Optimized, Unequal Pulse Spacing in Multiple Echo Sequences Improves Refocusing

TL;DR

This paper demonstrates that optimized, unequal pulse spacing in multiple echo sequences, based on the Uhrig dynamical decoupling (UDD) sequence, enhances T2-weighted contrast in magnetic resonance imaging of structured tissues, both theoretically and experimentally.

Contribution

It introduces the application of the UDD sequence to magnetic resonance imaging, showing its advantages over conventional equal-spaced sequences in structured tissue environments.

Findings

UDD sequences provide different T2 contrast than CPMG sequences.

Experimental results show substantial contrast enhancement in tissue and tumor models.

Theoretically, UDD sequences are advantageous for imaging structured materials with fluctuating fields.

Abstract

A recent quantum computing paper (G. S. Uhrig, Phys Rev Lett 98 100504 (2007)) analytically derived optimal pulse spacings for a multiple spin echo sequence designed to remove decoherence in a two level system coupled to a bath. The spacings in what has been called a UDD sequence differ dramatically from the conventional, equal pulse spacing of a Carr-Purcell-Meiboom-Gill (CPMG) multiple spin echo sequence. The UDD sequence was derived for a model that is unrelated to magnetic resonance, but was recently shown theoretically to be more general. Here we show that the UDD sequence has theoretical advantages for magnetic resonance imaging of structured materials such as tissue, where diffusion in compartmentalized and microstructured environments leads to fluctuating fields on a range of different timescales. We also show experimentally, both in excised tissue and in a live mouse tumor model, that optimal UDD sequences produce different T2-weighted contrast than do CPMG sequences with the same number of pulses and total delay, with substantial enhancements in most regions.