T₁ mapping with the variable flip angle technique: A simple correction for insufficient spoiling of transverse...

Area of Science:

• Magnetic Resonance Imaging (MRI)
• Quantitative MRI
• Biomedical Engineering

Background:

• The variable flip angle method is crucial for deriving T1 maps from radiofrequency-spoiled gradient-echo data.
• Errors in T1 estimation arise from insufficient spoiling of transverse magnetization, violating the Ernst equation assumptions.
• The radiofrequency-spoiling phase increment (Δϕ) significantly influences these errors.

Purpose of the Study:

• To present a versatile correction method using modified flip angles (α') to restore the validity of the Ernst equation.
• To address and correct errors in T1 estimation caused by insufficient spoiling in gradient-echo MRI.
• To improve the accuracy of T1 maps generated by the variable flip angle technique.

Main Methods:

• Simulated spoiled gradient-echo signals across various flip angles (α), repetition times (TR), T1, and T2 values (85 ms).
• Determined modified flip angles (α') and correction factors (CΔϕ) for common phase increments (Δϕ = 50°/117°/150°).
• Fitted correction factors as a polynomial function of α and TR, independent of T1, and validated the method in vitro and in vivo.

Main Results:

• The correction factor CΔϕ was found to be independent of T1 and could be fitted as a polynomial CΔϕ(α, TR).
• The developed method demonstrated a significant improvement in the accuracy of T1 maps.
• The technique proved effective for variable flip angle scans with diverse acquisition parameters, both in vitro and in vivo.

Conclusions:

• The proposed method offers a straightforward correction for insufficient spoiling in gradient-echo MRI data.
• Polynomial parameters for three common Δϕ values are provided, facilitating practical application.
• This technique enhances the reliability and accuracy of T1 mapping in clinical and research settings.