A small ultrasound-based sensor was developed for use in MRI. It is about 3x3x1cm in size, can be fixed to the skin, readily fits under imaging coils, and provides up to 20,000 data points in a 0.2ms period. A switching circuit was developed allowing up to four such sensors to be used at a time. Examples are show where a sensor was attached over a carotid artery, over the heart or below the ribs, providing information about blood influx to the brain, cardiac motion or breathing, respectively. These devices and associated processing may prove especially useful at high field, where ECG detection proves ineffective.