Magnetic Resonance Imaging: The Underlying Principles

The creation of a magnetic resonance image (MRI) and its inherent contrast are controlled by a variety of anatomical structure- and sequence-dependent parameters. While these may seem confusing to the uninitiated, they provide MRI with great flexibility and make it a powerful clinical tool. This article describes the principles of basic physics behind magnetic resonance spectroscopy (MRS) and imaging, including a basic description of the properties of magnetic resonance compatible nuclei, how a radiofrequency (RE) pulse produces a signal, and how this signal can be spatially encoded to produce an image. The relaxation properties of the MRI signal depend on biological tissue type and can provide information on tissue composition, environment, and pathological changes. The contrast properties within an image can be manipulated based on the relaxation properties of the anatomical sample and the nature of the imaging sequence. The benefits of T1- and T2-weighted images in musculoskeletal imaging and the common sequences used (including turbo spin echo [TSE], fat suppression sequences such as STIR, and rapid breath-hold sequences such as HASTE and FISP) are discussed. The principles behind contrast agents and diffusion-weighted imaging and how they can be applied in the body are considered. J Orthop Sports Phys Ther 2011;41(11):806-819, Epub 7 June 2011. doi:10.2519/jospt.2011.3576