Fast T1 mapping determined using incomplete inversion recovery look-locker 3D balanced SSFP acquisition and a simple two-parameter model fit

Abstract

Purpose: To evaluate a fast T1 mapping technique using incomplete inversion recovery 3D balanced steady-state free precession acquisition along with a two-parameter model fit.

Materials and methods: Using Bloch simulations, we explored the two-parameter model fit for data acquired using such an acquisition scheme. The parameter space over which the fit holds good was determined through simulations. A linear correction was derived for the R1* (1/T1*) values so determined. Two phantoms and six volunteers were scanned using the described technique. Comparison scans using full recovery as well as gold standard inversion recovery spin echo were also performed.

Results: The two-parameter fit works exceedingly well over a large parameter space. T1 values in the phantoms showed an error of 4.9% and 39% before correction and 0.9% and 1.6% after correction. For the six volunteers, error in T1 value was 5.3% for white matter (WM) and 2.4% for gray matter (GM) after correction, while it was 11.2% and 18.2% before correction.

Conclusion: The work presented here allows for T1 map determination with higher resolution and shorter acquisition time than previously possible. The technique is especially well suited for GM/WM T1 mapping.

Figure 1

Longitudinal magnetization (prior to application of the inversion pulse) as a function of the number of times the inversion pulse is applied. Each inversion pulse is followed by b-SSFP acquisition with TR/TE = 4/2 ms, α as indicated in the legend and two different echo train lengths. Shorter etl (270) allows for some dead time between end of acquisition and the inversion pulse. TR_seq = 2000 ms is the time between inversion pulses. Initial M_z = 1 (prior to first inversion pulse) while T1/T2 = 1500/60 ms.

Figure 2

Shows the two parameter and three parameter fit for inversion recovery b-SSFP (in steady-state) with TR/TE = 4/2 ms, TR_seq = 2.5 ms, α = 45° and continuous sampling along the recovery curve. T1/T2 were assumed to be 800/60 ms for the simulation. Every 50^th sampled point is shown for clarity. M_z shown is in the steady-state.

Figure 3

(A) Relationship between M₀* (two parameter model) and A (three parameter model) and (B) between β (two parameter model) and R1* (three parameter model) for a selected scan parameter set. R1 values used for the simulation correspond to T1 = [400 600 800 1000] ms.

Figure 4

(A) Chi-square test and (C) Pearson correlation for parameter space when TR/TE = 3.6/1.8 ms, total etl = 495, TR_seq = 3 s (Rx1) while (B) and (C) are the corresponding tests for a different parameter space where TR/TE = 4/2 ms, total etl = 270, TR_seq = 1.5 s. (Note that χ² = 9.48 and r = 0.81 for a level of significance α = 0.05.) This shows that the two parameter model provides an excellent fit over most of the parameter space.

Figure 5

Shows the linear correspondence between the calculated R1* value for a two parameter fit and the true R1 value (T1 = 400, 600, 800 and 1000 ms) for three different flip angles. Other scan parameters are TR_seq = 2 s, b-SSFP acquisition: TR/TE = 4/2 ms.

Figure 6

T1 values before and after correction for two different T1 phantoms are compared with the T1 value obtained from a full magnetization recovery study. Note the substantial improvement in accuracy for the long T1 phantom after correction.

Figure 7

(A) Uncorrected and corrected (B) T1 maps of the brain compared to a map obtained with (C) full inversion recovery (TR_seq = 10 s) for gray and white matter. Inset (D) shows the corresponding M₀* map with B1 inhomogeneity.

Figure 8

T1 values for white and gray matter prior to and after correction across six volunteers compared to values obtained with full recovery. Bars denote the mean values obtained for the six volunteers while the error bars denote the standard deviation of the mean values for the six volunteers.

Figure 9

Rapid high resolution T1 imaging (0.9×0.9×1 mm) with short TR (TR_seq = 1.5 s) and SENSE along slice and phase encoding directions (SENSE factor = 2). Uncorrected (A) and corrected (B) images on the same gray scale depict the improvement in T1 contrast between gray and white matter after correction. 27 slices were obtained in 26 s for this acquisition. When compared with full recovery IR-BSSFP, error was 41% and 56% in WM and GM, respectively, while after correction it was 1.7% and 5.8% after correction.