DO NOT!!! grab the Elgato Game Capture HD. It is a USB 2.0 device, and will have a 2-second delay on any capture sources coming from it due to the need to compress the video before sending it over the USB connection, as USB 2.0 does not have the bandwidth needed.
There are many devices based on the Yuan capture chipset out there that may work, if one with s-video is available. Just make sure it's USB 3.0 and not 2.0.
If you are working in a medical environment (as it sounds like you are, with an ultrasound) I'd really recommend the Magewell USB Capture AIO. It carries a significant price tag, but is a no-compromises solution for a medical grade application.
USB Capture AIO supports HDMI/SDI/DVI/VGA/CVBS/S-video/Component video capture with resolution up to 2048x2160 to any computer for video conferencing, streaming and more via a USB 3.0 interface.
www.magewell.com