3D volumetric parallel excitation at 9.4T using the trajectory container concept

Tingting Shao^{1}, Nikolai I. Avdievich^{1}, and Anke Henning^{1,2}

*B1 map and basic imaging settings* The B_{1}+ field maps were acquired with a home-built
8-channel TxRx array head coil [3] and a spherical spectroscopy phantom filled
with an aqueous solution of acetate and lactate on a 9.4*T* whole body human MRI scanner
(SIEMENS Healthcare, Erlangen, Germany), using the 3D actual flip angle imaging
(AFI) method [4]. The B_{1}+ field map acquired in CP mode is shown in Figure 1. The desired excitation patterns are a uniform cube (4*cm**4*cm**4*cm*),
a uniform slab (thickness = 4*cm*) and a specified “MPI” pattern (thickness = 4*cm*),
all blurred by convolution with a Gaussian kernel of FWHM = 1.2*cm*. The 3D parallel excitation
experiments are based on a fast 3D gradient echo sequence (GRE) (TE 10*ms* / TR
100*ms* / resolution 1.72*mm**1.72*mm**5*mm*).

*K-space trajectory
determination* Assuming
that all the channels fully contribute to the RF modulation during the entire
duration of the k-space trajectory, the Point-Spread-Function (PSF) induced
by RF-modulation of all channels can be calculated by summing up the absolute
values of the inverse FFT of the B_{1}+ maps from all
individual channels. The de-convolution of the RF distribution of the
spatial target profile (its inverse FFT) with the PSF leads to a weighting map
that demonstrates the distribution of the required deposition of RF energy throughout
the excitation k-space. By finding a certain weighting threshold based on the
histogram of the weighting map, the boundary of the high weighting area can be
used to define the “k-space trajectory container”. As seen in Fig. 2 that corresponds
to the excitation patterns for the cube (c-d) and the slab (g-h) the resulting
k-space trajectories are restricted to traverse through the pre-defined k-space
container.

To maintain the continuity of the k-space sampling and to meet the Nyquist criterion, the k-space trajectory is designed to connect separated parts of the trajectory container with the constraint of being k-space central-point-symmetrically shaped. To efficiently cover the interior space of the “trajectory container”, an acceleration factor of 4 is chosen for the transverse direction and 2 is chosen for the longitudinal direction, according to the dimension of the estimated PSF. In this work we used a stack-spiral trajectory and tailored its shape according to these criteria, as can be seen in Fig 3. Stack-spiral trajectory is chosen because of its traversing efficiency and the accordance of its gradient amplitude and the weighting map distribution.

*RF design method* The parallel RF pulses are designed based on the iterative small-tip-angle
(STA) approach [5] after trajectory optimization. A constraint on maximum RF
amplitude is applied during the RF pulse calculation. The pulse duration was
limited to a maximum of 16*ms* considering the ultra-short T2 relaxation time at
9.4*T*.

Fig
1 (a) transversal view and (b) coronal view of the AFI B_{1}+ map (*µT*/*V*) of a single-row
8-channel array while driven in the CP mode.

Fig
2 (a) longitudinal (*k*_{x}-*k*_{z}) view and (b) transversal (*k*_{x}-*k*_{y}) view of the RF-energy
weighting map for the cube case. (c-d) longitudinal and transversal views of
k-space container for the cube case. (e-f) longitudinal and transversal views
of the RF-energy weighting map for the slab case. (g-h) longitudinal and
transversal views of k-space container for the slab case.

Fig
3 The designed k-space trajectory of the cube case (a) and the slab case (b).

Fig 4 (a, c, e) transversal views
and (b, d, f) coronal views of the measured excitation profiles of case cube,
slab and “MPI” pattern, acquired with GRE sequence including the corresponding pTX
excitation pulses.

Table I. Evaluation of the excitation cases.

Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)

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