The inversion time (TI) in an inversion recovery MRI sequence is chosen to null the signal from a specific tissue. To null CSF signal (producing FLAIR), the TI for CSF (T1 ~4000 ms) at 1.5T is approximately:
- A TI = 170 ms (to null fat, as in STIR)
- B TI = 2200–2500 ms (to null CSF, as in FLAIR) ✓
- C TI = 600–700 ms (to produce T1 contrast)
- D TI = 10000 ms (beyond T1 of all tissues)
Correct answer: B. TI = 2200–2500 ms (to null CSF, as in FLAIR)
Explanation
In inversion recovery sequences, signal nulling occurs when TI = T1 × ln2 (≈0.693 × T1). For CSF with T1 ~4000 ms at 1.5T: TI = 0.693 × 4000 ≈ 2772 ms, practically 2200–2500 ms range is used (accounting for TR and sequence variations). This produces FLAIR (Fluid Attenuated Inversion Recovery), where CSF appears dark. STIR uses TI ≈ 150–170 ms to null fat (fat T1 at 1.5T ≈ 270 ms, TI = 0.693 × 270 ≈ 187 ms). FLAIR is particularly valuable for detecting periventricular and cortical lesions hidden by adjacent bright CSF on T2 sequences.
Reference: Grainger & Allison's Diagnostic Radiology, 7th ed.
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