Small particles of ferrite are used as superparamagnetic contrast medium in MR imaging (appearing predominantly dark on MRI). These agents exhibit strong T1 relaxation properties, and due to susceptibility differences to their surroundings also produce a strongly varying local magnetic field, which enhances T2 relaxation to darken the contrast media containing structures.
Superparamagnetic contrast agents are also known by the abbreviation SPIO's (small particle iron oxide or superparamagnetic iron oxide) and USPIO's (ultrasmall particle iron oxide or ultrasmall superparamagnetic iron oxide).
Two types of USPIO will be available on the market as blood pool agents, while SPIO's have been used as darkening contrast agents for liver imaging. As particulate matter they are taken up by the RES. Very small particles of less than 300 nanometers also remain intravascular for a prolonged period of time and thus can serve as blood pool agents.

See also the related poll result: 'The development of contrast agents in MRI is'

The use of gas as a contrast medium has significant potential to avoid limitations of conventional contrast agents. Gases can transit smaller vascular conduits and can be injected through smaller and less traumatic access systems than liquids. Highly soluble gases (such as CO2) can be imaged as a bolus. Blood is displaced by the gas, with the result of negative image contrast.
Because gases are compressible, standard liquid injectors cannot be used. The design for a gasinjector should have the option for individual adaptation of blood flow rate, vessel diameter, pulse pressure, and heart rate.