Hyperpolarized ¹³ C spectroscopic imaging using single-shot 3D sequences with unpaired adiabatic refocusing pulses

Hyperpolarized MRI with ¹³ C-labeled metabolites has enabled metabolic imaging of tumors in vivo. The heterogeneous nature of tumors and the limited lifetime of the hyperpolarization require high resolution, both temporally and spatially. We describe two sequences that make more efficient use of the ¹³ C polarization than previously described single-shot 3D sequences. With these sequences, the target metabolite resonances were excited using spectral-spatial pulses and the data acquired using spiral readouts from a series of echoes created using a fast-spin-echo sequence employing adiabatic 180° pulses. The third dimension was encoded with blipped gradients applied in an interleaved order to the echo train. Adiabatic inversion pulses applied in the absence of slice selection gradients allowed acquisition of signal from odd echoes, formed by unpaired adiabatic pulses, as well as from even echoes. The sequences were tested on tumor-bearing mice following intravenous injection of hyperpolarized [1-¹³ C]pyruvate. [1-¹³ C] pyruvate and [1-¹³ C] lactate images were acquired in vivo with a 4 × 4 × 2 cm³ field of view and a 32 × 32 × 16 matrix, leading to a nominal resolution of 1.25 × 1.25 × 1.25 mm³ and an effective resolution of 1.25 × 1.25 × 4.5 mm³ when the z-direction point spread function was taken into account. The acquisition of signal from more echoes also allowed for an improvement in the signal-to-noise ratio for resonances with longer T₂ relaxation times. The pulse sequences described here produced hyperpolarized ¹³ C images with improved resolution and signal-to-noise ratio when compared with similar sequences described previously.