PASADENA hyperpolarization of 13C biomolecules: equipment design and installation

Abstract

Object: The PASADENA method has achieved hyperpolarization of 16-20% (exceeding 40,000-fold signal enhancement at 4.7 T), in liquid samples of biological molecules relevant to in vivo MRI and MRS. However, there exists no commercial apparatus to perform this experiment conveniently and reproducibly on the routine basis necessary for translation of PASADENA to questions of biomedical importance. The present paper describes equipment designed for rapid production of six to eight liquid samples per hour with high reproducibility of hyperpolarization.

Materials and methods: Drawing on an earlier, but unpublished, prototype, we provide diagrams of a delivery circuit, a laminar-flow reaction chamber within a low field NMR contained in a compact, movable housing. Assembly instructions are provided from which a computer driven, semi-automated PASADENA polarizer can be constructed.

Results: Together with an available parahydrogen generator, the polarizer, which can be operated by a single investigator, completes one cycle of hyperpolarization each 52 s. Evidence of efficacy is presented. In contrast to competing, commercially available devices for dynamic nuclear polarization which characteristically require 90 min per cycle, PASADENA provides a low-cost alternative for high throughput.

Conclusions: This equipment is suited to investigators who have an established small animal NMR and wish to explore the potential of heteronuclear ((13)C and (15)N) MRI, MRS, which harnesses the enormous sensitivity gain offered by hyperpolarization.

Fig. 1

PASADENA Laminar flow polarizer. a Photograph and b schematic view of the experimental setup. a Low-pass filter, r.f. amplifier, gauss meter, B₀ power supply and reaction compartment, on top of the rack for the computer and synthesizer, parahydrogen (left) and nitrogen (right) gas tanks (top to bottom). b Functional elements in the reaction compartment: B₀ coil, B₁ coil, and reactor. The valves for fluid control are indicated with number {V1}–{V5}. During a PASADENA experiment, 1 the reactor is filled with parahydrogen to a pressure of 10 bar, 2 the precursor molecule is injected through the center of the injection cap, and 3 the r.f. spin order transfer sequence is applied after a reaction time of 3 s. Thereafter, the hyperpolarized agent is withdrawn through the end cap and delivered to the detecting MR unit

Fig. 2

a Engineering drawing and b photograph of the laminar flow reaction chamber, to hold the hydrogenation reaction. The reactor is placed within the B₁ and B₀ coils. Dimensions are in mm

Fig. 3

NMR unit of the PASADENA polarizer. a Engineering drawing and b photograph of B₁ r.f. transmission coil. The reaction chamber (Fig. 2) is held in place by two mountings in the coil at the isocenter. The tip of the gauss meter probe, which is introduced from the top of the coil assembly is located outside the reaction chamber and close to isocenter. A BNC connector at the top of the NMR unit connects the cable delivering r.f. to the coil. Dimensions are in mm

Fig. 4

NMR unit of PASADENA polarizer. a Engineering drawing and b photograph of the solenoid coil for the generation of the static field (B₀) and c photograph of the complete assembly of the parts described in Figs. 2 and 3. For improved homogeneity, the coil (shown in b) is constructed in three sections, which allow for individual control of the magnetic field (resistors in parallel to each section). The current is provided by a precision power supply, which is adjusted to the field determined with the gauss meter probe. Dimensions are in mm

Fig. 5

Process control unit of the PASADENA polarizer. The sequence of the PASADENA experiment (fluid control: valves 1–5, and spin order transfer B₁) is controlled by one central computer (C). The static field (B₀) and the temperature of the reaction compartment (Tp) are set by separate controllers (DC, Tc, respectively)

Fig. 6

Schematic of a parahydrogen unit for the PASADENA polarizer. Commercially available hydrogen gas (25% parahydrogen, 75% orthohydrogen) is cooled to ~ 20 K and passed through a catalyst (granular ferric oxide, A) for >97% conversion to parahydrogen. The two stages of the cold head are surrounded by copper tubing packed with the catalyst: 1 Pressure regulator for incoming hydrogen; 2 flow meter; 3 flow controller; 4 pressure gauge; 5 pressure display; 6 relay; 7 vacuum pump valve; 8 helium compressor; 9 water cooler. A setup similar to the one published by Tam and Fajardo [10] is available commercially

Fig. 7

Quantification of the polarization achieved with PASADENA. ¹³C hyperpolarized spectrum of 2.7 mmol/L HEP 1-¹³C,2,3,3 -d₃ at 4.7 T in vitro (a). Polarization of ~18% was achieved in this representative example. The inset spectrum shows a reference signal obtained from natural abundance ethanol; ¹³C concentration = 188 mmol/L per site (b). Note that the scale of the inset spectrum was magnified by 512 times