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  • Open Access

Aortic 4D flow: quantifying the effects of contrast and field strength at 1.5 T, 3T and 7T

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Journal of Cardiovascular Magnetic Resonance201416 (Suppl 1) :P169

https://doi.org/10.1186/1532-429X-16-S1-P169

  • Published:

Keywords

  • Contrast Agent
  • Field Strength
  • Flip Angle
  • Aortic Flow
  • Magnitude Image

Background

4D flow is a promising new method for assessment of aortic pathology, but is limited by signal-to-noise ratio (SNR) leading to long acquisition times. Higher field strength may prove a solution. SNR in aortic 4D flow increases at 3T versus 1.5T (Strecker et al, JMRI 2012) and further by adding a contrast agent (Bock et al, MRM 2010). This work extends this comparison to human 7 Tesla and quantifies the field strength dependent effects of contrast agents.

Methods

Four healthy male volunteers were scanned six times: both with and without contrast (MultiHance, Braco, Milano, Italy) at each field 1.5 T, 3 T, and 7 T. All scans were acquired within four weeks and on Siemens scanners. Identical protocols were used, TR/TE 4.33/2.5 ms, temporal resolution 52 ms, bandwidth 1502 Hz/pixel, matrix 192 × 124 × 24, field of view 384 × 310 × 60 mm3, resolution 2.0 × 2.5 × 2.5 mm3, flip angle 7°, segmentation 3, GRAPPA 2, VENC 150 cm/s. At 7 T no RF spoiling was employed and a flip angle post B1 shimming ranging from 5° to 7° through the aorta. 7 T scans employed dynamic B1 shimming alternating between navigator and aorta specific shim. SNR was calculated by taking the difference of two symmetrically flow-encoded in one direction magnitude images (enc1 and enc3). SNR(r) = mean(enc1(r, t) + enc3(r, t))/√2 (std(enc1(r, t) - enc3(r, t))), r is the spatial coordinate and t are the temporal frames during diastole. SNR was assessed in the descending aorta over a 40 × 7.5 mm2 × aorta diameter ROI centered 60 mm below the midpoint of the aortic arch.

Results

SNR for each of the six scans for each subject are plotted and a set of stream lines seeded in the LV (7 T data) are shown. The mean ± SD increase in SNR due to contrast agent is 1.8 ± 0.2, 1.8 ± 0.5 and 1.4 ± 0.2 for 1.5 T, 3 T and 7 T respectively. The mean ± SD increase in SNR due to field strength without and with the contrast agent is 1.8 ± 0.4 and 1.7 ± 0.1 for 1.5 T to 3 T and 2.1 ± 0.7 and 1.7 ± 0.4 for 3 T to 7 T. The average difference in peak net flow rate at the same location in descending aorta at 7 T compared to 3 T was 7 ± 7 ml/s. Of interest is that increases in SNR by stepping up in field strength are comparable to the increase from contrast.

Conclusions

4D aortic flow is feasible at 7 Tesla and yields substantial SNR improvements over lower field strengths. Future work will exploit this higher SNR to explore improved spatial and/or temporal resolution.

Funding

Medical Research Council (UK) and British Heart Foundation.
Figure 1
Figure 1

Box and whisker plots of SNR at each field strength in the descending aorta at 1.5 T, 3 T, and 7 T both without and with a contrast agent (C).

Figure 2
Figure 2

Streamlines generated from a 7 T scan with contrast using software from Siemens and by seeding the streamlines in the left ventricle at 155 ms after the ECG R wave.

Authors’ Affiliations

(1)
University of Oxford Centre for Clinical Magnetic Resonance Research, Oxford, UK

Copyright

© Hess et al.; licensee BioMed Central Ltd. 2014

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 (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated.

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