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Research group 8.11

Parallel transmission

The term "parallel transmission" (pTx) refers to the excitation of the nuclear spin system with multi-channel transmit coils excited in parallel.

The goal is to optimize the field distributions of the magnetic and electric RF fields B1 and E1 for the respective application by exciting the individual elements of a coil array with individually selected amplitudes and phases.

In addition to procedures for static RF homogenization (B1+ shimming), pTx procedures for the selective excitation of magnetization patterns with freely selectable spatial distribution are increasingly important (Fig.1, 2). These techniques are based on the simultaneous generation of individual RF waveforming on the individual coil elements, while the gradient system generates a 2 or 3-dimensional k-space trajectory. These procedures are of particular importance for high- and ultrahigh-field MRI and for the active E-field steering in the MR scanner to reduce RF heating of metallic implants (Fig.3).

 

Fig.1: 2D selective excitation at 7 tesla: Selection of the region to be imaged (left), imaging of the selected region with low resolution (center), 'zoomed' MR imaging with high resolution at reduced field-of-view (right)   

Fig.2: pTx-MR spectroscopy in the brain: MR spectrum at 3T after selective excitation of an irregularly shaped voxel (A)

Fig.3: Reduction of the tissue heating caused by an wire type implant mimicking a DBS electrode simulated relative |B1+| distribution (top), simulated tissue temperatures in °C (bottom).

Left: pTx excitation in the CP mode with significant heating and B1+ artifacts in the area of the wire.

Right: pTx excitation in a modified CP mode with suppressed RF absorption of the wire. The tissue heating due to the implant and the B1+ artifacts are completely suppressed, whereas the |B1+| distribution remains usable for MR imaging.

In the case of most MR systems which are available today, only a single channel body coil is provided for the excitation of the spin system. In contrast to this, PTB's 3-tesla and 7-tesla MR systems are pTx-capable, i.e. they are each equipped with an 8-channel RF transmission system. Decisive for the use of MR measurement procedures based on pTx is the availability of efficient multi-channel transmit/receive coil systems as well as the guarantee of patient safety in this considerably more complex MR measurement procedure. The work of RG 8.11 related with parallel transmission is, therefore, also closely related with the further development of standard IEC 60601-2-33.

Selected references

G. Weidemann, F. Seifert, W. Hoffmann, H. Pfeiffer, R. Seemann, B. Ittermann
Measurements of RF power reflected and radiated by multichannel transmit MR coils at 7T
Opens external link in new windowMagn Reson Mater Phy DOI 10.1007/s10334-016-0551-6.

F. Seifert, G. Weidemann, B. Ittermann
Correlation of psSAR and tissue specific temperature for 7T pTx head coils - a large scale simulation study
Proc. Intl. Soc. Mag. Reson. Med. 23, 380 (2015).

F. Seifert, G. Weidemann, B. Ittermann
Q matrix approach to control implant heating by transmit array coils
Proc. Intl. Soc. Mag. Reson. Med. 23, 3212 (2015).

A. Kuehne, P. Waxmann, W. Hoffmann, H. Pfeiffer, R. Seemann, F. Seifert, B. Ittermann
Parallel transmission experiments using an extensible RF pulse generator
Proc. Intl. Soc. Mag. Reson. Med. 21, 4404 (2013).

T. D. Lindel, A. Kuehne, P. Waxmann, F. Seifert, T. Niendorf, B. Ittermann
Spatially Selective Excitation (SSE) for Brain Imaging at 7 T
Proc. Intl. Soc. Mag. Reson. Med. 20, 1061 (2012).

P. Waxmann, T. D. Lindel, F. Schubert, B. Ittermann, R. Mekle
Combination of SPECIAL and 2D SSE Parallel Transmit Pulses for Volume Selection in MR Spectroscopy
Proc. Intl. Soc. Mag. Reson. Med. 20, 1729 (2012).

A. Kühne, S. Goluch, P. Waxmann, F. Seifert, B. Ittermann, E. Moser, E. Laistler
Power balance and loss mechanism analysis in RF transmit coil arrays
Opens external link in new windowMagn Reson Med 74, 1165-76 (2012).

F. Seifert, E. Kirilina, T. Riemer
Transmitter/receiver antenna for MR with improved decoupling between antenna elements
United States Patent US 7,916,920 B2.

E. Kirilina, A. Kühne, T. Lindel, W. Hoffmann, K. H. Rhein, T. Riemer & F. Seifert
Current CONtrolled Transmit And Receive Coil Elements (C2ONTAR) for Parallel Acquisition and Parallel Excitation Techniques at High-Field MRI
Opens external link in new windowAppl Magn Reson 41, 507-23 (2011).

F. Seifert, G. Wübbeler, S. Junge, B. Ittermann, H. Rinneberg
Patient safety concept for multichannel transmit coils
J Magn Reson Imaging 26, 1315-21 (2007)

P. Ullmann,S. Junge, M. Wick, F. Seifert, W. Ruhm, J. Hennig
Experimental analysis of parallel excitation using dedicated coil setups and simultaneous RF transmission on multiple channels
Magn Reson Med 54, 994-1001 (2005)

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