MRI background

Next to X-rays, magnetic resonance imaging (MRI) is one of the most valuable imaging tools used for diagnosis.   An MRI machine consists of a large static magentic field, three orthogonal controllable magnetic gradients (X, Y, and Z direction), and an RF transmitter and receiver.   When controlled magnetic fields and radio frequency (RF) signals are applied across tissue in the body, certain molecules and atoms respond.   A great example is hydrogen in water, which makes up much of a person's body weight.   A large static field is applied across the body in the z-direction.   The effect of the field is the alignment of all the hydrogen spin axes in the same direction.   When a pulsed RF signal of particular frequency is applied to tissue, the proton in hydrogen's nucleus spins in a direction different than its normally aligned direction (e.g. it spins perpendicular to the static field).   When the RF signal is turned off the spin of the proton goes through a relaxation process until it returns to its orginal alignment with the static field.   As it relaxes, RF signals of a certain frequency unique to particular tissues are detected and recorded.   Using 2D and 3D Fourier analysis, images are produced from the RF data.   The three orthogonal magnetic gradients are useful for accurately pinpointing the desired tissue cross-sections.  

The image details of certain tissues are not always present after MRI scans.   Image contrast enhancements have proven to be useful in exploring difficult areas.   Current research and development of nanoparticles have demonstrated signifant contrast enhancement.   The nanoparticles are composed of iron oxide surrounded by an organic covering.   The properties of the particles have lead to the term superparamagnetic iron oxide (SPIO) nanoparticles.   If using T1 imagery, areas treated with the nanoparticles will appear brighter in the final images, thus giving contrast between treated and untreated tissues.   T2 images will show the nanoparticles to be black.  

Left: Non-enhanced T1-weighted image brainstem infarct (arrow) Right: Images 48 h (D) following SPIO infusion Image courtesy of Annals of Biomedical Engineering, Vol. 34, No. 1, January 2006 pp. 23–38