Longitudinal Magnetization Recovery (Mz)
After 180° inversion pulse — vertical line marks current TI. Dashed zero-crossing = tissue null point.
Signal vs. TI — All Tissues
Final image signal |Mz| (at current TE/TR). Current TI marked by vertical line.
Relative Signal at Current TI
Clinical IR Sequences

Simulated brain contrast based on T1 signal model. Hover each card for tissue signal breakdown.

Physics Reference

Inversion Recovery Signal Equation

After a 180° inversion pulse, longitudinal magnetization (Mz) recovers toward equilibrium (M₀) via T1 relaxation. Signal in IR sequences depends on TI, TR, and T1:

Mz(TI) = M₀ · [1 − 2·exp(−TI/T1) + exp(−TR/T1)]

|SI| = |Mz(TI)| · exp(−TE/T2*)

TI_null = T1 · ln(2) ≈ 0.693 · T1

For magnitude reconstruction, the absolute value is displayed — a tissue at its null point appears dark regardless of sign. Phase-sensitive reconstruction preserves the sign, showing suppressed tissue as a dark band and recovering tissue as bright.

T1 / T2 Values by Field Strength

Tissue T1 (ms) T2 (ms) T1_null (ms)

FLAIR — Fluid-Attenuated IR

Long TI (≈1800–2400 ms at 3T) chosen to null CSF. Suppresses CSF signal while maintaining T2 contrast in adjacent tissue. Critical for detecting periventricular lesions (MS plaques), cortical/juxtacortical lesions, subarachnoid space pathology.

TI_FLAIR ≈ T1_CSF · ln(2)
3T: TI ≈ 2300–2500 ms
1.5T: TI ≈ 1800–2200 ms
TR typically > 6000 ms

STIR — Short TI IR

Short TI (≈150–200 ms) nulls fat signal by inverting at peak fat suppression. T1-additive (short T1 = dark), robust to field inhomogeneity. Used in MSK, head/neck, spine. Cannot be combined with Gd contrast (enhancing tissue also nulled).

TI_STIR ≈ T1_fat · ln(2)
3T: TI ≈ 155–175 ms
1.5T: TI ≈ 150–180 ms
TR typically > 3000 ms

DIR — Double Inversion Recovery

Two sequential 180° pulses null both CSF and white matter. Produces striking cortical gray matter contrast — critical for cortical lesion detection in MS (invisible on standard FLAIR). Technically challenging: more complex timing, lower SNR.

Two TI values (TI1 > TI2):
TI1 ≈ T1_CSF · ln(2)
TI2 ≈ T1_WM · ln(2)
Both CSF + WM suppressed simultaneously

Null Point & Contrast Optimization

The null point (TI = T1·ln2) defines where a tissue's Mz crosses zero. Setting TI at or near a tissue's null point suppresses that tissue. Contrast between remaining tissues depends on their Mz values at that TI. Increasing TR beyond 5×T1 improves signal but lengthens scan time.

Contrast ratio = |Mz_A − Mz_B| / Mz_max

CNR ∝ (Mz_A − Mz_B) · exp(−TE/T2*)

Optimal TI: between null points of two tissues of interest