Quality assurance of PASADENA hyperpolarization for 13C biomolecules

Abstract

Object: Define MR quality assurance procedures for maximal PASADENA hyperpolarization of a biological (13)C molecular imaging reagent.

Materials and methods: An automated PASADENA polarizer and a parahydrogen generator were installed. (13)C enriched hydroxyethyl acrylate, 1-(13)C, 2,3,3-d(3) (HEA), was converted to hyperpolarized hydroxyethyl propionate, 1-(13)C, 2,3,3-d(3) (HEP) and fumaric acid, 1-(13)C, 2,3-d(2) (FUM) to hyperpolarized succinic acid, 1-(13)C, 2,3-d(2) (SUC), by reaction with parahydrogen and norbornadiene rhodium catalyst. Incremental optimization of successive steps in PASADENA was implemented. MR spectra and in vivo images of hyperpolarized (13)C imaging agents were acquired at 1.5 and 4.7 T.

Results: Application of quality assurance (QA) criteria resulted in incremental optimization of the individual steps in PASADENA implementation. Optimal hyperpolarization of HEP of P = 20% was achieved by calibration of the NMR unit of the polarizer (B (0) field strength +/- 0.002 mT). Mean hyperpolarization of SUC, P = [15.3 +/- 1.9]% (N = 16) in D (2)O, and P = [12.8 +/- 3.1]% (N = 12) in H (2)O, was achieved every 5-8 min (range 13-20%). An in vivo (13)C succinate image of a rat was produced.

Conclusion: PASADENA spin hyperpolarization of SUC to 15.3% in average was demonstrated (37,400 fold signal enhancement at 4.7 T). The biological fate of (13)C succinate, a normally occurring cellular intermediate, might be monitored with enhanced sensitivity.

Fig. 1

Chemistry and spin physics of the PASADENA experiment. (a) (left) molecular parahydrogen ( ) is added to the precursor ( ) by catalytic hydrogenation; (center) manipulations of the spins of the molecule by spin order transfer r.f. pulse sequence, involving the ²J_CHa,³ J_CHb,³ J_HaHb couplings; (right) net spin polarization is formed on ¹³C. (b) Hydrogenation of precursor FUM forms the product SUC. (c) Hydrogenation reaction cycle of Rhodium based catalyst: the norbornadiene moeity is hydrogenated and becomes labile and combines with the bisphosphine ligand to form the catalyst. The PASADENA precursor attaches itself to this active catalyst and hydrogenation occurs by addition [15,16]

Fig. 2

Experiment to center the frequency of the low field MR unit of the polarizer. A constant ¹³C excitation pulse was applied to a sample of saturated 1-¹³C sodium acetate in D₂O, followed by the transfer (~14 s) and a pulse-acquisition detection in a high field spectrometer. This procedure was repeated for eight settings of the static magnetic field to determine the optimum (1.763 mT). The scale refers to the offset from this optimum

Fig. 3

¹³C and ¹H flip angle calibration of the low field MR unit in the polarizer. A ¹³C or ¹H square excitation pulse was applied to a sample of saturated 1-¹³C sodium acetate in D₂O, followed by the transfer (~14 s) and a pulse-acquisition detection in a high field spectrometer. This procedure was repeated with 22 and 31 different pulse widths for ¹³C, ¹H, respectively, at constant amplitudes (¹H 31.8 V, ¹³C 9.6 V). Inversion pulses were determined to be 157 μs (¹H) and 550 μs (¹³C), respectively

Fig. 4

Determination of the relevant J-couplings for the hyperpolarization of succinate. (a) ¹³C NMR spectrum of natural abundance succinic acid at pH 7.4 (14 T Varian spectrometer, 256 acquisitions, H/C/N probe, no decoupling). (b) ¹³C NMR spectra of carboxyl (1, 4-¹³C, ~185 ppm) and methylene (2, 3-¹³C, ~35 ppm) groups at pH 7.4. Note the line broadening of the former. (c) Simulations (red) and measured ¹³C NMR spectra of carboxyl and methylene groups at pH 2.95. Note that at pH 2.95 improved resolution of the line splittings in carboxyl resonances are observed. The J-couplings extracted of the simulated spectrum were ²J_Cha = −7.15 Hz, ³J_CHb = 5.82 Hz and ³J_HaHb = 7.41 Hz. These values were employed to calculate the spin order transfer sequence for succinate

Fig. 5

Calibration of the center frequency of the low field MR unit. Maximal polarization achieved in this experimental set-up was 21%, somewhat lower than previously reported [18]. The PASADENA experiment was conducted at different strengths of the static field using HEP (0.1 mM). This was very sensitive to variations of the static field: Half of the polarization was lost at a field offset of ±0.009 mT (full width at half maximum (FWHM) = 0.018 mT). The amplitudes and durations of the B₁ pulses of the SOT used are shown in Fig. 3

Fig. 6

Impact of r.f. amplifier power to hyperpolarization. (left) New r.f. hardware allowed for increase in r.f. power (previously 25V/16V in Fig. 5, now 25V/50V (for carbon, hydrogen, respectively): 3.1-fold increase) and less sensitivity of hyperpolarization to off-resonance frequency (previously FWHM = 0.018 mT, now FWHM = 0.062 mT, 3.4-fold increase). Adaptation of the spin order transfer sequence allowed for hyperpolarization of SUC, which was conducted ten times at different strengths of the static field (right)

Fig. 7

Optimization of SUC hyperpolarization yield by two-dimensional optimization of the B₁ flip angles. First, the PASADENA experiment was conducted 8 times. Each time the pulse width of the ¹H pulses in the SOT was varied (a). The optimum was determined to be at 115 μs (50 V). Next, the experiment was repeated under variation of the ¹³C pulse widths in the SOT (b). The optimum found was 230 μs (25 V). (c) demonstration of the multidimensional character of this optimization

Fig. 8

T₁ measurement of hyperpolarized SUC in D₂O (T₁= [40 ± 2] s) and in H₂O (T₁= [27 ± 3] s). The decaying polarization was probed by small angle pulse-acquisition experiments in a high field system (4.7 T)

Fig. 9

Quantification of polarization: (a) ¹³C spectra of SUC (1.95 mM) hyperpolarized to 19%, corresponding to an enhancement of ~47,000 at 4.7 T and (b) unlabeled thermally polarized (P = 4.1 × 10⁻⁴%) ethanol (¹³C concentration 188 mM per site natural abundance)

Fig. 10

Reproducibility of hyperpolarization. $P_{\text{HP}}^{t=0}=[15.3\pm 1.9]\%$ was achieved in 16 hyperpolarization experiments on SUC, in D₂O conducted on successive days. Details are provided in Table 2

Fig. 11

In vivo ¹³C sub-second image (0.3 s) of a rat brain, acquired 9 s after close-arterial injection of 1 ml, 25 mM hyperpolarized SUC (in color). The ¹³C image was overlaid on a coronal ¹H fast gradient echo image with matching field-of-view (FOV) and slice location acquired prior to infusion to provide anatomical correlation (3D FIESTA, TR/TE = 6.3/3.1 ms, 5 × 5 × 5 mm³ spatial resolution, FOV = 220 mm/320 mm, 44 phase encoding steps/64 readout points, respectively)