Exciting RF: Mapping, Design & Applications

Friday 16 May 2014

Kosuke Ito¹, Yukio Kaneko², Yoshihisa Soutome², and Masahiro Takizawa^1
1Hitachi Medical corporation, Kashiwa, Chiba, Japan, ²Hitachi Ltd, Central Research Laboratory, Kokubunji, Japan

The regional RF shimming using 4-channel RF transmit coil was evaluated for breast imaging in vivo at 3 T. The homogeneity of B1 and relative RF transmit power were compared with QD mode, volume RF shimming mode, and regional RF shimming mode. By using regional RF shimming mode, B1 homogeneity in breast region was improved about 35%, and relative RF transmit power decreased about 18% compared to QD mode. These results show the regional RF shimming can improve B1 homogeneity and decrease RF transmit power simultaneously.

Mathias Davids¹, Michaela Ruttorf¹, Frank G. Zoellner¹, and Lothar R. Schad^1
1Computer Assisted Clinical Medicine, Medical Faculty Mannheim, Heidelberg University, Mannheim, BW, Germany

Especially 3D parallel selective excitation pulses require the synthesis of fast non-uniform k-space trajectories. Since numerical approaches are usually very time-consuming and impaired by numerical errors, a novel analytic framework on rapidly designing time-optimal trajectories was developed. The trajectory is represented by analytic gradient basis functions that are symbolically solved to traverse given k-space control points. The trajectory is then globally accelerated to fully utilize given hardware constraints Gmax and Smax, yielding time-optimal analytic gradients. Furthermore, this optimization uses an analytically derived Jacobian which guarantees convergence within seconds. Any arbitrarily shaped 2D/3D trajectory can be modeled and optimized using the method.

Michael Stephen Poole¹, Desmond H Y Tse¹, Kaveh Vahedipour¹, and N Jon Shah^1,2
1INM-4, Forschungszentrum Jülich, Jülich, Germany, ²Department of Neurology, RWTH Aachen University, Aachen, Germany

B1+ homogenisation was performed at 9.4 T repeatably and robustly using kt-points and a region-growing algorithm in a PBS phantom, an ex vivo human brain and in vivo. DREAM was used to obtain B1+ maps of 8 parallel transmitters. Small tip-angle gradient echo images were aquired. We hypothesise that there exists a smooth continuum from the small region solution (essentially transmitter phase coherence in the centre) to the globally optimum solution for the full homogenisation volume.

Aurélien Massire¹, Alexandre Vignaud¹, Alexis Amadon¹, Benjamin Robert², Denis Le Bihan¹, and Nicolas Boulant^1
1CEA DSV I2BM NeuroSpin UNIRS, Gif-sur-Yvette, France, ²Siemens Healthcare, St Denis, France

We use the gradient ascent pulse engineering algorithm combined with the kT-point method to design all the non-selective refocusing pulses of a SPACE sequence that mitigate severe B1+ and ΔB0 inhomogeneities. The novelty of the method lays in the optimization of the rotation matrices themselves rather than magnetization states. Three healthy subjects were scanned with a 7 Tesla scanner equipped with an 8-channel transceiver array. Exploiting the full potential of parallel transmission with the proposed methodology produced high quality whole brain T2-weighted images with uniform signal and contrast, requiring only 10 minutes of subject-specific data acquisition and pulse design.

Nicolas Boulant¹, Aurelien Massire¹, Alexis Amadon¹, and Alexandre Vignaud^1
1Neurospin, CEA, Saclay, Ile de France, France

Although it seems that there is a general consensus that temperature is the true relevant safety parameter, tracking the SAR in MR exams and in RF pulse design has remained the gold standard, likely due to simplicity reasons. Here we investigate numerically a parallel transmission RF pulse design algorithm under strict temperature constraints. With the example of a 10 min TOF sequence at 7T, we show that the SAR guidelines in this instance can be quite conservative and that more performance at UHF thus is within reach through the use of these thermo-regulated pulses.

Sebastian Schmitter¹, Xiaoping Wu¹, Kamil Ugurbil¹, and Pierre-Francois van de Moortele^1
1Center for Magnetic Resonance Research, University of Minnesota, Minneapolis, MN, United States

Cardiac MRI at ultra-high fields is challenged by the short RF wavelength that induces flip-angle variations. We have addressed this problem successfully in 7T cardiac CINE imaging using parallel transmission (pTX) with 2-spoke RF pulses. Sometimes, however, we observed deviations from our expectations. We suspected, that the subject’s respiratory state during cardiac CINE breath-hold scans is different than during transmit B1 calibration scans (also breath-hold). Here we investigate the impact of breath-hold position on 7T cardiac CINE imaging using 2-spoke pTX pulses and demonstrate a pulse design strategy to increase the robustness of spoke pulses to variations in breath-hold position.

Shaihan J Malik¹ and Joseph V Hajnal^1,2
1Division of Imaging Sciences and Biomedical Engineering, Kings College London, London, London, United Kingdom, ²Centre for the Developing Brain, Kings College London, London, London, United Kingdom

The 3D Fast Spin Echo sequence offers a very good candidate for inner volume imaging (IVI) since magnetization that is not transverse after the excitation pulse does not contribute any signal thereafter. In this work 3D localized excitations have been realised in-vivo using parallel transmission to achieve reasonable pulse durations (~12ms) with on-line subject specific optimization. The pulses were coupled with an otherwise standard T2 weighted 3D-FSE protocol in the human brain, employing non-selective variable flip angle refocusing pulses. High resolution IVI achieved comparable image quality to the standard sequence with large reductions in imaging time.

Feng Zhao¹, Jeffrey A Fessler², and Douglas C Noll^1
1Biomedical Engineering, The University of Michigan, Ann Arbor, MI, United States, ²EECS, The University of Michigan, Ann Arbor, MI, United States

The conventional spectrally selective fat saturation pulse may perform poorly with inhomogeneous B0 and/or B1 fields at high fields, and the pulse length may be too long for low field applications. We investigated a 4D spectral-spatial fat sat pulse which mitigates the field inhomogeneity problem in fat sat and also largely reduces the pulse length.

Jieying Luo¹, Nii Okai Addy¹, R. Reeve Ingle¹, Brian A. Hargreaves², Bob S. Hu³, Dwight G. Nishimura¹, and Taehoon Shin^4
1Electrical Engineering, Stanford University, Stanford, California, United States, ²Radiology, Stanford University, Stanford, California, United States,³Palo Alto Medical Foundation, Palo Alto, California, United States, ⁴University of Maryland, Baltimore, Maryland, United States

A magnetization preparation sequence that efficiently combines T2-Prep and outer volume suppression is designed and tested for coronary angiography. The proposed sequence is composed of a BIR-4 90° tip-down pulse, two adiabatic refocusing pulses and a 2D spiral 90° tip-up pulse. Its performance is demonstrated with numerical simulation, phantom and in vivo experiments. This sequence induces T2 weighting to improve blood-myocardium contrast while suppressing outer volume signals to facilitate scan acceleration and reduce motion artifacts.

David Otto Brunner¹ and Klaas Paul Pruessmann^1
1Institute for Biomedical Engineering, University and ETH Zurich, Zurich, Switzerland

Coil ringdown due to the high Q resonance of NMR detectors limits the speed at which the system can switch from high power transmission to sensitive signal acquisition and is a consequence of the narrowband behaviour of the coil. We present an analytic broadband pulse design approach to shorten the ringdown by precompensating the Q of the coil with no excess peak power requirements, no additional SAR deposition and no changes to the existing RF coil hardware.