Synopsis

For hyperpolarized ¹³C MRI acquisitions aimed at metabolic rate constant estimation, the Bloch-Siegert shift enables encoding of the transmit field (B₁⁺-field) amplitude within a single hyperpolarized substrate injection. This ability is needed since most clinical hyperpolarized MRI studies use inhomogeneous transmit coils, and because kinetic modeling based on incorrect flip angles can lead to incorrect rate constant estimations. This study demonstrates the feasibility of integrated B₁⁺ mapping for large volume thermal and hyperpolarized phantoms in a clinical setup using a clamshell transmit coil and a 16-channel receive array, and a 3D stack-of-spirals sequence. Phase-sensitive coil-combination was achieved using ESPIRiT.

Introduction

Methods

Bloch-Siegert B₁⁺ mapping was integrated in a 3D stack-of-spirals sequence with singleband spectral-spatial (SPSP) excitation. By alternating between positive and negative off-resonance Bloch-Siegert pulses through the full dynamic acquisition, B₁⁺ mapping is possible across multiple time points. This makes the method robust against spatially varying signal support over time; expected in vivo.

All data were acquired on an MR750 3T scanner (GE Healthcare, Waukesha, WI, USA) with a ¹³C clamshell coil for excitation and a 16-channel coil for reception (Rapid Biomedical, Rimpar, Germany). See coil setup in Figure 1.

Multi-compartment thermal phantom: Image data were acquired for a 32x32x12 cm³ FOV with a 30-ms single-shot-spiral readout in the axial plane and 10 phase-encodes (superior/inferior). Two measurements were conducted with 4-ms Bloch-Siegert pulses applied +/-2 kHz off-resonance. Other scan parameters: TR 1 s; flip angle 70°; NEX 32; SPSP frequencies four (Ethylene Glycol, Urea, Alanine, Acetate (∆f = [0,3177,3598,3803] Hz)). The spirals were gridded to a 54x54 matrix and filtered to result in an isotropic resolution of 1.2 cm. The data were coil-combined using ESPIRiT for each 3D volume, across frequencies and time points, before estimating B₁⁺ maps as previously described.¹

Hyperpolarized [1-¹³C]pyruvate phantom: 1.47 g of [1-¹³C]pyruvic acid doped with 15 mM electron paramagnetic agent was polarized for 3 hours in a 5T Spinlab at 0.78 K. The sample was rapidly dissolved and neutralized yielding 42 mL pyruvate (259 mM, 44 % polarization). This was added to a sphere containing ~2.5 L water. Imaging started 43 s after injection. The sequence was run three times with 16 time points for each run and a 20-s and 10-s delay between runs. Other scan parameters: TR 200 ms; flip angle 5°. FOV, spiral, Bloch-Siegert pulse, and reconstruction method were identical to those for the thermal phantom experiment. Due to long relaxation times and the short TR, the measured signal depends critically on T₁, T₂, B₁ and B₀, so fitting of the measured signal to a signal equation was not attempted. This could have provided validation for the B₁⁺ maps.

Results and Discussion

Figure 2(b-c) illustrate how coil-combination by ESPIRiT results in similar or higher SNR levels compared to sum-of-squares reconstruction. Figure 3 shows the B₁⁺ mapping results together with a simulated B₁⁺-field for the clamshell transmitter. All maps show the same pattern. The slightly smaller inhomogeneity of the simulated map compared to the Bloch-Siegert maps is likely explained by a geometry symmetry assumption for the simulation, while the B₁⁺ field of the clamshell coil has experimentally been shown to be asymmetric as visible in Figure 3(a).

Figure 4 shows 24 B₁⁺ maps extracted from the 24 pairs of consecutive time points of the hyperpolarized experiment. Ideally these would be identical, but as SNR decreases so does reliability of B₁⁺ mapping. In the higher SNR region, flip angles were estimated consistently up to 100 s after acquisition start (70 s active scan time), while in low SNR regions estimates became less consistent after 40 s active scan time. A minimum SNR of ~40 is needed for accurate B₁⁺ estimation. Figure 5 shows scatter plots for the voxel-wise relative flip angle values across experiments. A higher linear correlation was observed for high relative flip angle voxels compared to voxels with low relative flip angle; as these voxels also had low SNR, this is not unexpected.

Conclusion

Acknowledgements

References

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