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 .
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].