START BELOW ZERO
Inversion recovery gives tissue a head start in the opposite direction.
An inversion pulse first turns longitudinal magnetization toward the negative Z direction. The tissue then begins recovering toward equilibrium. The time between that inversion pulse and the excitation pulse is called inversion time, or TI.
Different tissues recover at different rates because they have different T1 values. That means each tissue passes through zero at a different time.
CATCH THE ZERO CROSSING
Invert, wait, excite, then read the signal.
A 180° inversion pulse turns longitudinal magnetization toward negative Z.
The tissue recovers during the chosen TI.
The excitation pulse is applied as the target tissue reaches zero.
Little longitudinal magnetization is available, so the target tissue produces little signal.
The exact null point is not one universal number. It depends on tissue T1, field strength, TR, sequence design, scanner, and other protocol choices.
TWO IMPORTANT APPLICATIONS
STIR targets fat signal. FLAIR targets fluid signal.
Uses a short TI to null tissue with the short T1 of fat. It is useful when broad, reliable fat suppression is needed.
Uses a long TI to null cerebrospinal fluid while preserving T2-sensitive contrast in many brain applications.
STIR is not chemically specific to fat.
STIR suppresses signal according to T1 recovery. Other short-T1 tissues, including enhancing tissue after gadolinium, may also lose signal. For that reason, STIR is generally not used to evaluate post-contrast enhancement. Always follow the approved protocol and local clinical guidance.
In the simplified case where longitudinal magnetization has fully recovered before inversion, the null point is approximately TI = 0.69 × tissue T1. Real sequences may require different timing, so scanner protocol values take priority.
THE ELEVATOR ANALOGY
Take the picture as the elevator passes the ground floor.
Imagine tissue recovery as an elevator rising from a basement floor through the lobby and upward. TI decides when the picture is taken. If the target tissue is crossing the lobby at zero when the excitation arrives, its signal is suppressed.
Short TI, fat signal suppressed.
Fluid signal attenuated.
Time the excitation near the target tissue's zero crossing.
Remember: Invert, wait for zero, then excite.
CHECK YOUR UNDERSTANDING
Match the timing to the tissue being suppressed.
What does TI measure?
TI is the time between the inversion pulse and the excitation pulse.
Which tissue signal is commonly suppressed by STIR?
STIR uses a short inversion time to suppress tissue with the short T1 of fat. It is not chemically specific to fat.
Which signal is commonly suppressed by FLAIR?
FLAIR uses a long inversion time to suppress cerebrospinal fluid, commonly while maintaining T2-sensitive image contrast.
LESSON 16 COMPLETE
You understand how inversion time can suppress selected tissue signal.Educational references
This lesson is educational and does not replace manufacturer instructions, institutional protocols, or clinical judgment. TI values and suppression behavior vary with the scanner, field strength, sequence, and tissue.