Benefits and Pitfalls

Benefits and Pitfalls#

It has been well reported in recent years that the accuracy of VFA T₁ estimates is very sensitive to pulse sequence implementations [Stikov et al., 2015, Baudrexel et al., 2017, Lutti and Weiskopf, 2013], and as such is less robust than the gold standard inversion recovery technique. In particular, the signal bias resulting from insufficient spoiling can result in inaccurate T₁ estimates of up to 30% relative to inversion recovery estimated values [Stikov et al., 2015]. VFA T₁ map accuracy and precision is also strongly dependent on the quality of the measured B₁ map [Lee et al., 2017], which can vary substantially between implementations [Boudreau et al., 2017]. Modern rapid B₁ mapping pulse sequences are not as widely available as VFA, resulting in some groups attempting alternative ways of removing the bias from the T₁ maps like generating an artificial B₁ map through the use of image processing techniques [Liberman et al., 2013] or omitting B₁ correction altogether [Yuan et al., 2012]. The latter is not recommended, because most MRI scanners have default pulse sequences that, with careful protocol settings, can provide B₁ maps of sufficient quality very rapidly [Boudreau et al., 2017, Samson et al., 2006, Wang et al., 2005].

Despite some drawbacks, VFA is still one of the most widely used T₁ mapping methods in research. Its rapid acquisition time, rapid image processing time, and widespread availability makes it a great candidate for use in other quantitative imaging acquisition protocols like quantitative magnetization transfer imaging [Cercignani et al., 2005, Yarnykh, 2002] and dynamic contrast enhanced imaging [Li et al., 2018, Sung et al., 2013].

References

1: Simon Baudrexel, Ulrike Nöth, Jan-Rüdiger Schüre, and Ralf Deichmann. T1 mapping with the variable flip angle technique: a simple correction for insufficient spoiling of transverse magnetization. Magnetic Resonance in Medicine, 79(6):3082–3092, October 2017. doi:10.1002/mrm.26979.
2(1,2): Mathieu Boudreau, Christine L. Tardif, Nikola Stikov, John G. Sled, Wayne Lee, and G. Bruce Pike. B1 mapping for bias-correction in quantitative t1 imaging of the brain at 3t using standard pulse sequences. Journal of Magnetic Resonance Imaging, 46(6):1673–1682, March 2017. doi:10.1002/jmri.25692.
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9(1,2): Nikola Stikov, Mathieu Boudreau, Ives R. Levesque, Christine L. Tardif, Joëlle K. Barral, and G. Bruce Pike. On the accuracy of t1 mapping: searching for common ground. Magnetic Resonance in Medicine, 73(2):514–522, 2015. arXiv:https://onlinelibrary.wiley.com/doi/pdf/10.1002/mrm.25135, doi:10.1002/mrm.25135.
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