MRI basics

Magnetic resonance imaging is a unique imaging concept since unlike radiography such as x-ray or Computed tomography (CT) scans, it doesn’t use ionizing radiation. It relies on the nuclear magnetic properties of the nuclei of atoms (mainly hydrogen). NMR active nuclei, have a magnetic moment and can be thought of as miniature bar magnets with a north and south pole. These poles are usually randomly orentated. (fig 1)

fig 1

Biological tissues consist predominantly of water and fat, which both contain protons (hydrogen nuclei). The total proton concentration of biological tissues is approximately 100 mM. When a body is placed in a magnetic field much stronger than that of the Earth (A 1 Telsa magnet is 20,000 times stronger than the earth’s magnetic field), the NMR active nuclei align themselves with the magnetic field (fig 2). There are 2 available energy states spin up (parallel to the magnetic field) and spin down (antiparallel to the magnetic field). Spin up is slightly lower in energy than spin down so there is a slight excess of spin up nuclei compared to spin down nuclei within the sample. This leads to an over all sample magnetisation. The difference in energy between the two spin states increases as the magnetic field strength increases.

(fig 2)

When placed in a magnetic field the nuclei are unable to completely align with the magnetic field and so they precess (rotate like a gyroscope) about their axis at a characteristic frequency known as the Larmor frequency. This precession is usually out of phase so there is no overall magnetic vector in the XY plane (fig 2).

If an electromagnetic pulse of the correct frequency (radio frequency, RF) is applied the distribution of the spin up and spin down states is perturbed and the phase of the spins is aligned (fig 3). If a 90 degree pulse is used the result is that the number of spin up and spin down states are equalised and the phases are aligned, therefore the overall magnetisation vector is transferred to the XY plane.

(fig 3)

From this excited state there are two forms of relaxation. The most rapid form of relaxation is the dephasing of the spins, this is known asT2 relaxation or transverse relaxation (fig 4). This has the effect of reducing the over all magnetisation vector in the XY plane.

(fig 4)

The second slower form of relaxation involves the return to equilibrium of spin up and spin down states, this is known as T1 relaxation or longitudinal relaxation. This has the effect of restoring the overall magnetisation vector in the Z direction (fig 5).

(fig 5)

Both forms of relaxation take place simultaneously, however T1 relaxation is much slower than T2 relaxation in solids but only slightly slower than the corresponding T2 relaxation rate in liquids. The properties of most biological tissues fall somewhere in between a classical solid or liquid thus different tissues have different T1 and T2 relaxation rates. This difference in relaxation rate leads to a difference in signal intensity at a given time period after the excitation pulse. This difference in signal intensity is visualised as a grey scale applied to the acquired image. By editing the pulse parameters of an MRI scanner it is possible to weight an image towards T1 relaxation,T2 relaxation or Proton Density weichting thus highlighting different tissue properties.

T1 FLASH image	T2* FLASH image	Proton density FLASH
T1 RARE image	T2 RARE image	Proton density RARE