It is the time taken to for the spins to dissipate energy so that they regain their longitudinal magnetization
It is an exponential process
It is tissue specific
Defined as time taken for 63% of the longitudinal magnetization to be recovered
The time taken for this extends from one RF pulse to the next (i.e., TR)
TR determines how much T1 recovery occurs in a tissue
T1 recovery is proportional to strength of external magnetic field. Stronger the field, longer the recovery times.
Fat
Absorbs energy very quickly
T1 recovery time is very short
200 ms
Water
Very poorly receives energy from spins
T1 recovery time is very long
2500 ms
What happens if you control the T1 recovery?
A short TR does not permit sufficient recovery of longitudinal magnetization. As a result, when a successive RF pulse is applied, it pushes the NMV beyond the transverse plane. This is called saturation.
When saturation occurs, signal from fat and water demonstrate a difference
On the contrary, a long TR permits sufficient time for recovery of longitudinal magnetization. As a result, there is no difference between fat and water other than the inherent contrast that is there due to the proton density. Proton density is an intrinsic contrast, and inherent to the tissue being imaged.
