Frequency correction method for improved spatial correlation of hyperpolarized 13C metabolites and anatomy

A vendor-neutral EPI sequence for hyperpolarized 13C MRI

PURPOSE

To develop a flexible, vendor-neutral EPI sequence for hyperpolarized 13C metabolic imaging.

METHODS

An open-source EPI sequence consisting of a metabolite-specific spectral-spatial RF excitation pulse and a customizable EPI readout was created using the Pulseq framework. To explore the flexibility of our sequence, we tested several versions of the sequence including a symmetric 3D readout with different spatial resolutions for each metabolite (1.0 cm3 and 1.5 cm3). A multichamber phantom constructed with a Shepp-Logan geometry, containing two chambers filled with either natural abundance 13C compounds or hyperpolarized (HP) [1-13C]pyruvate, was used to test each sequence. For experiments involving HP [1-13C]pyruvate, a single chamber was prefilled with nicotinamide adenine dinucleotide hydride and lactate dehydrogenase to facilitate the conversion of [1-13C]pyruvate to [1-13C]lactate. All experiments were performed on a Siemens Prisma 3T scanner.

RESULTS

All the sequence variations localized natural-abundance 13C ethylene glycol and methanol to the appropriate compartment of the multichamber phantom. [1-13C]pyruvate was detectable in both chambers following the injection of HP [1-13C]pyruvate, whereas [1-13C]lactate was only found in the chamber containing nicotinamide adenine dinucleotide hydride and lactate dehydrogenase. The conversion rate from [1-13C]pyruvate to [1-13C]lactate (kPL) was 0.01 s-1 (95% confidence interval [0.00, 0.02]).

CONCLUSION

We have developed and tested a vendor-neutral EPI sequence for imaging HP 13C agents. We have made all of our sequence creation and image reconstruction code freely available online for other investigators to use.

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.

Improved reconstruction stability for chemical shift encoded hyperpolarized 13 C magnetic resonance spectroscopic imaging using k-t spiral acquisitions

PURPOSE

A multiecho, field of view (FOV)-oversampled k-t spiral acquisition and direct iterative decomposition of water and fat with echo asymmetry and least-squares estimation reconstruction is demonstrated to improve the stability of hyperpolarized ¹³ C magnetic resonance spectroscopic imaging (MRSI) in the presence of signal ambiguities attributed to low-SNR (signal-to-noise-ratio) species, local uncertainties in metabolite peaks, and echo-to-echo signal inconsistencies.

THEORY

k-t spiral acquisitions redistribute readout points to be more densely spaced radially in k-space by acquiring an FOV and matrix that are oversampled by η. These more densely spaced spiral turns constitute effective intraspiral echoes and can supplement conventional interspiral echoes to improve spectral separation and reduce spectral cross-talk to better resolve ¹³ C-labeled species for spectroscopic imaging.

METHODS

Digital simulations and imaging phantom experiments were performed for a range of interspiral echo spacings and η using multiecho, k-t spiral acquisitions. Image spectral cross-talk artifacts were evaluated both qualitatively and quantitatively as the percent error in measured metabolite ratios. In vivo murine experiments evaluated the feasibility of multiecho, k-t spiral [1-¹³ C]pyruvate MRSI to reduce spectral cross-talk for 3 scenarios of different expected reconstruction stability.

RESULTS

Digital simulations and imaging phantom experiments both demonstrated reduced or comparable image spectral cross-talk and percent errors in measured metabolite ratios with increasing η and better choices of echo spacings. In vivo images displayed markedly reduced spectral cross-talk in lactate images acquired with η = 7 versus η = 1.

CONCLUSION

The precision of hyperpolarized ¹³ C metabolic imaging and quantification in the presence of low-SNR species, local uncertainties in metabolite resonances, and echo-to-echo signal inconsistencies can be improved with the use of FOV-oversampled k-t spiral acquisitions.

Partial Fourier reconstruction for improved resolution in 3D hyperpolarized 13 C EPI

PURPOSE

Asymmetric in-plane k-space sampling of EPI can reduce the minimum achievable TE in hyperpolarized 13 C with spectral-spatial radio frequency pulses, thereby reducing T 2 * weighting and signal-losses. Partial Fourier image reconstruction exploits the approximate Hermitian symmetry of k-space data and can be applied to asymmetric data sets to synthesize unmeasured data. Here we tested whether the application of partial Fourier image reconstruction would improve spatial resolution from hyperpolarized [1- 13 C ]pyruvate scans in the human brain.

METHODS

Fifteen healthy control subjects were imaged using a volumetric dual-echo echo-planar imaging sequence with spectral-spatial radio frequency excitation. Images were reconstructed by zero-filling as well as with the partial Fourier reconstruction algorithm projection-on-convex-sets. Resulting images were quantitatively evaluated with a no-reference image quality assessment.

RESULTS

The no-reference image sharpness metric agreed with perceived improvements in image resolution and contrast. The [1- 13 C ]lactate images benefitted most, followed by the [1- 13 C ]pyruvate images. The 13 C -bicarbonate images were improved by the smallest degree, likely owing to relatively lower SNR.

CONCLUSIONS

Partial Fourier imaging and reconstruction were shown to improve the sharpness and contrast of human HP 13 C brain data and is a viable method for enhancing resolution.

Translation of Carbon-13 EPI for hyperpolarized MR molecular imaging of prostate and brain cancer patients

PURPOSE

To develop and translate a metabolite-specific imaging sequence using a symmetric echo planar readout for clinical hyperpolarized (HP) Carbon-13 (¹³ C) applications.

METHODS

Initial data were acquired from patients with prostate cancer (N = 3) and high-grade brain tumors (N = 3) on a 3T scanner. Samples of [1-¹³ C]pyruvate were polarized for at least 2 h using a 5T SPINlab system operating at 0.8 K. Following injection of the HP substrate, pyruvate, lactate, and bicarbonate (for brain studies) were sequentially excited with a singleband spectral-spatial RF pulse and signal was rapidly encoded with a single-shot echo planar readout on a slice-by-slice basis. Data were acquired dynamically with a temporal resolution of 2 s for prostate studies and 3 s for brain studies.

RESULTS

High pyruvate signal was seen throughout the prostate and brain, with conversion to lactate being shown across studies, whereas bicarbonate production was also detected in the brain. No Nyquist ghost artifacts or obvious geometric distortion from the echo planar readout were observed. The average error in center frequency was 1.2 ± 17.0 and 4.5 ± 1.4 Hz for prostate and brain studies, respectively, below the threshold for spatial shift because of bulk off-resonance.

CONCLUSION

This study demonstrated the feasibility of symmetric EPI to acquire HP ¹³ C metabolite maps in a clinical setting. As an advance over prior single-slice dynamic or single time point volumetric spectroscopic imaging approaches, this metabolite-specific EPI acquisition provided robust whole-organ coverage for brain and prostate studies while retaining high SNR, spatial resolution, and dynamic temporal resolution.

Cardiac applications of hyperpolarised magnetic resonance

Cardiovascular disease is the leading cause of death world-wide. It is increasingly recognised that cardiac pathologies show, or may even be caused by, changes in metabolism, leading to impaired cardiac energetics. The heart turns over 15 times its own weight in ATP every day and thus relies heavily on the availability of substrates and on efficient oxidation to generate this ATP. A number of old and emerging drugs that target different aspects of metabolism are showing promising results with regard to improved cardiac outcomes in patients. A non-invasive imaging technique that could assess the role of different aspects of metabolism in heart disease, as well as measure changes in cardiac energetics due to treatment, would be valuable in the routine clinical care of cardiac patients. Hyperpolarised magnetic resonance spectroscopy and imaging have revolutionised metabolic imaging, allowing real-time metabolic flux assessment in vivo for the first time. In this review we summarise metabolism in the healthy and diseased heart, give an introduction to the hyperpolarisation technique, 'dynamic nuclear polarisation' (DNP), and review the preclinical studies that have thus far explored healthy cardiac metabolism and different models of human heart disease. We furthermore show what advances have been made to translate this technique into the clinic, what technical challenges still remain and what unmet clinical needs and unexplored metabolic substrates still need to be assessed by researchers in this exciting and fast-moving field.

Susceptibility-induced distortion correction in hyperpolarized echo planar imaging

Purpose

Echo planar imaging is an attractive rapid imaging readout that can image hyperpolarized compounds in vivo. By alternating the sign of the phase encoding gradient waveform, spatial offsets arising from uncertain frequency shifts can be determined. We show here that blip‐reversed echo planar imaging can also be used to correct for susceptibility and B₀ inhomogeneity effects that would otherwise produce image‐domain distortion in the heart.

Methods

Previously acquired blip‐reversed cardiac 3D‐Spectral‐Spatial echo planar imaging volumetric timecourses of hyperpolarized [1‐¹³C]pyruvate were distortion corrected by a deformation field estimated by reconstructing signal‐to‐noise ratio (SNR)‐weighted progressively subsampled temporally summed images of each metabolite.

Results

Reconstructing blip‐reversed data as proposed produced volumetric timecourses that overlaid with proton reference images more consistently than without such corrections.

Conclusion

The method proposed may form an attractive method to correct for image‐domain distortions in hyperpolarized echo planar imaging experiments. Magn Reson Med 79:2135–2141, 2018. © 2017 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

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.

Dual-Echo EPI sequence for integrated distortion correction in 3D time-resolved hyperpolarized 13 C MRI

PURPOSE

To provide built-in off-resonance correction in time-resolved, volumetric hyperpolarized ¹³ C metabolic imaging by implementing a novel dual-echo 3D echo-planar imaging (EPI) sequence and reconstruction.

METHODS

A spectral-spatial pulse for single-resonance excitation followed by a dual-echo 3D EPI readout was implemented to provide 64 × 8 × 6 cm³ coverage at 5 × 5 × 5 mm³ nominal resolution. Multiple sources of EPI distortions were encoded using a multi-echo ¹ H EPI reference scan. Phase maps computed from the reference scans were combined with a bulk ¹³ C frequency offset encoded in the dual-echo [1-¹³ C]pyruvate images to correct geometric distortion and improve spatial registration. The proposed scheme was validated in a phantom study, and in vivo [1-¹³ C]pyruvate and [1-¹³ C]lactate rat images were acquired with intentional transmit frequency deviations to assess the dual-echo 3D EPI sequence.

RESULTS

The phantom study demonstrated improved spatial registration in off-resonance corrected images. Close agreement was observed between metabolic kidney signal and the underlying anatomy in rat imaging experiments. Relative to a single-echo acquisition, the coherent addition of the two corrected echoes provided the expected increase in signal-to-noise ratio by approximately 2.

CONCLUSION

A novel dual-echo 3D EPI acquisition sequence for integrated off-resonance correction in hyperpolarized ¹³ C imaging was developed and demonstrated. The proposed sequence offers clear advantages over flyback EPI for time-resolved metabolic mapping. Magn Reson Med 79:643-653, 2018. © 2017 International Society for Magnetic Resonance in Medicine.

Magnetic resonance imaging with hyperpolarized agents: methods and applications

In the past decade, hyperpolarized (HP) contrast agents have been under active development for MRI applications to address the twin challenges of functional and quantitative imaging. Both HP helium (³He) and xenon (¹²⁹Xe) gases have reached the stage where they are under study in clinical research. HP ¹²⁹Xe, in particular, is poised for larger scale clinical research to investigate asthma, chronic obstructive pulmonary disease, and fibrotic lung diseases. With advances in polarizer technology and unique capabilities for imaging of ¹²⁹Xe gas exchange into lung tissue and blood, HP ¹²⁹Xe MRI is attracting new attention. In parallel, HP ¹³C and ¹⁵N MRI methods have steadily advanced in a wide range of pre-clinical research applications for imaging metabolism in various cancers and cardiac disease. The HP [1-¹³C] pyruvate MRI technique, in particular, has undergone phase I trials in prostate cancer and is poised for investigational new drug trials at multiple institutions in cancer and cardiac applications. This review treats the methodology behind both HP gases and HP ¹³C and ¹⁵N liquid state agents. Gas and liquid phase HP agents share similar technologies for achieving non-equilibrium polarization outside the field of the MRI scanner, strategies for image data acquisition, and translational challenges in moving from pre-clinical to clinical research. To cover the wide array of methods and applications, this review is organized by numerical section into (1) a brief introduction, (2) the physical and biological properties of the most common polarized agents with a brief summary of applications and methods of polarization, (3) methods for image acquisition and reconstruction specific to improving data acquisition efficiency for HP MRI, (4) the main physical properties that enable unique measures of physiology or metabolic pathways, followed by a more detailed review of the literature describing the use of HP agents to study: (5) metabolic pathways in cancer and cardiac disease and (6) lung function in both pre-clinical and clinical research studies, concluding with (7) some future directions and challenges, and (8) an overall summary.

Voxel-by-voxel correlations of perfusion, substrate, and metabolite signals in dynamic hyperpolarized (13) C imaging

In this study, a mixture of pyruvic acid and the perfusion agent HP001 was co-polarized for simultaneous assessment of perfusion and metabolism in vivo. The pre-polarized mixture was administered to rats with subcutaneous MDA-MB-231 breast cancer xenografts and imaged using an interleaved sequence with designed spectral-spatial pulses and flyback echo-planar readouts. Voxel-by-voxel signal correlations from 10 animals (15 data sets) were analyzed for tumour, kidney, and muscle regions of interest. The relationship between perfusion and hyperpolarized signal was explored on a voxel-by-voxel basis in various metabolically active tissues, including tumour, healthy kidneys, and skeletal muscle. Positive pairwise correlations between lactate, pyruvate, and HP001 observed in all 10 tumours suggested that substrate delivery was the dominant factor limiting the conversion of pyruvate to lactate in the tumour model used in this study. On the other hand, in cases where conversion is the limiting factor, such as in healthy kidneys, both pyruvate and lactate can act as excellent perfusion markers. In intermediate cases between the two limits, such as in skeletal muscle, some perfusion information may be inferred from the (pyruvate + lactate) signal distribution. Co-administration of pyruvate with a dynamic nuclear polarization (DNP) perfusion agent is an effective approach for distinguishing between slow metabolism and poor perfusion and a practical strategy for lactate signal normalization to account for substrate delivery, especially in cases of rapid pyruvate-to-lactate conversion and in poorly perfused regions with inadequate pyruvate signal-to-noise ratio for reliable determination of the lactate-to-pyruvate ratio. Copyright © 2016 John Wiley & Sons, Ltd.

Use of a Multi-compartment Dynamic Single Enzyme Phantom for Studies of Hyperpolarized Magnetic Resonance Agents

Imaging of hyperpolarized substrates by magnetic resonance shows great clinical promise for assessment of critical biochemical processes in real time. Due to fundamental constraints imposed by the hyperpolarized state, exotic imaging and reconstruction techniques are commonly used. A practical system for characterization of dynamic, multi-spectral imaging methods is critically needed. Such a system must reproducibly recapitulate the relevant chemical dynamics of normal and pathological tissues. The most widely utilized substrate to date is hyperpolarized [1-(13)C]-pyruvate for assessment of cancer metabolism. We describe an enzyme-based phantom system that mediates the conversion of pyruvate to lactate. The reaction is initiated by injection of the hyperpolarized agent into multiple chambers within the phantom, each of which contains varying concentrations of reagents that control the reaction rate. Multiple compartments are necessary to ensure that imaging sequences faithfully capture the spatial and metabolic heterogeneity of tissue. This system will aid the development and validation of advanced imaging strategies by providing chemical dynamics that are not available from conventional phantoms, as well as control and reproducibility that is not possible in vivo.

Robust and high resolution hyperpolarized metabolic imaging of the rat heart at 7 T with 3D spectral-spatial EPI

PURPOSE

Hyperpolarized metabolic imaging has the potential to revolutionize the diagnosis and management of diseases where metabolism is dysregulated, such as heart disease. We investigated the feasibility of imaging rodent myocardial metabolism at high resolution at 7 T.

METHODS

We present here a fly-back spectral-spatial radiofrequency pulse that sidestepped maximum gradient strength requirements and enabled high resolution metabolic imaging of the rodent myocardium. A 3D echo-planar imaging readout followed, with centric ordered z-phase encoding. The cardiac gated sequence was used to image metabolism in rodents whose metabolic state had been manipulated by being fasted, fed, or fed and given the pyruvate dehydrogenase kinase inhibitor dichloroacetate.

RESULTS

We imaged hyperpolarized metabolites with a spatial resolution of 2×2×3.8 mm(3) and a temporal resolution of 1.8 s in the rat heart at 7 T. Significant differences in myocardial pyruvate dehydrogenase flux were observed between the three groups of animals, concomitant with the known biochemistry.

CONCLUSION

The proposed sequence was able to image in vivo metabolism with excellent spatial resolution in the rat heart. The field of view enabled the simultaneous multi-organ acquisition of metabolic information from the rat, which is of great utility for preclinical research in cardiovascular disease. Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance.

Accelerated 3D echo-planar imaging with compressed sensing for time-resolved hyperpolarized 13 C studies

PURPOSE

To enable large field-of-view, time-resolved volumetric coverage in hyperpolarized ¹³ C metabolic imaging by implementing a novel data acquisition and image reconstruction method based on the compressed sensing framework.

METHODS

A spectral-spatial pulse for single-resonance excitation followed by a symmetric echo-planar imaging (EPI) readout was implemented for encoding a 72 × 18 cm² field of view at 5 × 5 mm² resolution. Random undersampling was achieved with blipped z-gradients during the ramp portion of the echo-planar imaging readout. The sequence and reconstruction were tested with phantom studies and consecutive in vivo hyperpolarized ¹³ C scans in rats. Retrospectively and prospectively undersampled data were compared on the basis of structural similarity in the reconstructed images and the quantification of the lactate-to-pyruvate ratio in rat kidneys.

RESULTS

No artifacts or loss of resolution are evident in the compressed sensing reconstructed images acquired with the proposed sequence. Structural similarity analysis indicate that compressed sensing reconstructions can accurately recover spatial features in the metabolic images evaluated.

CONCLUSION

A novel z-blip acquisition sequence for compressed sensing accelerated hyperpolarized ¹³ C 3D echo-planar imaging was developed and demonstrated. The close agreement in lactate-to-pyruvate ratios from both retrospectively and prospectively undersampled data from rats shows that metabolic information is preserved with acceleration factors up to 3-fold with the developed method. Magn Reson Med 77:538-546, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Studies of Metabolism Using (13)C MRS of Hyperpolarized Probes

First described in 2003, the dissolution dynamic nuclear polarization (DNP) technique, combined with (13)C magnetic resonance spectroscopy (MRS), has since been used in numerous metabolic studies and has become a valuable metabolic imaging method. DNP dramatically increases the level of polarization of (13)C-labeled compounds resulting in an increase in the signal-to-noise ratio (SNR) of over 50,000 fold for the MRS spectrum of hyperpolarized compounds. The high SNR enables rapid real-time detection of metabolism in cells, tissues, and in vivo. This chapter will present a comprehensive review of the DNP approaches that have been used to monitor metabolism in living systems. First, the list of (13)C DNP probes developed to date will be presented, with a particular focus on the most commonly used probe, namely [1-(13)C] pyruvate. In the next four sections, we will then describe the different factors that need to be considered when designing (13)C DNP probes for metabolic studies, conducting in vitro or in vivo hyperpolarized experiments, as well as acquiring, analyzing, and modeling hyperpolarized (13)C data.