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

Abstract

Hyperpolarized MRI with 13C‐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 13C 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 [1‐13C]pyruvate. [1‐13C] pyruvate and [1‐13C] 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 T 2 relaxation times. The pulse sequences described here produced hyperpolarized 13C images with improved resolution and signal‐to‐noise ratio when compared with similar sequences described previously.

Detection of lung transplant rejection in a rat model using hyperpolarized [1‐13C] pyruvate‐based metabolic imaging

“…for translation to modern clinical MRI scanners. It should be noted, however, that the implementation of 13 C HASTE imaging would provide enhanced in‐plane spatial resolution comparable with that of UTE . In a different murine study, FDG‐PET differentiated between allografts and isografts, showing a significant difference in injected dose per cc of lung (%ID/cc) by day 7; this study further showed that the largest glucose utilization was by CD8+ T cells in particular, a finding echoed in our study.…”

Comparison of 13C MRI of hyperpolarized [1‐13C]pyruvate and lactate with the corresponding mass spectrometry images in a murine lymphoma model

“…Following injection of hyperpolarized [1- 13 C]pyruvate into an EL4 tumor-bearing mouse, 16 axial images of [1- 13 C]lactate and [1- 13 C]pyruvate were acquired using a single-shot, 3D volume imaging sequence. 15 The subcutaneous tumors showed pyruvate uptake and conversion to lactate, with consistently a region of high 13 C lactate signal intensity at the tumor base (Figures 3 and 4) (Supporting Information Figure S1). [1- 13 C]lactate and [ 12 C]lactate were detected in excised tumor sections by DESI MSI and metabolic maps generated from their peak intensities (Figure 3 and Supporting Information Figure S1).…”

“…MRI data were processed using an in-house MatLab (version R2019b, MathWorks, Natick, MA) script. 15 The MSI .raw data files were converted into .mzML files using ProteoWizard msConvert version 3 (open source) 18 and compiled into an .imzML file. 19 Subsequent data processing was performed in SCiLS Lab (version 2019c, Bruker Daltonik, Bremen, Germany).…”

“…Following intravenous injection of hyperpolarized [1- 13 C]pyruvate, 13 C images were acquired with a single-shot 3D volume imaging sequence. 15 Spectral spatial pulses excited alternately the [1- 13 C]pyruvate and [1- 13 C]lactate resonances, with flip angles of 7° and 45°, respectively. Signal was acquired from a 40 × 40 × 20 mm 3 volume with a matrix size of 32 × 32 × 16 data points using a stack of spiral acquisitions.…”

Hyperpolarized 13C MRI data acquisition and analysis in prostate and brain at University of California, San Francisco

“…Spin-echo or steady-state methods are promising for providing substantial improvements in SNR and resolution. 80,81 Autonomous scanning methods with integrated frequency, power and timing corrections 82 have the potential to provide more robust and reproducible results. There are promising new kinetic models that can remove confounding factors such as perfusion, and more robust analysis methods.…”