Relaxation time contrast


Typical radiologist statement after MRI exam:

"PD- and T1-weighted imaging were normal. 
  T2-weighted imaging revealed a subcortical lesion".
"PD- and T1-weighted imaging were normal. 
  T2-weighted imaging revealed a subcortical lesion".

T1, T2 and proton density (PD) are parameters characterizing tissue:
just like "temperature" or "water content".
The "proton density" is, in fact, the water content.

T1 and T2 time-constants are somewhat special:
Can only be determined by MRI (they are "MR contrast parameters")
Reflect aspects of consistency (molecular mobility)
"PD- and T1-weighted imaging were normal. 
  T2-weighted imaging revealed a subcortical lesion".

T1, T2 and proton density (PD) are parameters characterizing tissue:
just like "temperature" or "water content".
The "proton density" is, in fact, the water content.

T1 and T2 time-constants are somewhat special:
Can only be determined by MRI (they are "MR contrast parameters")
Reflect aspects of consistency (molecular mobility)

So what is "weighting" ??
The parameters above are seldom measured quantitatively...
...but their relative values may be apparent in the images.
i.e: The contrast in a "T1-weighted" image comes mostly from T1-differences.
So why all this talk about T1 and T2?