Wouthuysen–Field coupling: Difference between revisions
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:''This is a sub-article to [[Physics of magnetic resonance imaging]] | |||
== Introduction == | |||
The Shinnar-LeRoux transform is a useful tool for generating optimized selective pulses in [[Magnetic Resonance Imaging]] [[Radio Frequency]] pulse sequence design. Selective pulses are used in [[MRI]] to isolate a slice through the subject.<ref name="Pauly1991">{{cite journal|last=Pauly|first=J|coauthors=P Le Roux, D Nishimura, A Macovski|title=Parameter relations for the Shinnar-Le Roux selective excitation pulse design algorithm.|journal=IEEE Transactions on Medical Imaging|year=1991|volume=10|issue=1|pages=53–65|doi=10.1109/42.75611|pmid=18222800|url=http://www.ncbi.nlm.nih.gov/pubmed/18222800|accessdate=30 November 2011}}</ref> | |||
Given a desired magnetization profile, determining the [[Radio frequency|RF]] pulse that will produce it is not trivial. At low tip angles, the RF excitation waveform can be approximated by the [[Fourier inversion theorem|inverse Fourier Transform]] of the desired frequency profile.<ref>{{cite book|last=Bernstein|first=Matt A.|title=Handbook of MRI Pulse Sequences|year=2005|publisher=Kevin E. King, Xiaohong Joe Zhou, and Wilson Fong|isbn=978-0-12-092861-3|url=http://www.amazon.com/Handbook-Pulse-Sequences-Matt-Bernstein/dp/0120928612}}</ref> | |||
The [[Radio frequency|RF]] pulse design problem is generally nonlinear due to the non-linearity of the [[Bloch equations]]. A direct solution to the pulse design problem was independently proposed by Shinnar <ref>M. Shinnar, L. Bolinger, and J. S. Leigh, “Use of finite impulse response filters in pulse design,” in Proc. 7th SMRM, Aug. 1988, p. 695.</ref><ref>M. Shinnar, L. Bolinger, and J. S. Leigh, “Synthesis of soft pulses with specified frequency responses,” in Proc. 7th SMRM, Aug. 1988, p. 1040.</ref><ref>M. Shinnar, S. Eleff, H. Subramanian, and J. S. Leigh, “The synthesis of pulse sequences yielding arbitrary magnetization vectors,” Magnet. Resonance Med., vol. 12, pp. 74-80, Oct. 1989.</ref><ref>M. Shinnar, L. Bolinger, and J. S. Leigh, “The use of finite impulse response filters in pulse design,” Magnetic Resonance Med., vol. 12, pp. 75-87, Oct. 1989.</ref> and Le Roux <ref>P. Le Roux, “Exact synthesis of radio frequency waveforms,” in Proc. 7th SMRM, Aug. 1988, p. 1049.</ref> based on a discrete approximation to the spin domain version of the Bloch equations. | |||
== Theory == | |||
The Shinnar-LeRoux algorithm simplifies the solution of the Bloch equations to the design of two polynomials, which can be solved using well-known digital filter design algorithms.<ref name="Pauly1991" /> | |||
<math>[B_1(t),\phi(t)]\Longleftarrow SLR \Longrightarrow [A_N(z),B_N(z)]</math> | |||
Where ''N'' is the number of bins, or hard pulse divisions that you wish to approximate with, and ''φ(t)'' is the phase of the ''B<sub>1</sub>(t)'' waveform at a given time ''t''. | |||
The mapping of the RF pulse into two complex polynomials will be denoted as the Forward SLR Transform. Given two polynomials <math>[A_N(z),B_N(z)]</math> the SLR transform can be inverted to calculate the RF pulse the produces these polynomials. The order of the polynomials <math>[A_N(z),B_N(z)]</math> is <math>N-1</math>. A minimum phase <math>A_N(z)</math> results in a minimum energy RF pulse. | |||
== References == | |||
{{Reflist}} | |||
[[Category:Nuclear magnetic resonance]] | |||
[[Category:Magnetic resonance imaging]] | |||
[[Category:Medical imaging]] | |||
Latest revision as of 06:49, 3 November 2013
- This is a sub-article to Physics of magnetic resonance imaging
Introduction
The Shinnar-LeRoux transform is a useful tool for generating optimized selective pulses in Magnetic Resonance Imaging Radio Frequency pulse sequence design. Selective pulses are used in MRI to isolate a slice through the subject.[1]
Given a desired magnetization profile, determining the RF pulse that will produce it is not trivial. At low tip angles, the RF excitation waveform can be approximated by the inverse Fourier Transform of the desired frequency profile.[2]
The RF pulse design problem is generally nonlinear due to the non-linearity of the Bloch equations. A direct solution to the pulse design problem was independently proposed by Shinnar [3][4][5][6] and Le Roux [7] based on a discrete approximation to the spin domain version of the Bloch equations.
Theory
The Shinnar-LeRoux algorithm simplifies the solution of the Bloch equations to the design of two polynomials, which can be solved using well-known digital filter design algorithms.[1]
Where N is the number of bins, or hard pulse divisions that you wish to approximate with, and φ(t) is the phase of the B1(t) waveform at a given time t.
The mapping of the RF pulse into two complex polynomials will be denoted as the Forward SLR Transform. Given two polynomials the SLR transform can be inverted to calculate the RF pulse the produces these polynomials. The order of the polynomials is . A minimum phase results in a minimum energy RF pulse.
References
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My blog: http://www.primaboinca.com/view_profile.php?userid=5889534 - ↑ M. Shinnar, L. Bolinger, and J. S. Leigh, “Use of finite impulse response filters in pulse design,” in Proc. 7th SMRM, Aug. 1988, p. 695.
- ↑ M. Shinnar, L. Bolinger, and J. S. Leigh, “Synthesis of soft pulses with specified frequency responses,” in Proc. 7th SMRM, Aug. 1988, p. 1040.
- ↑ M. Shinnar, S. Eleff, H. Subramanian, and J. S. Leigh, “The synthesis of pulse sequences yielding arbitrary magnetization vectors,” Magnet. Resonance Med., vol. 12, pp. 74-80, Oct. 1989.
- ↑ M. Shinnar, L. Bolinger, and J. S. Leigh, “The use of finite impulse response filters in pulse design,” Magnetic Resonance Med., vol. 12, pp. 75-87, Oct. 1989.
- ↑ P. Le Roux, “Exact synthesis of radio frequency waveforms,” in Proc. 7th SMRM, Aug. 1988, p. 1049.