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T2 accuracy on a whole-body imager
W D Foltz1, J A Stainsby, G A Wright
1Department of Medical Biophysics, University of Toronto, Ontario, Canada.
Abstract:
MR oximetry requires a T2 measurement that is accurate within 5% in vivo. Simple methods are susceptible to signal loss and tend to underestimate T2. Current methods utilize RF pulses or RF cycling patterns that prevent signal loss at each data acquisition. However, using these methods with imperfect pulses, T2 tends to be overestimated due to temporary storage of the magnetization along the longitudinal axis where it decays more slowly with a time constant T1 > T2. To reduce the T1 dependence while preventing signal loss, we utilize simple 90x180y90x composite pulses and good RF cycling patterns. These trains are critical for T2 accuracy over typical ranges of RF and static field inhomogeneities and refocusing intervals. T1 signal decay during each 90x180y90x pulse must be accounted for to yield accuracy within 5% when the pulse-width is 10% or more of the refocusing interval. A simple correction scheme compensates for this T1-related error effectively.
Insights
Accurate in vivo T2 measurement for MR oximetry is crucial. This study introduces a method using composite pulses and RF cycling to prevent signal loss and reduce T1 dependence, achieving T2 accuracy within 5%.
Area of Science:
- Magnetic Resonance Imaging (MRI)
- Biomedical Engineering
- Medical Physics
Background:
- Accurate in vivo T2 measurement is essential for MR oximetry, requiring precision within 5%.
- Existing methods often suffer from signal loss, leading to T2 underestimation.
- Imperfect radiofrequency (RF) pulses can cause T2 overestimation due to T1-dependent signal decay.
Purpose of the Study:
- To develop an MR oximetry method that achieves T2 accuracy within 5% in vivo.
- To minimize signal loss and T1 dependence in T2 measurements.
- To improve the reliability of T2 measurements under RF and static field inhomogeneities.
Main Methods:
- Utilized simple 90x180y90x composite pulses and optimized RF cycling patterns.
- Implemented strategies to prevent signal loss during data acquisition.
- Accounted for T1 signal decay during the composite pulse sequence.
Main Results:
- The proposed method effectively prevents signal loss, a common issue in T2 measurements.
- T1 dependence was significantly reduced, mitigating T2 overestimation.
- Achieved T2 accuracy within the critical 5% threshold for MR oximetry.
Conclusions:
- The combination of composite pulses and RF cycling enhances T2 accuracy for MR oximetry.
- A simple correction scheme effectively compensates for T1-related errors.
- This approach offers a robust solution for reliable in vivo T2 quantification.