T1 is a rate of longitudinal relaxation. When we tip the magnetization in tissue away from its alignment with the scanner’s magnetic field, it takes a little bit of time for it to go back to its equilibrium low energy. That rate of change is T1.
T2 is a rate of transverse relaxation. I think “spin-spin” is a confusing term, though it is commonly used. After we tip magnetization away from its alignment with the field axis, it precesses (rotates) around that axis, kinda like a gyroscope or a precessing spinning top. Neighboring ensembles don’t have the exact same precession frequency. There is a spread in these frequencies. Therefore, neighboring ensembles accumulate a phase relative to each other resulting in their signals gradually cancelling each other out, until the signal disappears. This rate of change is T2 (actually, it’s T2* – “Tee two star”, which is strongly related to T2).
T1 is different in different tissue types, as is T2, and T2*. These values also change with some pathology. relaxation rates are one form of tissue contrast. We can get an image that’s T1-weighted, or we can actually do a fitting and get a quantitative T1 map. The same is true for T2 or T2*. We can get a qualitative T2-weighted image, or a quantitative T2 map. I think radiologists need to get used to the quantitative maps, as the qualitative data may not be as reliable, and doesn’t represent a precise measurement. It can vary substantially based on measurement conditions and the setup. Yet, change apparently is tough – radiologists still rely heavily on qualitative data instead of the alternative, which actually can be used to make statistical inferences.
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 system, 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.