MRI Basics

What is Magnetic Resonance Imaging (MRI)>It is a modality used to image the human body without using x-rays. It uses a large magnet, radiowaves and a computer.
Non-invasive medical imaging method, like ultrasound and X-ray. Clinically used in a wide variety of specialties. Advantages are excellent / flexible contrast, non-invasive, no ionizing radiation.

Magnetic Resonance
Certain atomic nuclei including 1H exhibit nuclear magnetic resonance. Nuclear “spins” are like magnetic dipoles. Hydrogen is the most common element in the body which has highest sensitivity to magnetic resonance. Hydrogen ion is positively charged.

Nuclear spin
A particle rotating upon its own axis is called Nuclear spin. Spinning particles contain a magnetic field. Human body is made up of tiny bar magnets. Spins are normally oriented randomly. In an applied magnetic field, the spins align with the applied field in their equilibrium state. Excess along B0 results in net magnetization.

Precession

Spins precess about applied magnetic field, B0, that is along z axis. During precession the tail of the vector is fixed and the head is revolving. This revolving motion is called Precession. Precessional frequency is the speed at which the hydrogen protons precesses.

Larmour Equation is f = g M

f = Frequency in Revolution /sec

M = Magnetic field strength in T

g = Gyromagnetic ratio [42.6]

Relaxation Processes

The return of M to its equilibrium state (the direction of the z-axis) is known as relaxation. There are three factors that influence the decay of M: magnetic field inhomogeneity, longitudinal T1 relaxation and transverse T2 relaxation. T1 relaxation (also known as spin-lattice relaxation) is the realignment of spins (and so of M) with the external magnetic field B0 (z-axis). T2 relaxation (also known as T2 decay, transverse relaxation or spin-spin relaxation) is the decrease in the x-y component of magnetisation.

T1 relaxation

Following termination of an RF pulse, nuclei will dissipate their excess energy as heat to the surrounding environment (or lattice) and revert to their equilibrium position. Realignment of the nuclei along B0, through a process known as recovery, leads to a gradual increase in the longitudinal magnetisation. The time taken for a nucleus to relax back to its equilibrium state depends on the rate that excess energy is dissipated to the lattice. Let M-0-long be the amount of magnetisation parallel with B0 before an RF pulse is applied. Let M-long be the z component of M at time t, following a 90 degree pulse at time t = 0. It can be shown that the process of equilibrium restoration is described by the equation

where T1 is the time taken for approximately 63% of the longitudinal magnetisation to be restored following a 90 degree pulse.

T2 relaxation

While nuclei dissipate their excess energy to the lattice following an RF pulse, the magnetic moments interact with each other causing a decrease in transverse magnetisation. This effect is similar to that produced by magnet inhomogeneity, but on a smaller scale. The decrease in transverse magnetisation (which does not involve the emission of energy) is called decay. The rate of decay is described by a time constant, T2*, that is the time it takes for the transverse magnetisation to decay to 37% of its original magnitude. T2* characterises dephasing due to both B0 inhomogeneity and transverse relaxation. Let M-0-trans be the amount of transverse magnetisation (Mx-y) immediately following an RF pulse. Let M-trans be the amount of transverse magnetisation at time t, following a 90 degree pulse at time t = 0. It can be shown that

Type of Images

• T1 weighted image
• T2 weighted image
• Proton density weighted image

T1 weighted image:
Image produced as a result of differences in T1 times of the tissue. The basis of T1 weighted imaging is the longitudinal relaxation. A T1 weighted magnetic resonance image is created typically by using short TE and TR times. TR less than T1 (typically £ 500 ms) and TE less than T2 (typically £ 30 ms).

T2 weighted image:
Image produced as a result of differences in T2 times of the tissue and it is the image made with a sequence with long TR and TE to show contrast in tissues with varying T2 relaxation times; water gives a strong signal. TR greater than T1 (typically £ 2 000 ms) and TE less than T2 (typically £ 100 ms).


Proton density weighted image
Image that is produced as a result of differences in proton densities of the tissues. An image produced by controlling the selection of scan parameters to minimize the effects of T1 and T2, resulting in an image dependent primarily on the density of protons in the imaging volume. Images are generated by choosing TR greater than T1 (typically £ 2 000 ms) and TE less than T2 (typically £ 30 ms).