Abstract
PurposeImaging tumor metabolism in vivo using hyperpolarized [1‐13C]pyruvate is a promising technique for detecting disease, monitoring disease progression, and assessing treatment response. However, the transient nature of the hyperpolarization and its depletion following excitation limits the available time for imaging. We describe here a single‐shot multi spin echo sequence, which improves on previously reported sequences, with a shorter readout time, isotropic point spread function (PSF), and better signal‐to‐noise ratio.MethodsThe sequence uses numerically optimized spectrally selective excitation pulses set to the resonant frequencies of pyruvate and lactate and a hyperbolic secant adiabatic refocusing pulse, all applied in the absence of slice selection gradients. The excitation pulses were designed to be resistant to the effects of B0 and B1 field inhomogeneity. The gradient readout uses a 3D cone trajectory composed of 13 cones, all fully refocused and distributed among 7 spin echoes. The maximal gradient amplitude and slew rate were set to 4 G/cm and 20 G/cm/ms, respectively, to demonstrate the feasibility of clinical translation.ResultsThe pulse sequence gave an isotropic PSF of 2.8 mm. The excitation profiles of the optimized pulses closely matched simulations and a 46.10 ± 0.04% gain in image SNR was observed compared to a conventional Shinnar–Le Roux excitation pulse. The sequence was demonstrated with dynamic imaging of hyperpolarized [1‐13C]pyruvate and [1‐13C]lactate in vivo.ConclusionThe pulse sequence was capable of dynamic imaging of hyperpolarized 13C labeled metabolites in vivo with relatively high spatial and temporal resolution and immunity to system imperfections.
Highlights
The principal limitation of the technique is the short lifetime of the spin hyperpolarization, which means that the signal is often observable for only a few minutes, requiring the use of very fast imaging methods.3,4 [1-13C]Pyruvate has been the most intensively investigated metabolite because of its central role in metabolism, the ease with which it can be hyperpolarized and its relatively fast membrane transport and subsequent metabolism when compared to the lifetime of the polarization.[5]
As well as being fast the imaging pulse sequence must make economical use of the polarization because each excitation pulse results in depletion of the polarization, in addition to that due to T1-dependent decay, degrading the signal-to-noise ratio (SNR) and decreasing the time window over which metabolism of the labelled substrate can be monitored
We have developed a single-shot multi echo sequence with a 3D cone gradient readout, which avoids the need for a spatially selective excitation pulse when the imaging field of view (FOV) exceeds the sensitive volume of the receiver coil
Summary
Dynamic nuclear spin polarization of isotopically labeled metabolites has proven to be a promising technique for dynamic magnetic resonance imaging measurements of metabolism in vivo.[1,2] The principal limitation of the technique is the short lifetime of the spin hyperpolarization, which means that the signal is often observable for only a few minutes, requiring the use of very fast imaging methods.3,4 [1-13C]Pyruvate has been the most intensively investigated metabolite because of its central role in metabolism, the ease with which it can be hyperpolarized and its relatively fast membrane transport and subsequent metabolism when compared to the lifetime of the polarization.[5]. Lau et al imaged hyperpolarized 13C-labeled pyruvate and bicarbonate in the heart with a rapid multislice sequence[14] and recently used a simultaneous multislice sequence for accelerated imaging over a larger field of view (FOV).[15] In the case of 3D acquisition strategies Miller et al introduced a 3D echo-planar imaging sequence[16] where the temporal resolution was traded for z-resolution. In order to improve the temporal resolution of 3D acquisitions Wang et al recently proposed metabolite selective single-shot spin echo-based sequences[17,18] whereas Chen et al used a 3D pulse sequence with echo planar spectroscopic imaging (EPSI) readout and compressed-sensing (CS).[19] The advantage of imaging approaches where a specific resonance is excited selectively is that they are faster than EPSI. Balanced steady state free precession imaging (bSSFP) can give higher SNR and efforts have been made to overcome the inherent frequency profile problems of this sequence.[20,21,22,23]
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