Why in 3T MRI T1 relaxation times get longer ?

T1 relaxation times get longer-requiring sequence parameter changes.

This may seem counterintuitive—you might at first think that a stronger magnetic field would “pull” the net magnetisation vector of any spin isochromat back to alignment with the external magnetic field more quickly—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. This has to do with the number of resonant protons which are available to transfer energy to the “lattice”, which depends on field strength. 

--> Longer T1 times of tissues means that pulse sequence parameter settings from lower field strengths may not simply be copied over to a 3.0T magnet. The slower recovery of longitudinal magnetisation usually means that a longer TR is required to maintain expected contrast between tissues. This change in TR has consequences on other parameters and metrics such as scan time and coverage. Similarly, preparation-pulse delay times require modification.

Let's consider just the structured-water protons (medium range of τ_c'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.

Source: http://www.revisemri.com/