16-channel pTx body MRI for reduced field of view lumbar spine and kidney imaging at 7 Tesla
Sascha Brunheim1,2, Stephan Orzada1, Sören Johst1, Marcel Gratz1,2, Jessica Kohl3, Mark E Ladd1,4, and Harald H Quick1,2

1Erwin L. Hahn Institute for MRI, University of Duisburg-Essen, Essen, Germany, 2High Field and Hybrid MR Imaging, University Hospital Essen, Essen, Germany, 3University of Applied Science Ruhr West, Mülheim an der Ruhr, Germany, 4Division of Medical Physics in Radiology, German Cancer Research Center (DKFZ), Heidelberg, Germany


In this work we present in-vivo pTx excitation results in the low flip angle regime with a 16-channel transceiver body array at 7 Tesla. The pTx pulse calculation was based on a jointly fast acquired B0 and single-channel B1+ dataset (B01TIAMO) of the central abdomen. The pTx pulse enabled us to acquire high-resolution reduced field of view images of the distal spinal cord and the unilateral left kidney. The results of the established workflow for abdominal pTx provide promising perspectives, especially for neuroradiological spine imaging.


Parallel transmit (pTx) body imaging[1,2] offers the possibility for higher resolution reduced field of view imaging (rFOV), without fold-in artifacts and incorporating the mitigation of the inherent RF transmit field (B1+) inhomogeneities at ultrahigh field (UHF) MRI. To make use of this technique, not only suitable hardware but also an exact knowledge of the in-vivo main magnetic (B0) and B1+ field distribution is necessary. Accordingly, B01TIAMO[3,4] has proven to provide accurate data of the abdomen even for many transmit channels during a single breath-hold. Here we present results for spinal cord and kidney rFOV pTx in the low flip angle regime with a 16-channel transceiver array.


Data were acquired on a 7T UHF MRI system (Magnetom 7T, Siemens Healthcare, Erlangen, Germany) equipped with a custom-built 16-channel Tx/Rx body array[5] (Figure 1). The amplitudes and phases of the individual transmit channels were controlled by an add-on system including online SAR supervision[6,7]. The 10s-average maximum forward input power per channel was below 10 W during all in-vivo pTx measurements.

One B0 offset map and 16 single-channel B1+ maps were jointly acquired with B01TIAMO in 12 seconds for a healthy male volunteer (1.89m; 80kg) at the axial position of the intervertebral disc between L2 and L3 (Figure 2). In a first step, error-prone low-SNR regions in the image center and air-filled compartments were masked to prevent false scaling. Afterwards, a linear least-square solver was applied for extrapolation of the respective voxels within the sparse matrix.

The pTx pulse calculation was solved for the RF excitation b = argmin{|| |Ab| - m ||2}, with system matrix A and target transverse magnetization m with a smooth and slowly varying spatial phase profile[8]. A 13-turn 2D spiral-in gradient trajectory with variable density was applied in combination with the resulting non-selective RF pulse of 5.6 ms duration for excitation of the spinal column and left kidney volume (Figure 3). An additional 2 ms non-selective fat-saturation RF pulse preceded the pTx excitation. The readout/phase-encoding direction of the 3D gradient echo (GRE) sequence was aligned to the longitudinal body axis to allow time-optimized acquisitions.

The full FOV 3D GRE pTx parameters for the spine/left kidney were TA 119 s/40 s, TE 8 ms, TR 25 ms, 256x160x104 matrix/128x80x52 matrix, 2 mm3/4 mm3 resolution, iPAT 2x2. For the zoomed-in rFOV pTx imaging of the spine/left kidney the parameters were TA 453 s/36 s, TE 8 ms, TR 25 ms, 770x192x160 matrix/320x60x52 matrix, 0.65 mm3/1.56 mm3 resolution, iPAT 2/2x2. For comparison, a standard 3D GRE with TIAMO[9] was conducted within a breath-hold: TA 48 s, TE 3 ms, TR 6 ms, 256x168x120 matrix, 2 mm3 resolution, partial Fourier 6/8(phase) 7/8(slice), iPAT 2x2.


In Figure 3 the results of the Bloch simulation flip angles α show a normalized root-mean-square error (nRMSE) of 32.69% for the spinal cord and 10.26% for the left kidney region in comparison to the respective pTx target excitation patterns. Especially with strict α scaling, there were some residual signal areas visible in the outer body region and sporadic miscalculated voxels in the area of the ventral intestines, which result in local phase φ irregularities. Nevertheless, it was possible to achieve sufficient magnitude background suppression for the non-target sectors with an overall smooth phase profile for both selective excitations.

Image planes of the lumbar spine for the different techniques are presented in Figure 4. Despite the reported low nRMSE score, the in-vivo pTx results provide evidence of a distinct excitation profile along the longitudinal axis. This enabled high-resolution rFOV imaging of the distal spinal canal with clear depiction of the conus medullaris, cauda equina, and filum terminale that was not compromised by motion artifacts.

Figure 5 presents the in-vivo pTx results for the left kidney region. The excitation profile includes abdominal regions affected by intestinal activity and the need for breath-holding limited the acquisition parameters. Therefore, the rFOV pTx images contain fold-in artifacts but the discrimination of the renal cortex, medulla, and pelvis was possible with a 2.6-times higher resolution as for full FOV pTx.

Discussion and Conclusion

16-channel high-resolution rFOV pTx imaging of the abdomen with satisfactory background suppression has been demonstrated for defined regions. The quality of the single axial slice B0/B1+ data was adequate for pTx pulse calculation, in contrast to our previous assumption4. The spinal column results are promising, and the signal-to-noise ratio could be improved with an additional receive-only coil array. The application of rFOV pTx for single-organ excitation, as successfully presented here for the kidney, shows the potential but also demonstrates the limits of in-vivo application.


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.


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Figure 1. A: The CAD schematic depicts the distribution of the 16 25-cm-long micro strip line elements with meanders for abdominal imaging. B: The subject was positioned within this 16ch Tx/Rx body array with the arms above the head for the B0/B1+ mapping and pTx experiments.

Figure 2. The axial slice B01TIAMO results at intervertebral disc L2/3 formed the basis for further pTx experiments. A: The B0 offset map was calculated from a 2-echo acquisition with TIAMO readout with central low-SNR regions and air-filled compartments masked out (left) and extrapolated B0 offset (right). B: Normalized single-channel B1+ magnitudes before (top) and after extrapolation (bottom). C: Single-channel B1+ phase distribution derived by geometric-decomposition coil compression (GCC) before (top) and after extrapolation (bottom).

Figure 3. A: From left to right: Expected flip angle α for a square root of Hann-filtered square target excitation of the spinal cord. Bloch simulated α based on B01TIAMO with a normalized root-mean-square error (nRMSE) for the target region, scaled to the absolute maximum and α = 3°. Corresponding target phase distribution φ and Bloch-simulated φ based on B01TIAMO. B: From left to right: Expected α distribution for a smaller square root of Hann-filtered square target excitation of the left kidney. Bloch simulated α, scaled to maximum and α = 2°. Target phase distribution φ and Bloch-simulated φ.

Figure 4. Comparison of a TIAMO 3D GRE to pTx and rFOV pTx of the spinal column at same window levels. A-C: Axial, sagittal, and coronal slice, respectively, from a 3D GRE with TIAMO during breath-hold (48 s, 2 mm3 resolution). D-F: Corresponding slices of a 3D GRE acquired in free breathing with selective excitation (119 s, 2 mm3 resolution). G-I: 3D GRE rFOV pTx (453 s, 0.65 mm3 resolution) with neuronal fiber bundle depiction (white arrows). J: Example multiplanar-reconstructed axial slices (1.2 mm3 voxel size) at different intervertebral disc levels in accordance with the Bloch simulation excitation pattern (arrow).

Figure 5. Comparison of a TIAMO 3D GRE sequence to pTx and rFOV pTx results of the left kidney at same window levels. A-C: Axial, sagittal, and coronal slice, respectively, from a 3D GRE with TIAMO during breath-hold (48 s, 2 mm3 resolution). D-F: Corresponding slices of a 3D GRE pTx during breath-hold (40 s, 4 mm3 resolution). G: Example 3D GRE rFOV pTx coronal slices with maximum intensity projection (1.56 mm3 voxel size) acquired within a 36 s breath-hold. The residual signal of the Bloch simulation excitation pattern (arrows) results in fold-in artifacts.

Proc. Intl. Soc. Mag. Reson. Med. 26 (2018)