Skip to main page content
U.S. flag

An official website of the United States government

Dot gov

The .gov means it’s official.
Federal government websites often end in .gov or .mil. Before sharing sensitive information, make sure you’re on a federal government site.

Https

The site is secure.
The https:// ensures that you are connecting to the official website and that any information you provide is encrypted and transmitted securely.

Access keys NCBI Homepage MyNCBI Homepage Main Content Main Navigation
. 2013 Dec;70(6):1669-73.
doi: 10.1002/mrm.24599. Epub 2013 Feb 7.

Efficient Bloch-Siegert B1 (+) mapping using spiral and echo-planar readouts

Manojkumar Saranathan  1 , Mohammad Mehdi Khalighi, Gary H Glover, Prachi Pandit, Brian K Rutt
Affiliations

Efficient Bloch-Siegert B1 (+) mapping using spiral and echo-planar readouts

Manojkumar Saranathan et al. Magn Reson Med. 2013 Dec.

Abstract

The Bloch-Siegert (B-S) B1 (+) mapping technique is a fast, phase-based method that is highly SAR limited especially at 7T, necessitating the use of long repetition times. Spiral and echo-planar readouts were incorporated in a gradient-echo based B-S sequence to reduce specific absoprtion rate (SAR) and improve its scan efficiency. A novel, numerically optimized 4 ms B-S off-resonant pulse at + 1960 Hz was used to increase sensitivity and further reduce SAR compared with the conventional 6 ms Fermi B-S pulse. Using echo-planar and spiral readouts, scan time reductions of 8-16 were achieved. By reducing the B-S pulse width by a factor of 1.5, SAR was reduced by a factor of 1.5 and overall sensitivity was increased by a factor of 1.33 due to the nearly halved resonance offset of the new B-S pulse. This was validated on phantoms and volunteers at 7 T.

Keywords: B1+ mapping; Bloch-Siegert method; echo-planar readout; spiral readout.

PubMed Disclaimer

Figures

Figure 1
Figure 1
RF pulse profiles (left column) and the corresponding frequency response plots (right column) for the 4ms quad B-S pulse (top row) and the conventional 6ms Fermi B-S pulse (bottom row). Note the high degree of stop-band suppression achieved at 1960 Hz using the 4ms pulse.
Figure 2
Figure 2
Comparison of mean B1+ maps (top panel), ANR (middle panel) and % error (bottom panel) from a head-neck phantom at 7T. The first two columns show conventional 2D gradient echo sequence using the 6 ms Fermi pulse (a) and the 4 ms quad pulse (b). The last three columns are echo-planar readouts with echo train lengths of 4 and 8 (c-d) and a spiral readout (e). The 4ms pulse was used for echo-planar and spiral readout cases.
Figure 3
Figure 3
Comparison of mean B1+ maps (top panel), ANR (middle panel) and % error (bottom panel) from a human brain at 7T. The first two columns show conventional 2D gradient echo sequence using the 6 ms Fermi pulse (a) and the 4 ms quad pulse (b). The last three columns are echo-planar readouts with echo train lengths of 4 and 8 (c-d) and a spiral readout (e). The 4ms pulse was used for echo-planar and spiral readout cases.
Figure 4
Figure 4
Whole brain coverage B1+ maps obtained using the spiral readout B-S sequence with 4 interleaves and the 4 ms B-S pulse in 12 seconds.
Figure 5
Figure 5
Whole brain coverage B1+ maps obtained using the single-shot spiral readout B-S sequence with the 4 ms B-S pulse in 6 seconds, highlighting the scan time efficiency improvement.
See this image and copyright information in PMC

References

    1. Hornak JP, Szumowski J, Bryant RG. Magnetic field mapping. Magn Reson Med. 1988;6:158–163. - PubMed
    1. Cunningham CH, Pauly JM, Nayak KS. Saturated double-angle method for rapid B1 mapping. Magn Reson Med. 2006;55:1326–1333. - PubMed
    1. Look D, Locker D. Time saving in measurement of NMR and EPR relaxation times. Rev Sci Instrum. 1970;41:250–251.
    1. Wade T, Rutt B. B1 correction using double angle look-locker (DALL). Proceedings of the 16th Annual Meeting of the International Society of Magnetic Resonance in Medicine; Toronto, Canada. 2008. p. p1246.
    1. Yarnykh VL. Actual flip angle imaging in the pulsed steady state: a method for rapid three-dimensional mapping of the transmitted radiofrequency field. Magn Reson Med. 2007;57:192–200. - PubMed

Publication types