Sampling Strategies for Hyperpolarized Carbon-13 Dynamic Imaging
In hyperpolarized C-13 imaging, the magnetization obtained is in a non-equilibrium state and decays exponentially towards thermal equilibrium based on the T1 relaxation and metabolic turnover rate. During imaging, RF excitation and T1 relaxation contributes to the irreversible decay of the hyperpolarized magnetization. Various RF excitation schemes have been designed to efficiently utilize magnetization for hyperpolarized metabolic imaging. In this work, we present optimal techniques for C-13 imaging by designing flip angles taking into consideration the total imaging acquisition time, repetition time, metabolism and the T1 relaxation to maximize the lactate signal acquisition from the available magnetization.
All simulations were done on Matlab. Verification of simulation results for the effect of total scan time and repetition time (TR) on the SNR was done with an excitation scheme defined by the optimal flip angle scheme. Images were acquired with a symmetric, ramp-sampled EPI readout. All scan parameters, other than the variable being verified (TR or total scan time) were kept constant (96 × 96 mm FOV, 32 × 32 matrix).
Optimal flip angle design is independent of the kinetic forward rate constant (kpl). Experimental data gives qualitatively similar images with varying TR and agree with total scan time simulations. An optimal study design is dictated by a total scan time window which is ~ 10s more than the relaxation time and a good T1 estimation.
Designing a flip angle scheme as a solution to maximize the substrate signal results in images with increased SNR images. The kpl value of tumors need not be considered when designing flip angles. Also, TR does not have a significant effect on the SNR. These results provide certain freedom to use various pulse sequences and acquisition schemes that require a range of TRs. A good approximation of the T1 relaxation time of the metabolite at the region of interest is important to define the total scan time and to ensure that maximum signal is obtained.