Effects of excitation angle strategy on quantitative analysis of hyperpolarized pyruvate

Abstract

Purpose Various excitation strategies have been proposed for dynamic imaging of hyperpolarized agents such as [1‐13C]‐pyruvate, but the impact of these strategies on quantitative evaluation of signal evolution remains unclear. To better understand their relative performance, we compared the accuracy and repeatability of measurements made using variable excitation angle strategies and conventional constant excitation angle strategies. Methods Signal evolution for constant and variable excitation angle schedules was simulated using a pharmacokinetic model of hyperpolarized pyruvate with 2 chemical pools and 2 physical compartments. Noisy synthetic data were then fit using the same pharmacokinetic model with the apparent chemical exchange term as an unknown, and fit results were compared with simulation parameters to determine accuracy and reproducibility. Results Constant excitations and a variable excitation strategy that maximizes the HP lactate signal yielded data that supported quantitative analyses with similar accuracy and repeatability. Variable excitation angle strategies that were designed to produce a constant signal level resulted in lower signal and worse quantitative accuracy and repeatability, particularly for longer acquisition times. Conclusions These results suggest that either constant excitation angle or variable excitation angles that attempt to maximize total signal, as opposed to maintaining a constant signal level, are preferred for metabolic quantification using hyperpolarized pyruvate.

Three‐dimensional accelerated acquisition for hyperpolarized 13C MR with blipped stack‐of‐spirals and conjugate‐gradient SENSE

“…At this stage, the choice of flip angles was based on a numerical analysis, thereby representing a rough estimate of the optimal flip angle due to uncertainties of exact parameter values in the 2-compartment model. A more analytical approach might be preferred for future studies, possibly inspired by the recent results presented by Walker et al, 35 which support application of a constant excitation strategy that maximizes product signal. Pyruvate images were summed through all frames, whereas the products were summed from the fourth frame through the last.…”

Fast Imaging for Hyperpolarized MR Metabolic Imaging

“…When acquiring multiple timepoints to measure the conversion kinetics, variable flip angles can be designed to maximize total SNR or improve estimates of conversion rates 90 . The main challenge using variable flip angles is that they are more sensitive to the timing of the acquisition relative to the bolus characteristics and to inhomogeneities in the transmit B 1 field 96 . Thus, they will work best when paired with the advanced calibration methods described above.…”

“…This is made more challenging by the fact that each excitation, particularly those on the substrate, affects all downstream metabolite measurements as well as later timepoints. In kinetic modeling, arbitrary flip angle and acquisition timings can be incorporated using a hybrid discrete‐continuous model 90,92,96,97 …”

Slice profile effects on quantitative analysis of hyperpolarized pyruvate

“…This approach models excitation losses as a discrete event. 30 The total signal observed after the n th excitation is a weighted combination of signals arising from vascular and extravascular spaces:…”

“…Finally, the signal curves were fit by the conventional pharmacokinetic model assuming uniform excitation across the slice profile, or using the slice-correction method. 30 A least-squares method was used to fit simulated data to the pharmacokinetic model and determine the amplitude scale factor for the vascular input function and the apparent rate of conversion from HP pyruvate to HP lactate (k pl ). All other parameters were assumed to be known as the values used during simulation.…”