Mono-Exponential Fitting in T2-Relaxometry: Relevance of Offset and First Echo

Area of Science:

• Quantitative Magnetic Resonance Imaging (MRI)
• Biophysics

Background:

• T2 relaxometry is crucial for quantitative MRI.
• The offset parameter in T2 relaxometry, used for signal floor correction, is influenced by refocusing flip angle imperfections.
• B1 field imperfections are a significant, understudied factor affecting the offset parameter.

Purpose of the Study:

• To demonstrate that B1 imperfections significantly contribute to the offset parameter in T2 relaxometry.
• To introduce a simple method to reduce systematic T2 errors.
• To validate the proposed method using simulations and phantom experiments.

Main Methods:

• Simulated T2 relaxometry signal curves using extended phase graph theory.
• Evaluated four post-processing methods: including/excluding the first echo, fitting with/without offset.
• Performed T2 relaxometry on a phantom using a 9.4T MRI scanner and single spin echo sequences.

Main Results:

• Simulation data revealed that systematic T2 error and offset depend on refocusing pulse, echo spacing, and echo train length.
• Discarding the first echo reduced systematic T2 error.
• Further reduction in T2 error was achieved by incorporating the offset as a fitting parameter.
• Phantom experiments corroborated simulation findings.

Conclusions:

• The fitted offset parameter in T2 relaxometry is demonstrably influenced by imperfect refocusing pulses.
• Employing offset fitting and excluding the first echo offers a rapid and effective approach to minimize T2 errors.
• This method is particularly beneficial for low to intermediate echo train lengths in quantitative MRI.