Optimizing SNR for multi-metabolite hyperpolarized carbon-13 MRI using a hybrid flip-angle scheme

Abstract

Purpose: To improve the SNR of hyperpolarized carbon-13 MRI of [1-¹³ C]pyruvate using a multispectral variable flip angle (msVFA) scheme in which the spectral profile and flip angle vary dynamically with time.

Methods: Each image acquisition in a time-resolved imaging experiment used a unique spectrally varying RF pulse shape for msVFA. Therefore, the flip angle for every acquisition was optimized for pyruvate and each of its metabolites to yield the highest SNR across the acquisition. Multispectral VFA was compared with a spectrally varying constant flip-angle excitation model through simulations and in vivo. A modified broadband chemical shift-encoded gradient-echo sequence was used for in vivo experiments on six pregnant guinea pigs. Regions of interest placed in the placentae, maternal liver, and maternal kidneys were used as areas for SNR measurement.

Results: In vivo experiments showed significant increases in SNR for msVFA relative to constant flip angle of up to 250% for multiple metabolites.

Conclusion: Hyperpolarized carbon-13 imaging with msVFA excitation produces improved SNR for all metabolites in organs of interest.

Keywords: RF pulse design; carbon-13; hyperpolarized MRI; metabolism; pyruvate.

Figure 1

The desired flip‐angle trajectories for each metabolite are shown for constant flip angle (CFA; top) and multispectral variable flip angle (msVFA; bottom) strategies. These flip angles were achieved using a double Gaussian RF pulse. Dotted vertical lines indicate the start time of each acquisition

Figure 2

Double Gaussian RF spectral profile used in each msVFA acquisition, centered on the pyruvate peak. The resonant frequency of each metabolite is indicated by vertical dashed lines. The magnitude and shape of the RF profile progressively changes with each acquisition to follow the msVFA trajectory

Figure 3

Mean SNR for each metabolite averaged over all placenta (A), maternal liver (B), and maternal kidney (C) regions of interest (ROIs). The mean SNR at each time point may be compared between msVFA (red squares) and CFA (blue circles) acquisitions. A, The increased SNR provided by the msVFA acquisition is apparent for all metabolites, except alanine. This is likely due to limited production of alanine in the placenta providing very little metabolite signal, which could be increased by the use of msVFA. B, The increased SNR provided by the msVFA acquisition is apparent for pyruvate and lactate, although there is limited improvement for alanine and bicarbonate. C, There appears to be an increased mean SNR using msVFA for all metabolites in maternal kidney ROIs; however, these trends were not found to be significant. Note that for all ROIs, the y‐axis SNR scale is larger for pyruvate due to a larger amount of signal from pyruvate compared with other metabolites

Figure 4

Typical hyperpolarized ¹³C metabolite images overlaid on axial T₁ of the same animal at 30 seconds after injection. Four pairs of images are shown here for each metabolite: magenta, pyruvate (top left); yellow, lactate (top right); green, alanine (bottom left); and cyan, bicarbonate (bottom right). For each pair of images, the image on the left was acquired using CFA and the image on the right acquired using msVFA. The image pairs have identical window and level for each metabolite. Placentae are indicated by blue arrows and outline, and fetal livers are indicated by red arrows and outline. A different slice is shown for alanine and bicarbonate, to show increased bicarbonate signal in the placenta using msVFA