SAR reduction in 7T C-spine imaging using a “dark modes” transmit array strategy

Magnetic Resonance in Medicine (Impact Factor: 3.57). 04/2014; 73(4). DOI: 10.1002/mrm.25246


PurposeLocal specific absorption rate (SAR) limits many applications of parallel transmit (pTx) in ultra high-field imaging. In this Note, we introduce the use of an array element, which is intentionally inefficient at generating spin excitation (a “dark mode”) to attempt a partial cancellation of the electric field from those elements that do generate excitation. We show that adding dipole elements oriented orthogonal to their conventional orientation to a linear array of conventional loop elements can lower the local SAR hotspot in a C-spine array at 7 T.Methods
We model electromagnetic fields in a head/torso model to calculate SAR and excitation B1+ patterns generated by conventional loop arrays and loop arrays with added electric dipole elements. We utilize the dark modes that are generated by the intentional and inefficient orientation of dipole elements in order to reduce peak 10g local SAR while maintaining excitation fidelity.ResultsFor B1+ shimming in the spine, the addition of dipole elements did not significantly alter the B1+ spatial pattern but reduced local SAR by 36%.Conclusion
The dipole elements provide a sufficiently complimentary B1+ and electric field pattern to the loop array that can be exploited by the radiofrequency shimming algorithm to reduce local SAR. Magn Reson Med, 2014. © 2014 Wiley Periodicals, Inc.

Download full-text


Available from: Yigitcan Eryaman, Jul 31, 2014
  • [Show abstract] [Hide abstract]
    ABSTRACT: The development of MRI systems operating at or above 7 T has provided researchers with a new window into the human body, yielding improved imaging speed, resolution and signal-to-noise ratio. In order to fully realise the potential of ultrahigh-field MRI, a range of technical hurdles must be overcome. The non-uniformity of the transmit field is one of such issues, as it leads to non-uniform images with spatially varying contrast. Parallel transmission (i.e. the use of multiple independent transmission channels) provides previously unavailable degrees of freedom that allow full spatial and temporal control of the radiofrequency (RF) fields. This review discusses the many ways in which these degrees of freedom can be used, ranging from making more uniform transmit fields to the design of subject-tailored RF pulses for both uniform excitation and spatial selection, and also the control of the specific absorption rate. © 2015 The Authors. NMR in Biomedicine published by John Wiley & Sons Ltd. © 2015 The Authors. NMR in Biomedicine published by John Wiley & Sons Ltd.
    NMR in Biomedicine 05/2015; DOI:10.1002/nbm.3313 · 3.04 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: The goal of this study was to explore the feasibility of using an array of electric dipole antennas for RF transmission in spine MRI at high fields. A two-channel transmit array based on an electric dipole design was quantitatively optimized for 7T spine imaging and integrated with a receive array combining eight loop coils. Using B1+ mapping, the transmit efficiency of the dipole array was compared with a design using quadrature loop pairs. The radiofrequency energy deposition for each array was measured using a home-built dielectric phantom and MR thermometry. The performance of the proposed array was qualitatively demonstrated in human studies. The results indicate dramatically improved transmit efficiency for the dipole design compared with the loop excitation. A gain of up to 76% was achieved within the spinal region. For imaging of the spine, electric dipole-based transmitters provide an attractive alternative to the traditional loop-based design. Easy integration with existing receive array technology facilitates practical use at high fields. Magn Reson Med, 2015. Published 2015. This article is a U.S. Government work and is in the public domain in the USA. Published 2015. This article is a U.S. Government work and is in the public domain in the USA.
    Magnetic Resonance in Medicine 07/2015; 74(4). DOI:10.1002/mrm.25817 · 3.57 Impact Factor