Ultrasound imaging is different from other forms of medical imaging. Diagnostic ultrasound machines were first developed in the early 1950s, leading to the publication of the first obstetrics ultrasound paper in the Lancet, showing an image of the fetal head. Compared to annual quality control (QC) checks of magnetic resonance imaging scanners, which typically involve assessing a handful of machines, QC of ultrasound scanners continues throughout the year. Medical ultrasound technology was developed following the Second World War based on Sound Navigation Ranging (SONAR) methods developed for detecting submarines. In SONAR, a “ping” of sound is emitted, and an echo intensity map is then generated by analysing the direction, intensity and timing of the resulting echoes. When ultrasound imaging of vessels and Doppler measurements are combined, this provides a powerful tool for displaying blood flow velocity profiles from specific vessels or regions of interest echoes. Doppler ultrasound is commonly used to image the heart and vasculature.