What is the difference between T1 and T2 imaging in MRI?

T1-weighted MRI enhances the signal of the fatty tissue and suppresses the signal of the water. T2-weighted MRI enhances the signal of the water. Consideration of all the information provided by these modalities is conducive to MRI image analysis and diagnosis.

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Image contrast is the goal in all imaging procedures. The imaging technique will emphasize certain contrast characteristics of anatomical structures and allow us to differentiate the structures and determine which structures are abnormal.

MRI structural image contrast is natively (i.e. without using contrast enhancing agents) superior than CT and other imaging techniques. In both CT and MRI, image contrast is a function of tissue density. For MRI in which the source of signal are the protons (especially hydrogen protons), the type of density that matters the most is proton density. In addition to tissue density, tissue relaxation properties contribute to image contrast in MRI (but not CT). There are two types of relaxation properties: T1 relaxation and T2 relaxation. Both types have been correctly described by the other responders but let me state it in a slightly different way. During the process of T1 relaxation, protons reorient resulting in recovery of longitudinal magnetization. During the process of T2 relaxation, protons dephase (spin becomes desynchronized) resulting in decay of transverse magnetization.

................. Intrinsic tissue properties therefore determine the native image contrast in MRI: proton density, T1 relaxation, and T2 relaxation.

1. Proton density is quantified as a relative concentration, i.e. the concentration of protons in the tissue of interest relative to that in water with the same volume and temperature. Being a relative measure PD is really a dimensionless quantity. The PD of CSF (water) is ~ 1 and the PD of white matter (high fat) is ~ 0.6.

2. T1 relaxation is measured using a time constant called T1 (usually reported in milliseconds, msec). T1 is defined as the time when 63% of the longitudinal magnetization has recovered; 3 x T1=95% recovery.

3. T2 relaxation is measured using a time constant called T2 (usually reported in milliseconds, msec). T2 is is defined as the time when 63% of the transverse magnetization has decayed; 3 x T2=95% decay.

................. Scanning parameters (e.g. TR=repetition time, TE= echo time) can be selected to emphasize certain image contrast properties (weighting). How this is done (called MRI pulse sequence) is complex and beyond the scope of this answer. I will limit the discussion to three basic image-weighting techniques using different TR/TE values. (Note: do not confuse the extrinsic parameters TR/TE with the intrinsic tissue relaxation time constants T1/T2).

1. PD-weighted imaging is used to differentiate anatomical structures based on their proton density; i.e. the scanning parameters are set (long TR/short TE) to minimize T1 and T2 relaxation effects.

2. T1-weighted imaging is used to differentiate anatomical structures mainly on the basis of T1 values; i.e. the scanning parameters are set (short TR/short TE) to minimize T2 relaxation effects. Tissues with high fat content (e.g. white matter) appear bright and compartments filled with water (e.g. CSF) appears dark. This is good for demonstrating anatomy.

3. T2-weighted imaging is used to differentiate anatomical structures mainly on the basis of T2 values; i.e. the scanning parameters are set (long TR/long TE) to minimize T1 relaxation effects. Compartments filled with water (e.g. CSF compartments) appear bright and tissues with high fat content (e.g. white matter) appear dark. This is good for demonstrating pathology since most (not all) lesions are associated with an increase in water content.

4. There are other weighting techniques, such as diffusion-weighting, perfusion-weighting, etc. No plans to include them in the discussion.

................. If you wish to get to the bottom of it, here's an excellent introductory book: MRI: The Basics.

................. Added on 1/13/15 in response to a comment:

Relaxation means restoration of the equilibrium state or going back to a low-energy level after excitation.

Spin-lattice (longitudinal) or T1 relaxation is the process by which the longitudinal magnetization is recovered (after the excitation pulse is applied) due to transfer of energy from the nuclear spin system to the neighboring molecules (the lattice). It occurs in the z-direction (z-axis is often depicted as a vertical line). The T1 relaxation time is a measure of the rate of transfer of energy from the nuclear spin system to the neighboring molecules (the lattice). It is the time when 63% of the longitudinal magnetization has recovered.

Spin-spin (transverse) or T2 relaxation is the process by which the transverse magnetization decays due to dephasing of proton spins (spins becoming desynchronized). After the excitation pulse is applied, the magnetization flips 90 degrees from the longitudinal axis to the xy-plane. The transverse magnetization is initially maximum (due to coherent nuclear spins) but this arrangement is gradually lost due to field inhomogeneities and/or direct interactions between the spins (without energy transfer to the lattice). T2 relaxation occurs on the xy-plane and is often depicted as the spreading of magnetic moments along the plane. The T2 relaxation time is a measure of the rate of the decay of transverse magnetization within the xy-plane. It is the time when 63% of the transverse magnetization has decayed.

See: Spin-lattice and spin-spin relaxation, page on ucl.ac.uk.

................. Excellent diagram showing relationship of T2 and T2* (T2 with asterisk)

Source: www.jcmr-online.com - Figure.