Figure 1.
Diagram of the MP2RAGE and Sa2RAGE sequences.
Inversion (Delay) times TI1 and TI2 (TD1 and TD2) are defined as the time from the middle of the inversion (saturation) pulse to the excitation corresponding to the center k-space line in the phase encoding in the slab selection direction. MP2RAGETR is the time between two successive inversion pulses and TR is the time between successive excitation pulses in the GRE kernel which is composed of n excitations. Features specifically associated with the MP2RAGE and Sa2RAGE are shown in black and light grey respectively (the adiabatic inversion and the saturation pulse). The gradient echo excitation pulses used for both the SA2RAGE and MP2RAGE were non-frequency selective.
Figure 2.
Plots of the theoretical relationship between T1 values and MP2RAGE intensity.
(a) MP2RAGE as a function of T1 for the parameters that optimize contrast over all values of T1 present in the brain (dark grey line) and the parameters that optimize contrast for T1 values between those of WM and GM (light grey). The dashed lines represent the variation in intensities when the effective B1+ is ±40% of the nominal value while dark dotted lines show the T1 values for which Protocol A was optimised and light dotted lines show the T1 values for which Protocol B was optimised. (b) Plot of the contrast to noise ratio between successive T1 values (δT1 = 0.05 s) for the different protocols as a function of T1. The dashed line shows the corrected CNR when the effect of the Partial Fourier in protocol B is taken into account.
Figure 3.
Lookup tables used to compute: (a) the R1 (1/T1) maps for the MP2RAGE sequence with MP2RAGETR/TI1/TI2 = 6/0.8/2.7 s, α1/α2 = 7/5 (protocol ii-A), (b) the B1 maps for the Sa2RAGE sequence with Sa2RAGETR/TD1/TD2 = 2.4/0.058/1.8 s, α1/α2 = 4/11.
Grey dashed lines define the typical range of B1 and T1 observed in the human brain at 7T (±40% of the nominal B1 value).
Figure 4.
Panels with representative data from the two different subjects (Subject 1 - top three rows, Subject 2 -bottom 4 rows).
The different columns show:(1st column) conventional MP2RAGE; (2nd column) MP2RAGE optimized for WM GM contrast;(3rd column) MP2RAGE MIP images and (4th column) full range MP2RAGEWMGMimages. Yellow arrows point regions where the increased WM T1 sensitivity of the new protocol allows the visualization of brain stem and pons sub-structures, while white arrows show the delineation the medio-dorsal, ventral lateral and pulvinar nuclei of the thalamus.
Table 1.
Table showing the mean contrast to noise ratio between different brain regions with T1 values in between that of cerebral white matter and cortical grey matter.
Figure 5.
Plots of the look up tables (a,c) of the MP2RAGE intensity as a function of the T1 values and the associated error (b,d) on the T1 calculation as a function of T1 when in the presence of B1 values that are ±40% different from the nominal value (light and dark gray respectively).
Protocols A and B are shown in the top (a,b) and bottom panel (c,d) respectively.
Figure 6.
Transverse and coronal slices of: (a) Uncorrected R1 maps using protocol ii-B; (b,c) Uncorrected R1 maps using using protocol ii-A with two different head and dielectric pad positions; (e) Corrected R1 maps using protocol ii-B; (f,g) Corrected R1 maps using using protocol ii-A with two different head and dielectric pad positions; (e,h) B1+ maps corresponding to the two different head and dielectric pad positions; R1 maps are shown in a short range from 0.75 to 0.95 s−1 to emphasize the sensitivity of the R1 maps to the B1 in-homogeneity, and the success of its correction.
Arrows point out regions of increased difference between in the B1+ maps that have clear implications on the uncorrected high resolution R1 maps. Blue arrows point out white matter fibber bundles with increased R1 values with respect to the remaining white matter. Red arrows point out regions of significant R1 differences from the ground truth prior to the B1+ correction, which are successfully corrected. The white arrow points out a region of very low B1+ field where the adiabatic condition was not reached and hence the R1 values are largely overestimated and not possible to correct with the proposed methodology.
Figure 7.
Transverse, sagittal and coronal slices of: (a) a corrected R1 map using protocol B and (b,c) uncorrected and corrected R1 maps calculated using protocol A.
The R1 maps are shown in a short range from 0.45 to 0.80 s−1 in order to emphasize the sensitivity of the cortical R1 maps to the B1 in-homogeneity. Panel (d) shows the reconstructed corrected R1 surface across the middle layer of the cortex in the right hemisphere as calculated by freesurfer. Blue arrows point out primary sensory cortices (visual, auditory and sensory-motor cortex) which can be associated with increased R1 values. Red arrows point out regions of significant R1 differences from the ground truth prior to the B1+ correction, which are successfully corrected: on the coronal slice a strong left right asymmetry between similar cortical regions, while on the sagittal slice a strong anterior posterior variation is highlighted.
Figure 8.
Transverse (a), sagittal (b) and coronal (c) slices covering the hippocampus of a high resolution corrected R1 map.
Arrows show hippocampal structures: CA1, CA2 3 (blue arrow); fimbria of hippocampus (yellow arrow); CA4 and DG (black arrow); subiculum (green arrow) [32]. Such structures are only discernible in the high resolution dataset.