T1, T2 Relaxations and Image Weighting | MRI

T1, T2 Relaxations and Image Weighting
MRI- T1, T2 Relaxations & Image Weighting

Relaxation means recovery of protons back towards equilibrium after been disturbed by RF excitation. Relaxation times of protons such as T1 and T2, and the number of protons in tissue (proton density) are the main determinant of the contrast in an MR image.

RF pulse causes tilting of magnetization in the transverse plane, where it rotates at Larmor frequency. This article discusses processes happening after this and their implications on the image contrast.

What happens when the RF pulse is switched off?

When Rf pulse is switched off, LM starts increasing along Z-axis and TM starts reducing in the transverse plane. The process of recovery of LM is called Longitudinal Relaxation while the reduction in the magnitude of TM is called Transverse Relaxation.

A signal vector can represent the components of magnetization in longitudinal and transverse planes (i.e. LM and TM). This vector represents the sum of these components and is called a net magnetization vector (NMV). NMV lies somewhere between LM and TM. If there is no magnetization in the transverse plane LM will be the same as NMV. Similarly, if there is no LM, TM will be equal to NMV.

MRI- Longitudinal Relaxation

When the RF pulse is switched off, spinning protons start losing their energy. The low-energy protons tend to align along the Z-axis. As more and more protons align along the positive side of the Z-axis there is a gradual increase in the magnitude (recover) of the LM. The energy released by the protons is transferred to the surrounding (the crystalline lattice of molecules) hence the longitudinal relaxation is also called 'spin-lattice relaxation. The transfer of energy involves dipole-dipole interaction of fluctuating small magnetic fields that originates in adjacent protons and electrons in the surrounding. For the transfer of energy to occur, these fluctuations must occur at the Larmor frequency.

The time taken by LM to recover to its original value after the RF pulse is switched off is called longitudinal relaxation time or T1.

MRI- Transverse Relaxation

The transverse magnetization represents a composition of magnetic forces of protons processing at a similar frequency. More the number of protons processing at the same frequency (in-phase) stronger will be the TM. These protons are constantly exposed to static or slowly fluctuating local magnetic fields. Hence they start losing phase after RF pulse is switched off. This going out of phase of protons (dephasing) results in a gradual decrease in TM magnitude and is termed transverse relaxation. Since the dephasing is related to the static or slowly fluctuating intrinsic fields caused by adjacent spins (protons), transverse relaxation is also called 'spin-spin relaxation. 

The time taken by TM to reduce to its original value is transverse relaxation time or T2. Even though they appear as parts of the same process, the longitudinal and transverse relaxations are different because the underlying mechanisms are different.

MRI- T1 Relaxations

T1 is the time taken by LM to recover after the RF pulse is switched off. This is not an exact time, but it is 'constant'. T1 is the time when LM reaches back to 63% of its original value. The curve showing the gradual recovery of LM against time is called the T1 curve. 1/T1 in the longitudinal relaxation rate.

T1 depends upon tissue composition, structure, and surroundings. If the surrounding matter has magnetic fields, which fluctuate at Larmor frequency, transfer of energy from protons to the surrounding is easy and fast. Protons in such water molecules move too rapidly the protons in water take a long time to transfer their energy. Hence water has long T1. On the other hand, the fluctuating magnetic field in the fatty acids has a frequency near the Larmor frequency. There is a fast energy transfer from fat protons to the surrounding. Hence fatty tissue has short T1.

T1 increases with the strength of the external magnetic field. T1 at 3T is longer than T1 of the same tissue at 1.5T.

MRI- T1 Weighted Image

The magnitude of LM indirectly determines the strength of the MR signal. Tilting of stronger LM by 90 degrees RF pulse will result in a greater magnitude of TM and stronger MR signal. The tissues with short T1 regain their maximum LM in a short time after the RF pulse is switched off. When the next RF pulse is sent, TM will be stronger and the resultant signal will also be stronger. Therefore, a material with short T1 has a bright signal on T1 weights images.

How does one make images T1 weighted?

This is done by keeping the TR short. If TR is long the tissues with long T1 will also regain maximum LM giving a stronger signal with the next RF pulse. This will result in no significant difference between signal intensity of tissue with different T1. With short TR only the tissues with short T1 will show the high signal intensity of tissues is due to their different T1.

MRI- T2 Relaxations

T2 is the time taken by TM to disappear. Similar to T1, it is a 'constant' and not an exact time. It is the time taken by the TM to reduce to 37% of its maximum value. The curve showing decrease in magnitude (decay) of the TM plotted against time is called the T2 curve. 1/T2 is the transverse relaxation rate.

T2 depends on the inhomogeneity of local magnetic fields within the tissues. As water molecules move very fast, their magnetic fields are within the tissues. These fluctuating magnetic fields cancel each other. So there are no big differences in magnetic field strength inside such a tissue. Because of the lack of much inhomogeneity protons stay in phase for a long-time resulting in long T2 for water. 

If the liquid is impure or the tissue has larger molecules, the molecules move at a slower rate. The maintains inhomogeneity of the intrinsic magnetic field within the tissue. As a result, protons go out of phase very fast. Hence impure liquids or tissues with larger molecules have short T2. Fat has shorter T2.

MRI- T2 Weight Image

Immediately after its formation TM has the greatest magnitude and produces the strongest signal. Thereafter it starts decreasing in magnitude because of dephasing, gradually reducing the intensity of the received signal. Different tissue depending on their T2, have variable times for which TM will remain strong enough to induce a useful signal in the receiver coil. Tissue or material with longer T2, such as water, will retain their signal for a longer time. Tissue with short T2 will lose their signal earlier after the RF pulse is turned off.

How does one make images T2 weighted?

The image is made T2- weighted by keeping the TE longer. At short TE, tissue with long, as well as short T2, have strong signals. Therefore, on the images acquired at short echo time, there will not be a significant signal intensity difference between tissue with short and long T2. At longer TE, only those tissue with long T2 will have sufficient strong signal and the signal difference amongst tissues (contrast) is determined by T2 of tissues. Tissues with long T2 are bright on T2-weighted images. TR is kept long for T2-weighted images to eliminate T1 effects.

Proton Density (PD) Image

The contrast in the PD image is determined by the density of protons in the tissue. T1 effect is reduced by keeping long TR and the T2 effect is reduced by keeping TE short. Hence long TR and short TE give PD-weighted images. The signal intensity difference amongst tissues is a function of the number of protons they have.

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