Rolf F Schulte^{1}, Haonan Wang^{2}, Anja CS Brau^{3}, and Martin A Janich^{1}

Two
different transmit B_{1}^{+} mapping techniques were implemented and investigated for
cardiac B_{1}^{+} shimming at 3T: (1) 2D cardiac-triggered spiral-Bloch-Siegert B_{1}^{+} mapping;
(2) 3D Zero-Echo-Time (ZTE) B_{1}^{+} phase mapping. B_{1}^{+} homogeneity was optimised and performance assessed
by evaluating the cardiac black-blood fast spin-echo sequence performance in
healthy volunteers.

The spiral-Bloch-Siegert method is based on rapid spiral
imaging with transmit magnitude |B_{1}^{+}|-mapping using the phase-sensitive
Bloch-Siegert method [1]. Four spiral arms are used to encode one
image (matrix size=100^{2}, FOV=(40cm)^{2}, spiral arm t=8.9ms;
Bloch-Siegert pulse t=4ms, f_{off}=2kHz), requiring two repetitions for
plus-minus the Bloch-Siegert frequency, and another two for the two transmit
channels I and Q (4×2×2=16 excitations in total). One excitation and encoding
takes 52.4ms (due to SAR constraints), hence the whole scan can be acquired
with cardiac gating in a single breath-hold and with multiple slices. The
transmit RF phase map ($$$\angle$$$B_{1}^{+}) is extracted from the coil-combined phase difference between
the two transmit channels.

The Zero-Echo-Time (ZTE) B_{1}^{+} phase mapping method is based on measuring a
low-resolution 3D-radial ZTE image and extracting the phase difference map ($$$\angle$$$B_{1}^{+}) between
the two transmit channels scan (matrix size=100^{3}, FOV=(40cm)^{3},
scan duration=27s). The validity of using only the transmit RF phase map ($$$\angle$$$B_{1}^{+}) for
B1 shimming assumes sufficiently flat |B_{1}^{+}| magnitudes of the two channels over
the region-of-interest. Thus, B_{1}^{+} inhomogeneities are mainly caused by spatially varying transmit RF phase.

The goal of the subsequent B_{1}^{+} optimisation is to combine the
two transmit channels optimising B_{1}^{+} homogeneity over the specified region of
interest. The cost function is given by $$$\sum(f-\bar f)^2$$$ with $$$f=|B_I+wB_Q|$$$, $$$B_I$$$ and $$$B_Q$$$ denoting
the two transmit channels I and Q combined with the complex weight $$$w=10^{-txAtten/200} \cdot \exp(i\cdot txPhase)$$$.
txAtten is constrained by the system configuration to
0...45[dB/10], while txPhase to -45°...0°.

Both sequences were implemented on a 3T GE MR750w scanner including
transmit-channel selection for an easy workflow. Seven healthy volunteers were
examined (2 with spiral-Bloch-Siegert, 6 with ZTE B_{1}^{+ }phase-mapping; body
weights 60-95kg). All reconstruction, post-processing and B_{1}^{+} optimisation was
implemented in Matlab.

The |B_{1}^{+}| magnitude images measured with the spiral-Bloch-Siegert
method (Fig. 1) are relatively flat within the region-of-interest, hence
supporting the assumption that $$$\angle$$$B_{1}^{+} phase maps are sufficient for cardiac B_{1}^{+} optimisation. A typical
ZTE $$$\angle$$$B_{1}^{+} phase map is shown in Fig. 2. While spiral-Bloch-Siegert yields
full B_{1}^{+} magnitude and phase information, it is offset by a cumbersome
measurement procedure, requiring cardiac gating, breath-hold, good shim, slice
planning and yields only (multi-slice) 2D maps with limited robustness. ZTE is
much simpler and more robust due to short TE and 3D radial properties of high
oversampling and averaging in the centre of k-space, although ZTE is lacking
|B_{1}^{+}|-information.

The B_{1}^{+} optimisation generally yields the same results for both B_{1}^{+}
mapping methods: maximum attenuating of one channel (txAtten=45[db/10]), while using
the same phase as for quadrature mode (txPhase=0°). Note, that phase-based B1+ shimming relies on the assumption of flat |B_{1}^{+}| I and Q magnitudes, which means a single channel has already optimal |B_{1}^{+}| homogeneity. The theoretically optimal solution would therefore be to completely switch off one channel, hence eliminating the perturbing effects of $$$\angle$$$B_{1}^{+}. As this would require doubling the power of the remaining channel, the system limits attenuation to a maximum of 45[dB/10].

Fast-spin-echo images measured with standard quadrature mode and the optimised dual-drive values are shown in Fig. 3.

The B_{1}^{+} shimming results in the heart are reproducible and encouraging
in the measured subjects, and further scans to confirm these initial findings
are ongoing. The ZTE phase method may be applicable to other body regions as
well, if the assumption of flat |B_{1}^{+}| with the region-of-interest holds.

- B1 mapping by Bloch-Siegert shift. Sacolick LI, Wiesinger F, Hancu I, Vogel MW. Magn Reson Med. 2010 May;63(5):1315-22. doi: 10.1002/mrm.22357.