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

Abstract

Purpose: Acquisition timing and B₁ calibration are two key factors that affect the quality and accuracy of hyperpolarized ¹³ C 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 ¹³ C 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 ¹³ C imaging was tested on healthy rats, tumor-bearing mice and a healthy volunteer on a clinical 3T scanner following hyperpolarized [1-¹³ C]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, k_PL ). 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 ¹³ C imaging.

Keywords: 13C; B1 mapping; Bloch-Siegert; Bolus tracking; Hyperpolarized; MRI; Pyruvate; Real-time; metabolic imaging.

Figure 1:

Overview of the proposed scheme. Bolus tracking starts before the bolus injection. Real-time center frequency calibration (‘CF Cal’) based on a slab FID is triggered upon ROI bolus arrival, while Bloch-Siegert B₁ mapping and real-time ROI B₁ calibration (‘B₁ Cal’) are triggered at the ROI bolus peak. The sequence triggered after B₁ calibration for most experiments in this study is alternate pyruvate/lactate dynamic imaging, and could be replaced by any hyperpolarized ¹³C sequence for other studies.

Figure 2:

Metabolite-specific imaging sequence used in this study, where all instances included a singleband spectral-spatial excitation pulse (passband 120 Hz, stopband 600 Hz) and a single-shot spiral readout. The sequence shown also includes an off-resonance Fermi pulse (T_RF = 12 ms, ω_RF = ±4.5 kHz) for Bloch-Siegert B1 mapping, while bolus tracking and pyruvate/lactate dynamic imaging also used this sequence but without Fermi pulse and its associated delay. Other key parameters for bolus tracking were FA 5° (pyruvate), TR 1s; and for Bloch-Siegert B1 mapping were FA 10° (pyruvate), TR 200ms.

Figure 3:

Validation of Bloch-Siegert B mapping on the ¹³C/¹H birdcage coil using a cylindrical ethylene glycol phantom (a-b) and on the ¹³C figure-8 transceiver coil using a cup filled with oil (c-e). For the latter experiment, the oil cup was placed on the top of the ¹³C coil. In both experiments, the desired B₁ of the Bloch-Siegert pulse was 0.3G, at which the excitation pulse would produce the theoretically accurate flip angle. (a) Axial ¹³C B₁ maps of birdcarge coil with 50%, 75%, 100% and 150% relative to the calibrated transmit power in pre-scan. (b) A plot of Bloch-Siegert phase difference versus relative transmit power. Each data point corresponds to the mean value of the phantom area of each B₁ image in(a). The data point at the relative power of zero is estimated to be zero. The quadratic curve is computed by a least-squares fitting. (c) Axial ¹³C image of transceiver coil with a nominal 180° flip angle. The dark band in the image corresponds to a 180° signal null. (d) Corresponding axial B₁ map of (c), where the dark band corresponds to ~0.3 G as expected. (e) A plot of B₁ value across the dark band and corresponding flip angles calculated based on the B₁ value.

Figure 4:

Results of a hyperpolarized [1-¹³C]pyruvate study in a normal rat using the proposed method (Fig. 1) with a birdcage coil. Real-time center frequency calibration was not performed in this study. The ROI for both bolus tracking and B₁ calibration was on the left kidney. Injection time was 8 s and initial power was purposely set to 120% of the calibrated power in pre-scan. Sequence parameters are presented in Table 1. (a) Proton localizer. (b) Normalized ¹³C B₁ map. Real-time B₁ scaling factor (~0.87) matched up with the initial transmit power (120%). (c) Normalized pyruvate signal curves in different ROIs. The ROI (left kidney) bolus peak was successfully detected. (d) ¹³C results displayed in the order of time. Every other timeframe is shown. The full set of images can be found in Sup. Fig. S1. Experiment recording: https://youtu.be/CN3mIrzmBT8.

Figure 5:

Results of a hyperpolarized [1-¹³C]pyruvate study in a TRAMP mouse using the proposed method (Fig. 1) with a birdcage coil. The ROI for both bolus tracking and B₁ calibration was the tumor. Injection time was 12 s and initial power was the same as the calibrated power in pre-scan. Sequence parameters are presented in Table 1. (a) Proton localizer. (b) Normalized ¹³C B₁ map. (c) Pyruvate-to-lactate conversion rate(k_PL) map with corrected flip angle. (d) Acquired frequency spectrum for center frequency calibration. “Center frequency” is abbreviated as “cf” in the gure. (e) ¹³C results displayed in the order of time. Every other timeframe is shown. The full set of images can be found in Sup. Fig. S2. Bolus tracking images before and after real-time center frequency calibration demonstrate that real-time center frequency calibration reduced off-resonance artifacts in real-time reconstructed images. Experiment recording: https://youtu.be/ViTDb3PzK3U.

Figure 6:

Results of two hyperpolarized [1-¹³C]pyruvate studies in a normal rat using the proposed method (Fig. 1) with a ¹³C surface transceiver coil. The two experiments were performed with the same parameters except for different tracking/calibrating ROIs: experiment #1 on right kidney and #2 was on left kidney, where the left kidney is closer to the coil. Injection time was 10 s and initial power was the same as the calibrated power in pre-scan. Sequence parameters are presented in Table 1. (a) Proton localizer. (b) Normalized ¹³C B₁ maps. B₁ maps acquired in two experiments are consistent. (c) Estimated k_PL with and without flip angle correction based on measured B₁ map. k_PL values for left kidney, right kidney and intestine are labeled in the maps. Using acquired B₁ maps to correct flip angle results in more consistent k_PL estimations of those ROIs between the two experiments, demonstrating the importance of flip angle correction for k_PL measurements. Experiment recordings: https://youtu.be/Mu3NW7Kog9M, https://youtu.be/fL5gVkPDw2o.

Figure 7:

Results of hyperpolarized [1-¹³C]pyruvate studies on a healthy human volunteer using the proposed method (Fig. 1) with in-house built birdcage transmit coil and 32-channel receive array. In this study, single-slice real-time B₁ calibration was triggered right after real-time center frequency calibration. The ROI for both bolus tracking and B₁ calibration was on the brain tissue near the superior sagittal sinus of slice 5 (see Fig. 8 for slice 1–8). A multi-slice 2D acquisition was used for pyruvate/lactate/bicarbonate dynamic imaging. Initial power was the same as the calibrated power in pre-scan. Sequence parameters are presented in Table 2. (a) Proton image of slice 5. (b) Normalized ¹³C B₁ maps. (c) Acquired frequency spectrum for center frequency calibration. (d) ¹³C results of slice 5 displayed in the order of time. Multi-slice dynamic images can be found in Sup. Fig. S3. Sum-over-time images can be found in Fig. 8. Experiment recordings: https://youtu.be/Oq36Z7ayQ0g.

Figure 8:

Sum of first twenty time points of multi-slice pyruvate/lactate/bicarbonate dynamic images as described in Fig. 7 and Sup. Fig. S3. Anatomical images are provided as coarse anatomical landmarks. Maximum SNRs of sum-over-time images for pyruvate, lactate and bicarbonate are 600, 58 and 19, respectively.