3 - HP acquisition methods: pulse sequences, reconstruction, and RF coils

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Abstract

In the 17 years since the introduction of dissolution dynamic nuclear polarization, significant advancements have been made in the development and acquisition of hyperpolarized data, with the field moving from preclinical studies using nonselective spectroscopy to clinical studies with whole organ coverage using rapid imaging sequences. The use of 13C labeled agents, primarily [1-13C]pyruvate, enables monitoring of key metabolic pathways with the ability to image both substrate and products to noninvasively measure real-time metabolism, but the nonrenewable magnetization requires specialized hardware and pulse sequences to efficiently encode this magnetization. This chapter will describe some of the unique challenges associated with HP imaging, the main classes of pulse sequences for HP imaging, and review the general tradeoffs between RF coils.

Introduction

In the 17 years since the introduction of dissolution dynamic nuclear polarization (DNP) [1], significant advancements have been made in the development and acquisition of hyperpolarized 13C data. The use of metabolically active 13C labeled agents, primarily [1-13C]pyruvate, enables noninvasive monitoring of key metabolic pathways, but the nonrenewable magnetization requires specialized hardware and pulse sequences to efficiently encode this magnetization. Since the early work of Golman et al. [2], significant advancements have been made in RF design, pulse sequence strategies, and coil hardware that have helped the field move from rodent studies using single-slice chemical shift imaging (CSI) to human studies using metabolite-selective pulse sequences that provide whole organ coverage over a large field of view (FOV). This chapter will describe some of the unique challenges associated with HP imaging, discuss the main classes of pulse sequences for HP imaging, and review the general trade-offs between RF coils. Although dissolution DNP has been the primary polarization mechanism for human applications, this chapter is equally applicable to substrates polarized via PHIP, SABRE, or other processes [3,4].

Section snippets

Hyperpolarized imaging considerations

The dissolution DNP process, discussed in more detail in Chapters 1 and 2, provides more than four orders of magnitude increase to nuclear polarization. This transient increase overcomes the small thermal equilibrium magnetization—on the order of parts-per-million at clinical field strengths and physiologic temperature—and enables dynamic imaging of hyperpolarized substrates that can be used to rapidly and efficiently encode 5D data (3 spatial + 1 spectral + 1 temporal dimension). However, this has

Pulse sequences and reconstruction

Data acquisition strategies in HP MRI experiments must account for multiple chemical shifts, efficiently utilize the nonrenewable HP magnetization, and acquire data quickly relative to metabolism and relaxation decay processes. Studies of inert HP molecules, such as 13C-urea [17,20] or 13C-t-butanol [21], have only a single resonance and can be imaged with any conventional pulse sequence combined with the HP RF pulse strategies described above. Studies of metabolically active HP molecules

RF coils

RF coils are an oft-overlooked aspect of an MRI experiment, but the choice of RF coil will directly impact the spatial resolution, sensitivity, volumetric coverage, and overall SNR in a hyperpolarized study. This section is intended to introduce the reader to the basic concepts and trade-offs between surface coils, volume coils, and multichannel arrays. See Refs. [61,62] for further reading on MRI coils.

Summary

HP 13C MRI provides novel metabolic information in a rapid, noninvasive manner. However, the transient nature of the HP magnetization and the need for both spectral encoding and temporal resolution place unique demands on RF and acquisition strategies. MATLAB code for many of the topics discussed in this chapter (prewhitening, coil combination, and RF pulse design) can be found on the Hyperpolarized MRI Toolbox: https://github.com/LarsonLab/hyperpolarized-mri-toolbox

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