Frequency-specific SSFP for hyperpolarized ¹³C metabolic imaging at 14.1 T

Abstract

Metabolic imaging of hyperpolarized [1-(13)C] pyruvate co-polarized with [(13)C]urea by dynamic nuclear polarization with rapid dissolution is a promising new method for assessing tumor metabolism and perfusion simultaneously in vivo. Novel pulse sequences are required to enable dynamic imaging of multiple (13)C spectral lines with high spatiotemporal resolution. The goal of this study was to investigate a new frequency-specific approach for rapid metabolic imaging of multiple (13)C resonances using the spectral selectivity of steady-state free precession pulse (SSFP) trains. Methods developed in simulations were implemented in a dynamic frequency-cycled balanced SSFP pulse sequence on a 14.1-T animal magnetic resonance imaging scanner. This acquisition was tested in thermal and hyperpolarized phantom imaging studies and in a transgenic mouse with prostate cancer.

Figure 1

Determination of allowable TR’s for SSFP metabolic imaging of [1-¹³C] pyruvate. Plot shows the minimum avoidance of overlap between any deliberately excited spectral peak’s spectral response (among pyruvate, lactate, and urea) and any other significant present peak (pyruvate, pyruvate hydrate, lactate, and urea), as a function of TR.

Figure 2

Small flip angle SSFP response of hyperpolarized magnetization near resonance, for phantom (column 1) and in vivo (columns 2&3) scan parameters described in text. Transverse magnitude (row A) and phase (B) of hyperpolarized magnetization during pulse train, and remaining longitudinal component (C).On-resonance PSF along phase direction due to signal variation during pulse train (solid black line), as compared to ideal PSF (dotted red line) (D).Column 3 shows the broadened composite frequency profile obtained by complex weighted combination of three acquisitions at different center frequencies (0 Hz, −25 Hz, 25 Hz).

Figure 3

In vivo shimming conditions for prostatic region of TRAMP mouse, derived from high resolution B₀ maps. Mean ¹³C Larmor frequency offset across sample (A) andintravoxel standard deviation (B), in axial slice with 2.5×2.5×10mm³ voxels (same slice as hyperpolarized data in Figure 6).

Figure 4

Frequency-specific axial bSSFP images of [¹³C]urea (left) and [¹³C]lactate (right) syringes, created by moving the center frequency of the RF pulse. Respective horizontal signal profiles through syringe are also shown.

Figure 5

Dynamic frequency-specific bSSFP images of syringe containing 40 mM solution of co-polarized pyruvate (top) and urea (bottom), acquired every 3 seconds (left-to-right). Plots show image SNR (solid line) and exponential fits to the data (dotted red line).

Figure 6

Axial dynamic imaging of hyperpolarized ¹³C pyruvate, lactate, and urea in prostate tumor of TRAMP mouse, and plots of mean dynamic hyperpolarized signals within tumor region as identified on anatomic imaging.