A Phase-Cycling Algorithm for Reducing Sidebands in Adiabatic Decoupling
✍ Scribed by Thomas E. Skinner; M.Robin Bendall
- Publisher
- Elsevier Science
- Year
- 1997
- Tongue
- English
- Weight
- 147 KB
- Volume
- 124
- Category
- Article
- ISSN
- 1090-7807
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✦ Synopsis
Adiabatic decoupling is as dependent on well-designed inversion pulse can be a sizable fraction of 1/J. However, phase cycles for performance as any broadband method. pulse length and RF field strength can be varied indepen-For example, the dramatic performance gains of STUD (1) dently in adiabatic decoupling, in contrast to compositecompared to DAP-16 (2) are a result, primarily, of the differpulse methods, where the length of the sequence is inversely ent phase cycles chosen for implementation of each scheme. proportional to the RF amplitude. The above fast-cycling Each method employs a sech/tanh (hyberbolic secant) incondition then becomes a more ambiguous gauge for sideversion pulse (3). In this article, we describe a simple algoband performance, since sideband levels can actually be derithm (4) for improving the performance of broadband-decreased by increasing T p when RF levels are below a threshcoupling schemes. New phase cycles constructed with this old determined by the desired decoupled bandwidth (8). technique can significantly reduce sidebands and increase Thus, there are mitigating factors, and sech/tanh decoupling decoupling efficiency, allowing either lower RF power to implemented with the efficient M4P5 phase cycle was shown achieve a desired decoupled bandwidth or larger bandwidth to provide performance for large bandwidths that signififor a given RF power level. Examples providing progressive cantly exceeds composite-pulse methods (1, 8). This phase improvement in decoupling performance are tabulated for cycle provides similar benefits for other adiabatic decoupling comparison.
methods (9, 10) compared to implementations using alterna-The M4P5 phase cycle introduced by Fujiwara and Nagative cycles (11,12). Further improvements in decoupling yama (5) has become a de facto standard for adiabatic deperformance can be found by considering the behavior of coupling. It consists of the specific P5 inversion cycle [0Њ, M4P5 in more detail. 150Њ, 60Њ, 150Њ, 0Њ] from the general prescription
The value d Å 150Њ used in M4P5 is based on a plausible assumption that maximizing the average inverted z magneti-[0Њ, d, 2d / 120Њ, 3d / 60Њ, 4d / 120Њ]
[1] zation over a desired frequency-offset range will optimize the performance of the phase cycle for decoupling (5). Deof Tycko and Pines (6), overlaid by MLEV-4 (7). Origicoupling methods are typically evaluated using a sample nally used in conjunction with frequency-switched composcontaining a single value for the coupling constant J CH . Offite inversion pulses (5), M4P5 was shown to provide an resonance performance is assessed by varying the decoupler exceptional method for broadband decoupling due to enoffset. We find that both the intensity of the maximum sidehanced off-resonance inversion provided by the P5 cycle. band at a given decoupler offset and the average sideband Although adiabatic pulses can also exhibit outstanding intensity over a given offset range are sensitive to the value broadband inversion properties, they were noted to cause of d in adiabatic decoupling. If we consider the distribution considerable sidebands (5). Adiabatic decoupling would of maximum sideband amplitudes as a function of decoupler typically violate the fast-cycling condition T c J Ӷ 1, where offset, we also find that changing d changes the particular T c is the time to return the irradated spins to their initial offset at which the maximum of the distribution occurs. Yet, orientation, since the length, T p , of even a single adiabatic in hyperbolic secant decoupling, for example, the maximum in the sideband distribution over an effective decoupled bandwidth equal to Ç90% of the adiabatic frequency sweep ‡ Present address: Department of Chemistry and Chemical Engineering, is relatively insensitive to the value of d. The strategy em-