Skip to content


  • Workshop presentation
  • Open Access

Improved excitation fidelity in cardiac imaging with 2-spoke parallel excitation at 7 Tesla

  • 1,
  • 1,
  • 1,
  • 2,
  • 3,
  • 1,
  • 1,
  • 1 and
  • 1
Journal of Cardiovascular Magnetic Resonance201315 (Suppl 1) :W1

  • Published:


  • Ultra High Field
  • Parallel Excitation
  • Homogeneous Excitation
  • Residual Inhomogeneity
  • SINC Shape


Cardiac MRI may greatly benefit from ultra high field (UHF) providing higher SNR and intrinsic contrast. But shorter RF wavelength yields transmit B1 (B1+) heterogeneity and contrast variations through the heart. These can be addressed by parallel transmission (pTX) using multi-spoke RF pulses as previously shown in other organs. However, applying pTX in cardiac MRI at 7T is challenging and requires rapid, multi-channel ECG triggered B1+ calibration and ECG synchronized, motion-insensitive pTX acquisitions. In this initial work we investigate the impact of 2-spoke RF pulse design on cardiac imaging at 7T using a 16-channel pTX system


Healthy volunteers were imaged at 7T (Siemens, Erlangen) using a 16 channel pTX system with 16 channel transceiver body coil. All sequences were modified to enable both ECG triggering and full pTX capability. B1+ maps of each transmit (TX) channel were obtained using a fast estimation technique [1]. Each map was acquired within 484ms during diastole. 1- and 2-spoke pTX RF pulses (SINC shape, BWTP 4) were designed based on magnitude least squares (MLS) optimization [2] aiming at homogeneous excitation in manually drawn ROI. 3 excitations were compared: a) 1-spoke (= standard excitation) with non-optimized initial RF phase setting (equal amplitude), b) 1-spoke with optimized RF magnitude/phase and c) 2-spoke RF pulse with magnitude/phase optimized for each spoke and channel. To keep a constant RF duration 1-spoke pulses were set to 1600us duration while 2-spoke subpulses were set to 800us. Coefficient of variation (CV, i.e. std/mean) in target ROI, total RF energy and peak energy per channel were computed for all RF pulses for same flip angle. Axial- and 4-chamber views were acquiried using a CINE GRE sequence (resolution 1.2x1.2x5mm3; 1 spoke: TR/TE=41.7/2.5ms, 32 phases; 2 spoke: TR/TE=44.8/2.7ms, 29 phases)


Fig.1a shows axial views of predicted B1+ maps and GRE cardiac images using 1- and 2-spokes RF pulses. Signal patterns are highly consistent with predictions based on measured B1+ maps, including in more posterior regions (e.g. see aorta; also Fig.1b). Homogeneity improved with 2-spoke, with CV dropping from 16.4% to 10.0% with only minor increase in RF energy (+12% - see Figure 2.1). Fidelity improvement with 2- vs. 1-spoke pulses occured especially close to the ROI edges and posteriorly. Fig. 1b shows GRE images obtained with 2-spoke excitation in 4-chamber view at different cycle phases. 1+2 spokes acquisition in 4 chamber view resulted in higher residual inhomogeneity and higher RF compared to axial views (Figure 2), likely due to larger ROI size and/or orientation. Note the absence of significant excitation pattern change through the cycle.
Figure 1
Figure 1

a) estimated B1+ maps for 1 and 2 spoke excitation (upper row) and last cardiac phase of corresponding CINE acquisition (lower row); b) 4 chamber view of different cardiac phases, acquired in CINE mode with 2 spokes at 7T. Upper left corner: corresponding B1+ estimation map (scale different to a) to appreciate overall pattern)

Figure 2
Figure 2

Summarized results of excitation fidelity, RF energy and max. of energy per channel. Gray shaded columns correspond to Fig.1


pTX 2-spoke excitation in the heart at 7T is feasible, providing improved excitation fidelity, and reduced peak RF amplitude and reduced total RF energy.


P41 EB015894, S10RR026783, R21-EB009138, KECK Foundation.

Authors’ Affiliations

Center for Magnetic Resonance Res earch, University of Minnesota, Minneapolis, MN, USA
Siemens Healthcare Sector, Erlangen, Bavaria, Germany
Siemens Medical Solutions USA, Inc., Center for Magnetic Resonance Research, University of Minnesota, Minneapolis, MN, USA


  1. Van de Moortele : ISMRM. 2009Google Scholar
  2. Setsompop : MRM. 2008Google Scholar


© Schmitter et al; licensee BioMed Central Ltd. 2013

This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.