Use of a Multi-compartment Dynamic Single Enzyme Phantom for Studies of Hyperpolarized Magnetic Resonance Agents

Summary

A multi-compartment dynamic phantom is used to simulate some biology of interest for metabolic studies using hyperpolarized magnet resonance agents.

Abstract

Imaging of hyperpolarized substrates by magnetic resonance shows great clinical promise for assessment of critical biochemical processes in real time. Due to fundamental constraints imposed by the hyperpolarized state, exotic imaging and reconstruction techniques are commonly used. A practical system for characterization of dynamic, multi-spectral imaging methods is critically needed. Such a system must reproducibly recapitulate the relevant chemical dynamics of normal and pathological tissues. The most widely utilized substrate to date is hyperpolarized [1-¹³C]-pyruvate for assessment of cancer metabolism. We describe an enzyme-based phantom system that mediates the conversion of pyruvate to lactate. The reaction is initiated by injection of the hyperpolarized agent into multiple chambers within the phantom, each of which contains varying concentrations of reagents that control the reaction rate. Multiple compartments are necessary to ensure that imaging sequences faithfully capture the spatial and metabolic heterogeneity of tissue. This system will aid the development and validation of advanced imaging strategies by providing chemical dynamics that are not available from conventional phantoms, as well as control and reproducibility that is not possible in vivo.

Introduction

The clinical impact of hyperpolarized magnetic resonance imaging (MRI) of ¹³C-labeled compounds is critically dependent on its ability to measure chemical conversion rates through real time magnetic resonance spectroscopy and spectroscopic imaging^1-5. During sequence development and verification, dynamic chemical conversion is generally achieved through in vivo or in vitro models^6-9 that offer limited control and reproducibility. For robust testing and quality assurance, a more controlled system that preserves the chemical conversion endemic to this measurement would be preferred. We outline a method to achieve this c....

Protocol

NOTE: (Phantom Design) Two 3 ml chambers were machined out of Ultem and fitted with PEEK tubing (1.5875 mm OD and 0.762 mm ID) for injection and exhaust. The Chambers were placed in a 50 ml centrifuge tube filled with water (Figure 1). To avoid signal voids created by bubbles, the chambers and the lines were pre filled with deionized water (dH₂O).

1. Solution Preparation

1. Prepare 1 L buffer solution (81.3 mM Tris pH 7.6, 203.3 mM NaCl). Weigh out 11.38 g Trizma preset crystals pH 7.6 and 11.88 g NaCl and dissolve in 1 L of dH₂O.
2. Prepare 50 mM NADH solution. Weigh out 17.8 mg of ....

Results

Slice-selective 2D images were acquired using a snapshot radEPSI sequence. Metabolite images were reconstructed using filtered back projection. The metabolite images were well aligned with proton images, as seen in Figure 2. In this system hyperpolarized lactate signal can only be generated from the enzymatic conversion of hyperpolarized pyruvate. In Figure 4, the bottom chamber, with higher LDH concentration, has a stronger lactate and weaker pyruvate si.......

Discussion

Real time imaging of hyperpolarized metabolites has many unique challenges for sequence design, validation, and quality control. The ability to resolve spatiotemporal and spectral heterogeneity offers substantial clinical potential but precludes QA and validation methods associated with conventional MRI. Complex imaging sequences or reconstruction algorithms can have subtle dependencies that render them difficult to characterize or validate outside of the imaging experiment. Biological heterogeneity and other practical c.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
BioSpect 7T	Bruker	BioSpec 70/30 USR	7 Tesla Pre-Clinical MRI Scanner
HyperSense	Oxford Instruments	Hypersense DNP Polarizer	Dynamic Nuclear Polarizer for MRI agents
1-13C-Pyrvic Acid	Sigma Aldrich	677175	Carbon 13 labled neat pyruvic acid
Trityl Radical	GE Healthcare	OX063	Free radical used in Dynamic Nuclear Polarization
NaOH	Sigma Aldrich	S8045
EDTA	Sigma Aldrich	E6758	Ethylenediaminetetraacetic acid
LDH	Worthingthon	LS002755	Lactate Dehydrogenase from rabbit muscle
NADH	Sigma Aldrich	N4505	β-Nicotinamide adenine dinucleotide, reduced dipotassium salt
Trizma	Sigma Aldrich	T7943	Trizma Pre-set crystals
NaCl	Sigma Aldrich	S7653