What is the difference between an ultrasound and an mri?

Different technique: Ultrasound uses sound waves to look inside your body. It is like sonar and measures the sound waves that bounce back from the organs inside your body to recreate an image. MRI uses a high power magnet and radio waves to see the organs and makes an image. Both have strengths and weaknesses and are not interchangeable. Your doctor or health provider needs to determine which is better to use.