To develop and optimize a B1-corrected variable flip angle T₁ mapping acquisition strategy at 3T, producing volumetric sodium T₁ maps of the brain.

6 healthy male volunteers, mean age 30 years ± 5 years were scanned using a GE MR750 3T system (Waukesha, WI) with a birdcage 23Na/1H head coil (Rapid Biomedical, Rimpar, Germany).

The variable flip angle (VFA) acquisition was performed with a spoiled gradient echo acquisition utilising a 3D cones readout [1]. FOV = 24cm, TE = 0.5ms, TR = 110ms, flip angles chosen from simulation of the sodium signal = 55, 75, 95, 105, 115 degrees, readout duration = 30ms, averages = 6, readouts per average = 197, nominal voxel size = 4.5x4.5x4.5 mm3, scan time = 2 minutes.

The sodium B1 acquisition scheme was performed with the same scan parameters as the VFA sequence, however TR = 220ms, data acquired with flip angles = 50 and 100 degrees.

The IR-gradient echo imaging was performed with an adiabatic inversion pulse, and a 3D cones readout. FOV = 24cm, TE = 0.5ms, TR= 180ms, inversion times chosen from simulation of the sodium signal = 19, 25, 30, 35, 40ms, readout duration = 30ms, averages = 6, readouts/average= 197, nominal voxel size = 4.5x4.5x4.5mm3, Scan time = 4 minutes.

A power noise correction was performed on all of the sodium images ². B1 maps were constructed from the B1 dual flip angle data ³. The raw sodium images were registered with a T₁ weighted proton scan (TE = 3.18 ms, TR = 8.16 ms, 1.5 mm isotropic resolution (reconstructed to 1mm isotropic) in SPM8, and the proton scan used to produce white matter, grey matter, and CSF masks, with masks gated on p > 0.95.

VFA T1 maps were constructed with linear fitting, iterative non-linear fitting, and eMOS, with and without B1 correction ^4,5. The IR maps were constructed with non-linear fitting. Processing was performed using Matlab.

Average sodium T₁ values for grey matter, white matter, and CSF obtained from the segmentation masks and T₁ maps, over the whole brain. Relative error maps between the VFA T₁ maps and the IR T₁ maps were produced according to equation 1, and the average error for each tissue type with and without B1 mapping was calculated, as seen in figure 1.

It is noted that, in comparison to Chavez, no site specific correction, normally obtained from the difference between an IR T₁ map and eMOS T₁ map, was applied to the eMOS T₁ maps produced in this study.

Equation 1

$$Error = 100\times(\frac{VFA_{T1Value}-IR_{T1Value}}{IR_{T1Value}}) $$

A two-tailed t-test was used to compare the difference in mean whole brain tissue T₁ values, data shown in figure 2, between data sets corrected and uncorrected for B1 non-uniformity. The results show that there is no significant difference between T₁ maps corrected or uncorrected for B1 at sodium frequency at 3T, with a birdcage head coil (p > 0.05).

A two-tailed t-test was used to compare the results from each individual VFA method with both Inversion recovery T₁ values at 3T, shown in figure 2, and average relative error for grey matter and white matter T₁ maps, shown in figure 1. The results of this test are shown in figure 4, showing that the eMOS method outperforms a linear fitting method for estimating T₁ values in both grey matter and white matter (p < 0.05 for both). However, it can be seen that the non-linear fitting performs as well as eMOS for estimating white matter T₁ (p > 0.05) but that eMOS performs better than non-linear fitting for grey matter T₁ estimation (p < 0.05). Finally, it has been shown that there is no significant difference between linear or non-linear fitting for estimating T₁ (p > 0.05 in both cases). Figure 3 shows a masked IR T₁ map for each tissue type.

In conclusion, a new application of eMOS has been demonstrated in estimating accurate sodium T₁ maps, using only a pair of two-minute sodium scans, with eMOS outperforming other VFA methods in producing accurate T₁ maps. The study has also shown that the spatial variance of B1 is negligible in a birdcage head coil. T₁ mapping is useful to aid in optimisation of sequence TR and flip angle, as well as assessing tissue T₁ changes associated with change in microstructure. Further to this study a comparison of eMOS against progressive saturation techniques, assessing the method at higher spatial resolutions, and finally studying sodium T₁ in cases of pathology, will be undertaken.