Physical principles of magnetic resonance imaging

When a nucleus contains either unpaired protons, neutrons, or both, it has angular momentum. This property provides the basis for magnetic resonance imaging (MRI). Because of its abundance in the human body, hydrogen is used for clinical MRI. Within the MR magnet, the atoms are aligned with the magnetic field. The MR signal is generated by subjecting the atoms to radiofrequency (RF) pulsations, and the pulse sequences selected will determine the appearance of the image. The repitition time (TR) is the time between the RF pulses, and the echo time (TE) is the time between the RF pulse and the recording of the signal. T1-weighted images (short TR/short TE) provide the best anatomic detail, but T2-weighted images (long TR/long TE) are often needed to demonstrate pathology. Many unfamiliar artifacts may be encountered on MR images. These can be due to several factors, including inhomogeneities in the magnetic field (for example, those resulting from metallic orthopedic applicances or prior surgery), extraneous RF interference, and motion. Surface coils are commonly used in musculoskeletal MR imaging to improve the quality of the examination by increasing the signal-to-noise ratio over the area of interest.