A novel method for simultaneous 3D B₁ and T₁ mapping: the method of slopes (MoS)

Abstract

A novel three-dimensional simultaneous B(1) and T(1) mapping method is introduced: the method of slopes (MoS). The linearity of the spoiled gradient recalled echo (SPGR) signal vs flip angle relation is exploited: B(1) mapping is achieved by a two-point extrapolation to signal null with a correction scheme while T(1) mapping uses the slopes of the SPGR signal vs flip angle curves near the origin and near the signal null. This new method improves upon the existing variable flip angle (VFA) T(1)-mapping method in that (i) consistency between B(1) and T(1) maps is ensured (ii) the sampling scheme is T(1)-independent (iii) the noise bias and singularity, associated with using a linear form for the SPGR signal equation, is eliminated by using the full equation. The method is shown to yield accurate and robust results via simulations. Initial estimates of B(1) and T(1) values are obtained from three data points via simple computations and straight line approximations. Initial estimates of B(1) values, for a range of values, are shown to be accurate due to the proposed B(1) correction scheme. The accuracy and robustness of T(1) values is achieved via a non-linear fitting algorithm which includes a fourth data point sampled at high SNR. The MoS was validated by comparing resulting B(1) and T(1) maps with those obtained using other standard methods. Finally, the ability to obtain brain B(1) and T(1) maps using the MoS was demonstrated by in vivo experiments. The MoS is expected to perform well on other motion-free anatomical regions as well.

On the accuracy of T1 mapping: Searching for common ground

“…These deviations cannot be explained by differences in the imaging parameters, because they were kept constant, and the same B 1 map was used for the LL and the VFA protocol. The low resolution of the scans could result in biased WM T 1 values due to partial volume effects . Yet, investigations into multicomponent T 1 behavior indicate opposite trends of what we observed in our experiments.…”

Intensity Nonuniformity Correction

“…Unlike coil sensitivity, excitation field strength B 1 þ is easily measured using stock imaging sequences (Chavez & Stanisz, 2012;Yarnykh, 2007) and can be done so rapidly because the needed spatial resolution for the field map is low. Signal ratios have been used extensively for quantitative MRI techniques including diffusion tensor imaging, magnetization transfer ratio, and quantitative T1 and T2 methods.…”

Mapping water exchange rates in rat tumor xenografts using the late-stage uptake following bolus injections of contrast agent

“…All MRI scans were performed on a 7 T Bruker Avance (Bruker Biospin, Germany) animal scanner with an 86 mm quadrature coil for excitation and 2 cm surface coil placed around the tumor for receive. To obtain T 1 data, a 3D SPGR sequence was chosen because it is fast (four flip angles can be obtained in ∼1 min) and accurate T 1 measurements can be obtained with appropriate flip angles, data fitting procedures and B 1 correction schemes . At each steady‐state concentration, four SPGR scans were acquired (α = 16.6–20, 11, 7, and 5°; TR = 20 ms; TE = 2.5 ms; 128 × 96 × 8 matrices; 4 × 3 × 0.8 cm 3 field‐of‐view).…”

“…The DCE-MRI sequence used to monitor the contrast agent concentration and its approach to steady state was an SPGR sequence at a single flip angle (a ¼16. [6][7][8][9][10][11][12][13][14][15][16][17][18][19][20] ; TR ¼ 20 ms; TE ¼ 2.5 ms; 64 Â 96 Â 8 matrices; 4 Â 3 Â 0.8 cm 3 field-of-view; temporal resolution 10.2 s).…”

“…Instead, these regions were identified by using Eq. [17] to determine whether the fit was sensitive to the value of the exchange rate parameter. Voxels where the confidence interval for the exchange rate constant was broad (i.e., k IE values from < 3 to > 5 s À1 did not significantly change the x 2 value from its minimum) were excluded from the final maps as unreliable.…”