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Abstract:
Previously published T1 relaxation times for brain tissue
at 7T vary greatly in results and in methods. We present T1 and T2 mapping sequences applicable at 7T. For T1 mapping, a Look-Locker sequence with an adiabatic inversion prepulse and a modified fitting routine was implemented.
For T2 mapping the vendor pre-implemented mixed imaging sequence [1] was validated. For cross validation purpose T1 and T2 relaxation times were measured at selected anatomical locations with SV-MRS parameter series.
All measurements were acquired on a 7T MR system.
Nine healthy volunteers were measured and gave informed consent in line with local ethics regulations. Unsuppressed single voxel water spectra using very small voxel sizes with flip angle-optimized outer volume suppression
has been used. For T1 measurement an inversion recovery series and for T2 measurements an echo time series were recorded. A three parameter model was fitted to the areas of the water peak to determine T1, and a two parameter
model to determine T2. For the Look-Locker sequence a highly adiabatic inversion pulse was used and the residual inversion imperfections due to B1 inhomogeneity were corrected by a modified fitting model. From the mixed
sequence only the T2 maps were validated with MRS.
Relaxation values from the spectroscopy experiments are summarized in Table 1. T1 values for white matter (wm) lie approximately in the mean of the literature data [2-8]. T1 values for nucleus caudate and gray matter (gm) are
in a good agreement with published data. T2 values for wm, nucleus caudate and gm are in general lower than the few published data. Except T1 values in wm, which were higher (p < 0.05) in the Look-Locker scan, T1 and T2 values
in the caudate nucleus and gm as well as T2 values in wm from the imaging sequences (Table 2) were in an excellent agreement with the MRS data.
optimized adiabatic inversion and consideration of inversion imperfection during fitting and validated this with single voxel MRS. We compared the T2 map from the implemented mixed sequence with MRS echo time series and
found an excellent agreement between the methods. This enables relaxation parameter mapping at 7T for T1 and T2 without the use of technically challenging B1 correction methods.
