Development of a Symmetric Echo-Planar Spectroscopy Imaging Framework for Hyperpolarized 13C Imaging in a Clinical PET/MR Scanner.

Abubakr Eldirdiri,Pernille Holst,Christina Schøier,Stefan Posse,Helle Hjorth Johannesen,Andreas Kjaer,Annemarie T Kristensen,Jan Henrik Ardenkjaer-Larsen,Adam E Hansen,Rie B Hansen,Lars G Hanson

doi:10.18383/j.tom.2018.00006

Abstract

Here, we developed a symmetric echo-planar spectroscopic imaging (EPSI) sequence for hyperpolarized 13C imaging on a clinical hybrid positron emission tomography/magnetic resonance imaging system. The pulse sequence uses parallel reconstruction pipelines to separately reconstruct data from odd-and-even gradient echoes to reduce artifacts from gradient imbalances. The ramp-sampled data in the spatiotemporal frequency space are regridded to compensate for the chemical-shift displacements. Unaliasing of nonoverlapping peaks outside of the sampled spectral width was performed to double the effective spectral width. The sequence was compared with conventional phase-encoded chemical-shift imaging (CSI) in phantoms, and it was evaluated in a canine cancer patient with ameloblastoma after injection of hyperpolarized [1-13C]pyruvate. The relative signal-to-noise ratio of EPSI with respect to CSI was 0.88, which is consistent with the decrease in sampling efficiency due to ramp sampling. Data regridding in the spatiotemporal frequency space significantly reduced spatial blurring compared with direct fast Fourier transform. EPSI captured the spatial distributions of both metabolites and their temporal dynamics in vivo with an in-plane spatial resolution of 5 × 9 mm2 and a temporal resolution of 3 seconds. Significantly higher spatial and temporal resolution for delineating anatomical structures in vivo was achieved for EPSI metabolic maps than for CSI maps, which suffered spatiotemporal blurring. The EPSI sequence showed promising results in terms of short acquisition time and sufficient spectral bandwidth of 500 Hz, allowing to adjust the trade-off between signal-to-noise ratio and encoding speed.

Highlights

Since the introduction of the dissolution dynamic nuclear polarization technique [1], magnetic resonance spectroscopic of hyperpolarized 13C has emerged as an attractive modality that enables imaging of metabolic activity in vivo [2,3,4,5]
The calculated relative signal-to-noise ratio (SNR) values of free induction decay (FID) echo-planar spectroscopic imaging (EPSI) with respect to chemical-shift imaging (CSI) were 0.87 Ϯ 0.02, 0.89 Ϯ 0.01, and 0.86 Ϯ 0.01 for 13C-bicarbonate, 13C-urea, and [1-13C]acetate, respectively, whereas the relative SNR values of spin echo (SE) EPSI were 0.61 Ϯ 0.03, 0.63 Ϯ 0.09, and 0.59 Ϯ 0.01 for the 3 metabolites
This work presents an implementation of a symmetric EPSI sequence for hyperpolarized 13C in a hybrid clinical positron emission tomography (PET)/magnetic resonance (MR) system

Summary

Introduction

Since the introduction of the dissolution dynamic nuclear polarization (dDNP) technique [1], magnetic resonance spectroscopic of hyperpolarized 13C has emerged as an attractive modality that enables imaging of metabolic activity in vivo [2,3,4,5]. Hyperpolarized 13C metabolic imaging is being widely conducted in combination with the more established positron emission tomography (PET) [6, 7] to study its potential for clinical use. The enhanced magnetization obtained from hyperpolarization methods is short-lived, and the magnetization consumed by excitations is nonrecoverable. This dictates the use of optimized imaging sequences that efficiently use the decaying magnetization. The conventional 2D phase-encoded chemical-shift imaging (CSI) sequence encodes the k-space using 2 phase-encoding gradients

Methods

Results

Conclusion