Dataset Information

Increasing the sensitivity of hyperpolarized [¹⁵ N₂ ]urea detection by serial transfer of polarization to spin-coupled protons.

ABSTRACT:

Purpose

Hyperpolarized ¹⁵ N-labeled molecules have been proposed as imaging agents for investigating tissue perfusion and pH. However, the sensitivity of direct ¹⁵ N detection is limited by the isotope's low gyromagnetic ratio. Sensitivity can be increased by transferring ¹⁵ N hyperpolarization to spin-coupled protons provided that there is not significant polarization loss during transfer. However, complete polarization transfer would limit the temporal window for imaging to the order of the proton T₁ (2-3 s). To exploit the long T₁ offered by storing polarization in ¹⁵ N and the higher sensitivity of ¹ H detection, we have developed a pulse sequence for partial polarization transfer.

Methods

A polarization transfer pulse sequence was modified to allow partial polarization transfer, as is required for dynamic measurements, and that can be implemented with inhomogeneous B₁ fields, as is often the case in vivo. The sequence was demonstrated with dynamic spectroscopy and imaging measurements with [¹⁵ N₂ ]urea.

Results

When compared to direct ¹⁵ N detection, the sequence increased the signal-to-noise ratio (SNR) by a factor of 1.72 ± 0.25, where both experiments depleted ~20% of the hyperpolarization (>10-fold when 100% of the hyperpolarization is used). Simulations with measured cross relaxation rates showed that this sequence gave up to a 50-fold increase in urea proton polarization when compared to spontaneous polarization transfer via cross relaxation.

Conclusion

The sequence gave an SNR increase that was close to the theoretical limit and can give a significant SNR benefit when compared to direct ¹³ C detection of hyperpolarized [¹³ C]urea.

SUBMITTER: Kreis F

PROVIDER: S-EPMC8629126 | biostudies-literature |

REPOSITORIES: biostudies-literature

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Project description:BackgroundHyperpolarized carbon-13 (HP-13C) MRI is a non-invasive imaging technique for probing brain metabolism, which may improve clinical cancer surveillance. This work aimed to characterize the consistency of serial HP-13C imaging in patients undergoing treatment for brain tumors and determine whether there is evidence of aberrant metabolism in the tumor lesion compared to normal-appearing tissue.MethodsSerial dynamic HP [1-13C]pyruvate MRI was performed on 3 healthy volunteers (6 total examinations) and 5 patients (21 total examinations) with diffuse infiltrating glioma during their course of treatment, using a frequency-selective echo-planar imaging (EPI) sequence. HP-13C imaging at routine clinical timepoints overlapped treatment, including radiotherapy (RT), temozolomide (TMZ) chemotherapy, and anti-angiogenic/investigational agents. Apparent rate constants for [1-13C]pyruvate conversion to [1-13C]lactate (kPL) and [13C]bicarbonate (kPB) were simultaneously quantified based on an inputless kinetic model within normal-appearing white matter (NAWM) and anatomic lesions defined from 1H MRI. The inter/intra-subject consistency of kPL-NAWM and kPB-NAWM was measured in terms of the coefficient of variation (CV).ResultsWhen excluding scans following anti-angiogenic therapy, patient values of kPL-NAWM and kPB-NAWM were 0.020 s-1 ± 23.8% and 0.0058 s-1 ± 27.7% (mean ± CV) across 17 HP-13C MRIs, with intra-patient serial kPL-NAWM/kPB-NAWM CVs ranging 6.8-16.6%/10.6-40.7%. In 4/5 patients, these values (0.018 s-1 ± 13.4% and 0.0058 s-1 ± 24.4%; n = 13) were more similar to those from healthy volunteers (0.018 s-1 ± 5.0% and 0.0043 s-1 ± 12.6%; n = 6) (mean ± CV). The anti-angiogenic agent bevacizumab was associated with global elevations in apparent rate constants, with maximum kPL-NAWM in 2 patients reaching 0.047 ± 0.001 and 0.047 ± 0.003 s-1 (±model error). In 3 patients with progressive disease, anatomic lesions showed elevated kPL relative to kPL-NAWM of 0.024 ± 0.001 s-1 (±model error) in the absence of gadolinium enhancement, and 0.032 ± 0.008, 0.040 ± 0.003 and 0.041 ± 0.009 s-1 with gadolinium enhancement. The lesion kPB in patients was reduced to unquantifiable values compared to kPB-NAWM.ConclusionSerial measures of HP [1-13C]pyruvate metabolism displayed consistency in the NAWM of healthy volunteers and patients. Both kPL and kPB were globally elevated following bevacizumab treatment, while progressive disease demonstrated elevated kPL in gadolinium-enhancing and non-enhancing lesions. Larger prospective studies with homogeneous patient populations are planned to evaluate metabolic changes following treatment.

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Increasing the sensitivity of hyperpolarized [¹⁵ N₂ ]urea detection by serial transfer of polarization to spin-coupled protons.

Purpose

Methods

Results

Conclusion

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Increasing the sensitivity of hyperpolarized [¹⁵ N₂ ]urea detection by serial transfer of polarization to spin-coupled protons.