Spectrally selective three-dimensional dynamic balanced steady-state free precession for hyperpolarized C-13 metabolic imaging with spectrally selective radiofrequency pulses

Quo Vadis Hyperpolarized 13C MRI?

Over the last two decades, hyperpolarized 13C MRI has gained significance in both preclinical and clinical studies, hereby relying on technologies like PHIP-SAH (ParaHydrogen-Induced Polarization-Side Arm Hydrogenation), SABRE (Signal Amplification by Reversible Exchange), and dDNP (dissolution Dynamic Nuclear Polarization), with dDNP being applied in humans. A clinical dDNP polarizer has enabled studies across 24 sites, despite challenges like high cost and slow polarization. Parahydrogen-based techniques like SABRE and PHIP offer faster, more cost-efficient alternatives but require molecule-specific optimization. The focus has been on imaging metabolism of hyperpolarized probes, which requires long T1, high polarization and rapid contrast generation. Efforts to establish novel probes, improve acquisition techniques and enhance data analysis methods including artificial intelligence are ongoing. Potential clinical value of hyperpolarized 13C MRI was demonstrated primarily for treatment response assessment in oncology, but also in cardiology, nephrology, hepatology and CNS characterization. In this review on biomedical hyperpolarized 13C MRI, we summarize important and recent advances in polarization techniques, probe development, acquisition and analysis methods as well as clinical trials. Starting from those we try to sketch a trajectory where the field of biomedical hyperpolarized 13C MRI might go.

Hyperpolarized Carbon-13 MRI in Breast Cancer

One of the hallmarks of cancer is metabolic reprogramming, including high levels of aerobic glycolysis (the Warburg effect). Pyruvate is a product of glucose metabolism, and ¹³C-MR imaging of the metabolism of hyperpolarized (HP) [1-¹³C]pyruvate (HP ¹³C-MRI) has been shown to be a potentially versatile tool for the clinical evaluation of tumor metabolism. Hyperpolarization of the ¹³C nuclear spin can increase the sensitivity of detection by 4-5 orders of magnitude. Therefore, following intravenous injection, the location of hyperpolarized ¹³C-labeled pyruvate in the body and its subsequent metabolism can be tracked using ¹³C-MRI. Hyperpolarized [¹³C]urea and [1,4-¹³C₂]fumarate are also likely to translate to the clinic in the near future as tools for imaging tissue perfusion and post-treatment tumor cell death, respectively. For clinical breast imaging, HP ¹³C-MRI can be combined with ¹H-MRI to address the need for detailed anatomical imaging combined with improved functional tumor phenotyping and very early identification of patients not responding to standard and novel neoadjuvant treatments. If the technical complexity of the hyperpolarization process and the relatively high associated costs can be reduced, then hyperpolarized ¹³C-MRI has the potential to become more widely available for large-scale clinical trials.

Quantification of Prostate Cancer Metabolism Using 3D Multiecho bSSFP and Hyperpolarized [1-13 C] Pyruvate: Metabolism Differs Between Tumors of the Same Gleason Grade

BACKGROUND

Three-dimensional (3D) multiecho balanced steady-state free precession (ME-bSSFP) has previously been demonstrated in preclinical hyperpolarized (HP) 13 C-MRI in vivo experiments, and it may be suitable for clinical metabolic imaging of prostate cancer (PCa).

PURPOSE

To validate a signal simulation framework for the use of sequence parameter optimization. To demonstrate the feasibility of ME-bSSFP for HP 13 C-MRI in patients. To evaluate the metabolism in PCa measured by ME-bSSFP.

STUDY TYPE

Retrospective single-center cohort study.

PHANTOMS/POPULATION

Phantoms containing aqueous solutions of [1-13 C] lactate (2.3 M) and [13 C] urea (8 M). Eight patients (mean age 67 ± 6 years) with biopsy-confirmed Gleason 3 + 4 (n = 7) and 4 + 3 (n = 1) PCa. FIELD STRENGTH/SEQUENCES: 1 H MRI at 3 T with T2 -weighted turbo spin-echo sequence used for spatial localization and spoiled dual gradient-echo sequence used for B0 -field measurement. ME-bSSFP sequence for 13 C MR spectroscopic imaging with retrospective multipoint IDEAL metabolite separation.

ASSESSMENT

The primary endpoint was the analysis of pyruvate-to-lactate conversion in PCa and healthy prostate regions of interest (ROIs) using model-free area under the curve (AUC) ratios and a one-directional kinetic model (kP ). The secondary objectives were to investigate the correlation between simulated and experimental ME-bSSFP metabolite signals for HP 13 C-MRI parameter optimization.

STATISTICAL TESTS

Pearson correlation coefficients with 95% confidence intervals and paired t-tests. The level of statistical significance was set at P < 0.05.

RESULTS

Strong correlations between simulated and empirical ME-bSSFP signals were found (r > 0.96). Therefore, the simulation framework was used for sequence optimization. Whole prostate metabolic HP 13 C-MRI, observing the conversion of pyruvate into lactate, with a temporal resolution of 6 seconds was demonstrated using ME-bSSFP. Both assessed metrics resulted in significant differences between PCa (mean ± SD) (AUC = 0.33 ± 012, kP  = 0.038 ± 0.014) and healthy (AUC = 0.15 ± 0.10, kP  = 0.011 ± 0.007) ROIs.

DATA CONCLUSION

Metabolic HP 13 C-MRI in the prostate using ME-bSSFP allows for differentiation between aggressive PCa and healthy tissue.

EVIDENCE LEVEL

2 TECHNICAL EFFICACY: Stage 1.

Spectrally selective bSSFP using off-resonant RF excitations permits high spatiotemporal resolution 3D metabolic imaging of hyperpolarized [1-13 C]Pyruvate-to-[1-13 C]lactate conversion

PURPOSE

To develop a high spatiotemporal resolution 3D dynamic pulse sequence for preclinical imaging of hyperpolarized [1-¹³ C]pyruvate-to-[1-¹³ C]lactate metabolism at 7T.

METHODS

A standard 3D balanced SSFP (bSSFP) sequence was modified to enable alternating-frequency excitations. RF pulses with 2.33 ms duration and 900 Hz FWHM were placed off-resonance of the target metabolites, [1-¹³ C]pyruvate (by approximately -245 Hz) and [1-¹³ C]lactate (by approximately 735 Hz), to selectively excite those resonances. Relatively broad bandwidth (compared to those metabolites' chemical shift offset) permits a short TR of 6.29 ms, enabling higher spatiotemporal resolution. Bloch equation simulations of the bSSFP response profile guided the sequence parameter selection to minimize spectral contamination between metabolites and preserve magnetization over time.

RESULTS

Bloch equation simulations, phantom studies, and in vivo studies demonstrated that the two target resonances could be cleanly imaged without substantial bSSFP banding artifacts and with little spectral contamination between lactate and pyruvate and from pyruvate hydrate. High spatiotemporal resolution 3D images were acquired of in vivo pyruvate-lactate metabolism in healthy wild-type and endogenous pancreatic tumor-bearing mice, with 1.212 s acquisition time per single-metabolite image and (1.75 mm)³ isotropic voxels with full mouse abdomen 56 × 28 × 21 mm³ FOV and fully-sampled k-space. Kidney and tumor lactate/pyruvate ratios of two consecutive measurements in one animal, 1 h apart, were consistent.

CONCLUSION

Spectrally selective bSSFP using off-resonant RF excitations can provide high spatio-temporal resolution 3D dynamic images of pyruvate-lactate metabolic conversion.

Metabolite-Specific Echo Planar Imaging for Preclinical Studies with Hyperpolarized 13C-Pyruvate MRI

Metabolite-specific echo-planar imaging (EPI) sequences with spectral–spatial (spsp) excitation are commonly used in clinical hyperpolarized [1-13C]pyruvate studies because of their speed, efficiency, and flexibility. In contrast, preclinical systems typically rely on slower spectroscopic methods, such as chemical shift imaging (CSI). In this study, a 2D spspEPI sequence was developed for use on a preclinical 3T Bruker system and tested on in vivo mice experiments with patient-derived xenograft renal cell carcinoma (RCC) or prostate cancer tissues implanted in the kidney or liver. Compared to spspEPI sequences, CSI were found to have a broader point spread function via simulations and exhibited signal bleeding between vasculature and tumors in vivo. Parameters for the spspEPI sequence were optimized using simulations and verified with in vivo data. The expected lactate SNR and pharmacokinetic modeling accuracy increased with lower pyruvate flip angles (less than 15°), intermediate lactate flip angles (25° to 40°), and temporal resolution of 3 s. Overall SNR was also higher with coarser spatial resolution (4 mm isotropic vs. 2 mm isotropic). Pharmacokinetic modelling used to fit kPL maps showed results consistent with the previous literature and across different sequences and tumor xenografts. This work describes and justifies the pulse design and parameter choices for preclinical spspEPI hyperpolarized 13C-pyruvate studies and shows superior image quality to CSI.

A reproducible dynamic phantom for sequence testing in hyperpolarised 13C-magnetic resonance

OBJECTIVE

To develop a phantom system which can be integrated with an automated injection system, eliminating the experimental variability that arises with manual injection; for the purposes of pulse sequence testing and metric derivation in hyperpolarised 13C-MR.

METHODS

The custom dynamic phantom was machined from Ultem and filled with a nicotinamide adenine dinucleotide and lactate dehydrogenase mixture dissolved in phosphate buffered saline. Hyperpolarised [1-13C]-pyruvate was then injected into the phantom (n = 8) via an automated syringe pump and the conversion of pyruvate to lactate monitored through a 13C imaging sequence.

RESULTS

The phantom showed low coefficient of variation for the lactate to pyruvate peak signal heights (11.6%) and dynamic area-under curve ratios (11.0%). The variance for the lactate dehydrogenase enzyme rate constant (kP) was also seen to be low at 15.6%.

CONCLUSION

The dynamic phantom demonstrates high reproducibility for quantification of 13C-hyperpolarised MR-derived metrics. Establishing such a phantom is needed to facilitate development of hyperpolarsed 13C-MR pulse sequenced; and moreover, to enable multisite hyperpolarised 13C-MR clinical trials where assessment of metric variability across sites is critical.

ADVANCES IN KNOWLEDGE

The dynamic phantom developed during the course of this study will be a useful tool in testing new pulse sequences and standardisation in future hyperpolarised work.

Hyperpolarized Metabolic MRI-Acquisition, Reconstruction, and Analysis Methods

Hyperpolarized metabolic MRI with 13C-labeled agents has emerged as a powerful technique for in vivo assessments of real-time metabolism that can be used across scales of cells, tissue slices, animal models, and human subjects. Hyperpolarized contrast agents have unique properties compared to conventional MRI scanning and MRI contrast agents that require specialized imaging methods. Hyperpolarized contrast agents have a limited amount of available signal, irreversible decay back to thermal equilibrium, bolus injection and perfusion kinetics, cellular uptake and metabolic conversion kinetics, and frequency shifts between metabolites. This article describes state-of-the-art methods for hyperpolarized metabolic MRI, summarizing data acquisition, reconstruction, and analysis methods in order to guide the design and execution of studies.

Probing hepatic metabolism of [2-13C]dihydroxyacetone in vivo with 1H-decoupled hyperpolarized 13C-MR

OBJECTIVES

To enhance detection of the products of hyperpolarized [2-13C]dihydroxyacetone metabolism for assessment of three metabolic pathways in the liver in vivo. Hyperpolarized [2-13C]DHAc emerged as a promising substrate to follow gluconeogenesis, glycolysis and the glycerol pathways. However, the use of [2-13C]DHAc in vivo has not taken off because (i) the chemical shift range of [2-13C]DHAc and its metabolic products span over 144 ppm, and (ii) 1H decoupling is required to increase spectral resolution and sensitivity. While these issues are trivial for high-field vertical-bore NMR spectrometers, horizontal-bore small-animal MR scanners are seldom equipped for such experiments.

METHODS

Real-time hepatic metabolism of three fed mice was probed by 1H-decoupled 13C-MR following injection of hyperpolarized [2-13C]DHAc. The spectra of [2-13C]DHAc and its metabolic products were acquired in a 7 T small-animal MR scanner using three purpose-designed spectral-spatial radiofrequency pulses that excited a spatial bandwidth of 8 mm with varying spectral bandwidths and central frequencies (chemical shifts).

RESULTS

The metabolic products detected in vivo include glycerol 3-phosphate, glycerol, phosphoenolpyruvate, lactate, alanine, glyceraldehyde 3-phosphate and glucose 6-phosphate. The metabolite-to-substrate ratios were comparable to those reported previously in perfused liver.

DISCUSSION

Three metabolic pathways can be probed simultaneously in the mouse liver in vivo, in real time,  using hyperpolarized DHAc.

Hyperpolarized Carbon (13C) MRI of the Kidney: Experimental Protocol

Alterations in renal metabolism are associated with both physiological and pathophysiologic events. The existing noninvasive analytic tools including medical imaging have limited capability for investigating these processes, which potentially limits current understanding of kidney disease and the precision of its clinical diagnosis. Hyperpolarized ¹³C MRI is a new medical imaging modality that can capture changes in the metabolic processing of certain rapidly metabolized substrates, as well as changes in kidney function. Here we describe experimental protocols for renal metabolic [1-¹³C]pyruvate and functional ¹³C-urea imaging step-by-step. These methods and protocols are useful for investigating renal blood flow and function as well as the renal metabolic status of rodents in vivo under various experimental (patho)physiological conditions.This chapter is based upon work from the COST Action PARENCHIMA, a community-driven network funded by the European Cooperation in Science and Technology (COST) program of the European Union, which aims to improve the reproducibility and standardization of renal MRI biomarkers. This experimental protocol is complemented by two separate chapters describing the basic concept and data analysis.

Correction and optimization of symmetric echo-planar spectroscopic imaging for hyperpolarized [1-13C]-pyruvate

Symmetric echo-planar spectroscopic imaging (EPSI) supports higher spectral bandwidth and improves signal-to-noise efficiency compared to flyback EPSI with the same readout bandwidth, but suffers from artifacts that are associated with non-uniform temporal sampling in k-t space. Our goal is to eliminate these artifacts and enhance observation of hyperpolarized [1-¹³C] pyruvate and its metabolites using symmetric EPSI. We used symmetric EPSI to efficiently acquire radially encoded spectroscopic imaging projections with a spectral under-sampling scheme that was optimized for HP pyruvate and its metabolites. A simple approach called selective correction of off-resonance effects (SCORE) was developed and applied to eliminate spectral artifacts. Simulations were used to assess the relative SNR performance of this technique, and a phantom study was carried out at 3 T to evaluate this method and compare it with alternative strategies. SCORE correction eliminated spectral artifacts due to chemical shift and non-uniform sampling in time. It is also compatible with established methods to eliminate artifacts caused by eddy currents. SCORE corrected symmetric EPSI supported maximal EPSI spectral bandwidth and improved SNR efficiency. Symmetric EPSI with SCORE correction offers a straightforward, efficient, and effective framework for assessment of hyperpolarized [1-¹³C] pyruvate and its metabolites.

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.

Acquisition strategies for spatially resolved magnetic resonance detection of hyperpolarized nuclei

Hyperpolarization is an emerging method in magnetic resonance imaging that allows nuclear spin polarization of gases or liquids to be temporarily enhanced by up to five or six orders of magnitude at clinically relevant field strengths and administered at high concentration to a subject at the time of measurement. This transient gain in signal has enabled the non-invasive detection and imaging of gas ventilation and diffusion in the lungs, perfusion in blood vessels and tissues, and metabolic conversion in cells, animals, and patients. The rapid development of this method is based on advances in polarizer technology, the availability of suitable probe isotopes and molecules, improved MRI hardware and pulse sequence development. Acquisition strategies for hyperpolarized nuclei are not yet standardized and are set up individually at most sites depending on the specific requirements of the probe, the object of interest, and the MRI hardware. This review provides a detailed introduction to spatially resolved detection of hyperpolarized nuclei and summarizes novel and previously established acquisition strategies for different key areas of application.

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.

Fast Imaging for Hyperpolarized MR Metabolic Imaging

MRI with hyperpolarized carbon-13 agents has created a new type of noninvasive, in vivo metabolic imaging that can be applied in cell, animal, and human studies. The use of ¹³ C-labeled agents, primarily [1-¹³ C]pyruvate, enables monitoring of key metabolic pathways with the ability to image substrate and products based on their chemical shift. Over 10 sites worldwide are now performing human studies with this new approach for studies of cancer, heart disease, liver disease, and kidney disease. Hyperpolarized metabolic imaging studies must be performed within several minutes following creation of the hyperpolarized agent due to irreversible decay of the net magnetization back to equilibrium, so fast imaging methods are critical. The imaging methods must include multiple metabolites, separated based on their chemical shift, which are also undergoing rapid metabolic conversion (via label exchange), further exacerbating the challenges of fast imaging. This review describes the state-of-the-art in fast imaging methods for hyperpolarized metabolic imaging. This includes the approach and tradeoffs between three major categories of fast imaging methods-fast spectroscopic imaging, model-based strategies, and metabolite specific imaging-as well additional options of parallel imaging, compressed sensing, tailored RF flip angles, refocused imaging methods, and calibration methods that can improve the scan coverage, speed, signal-to-noise ratio (SNR), resolution, and/or robustness of these studies. To date, these approaches have produced extremely promising initial human imaging results. Improvements to fast hyperpolarized metabolic imaging methods will provide better coverage, SNR, resolution, and reproducibility for future human imaging studies. LEVEL OF EVIDENCE: 5 TECHNICAL EFFICACY STAGE: 1.

Cancer in the crosshairs: targeting cancer metabolism with hyperpolarized carbon-13 MRI technology

Magnetic resonance (MR)-based hyperpolarized (HP) 13 C metabolic imaging is under active pursuit as a new clinical diagnostic method for cancer detection, grading, and monitoring of therapeutic response. Following the tremendous success of metabolic imaging by positron emission tomography, which already plays major roles in clinical oncology, the added value of HP 13 C MRI is emerging. Aberrant glycolysis and central carbon metabolism is a hallmark of many forms of cancer. The chemical transformations associated with these pathways produce metabolites ranging in general from three to six carbons, and are dependent on the redox state and energy charge of the tissue. The significant changes in chemistry associated with flux through these pathways imply that HP imaging can take advantage of the underlying chemical shift information encoded into an MR experiment to produce images of the injected substrate as well as its metabolites. However, imaging of HP metabolites poses unique constraints on pulse sequence design related to detection of X-nuclei, decay of the HP magnetization due to T1 , and the consumption of HP signal by the inspection pulses. Advancements in the field continue to depend critically on customization of MRI systems and pulse sequences for optimized detection of HP 13 C signals, focused largely on extracting the maximum amount of information during the short lifetime of the HP magnetization. From a clinical perspective, the success of HP 13 C MRI of cancer will largely depend upon the utility of HP pyruvate for the detection of lactate pools associated with the Warburg effect, though several other agents are also under investigation, with novel agents continually being formulated. In this review, the salient aspects of HP 13 C imaging will be highlighted, with an emphasis on both technological challenges and the biochemical aspects of HP experimental design.

High spatiotemporal resolution bSSFP imaging of hyperpolarized [1-13 C]pyruvate and [1-13 C]lactate with spectral suppression of alanine and pyruvate-hydrate

Purpose

The bSSFP acquisition enables high spatiotemporal resolution for hyperpolarized 13C MRI at 3T, but is limited by spectral contamination from adjacent resonances. The purpose of this study was to develop a framework for in vivo dynamic high resolution imaging of hyperpolarized [1-13C]pyruvate and [1-13C]lactate generated in vivo at 3T by simplifying the spectrum through the use of selective suppression pulses.

Methods

Spectral suppression pulses were incorporated into the bSSFP sequence for suppression of [1-13C]alanine and [1-13C]pyruvate-hydrate signals, leaving only the pyruvate and lactate resonances. Subsequently, the bSSFP pulse width, time-bandwidth, and repetition time were optimized for imaging these dual resonances.

Results

The spectral suppression reduced both the alanine and pyruvate-hydrate signals by 85.5 ± 4.9% and had no significant effect on quantitation of pyruvate to lactate conversion (liver: P = 0.400, kidney: P = 0.499). High resolution (2 × 2 mm2 and 3 × 3 mm2) sub-second 2D coronal projections and 3D 2.5 mm isotropic images were obtained in rats and tumor-bearing mice with 1.8-5 s temporal resolution, allowing for calculation of lactate-to-pyruvate ratios and k PL.

Conclusion

The developed framework presented here shows the capability for dynamic high resolution volumetric hyperpolarized bSSFP imaging of pyruvate-to-lactate conversion on a clinical 3T MR scanner.

Using a local low rank plus sparse reconstruction to accelerate dynamic hyperpolarized 13C imaging using the bSSFP sequence

Acceleration of dynamic 2D (T2 Mapping) and 3D hyperpolarized 13C MRI acquisitions using the balanced steady-state free precession sequence was achieved with a specialized reconstruction method, based on the combination of low rank plus sparse and local low rank reconstructions. Methods were validated using both retrospectively and prospectively undersampled in vivo data from normal rats and tumor-bearing mice. Four-fold acceleration of 1-2 mm isotropic 3D dynamic acquisitions with 2-5 s temporal resolution and two-fold acceleration of 0.25-1 mm2 2D dynamic acquisitions was achieved. This enabled visualization of the biodistribution of [2-13C]pyruvate, [1-13C]lactate, [13C, 15N2]urea, and HP001 within heart, kidneys, vasculature, and tumor, as well as calculation of high resolution T2 maps.

High resolution hyperpolarized 13 C MRSI using SPICE at 9.4T

PURPOSE

To test the feasibility of using the SPICE (SPectroscopic Imaging by exploiting spatiospectral CorrElation) technique, which uses the partial separability of spectroscopic data, for high resolution hyperpolarized (HP) ¹³ C spectroscopic imaging.

METHODS

Numerical simulations were performed to investigate the impact of transient HP signals on SPICE reconstruction. Furthermore, spectroscopic imaging exams from SPICE and conventional EPSI (echo-planar spectroscopic imaging) were simulated for comparison. For in vivo experiments, HP ¹³ C SPICE was performed in a mouse kidney by means of the injection of HP [1-¹³ C] pyruvate at 9.4T.

RESULTS

The variation of lactate/pyruvate from the simulated SPICE was less than 4% under various factors that affect the transient HP signal, suggesting that the impact is negligible. We found that while HP ¹³ C EPSI was limited to the low signal-to-noise ratio (SNR) of lactate, these limitations were mitigated through HP ¹³ C SPICE, facilitating the improved SNR of lactate and the distinction of tissues. Acquisition of a high resolution HP ¹³ C spectroscopic image was possible for the in vivo experiments. With the fine structural information, the acquired image showed higher signal of pyruvate and lactate in the renal cortices than in the medullas, which is known to be attributed to higher activity of lactate dehydrogenase.

CONCLUSION

The feasibility of HP ¹³ C SPICE was investigated. Simulation studies were conducted and in vivo experiments were performed in the mouse kidney at 9.4T. Results confirmed that a high resolution HP ¹³ C spectroscopic image with adequate spectral resolution can be obtained. Magn Reson Med 80:703-710, 2018. © 2018 International Society for Magnetic Resonance in Medicine.

Noninvasive Interrogation of Cancer Metabolism with Hyperpolarized 13C MRI

This review will highlight recent advances in hyperpolarized ¹³C MR spectroscopic imaging, which can be used to noninvasively interrogate tumor metabolism. After providing an overview of MR and hyperpolarization, we will discuss the latest advances in data acquisition techniques. Next, we will shift our focus to hyperpolarized probe design and provide an overview of the latest hyperpolarized ¹³C MR spectroscopic imaging probes developed in the last several years.

Development of high resolution 3D hyperpolarized carbon-13 MR molecular imaging techniques

The goal of this project was to develop and apply techniques for T2 mapping and 3D high resolution (1.5mm isotropic; 0.003cm3) 13C imaging of hyperpolarized (HP) probes [1-13C]lactate, [1-13C]pyruvate, [2-13C]pyruvate, and [13C,15N2]urea in vivo. A specialized 2D bSSFP sequence was implemented on a clinical 3T scanner and used to obtain the first high resolution T2 maps of these different hyperpolarized compounds in both rats and tumor-bearing mice. These maps were first used to optimize timings for highest SNR for single time-point 3D bSSFP acquisitions with a 1.5mm isotropic spatial resolution of normal rats. This 3D acquisition approach was extended to serial dynamic imaging with 2-fold compressed sensing acceleration without changing spatial resolution. The T2 mapping experiments yielded measurements of T2 values of >1s for all compounds within rat kidneys/vasculature and TRAMP tumors, except for [2-13C]pyruvate which was ~730ms and ~320ms, respectively. The high resolution 3D imaging enabled visualization the biodistribution of [1-13C]lactate, [1-13C]pyruvate, and [2-13C]pyruvate within different kidney compartments as well as in the vasculature. While the mouse anatomy is smaller, the resolution was also sufficient to image the distribution of all compounds within kidney, vasculature, and tumor. The development of the specialized 3D sequence with compressed sensing provided improved structural and functional assessments at a high (0.003cm3) spatial and 2s temporal resolution in vivo utilizing HP 13C substrates by exploiting their long T2 values. This 1.5mm isotropic resolution is comparable to 1H imaging and application of this approach could be extended to future studies of uptake, metabolism, and perfusion in cancer and other disease models and may ultimately be of value for clinical imaging.