A simple B ₁ correction method for dynamic contrast-enhanced MRI

Abstract

The double-angle method (DAM) is commonly used as a reference standard in radiofrequency field (B ) mapping studies. This study explored two aspects of DAM: (i) use of small flip angle pairs to reduce the repetition time (TR) needed for adequate longitudinal relaxation (T); and (ii) the effect of using different flip angle ratios for B mapping. Results of phantom studies show that B correction using small flip angle pairs ≤ 60° with TR = 5000 ms can allow for accurate estimation of T up to about 1500 ms; and that increasing the ratio of the two flip angles used for B correction resulted in more accurate estimation of T . These modifications allow 3-dimensional (3D) B mapping to be consistently performed with the same 3D spoiled gradient echo sequence used for T mapping in dynamic contrast-enhanced MRI.

Combined estimation of B1 and T1 for dynamic contrast-enhanced MRI by accounting for incomplete spoiling of transverse magnetization

“…In the present study, the DAM B 1 maps shown in figure 7(b) were acquired using the same 3D-SPGR sequence implemented for VFA imaging, but over a coarser acquisition matrix (64 × 64) and with a prolonged TR of 5000ms for in vivo B 1 imaging. For gray and white matter tissues with T 1 ranging from ∼800-1500ms, a prolonged TR of 5000ms ( 3 × 1500ms) used for DAM B 1 mapping would be adequate to remove residual T 1 effects in the B 1 images (Li et al 2018). However, regions with cerebrospinal fluid (CSF, such as the ventricles) that are associated with higher T 1  2500ms would require an even longer TR in order to yield accurate B 1 estimates.…”

“…While VFA methods (with no B 1 correction) have been found to yield either lower (Deoni 2007, Cheng and Wright 2006, Li et al 2018 or higher (Stikov et al 2015) T 1 estimates than IR in aqueous phantoms, VFA T 1 estimates were consistently higher than those measured by IR for in vivo studies (Stikov et al 2015). The exact reason for this observation is unclear and one could speculate that factors which may be greater in vivo than in phantoms, such as diffusion anisotropy, tissue conductivity and multiple-pool magnetization transfer, could result in the observed discrepancies between phantom and in vivo measurements (Stikov et al 2015).…”

“…B 1 inhomogeneities were expected to vary gradually and a coarse acquisition matrix was implemented for time efficiency. The 3D-SPGR images acquired for B 1 mapping have similar parameters as the DCE images, except for the following changes: acquisition matrix 64 × 64 (Fourier reconstruct to 256 × 256), BW = 187 Hz/pixel, flip angles 10°and 60°; and TR = 15000ms was used in the phantom study, while TR = 5000ms was used for in vivo imaging (Li et al 2018). A flip angle ratio of 6 (i.e.…”

Quantitative T1 MRI

“…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).…”