performing satisfactorily. However, characterization and measurements of the ultrasound fields are difficult because of the size of the hydrophones' effective aperture. Since the spatial averaging effect mainly depends on the effective diameter of the hydrophone probes, this work explains an experimental technique used to measure the size of the ultrasonic hydrophones' effective aperture. Three ultrasonic transducer sources with three different resonance frequencies, 5 MHz, 7.5 MHz, and 10 MHz were utilized with a pulse-echo technique. The measurements in this work were performed with a bilaminar PVDF membrane hydrophone probe having nominal diameter of 400 μm. The results show that the current measurement system offers the ability to effectively determine the directivity pattern and thus effective diameter for circular aperture membrane hydrophones.