The purpose of this page is to provide some guidance for T1 mapping and a methodology to evaluate new techniques.
Almost everybody in MR does T1 mapping at one point or another. There is a huge body of literature about different T1 mapping methods. However, the published T1 values of specific tissues in healthy subjects vary well-beyond the expected inter-subject variability. We have two objectives: first, provide the tools to perform reliable T1 mapping and avoid common pitfalls; second, provide a gold standard method and a framework to evaluate new methods. This is a work in progress and your feedback will be greatly appreciated: jbarral AT mrsrl DOT stanford DOT edu.
You may want to do T1 mapping to:
Example of 14 gold standard T1 mapping scans of chicken muscle performed overnight. Temperature was monitored with a Luxtron® probe. |
We define the gold standard as the single slice 2D spin-echo inversion recovery (SE-IR) sequence.
A typical protocol is: FOV 10 cm, matrix size 512x128, slice
thickness 2 mm, TR 2550 ms, TE 10 ms, BW 32 kHz, Frequency direction Left/Right, 4 inversion times
TIs: 50, 400, 1100 and 2500 ms.
Pulse profile and zoom in the passband. A Silver-Hoult adiabatic pulse of length 8.64 ms was simulated,
with a prescribed slice thickness of 2 mm. T1 and T2 values of muscle at 1.5 T were used [1]. |
Don't make assumptions on the signal equation! For example, the flip angle depends on T1, since the inversion pulse is typically 8 ms long. You can use Bloch simulations if you need to be convinced. For a SE-IR sequence with different inversion times TIn, the correct model is S(TIn) = exp(i φ) (ra + rb exp(-TIn/T1)), with φ, ra, and rb real parameters. The model can be generalized into a+b exp(-TIn/T1), with a and b complex parameters.
Our fitting procedure uses a non-linear least squares method and a grid search, since we consider that it is enough to know T1 with 1 ms precision (you can use a finer grid if you like), and we know that T1 can be found within a given range, say 1 ms to 5000 ms. The main advantage of our procedure over a standard Levenberg-Marquardt algorithm is its speed.
The chopping procedure during Prescan can give an additional 180° phase on the whole image for certain TIs. If you have to fix the phase, you can do it manually once you have all the data.
Four images taken at TI = [50, 300, 1000, 2000] (magnitude is shown). |
Global T1 map (a montage is displayed if several slices were imaged). |
Check of the fit at a specific voxel. |
T1 histogram of the chosen ROI.
Results obtained with 10,000 simulations and the following parameters:
Noise Standard Deviation 0.03, Flip Angle 172, TR 2550, TI = [50,
400, 1100, 2500].