On-resonant balanced Steady-State Free Precession imaging at 9.4T

Damien Nguyen^{1,2}, Tom Hilbert^{3,4,5}, Philipp Ehses^{6,7}, Klaus Scheffler^{6,7}, Jean-Philippe Thiran^{4,5}, Oliver Bieri^{1,2}, and Tobias Kober^{3,4,5}

After written consent was obtained, a healthy volunteer was imaged on a 9.4T research system (Siemens, Erlangen, Germany) with a maximum gradient strength and slew rate of 70 mT/m and 200 T/m/s respectively, using a custom-built head coil [2] for RF transmission and reception (16 transmit/31 receive channels) in combination with a recently proposed highly undersampled phase-cycled 3D balanced Steady-State Free Precession (bSSFP) prototype trueCISS sequence [1] (1x1x1mm^{3}, TR 4.26 ms, TE 2.13 ms, flip-angle 15°, 16 phases $$$\phi$$$ = 0°, 45°, 90°, ..., 315°, 8-fold undersampling, TA 2:40). First, the phase-cycled images were reconstructed using a sparse iterative reconstruction. Subsequently, the bSSFP signal-model (cf. equation below with $$$M_0$$$ equilibrium magnetization, $$$\Lambda$$$ relaxation time ratio $$$T_1/T_2$$$, $$$\alpha$$$ flip-angle, $$$\Delta\phi$$$ local phase offset relating to field inhomogeneity) was fitted voxel-wise onto these images, effectively estimating three parameter maps: $$$M_0$$$, $$$\Lambda$$$, and $$$\Delta\phi$$$. The resulting parameter maps were used to synthesize an on-resonant bSSFP signal image by applying the forward signal model with $$$\phi - \Delta\phi$$$ = 0°.

$$M = M_0 \left\lvert\frac{2\sin\alpha \cos\left(\frac{\phi - \Delta\phi}{2}\right)}{1 + \cos\alpha + 2\cos\left(\phi - \Delta\phi\right) + \left(4\Lambda - \cos^2\left(\phi-\Delta\phi\right)\sin^2\left(\frac{\alpha}{2}\right)\right)}\right\rvert$$

For comparison, Fig. 1a shows two slices of a reconstructed on-resonant trueCISS image for $$$\alpha$$$ = 15° inside a human brain measured at 3T [1]. Fig. 1b shows two similarly located slices in an acquisition done at 9.4T with $$$\alpha$$$ = 15°. The ultra-high field images show improved contrast in the red nucleus (blue arrow), substantia nigra (red arrow) and globus pallidus (white arrows) compared to the 3T image. Due to severe SAR limitations when scanning at ultra-high field strengths, it is in practice impossible to measure with high flip angles (ie. $$$\alpha$$$ > 15°) and short TR. In the present case, however, since the flip angle is an independent parameter within the signal model and all other parameters are known from the fitting procedure, it is possible to synthetically generate an on-resonant bSSFP image of any flip angle. To illustrate this, a synthetic on-resonant bSSFP image for a flip angle of 70° was generated and is presented in Fig. 1c for the same slices as before. Lastly, Fig. 1d shows the same two slices reconstructed using the maximum bSSFP signal computed voxel-wise: $$$M_\max\lvert_{\theta = \theta_{opt}} \approx \frac{1}{2}M_0\Lambda^{-1/2}$$$ where $$$\theta_{opt} \approx \cos^{-1}\left(\frac{\Lambda-1}{\Lambda+1}\right) $$$ [3].

It should be noted that the trueCISS acquisition presented above only requires about the same scan time as a single fully-sampled bSSFP image and thus not only offers high quality images in a time-efficient manner, but also delivers genuine bSSFP contrast images that can be used as basis for further quantification methods. One issue with the current approach is the stability of the fitting procedure due to the low number of phase-cycles acquired. While increasing the number of phase-cycles greatly improves the robustness of the method, it is time-consuming but can be partially compensated by further increasing the undersampling of K-space.

1. T. Hilbert, D. Nguyen, T. Kober, J.-P.Thiran, G. Krueger and O. Bieri. TrueCISS: Genuine bSSFP Signal Reconstruction from Undersampled Multiple-Acquisition SSFP Using Model-Based Iterative Non-Linear Inversion. Proc. Intl. Soc. Mag. Reson. Med.. Toronto, Canada. 2015

2. G. Shajan, M. Kozlov, J Hoffmann, R. Turner, K. Scheffler and R. Pohmann, A 16-channel dual-row transmit array in combination with a 31-element receive array for human brain imaging at 9.4 T, Magn Reson Med 2006;56:1067–1074

3. R.W. Brown, Y.C.N. Cheng, E.M. Haacke, M.R. Thompson and R. Venkatesan. Fast Imaging in the Steady-State. In: Magnetic Resonance Imaging: Physical Principles and Sequence Design. Wiley; 1999. p. 451-512

**Fig. 1 a)** Two slices from an on-resonant trueCISS image from a healthy volunteer at 3T for $$$\alpha$$$ = 15° [1].

**b-d)**
Two similarly located slices from a reconstructed on-resonant trueCISS image of a brain
at 9.4T for $$$\alpha$$$ = 15° and $$$\alpha$$$ = 70° respectively, as well as a trueCISS maximum bSSFP signal
image. Arrows show interesting contrast in the red nucleus (blue), substantia nigra (red) as well as globus pallidus (white).

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

0747