Ultrasound has 0 radiation, but MRI's don't have any radiation either, plus their image quality is truly the gold standard of all soft tissue imaging. Ultrasound is grainy and does not have the resolution or exquisite detail of an MRI. CT would be the second highest image quality.

There is no comparison of either CT or MRI to ultrasound, they are the gold standard of diagnostic imaging.

The only typical ultrasound studies, that I am aware of, done on the neck, have to do with calcium scoring of the carotid artery for heart attack risk evaluation. It just doesn't have the resolution for complex structures like the head and neck region and the signals will bounce off of bones as well.

Progress is constantly being made in the area of ultrasound image quality/resolution and I would anticipate that eventually it will emerge as a more mature imaging modality than it is today. Not that it doesn't have many uses already, especially OB/GYN imaging...