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Dependence on T1 of the echo amplitudes from multiple spin-echo sequences with equidistant echoes: simulation studies
A Fransson1, A Ericsson, G O Sperber
1Department of Radiation Physics, Karolinska Institutet, Stockholm, Sweden.
Magnetic Resonance Imaging
|January 1, 1993
Summary
The Carr-Purcell-Meiboom-Gill (CPMG) MRI protocol offers more accurate transverse relaxation time (T2) estimates than the Phase-Alternating-Phase-Shift (PHAPS) protocol, especially with nonselective refocusing pulses.
Area of Science:
- Magnetic Resonance Imaging (MRI)
- Biophysical Chemistry
- Medical Physics
Background:
- Accurate estimation of transverse relaxation times (T2) is crucial for MRI-based tissue characterization.
- T1 relaxation can influence T2 measurements through stimulated echo components in multi-echo sequences.
- Various multi-echo spin-echo protocols exist, each with potential biases in T2 estimation.
Purpose of the Study:
- To numerically simulate and evaluate the accuracy of T2 estimation using different multi-echo MRI protocols.
- To investigate the impact of T1 relaxation and intravoxel static magnetic field inhomogeneity on T2 estimates.
- To compare the robustness of Carr-Purcell (CP), Carr-Purcell-Meiboom-Gill (CPMG), and Phase-Alternating-Phase-Shift (PHAPS) protocols.
Main Methods:
- Numerical simulations of spin systems under multi-echo spin-echo MRI protocols (CP, CPMG, PHAPS).
- Inclusion of T1 relaxation effects and stimulated echo components in echo amplitude calculations.
- Simulation of intravoxel static magnetic field inhomogeneities (0.1, 1, and 10 ppm) with selective and nonselective refocusing pulses.
Main Results:
- T2 estimates showed notable dependence on T1 for values below approximately 800 msec across all protocols.
- The PHAPS protocol yielded underestimated T2 values with selective refocusing but overestimated values with nonselective refocusing, showing high sensitivity to field inhomogeneity.
- Long T2 values were generally erroneous with the PHAPS protocol, while CPMG demonstrated greater robustness.
Conclusions:
- The CPMG protocol structure is a more robust method for T2 estimation compared to the PHAPS protocol.
- T1 relaxation significantly affects T2 measurements, particularly for tissues with shorter T1 values.
- The choice of refocusing pulse (selective vs. nonselective) critically influences the accuracy of T2 estimates, especially with the PHAPS sequence.