Longer T1 at Higher Field
First of all, note that when we "increase the magnetic field strength" (B0) we are usually referring to removing the sample from one magnet and putting it in another. For example, going from a 1.5 T magnet to a 3.0 T magnet.
You may at first assume that because a higher magnetic field is stronger, it will pull the net magnetisation vector back to it's original position in the z-direction more quickly, producing shorter T1 times. But this classical picture does not help us here. In fact the T1s usually get longer—slower regrowth of the net magnetisation vector in the z-direction.
Why is this so? It has to do with the number of resonant protons that are available to transfer energy to the "lattice".
Refer to a previous page to learn about how the environment of hydrogen protons affects their transfer of energy to that environment, and thus affects T1. Structured water and correlation times are introduced.
Let's consider just the structured-water protons (medium range of τc's). How many protons within this set experience an oscillating magnetic field at or near the Larmor frequency? Actually, not all of them, and the proportion is related to the Larmor frequency. Consider this graph:
This graph indicates the number (the spectral- (relating to frequency) density) of protons which are tumbling at or near the Larmor frequency. We are interested in the blue line. If we increase B0, the Larmor frequency increases (move the red line to the right), and so the spectral density at the Larmor frequency reduces. (Don't forget, the Larmor frequency is proportional to B0). So increase the magnetic field strength, and there are fewer protons available to transfer energy efficiently to the lattice, and the T1 time is lengthened.