Metabolic MRI with hyperpolarized [1-13C]pyruvate separates benign oligemia from infarcting penumbra in porcine stroke

Acute ischemic stroke patients benefit from reperfusion in a short time-window after debut. Later treatment may be indicated if viable brain tissue is demonstrated and this outweighs the inherent risks of late reperfusion. Magnetic resonance imaging (MRI) with hyperpolarized [1-13C]pyruvate is an emerging technology that directly images metabolism. Here, we investigated its potential to detect viable tissue in ischemic stroke. Stroke was induced in pigs by intracerebral injection of endothelin 1. During ischemia, the rate constant of pyruvate-to-lactate conversion, kPL, was 52% larger in penumbra and 85% larger in the infarct compared to the contralateral hemisphere (P = 0.0001). Within the penumbra, the kPL was 50% higher in the regions that later infarcted compared to non-progressing regions (P = 0.026). After reperfusion, measures of pyruvate-to-lactate conversion were slightly decreased in the infarct compared to contralateral. In addition to metabolic imaging, we used hyperpolarized pyruvate for perfusion-weighted imaging. This was consistent with conventional imaging for assessment of infarct size and blood flow. Lastly, we confirmed the translatability of simultaneous assessment of metabolism and perfusion with hyperpolarized MRI in healthy volunteers. In conclusion, hyperpolarized [1-13C]pyruvate may aid penumbral characterization and increase access to reperfusion therapy for late presenting patients.

Current human brain applications and challenges of dynamic hyperpolarized carbon-13 labeled pyruvate MR metabolic imaging

The ability of hyperpolarized carbon-13 MR metabolic imaging to acquire dynamic metabolic information in real time is crucial to gain mechanistic insights into metabolic pathways, which are complementary to anatomic and other functional imaging methods. This review presents the advantages of this emerging functional imaging technology, describes considerations in clinical translations, and summarizes current human brain applications. Despite rapid development in methodologies, significant technological and physiological related challenges continue to impede broader clinical translation.

Clinical translation of hyperpolarized 13 C pyruvate and urea MRI for simultaneous metabolic and perfusion imaging

Purpose

The combined hyperpolarized (HP) ¹³C pyruvate and urea MRI has provided a simultaneous assessment of glycolytic metabolism and tissue perfusion for improved cancer diagnosis and therapeutic evaluation in preclinical studies. This work aims to translate this dual‐probe HP imaging technique to clinical research.

Methods

A co‐polarization system was developed where [1‐¹³C]pyruvic acid (PA) and [¹³C, ¹⁵N₂]urea in water solution were homogeneously mixed and polarized on a 5T SPINlab system. Physical and chemical characterizations and toxicology studies of the combined probe were performed. Simultaneous metabolic and perfusion imaging was performed on a 3T clinical MR scanner by alternatively applying a multi‐slice 2D spiral sequence for [1‐¹³C]pyruvate and its downstream metabolites and a 3D balanced steady‐state free precession (bSSFP) sequence for [¹³C, ¹⁵N₂]urea.

Results

The combined PA/urea probe has a glass‐formation ability similar to neat PA and can generate nearly 40% liquid‐state ¹³C polarization for both pyruvate and urea in 3‐4 h. A standard operating procedure for routine on‐site production was developed and validated to produce 40 mL injection product of approximately 150 mM pyruvate and 35 mM urea. The toxicology study demonstrated the safety profile of the combined probe. Dynamic metabolite‐specific imaging of [1‐¹³C]pyruvate, [1‐¹³C]lactate, [1‐¹³C]alanine, and [¹³C, ¹⁵N₂]urea was achieved with adequate spatial (2.6 mm × 2.6 mm) and temporal resolution (4.2 s), and urea images showed reduced off‐resonance artifacts due to the J_CN coupling.

Conclusion

The reported technical development and translational studies will lead to the first‐in‐human dual‐agent HP MRI study and mark the clinical translation of the first HP ¹³C MRI probe after pyruvate.

Specialized computational methods for denoising, B1 correction, and kinetic modeling in hyperpolarized 13 C MR EPSI studies of liver tumors

PURPOSE

To develop a novel post-processing pipeline for hyperpolarized (HP) ¹³ C MRSI that integrates tensor denoising and B 1 + correction to measure pyruvate-to-lactate conversion rates (k_PL ) in patients with liver tumors.

METHODS

Seven HP ¹³ C MR scans of progressing liver tumors were acquired using a custom ¹³ C surface transmit/receive coil and the echo-planar spectroscopic imaging (EPSI) data analysis included B₀ correction, tensor rank truncation, and zero- and first-order phase corrections to recover metabolite signals that would otherwise be obscured by spectral noise as well as a correction for inhomogeneous transmit ( B 1 + ) using a B 1 + map aligned to the coil position for each patient scan. Processed HP data and corrected flip angles were analyzed with an inputless two-site exchange model to calculate k_PL .

RESULTS

Denoising averages SNR increases of pyruvate, lactate, and alanine were 37.4-, 34.0-, and 20.1-fold, respectively, with lactate and alanine dynamics most noticeably recovered and better defined. In agreement with Monte Carlo simulations, over-flipped regions underestimated k_PL and under-flipped regions overestimated k_PL . B 1 + correction addressed this issue.

CONCLUSION

The new HP ¹³ C EPSI post-processing pipeline integrated tensor denoising and B 1 + correction to measure k_PL in patients with liver tumors. These technical developments not only recovered metabolite signals in voxels that did not receive the prescribed flip angle, but also increased the extent and accuracy of k_PL estimations throughout the tumor and adjacent regions including normal-appearing tissue and additional lesions.

Denoising of hyperpolarized 13 C MR images of the human brain using patch-based higher-order singular value decomposition

PURPOSE

To improve hyperpolarized ¹³ C (HP-¹³ C) MRI by image denoising with a new approach, patch-based higher-order singular value decomposition (HOSVD).

METHODS

The benefit of using a patch-based HOSVD method to denoise dynamic HP-¹³ C MR imaging data was investigated. Image quality and the accuracy of quantitative analyses following denoising were evaluated first using simulated data of [1-¹³ C]pyruvate and its metabolic product, [1-¹³ C]lactate, and compared the results to a global HOSVD method. The patch-based HOSVD method was then applied to healthy volunteer HP [1-¹³ C]pyruvate EPI studies. Voxel-wise kinetic modeling was performed on both non-denoised and denoised data to compare the number of voxels quantifiable based on SNR criteria and fitting error.

RESULTS

Simulation results demonstrated an 8-fold increase in the calculated SNR of [1-¹³ C]pyruvate and [1-¹³ C]lactate with the patch-based HOSVD denoising. The voxel-wise quantification of k_PL (pyruvate-to-lactate conversion rate) showed a 9-fold decrease in standard errors for the fitted k_PL after denoising. The patch-based denoising performed superior to the global denoising in recovering k_PL information. In volunteer data sets, [1-¹³ C]lactate and [¹³ C]bicarbonate signals became distinguishable from noise across captured time points with over a 5-fold apparent SNR gain. This resulted in >3-fold increase in the number of voxels quantifiable for mapping k_PB (pyruvate-to-bicarbonate conversion rate) and whole brain coverage for mapping k_PL .

CONCLUSIONS

Sensitivity enhancement provided by this denoising significantly improved quantification of metabolite dynamics and could benefit future studies by improving image quality, enabling higher spatial resolution, and facilitating the extraction of metabolic information for clinical research.

Background-free dual-mode optical and 13C magnetic resonance imaging in diamond particles

Multimodal imaging-the ability to acquire images of an object through more than one imaging mode simultaneously-has opened additional perspectives in areas ranging from astronomy to medicine. In this paper, we report progress toward combining optical and magnetic resonance (MR) imaging in such a "dual" imaging mode. They are attractive in combination because they offer complementary advantages of resolution and speed, especially in the context of imaging in scattering environments. Our approach relies on a specific material platform, microdiamond particles hosting nitrogen vacancy (NV) defect centers that fluoresce brightly under optical excitation and simultaneously "hyperpolarize" lattice [Formula: see text] nuclei, making them bright under MR imaging. We highlight advantages of dual-mode optical and MR imaging in allowing background-free particle imaging and describe regimes in which either mode can enhance the other. Leveraging the fact that the two imaging modes proceed in Fourier-reciprocal domains (real and k-space), we propose a sampling protocol that accelerates image reconstruction in sparse-imaging scenarios. Our work suggests interesting possibilities for the simultaneous optical and low-field MR imaging of targeted diamond nanoparticles.

Hyperpolarized 13 C MRI data acquisition and analysis in prostate and brain at University of California, San Francisco

Based on the expanding set of applications for hyperpolarized carbon-13 (HP-¹³ C) MRI, this work aims to communicate standardized methodology implemented at the University of California, San Francisco, as a primer for conducting reproducible metabolic imaging studies of the prostate and brain. Current state-of-the-art HP-¹³ C acquisition, data processing/reconstruction and kinetic modeling approaches utilized in patient studies are presented together with the rationale underpinning their usage. Organized around spectroscopic and imaging-based methods, this guide provides an extensible framework for handling a variety of HP-¹³ C applications, which derives from two examples with dynamic acquisitions: 3D echo-planar spectroscopic imaging of the human prostate and frequency-specific 2D multislice echo-planar imaging of the human brain. Details of sequence-specific parameters and processing techniques contained in these examples should enable investigators to effectively tailor studies around individual-use cases. Given the importance of clinical integration in improving the utility of HP exams, practical aspects of standardizing data formats for reconstruction, analysis and visualization are also addressed alongside open-source software packages that enhance institutional interoperability and validation of methodology. To facilitate the adoption and further development of this methodology, example datasets and analysis pipelines have been made available in the supporting information.

Metabolic imaging with hyperpolarized 13 C pyruvate magnetic resonance imaging in patients with renal tumors-Initial experience

BACKGROUND

Optimal treatment selection for localized renal tumors is challenging because of their variable biologic behavior and limitations in the preoperative assessment of tumor aggressiveness. The authors investigated the emerging hyperpolarized (HP) ¹³ C magnetic resonance imaging (MRI) technique to noninvasively assess tumor lactate production, which is strongly associated with tumor aggressiveness.

METHODS

Eleven patients with renal tumors underwent HP ¹³ C pyruvate MRI before surgical resection. Tumor ¹³ C pyruvate and ¹³ C lactate images were acquired dynamically. Five patients underwent 2 scans on the same day to assess the intrapatient reproducibility of HP ¹³ C pyruvate MRI. Tumor metabolic data were compared with histopathology findings.

RESULTS

Eight patients had tumors with a sufficient metabolite signal-to-noise ratio for analysis; an insufficient tumor signal-to-noise ratio was noted in 2 patients, likely caused by poor tumor perfusion and, in 1 patient, because of technical errors. Of the 8 patients, 3 had high-grade clear cell renal cell carcinoma (ccRCC), 3 had low-grade ccRCC, and 2 had chromophobe RCC. There was a trend toward a higher lactate-to-pyruvate ratio in high-grade ccRCCs compared with low-grade ccRCCs. Both chromophobe RCCs had relatively high lactate-to-pyruvate ratios. Good reproducibility was noted across the 5 patients who underwent 2 HP ¹³ C pyruvate MRI scans on the same day.

CONCLUSIONS

The current results demonstrate the feasibility of HP ¹³ C pyruvate MRI for investigating the metabolic phenotype of localized renal tumors. The initial data indicate good reproducibility of metabolite measurements. In addition, the metabolic data indicate a trend toward differentiating low-grade and high-grade ccRCCs, the most common subtype of renal cancer.

LAY SUMMARY

Renal tumors are frequently discovered incidentally because of the increased use of medical imaging, but it is challenging to identify which aggressive tumors should be treated. A new metabolic imaging technique was applied to noninvasively predict renal tumor aggressiveness. The imaging results were compared with tumor samples taken during surgery and showed a trend toward differentiating between low-grade and high-grade clear cell renal cell carcinomas, which are the most common type of renal cancers.

Resistance to Androgen Deprivation Leads to Altered Metabolism in Human and Murine Prostate Cancer Cell and Tumor Models

Currently, no clinical methods reliably predict the development of castration-resistant prostate cancer (CRPC) that occurs almost universally in men undergoing androgen deprivation therapy. Hyperpolarized (HP) ¹³C magnetic resonance imaging (MRI) could potentially detect the incipient emergence of CRPC based on early metabolic changes. To characterize metabolic shifts occurring upon the transition from androgen-dependent to castration-resistant prostate cancer (PCa), the metabolism of [U-¹³C]glucose and [U-¹³C]glutamine was analyzed by nuclear magnetic resonance spectroscopy. Comparison of steady-state metabolite concentrations and fractional enrichment in androgen-dependent LNCaP cells and transgenic adenocarcinoma of the murine prostate (TRAMP) murine tumors versus castration-resistant PC-3 cells and treatment-driven CRPC TRAMP tumors demonstrated that CRPC was associated with upregulation of glycolysis, tricarboxylic acid metabolism of pyruvate; and glutamine, glutaminolysis, and glutathione synthesis. These findings were supported by ¹³C isotopomer modeling showing increased flux through pyruvate dehydrogenase (PDH) and anaplerosis; enzymatic assays showing increased lactate dehydrogenase, PDH and glutaminase activity; and oxygen consumption measurements demonstrating increased dependence on anaplerotic fuel sources for mitochondrial respiration in CRPC. Consistent with ex vivo metabolomic studies, HP [1-¹³C]pyruvate distinguished androgen-dependent PCa from CRPC in cell and tumor models based on significantly increased HP [1-¹³C]lactate.

Tumor Metabolism and Neurocognition in CNS Lymphoma

BACKGROUND

The mechanistic basis for neurocognitive deficits in CNS lymphoma and other brain tumors is incompletely understood. We tested the hypothesis that tumor metabolism impairs neurotransmitter pathways and neurocognitive function.

METHODS

We performed serial cerebrospinal fluid (CSF) metabolomic analyses using liquid chromatography-electrospray tandem mass spectrometry to evaluate changes in the tumor microenvironment in 14 patients with recurrent CNS lymphoma, focusing on 18 metabolites involved in neurotransmission and bioenergetics. These were paired with serial mini-mental state examinations (MMSE) and MRI studies for tumor volumetric analyses. Patients were analyzed in the setting of the phase I trial of lenalidomide/rituximab. Associations were assessed by Pearson and Spearman correlation coefficient. Generalized estimating equation (gee) models were also established, adjusting for within-subject repeated measures.

RESULTS

Of 18 metabolites, elevated CSF lactate correlated most strongly with lower MMSE score (p<8E-8, rho=-0.67). High lactate was associated with lower GABA, higher glutamate/GABA ratio and dopamine. Conversely, high succinate correlated with higher MMSE score. Serial analysis demonstrated a reproducible, time-dependent, reciprocal correlation between changes in lactate and GABA concentrations. While high lactate and low GABA correlated with tumor contrast enhancing volume, they correlated more significantly with lower MMSE scores than tumor volumes.

CONCLUSIONS

We provide evidence that lactate production and Warburg metabolism may impact neurotransmitter dysregulation and neurocognition in CNS lymphomas. We identify novel metabolomic biomarkers that may be applied in future studies of neurocognition in CNS lymphomas. Elucidation of mechanistic interactions between lymphoma metabolism, neurotransmitter imbalance and neurocognition may promote interventions that preserve cognitive function.

Pilot Study of Hyperpolarized 13C Metabolic Imaging in Pediatric Patients with Diffuse Intrinsic Pontine Glioma and Other CNS Cancers

BACKGROUND AND PURPOSE

Pediatric CNS tumors commonly present challenges for radiographic interpretation on conventional MR imaging. This study sought to investigate the safety and tolerability of hyperpolarized carbon-13 (HP-¹³C) metabolic imaging in pediatric patients with brain tumors.

MATERIALS AND METHODS

Pediatric patients 3 to 18 years of age who were previously diagnosed with a brain tumor and could undergo MR imaging without sedation were eligible to enroll in this safety study of HP [1-¹³C]pyruvate. Participants received a one-time injection of HP [1-¹³C]pyruvate and were imaged using dynamic HP-¹³C MR imaging. We assessed 2 dose levels: 0.34 mL/kg and the highest tolerated adult dose of 0.43 mL/kg. Participants were monitored throughout imaging and for 60 minutes postinjection, including pre- and postinjection electrocardiograms and vital sign measurements.

RESULTS

Between February 2017 and July 2019, ten participants (9 males; median age, 14 years; range, 10-17 years) were enrolled, of whom 6 completed injection of HP [1-¹³C]pyruvate and dynamic HP-¹³C MR imaging. Four participants failed to undergo HP-¹³C MR imaging due to technical failures related to generating HP [1-¹³C]pyruvate or MR imaging operability. HP [1-¹³C]pyruvate was well-tolerated in all participants who completed the study, with no dose-limiting toxicities or adverse events observed at either 0.34 (n = 3) or 0.43 (n = 3) mL/kg. HP [1-¹³C]pyruvate demonstrated characteristic conversion to [1-¹³C]lactate and [¹³C]bicarbonate in the brain. Due to poor accrual, the study was closed after only 3 participants were enrolled at the highest dose level.

CONCLUSIONS

Dynamic HP-¹³C MR imaging was safely performed in 6 pediatric patients with CNS tumors and demonstrated HP [1-¹³C]pyruvate brain metabolism.

Tensor image enhancement and optimal multichannel receiver combination analyses for human hyperpolarized 13 C MRSI

PURPOSE

With the initiation of human hyperpolarized ¹³ C (HP-¹³ C) trials at multiple sites and the development of improved acquisition methods, there is an imminent need to maximally extract diagnostic information to facilitate clinical interpretation. This study aims to improve human HP-¹³ C MR spectroscopic imaging through means of Tensor Rank truncation-Image enhancement (TRI) and optimal receiver combination (ORC).

METHODS

A data-driven processing framework for dynamic HP ¹³ C MR spectroscopic imaging (MRSI) was developed. Using patient data sets acquired with both multichannel arrays and single-element receivers from the brain, abdomen, and pelvis, we examined the theory and application of TRI, as well as 2 ORC techniques: whitened singular value decomposition (WSVD) and first-point phasing. Optimal conditions for TRI were derived based on bias-variance trade-off.

RESULTS

TRI and ORC techniques together provided a 63-fold mean apparent signal-to-noise ratio (aSNR) gain for receiver arrays and a 31-fold gain for single-element configurations, which particularly improved quantification of the lower-SNR-[¹³ C]bicarbonate and [1-¹³ C]alanine signals that were otherwise not detectable in many cases. Substantial SNR enhancements were observed for data sets that were acquired even with suboptimal experimental conditions, including delayed (114 s) injection (8× aSNR gain solely by TRI), or from challenging anatomy or geometry, as in the case of a pediatric patient with brainstem tumor (597× using combined TRI and WSVD). Improved correlation between elevated pyruvate-to-lactate conversion, biopsy-confirmed cancer, and mp-MRI lesions demonstrated that TRI recovered quantitative diagnostic information.

CONCLUSION

Overall, this combined approach was effective across imaging targets and receiver configurations and could greatly benefit ongoing and future HP ¹³ C MRI research through major aSNR improvements.

A variable resolution approach for improved acquisition of hyperpolarized 13 C metabolic MRI

PURPOSE

To ameliorate tradeoffs between a fixed spatial resolution and signal-to-noise ratio (SNR) for hyperpolarized 13 C MRI.

METHODS

In MRI, SNR is proportional to voxel volume but retrospective downsampling or voxel averaging only improves SNR by the square root of voxel size. This can be exploited with a metabolite-selective imaging approach that independently encodes each compound, yielding high-resolution images for the injected substrate and coarser resolution images for downstream metabolites, while maintaining adequate SNR for each. To assess the efficacy of this approach, hyperpolarized [1-13 C]pyruvate data were acquired in healthy Sprague-Dawley rats (n = 4) and in two healthy human subjects.

RESULTS

Compared with a constant resolution acquisition, variable-resolution data sets showed improved detectability of metabolites in pre-clinical renal studies with a 3.5-fold, 8.7-fold, and 6.0-fold increase in SNR for lactate, alanine, and bicarbonate data, respectively. Variable-resolution data sets from healthy human subjects showed cardiac structure and neuro-vasculature in the higher resolution pyruvate images (6.0 × 6.0 mm2 for cardiac and 7.5 × 7.5 mm2 for brain) that would otherwise be missed due to partial-volume effects and illustrates the level of detail that can be achieved with hyperpolarized substrates in a clinical setting.

CONCLUSION

We developed a variable-resolution strategy for hyperpolarized 13 C MRI using metabolite-selective imaging and demonstrated that it mitigates tradeoffs between a fixed spatial resolution and SNR for hyperpolarized substrates, providing both high resolution pyruvate and coarse resolution metabolite data sets in a single exam. This technique shows promise to improve future studies by maximizing metabolite SNR while minimizing partial-volume effects from the injected substrate.

Slice profile effects on quantitative analysis of hyperpolarized pyruvate

Magnetic resonance imaging of hyperpolarized pyruvate provides a new imaging biomarker for cancer metabolism, based on the dynamic in vivo conversion of hyperpolarized pyruvate to lactate. Methods for quantification of signal evolution need to be robust and reproducible across a range of experimental conditions. Pharmacokinetic analysis of dynamic spectroscopic imaging data from hyperpolarized pyruvate and its metabolites generally assumes that signal arises from ideal rectangular slice excitation profiles. In this study, we examined whether this assumption could lead to bias in kinetic analysis of hyperpolarized pyruvate and, if so, whether such a bias can be corrected. A Bloch-McConnell simulator was used to generate synthetic data using a known set of "ground truth" pharmacokinetic parameter values. Signal evolution was then analyzed using analysis software that either assumed a uniform slice profile, or incorporated information about the slice profile into the analysis. To correct for slice profile effects, the expected slice profile was subdivided into multiple sub-slices to account for variable excitation angles along the slice dimension. An ensemble of sub-slices was then used to fit the measured signal evolution. A mismatch between slice profiles used for data acquisition and those assumed during kinetic analysis was identified as a source of quantification bias. Results indicate that imperfect slice profiles preferentially increase detected lactate signal, leading to an overestimation of the apparent metabolic exchange rate. The slice profile-correction algorithm was tested in simulation, in phantom measurements, and applied to data acquired from a patient with prostate cancer. The results demonstrated that slice profile-induced biases can be minimized by accounting for the slice profile during pharmacokinetic analysis. This algorithm can be used to correct data from either single or multislice acquisitions.

55 Mn-based fiducial markers for rapid and automated RF coil localization for hyperpolarized 13 C MRI

PURPOSE

To use fiducial markers containing manganese 55 to rapidly localize carbon 13 (¹³ C) RF coils for correcting images for B₁ variation.

METHODS

Hollow high-density polyethylene spheres were filled with 3M sodium permanganate and affixed to a rectangular ¹³ C-tuned RF coil. The relative positions of the markers and coil conductors were mapped using CT. Marker positions were measured by MRI using a series of 1D projections and automated peak detection. Once the coil location was determined, coil sensitivity was estimated using a quasi-static calculation. Simulations were performed to determine the minimum number of projections required for robust localization. Phantom experiments were used to confirm the accuracy of marker localization as well as the calculated coil sensitivity. Finally, in vivo validation was performed using hyperpolarized ¹³ C pyruvate in a rat model.

RESULTS

In simulations, our algorithm was accurate in determining marker positions when at least 6 projections were used (RMSE 1.4 ± 0.9 mm). These estimates were verified in phantom experiments, where markers locations were determined with an RMS accuracy of 1.3 mm. A minimum SNR of 4 was required for automated detection to perform accurately. Computed coil sensitivity had a median error of 17% when taken over the entire measured area and 5.7% over a central region. In a rat, correction for nonuniform reception and flip angle was able to normalize the signals arising from asymmetrically positioned kidneys.

CONCLUSION

Manganese 55 fiducial markers are an inexpensive and reliable method for rapidly localizing ¹³ C RF coils and correcting ¹³ C images for B₁ variation without user intervention.

Characterization of serial hyperpolarized 13C metabolic imaging in patients with glioma

•

Serial HP ¹³C MRI was evaluated for data consistency and abnormal metabolism.

•

Metabolism of [1-¹³C]pyruvate to lactate and bicarbonate was kinetically modeled.

•

Conversion rates within NAWM were consistent in healthy volunteer and patient scans

•

Progressed tumor lesions showed higher relative conversion rates to [1-¹³C]lactate.

•

Globally elevated rate constants were observed with anti-angiogenic treatment.

Background

Hyperpolarized 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.

Methods

Serial 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 (k_PL) and [¹³C]bicarbonate (k_PB) 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 k_PL-NAWM and k_PB-NAWM was measured in terms of the coefficient of variation (CV).

Results

When excluding scans following anti-angiogenic therapy, patient values of k_PL-NAWM and k_PB-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 k_PL-NAWM/k_PB-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 k_PL-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 k_PL relative to k_PL-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 k_PB in patients was reduced to unquantifiable values compared to k_PB-NAWM.

Conclusion

Serial measures of HP [1-¹³C]pyruvate metabolism displayed consistency in the NAWM of healthy volunteers and patients. Both k_PL and k_PB were globally elevated following bevacizumab treatment, while progressive disease demonstrated elevated k_PL in gadolinium-enhancing and non-enhancing lesions. Larger prospective studies with homogeneous patient populations are planned to evaluate metabolic changes following treatment.

Simultaneous Metabolic and Perfusion Imaging Using Hyperpolarized 13C MRI Can Evaluate Early and Dose-Dependent Response to Radiation Therapy in a Prostate Cancer Mouse Model

PURPOSE

To investigate use of a novel imaging approach, hyperpolarized (HP) ¹³C magnetic resonance imaging (MRI) for simultaneous metabolism and perfusion assessment, to evaluate early and dose-dependent response to radiation therapy (RT) in a prostate cancer mouse model.

METHODS AND MATERIALS

Transgenic Adenocarcinoma of Mouse Prostate (TRAMP) mice (n = 18) underwent single-fraction RT (4-14 Gy steep dose across the tumor) and were imaged serially at pre-RT baseline and 1, 4, and 7 days after RT using HP ¹³C MRI with combined [1-¹³C]pyruvate (metabolic active agent) and [¹³C]urea (perfusion agent), coupled with conventional multiparametric ¹H MRI including T2-weighted, dynamic contrast-enhanced, and diffusion-weighted imaging. Tumor tissues were collected 4 and 7 days after RT for biological correlative studies.

RESULTS

We found a significant decrease in HP pyruvate-to-lactate conversion in tumors responding to RT, with concomitant significant increases in HP pyruvate-to-alanine conversion and HP urea signal; the opposite changes were observed in tumors resistant to RT. Moreover, HP lactate change was dependent on radiation dose; tumor regions treated with higher radiation doses (10-14 Gy) exhibited a greater decrease in HP lactate signal than low-dose regions (4-7 Gy) as early as 1 day post-RT, consistent with lactate dehydrogenase enzyme activity and expression data. We also found that HP [¹³C]urea MRI provided assessments of tumor perfusion similar to those provided by ¹H dynamic contrast-enhanced MRI in this animal model. However, apparent diffusion coefficien , a conventional ¹H MRI functional biomarker, did not exhibit statistically significant changes within 7 days after RT.

CONCLUSION

These results demonstrate the ability of HP ¹³C MRI to monitor radiation-induced physiologic changes in a timely and dose-dependent manner, providing the basic science premise for further clinical investigation and translation.

Simultaneous T1 and T2 mapping of hyperpolarized 13C compounds using the bSSFP sequence

As in conventional ¹H MRI, T₁ and T₂ relaxation times of hyperpolarized (HP) ¹³C nuclei can provide important biomedical information. Two new approaches were developed for simultaneous T₁ and T₂ mapping of HP ¹³C probes based on balanced steady state free precession (bSSFP) acquisitions: a method based on sequential T₁ and T₂ mapping modules, and a model-based joint T₁/T₂ approach analogous to MR fingerprinting. These new methods were tested in simulations, HP ¹³C phantoms, and in vivo in normal Sprague-Dawley rats. Non-localized T₁ values, low flip angle EPI T₁ maps, bSSFP T₂ maps, and Bloch-Siegert B₁ maps were also acquired for comparison. T₁ and T₂ maps acquired using both approaches were in good agreement with both literature values and data from comparative acquisitions. Multiple HP ¹³C compounds were successfully mapped, with their relaxation time parameters measured within heart, liver, kidneys, and vasculature in one acquisition for the first time.

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

PURPOSE

The balanced steady-state free precession sequence has been previously explored to improve the efficient use of nonrecoverable hyperpolarized ¹³C magnetization, but suffers from poor spectral selectivity and long acquisition time. The purpose of this study was to develop a novel metabolite-specific 3D bSSFP ("MS-3DSSFP") sequence with stack-of-spiral readouts for improved lactate imaging in hyperpolarized [1-¹³ C]pyruvate studies on a clinical 3T scanner.

METHODS

Simulations were performed to evaluate the spectral response of the MS-3DSSFP sequence. Thermal ¹³C phantom experiments were performed to validate the MS-3DSSFP sequence. In vivo hyperpolarized [1-¹³ C], pyruvate studies were performed to compare the MS-3DSSFP sequence with metabolite-specific gradient echo ("MS-GRE") sequences for lactate imaging.

RESULTS

Simulations, phantom, and in vivo studies demonstrate that the MS-3DSSFP sequence achieved spectrally selective excitation on lactate while minimally perturbing other metabolites. Compared with MS-GRE sequences, the MS-3DSSFP sequence showed approximately a 2.5-fold SNR improvement for lactate imaging in rat kidneys, prostate tumors in a mouse model, and human kidneys.

CONCLUSIONS

Improved lactate imaging using the MS-3DSSFP sequence in hyperpolarized [1-¹³ C]pyruvate studies was demonstrated in animals and humans. The MS-3DSSFP sequence could be applied for other clinical applications such as in the brain or adapted for imaging other metabolites such as pyruvate and bicarbonate.

Kinetic Modeling of Hyperpolarized Carbon-13 Pyruvate Metabolism in the Human Brain

Kinetic modeling of the in vivo pyruvate-to-lactate conversion is crucial to investigating aberrant cancer metabolism that demonstrates Warburg effect modifications. Non-invasive detection of alterations to metabolic flux might offer prognostic value and improve the monitoring of response to treatment. In this clinical research project, hyperpolarized [1-¹³C] pyruvate was intravenously injected in a total of 10 brain tumor patients to measure its rate of conversion to lactate ( k_PL ) and bicarbonate ( k_PB ) via echo-planar imaging. Our aim was to investigate new methods to provide k_PL and k_PB maps with whole-brain coverage. The approach was data-driven and addressed two main issues: selecting the optimal model for fitting our data and determining an appropriate goodness-of-fit metric. The statistical analysis suggested that an input-less model had the best agreement with the data. It was also found that selecting voxels based on post-fitting error criteria provided improved precision and wider spatial coverage compared to using signal-to-noise cutoffs alone.

First hyperpolarized [2-13C]pyruvate MR studies of human brain metabolism

We developed methods for the preparation of hyperpolarized (HP) sterile [2-¹³C]pyruvate to test its feasibility in first-ever human NMR studies following FDA-IND & IRB approval. Spectral results using this MR stable-isotope imaging approach demonstrated the feasibility of investigating human cerebral energy metabolism by measuring the dynamic conversion of HP [2-¹³C]pyruvate to [2-¹³C]lactate and [5-¹³C]glutamate in the brain of four healthy volunteers. Metabolite kinetics, signal-to-noise (SNR) and area-under-curve (AUC) ratios, and calculated [2-¹³C]pyruvate to [2-¹³C]lactate conversion rates (k_PL) were measured and showed similar but not identical inter-subject values. The k_PL measurements were equivalent with prior human HP [1-¹³C]pyruvate measurements.

Hyperpolarized 13C-pyruvate MRI detects real-time metabolic flux in prostate cancer metastases to bone and liver: a clinical feasibility study

Background

Hyperpolarized (HP) ¹³C-pyruvate MRI is a stable-isotope molecular imaging modality that provides real-time assessment of the rate of metabolism through glycolytic pathways in human prostate cancer. Heretofore this imaging modality has been successfully utilized in prostate cancer only in localized disease. This pilot clinical study investigated the feasibility and imaging performance of HP ¹³C-pyruvate MR metabolic imaging in prostate cancer patients with metastases to the bone and/or viscera.

Methods

Six patients who had metastatic castration-resistant prostate cancer were recruited. Carbon-13 MR examination were conducted on a clinical 3T MRI following injection of 250 mM hyperpolarized ¹³C-pyruvate, where pyruvate-to-lactate conversion rate (k_PL) was calculated. Paired metastatic tumor biopsy was performed with histopathological and RNA-seq analyses.

Results

We observed a high rate of glycolytic metabolism in prostate cancer metastases, with a mean k_PL value of 0.020 ± 0.006 (s⁻¹) and 0.026 ± 0.000 (s⁻¹) in bone (N = 4) and liver (N = 2) metastases, respectively. Overall, high k_PL showed concordance with biopsy-confirmed high-grade prostate cancer including neuroendocrine differentiation in one case. Interval decrease of k_PL from 0.026 at baseline to 0.015 (s⁻¹) was observed in a liver metastasis 2 months after the initiation of taxane plus platinum chemotherapy. RNA-seq found higher levels of the lactate dehydrogenase isoform A (Ldha,15.7 ± 0.7) expression relative to the dominant isoform of pyruvate dehydrogenase (Pdha1, 12.8 ± 0.9).

Conclusions

HP ¹³C-pyruvate MRI can detect real-time glycolytic metabolism within prostate cancer metastases, and can measure changes in quantitative k_PL values following treatment response at early time points. This first feasibility study supports future clinical studies of HP ¹³C-pyruvate MRI in the setting of advanced prostate cancer.

Assessing high-intensity focused ultrasound treatment of prostate cancer with hyperpolarized 13 C dual-agent imaging of metabolism and perfusion

The goal of the study was to establish early hyperpolarized (HP) ¹³ C MRI metabolic and perfusion changes that predict effective high-intensity focused ultrasound (HIFU) ablation and lead to improved adjuvant treatment of partially treated regions. To accomplish this a combined HP dual-agent (¹³ C pyruvate and ¹³ C urea) ¹³ C MRI/multiparametric ¹ H MRI approach was used to measure prostate cancer metabolism and perfusion 3-4 h, 1 d, and 5 d after exposure to ablative and sub-lethal doses of HIFU within adenocarcinoma of mouse prostate tumors using a focused ultrasound applicator designed for murine studies. Pathologic and immunohistochemical analysis of the ablated tumor demonstrated fragmented, non-viable cells and vasculature consistent with coagulative necrosis, and a mixture of destroyed tissue and highly proliferative, poorly differentiated tumor cells in tumor tissues exposed to sub-lethal heat doses in the ablative margin. In ablated regions, the intensity of HP ¹³ C lactate or HP ¹³ C urea and dynamic contrast-enhanced (DCE) MRI area under the curve images were reduced to the level of background noise by 3-4 h after treatment with no recovery by the 5 d time point in either case. In the tissues that received sub-lethal heat dose, there was a significant 60% ± 12.4% drop in HP ¹³ C lactate production and a significant 30 ± 13.7% drop in urea perfusion 3-4 h after treatment, followed by recovery to baseline by 5 d after treatment. DCE MRI K^trans showed a similar trend to HP ¹³ C urea, demonstrating a complete loss of perfusion with no recovery in the ablated region, while having a 40%-50% decrease 3-4 h after treatment followed by recovery to baseline values by 5 d in the margin region. The utility of the HP ¹³ C MR measures of perfusion and metabolism in optimizing focal HIFU, either alone or in combination with adjuvant therapy, deserves further testing in future studies.

Methodological consensus on clinical proton MRS of the brain: Review and recommendations

Proton MRS (1 H MRS) provides noninvasive, quantitative metabolite profiles of tissue and has been shown to aid the clinical management of several brain diseases. Although most modern clinical MR scanners support MRS capabilities, routine use is largely restricted to specialized centers with good access to MR research support. Widespread adoption has been slow for several reasons, and technical challenges toward obtaining reliable good-quality results have been identified as a contributing factor. Considerable progress has been made by the research community to address many of these challenges, and in this paper a consensus is presented on deficiencies in widely available MRS methodology and validated improvements that are currently in routine use at several clinical research institutions. In particular, the localization error for the PRESS localization sequence was found to be unacceptably high at 3 T, and use of the semi-adiabatic localization by adiabatic selective refocusing sequence is a recommended solution. Incorporation of simulated metabolite basis sets into analysis routines is recommended for reliably capturing the full spectral detail available from short TE acquisitions. In addition, the importance of achieving a highly homogenous static magnetic field (B0 ) in the acquisition region is emphasized, and the limitations of current methods and hardware are discussed. Most recommendations require only software improvements, greatly enhancing the capabilities of clinical MRS on existing hardware. Implementation of these recommendations should strengthen current clinical applications and advance progress toward developing and validating new MRS biomarkers for clinical use.

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

PURPOSE

Rapid chemical exchange can affect SNR and pH measurement accuracy for hyperpolarized pH imaging with [¹³ C]bicarbonate. The purpose of this work was to investigate chemical exchange effects on hyperpolarized imaging sequences to identify optimal sequence parameters for high SNR and pH accuracy.

METHODS

Simulations were performed under varying rates of bicarbonate-CO₂ chemical exchange to analyze exchange effects on pH quantification accuracy and SNR under different sampling schemes. Four pulse sequences, including 1 new technique, a multiple-excitation 2D EPI (multi-EPI) sequence, were compared in phantoms using hyperpolarized [¹³ C]bicarbonate, varying parameters such as tip angles, repetition time, order of metabolite excitation, and refocusing pulse design. In vivo hyperpolarized bicarbonate-CO₂ exchange measurements were made in transgenic murine prostate tumors to select in vivo imaging parameters.

RESULTS

Modeling of bicarbonate-CO₂ exchange identified a multiple-excitation scheme for increasing CO₂ SNR by up to a factor of 2.7. When implemented in phantom imaging experiments, these sampling schemes were confirmed to yield high pH accuracy and SNR gains. Based on measured bicarbonate-CO₂ exchange in vivo, a 47% CO₂ SNR gain is predicted.

CONCLUSION

The novel multi-EPI pulse sequence can boost CO₂ imaging signal in hyperpolarized ¹³ C bicarbonate imaging while introducing minimal pH bias, helping to surmount a major hurdle in hyperpolarized pH imaging.