In vivo single-shot 13C spectroscopic imaging of hyperpolarized metabolites by spatiotemporal encoding

Abstract

Hyperpolarized metabolic imaging is a growing field that has provided a new tool for analyzing metabolism, particularly in cancer. Given the short life times of the hyperpolarized signal, fast and effective spectroscopic imaging methods compatible with dynamic metabolic characterizations are necessary. Several approaches have been customized for hyperpolarized (13)C MRI, including CSI with a center-out k-space encoding, EPSI, and spectrally selective pulses in combination with spiral EPI acquisitions. Recent studies have described the potential of single-shot alternatives based on spatiotemporal encoding (SPEN) principles, to derive chemical-shift images within a sub-second period. By contrast to EPSI, SPEN does not require oscillating acquisition gradients to deliver chemical-shift information: its signal encodes both spatial as well as chemical shift information, at no extra cost in experimental complexity. SPEN MRI sequences with slice-selection and arbitrary excitation pulses can also be devised, endowing SPEN with the potential to deliver single-shot multi-slice chemical shift images, with a temporal resolution required for hyperpolarized dynamic metabolic imaging. The present work demonstrates this with initial in vivo results obtained from SPEN-based imaging of pyruvate and its metabolic products, after injection of hyperpolarized [1-(13)C]pyruvate. Multi-slice chemical-shift images of healthy rats were obtained at 4.7T in the region of the kidney, and 4D (2D spatial, 1D spectral, 1D temporal) data sets were obtained at 7T from a murine lymphoma tumor model.

Keywords: Cancer; Chemical shift imaging; DNP; Hyperpolarized MRI; Hyperpolarized dynamic imaging; Spatiotemporal encoding; Spectroscopic imaging; Ultrafast MRI.

Figure 1

(a,b) SPEN spectroscopic 1D MRSI schemes based on excitation or refocusing chirp pulses. (c) Echo Planar Spectroscopic 1D imaging

Figure 2

Comparison of SPEN and CSI results obtained upon ¹³C chemical shift imaging of a phantom. SPEN and CSI sequences are displayed in (a) and (b), respectively. (c) ¹H reference image of two tubes containing [1-¹³C]acetate and [¹³C]urea. (f) ¹³C spectrum of the two tubes. (d, g) Urea and acetate ¹³C chemical-shift images obtained with CSI and SPEN acquisitions after suitable reconstruction, focusing on the labeled sites indicated in bold. (e, h) ¹³C chemical-shift images overlaid on the reference ¹H image. See text for further experimental details.

Figure 3

Measurements of signal decay following injection of hyperpolarized [1-¹³C]pyruvate into a tube using SPEN-based spectroscopic imaging. (a) Representative subset of pyruvate images acquired using the Hybrid SPEN sequence in Fig. 2a with flip angles α ∼ 15° (left) and ∼ 60° (right). (b) 1D profiles from the integrated images as a function of time, for the full train of repetitions.

Figure 4

Representative metabolic images from a rat following injection of hyperpolarized [1-¹³C]pyruvate, comparing the results obtained with CSI and single-shot spectrally-resolved experiments. (a) ¹³C metabolic images obtained with CSI and SPEN for lactate, alanine, pyruvate and bicarbonate (the images are scaled independently - SNR values in the dominant peak regions of each image are displayed). (b) Pyruvate and lactate ¹³C images overlaid on their corresponding ¹H anatomical images.

Figure 5

Representative ¹³C images acquired after injection of [1-¹³C]pyruvate in mice. (a, b) ¹H anatomic images overlaid with ¹³C images of pyruvate and lactate obtained using CSI and SPEN. The region indicated by “T” marks the approximate tumor location. (c) Comparison of the pyruvate and lactate time courses obtained with SPEN-based spectroscopic imaging and with 1D experiments. The SPEN time-course was obtained by averaging over a region of interest where the signal was most intense (from “T”). A flip angle of 20° and a repetition time of 2 s were used in both experiments.