Molecular tumbling rate effects on T1 and T2

The average rate at which molecules tumble (and therefore T1 and T2 time) is related to the molecular size. Small molecules (e.g. water/CSF) have a broad distribution of motional frequencies with poor matching with the Larmor frequency and therefore have long T1 values. Medium sized molecules (e.g. lipids) have a narrower distribution of tumbling rates matched to typical resonant frequencies and therefore have relatively short T1 values. Large molecules (e.g. protein, DNA) tumble too slowly to be effective in causing relaxation and therefore also have long T1 values.

T2 time is primarily related to the nearly static intrinsic field caused by protons at low or zero re-orientation frequency. Large macromolecules have short 2 values, since they have the largest intrinsic fields at low re-orientation rates. Smaller molecules have longer T2 values due to lower intrinsic fields.

Thus, in the absence of external field inhomogeneities, the free induction decay will decay with the time constant T2. Gadolinium has seven unpaired electrons, which increases field inhomogeneity, thus reducing the T2 relaxation time.

The T2 time constant is dependent on:

• specific tissue – i- i.e. mobility of nuclear species, the presence of macromolecules. T2 increases with decreased molecular size and increased molecular mobility.
• temperature
• presence of paramagnetic ions/molecules (e.g. Gd) – shortens- shortens T2 time