T1 VFA

Class: NodeT1VFA

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Calculates a T1 map using the variable flip angle (VFA) method. At least two images are required, however, more images are advised if a larger range of T1 values are expected. Flip angle (FA) and repetition time (TR) are detected automatically and it is required that the TR is the same in all images.

In addition to the images with varying flip angle, a B1 map can also be provided to improve the accuracy, in particular at high field strengths. The B1 map represents a scaling of FA, hence value 1 indicates that the nominal FA is achieved. An optional mask can also be provided to limit the calculations to a region.

Produces a T1 map in the same units as TR, and a \(S_0\) map which is a proton and \(T2^*\)-weighted image. The parameter values are obtained by fitting the data to a linearized version of the spoiled gradient echo signal equation [1]:

\[ \begin{equation} S_i = \frac{S_0(1-e^{-TR/T1})\sin(FA_i \cdot B1)}{1-\cos(FA_i \cdot B1)e^{-TR/T1}},~~~~i = 1, 2, ...,n \label{eq:sample} \end{equation} \]

where \(n\) is the number of images used.

Example Workflows

VFA example

Inputs

Dataset

A set spoiled gradient echo images with equal TR and varying flip angles (at least 2 diffrent angles are needed). Note that TE should be the same for all images too.

MetaData: The fields RepetitionTime and FlipAngle must be present in the metadata for all images. The flip angle must be given in degrees.

Type: Image4DFloat, Required, Multiple (Minimum = 2)

Mask

An (optional mask) that specifies which pixels to analyse.

Type: Image4DBool, Optional, Single

B1 Correction

An (optional) B1 map that corrects for spatial variations in the flip angles. The values in the map are relative to the nominal flip angle. For example if the actual produced flip angle in a voxel is 8° while the nominal flip angle is 10°, the B1 value should be 0.8 in this voxel.

Type: Image4DFloat, Optional, Single

Outputs

T1

A T1 map calculated for the voxels specified by the mask or in all voxels if no mask is provided. The T1 map has the same unit as the TR, typically given in ms.

Type: Image4DFloat

S0

A proton and \(T_2^*\) weighted image.

Type: Image4DFloat

References

[1] H. L. M. Cheng and G. A. Wright, “Rapid high-resolution T1 mapping by variable flip angles: Accurate and precise measurements in the presence of radiofrequency field inhomogeneity,” Magn. Reson. Med., vol. 55, no. 3, pp. 566–574, 2006.

Keywords: Variable flip angle mehtod, VFA, T1 mapping.