Abstract
The aim of this study was to acquire the transient MRI signal of hyperpolarized tracers and their metabolites efficiently, for which specialized imaging sequences are required. In this work, a multi-echo balanced steady-state free precession (me-bSSFP) sequence with Iterative Decomposition with Echo Asymmetry and Least squares estimation (IDEAL) reconstruction was implemented on a clinical 3 T positron-emission tomography/MRI system for fast 2D and 3D metabolic imaging. Simulations were conducted to obtain signal-efficient sequence protocols for the metabolic imaging of hyperpolarized biomolecules. The sequence was applied in vitro and in vivo for probing the enzymatic exchange of hyperpolarized [1-13 C]pyruvate and [1-13 C]lactate. Chemical shift resolution was achieved using a least-square, iterative chemical species separation algorithm in the reconstruction. In vitro, metabolic conversion rate measurements from me-bSSFP were compared with NMR spectroscopy and free induction decay-chemical shift imaging (FID-CSI). In vivo, a rat MAT-B-III tumor model was imaged with me-bSSFP and FID-CSI. 2D metabolite maps of [1-13 C]pyruvate and [1-13 C]lactate acquired with me-bSSFP showed the same spatial distributions as FID-CSI. The pyruvate-lactate conversion kinetics measured with me-bSSFP and NMR corresponded well. Dynamic 2D metabolite mapping with me-bSSFP enabled the acquisition of up to 420 time frames (scan time: 180-350 ms/frame) before the hyperpolarized [1-13 C]pyruvate was relaxed below noise level. 3D metabolite mapping with a large field of view (180 × 180 × 48 mm3 ) and high spatial resolution (5.6 × 5.6 × 2 mm3 ) was conducted with me-bSSFP in a scan time of 8.2 seconds. It was concluded that Me-bSSFP improves the spatial and temporal resolution for metabolic imaging of hyperpolarized [1-13 C]pyruvate and [1-13 C]lactate compared with either of the FID-CSI or EPSI methods reported at 3 T, providing new possibilities for clinical and preclinical applications.
Highlights
Nuclear magnetic resonance (NMR) is a versatile phenomenon that is used for chemical analysis and in vivo imaging alike
Metabolic conversion rate measurements from me-balanced steady-state free precision (bSSFP) were compared with NMR spectroscopy and free induction decay-chemical shift imaging (FID-CSI)
The results obtained from dynamic me-bSSFP28,29 and IDEAL26 reconstruction provided fast and very efficient imaging of HP pyruvate and lactate
Summary
Nuclear magnetic resonance (NMR) is a versatile phenomenon that is used for chemical analysis and in vivo imaging alike. There are sequences that acquire full spatial but minimal spectral resolution These methods include Dixon techniques, where only a few spectral points are acquired, for example, to produce one image of fat and one image of water.[14,15,16,17] Multi-point Dixon techniques use an iterative reconstruction method that allows the reconstruction of images at specific predefined chemical shifts.[18,19] Instead of acquiring full free induction decays (FIDs), this allows acquisition of fast spiral CSIs with few spectral points if prior knowledge about the resonance frequencies exists.[20,21,22,23] Several improvements have been suggested, including reducing the effect of T2*-relaxation,[24] multiple lipid resonances[25] and phase errors induced by bipolar gradients.[26]. The spatial and temporal resolution were substantially improved, such that observation of the metabolic conversion in one slice or 3D metabolite mapping became possible
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