MR Physics

Magnetic Resonance Imaging (MRI), also known as Magnetic Resonance Tomography or Nuclear Magnetic Resonance Imaging, is one of the non-invasive imaging techniques that have superior soft tissue contrasts and potential physiological and functional applications. This type of radiation has not enough energy to remove an electron from an atom but just to excite it to a higher energy state. Since the 1980s, MRI has been a mainstay of non-invasive diagnostic radiology because it does not expose the body to radiation. It is frequently used in neuroimaging for the diagnosis and monitoring of diseases, and it has not yet shown any adverse effects from exposure, which is a major benefit over other imaging modalities. MRI enables to perform dynamic studies due to its speed of acquisition.[Hovet et al., 2018, Berger, 2002]

Basic Physics

Any atomic nucleus with an odd number of neutrons has spin different from zero and so, a magnetic moment (magnetic dipole). In the body, we can find several atoms with magnetic moment such as H, P, C, F, Na, which are sensitive to magnetic resonance. Around 60% of the human body is made up of water that contains hydrogen, which is also present in proteins and lipids. For this reason, hydrogen is the most widely used in MRI.

MRI bore contains a powerful magnet which generates an uniform magnetic field B0. Patiens are introduced in this magnetic field and hydrogen atoms align to the magnetic field. According to Larmour’s law, a magnetic dipole inside a magnetic field precesses (spins) around the magnetic field with a frequency proportional to the magnetic field strength. Hence, hydrogen atoms precess around the magnetic field generated by the MR with a frequency (Larmor frequency) that follows the equation:

w = γ B0

This precession can be parallel or antiparallel to B0. In the body the number of atoms that precess parallel is different to the ones that precess antiparallel producing an small magnetic field which is proportional to B0 and also depends on the density of hydrogen nuclei. So, the static magnetic field (B0) induces a slight magnetization of tissues.

Then, a radiofrequency pulse is emitted perpendicular to B0 with the same frequency that the spin precession frequency. Hydrogen atoms abrorb energy and spin out of equilibrium. Longitudinal magnetization (Mz) of protons in a parallel direction to B0 decreases, and a transverse magnetization (Mx, My) appears.

Then, when the RF disappears, the magnetic momentum gradually goes back to te minimum energy position (magnetic relaxation) while releasing energy. These emitted signals are measured into the k-space, which is an array of numbers representing spatial frequencies in the MR image. (Each k-space point contains spatial frequency and phase information about every pixel in the final image). Fourier transforme is performed to the k-space to obtain the final image. By varying the sequence of RF pulses applied & collected, different types of images are created.

MRI Sequences

It’s important to understand the meaning of repetition time (TR) and echo time (TE) in order to comprehend the main MRI sequences. Time to Echo (TE) is the time between the delivery of the RF pulse and the receipt of the echo signal and the interval between subsequent pulse sequences delivered to the same slice is known as the repetition time (TR).

The most common sequences are T1-weighted and T2-weighted images. In neuroimaging, T1-weighted images are commonly used in anatomical related studies, they are based on the study of the relaxation of the nuclei in the longitudinal component (Mz) of the magnetization vector and are produced with short TR and TE. T2-weighted images are produced with longer TR and TE. They are based on study of the variations of the component on the transverse plane of the magnetization during the relaxation, known as transverse relaxation (Mxy).

There are many sequences that can be used depending on the objective.