Medical imaging
Because CT and MRI are sensitive to different tissue properties, the appearances of the images obtained with the two techniques differ markedly. In CT, X-rays must be blocked by some form of dense tissue to create an image, so the image quality when looking at soft tissues will be poor. In MRI, while any nucleus with a net nuclear spin can be used, the proton of the hydrogen atom remains the most widely used, especially in the clinical setting, because it is so ubiquitous and returns a large signal. This nucleus, present in water molecules, allows the excellent soft-tissue contrast achievable with MRI.
Ultrasound computer tomography
USCT systems designed for medical imaging of soft tissue typically aim for resolution in order of centimeters to millimeters and require therefore ultrasound waves in the order of mega-hertz frequency. This requires typically water as low-attenuating transmission medium between ultrasound transducers and object to retain suitable sound pressures.