Combining B1 mapping with TIAMO for fast and accurate multi-channel RF shimming in 7 Tesla body MRI
Sascha Brunheim1,2, Stephan Orzada1, Soeren Johst1, Marcel Gratz1,2, Maximilian N. Voelker1, Oliver Kraff1, Martina Floeser3, Andreas K. Bitz3, Mark E. Ladd1,3, and Harald H. Quick1,2

1Erwin L. Hahn Institute for Magentic Resonance Imaging, University Duisburg-Essen, Essen, Germany, 2High Field and Hybrid MR Imaging, University Hospital Essen, Essen, Germany, 3Medical Physics in Radiology, German Cancer Research Center (DKFZ), Heidelberg, Germany

Synopsis

With current methods the mitigation of transmit field inhomogeneity at ultrahigh field by multi-channel RF shimming with conventional methods is relatively time consuming. This applies in particular for parallel transmit/receive in-vivo body imaging within breath-hold and during organ motion. Therefore, we propose a new technique merging fast acquired relative single channel maps and the spatial-dependent flip-angle distribution of two complementary shims to define absolute transmit coil maps for fast and accurate RF shim calculation. The performance of this technique is validated against established methods in phantom measurements and its reliability is shown in comparison to simulation data serving as reference.

Purpose

Fast and accurate mapping of the transmit field (B1+) is an essential prerequisite for parallel excitation applications. Many established techniques like actual flip angle imaging (AFI)[1] and dual refocusing echo acquisition (DREAM)[2] work well for stationary body regions and for the abdomen at 3T[3]. However acquisition times for single channel maps are unsuitably long for multi-channel body imaging and their dynamic flip angle (FA) range is limited especially for lower values (< 20°). Furthermore, the B1+ distribution at ultrahigh field has severe signal voids which compromise mapping sensitivity. In this work we propose a method to measure single channel B1+ maps by using a fast measurement of relative maps acquired in an interleaved manner that are normalized through the use of two absolute FA maps acquired by pre-saturation TurboFLASH (TFL)[4] in a time interleaved acquisition of modes (TIAMO)[5]. This sequence as a whole is referred to as B1TIAMO.

Methods

The 2D-B1TIAMO sequence consists of two consecutive parts: A series of gradient-echo (GRE) images with a single channel transmitting at a time[6] (TR=1s). Followed by two magnetization prepared sequences with TFL readout (TR=3.5s) using two complementary shims to achieve sufficient signal in every voxel even for low FA values. The information on absolute FA distribution is used to calculate a voxel-wise scaling factor for the relative B1-maps, utilizing a local weighting of the two measurements that takes into account the signal amplitude of their respective images. The total measurement time for a single axial slice was TA=22s for an 8ch transceiver array (case 0). For further improvement to prevailing procedures, the relative single channel maps were acquired continuously in an interleaved manner with the minimum achievable echo time (TE=1.05ms; TA=15s). The transmit channel was changed clockwise per phase encoding step in a centric-down (case 1) or linear-ascending way (case 2). In contrast to the normal centric-up sampling (case 0), these data were low-pass filtered accordingly post-hoc. Data were acquired on a Siemens 7T system (Magnetom 7T, Siemens Healthcare, Germany) with a custom 8ch Tx/Rx head[7] and 8ch Tx/Rx flexible body array[8]. Figure 1 depicts the setup for both phantom measurements and the voxelwise receive channel assignment for maximum signal contribution. A spherical 3.1Liter head phantom (T1=850ms; T2=70ms) was used for evaluation of 2D-B1TIAMO performance for one transversal slice versus a 2D-DREAM sequence (non-selective rectangular pulse; TR=6.3ms; TE1=1.98ms; TR2=3.95ms; TA=7.5s) and the 3D-AFI method (slab-selective sinc pulse for eight slices; TR=100ms; TE=3ms; TA=56s; TAslice=7s). Moreover, the experimental results of a large 32Liter body phantom (T1=530ms; T2=300) were qualitatively compared for one axial plane between 2D-B1TIAMO, 3D-AFI and simulation data. Overall matrix size was 64x64 with a voxel volume of 5.5x5.5x10mm3.

Results

In Figure 2 a B1+ comparison of the directly measured pre-saturated TFL and the 2D-B1TIAMO technique against the two established methods for the 8ch transceiver head array is shown. Further is an excerpt of the different case sequence timing diagrams depicted. Each method should yield a 60° FA in the middle of the head phantom in CP+ mode. In general both the pre-saturated TFL and the 2D-B1TIAMO method (CP+ mode reconstructed from single channel maps) showed good agreement with the standard of reference. Furthermore, most of the errors present in the pre-saturated TFL maps due to low signal-to-noise areas inside a body phantom (Figure 3) could be resolved by the B1TIAMO technique when reconstructing the distributions from the single channel maps. Improvements by B1TIAMO could be seen especially within the large-volume body-phantom (Figure 4) in comparison to the AFI technique which fails due to insufficient signal within the phantom for absolute single channel FA maps. Figure 5 shows the effect of Gaussian filtering on the single channel maps. Artifacts could be greatly reduced and good agreement between measurement and simulation is achieved.

Discussion

The usefulness of the combined B1TIAMO technique has been shown in phantom experiments. Two prerequisites have been fulfilled: First, the acquisition of 8 relative single channel maps was shortened from 22s to 15s due to the interleaved coil cycling scheme. Second, information about the B1+ distribution could be obtained for every voxel even within the large-volume body phantom. This was achieved by the combination of two complementary absolute FA maps. Finally, for more than 8 transceiver channels in body imaging it is highly desirable to accelerate the acquisition by employing the proposed interleaved coil cycling scheme. The gain in acquisition speed achieved with 2D-B1TIAMO rises with the number of channels: For a 32ch Tx/Rx body coil system currently being developed, the total measurement time could be reduced from 45s to 18s.

Acknowledgements

The research leading to these results has received funding from the European Research Council under the European Union's Seventh Framework Programme (FP/2007-2013) / ERC Grant Agreement n. 291903 MRexcite.

References

[1] Yarnykh VL “Actual flip-angle imaging in the pulsed steady state: A method for rapid three-dimensional mapping of the transmitted radiofrequency field.” (2007) MRM 57(1): 192-200. [2] Nehrke K, Börnert P “DREAM - A Novel Approach for robust, ultra-fast, multislice B1 mapping.” (2012) MRM 68(5): 1517-1526. [3] Nehrke K, Springkart AM, Börnert P “An in vivo comparison of the DREAM sequence with current RF shim technology.” (2015) MAGMA 28(2): 185-194. [4] Fautz HP, Vogel M, Gross P et al. “B1 mapping of coil arrays for parallel transmission.” (2008) Proc. Intl. Soc. Mag. Reson. Med. 16: p. 1247. [5] Orzada S, Maderwald S, Poser BA et al. “RF excitation using time interleaved acquisition of modes (TIAMO) to address B1 inhomogeneity in high-field MRI” (2010) MRM 64(2): 327-333. [6] Van de Moortele PF, Snyder C, DelaBarre et al. “Calibration tools for RF shim at very high field with multiple element RF coils: From ultra fast local relative phase to absolute magnitude B1+ mapping.” (2007) Proc. Intl. Soc. Mag. Reson. Med. 15: p. 1676. [7] Orzada S, Kraff O, Schäfer LC “8-channel transmit/receive head coil for 7 T human imaging using intrinsically decoupled strip line elements with meanders” (2009) Proc. Intl. Soc. Mag. Reson. Med. 17: p. 3010. [8] Orzada S, Quick HH, Ladd ME et al. “A flexible 8-channel transmit/receive body coil for 7 T human imaging” (2009) Proc. Intl. Soc. Mag. Reson. Med. 17: p. 2999.

Figures

Fig.1 Photos and schematic layouts of the transceiver element positions for the 8ch Tx/Rx head (left) and the 8ch Tx/Rx body array (right). Receive channel assignment based on the maximum signal contribution is color coded.

Fig.2 Quantitative comparison for the spherical phantom with the 8ch Tx/Rx head array in CP+ mode. A: FA slice profile through the axial plane acquired with the pre-saturation TFL method. B: 2D-B1TIAMO profile (CP+ reconstructed from single channel maps). C: Corresponding slice of the 3D-slab with AFI. D: 2D-DREAM profile.

Fig.3 A&C: Absolute FA maps of two complementary modes (CP+/CP2+) of the body phantom (8ch Tx/Rx flexible body array) with pre-saturation TFL sequence. B&D: Corresponding absolute FA maps for the B1TIAMO technique. Arrows indicate errors of the pre-saturation TFL-only method because of low FA values; these could be resolved by B1TIAMO.

Fig.4 A: GRE relative single channel maps for a normal acquisition (case 0). B: B1TIAMO results for absolute single channel maps (case 0). C: Corresponding AFI absolute single channel maps with eight times longer imaging time and severe signal voids visible. D: Simulation data agrees well with the B1TIAMO results (B).

Fig.5 Inspection of an example relative single channel. A: Simulation. B: B1TIAMO (case 0). C&D: Comparison of normal acquisition (B) and the interleaved coil cycling schemes showed an inherent low-pass filtering of k-space for case 0. E&F: By Gaussian filtering, the case 1&2 images even match the simulation better than case 0.



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