Abstract
Single‐shot echo planar imaging (EPI), which allows an image to be acquired using a single excitation pulse, is used widely for imaging the metabolism of hyperpolarized 13C‐labelled metabolites in vivo as the technique is rapid and minimizes the depletion of the hyperpolarized signal. However, EPI suffers from Nyquist ghosting, which normally is corrected for by acquiring a reference scan. In a dynamic acquisition of a series of images, this results in the sacrifice of a time point if the reference scan involves a full readout train with no phase encoding. This time penalty is negligible if an integrated navigator echo is used, but at the cost of a lower signal‐to‐noise ratio (SNR) as a result of prolonged T 2* decay. We describe here a workflow for hyperpolarized 13C EPI that requires no reference scan. This involves the selection of a ghost‐containing background from a 13C image of a single metabolite at a single time point, the identification of phase correction coefficients that minimize signal in the selected area, and the application of these coefficients to images acquired at all time points and from all metabolites. The workflow was compared in phantom experiments with phase correction using a 13C reference scan, and yielded similar results in situations with a regular field of view (FOV), a restricted FOV and where there were multiple signal sources. When compared with alternative phase correction methods, the workflow showed an SNR benefit relative to integrated 13C reference echoes (>15%) or better ghost removal relative to a 1H reference scan. The residual ghosting in a slightly de‐shimmed B 0 field was 1.6% using the proposed workflow and 3.8% using a 1H reference scan. The workflow was implemented with a series of dynamically acquired hyperpolarized [1‐13C]pyruvate and [1‐13C]lactate images in vivo, resulting in images with no observable ghosting and which were quantitatively similar to images corrected using a 13C reference scan.
Highlights
Two‐dimensional (2D) spatial and one‐dimensional (1D) spectral information has been acquired using spatiotemporal encoding after a single excitation pulse,[22] and we recently proposed a single‐shot three‐dimensional (3D) sequence, which is based on a stack of spiral acquisitions.[23]
The 1D search method resulted in images of similar quality to those obtained using a 13C reference scan and integrated 13C reference echoes, with a large field of view (FOV), a restricted FOV and with multiple signal sources (Figure 3A)
The images were better than the images corrected using 1H reference data when the magnetic field homogeneity was degraded: residual ghosting was 1.4% under well‐shimmed and 3.8% under de‐shimmed conditions (Figure 4) in images corrected using 1H reference data
Summary
Dynamic nuclear polarization (DNP) can increase the signal intensities of 13C‐labelled molecules by a factor of 104–105, making possible the imaging of tissue metabolism in vivo.[1,2,3,4,5] For example, the imaging of hyperpolarized [1‐13C]pyruvate and its downstream metabolic products has Abbreviations used: 1D/2D/3D, one‐/two‐/three‐dimensional; CSI, chemical shift imaging; DNP, dynamic nuclear polarization; EPI, echo planar imaging; FOV, field of view; FSE, fast spin echo; MRI, magnetic resonance imaging; RF, radiofrequency; ROI, region of interest; SNR, signal‐to‐noise ratio; SpSp, spectral‐spatial; SSIM, structural similarity; TE, echo time. Fly‐back designs avoid bipolar readout gradients and misalignment between alternate k‐space lines[26]; they are more prone to geometric distortions and give a lower signal‐to‐noise ratio (SNR).[27]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
