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1096 Simultaneous B0- and B1-map acquisition in a breath-hold for localized shim, frequency and RF power determination
Journal of Cardiovascular Magnetic Resonance volume 10, Article number: A221 (2008)
Cardiac MR at 3 T is challenged by the increased inhomogeneity of the static magnetic field B0, especially when using balanced steady-state free precession sequences. One solution is to determine localized second order shim corrections and a localized on-resonance frequency F0 based on an acquired B0-map . Furthermore, both numerical simulations  and measurements  have shown that also the transmit radiofrequency (RF) field B1 in the heart is more inhomogeneous at 3 T as compared to 1.5 T. Conventional methods to measure B1-maps, such as the dual-TR method , are rather lengthy. Recently, the saturated double angle method (SDAM) to acquire a B1-map covering the heart within a single breath-hold was introduced . Applying SDAM, Sung et al reported a flip angle distribution from 34° to 63° across the entire left ventricle (LV) for a nominal flip angle of 60° . This suggests that not only the B1 field over the LV is inhomogeneous by ± 30%, but that the average flip angle (power setting) is about 20% lower than the requested 60°. However, inadequate power settings may lead to signal reduction, changes in contrast, and eventually to biased quantitative measures.
To combine the acquisition of the B0- and the B1-map into one single breath-hold for fast determination of localized shim values, F0, and RF power settings.
The SDAM B1-map acquisition method was adapted to additionally acquire a second image for each slice with a longer echo time to simultaneously generate a B0-map (Figure 1).
As the measured signal in a multi-slice acquisition depends on the excitation profile of the applied pulse shape as well as on the RF excitation angle, the integral of the signal along the excitation profile was simulated based on Bloch-equation simulations of two different pulse shapes (sinc-Gaussian SG100 (tBW = 2.14) and Optex1 (tBW = 9.56)) and for a range of RF excitation angles from 0°–180°. This was used to calculate a correction lookup table to determine the local B1-field.
The proposed method was tested in a phantom, in the head and the heart of a volunteer. For verification purposes a regular B0-map and a dual-TR B1-map were acquired in the phantom and the head scans. The dual-TR B1-map acquisition took 14 minutes and cannot be triggered for a cardiac scan.
The following parameters were used for the phantom and head acquisitions: TR/TE = 14.4/2.3 ms (20.4/4.6 ms) using the SG100 (Optex1) pulse shape, α = 60°, 6 slices, 10 mm slice thickness, 10 mm gap, FOV = (400 mm)2, matrix = 802, 9 spiral interleaves, 7.5 ms readout, TSR = 600 ms. For the cardiac acquisition along the short axis orientation, the SG100 pulse shape was applied with a reduced slice thickness (5 mm) and acquisition window (3 ms) leading to a breath-hold duration of 18 heart beats.
Profiles of the measured B1-maps of the phantom and the head are shown in Figure 2. Calculated shim terms from the combined acquisition in the phantom and the head corresponded well with terms calculated from the conventional B0-map (mean difference of linear (second order) terms was 0.0015 mT/m (0.0276 mT/m2).
The simultaneously acquired cardiac B0- and B1-map are shown in Figure 3.
The proposed method allows simultaneous acquisition of a B0- and a B1-map in the heart within one breath-hold only, enabling fast localized shim, F0 and RF power setting determination. While optimized RF power settings should improve the imaging performance, actual B1-shimming, using a multi-channel transmit system, will likely be superior. The proposed slice profile correction for the B1-map is necessary, and may explain why others [4, 5] reported a much lower average flip angle than the one requested.
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Schär, M., Vonken, E.P. & Stuber, M. 1096 Simultaneous B0- and B1-map acquisition in a breath-hold for localized shim, frequency and RF power determination. J Cardiovasc Magn Reson 10, A221 (2008). https://doi.org/10.1186/1532-429X-10-S1-A221
- Pulse Shape
- Excitation Profile
- Short Axis Orientation
- Localize Shim
- Spiral Interleaf