Abstract
We present an improved and general approach for implementing echo train acquisition (ETA) in magnetic resonance spectroscopy, particularly where the conventional approach of Carr-Purcell-Meiboom-Gill (CPMG) acquisition would produce numerous artifacts. Generally, adding ETA to any N-dimensional experiment creates an N + 1 dimensional experiment, with an additional dimension associated with the echo count, n, or an evolution time that is an integer multiple of the spacing between echo maxima. Here we present a modified approach, called phase incremented echo train acquisition (PIETA), where the phase of the mixing pulse and every other refocusing pulse, φ(P), is incremented as a single variable, creating an additional phase dimension in what becomes an N + 2 dimensional experiment. A Fourier transform with respect to the PIETA phase, φ(P), converts the φ(P) dimension into a Δp dimension where desired signals can be easily separated from undesired coherence transfer pathway signals, thereby avoiding cumbersome or intractable phase cycling schemes where the receiver phase must follow a master equation. This simple modification eliminates numerous artifacts present in NMR experiments employing CPMG acquisition and allows "single-scan" measurements of transverse relaxation and J-couplings. Additionally, unlike CPMG, we show how PIETA can be appended to experiments with phase modulated signals after the mixing pulse.
Highlights
The primary benefits of Echo train acquisition (ETA) are reduced experiment time, enhanced sensitivity, and the ability to separate frequency contributions that are refocused during the echo train pulse sequence from those that are not
Each echo observed in a CPMG experiment is the result of a coherence transfer from p = +1 → −1 by a single rf pulse
In ETA even the smallest inefficiencies lead to significant signal loss since the intensity of the echo after n transfers is reduced by the transfer efficiency to the nth power
Summary
Echo train acquisition (ETA) is a powerful approach in magnetic resonance, forming the basis of a number of diverse and advanced magnetic techniques such as echo planar imaging1 in MRI, diffusion and transverse relaxation measurements,2–6 ultra-fast multi-dimensional liquid state NMR,7 and sensitivity enhancement in solid-state NMR.8 The primary benefits of ETA are reduced experiment time, enhanced sensitivity, and the ability to separate frequency contributions that are refocused during the echo train pulse sequence from those that are not. In CPMG acquisition, shown, a train of rf pulses produces a train of echo signals arising from the refocusing of frequency contributions with odd symmetry in their spin transition function.9 Frequency contributions lacking odd symmetry result in a modulation or decay of the echo train signal. Known,4,5,11–14 that CPMG acquisition can often fail to produce desired results because non-ideal rf pulses introduce contaminating signals from undesired coherence transfer pathways.
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