HISTORY AND BASIS OF MRI

 

The first detection of magnetic resonance was achieved in 1945 at Harvard by the team of Purcell and at Stanford by the team of Bloch. For this work, Purcell and Bloch jointly received the Nobel Prize for Physics in 1952.

In 1991, the Nobel Prize for Chemistry was awarded to R.R. Ernst for establishing the technological basis of  magnetic resonance imaging (MRI) by using radio frequency pulses and the mathematical process of two-dimensional Fourier transformation.

In 2003, the American chemist Paul C. Lauterbur and the British physicist Peter Mansfield  were awarded the Nobel Prize for Medicine. Their research during the 1970s brought about an improvement in the existing techniques, opening the way to ultrafast imaging sequences. Thanks to their work, MRI equipment became available in the early 1980s. Its spread was rapid and nowadays the protocols continue to improve with new magnets and the use of new techniques such as MR angiography, diffusion-weighted and perfusion-weighted MRI (for depicting metabolic and structural abnormalities while qualifying abnormal blood flow), real time MRI sequences (used during surgery) and MRI-spectroscopy, which provides chemical information about cellular metabolism.

MRI images are obtained by applying an oscillating electromagnetic field (radiofrequency pulse or RF pulse) to hydrogen protons when they are under the effect of a static magnetic field (a magnet). In this situation, the protons capture the energy (excitation) and then release it to the environment (relaxation). The signal or echo is captured by a receiver and encoded for storage. The subsequent decoding allows the signal to be displayed as an image.

This is a highly sensitive technique but its specificity is limited because many disorders share a similar signal. In veterinary neurology, it is considered an ancillary diagnostic test and, if necessary, should be performed after localizing the lesion in order to rule out possible causes in the differential diagnosis.

In this section we will explain the more basic sequences used in veterinary clinics with a low field magnet: spin echo and inversion recovery sequences.

The hydrogen atom is the simplest,  having a single proton and no neutrons. It has a large magnetic moment and is the most abundant in the body (about 70% belonging to water and the rest to triglycerides and to other substances). Therefore, the frequency of the electromagnetic wave applied in MRI is specific to the proton of hydrogen.