Optimization of fast spiral chemical shift imaging using least squares reconstruction: application for hyperpolarized (13)C metabolic imaging

Dynamic T 2 * relaxometry of hyperpolarized [1-13 C]pyruvate MRI in the human brain and kidneys

PURPOSE

This study aimed to quantifyT 2 * $$ {T}_2^{\ast } $$ for hyperpolarized [1-13 C]pyruvate and metabolites in the healthy human brain and renal cell carcinoma (RCC) patients at 3 T.

METHODS

DynamicT 2 * $$ {T}_2^{\ast } $$ values were measured with a metabolite-specific multi-echo spiral sequence. The dynamicT 2 * $$ {T}_2^{\ast } $$ of [1-13 C]pyruvate, [1-13 C]lactate, and 13 C-bicarbonate was estimated in regions of interest in the whole brain, sinus vein, gray matter, and white matter in healthy volunteers, as well as in kidney tumors and the contralateral healthy kidneys in a separate group of RCC patients.T 2 * $$ {T}_2^{\ast } $$ was fit using a mono-exponential function; and metabolism was quantified using pyruvate-to-lactate conversion rate maps and lactate-to-pyruvate ratio maps, which were compared with and without an estimatedT 2 * $$ {T}_2^{\ast } $$ correction.

RESULTS

TheT 2 * $$ {T}_2^{\ast } $$ of pyruvate was shown to vary during the acquisition, whereas theT 2 * $$ {T}_2^{\ast } $$ of lactate and bicarbonate were relatively constant through time and across the organs studied. TheT 2 * $$ {T}_2^{\ast } $$ of lactate was similar in gray matter (29.75 ± 1.04 ms), white matter (32.89 ± 0.9 ms), healthy kidney (34.61 ± 4.07 ms), and kidney tumor (33.01 ± 2.31 ms); and theT 2 * $$ {T}_2^{\ast } $$ of bicarbonate was different between whole-brain (108.17 ± 14.05 ms) and healthy kidney (58.45 ± 6.63 ms). TheT 2 * $$ {T}_2^{\ast } $$ of pyruvate had similar trends in both brain and RCC studies, reducing from 75.56 ± 2.23 ms to 22.24 ± 1.24 ms in the brain and reducing from 122.72 ± 9.86 ms to 57.38 ± 7.65 ms in the kidneys.

CONCLUSION

Multi-echo dynamic imaging can quantifyT 2 * $$ {T}_2^{\ast } $$ and metabolism in a single integrated acquisition. Clear differences were observed in theT 2 * $$ {T}_2^{\ast } $$ of metabolites and in their behavior throughout the timecourse.

MR-Nucleomics: The study of pathological cellular processes with multinuclear magnetic resonance spectroscopy and imaging in vivo

Clinical medicine has experienced a rapid development in recent decades, during which therapies targeting specific cellular signaling pathways, or specific cell surface receptors, have been increasingly adopted. While these developments in clinical medicine call for improved precision in diagnosis and treatment monitoring, modern medical imaging methods are restricted mainly to anatomical imaging, lagging behind the requirements of precision medicine. Although positron emission tomography and single photon emission computed tomography have been used clinically for studies of metabolism, their applications have been limited by the exposure risk to ionizing radiation, the subsequent limitation in repeated and longitudinal studies, and the incapability in assessing downstream metabolism. Magnetic resonance spectroscopy (MRS) or spectroscopic imaging (MRSI) are, in theory, capable of assessing molecular activities in vivo, although they are often limited by sensitivity. Here, we review some recent developments in MRS and MRSI of multiple nuclei that have potential as molecular imaging tools in the clinic.

Accelerated MR spectroscopic imaging-a review of current and emerging techniques

Over more than 30 years in vivo MR spectroscopic imaging (MRSI) has undergone an enormous evolution from theoretical concepts in the early 1980s to the robust imaging technique that it is today. The development of both fast and efficient sampling and reconstruction techniques has played a fundamental role in this process. State-of-the-art MRSI has grown from a slow purely phase-encoded acquisition technique to a method that today combines the benefits of different acceleration techniques. These include shortening of repetition times, spatial-spectral encoding, undersampling of k-space and time domain, and use of spatial-spectral prior knowledge in the reconstruction. In this way in vivo MRSI has considerably advanced in terms of spatial coverage, spatial resolution, acquisition speed, artifact suppression, number of detectable metabolites and quantification precision. Acceleration not only has been the enabling factor in high-resolution whole-brain 1 H-MRSI, but today is also common in non-proton MRSI (31 P, 2 H and 13 C) and applied in many different organs. In this process, MRSI techniques had to constantly adapt, but have also benefitted from the significant increase of magnetic field strength boosting the signal-to-noise ratio along with high gradient fidelity and high-density receive arrays. In combination with recent trends in image reconstruction and much improved computation power, these advances led to a number of novel developments with respect to MRSI acceleration. Today MRSI allows for non-invasive and non-ionizing mapping of the spatial distribution of various metabolites' tissue concentrations in animals or humans, is applied for clinical diagnostics and has been established as an important tool for neuro-scientific and metabolism research. This review highlights the developments of the last five years and puts them into the context of earlier MRSI acceleration techniques. In addition to 1 H-MRSI it also includes other relevant nuclei and is not limited to certain body regions or specific applications.

Dynamic volumetric hyperpolarized 13 C imaging with multi-echo EPI

PURPOSE

To generate dynamic, volumetric maps of hyperpolarized [1-¹³ C]pyruvate and its metabolic products in vivo.

METHODS

Maps of chemical species were generated with iterative least squares (IDEAL) reconstruction from multiecho echo-planar imaging (EPI) of phantoms of thermally polarized ¹³ C-labeled chemicals and mice injected with hyperpolarized [1-¹³ C]pyruvate on a preclinical 3T scanner. The quality of the IDEAL decomposition of single-shot and multishot phantom images was evaluated using quantitative results from a simple pulse-and-acquire sequence as the gold standard. Time course and area-under-the-curve plots were created to analyze the distribution of metabolites in vivo.

RESULTS

Improved separation of chemical species by IDEAL, evaluated by the amount of residual signal measured for chemicals not present in the phantoms, was observed as the number of EPI shots was increased from one to four. Dynamic three-dimensional metabolite maps of [1-¹³ C]pyruvate,[1-¹³ C]pyruvatehydrate, [1-¹³ C]lactate, [1-¹³ C]bicarbonate, and [1-¹³ C]alanine generated by IDEAL from interleaved multishot multiecho EPI of live mice were used to construct time course and area-under-the-curve graphs for the heart, kidneys, and liver, which showed good agreement with previously published results.

CONCLUSIONS

IDEAL decomposition of multishot multiecho 13C EPI images is a simple, yet robust method for generating high-quality dynamic volumetric maps of hyperpolarized [1-¹³ C]pyruvate and its products in vivo and has potential applications for the assessment of multiorgan metabolic phenomena.

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.

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.

Fast data acquisition techniques in magnetic resonance spectroscopic imaging

Magnetic resonance spectroscopic imaging (MRSI) is an important technique for assessing the spatial variation of metabolites in vivo. The long scan times in MRSI limit clinical applicability due to patient discomfort, increased costs, motion artifacts, and limited protocol flexibility. Faster acquisition strategies can address these limitations and could potentially facilitate increased adoption of MRSI into routine clinical protocols with minimal addition to the current anatomical and functional acquisition protocols in terms of imaging time. Not surprisingly, a lot of effort has been devoted to the development of faster MRSI techniques that aim to capture the same underlying metabolic information (relative metabolite peak areas and spatial distribution) as obtained by conventional MRSI, in greatly reduced time. The gain in imaging time results, in some cases, in a loss of signal-to-noise ratio and/or in spatial and spectral blurring. This review examines the current techniques and advances in fast MRSI in two and three spatial dimensions and their applications. This review categorizes the acceleration techniques according to their strategy for acquisition of the k-space. Techniques such as fast/turbo-spin echo MRSI, echo-planar spectroscopic imaging, and non-Cartesian MRSI effectively cover the full k-space in a more efficient manner per T_R . On the other hand, techniques such as parallel imaging and compressed sensing acquire fewer k-space points and employ advanced reconstruction algorithms to recreate the spatial-spectral information, which maintains statistical fidelity in test conditions (ie no statistically significant differences on voxel-wise comparisions) with the fully sampled data. The advantages and limitations of each state-of-the-art technique are reviewed in detail, concluding with a note on future directions and challenges in the field of fast spectroscopic imaging.

An indirect method for in vivo T2 mapping of [1-13 C] pyruvate using hyperpolarized 13 C CSI

An indirect method for in vivo T₂ mapping of ¹³ C-labeled metabolites using T₂ and T₂ * information of water protons obtained a priori is proposed. The T₂ values of ¹³ C metabolites are inferred using the relationship to T₂ ' of coexisting ¹ H and the T₂ * of ¹³ C metabolites, which is measured using routine hyperpolarized ¹³ C CSI data. The concept is verified with phantom studies. Simulations were performed to evaluate the extent of T₂ estimation accuracy due to errors in the other measurements. Also, bias in the ¹³ C T₂ * estimation from the ¹³ C CSI data was studied. In vivo experiments were performed from the brains of normal rats and a rat with C6 glioma. Simulation results indicate that the proposed method provides accurate and unbiased ¹³ C T₂ values within typical experimental settings. The in vivo studies found that the estimated T₂ of [1-¹³ C] pyruvate using the indirect method was longer in tumor than in normal tissues and gave values similar to previous reports. This method can estimate localized T₂ relaxation times from multiple voxels using conventional hyperpolarized ¹³ C CSI and can potentially be used with time resolved fast CSI.

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.

Single shot three-dimensional pulse sequence for hyperpolarized 13 C MRI

PURPOSE

Metabolic imaging with hyperpolarized 13 C-labeled cell substrates is a promising technique for imaging tissue metabolism in vivo. However, the transient nature of the hyperpolarization, and its depletion following excitation, limits the imaging time and the number of excitation pulses that can be used. We describe here a single-shot three-dimensional (3D) imaging sequence and demonstrate its capability to generate 13 C MR images in tumor-bearing mice injected with hyperpolarized [1-13 C]pyruvate.

METHODS

The pulse sequence acquires a stack-of-spirals at two spin echoes after a single excitation pulse and encodes the kz-dimension in an interleaved manner to enhance robustness to B0 inhomogeneity. Spectral-spatial pulses are used to acquire dynamic 3D images from selected hyperpolarized 13 C-labeled metabolites.

RESULTS

A nominal spatial/temporal resolution of 1.25 × 1.25 × 2.5 mm3  × 2 s was achieved in tumor images of hyperpolarized [1-13 C]pyruvate and [1-13 C]lactate acquired in vivo. Higher resolution in the z-direction, with a different k-space trajectory, was demonstrated in measurements on a thermally polarized [1-13 C]lactate phantom.

CONCLUSION

The pulse sequence is capable of imaging hyperpolarized 13 C-labeled substrates at relatively high spatial and temporal resolutions and is robust to moderate system imperfections. Magn Reson Med 77:740-752, 2017. © 2016 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.

A general chemical shift decomposition method for hyperpolarized (13) C metabolite magnetic resonance imaging

Metabolic imaging with hyperpolarized carbon-13 allows sequential steps of metabolism to be detected in vivo. Potential applications in cancer, brain, muscular, myocardial, and hepatic metabolism suggest that clinical applications could be readily developed. A primary concern in imaging hyperpolarized nuclei is the irreversible decay of the enhanced magnetization back to thermal equilibrium. Multiple methods for rapid imaging of hyperpolarized substrates and their products have been proposed with a multi-point Dixon method distinguishing itself as a robust protocol for imaging [1-(13) C]pyruvate. We describe here a generalized chemical shift decomposition method that incorporates a single-shot spiral imaging sequence plus a spectroscopic sequence to retain as much spin polarization as possible while allowing detection of metabolites that have a wide range of chemical shift values. The new method is demonstrated for hyperpolarized [1-(13) C]pyruvate, [1-(13) C]acetoacetate, and [2-(13) C]dihydroxyacetone. Copyright © 2016 John Wiley & Sons, Ltd.

Accelerated chemical shift imaging of hyperpolarized (13) C metabolites

PURPOSE

Chemical shift imaging (CSI) has long been considered the gold standard method for in vivo hyperpolarized (13) C metabolite imaging because of its high sensitivity. However, CSI requires a large number of excitations so it is desirable to reduce the number of RF excitations and the total acquisition time.

METHODS

Centric phase encoding and three-dimensional compressed sensing methods were adopted into a CSI acquisition to improve efficiency and reduce the number of excitations required for imaging hyperpolarized metabolites. The new method was implemented on a GE MR750W scanner for routine real time metabolic imaging experiments.

RESULTS

Imaging results from phantoms and in vivo animals using hyperpolarized (13) C tracers demonstrate that when the entire CSI dataset is treated as a single object, compressed sensing can be satisfactorily applied to spectroscopic CSI. Centric k-space trajectory data collection also greatly improves the acquisition efficiency. This combination of compressed sensing CSI and acquisition time reduction was used to perform a hyperpolarized (13) C dynamic study.

CONCLUSION

Compressed sensing can be satisfactorily applied to conventional CSI in hyperpolarized (13) C metabolite MR imaging to reduce the number of RF excitations and accelerate the imaging speed to take advantage of conventional CSI in providing high sensitivity and a large spectral bandwidth. Magn Reson Med 76:1033-1038, 2016. © 2016 Wiley Periodicals, Inc.

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 hyperpolarized (13)C metabolic imaging with selective non-excitation of pyruvate (SNEP)

In vivo metabolic imaging using hyperpolarized [1-(13)C]pyruvate provides localized biochemical information and is particularly useful in detecting early disease changes, as well as monitoring disease progression and treatment response. However, a major limitation of hyperpolarized magnetization is its unrecoverable decay, due not only to T1 relaxation but also to radio-frequency (RF) excitation. RF excitation schemes used in metabolic imaging must therefore be able to utilize available hyperpolarized magnetization efficiently and robustly for the optimal detection of substrate and metabolite activities. In this work, a novel RF excitation scheme called selective non-excitation of pyruvate (SNEP) is presented. This excitation scheme involves the use of a spectral selective RF pulse to specifically exclude the excitation of [1-(13)C]pyruvate, while uniformly exciting the key metabolites of interest (namely [1-(13)C]lactate and [1-(13)C]alanine) and [1-(13)C]pyruvate-hydrate. By eliminating the loss of hyperpolarized [1-(13)C]pyruvate magnetization due to RF excitation, the signal from downstream metabolite pools is increased together with enhanced dynamic range. Simulation results, together with phantom measurements and in vivo experiments, demonstrated the improvement in signal-to-noise ratio (SNR) and the extension of the lifetime of the [1-(13)C]lactate and [1-(13)C]alanine pools when compared with conventional non-spectral selective (NS) excitation. SNEP has also been shown to perform comparably well with multi-band (MB) excitation, yet SNEP possesses distinct advantages, including ease of implementation, less stringent demands on gradient performance, increased robustness to frequency drifts and B0 inhomogeneity as well as easier quantification involving the use of [1-(13)C]pyruvate-hydrate as a proxy for the actual [1-(13)C] pyruvate signal. SNEP is therefore a promising alternative for robust hyperpolarized [1-(13)C]pyruvate metabolic imaging with high fidelity.

Comparison of acquisition schemes for hyperpolarised ¹³C imaging

The aim of this study was to characterise and compare widely used acquisition strategies for hyperpolarised (13)C imaging. Free induction decay chemical shift imaging (FIDCSI), echo-planar spectroscopic imaging (EPSI), IDEAL spiral chemical shift imaging (ISPCSI) and spiral chemical shift imaging (SPCSI) sequences were designed for two different regimes of spatial resolution. Their characteristics were studied in simulations and in tumour-bearing rats after injection of hyperpolarised [1-(13)C]pyruvate on a clinical 3-T scanner. Two or three different sequences were used on the same rat in random order for direct comparison. The experimentally obtained lactate signal-to-noise ratio (SNR) in the tumour matched the simulations. Differences between the sequences were mainly found in the encoding efficiency, gradient demand and artefact behaviour. Although ISPCSI and SPCSI offer high encoding efficiencies, these non-Cartesian trajectories are more prone than EPSI and FIDCSI to artefacts from various sources. If the encoding efficiency is sufficient for the desired application, EPSI has been proven to be a robust choice. Otherwise, faster spiral acquisition schemes are recommended. The conclusions found in this work can be applied directly to clinical applications.

In vivo single-shot 13C spectroscopic imaging of hyperpolarized metabolites by spatiotemporal encoding

Hyperpolarized metabolic imaging is a growing field that has provided a new tool for analyzing metabolism, particularly in cancer. Given the short life times of the hyperpolarized signal, fast and effective spectroscopic imaging methods compatible with dynamic metabolic characterizations are necessary. Several approaches have been customized for hyperpolarized (13)C MRI, including CSI with a center-out k-space encoding, EPSI, and spectrally selective pulses in combination with spiral EPI acquisitions. Recent studies have described the potential of single-shot alternatives based on spatiotemporal encoding (SPEN) principles, to derive chemical-shift images within a sub-second period. By contrast to EPSI, SPEN does not require oscillating acquisition gradients to deliver chemical-shift information: its signal encodes both spatial as well as chemical shift information, at no extra cost in experimental complexity. SPEN MRI sequences with slice-selection and arbitrary excitation pulses can also be devised, endowing SPEN with the potential to deliver single-shot multi-slice chemical shift images, with a temporal resolution required for hyperpolarized dynamic metabolic imaging. The present work demonstrates this with initial in vivo results obtained from SPEN-based imaging of pyruvate and its metabolic products, after injection of hyperpolarized [1-(13)C]pyruvate. Multi-slice chemical-shift images of healthy rats were obtained at 4.7T in the region of the kidney, and 4D (2D spatial, 1D spectral, 1D temporal) data sets were obtained at 7T from a murine lymphoma tumor model.

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

Blip-reversed echo-planar imaging (EPI) is investigated as a method for measuring and correcting the spatial shifts that occur due to bulk frequency offsets in (13)C metabolic imaging in vivo. By reversing the k-space trajectory for every other time point, the direction of the spatial shift for a given frequency is reversed. Here, mutual information is used to find the 'best' alignment between images and thereby measure the frequency offset. Time-resolved 3D images of pyruvate/lactate/urea were acquired with 5 s temporal resolution over a 1 min duration in rats (N = 6). For each rat, a second injection was performed with the demodulation frequency purposely mis-set by +35 Hz, to test the correction for erroneous shifts in the images. Overall, the shift induced by the 35 Hz frequency offset was 5.9 ± 0.6 mm (mean ± standard deviation). This agrees well with the expected 5.7 mm shift based on the 2.02 ms delay between k-space lines (giving 30.9 Hz per pixel). The 0.6 mm standard deviation in the correction corresponds to a frequency-detection accuracy of 4 Hz. A method was presented for ensuring the spatial registration between (13)C metabolic images and conventional anatomical images when long echo-planar readouts are used. The frequency correction method was shown to have an accuracy of 4 Hz. Summing the spatially corrected frames gave a signal-to-noise ratio (SNR) improvement factor of 2 or greater, compared with the highest single frame.

Accelerating hyperpolarized metabolic imaging of the heart by exploiting spatiotemporal correlations

Hyperpolarized (13)C-labeled pyruvate is a promising tool to investigate cardiac metabolism. It has been shown that changes in substrate metabolism occur following the induction of ischemia. To investigate the metabolic changes that are confined to spatial regions, high spatiotemporal resolution is required. The present work exploits both spatial and temporal correlations using k-t principal component analysis (PCA) to undersample the spatiotemporal domain, thereby speeding up data acquisition. A numerical model was implemented to investigate optimal acquisition and reconstruction parameters for pyruvate, lactate and bicarbonate maps of the heart. Subsequently, prospectively undersampled in vivo data on rat hearts were acquired using a combination of spectral-spatial signal excitation and a variable-density single-shot echo planar readout. Using five-fold k-t PCA, a spatial resolution of 1 × 1 mm(2) at a temporal resolution of 3 s was achieved.

A catalyzing phantom for reproducible dynamic conversion of hyperpolarized [1-¹³C]-pyruvate

In vivo real time spectroscopic imaging of hyperpolarized ¹³C labeled metabolites shows substantial promise for the assessment of physiological processes that were previously inaccessible. However, reliable and reproducible methods of measurement are necessary to maximize the effectiveness of imaging biomarkers that may one day guide personalized care for diseases such as cancer. Animal models of human disease serve as poor reference standards due to the complexity, heterogeneity, and transient nature of advancing disease. In this study, we describe the reproducible conversion of hyperpolarized [1-¹³C]-pyruvate to [1-¹³C]-lactate using a novel synthetic enzyme phantom system. The rate of reaction can be controlled and tuned to mimic normal or pathologic conditions of varying degree. Variations observed in the use of this phantom compare favorably against within-group variations observed in recent animal studies. This novel phantom system provides crucial capabilities as a reference standard for the optimization, comparison, and certification of quantitative imaging strategies for hyperpolarized tracers.

Hyperpolarized 13C metabolic imaging using dissolution dynamic nuclear polarization

This article describes the basic physics of dissolution dynamic nuclear polarization (dissolution-DNP), and the impact of the resulting highly nonequilibrium spin states, on the physics of magnetic resonance imaging (MRI) detection. The hardware requirements for clinical translation of this technology are also presented. For studies that allow the use of externally administered agents, hyperpolarization offers a way to overcome normal magnetic resonance sensitivity limitations, at least for a brief T(1)-dependent observation window. A 10,000-100,000-fold signal-to-noise advantage provides an avenue for real-time measurement of perfusion, metabolite transport, exchange, and metabolism. The principles behind these measurements, as well as the choice of agent, and progress toward the application of hyperpolarized (13)C metabolic imaging in oncology, cardiology, and neurology are reviewed.

Molecular body imaging: MR imaging, CT, and US. Part II. Applications

Molecular imaging is expected to have a major impact on the early diagnosis of diseases and disease monitoring in the next decade. Traditionally, nuclear imaging techniques have been the mainstay of molecular imaging in the clinical arena. However, with continued development of molecularly targeted contrast agents for nonnuclear imaging techniques such as magnetic resonance (MR), computed tomography (CT), and ultrasonography (US), the spectrum of clinical molecular imaging applications is expanding. In the second part of this review series, an overview of applications of molecular MR imaging-, CT-, and US-based imaging strategies that show promise for clinical translation is presented, and key challenges that need to be addressed to successfully translate these promising techniques in the future are discussed. © RSNA, 2012.

Saturation-recovery metabolic-exchange rate imaging with hyperpolarized [1-13C] pyruvate using spectral-spatial excitation

Within the last decade hyperpolarized [1-13C] pyruvate chemical-shift imaging has demonstrated impressive potential for metabolic MR imaging for a wide range of applications in oncology, cardiology, and neurology. In this work, a highly efficient pulse sequence is described for time-resolved, multislice chemical shift imaging of the injected substrate and obtained downstream metabolites. Using spectral-spatial excitation in combination with single-shot spiral data acquisition, the overall encoding is evenly distributed between excitation and signal reception, allowing the encoding of one full two-dimensional metabolite image per excitation. The signal-to-noise ratio can be flexibly adjusted and optimized using lower flip angles for the pyruvate substrate and larger ones for the downstream metabolites. Selectively adjusting the excitation of the down-stream metabolites to 90° leads to a so-called "saturation-recovery" scheme with the detected signal content being determined by forward conversion of the available pyruvate. In case of repetitive excitations, the polarization is preserved using smaller flip angles for pyruvate. Metabolic exchange rates are determined spatially resolved from the metabolite images using a simplified two-site exchange model. This novel contrast is an important step toward more quantitative metabolic imaging. Goal of this work was to derive, analyze, and implement this "saturation-recovery metabolic exchange rate imaging" and demonstrate its capabilities in four rats bearing subcutaneous tumors.

A method for simultaneous echo planar imaging of hyperpolarized ¹³C pyruvate and ¹³C lactate

A rapid echo planar imaging sequence for dynamic imaging of [1-(13)C] lactate and [1-(13)C] pyruvate simultaneously was developed. Frequency-based separation of these metabolites was achieved by spatial shifting in the phase-encoded direction with the appropriate choice of echo spacing. Suppression of the pyruvate-hydrate and alanine resonances is achieved through an optimized spectral-spatial RF waveform. Signal sampling efficiency as a function of pyruvate and lactate excitation angle was simulated using two site exchange models. Dynamic imaging is demonstrated in a transgenic mouse model, and phantom validations of the RF pulse frequency selectivity were performed.

Generating super stimulated-echoes in MRI and their application to hyperpolarized C-13 diffusion metabolic imaging

Stimulated-echoes in MR can be used to provide high sensitivity to motion and flow, creating diffusion and perfusion weighting as well as T(1) contrast, but conventional approaches inherently suffer from a 50% signal loss. The super stimulated-echo, which uses a specialized radio-frequency (RF) pulse train, has been proposed in order to improve the signal while preserving motion and T(1) sensitivity. This paper presents a novel and straightforward method for designing the super stimulated-echo pulse train using inversion pulse design techniques. This method can also create adiabatic designs with an improved response to RF transmit field variations. The scheme was validated in phantom experiments and shown in vivo to improve signal-to-noise ratio (SNR). We have applied a super stimulated-echo to metabolic MRI with hyperpolarized (13)C-labeled molecules. For spectroscopic imaging of hyperpolarized agents, several repetition times are required but only a single stimulated-echo encoding is feasible, which can lead to unwanted motion blurring. To address this, a super stimulated-echo preparation scheme was used in which the diffusion weighting is terminated prior to the acquisition, and we observed a SNR increases of 60% in phantoms and 49% in vivo over a conventional stimulated-echo. Experiments following injection of hyperpolarized [1-(13)C] -pyruvate in murine transgenic cancer models have shown improved delineation for tumors since signals from metabolites within tumor tissues are retained while those from the vasculature are suppressed by the diffusion preparation scheme.

Effects of pyruvate dose on in vivo metabolism and quantification of hyperpolarized ¹³C spectra

Real-time in vivo measurements of metabolites are performed by signal enhancement of [1-(13)C]pyruvate using dynamic nuclear polarization, rapid dissolution and intravenous injection, acquisition of free induction decay signals and subsequent quantification of spectra. The commonly injected dose of hyperpolarized pyruvate is larger than typical tracer doses, with measurement before complete dilution of the injected bolus. Pyruvate is in exchange with its downstream metabolites lactate, alanine and bicarbonate. A transient exposure to high pyruvate blood concentrations may cause the saturation of cellular uptake and metabolic conversion. The goal of this study was to examine the effects of a [1-(13)C]pyruvate bolus on metabolic conversion in vivo. Spectra were quantified by three different methods: frequency-domain fitting with LCModel, time-domain fitting with AMARES and simple linear least-squares fitting in the time domain. Since the simple linear least-squares approach showed bleeding artifacts and LCModel produced noisier time signals. AMARES performed best in the quantification of in vivo hyperpolarized pyruvate spectra. We examined pyruvate doses of 0.1-0.4 mmol/kg (body mass) in male Wistar rats by acquiring slice-selective free induction decay signals in slices dominated by heart, liver and kidney tissue. Dose effects were noted in all cases, except for alanine in the cardiac slice below the dose of 0.2 mmol/kg. Our results indicate unlimited cellular uptake of pyruvate up to this dose and limited enzymatic activity of lactate dehydrogenase. In the cardiac slice above 0.2 mmol/kg and in liver and kidney slices, reflect limited cellular uptake or enzymatic activity, or a combination of both effects. The results indicate that the dose of pyruvate must be recognized as an important determinant for metabolic tissue kinetics, and saturation effects must be taken into account for the quantitative interpretation of the observed results.

Analysis of cancer metabolism by imaging hyperpolarized nuclei: prospects for translation to clinical research

A major challenge in cancer biology is to monitor and understand cancer metabolism in vivo with the goal of improved diagnosis and perhaps therapy. Because of the complexity of biochemical pathways, tracer methods are required for detecting specific enzyme-catalyzed reactions. Stable isotopes such as (13)C or (15)N with detection by nuclear magnetic resonance provide the necessary information about tissue biochemistry, but the crucial metabolites are present in low concentration and therefore are beyond the detection threshold of traditional magnetic resonance methods. A solution is to improve sensitivity by a factor of 10,000 or more by temporarily redistributing the populations of nuclear spins in a magnetic field, a process termed hyperpolarization. Although this effect is short-lived, hyperpolarized molecules can be generated in an aqueous solution and infused in vivo where metabolism generates products that can be imaged. This discovery lifts the primary constraint on magnetic resonance imaging for monitoring metabolism-poor sensitivity-while preserving the advantage of biochemical information. The purpose of this report was to briefly summarize the known abnormalities in cancer metabolism, the value and limitations of current imaging methods for metabolism, and the principles of hyperpolarization. Recent preclinical applications are described. Hyperpolarization technology is still in its infancy, and current polarizer equipment and methods are suboptimal. Nevertheless, there are no fundamental barriers to rapid translation of this exciting technology to clinical research and perhaps clinical care.

Tumor imaging using hyperpolarized 13C magnetic resonance spectroscopy

Dynamic nuclear polarization is an emerging technique for increasing the sensitivity of magnetic resonance imaging and spectroscopy, particularly for low-γ nuclei. The technique has been applied recently to a number of 13C-labeled cell metabolites in biological systems: the increase in signal-to-noise allows the spatial distribution of an injected molecule to be imaged as well as its metabolic product or products. This review highlights the most significant molecules investigated to date in preclinical cancer models, either in terms of their demonstrated metabolism in vivo or the biological processes that they can probe. In particular, label exchange between hyperpolarized 13C-labeled pyruvate and lactate, catalyzed by lactate dehydrogenase, has been shown to have a number of potential applications. Finally, techniques to image these molecules are also discussed as well as methods that may extend the lifetime of the hyperpolarized signal. Hyperpolarized magnetic resonance imaging and magnetic resonance spectroscopic imaging have shown great promise for the imaging of cancer in preclinical work, both for diagnosis and for monitoring therapy response. If the challenges in translating this technique to human imaging can be overcome, then it has the potential to significantly alter the management of cancer patients.

Rapid multislice imaging of hyperpolarized 13C pyruvate and bicarbonate in the heart

Hyperpolarization of spins via dynamic nuclear polarization (DNP) has been explored as a method to non-invasively study real-time metabolic processes occurring in vivo using (13)C-labeled substrates. Recently, hyperpolarized (13)C pyruvate has been used to characterize in vivo cardiac metabolism in the rat and pig. Conventional 3D spectroscopic imaging methods require in excess of 100 excitations, making it challenging to acquire a full cardiac-gated, breath-held, whole-heart volume. In this article, the development of a rapid multislice cardiac-gated spiral (13)C imaging pulse sequence consisting of a large flip-angle spectral-spatial excitation RF pulse combined with a single-shot spiral k-space trajectory for rapid imaging of cardiac metabolism is described. This sequence permits whole-heart coverage (6 slices, 8.8-mm in-plane resolution) in any plane, allowing imaging of the metabolites of interest, [1-(13)C] pyruvate, [1-(13)C] lactate, and (13)C bicarbonate, within a single breathhold. Pyruvate and bicarbonate cardiac volumes were acquired, while lactate images were not acquired due to low lactate levels in the animal model studied. The sequence was demonstrated with phantom experiments and in vivo testing in a pig model.

Frequency response of multipoint chemical shift-based spectral decomposition

PURPOSE

To provide a framework for characterizing the frequency response of multipoint chemical shift based species separation techniques.

MATERIALS AND METHODS

Multipoint chemical shift based species separation techniques acquire complex images at multiple echo times and perform maximum likelihood estimation to decompose signal from different species into separate images. In general, after a nonlinear process of estimating and demodulating the field map, these decomposition methods are linear transforms from the echo-time domain to the chemical-shift-frequency domain, analogous to the discrete Fourier transform (DFT). In this work we describe a technique for finding the magnitude and phase of chemical shift decomposition for input signals over a range of frequencies using numerical and experimental modeling and examine several important cases of species separation.

RESULTS

Simple expressions can be derived to describe the response to a wide variety of input signals. Agreement between numerical modeling and experimental results is very good.

CONCLUSION

Chemical shift-based species separation is linear, and therefore can be fully described by the magnitude and phase curves of the frequency response. The periodic nature of the frequency response has important implications for the robustness of various techniques for resolving ambiguities in field inhomogeneity.

Applications of hyperpolarized agents in solutions

This chapter provides an overview of pulse sequences adapted to hyperpolarized MR imaging. Applications of hyperpolarized agents in aqueous solution are reviewed. Vascular (e.g., angiography, perfusion, and catheter tracking) as well as metabolic (e.g., oncology, cardiology, neurology, and pH mapping) applications are covered. Due to the rapid development of new applications for hyperpolarized agents, a review format has been used for this chapter instead of a strict protocol/procedure structure.

Application of compressed sensing to in vivo 3D ¹⁹F CSI

This study shows how applying compressed sensing (CS) to (19)F chemical shift imaging (CSI) makes highly accurate and reproducible reconstructions from undersampled datasets possible. The missing background signal in (19)F CSI provides the required sparsity needed for application of CS. Simulations were performed to test the influence of different CS-related parameters on reconstruction quality. To test the proposed method on a realistic signal distribution, the simulation results were validated by ex vivo experiments. Additionally, undersampled in vivo 3D CSI mouse datasets were successfully reconstructed using CS. The study results suggest that CS can be used to accurately and reproducibly reconstruct undersampled (19)F spectroscopic datasets. Thus, the scanning time of in vivo(19)F CSI experiments can be significantly reduced while preserving the ability to distinguish between different (19)F markers. The gain in scan time provides high flexibility in adjusting measurement parameters. These features make this technique a useful tool for multiple biological and medical applications.

Microtesla MRI with dynamic nuclear polarization

Magnetic resonance imaging at microtesla fields is a promising imaging method that combines the pre-polarization technique and broadband signal reception by superconducting quantum interference device (SQUID) sensors to enable in vivo MRI at microtesla-range magnetic fields similar in strength to the Earth magnetic field. Despite significant advances in recent years, the potential of microtesla MRI for biomedical imaging is limited by its insufficient signal-to-noise ratio due to a relatively low sample polarization. Dynamic nuclear polarization (DNP) is a widely used approach that allows polarization enhancement by 2-4 orders of magnitude without an increase in the polarizing field strength. In this work, the first implementation of microtesla MRI with Overhauser DNP and SQUID signal detection is described. The first measurements of carbon-13 NMR spectra at microtesla fields are also reported. The experiments were performed at the measurement field of 96 μT, corresponding to Larmor frequency of 4 kHz for protons and 1 kHz for carbon-13. The Overhauser DNP was carried out at 3.5-5.7 mT fields using rf irradiation at 120 MHz. Objects for imaging included water phantoms and a cactus plant. Aqueous solutions of metabolically relevant sodium bicarbonate, pyruvate, alanine, and lactate, labeled with carbon-13, were used for NMR studies. All the samples were doped with TEMPO free radicals. The Overhauser DNP enabled nuclear polarization enhancement by factor as large as -95 for protons and as large as -200 for carbon-13, corresponding to thermal polarizations at 0.33 T and 1.1 T fields, respectively. These results demonstrate that SQUID-based microtesla MRI can be naturally combined with Overhauser DNP in one system, and that its signal-to-noise performance is greatly improved in this case. They also suggest that microtesla MRI can become an efficient tool for in vivo imaging of hyperpolarized carbon-13, produced by low-temperature dissolution DNP.

k-space water-fat decomposition with T2* estimation and multifrequency fat spectrum modeling for ultrashort echo time imaging

PURPOSE

To demonstrate the feasibility of combining a chemical shift-based water-fat separation method (IDEAL) with a 2D ultrashort echo time (UTE) sequence for imaging and quantification of the short T(2) tissues with robust fat suppression.

MATERIALS AND METHODS

A 2D multislice UTE data acquisition scheme was combined with IDEAL processing, including T(2)* estimation, chemical shift artifacts correction, and multifrequency modeling of the fat spectrum to image short T(2) tissues such as the Achilles tendon and meniscus both in vitro and in vivo. The integration of an advanced field map estimation technique into this combined method, such as region growing (RG), is also investigated.

RESULTS

The combination of IDEAL with UTE imaging is feasible and excellent water-fat separation can be achieved for the Achilles tendon and meniscus with simultaneous T(2)* estimation and chemical shift artifact correction. Multifrequency modeling of the fat spectrum yields more complete water-fat separation with more accurate correction for chemical shift artifacts. The RG scheme helps to avoid water-fat swapping.

CONCLUSION

The combination of UTE data acquisition with IDEAL has potential applications in imaging and quantifying short T(2) tissues, eliminating the necessity for fat suppression pulses that may directly suppress the short T(2) signals.

Fast dynamic 3D MR spectroscopic imaging with compressed sensing and multiband excitation pulses for hyperpolarized 13C studies

Hyperpolarized 13C MR spectroscopic imaging can detect not only the uptake of the pre-polarized molecule but also its metabolic products in vivo, thus providing a powerful new method to study cellular metabolism. Imaging the dynamic perfusion and conversion of these metabolites provides additional tissue information but requires methods for efficient hyperpolarization usage and rapid acquisitions. In this work, we have developed a time-resolved 3D MR spectroscopic imaging method for acquiring hyperpolarized 13C data by combining compressed sensing methods for acceleration and multiband excitation pulses to efficiently use the magnetization. This method achieved a 2 sec temporal resolution with full volumetric coverage of a mouse, and metabolites were observed for up to 60 sec following injection of hyperpolarized [1-(13)C]-pyruvate. The compressed sensing acquisition used random phase encode gradient blips to create a novel random undersampling pattern tailored to dynamic MR spectroscopic imaging with sampling incoherency in four (time, frequency, and two spatial) dimensions. The reconstruction was also tailored to dynamic MR spectroscopic imaging by applying a temporal wavelet sparsifying transform to exploit the inherent temporal sparsity. Customized multiband excitation pulses were designed with a lower flip angle for the [1-(13)C]-pyruvate substrate given its higher concentration than its metabolic products ([1-(13)C]-lactate and [1-(13)C]-alanine), thus using less hyperpolarization per excitation. This approach has enabled the monitoring of perfusion and uptake of the pyruvate, and the conversion dynamics to lactate and alanine throughout a volume with high spatial and temporal resolution.

In vivo application of sub-second spiral chemical shift imaging (CSI) to hyperpolarized 13C metabolic imaging: comparison with phase-encoded CSI

A fast spiral chemical shift imaging (CSI) has been developed to address the challenge of the limited acquisition window in hyperpolarized (13)C metabolic imaging. The sequence exploits the sparsity of the spectra and prior knowledge of resonance frequencies to reduce the measurement time by undersampling the data in the spectral domain. As a consequence, multiple reconstructions are necessary for any given data set as only frequency components within a selected bandwidth are reconstructed "in-focus" while components outside that band are severely blurred ("spectral tomosynthesis"). A variable-flip-angle scheme was used for optimal use of the longitudinal magnetization. The sequence was applied to sub-second metabolic imaging of the rat in vivo after injection of hyperpolarized [1-(13)C]-pyruvate on a clinical 3T MR scanner. The comparison with conventional CSI based on phase encoding showed similar signal-to-noise ratio (SNR) and spatial resolution in metabolic maps for the substrate and its metabolic products lactate, alanine, and bicarbonate, despite a 50-fold reduction in scan time for the spiral CSI acquisition. The presented results demonstrate that dramatic reductions in scan time are feasible in hyperpolarized (13)C metabolic imaging without a penalty in SNR or spatial resolution.

Fast volumetric spatial-spectral MR imaging of hyperpolarized 13C-labeled compounds using multiple echo 3D bSSFP

PURPOSE

The goal of this work was to develop a fast 3D chemical shift imaging technique for the noninvasive measurement of hyperpolarized (13)C-labeled substrates and metabolic products at low concentration.

MATERIALS AND METHODS

Multiple echo 3D balanced steady state magnetic resonance imaging (ME-3DbSSFP) was performed in vitro on a syringe containing hyperpolarized [1,3,3-2H3; 1-(13)C]2-hydroxyethylpropionate (HEP) adjacent to a (13)C-enriched acetate phantom, and in vivo on a rat before and after intravenous injection of hyperpolarized HEP at 1.5 T. Chemical shift images of the hyperpolarized HEP were derived from the multiple echo data by Fourier transformation along the echoes on a voxel by voxel basis for each slice of the 3D data set.

RESULTS

ME-3DbSSFP imaging was able to provide chemical shift images of hyperpolarized HEP in vitro, and in a rat with isotropic 7-mm spatial resolution, 93 Hz spectral resolution and 16-s temporal resolution for a period greater than 45 s.

CONCLUSION

Multiple echo 3D bSSFP imaging can provide chemical shift images of hyperpolarized (13)C-labeled compounds in vivo with relatively high spatial resolution and moderate spectral resolution. The increased signal-to-noise ratio of this 3D technique will enable the detection of hyperpolarized (13)C-labeled metabolites at lower concentrations as compared to a 2D technique.

Spiral demystified

Spiral acquisition schemes offer unique advantages such as flow compensation, efficient k-space sampling and robustness against motion that make this option a viable choice among other non-Cartesian sampling schemes. For this reason, the main applications of spiral imaging lie in dynamic magnetic resonance imaging such as cardiac imaging and functional brain imaging. However, these advantages are counterbalanced by practical difficulties that render spiral imaging quite challenging. Firstly, the design of gradient waveforms and its hardware requires specific attention. Secondly, the reconstruction of such data is no longer straightforward because k-space samples are no longer aligned on a Cartesian grid. Thirdly, to take advantage of parallel imaging techniques, the common generalized autocalibrating partially parallel acquisitions (GRAPPA) or sensitivity encoding (SENSE) algorithms need to be extended. Finally, and most notably, spiral images are prone to particular artifacts such as blurring due to gradient deviations and off-resonance effects caused by B(0) inhomogeneity and concomitant gradient fields. In this article, various difficulties that spiral imaging brings along, and the solutions, which have been developed and proposed in literature, will be reviewed in detail.

3D compressed sensing for highly accelerated hyperpolarized (13)C MRSI with in vivo applications to transgenic mouse models of cancer

High polarization of nuclear spins in liquid state through hyperpolarized technology utilizing dynamic nuclear polarization has enabled the direct monitoring of (13)C metabolites in vivo at a high signal-to-noise ratio. Acquisition time limitations due to T(1) decay of the hyperpolarized signal require accelerated imaging methods, such as compressed sensing, for optimal speed and spatial coverage. In this paper, the design and testing of a new echo-planar (13)C three-dimensional magnetic resonance spectroscopic imaging (MRSI) compressed sensing sequence is presented. The sequence provides up to a factor of 7.53 in acceleration with minimal reconstruction artifacts. The key to the design is employing x and y gradient blips during a fly-back readout to pseudorandomly undersample k(f)-k(x)-k(y) space. The design was validated in simulations and phantom experiments where the limits of undersampling and the effects of noise on the compressed sensing nonlinear reconstruction were tested. Finally, this new pulse sequence was applied in vivo in preclinical studies involving transgenic prostate cancer and transgenic liver cancer murine models to obtain much higher spatial and temporal resolution than possible with conventional echo-planar spectroscopic imaging methods.

Application of subsecond spiral chemical shift imaging to real-time multislice metabolic imaging of the rat in vivo after injection of hyperpolarized 13C1-pyruvate

Dynamic nuclear polarization can create hyperpolarized compounds with MR signal-to-noise ratio enhancements on the order of 10,000-fold. Both exogenous and normally occurring endogenous compounds can be polarized, and their initial concentration and downstream metabolic products can be assessed using MR spectroscopy. Given the transient nature of the hyperpolarized signal enhancement, fast imaging techniques are a critical requirement for real-time metabolic imaging. We report on the development of an ultrafast, multislice, spiral chemical shift imaging sequence, with subsecond acquisition time, achieved on a clinical MR scanner. The technique was used for dynamic metabolic imaging in rats, with measurement of time-resolved spatial distributions of hyperpolarized (13)C(1)-pyruvate and metabolic products (13)C(1)-lactate and (13)C(1)-alanine, with a temporal resolution of as fast as 1 s. Metabolic imaging revealed different signal time courses in liver from kidney. These results demonstrate the feasibility of real-time, hyperpolarized metabolic imaging and highlight its potential in assessing organ-specific kinetic parameters.

Quantification of hepatic steatosis with MRI: the effects of accurate fat spectral modeling

PURPOSE

To develop a chemical-shift-based imaging method for fat quantification that accounts for the complex spectrum of fat, and to compare this method with MR spectroscopy (MRS). Quantitative noninvasive biomarkers of hepatic steatosis are urgently needed for the diagnosis and management of nonalcoholic fatty liver disease (NAFLD).

MATERIALS AND METHODS

Hepatic steatosis was measured with "fat-fraction" images in 31 patients using a multiecho chemical-shift-based water-fat separation method at 1.5T. Fat-fraction images were reconstructed using a conventional signal model that considers fat as a single peak at -210 Hz relative to water ("single peak" reconstruction). Fat-fraction images were also reconstructed from the same source images using two methods that account for the complex spectrum of fat; precalibrated and self-calibrated "multipeak" reconstruction. Single-voxel MRS that was coregistered with imaging was performed for comparison.

RESULTS

Imaging and MRS demonstrated excellent correlation with single peak reconstruction (r(2) = 0.91), precalibrated multipeak reconstruction (r(2) = 0.94), and self-calibrated multipeak reconstruction (r(2) = 0.91). However, precalibrated multipeak reconstruction demonstrated the best agreement with MRS, with a slope statistically equivalent to 1 (0.96 +/- 0.04; P = 0.4), compared to self-calibrated multipeak reconstruction (0.83 +/- 0.05, P = 0.001) and single-peak reconstruction (0.67 +/- 0.04, P < 0.001).

CONCLUSION

Accurate spectral modeling is necessary for accurate quantification of hepatic steatosis with MRI.

Fast multiecho balanced SSFP metabolite mapping of (1)H and hyperpolarized (13)C compounds

OBJECT

To investigate the feasibility of multiecho balanced steady-state free precession (bSSFP)-based fast chemical shift mapping hyperpolarized (13)C metabolites. The overall goal was to reduce total imaging time and to increase spatial resolution compared to common chemical shift imaging (CSI).

MATERIALS AND METHODS

A multiecho bSSFP sequence in combination with an iterative reconstruction algorithm was implemented. (1)H experiments were performed on phantoms and on a human volunteer in order to investigate the feasibility of the method on a system with metabolite maps that are known beforehand. (13)C experiments were performed in vivo on pigs, where CSI images were acquired also for comparison.

RESULTS

Chemical shift images of three and four distinct (1)H resonance frequencies as well as chemical shift images of up to five hyperpolarized (13)C metabolites were successfully obtained.

CONCLUSION

Fast metabolite mapping based on multiecho balanced SSFP in combination with an iterative reconstruction approach could successfully separate several (1)H resonances and hyperpolarized (13)C metabolites.

Single-shot magnetic resonance spectroscopic imaging with partial parallel imaging

A magnetic resonance spectroscopic imaging (MRSI) pulse sequence based on proton-echo-planar-spectroscopic-imaging (PEPSI) is introduced that measures two-dimensional metabolite maps in a single excitation. Echo-planar spatial-spectral encoding was combined with interleaved phase encoding and parallel imaging using SENSE to reconstruct absorption mode spectra. The symmetrical k-space trajectory compensates phase errors due to convolution of spatial and spectral encoding. Single-shot MRSI at short TE was evaluated in phantoms and in vivo on a 3-T whole-body scanner equipped with a 12-channel array coil. Four-step interleaved phase encoding and fourfold SENSE acceleration were used to encode a 16 x 16 spatial matrix with a 390-Hz spectral width. Comparison with conventional PEPSI and PEPSI with fourfold SENSE acceleration demonstrated comparable sensitivity per unit time when taking into account g-factor-related noise increases and differences in sampling efficiency. LCModel fitting enabled quantification of inositol, choline, creatine, and N-acetyl-aspartate (NAA) in vivo with concentration values in the ranges measured with conventional PEPSI and SENSE-accelerated PEPSI. Cramer-Rao lower bounds were comparable to those obtained with conventional SENSE-accelerated PEPSI at the same voxel size and measurement time. This single-shot MRSI method is therefore suitable for applications that require high temporal resolution to monitor temporal dynamics or to reduce sensitivity to tissue movement.

Generalized k-space decomposition with chemical shift correction for non-Cartesian water-fat imaging

Chemical-shift artifacts associated with non-Cartesian imaging are more complex to model and less clinically acceptable than the bulk fat shift that occurs with conventional spin-warp Cartesian imaging. A novel k-space based iterative decomposition of water and fat with echo asymmetry and least-squares estimation (IDEAL) approach is introduced that decomposes multiple species while simultaneously correcting distortion of off-resonant species. The new signal model accounts for the additional phase accumulated by off-resonant spins at each point in the k-space acquisition trajectory. This phase can then be corrected by adjusting the decomposition matrix for each k-space point during the final IDEAL processing step with little increase in reconstruction time. The technique is demonstrated with water-fat decomposition using projection reconstruction (PR)/radial, spiral, and Cartesian spin-warp imaging of phantoms and human subjects, in each case achieving substantial correction of chemical-shift artifacts. Simulations of the point-spread-function (PSF) for off-resonant spins are examined to show the nature of the chemical-shift distortion for each acquisition. Also introduced is an approach to improve the signal model for species which have multiple resonant peaks. Many chemical species, including fat, have multiple resonant peaks, although such species are often approximated as a single peak. The improved multipeak decomposition is demonstrated with water-fat imaging, showing a substantial improvement in water-fat separation.

Compressed sensing for resolution enhancement of hyperpolarized 13C flyback 3D-MRSI

High polarization of nuclear spins in liquid state through dynamic nuclear polarization has enabled the direct monitoring of 13C metabolites in vivo at very high signal-to-noise, allowing for rapid assessment of tissue metabolism. The abundant SNR afforded by this hyperpolarization technique makes high-resolution 13C 3D-MRSI feasible. However, the number of phase encodes that can be fit into the short acquisition time for hyperpolarized imaging limits spatial coverage and resolution. To take advantage of the high SNR available from hyperpolarization, we have applied compressed sensing to achieve a factor of 2 enhancement in spatial resolution without increasing acquisition time or decreasing coverage. In this paper, the design and testing of compressed sensing suited for a flyback 13C 3D-MRSI sequence are presented. The key to this design was the undersampling of spectral k-space using a novel blipped scheme, thus taking advantage of the considerable sparsity in typical hyperpolarized 13C spectra. Phantom tests validated the accuracy of the compressed sensing approach and initial mouse experiments demonstrated in vivo feasibility.