Distinguishing Lipid Subtypes by Amplifying Contrast from J-Coupling

Ifeanyi K Uche,Gigi Galiana

doi:10.1038/s41598-019-39780-4

Ifeanyi K Uche, Gigi Galiana

Open Access

https://doi.org/10.1038/s41598-019-39780-4

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Abstract

Previous work has highlighted the complicated and distinctive dynamics that set signal evolution during a train of spin echoes, especially with nonuniform echo spacing applied to complex molecules like fats. The work presented here regards those signal patterns as codes that can be used as a contrast mechanism, capable of distinguishing mixtures of molecules with an imaging sequence, sidestepping many challenges of spectroscopy. For particular arrays of echo spacings, non-monotonic and distinctive signal evolution can be enhanced to improve contrast between target species. This work presents simulations that show how contrast between two molecules: (a) depends on the specific sequence of echo spacing, (b) is directly linked to the presence of J-coupling, and (c) can be relatively insensitive to variations in B0, T2 and B1. Imaging studies with oils demonstrate this phenomenon experimentally and also show that spin echo codes can be used for quantification. Finally, preliminary experiments apply the method to human liver in vivo, verifying that the presence of fat can lead to nonmonotonic codes like those seen in vitro. In summary, nonuniformly spaced echo trains introduce a new approach to molecular imaging of J-coupled species, such as lipids, which may have implications diagnosing metabolic diseases.

Highlights

The behavior of coupled spins in a spin echo train have been known since the early days of MR to depend on both spectroscopic parameters specific to the molecule and on experimental parameters specific to the sequence[1,2,3,4]
The blue and cyan signals correspond to signal from Molecule A (Mol A) under a nonuniformly spaced and evenly spaced spin echo train, respectively
The red and magenta signals correspond to Molecule B (Mol B) under a nonuniformly spaced and evenly spaced spin echo train, respectively

Summary

Introduction

The behavior of coupled spins in a spin echo train have been known since the early days of MR (magnetic resonance) to depend on both spectroscopic parameters specific to the molecule and on experimental parameters specific to the sequence[1,2,3,4]. S(nτ) ∝ ∑ cm cos(ωmn) + c m=0 where m indexes over each spin pair in the N-spin coupling network, and cm and ωm are constants, which depend on the J and Δω of each coupled spin pair as well as the echo spacing of the pulse sequence, τ While this equation describes a simple sum of sinusoids, the complication is that cm and ωm are not straightforward functions of J, Δω and τ. Stables et al performed extensive simulations studying more complicated spin systems (A3B2 and A3B2C2) in echo train experiments of different echo spacing[3] Those studies documented a wealth of erratic behavior, including a narrow range of τ where dynamics transitioned from relatively smooth evolution, where S(nτ) contains just a www.nature.com/scientificreports/. This profound variability was observed even for these studies of uniformly spaced CPMG pulse sequences

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