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Updated: Aug 7, 2025
A Magnetic Resonance Imaging Protocol for Stroke Onset Time Estimation in Permanent Cerebral Ischemia
Published on: September 16, 2017
Combined estimation of B1and T1for dynamic contrast-enhanced MRI by accounting for incomplete spoiling of transverse
1Department of Oncologic Imaging, National Cancer Centre, 169610, Singapore.
Abstract:
Objective.The variable flip angle (VFA) method for longitudinal relaxation time (T1) measurement is inherently sensitive to inaccuracies in the radiofRequency transmit field (B1) and incomplete spoiling of transverse magnetization. The objective of this study is to devise a computational method that addresses the problems of incomplete spoiling andB1inhomogeneity in the estimation ofT1using VFA method.Approach. Using an analytical expression of the gradient echo signal with account of incomplete spoiling, we first showed that ill-posedness in the simultaneous estimation ofB1andT1can be lifted with the use of flip angles larger than the Ernst angle. We then devised a nonlinear optimization method based on this signal model of incomplete spoiling for simultaneous estimation ofB1andT1.Main results. We evaluated the proposed method on a graded-concentration phantom to show that the derivedT1estimates offers an improvement over the regular VFA method and compares well with reference values measured by inversion recovery. Reduction of the number of flip angles from 17 to 5 yielded consistent results indicating that the proposed method is numerically stable.T1estimates derived from in-vivo brain imaging were consistent with literature values for gray and white matter tissues.Significance. Contrary to the common notion thatB1correction in the VFA method forT1mapping should be performed separately, we show that combined estimation ofB1andT1is feasible by the proposed method simply with the acquisition of 5 flip angles, as demonstrated on both phantom and in-vivo imaging data.
Insights
This study introduces a new computational method for accurate longitudinal relaxation time (T1) measurement using the variable flip angle (VFA) technique. The method simultaneously corrects for radiofrequency transmit field (B1) inaccuracies and incomplete spoiling, improving T1 estimation with fewer measurements.
Area of Science:
- Magnetic Resonance Imaging
- Quantitative MRI
- Biomedical Engineering
Background:
- The variable flip angle (VFA) method for longitudinal relaxation time (T1) measurement is susceptible to radiofrequency transmit field (B1) inaccuracies and incomplete transverse magnetization spoiling.
- Accurate T1 mapping is crucial for various clinical and research applications of MRI.
- Existing methods often require separate B1 correction or more complex acquisition schemes.
Purpose of the Study:
- To develop a computational method for simultaneous estimation of T1 and B1 using the VFA technique.
- To address the challenges of incomplete spoiling and B1 inhomogeneity in T1 mapping.
- To improve the accuracy and efficiency of T1 quantification.
Main Methods:
- Derived an analytical expression for the gradient echo signal accounting for incomplete spoiling.
- Demonstrated that using flip angles larger than the Ernst angle resolves ill-posedness in simultaneous B1 and T1 estimation.
- Developed a nonlinear optimization method based on the derived signal model for joint B1 and T1 estimation.
Main Results:
- The proposed method showed improved T1 estimates compared to the standard VFA method on a phantom, with results comparable to inversion recovery measurements.
- Consistent results were obtained with only 5 flip angles, indicating numerical stability and reduced acquisition time.
- In vivo brain imaging yielded T1 estimates consistent with literature values for gray and white matter.
Conclusions:
- Combined estimation of B1 and T1 is feasible with the proposed method, challenging the notion of separate correction steps.
- The method provides accurate T1 mapping using a reduced number of flip angles (5), enhancing efficiency.
- This approach offers a robust and practical solution for improved T1 quantification in MRI.
