|Transmit Concepts & Arrays|
A Comparison of Matching Strategies for RF
Transmission Arrays Based on Network Theory
David Otto Brunner1, Nicola De Zanche1, Klaas Paul Pruessmann1
1University and ETH Zurich, Zurich, Switzerland
RF transmission arrays are used in various forms in order to mitigate inhomogeneous RF excitation or accelerate spatially selective pulses. Since multiple ports must be fed by power amplifiers, the power loss is not only given by the reflection coefficient of each port but also by the coupling among ports. In this work we study the impact of different types of single-port matching setups on the coupling and on the power loss for different applications such as RF shimming and Transmit SENSE.
A Simplified 16-Channel Butler Matrix for Parallel
Excitation with the Birdcage Modes at 7T
Vijayanand Alagappan1, 2, Kawin Setsompop3, Juergen Nistler4, Andreas Potthast5, Franz Schmitt4, Elfar Adalsteinsson3, Lawrence L. Wald1
1Athinoula A. Martinos Center for Biomedical Imaging, Charlestown, Massachusetts, USA; 2Tufts University, Charlestown, Massachusetts, USA; 3MIT, Cambridge, Massachusetts, USA; 4Siemens Medical Solutions, Erlangen, Germany; 5Siemens Medical Solutions, Charlestown, USA
A Butler matrix drives the individual rungs in linear combinations to form the well know uniform birdcage mode and higher gradient modes potentially capturing a majority of the sensitivity and acceleration capabilities in a subset of the channels. In this work we develop and test a simplified 16-channel high-power Butler matrix at 300 MHz with a 16 channel transmit/receive stripline array coil.
An Extensible Transmit Array System Using Vector
Modulation and Measurement
Pascal Stang1, Adam Kerr1, John Pauly1, Greig Scott1
1Stanford University, Stanford, California , USA
Transmit arrays offer important enhancements including improved RF fidelity, selectivity, and pulse acceleration. However, significant challenges remain in the hardware implementation of Tx array systems, and their integration with existing scanners. We present a 4-channel modular expandable vector-modulated transmit array system that can be easily adapted to most scanners. Our system also incorporates coil current sensors and RF instrumentation to permit automated measurement and feed-forward compensation for coil coupling and other non-ideal RF effects. We successfully perform coil decoupling, static-vector B1-shimming, and a ‘spokes’ fully-parallel transmit sequence.
General Signal Vector Decoupling for Transmit Arrays
Greig C. Scott1, Pascal Stang1, Adam Kerr1, John Pauly1
1Stanford University, Stanford, USA
In transmit arrays, control of coil coupling is much more difficult to control. However, a linear basis of signal vectors always exists to uniquely excite each coil. Using an integrated vector modulator/vector analyzer system, we present a general signal vector calibration technique that works in transmit-only and transceive coils that lack the ability to PIN switch individual coils. Practical sensor issues are also addressed.
Intrinsically Decoupled Current CONtrolled Transmit
and Receive (2CONTAR) Coil Elements for Arbitrarily Arranged Transceive
Evgenia Kirilina1, Thomas Riemer2, Frank Seifert3
1Physikalisch Technische Bundesanstalt , Berlin, Germany; 2IZKF University if Leipzig, Leipzig, Germany; 3Physikalisch Technische Bundesanstalt, Berlin, Germany
Transmit coil arrays are usually driven by voltage sources with fixed source impedance. Due to mutual inductance this inherently leads to a coupling of the individual coil elements within the array resulting in RF field distortions during transmission and inappropriate sensitivity profiles during reception. In this contribution we present a novel parallel driven current controlled transmit/receive array. It combines the current source decoupling for transmission with a transmit/receive switch to apply the well established preamplifier decoupling for reception. Avoiding complicated feedback circuits a coil array is implemented which guarantees full control over the RF fields within the object.
A Close-Fitting 7 Tesla 8 Channel Transmit/Receive
Helmet Array with Dodecahedral Symmetry and B1 Variation Along Z
Graham C. Wiggins1, Azma Mareyam1, Kawin Setsompop2, Vijay Alagappan1, Andreas Potthast3, Lawrence L. Wald1, 4
1Massachusetts General Hospital, Charlestown, Massachusetts, USA; 2MIT, Cambridge, Massachusetts, USA; 3Siemens Medical Solutions,Inc, Charlestown, Massachusetts, USA; 4Harvard-MIT Division of Health Sciences and Technology, Cambridge, Massachusetts, USA
An 8 channel transmit/receive array has been constructed based on 8 of the 12 pentagonal faces of a dodecahedron. The pentagonal loop elements are capacitively decoupled at their common edges. The array forms a close-fitting helmet with high transmit efficiency and good receive sensitivity. The coil elements form spatially distinct B1 transmit profiles which also vary along Z, which should allow greater control of the excitation profile throughout the brain.
Hybrid TEM/Loop Coil Array for Parallel High Field
Jan Paska1, Christoph Leussler2
1Technical University of Hamburg-Harburg, Hamburg, Germany; 2Philips Research Europe Hamburg, Hamburg, Germany
We investigated the advantages of a new type of coil. We present a coil array consisting of hybrid TEM/loop coil elements. The single hybrid coil element is intrinsically decoupled, and therefore no additional decoupling circuitry is necessary. The hybrid coil array, as compared to TEM or loop coil arrays, has additional degrees of freedom to achieve a uniform RF-excitation. The coil array is also suitable for parallel imaging techniques due to the orthogonal fields. Since each hybrid coil element has an own RF-shield it is configurable and can be used for imaging of different body parts.
|Switchable Short Quadrature Body Coil with Two Axial
Zhiyong Zhai1, Gordon DeMeester1, Michael Morich1, Robert Gauss1, Paul Harvey2
1Philips Medical Systems, Cleveland, Ohio, USA; 2Philips Medical Systems, Netherlands
For compact high field MRI systems such as at 3T, it is desirable to have a short quadrature body coil (QBC). A short QBC has the advantages of less required peak power, less exposed RF power for a patient and less SAR than a longer QBC. Typically a QBC is a shielded birdcage coil. Shortening the QBC has some impact on the axial uniformity and, in a few applications, a more uniform transmit field can be beneficial. Here we describe a switchable QBC which can switch between a short and long uniformity mode as needed. As an example, the Finite Difference Time Domain (FDTD) method is used to calculate B1-field for a switchable 3T QBC.
A Novel 8-Channel Transceive Volume-Array for a 9.4T
Ewald Weber1, Bing Keong Li1, Feng Liu1, Yu Li1, Peter Ullmann2, Hector Sanchez Lopez1, Stuart Crozier1
1The University of Queensland, Brisbane, Australia; 2Bruker BioSpin MRI GmbH, Germany
This work focuses on the design of a novel 8-element transceive volume-array for high field animal MRI. A dedicated, shielded 8-element transceive volume-array for large rat MRI applications at 9.4T, has been developed and constructed. Preliminary phantom images acquired using this prototype show that homogenous B1 fields can be attained. In addition, the Transmit SENSE images obtained reveal that the design of this transceive volume array is well suited for accelerated spatially-selective excitation and that it worked well with GRAPPA.
Whole Body Imaging at 7T with a
16 Channel Body Coil and B1 Shimming
Thomas Vaughan1, Lance DelaBarre1, Carl Snyder1, Silvia Mangia1, Jinfeng Tian2, Matthew Waks2, Scott Shillak2, Labros Petropoulos2, Gregor Adriany1, Peter Andersen1, John Strupp1, Kamil Ugurbil1, 3
1University of Minnesota, Minneapolis, Minnesota, USA; 2MR Instruments, Inc., Minneapolis, Minnesota, USA; 3Max Planck Institute for Cybernetics, Tubingen, Germany
Of the thirty 7T whole body MRI systems installed or on order, none are equipped for whole body imaging with body coils. RF artifacts and SAR are two of the challenges to be met before body coil imaging is possible. In this work, a 16 channel TEM body coil system was developed and applied to whole body, 7T imaging. B1 shimming methods developed in-house were used to correct RF artifacts in anatomic ROIs. By this technology and method, whole body imaging with body coils appears feasible at 7T.