Semi-blinded freehand 3D ultrasound with novice users

We have built upon an existing freehand 3D ultrasound imaging technique to enable display-less scanning at a local site by novice users and remote reading by integrating an electromagnetic tracker with a 2D probe. Seventy-two volumes are generated using a reconstruction algorithm from data collected by three users in a single longitudinal sweep across a 23week fetus phantom in four different configurations for six scan durations ranging from 5-s to 30-s. The acquisition is semi-blinded: the user knows the fetal orientation but scans without image display and guidance of a conventional scan. Three non-expert readers and one expert Radiologist extract the clinically relevant planes and measure four key biometric features from the 3D images. In this paper, we propose (1) a risk metric R to rate the quality of the scan as a function of probe motion and contact and (2) a measurability index M for the availability of the 2D planes within the volume and visibility of the biometric features. Our analysis shows that R is the lowest and M the highest for 15-s acquisitions corresponding to an average transducer sweep speed of 2.4-cm/s. The finding is consistent with a reported speed range of 3-4 cm/s recommended for a low cost teleradiology solution for 2D ultrasound. The errors in average biometric measurements compared to the 50th percentile values in the fetal biometry tables for corresponding gestational week are within -3.8 to 5.7%. R, M, accuracy and precision of measurements are useful indicators of performance of the 3D ultrasound system.