Automatic measurement of biparietal diameter with a portable ultrasound device

Knowledge of the exact gestational age and expected day of delivery is essential for providing optimal medical surveillance. Fetal biparietal diameter (BPD) computed by ultrasound has been shown to correlate well with gestational age (before week 22) or it can be used for detecting growth abnormalities, later in the pregnancy. We have been developing a portable ultrasound scanner (the Umoja scanner) that can be used by midwives in LMIC countries with limited ultrasound and technological expertise. The aim of this work was to develop a technique for automatized computation of BPD which can run on tablet devices. An ultrasound image containing a contour of a fetal head is recorded with the prototype scanner. The image is preprocessed: converted to grayscale, gain adjusted, smoothed, dilated/eroded and finally binary thresholded. The potential contour candidates and their Cartesian coordinates are identified by applying the Canny edge detector. A line connecting the two most distant edge contours across the skull is computed. The original grayscale values along this line are used to identify the top and the bottom edge points which are used for measuring the BPD value. All image processing is performed using OpenCV (Open source Computer Vision), which is optimized for tablet devices. 27 ultrasound images suitable for BPD measurement were acquired by an experienced midwife and 9 student midwives with limited or no prior ultrasound experience, on 8 different fetuses from 18 to 34 weeks. Both manual (experienced midwife) and automatic BPD measurements were computed. The correlation plot and the error versus reference plot are produced, the mean error ± 1.96*STD was 0.72±3.62 [mm], while the correlation coefficient was R=0.9932. The automatic measurement failed in 4 cases. The overall computation time on a Nexus 10 tablet was 3.47 seconds, therefore our tool is suited for a portable device with limited computation power. The agreement of the proposed algor- thm with the reference measurements is comparable to the interobserver agreement for BPD (2.6 to 3.1 mm from literature study). Testing of the method on an extended dataset, including fetuses at different gestational ages is ongoing.