MRI: Magnetic Resonance Imaging
What is it?
Magnetic Resonance Imaging is often used to describe the medical imaging aspect of a larger topic termed Nuclear Magnetic Resonance, that may provide either images or spectroscopy analysis. MRI essentially images the amount and the environment of water molecules within our body. To fully understand how it works can rapidly become quite complex. Fundamentally, it is based upon the fact that water molecules contain two hydrogen protons that have what is termed a ´magnetic dipole moment´ that spin (precess) –; similar to a ´spinning top′. We may align the central axis of all of these spinning magnetic dipoles using a high strength magnetic field. We may then add an oscillating (Radio-Frequency, RF) magnetic field that has the same (Larmour) frequency as the spinning rate of the magnetic dipoles, that makes the angle of spin have a certain value &ndash which may be 180 degrees, ie spinning within a plane normal to the central axis. When the RF field is switched off, the spinning magnetic dipoles return to their original spinning angle, but in doing so, release energy of a certain amount and at a certain rate. There are number of parameters that may describe this process; the T1 time constant is related to ´axial magnetisation″ and the T2 time constant is related to ´transverse magnetisation″. A contrast agent such as gadolinium may be injected into the patient.
Interestingly, when imaging bone with MRI, we do not obtain a signal from the bone tissue since it contains very few water hydrogen protons – we obtain a signal from the surrounding marrow and other surrounding soft tissues.