A regional bolus tracking and real‐time B₁ calibration method for hyperpolarized ¹³C MRI

Abstract

Purpose Acquisition timing and B1 calibration are two key factors that affect the quality and accuracy of hyperpolarized 13C MRI. The goal of this project was to develop a new approach using regional bolus tracking to trigger Bloch‐Siegert B1 mapping and real‐time B1 calibration based on regional B1 measurements, followed by dynamic imaging of hyperpolarized 13C metabolites in vivo. Methods The proposed approach was implemented on a system which allows real‐time data processing and real‐time control on the sequence. Real‐time center frequency calibration upon the bolus arrival was also added. The feasibility of applying the proposed framework for in vivo hyperpolarized 13C imaging was tested on healthy rats, tumor‐bearing mice and a healthy volunteer on a clinical 3T scanner following hyperpolarized [1‐13C]pyruvate injection. Multichannel receive coils were used in the human study. Results Automatic acquisition timing based on either regional bolus peak or bolus arrival was achieved with the proposed framework. Reduced blurring artifacts in real‐time reconstructed images were observed with real‐time center frequency calibration. Real‐time computed B1 scaling factors agreed with real‐time acquired B1 maps. Flip angle correction using B1 maps results in a more consistent quantification of metabolic activity (i.e, pyruvate‐to‐lactate conversion, kPL). Experiment recordings are provided to demonstrate the real‐time actions during the experiment. Conclusions The proposed method was successfully demonstrated on animals and a human volunteer, and is anticipated to improve the efficient use of the hyperpolarized signal as well as the accuracy and robustness of hyperpolarized 13C imaging.

Investigation of analysis methods for hyperpolarized 13C‐pyruvate metabolic MRI in prostate cancer patients

“…The bias introduced by flip‐angle errors can be eliminated through B 1 mapping. Fast Bloch–Siegert B 1 mapping has been demonstrated for HP 13 C,() and more recently has been performed in real‐time during the HP experiment to minimize bias due to inaccurate B 1 calibration or unknown B 1 field variations.…”

Using bidirectional chemical exchange for improved hyperpolarized [13C]bicarbonate pH imaging

“…Second, transmit B 1 inhomogeneity can further lead to inaccuracy in pH measurement. To correct for these, pulse calibration and B 1 mapping could be performed on a 13 C thermal phantom before HP imaging, or B 1 mapping could potentially be performed during the HP experiment using a Bloch‐Siegert‐based approach in real time . Third, miscalibration of 180° pulse power in spin‐echo sequences can lead to signal losses in the second excited resonance via incomplete inversion.…”

A metabolite‐specific 3D stack‐of‐spiral bSSFP sequence for improved lactate imaging in hyperpolarized [1‐13C]pyruvate studies on a 3T clinical scanner

“…All acquisitions were started 6 seconds after the end of pyruvate injection. For each experiment, a 13 C frequency spectrum was acquired and real-time 13 C B 1 calibration 33 was performed right before metabolite acquisition.…”

“…13 C dynamic imaging was started 6 seconds after the bolus arrival in kidney which was monitored by a bolus tracking sequence. 33 Initial prescan frequency and power calibration were performed on 13 C urea phantom attached outside the receive coil, which was removed before pyruvate injection. Real-time 13 C frequency and power calibration 33 were performed on the renal tumor and triggered upon bolus arrival.…”

“…33 Initial prescan frequency and power calibration were performed on 13 C urea phantom attached outside the receive coil, which was removed before pyruvate injection. Real-time 13 C frequency and power calibration 33 were performed on the renal tumor and triggered upon bolus arrival. Proton anatomical reference was acquired with a four-channel paddle receive coil, using a 2D SSFSE sequence with FOV 38 × 38 cm, matrix size 512 × 512.…”