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.
Femoroacetabular impingement
Radiographic assessment is often considered first line for FAI, providing a two-dimensional view of the hip joints. Anterior-posterior pelvis and a lateral image of the hip in question should be attained. A 45-degree Dunn view is also recommended. MRI imaging may follow, particularly if there is no specific evidence on radiographs, producing a three-dimensional reconstruction of the joint for better definition, to evaluate the hip cartilage, or measure hip socket angles (e.g. the alpha-angle as described by Nötzli in 2-D and by Siebenrock in 3-D). MR arthrogram had been used in the past, as it was more sensitive for picking up soft tissue lesions; however, due to improvement in technology, MRI is now considered comparable for picking up such lesions. CT is not usually used due to radiation exposure and no benefit above MRI. It is possible to perform dynamic simulation of hip motion with CT or MRI assisting to establish whether, where, and to what extent, impingement is occurring.