Introductory Biomedical Imaging

Magnetic Resonance Imaging Simulations

Magnetic resonance imaging (MRI) is an imaging variant of nuclear magnetic resonance (NMR). MRI is renowned for its high-resolution, exquisite soft-tissue contrast, ability to monitor anatomy and function, and safety. MRI uses an unusual localization strategy to visualize protons in water in living tissue: signals arising from protons in different regions in the body are distinguished, and their distinct points of origin are identified, based on frequency. This approach, which is known as frequency encoding, is implemented by placing protons in a spatially varying magnetic field, which creates signals with frequences that are determined by their location. Like other approaches, image contrast in MRI is based on differences in signal amplitudes. One parameter that affects signal amplitude is proton density, which can be used as a source of contrast. However, soft tissues tend to have similar proton densities; thus, “density-weighted” images are relatively low in contrast. MRI’s strength for soft-tissue imaging arises because the MRI signal amplitude can be made to depend on other parameters, such as the local environment, that generate much better contrast. Weaknesses of MRI include relatively slow data acquisition, high equipment cost, and incompatibility with metal implants. MRI is notable for providing both anatomic and functional information.

The following simulations explore nuclear magnetism and how time-varying magnetic fields can be used to create structural and functional images. The simulations are listed in the recommended order of performance. Each simulation includes a home page from which the simulation can be run, together with links to simulation-specific Information, Background, and Activity documents.