Synopsis

Keywords: Pulse Sequence Design, Non-Proton, Sodium, 2-Nuclei

Extensive evidence suggests abnormal metabolism, sodium homeostasis, and tumor acidity are interconnected and play critical roles in brain tumor formation, progression, seizure activity, treatment resistance, and immune suppression. While Na⁺ and advanced H⁺ -based MRI techniques can provide this information, Na⁺ and H⁺ images are traditionally acquired sequentially, resulting in a total scan time exceeding what is clinically reasonable. In the current study, we demonstrate utility of a new interleaved sodium gradient echo and proton-based pH-sensitive amine chemical exchange saturation transfer echoplanar imaging (Na⁺-GRE/H⁺-CEST-EPI) sequence in 15 minutes, making it feasible to study patients with brain tumors.

Purpose

Extensive in vitro, preclinical, and clinical evidence have shown that abnormal metabolism, sodium homeostasis, and tumor acidity are interconnected and play critical roles in brain tumor formation, progression, seizure activity, treatment resistance, and immune suppression. While Na⁺ and advanced H⁺-based MRI can provide this information, Na⁺ and H⁺ images are traditionally acquired sequentially, resulting in a total scan time (approximately 20 to 25 minutes) exceeding what is clinically reasonable. In the current study, we developed a new interleaved sodium gradient echo and proton-based pH-sensitive amine chemical exchange saturation transfer echoplanar imaging (Na⁺-GRE/H⁺-CEST-EPI) sequence in approximately 15 minutes, making it clinically and economically feasible, particularly to study patients with brain tumors.

Methods

Phantom Preparation: The phantom was composed 3 glycine phantom solutions (pH~6.00 with different [Na⁺] of ~1.00M, ~0.50M and ~0.25M) and 2 deionized water (pH=7.00 with different [Na⁺] of ~1.00M and ~0.50M), all stored in Nalgene 30 ml polypropylene bottles (Thermo Fisher Scientific, Waltham, MA, USA). The detailed composition, including Na⁺ concentration and measured pH of each sample were summarized in Table 1.
Table 1 Phantom solution composition

Sample	Volume	Phosphate	Glycine	K2HPO4/KH2PO4 Ratio	Measured pH	Na+
#1	50.00 mL	10.00 mM	20.00 mM	0.19	5.99	1.00 M
#2	50.00 mL	10.00 mM	20.00 mM	0.19	6.01	0.50 M
#3	50.00 mL	10.00 mM	20.00 mM	0.19	6.00	0.25 M
#4	50.00 mL	10.00 mM	20.00 mM	0.19	7.00	1.00 M
#5	50.00 mL	10.00 mM	20.00 mM	0.19	7.00	0.50 M

MRI Acquisition: MRI measurements were performed on a Siemens 3T scanner (MAGNETOM Prisma, Siemens Healthcare, Erlangen, Germany) with a custom multinuclear coil (1-channel Na⁺/1-channel H⁺, RAPID, Columbus OH). Na⁺ and pH-sensitive images were acquired by a custom sequence performing interleaved acquisitions of a Na⁺-GRE and H⁺-CEST-EPI (Figure 1). Acquisition parameters included field-of-view (FOV)=500×500mm, matrix size=128×128, slice thickness=2.8mm with 50% interslice gap, bandwidth=902Hz/Pixel, partial Fourier=6/8, TE/TR=4.0ms/6.6ms for Na⁺-GRE, and TE/TR=48ms/500ms for H⁺-CEST-EPI. CEST off-resonance saturation was applied using a pulse train of 3 × 100 ms Gaussian pulses with peak amplitude of 6µT [1]. A total of 29 CEST off-resonance frequencies were sampled at −3.5 to −2.5ppm, −0.3 to +0.3 ppm, and +2.5 to +3.5ppm, with increments of 0.1 ppm. Four reference S₀ scans were collected with the same acquisition parameters, without the saturation pulses. Additionally, high-resolution T1-weighted MPRAGE images were acquired for anatomic reference.

CEST Image Processing: Following the B₀ correction via a z-spectra based k-means clustering and Lorentzian fitting algorithm [2], an integral width of 0.4 ppm was taken around both the −3.0 and +3.0 ppm spectral points of the inhomogeneity-corrected data. These data points were combined with the S₀ image to calculate the asymmetry in the magnetization transfer ratio (MTR_asym) at 3.0 ppm as defined by equation MTR_asym(ω) = S(−ω)/S₀ − S(ω)/S₀, where ω is the offset frequency of interest (3.0 ppm). The signal-to-noise ratio (SNR) of each phantom sample was calculated using the division of the mean signal intensity over the standard deviation of signal intensity.

Results

The Na⁺-GRE sequence was interleaved with the H⁺-CEST-EPI sequence for each image slice (Figure 1). Figure 2 shows phantom results, demonstrating the ability to construct Na⁺ (33 averages) and pH-weighted CEST-EPI (MTR_asym at 3.0ppm) images simultaneously. The scan time for one slice of Na⁺ (33 averages) and pH-weighted CEST-EPI (MTR_asym at 3.0ppm) images is 37 seconds. Therefore, the total scan time for interleaving Na⁺ GRE and H⁺-CEST-EPI of 25 image slices is approximately 15 minutes. In contrast, besides shimming time, the total scan time for H⁺-CEST-EPI of 25 image slices only was approximately 7.5 minutes, and the total scan time for Na⁺-GRE of 25 image slices only was approximately 12 to 19 minutes. Using the interleaved Na⁺-GRE/H⁺-CEST-EPI sequence, the SNR of the CEST image for five phantom samples were 7.38, 6.34, 8.88, 8.82, and 7.84, respectively (7.87±1.05). In contrast, using the clinical H⁺-CEST-EPI sequence [1], the SNR of the CEST image for five phantom samples were 6.58, 7.86, 7.22, 11.26, and 4.34, respectively (7.46±2.51).

Discussion

Results demonstrate that an interleaved Na⁺-GRE/H⁺-CEST-EPI sequence allows for simultaneous acquisition of sodium- and pH-weighted molecular MR images. These images can be acquired in clinically reasonable scan times and with no added contrast or risk to patients. This innovative approach will potentially be useful for understanding brain tumor biology, including identifying unique and exploitable metabolic characteristics and evaluating new drugs that may impact or be influenced by tumor metabolism.

Acknowledgements

This project was funded by DOD CA200290 and the UCLA Sodium MRI Program. We also thank Dr. Nathanson's lab to provide the workspace for preparing phantom.