Least-squares chemical shift separation for (13)C metabolic imaging

Advancements, Challenges, and Future Prospects in Clinical Hyperpolarized Magnetic Resonance Imaging: A Comprehensive Review

Hyperpolarized (HP) magnetic resonance imaging (MRI) is a groundbreaking imaging platform advancing from research to clinical practice, offering new possibilities for real-time, non-invasive metabolic imaging. This review explores the latest advancements, challenges, and future directions of HP MRI, emphasizing its transformative impact on both translational research and clinical applications. By employing techniques such as dissolution Dynamic Nuclear Polarization (dDNP), Parahydrogen-Induced Polarization (PHIP), Signal Amplification by Reversible Exchange (SABRE), and Spin-Exchange Optical Pumping (SEOP), HP MRI achieves enhanced nuclear spin polarization, enabling in vivo visualization of metabolic pathways with exceptional sensitivity. Current challenges, such as limited imaging windows, complex pre-scan protocols, and data processing difficulties, are addressed through innovative solutions like advanced pulse sequences, bolus tracking, and kinetic modeling. We highlight the evolution of HP MRI technology, focusing on its potential to revolutionize disease diagnosis and monitoring by revealing metabolic processes beyond the reach of conventional MRI and positron emission tomography (PET). Key advancements include the development of novel tracers like [2-13C]pyruvate and [1-13C]-alpha-ketoglutarate and improved data analysis techniques, broadening the scope of clinical metabolic imaging. Future prospects emphasize integrating artificial intelligence, standardizing imaging protocols, and developing new hyperpolarized agents to enhance reproducibility and expand clinical capabilities particularly in oncology, cardiology, and neurology. Ultimately, we envisioned HP MRI as a standardized modality for dynamic metabolic imaging in clinical practice.

Model-constrained reconstruction accelerated with Fourier-based undersampling for hyperpolarized [1-13 C] pyruvate imaging

PURPOSE

Model-constrained reconstruction with Fourier-based undersampling (MoReFUn) is introduced to accelerate the acquisition of dynamic MRI using hyperpolarized [1-13 C]-pyruvate.

METHODS

The MoReFUn method resolves spatial aliasing using constraints introduced by a pharmacokinetic model that describes the signal evolution of both pyruvate and lactate. Acceleration was evaluated on three single-channel data sets: a numerical digital phantom that is used to validate the accuracy of reconstruction and model parameter restoration under various SNR and undersampling ratios, prospectively and retrospectively sampled data of an in vitro dynamic multispectral phantom, and retrospectively undersampled imaging data from a prostate cancer patient to test the fidelity of reconstructed metabolite time series.

RESULTS

All three data sets showed successful reconstruction using MoReFUn. In simulation and retrospective phantom data, the restored time series of pyruvate and lactate maintained the image details, and the mean square residual error of the accelerated reconstruction increased only slightly (< 10%) at a reduction factor up to 8. In prostate data, the quantitative estimation of the conversion-rate constant of pyruvate to lactate was achieved with high accuracy of less than 10% error at a reduction factor of 2 compared with the conversion rate derived from unaccelerated data.

CONCLUSION

The MoReFUn technique can be used as an effective and reliable imaging acceleration method for metabolic imaging using hyperpolarized [1-13 C]-pyruvate.

Spurious phase correction in rapid metabolic imaging

IDEAL-type magnetic resonance spectroscopic imaging (MRSI) sequences require the acquisition of several datasets using optimized sampling in the time domain to reconstruct metabolite maps. Each unitary scan consists of a selective slice (2D) or slab (3D) excitation followed by an evolution time and then the acquisition of the spatially encoded signal. It is critical that the phase variation during the evolution time for each scan is only dependent on chemical shifts. In this paper, we described the apparition of spurious phase due to either the transmit or the receive frequency. The presence of this unwanted phase depends on (i) where the commutation between these two frequencies is performed and (ii) how it is done, as there are two phase commutation modes: continuous and coherent. We present the correction needed in function of the different cases. It appears that some solutions are universal. However, it is critical to know which case is implemented on the MRI scanner, which is not always easy information to have. We illustrated several cases with our preclinical MRI by using the IDEAL spiral method on a 13C phantom.

Improving deuterium metabolic imaging (DMI) signal-to-noise ratio by spectroscopic multi-echo bSSFP: A pancreatic cancer investigation

PURPOSE

Deuterium metabolic imaging (DMI) maps the uptake of deuterated precursors and their conversion into lactate and other markers of tumor metabolism. Even after leveraging ² H's short T₁ s, DMI's signal-to-noise ratio (SNR) is limited. We hypothesize that a multi-echo balanced steady-state free precession (ME-bSSFP) approach would increase SNR compared to chemical shift imaging (CSI), while achieving spectral isolation of the metabolic precursors and products.

METHODS

Suitably tuned ² H ME-bSSFP (five echo times [TEs], ΔTE = 2.2 ms, repetition time [TR]/flip-angle = 12 ms/60°) was implemented at 15.2T and compared to CSI (TR/flip-angle = 95 ms/90°) regarding SNR and spectral isolation, in simulations, in deuterated phantoms and for the in vivo diagnosis of a mouse tumor model of pancreatic adenocarcinoma (N = 10).

RESULTS

Simulations predicted an SNR increase vs. CSI of 3-5, and that the peaks of ² H-water, ² H_6,6' -glucose, and ² H_3,3' -lactate can be well isolated by ME-bSSFP; phantoms confirmed this. In vivo, at equal spatial resolution (1.25 × 1.25 mm² ) and scan time (10 min), ² H_6,6' -glucose's and ² H_3,3' -lactate's SNR were indeed higher for bSSFP than for CSI, three-fold for glucose (57 ± 30 vs. 19 ± 11, P < .001), doubled for water (13 ± 5 vs. 7 ± 3, P = .005). The time courses and overall localization of all metabolites agreed well, comparing CSI against ME-bSSFP. However, a clearer localization of glucose in kidneys and bladder, the detection of glucose-avid rims in certain tumors, and a heterogeneous pattern of intra-tumor lactate production could only be observed using ME-bSSFP's higher resolution.

CONCLUSIONS

ME-bSSFP provides greater SNR per unit time than CSI, providing for higher spatial resolution mapping of glucose uptake and lactate production in tumors.

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.

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.

Comparison of selective excitation and multi-echo chemical shift encoding for imaging of hyperpolarized [1-13C]pyruvate

Imaging methods for hyperpolarized (HP) ¹³C agents must sample the evolution of signal from multiple agents with distinct chemical shifts within a very brief timeframe (typically < 1 min), which is challenging using conventional imaging methods. In this work, we compare two of the most commonly used HP spectroscopic imaging methods, spectral-spatial selective excitation and multi-echo chemical shift encoding (CSE, also referred to as IDEAL), for a typical preclinical HP [1-¹³C]pyruvate imaging scan at 7 T. Both spectroscopic encoding techniques were implemented and validated in HP experiments imaging enzyme phantoms and the murine kidney. SNR performance of these two spectroscopic imaging approaches was compared in numerical simulations and phantom experiments using a single-shot flyback EPI readout for spatial encoding. With identical effective excitation angles, the SNR of images acquired with spectral-spatial excitations and CSE were found to be effectively equivalent.

Biomedical Applications of the Dynamic Nuclear Polarization and Parahydrogen Induced Polarization Techniques for Hyperpolarized 13C MR Imaging

Since the first pioneering report of hyperpolarized [1-13C]pyruvate magnetic resonance imaging (MRI) of the Warburg effect in prostate cancer patients, clinical dissemination of the technique has been rapid; close to 10 sites worldwide now possess a polarizer fit for the clinic, and more than 30 clinical trials, predominantly for oncological applications, are already registered on the US and European clinical trials databases. Hyperpolarized 13C probes to study pathophysiological processes beyond the Warburg effect, including tricarboxylic acid cycle metabolism, intra-cellular pH and cellular necrosis have also been demonstrated in the preclinical arena and are pending clinical translation, and the simultaneous injection of multiple co-polarized agents is opening the door to high-sensitivity, multi-functional molecular MRI with a single dose. Here, we review the biomedical applications to date of the two polarization methods that have been used for in vivo hyperpolarized 13C molecular MRI; namely, dissolution dynamic nuclear polarization and parahydrogen-induced polarization. The basic concept of hyperpolarization and the fundamental theory underpinning these two key 13C hyperpolarization methods, along with recent technological advances that have facilitated biomedical realization, are also covered.

In vivo methods and applications of xenon-129 magnetic resonance

Hyperpolarised gas lung MRI using xenon-129 can provide detailed 3D images of the ventilated lung airspaces, and can be applied to quantify lung microstructure and detailed aspects of lung function such as gas exchange. It is sensitive to functional and structural changes in early lung disease and can be used in longitudinal studies of disease progression and therapy response. The ability of 129Xe to dissolve into the blood stream and its chemical shift sensitivity to its local environment allow monitoring of gas exchange in the lungs, perfusion of the brain and kidneys, and blood oxygenation. This article reviews the methods and applications of in vivo129Xe MR in humans, with a focus on the physics of polarisation by optical pumping, radiofrequency coil and pulse sequence design, and the in vivo applications of 129Xe MRI and MRS to examine lung ventilation, microstructure and gas exchange, blood oxygenation, and perfusion of the brain and kidneys.

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.

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.

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.

3D hyperpolarized C-13 EPI with calibrationless parallel imaging

With the translation of metabolic MRI with hyperpolarized 13C agents into the clinic, imaging approaches will require large volumetric FOVs to support clinical applications. Parallel imaging techniques will be crucial to increasing volumetric scan coverage while minimizing RF requirements and temporal resolution. Calibrationless parallel imaging approaches are well-suited for this application because they eliminate the need to acquire coil profile maps or auto-calibration data. In this work, we explored the utility of a calibrationless parallel imaging method (SAKE) and corresponding sampling strategies to accelerate and undersample hyperpolarized 13C data using 3D blipped EPI acquisitions and multichannel receive coils, and demonstrated its application in a human study of [1-13C]pyruvate metabolism.

An open source, 3D printed preclinical MRI phantom for repeated measures of contrast agents and reference standards

In medical imaging, clinicians, researchers and technicians have begun to use 3D printing to create specialized phantoms to replace commercial ones due to their customizable and iterative nature. Presented here is the design of a 3D printed open source, reusable magnetic resonance imaging (MRI) phantom, capable of flood-filling, with removable samples for measurements of contrast agent solutions and reference standards, and for use in evaluating acquisition techniques and image reconstruction performance. The phantom was designed using SolidWorks, a computer-aided design software package. The phantom consists of custom and off-the-shelf parts and incorporates an air hole and Luer Lock system to aid in flood filling, a marker for orientation of samples in the filled mode and bolt and tube holes for assembly. The cost of construction for all materials is under $90. All design files are open-source and available for download. To demonstrate utility, B₀ field mapping was performed using a series of gadolinium concentrations in both the unfilled and flood-filled mode. An excellent linear agreement (R²>0.998) was observed between measured relaxation rates (R₁/R₂) and gadolinium concentration. The phantom provides a reliable setup to test data acquisition and reconstruction methods and verify physical alignment in alternative nuclei MRI techniques (e.g. carbon-13 and fluorine-19 MRI). A cost-effective, open-source MRI phantom design for repeated quantitative measurement of contrast agents and reference standards in preclinical research is presented. Specifically, the work is an example of how the emerging technology of 3D printing improves flexibility and access for custom phantom design.

Cross-sectional correlation between hepatic R2* and proton density fat fraction (PDFF) in children with hepatic steatosis

PURPOSE

To determine the relationship between hepatic proton density fat fraction (PDFF) and R2* in vivo.

MATERIALS AND METHODS

In this Health Insurance Portability and Accountability Act (HIPAA)-compliant, Institutional Review Board (IRB)-approved, cross-sectional study, we conducted a secondary analysis of 3T magnetic resonance imaging (MRI) exams performed as part of prospective research studies in children in whom conditions associated with iron overload were excluded clinically. Each exam included low-flip-angle, multiecho magnitude (-M) and complex (-C) based chemical-shift-encoded MRI techniques with spectral modeling of fat to generate hepatic PDFF and R2* parametric maps. For each technique and each patient, regions of interest were placed on the maps in each of the nine Couinaud segments, and composite whole-liver PDFF and R2* values were calculated. Pearson's correlation coefficients between PDFF and R2* were computed for each MRI technique. Correlations were compared using Steiger's test.

RESULTS

In all, 184 children (123 boys, 61 girls) were included in this analysis. PDFF estimated by MRI-M and MRI-C ranged from 1.1-35.4% (9.44 ± 8.76) and 2.1-38.1% (10.1 ± 8.7), respectively. R2* estimated by MRI-M and MRI-C ranged from 32.6-78.7 s-1 (48.4 ± 9.8) and 27.2-71.5 s-1 (42.2 ± 8.6), respectively. There were strong and significant correlations between hepatic PDFF and R2* values estimated by MRI-M (r = 0.874; P < 0.0001) and MRI-C (r = 0.853; P < 0.0001). The correlation coefficients (0.874 vs. 0.853) were not significantly different (P = 0.15).

CONCLUSION

Hepatic PDFF and R2* are strongly correlated with each other in vivo. This relationship was observed using two different MRI techniques.

LEVEL OF EVIDENCE

2 Technical Efficacy: Stage 1 J. Magn. Reson. Imaging 2018;47:418-424.

MRI proton density fat fraction is robust across the biologically plausible range of triglyceride spectra in adults with nonalcoholic steatohepatitis

BACKGROUND

Proton density fat fraction (PDFF) estimation requires spectral modeling of the hepatic triglyceride (TG) signal. Deviations in the TG spectrum may occur, leading to bias in PDFF quantification.

PURPOSE

To investigate the effects of varying six-peak TG spectral models on PDFF estimation bias.

STUDY TYPE

Retrospective secondary analysis of prospectively acquired clinical research data.

POPULATION

Forty-four adults with biopsy-confirmed nonalcoholic steatohepatitis.

FIELD STRENGTH/SEQUENCE

Confounder-corrected chemical-shift-encoded 3T MRI (using a 2D multiecho gradient-recalled echo technique with magnitude reconstruction) and MR spectroscopy.

ASSESSMENT

In each patient, 61 pairs of colocalized MRI-PDFF and MRS-PDFF values were estimated: one pair used the standard six-peak spectral model, the other 60 were six-peak variants calculated by adjusting spectral model parameters over their biologically plausible ranges. MRI-PDFF values calculated using each variant model and the standard model were compared, and the agreement between MRI-PDFF and MRS-PDFF was assessed.

STATISTICAL TESTS

MRS-PDFF and MRI-PDFF were summarized descriptively. Bland-Altman (BA) analyses were performed between PDFF values calculated using each variant model and the standard model. Linear regressions were performed between BA biases and mean PDFF values for each variant model, and between MRI-PDFF and MRS-PDFF.

RESULTS

Using the standard model, mean MRS-PDFF of the study population was 17.9 ± 8.0% (range: 4.1-34.3%). The difference between the highest and lowest mean variant MRI-PDFF values was 1.5%. Relative to the standard model, the model with the greatest absolute BA bias overestimated PDFF by 1.2%. Bias increased with increasing PDFF (P < 0.0001 for 59 of the 60 variant models). MRI-PDFF and MRS-PDFF agreed closely for all variant models (R²  = 0.980, P < 0.0001).

DATA CONCLUSION

Over a wide range of hepatic fat content, PDFF estimation is robust across the biologically plausible range of TG spectra. Although absolute estimation bias increased with higher PDFF, its magnitude was small and unlikely to be clinically meaningful.

LEVEL OF EVIDENCE

3 Technical Efficacy: Stage 2 J. Magn. Reson. Imaging 2018;47:995-1002.

A chemical shift encoding (CSE) approach for spectral selection in fluorine-19 MRI

PURPOSE

To develop a chemical shift encoding (CSE) approach for fluorine-19 MRI of perfluorocarbons in the presence of multiple known fluorinated chemical species.

THEORY AND METHODS

A multi-echo CSE technique is applied for spectral separation of the perfluorocarbon perfluoro-15-crown-5-ether (PFCE) and isoflurane (ISO) based on their chemical shifts at 4.7 T. Cramér-Rao lower bound analysis is used to identify echo combinations with optimal signal-to-noise performance. Signal contributions are fit with a multispectral fluorine signal model using a non-linear least squares estimation reconstruction directly from k-space data. This CSE approach is tested in fluorine-19 phantoms and in a mouse with a 2D and 3D spoiled gradient-echo acquisition using multiple echo times determined from Cramér-Rao lower bound analysis.

RESULTS

Cramér-Rao lower bound analysis for PFCE and ISO separation shows signal-to-noise performance is maximized with a 0.33 ms echo separation. A linear behavior (R²  = 0.987) between PFCE signal and known relative PFCE volume is observed in CSE reconstructed images using a mixed PFCE/ISO phantom. Effective spatial and spectral separation of PFCE and ISO is shown in phantoms and in vivo.

CONCLUSION

Feasibility of a gradient-echo CSE acquisition and image reconstruction approach with optimized noise performance is demonstrated through fluorine-19 MRI of PFCE with effective removal of ISO signal contributions. Magn Reson Med 79:2183-2189, 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.

The use of hyperpolarized carbon-13 magnetic resonance for molecular imaging

Until recently, molecular imaging using magnetic resonance (MR) has been limited by the modality's low sensitivity, especially with non-proton nuclei. The advent of hyperpolarized (HP) MR overcomes this limitation by substantially enhancing the signal of certain biologically important probes through a process known as external nuclear polarization, enabling real-time assessment of tissue function and metabolism. The metabolic information obtained by HP MR imaging holds significant promise in the clinic, where it could play a critical role in disease diagnosis and therapeutic monitoring. This review will provide a comprehensive overview of the developments made in the field of hyperpolarized MR, including advancements in polarization techniques and delivery, probe development, pulse sequence optimization, characterization of healthy and diseased tissues, and the steps made towards clinical translation.

Efficient spectroscopic imaging by an optimized encoding of pretargeted resonances

PURPOSE

A "relaxation-enhanced" (RE) approach to acquire in vivo localized spectra with flat baselines and good sensitivity has been recently proposed. As RE MR spectroscopy (MRS) targets a subset of a priori known resonances, new possibilities arise to acquire spectroscopic imaging data in faster, more efficient manners. This is hereby illustrated by Spectroscopically Encoded Chemical Shift Imaging (SECSI).

METHODS

SECSI delivers spectral/spatial correlations by collecting gradient echo trains whose timings are defined by the shifts of the resonances to be disentangled. Condition number considerations allow one to unravel these image contributions for various sites by a simple matrix inversion. The efficiency of the ensuing method is high enough to enable a sampling of additional spatial axes by means of their phase encoding in spin-echo trains.

RESULTS

The one-dimensional (1D) spectral / 2D spatial SECSI acquisitions were implemented on phantom, ex vivo, and in vivo models. In all cases, quality site-resolved images were obtained. The experimentally observed enhancements were consistent with theoretical signal-to-noise ratio derivations.

CONCLUSION

While still bound by MRSI's sensitivity limitations, a novel spectroscopic imaging protocol exploiting a priori information, selective excitations and multiple echo encodings, was proposed and demonstrated. The method is promising when dealing with high T2 / T2* ratios, sparse data, or hyperpolarization studies. Magn Reson Med 77:511-519, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

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

PURPOSE

Balanced steady-state free precession (bSSFP) sequences can provide superior signal-to-noise ratio efficiency for hyperpolarized (HP) carbon-13 (¹³ C) magnetic resonance imaging by efficiently utilizing the nonrecoverable magnetization, but managing their spectral response is challenging in the context of metabolic imaging. A new spectrally selective bSSFP sequence was developed for fast imaging of multiple HP ¹³ C metabolites with high spatiotemporal resolution.

THEORY AND METHODS

This novel approach for bSSFP spectral selectivity incorporates optimized short-duration spectrally selective radiofrequency pulses within a bSSFP pulse train and a carefully chosen repetition time to avoid banding artifacts.

RESULTS

The sequence enabled subsecond 3D dynamic spectrally selective imaging of ¹³ C metabolites of copolarized [1-¹³ C]pyruvate and [¹³ C]urea at 2-mm isotropic resolution, with excellent spectral selectivity (∼100:1). The sequence was successfully tested in phantom studies and in vivo studies with normal mice.

CONCLUSION

This sequence is expected to benefit applications requiring dynamic volumetric imaging of metabolically active ¹³ C compounds at high spatiotemporal resolution, including preclinical studies at high field and, potentially, clinical studies. Magn Reson Med 78:963-975, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

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.

Multiparametric human hepatocellular carcinoma characterization and therapy response evaluation by hyperpolarized (13) C MRSI

Individual tumor characterization and treatment response monitoring based on current medical imaging methods remain challenging. This work investigates hyperpolarized (13) C compounds in an orthotopic rat hepatocellular carcinoma (HCC) model system before and after transcatheter arterial embolization (TAE). HCC ranks amongst the top six most common cancer types in humans and accounts for one-third of cancer-related deaths worldwide. Early therapy response monitoring could aid in the development of personalized therapy approaches and novel therapeutic concepts. Measurements with selectively (13) C-labeled and hyperpolarized urea, pyruvate and fumarate were performed in tumor-bearing rats before and after TAE. Two-dimensional, slice-selective MRSI was used to obtain spatially resolved maps of tumor perfusion, cell energy metabolic conversion rates and necrosis, which were additionally correlated with immunohistochemistry. All three injected compounds, taken together with their respective metabolites, exhibited similar signal distributions. TAE induced a decrease in blood flow into the tumor and thus a decrease in tumor to muscle and tumor to liver ratios of urea, pyruvate and its metabolites, alanine and lactate, whereas conversion rates remained stable or increased on TAE in tumor, muscle and liver tissue. Conversion from fumarate to malate successfully indicated individual levels of necrosis, and global malate signals after TAE suggested the washout of fumarase or malate itself on necrosis. This study presents a combination of three (13) C compounds as novel candidate biomarkers for a comprehensive characterization of genetically and molecularly diverse HCC using hyperpolarized MRSI, enabling the simultaneous detection of differences in tumor perfusion, metabolism and necrosis. If, as in this study, bolus dynamics are not required and qualitative perfusion information is sufficient, the desired information could be extracted from hyperpolarized fumarate and pyruvate alone, acquired at higher fields with better spectral separation. Copyright © 2016 John Wiley & Sons, Ltd.

Simultaneous assessment of cardiac metabolism and perfusion using copolarized [1-13 C]pyruvate and 13 C-urea

Purpose

To demonstrate the feasibility of imaging a bolus of co‐polarized [1‐¹³C]pyruvate and ¹³C‐urea to simultaneously assess both metabolism and perfusion in the rodent heart.

Methods

Copolarized [1‐¹³C]pyruvate and ¹³C‐urea was imaged using a multi‐echo, flow‐sensitized spiral pulse sequence. Healthy rats were scanned in a two‐factor factorial design (n = 12 total; metabolism: overnight fasting versus fed with dichloroacetate injection; perfusion: rest versus adenosine stress‐induced hyperemia).

Results

Alterations in metabolism were detected by changes in pyruvate metabolism into ¹³C‐bicarbonate. Statistically independent alterations in perfusion were detected by changes in myocardial pyruvate and urea signals.

Conclusion

The new pulse sequence was used to obtain maps of metabolism and perfusion in the rodent heart in a single acquisition. This hyperpolarized ¹³C imaging test is expected to enable new studies in which the cardiac metabolism/perfusion mismatch can be studied in the acute environment. Magn Reson Med 77:151–158, 2017. © 2016 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine

Selective spectroscopic imaging of hyperpolarized pyruvate and its metabolites using a single-echo variable phase advance method in balanced SSFP

PURPOSE

In balanced steady state free precession (bSSFP), the signal intensity has a well-known dependence on the off-resonance frequency, or, equivalently, the phase advance between successive radiofrequency (RF) pulses. The signal profile can be used to resolve the contributions from the spectrally separated metabolites. This work describes a method based on use of a variable RF phase advance to acquire spatial and spectral data in a time-efficient manner for hyperpolarized 13C MRI.

THEORY AND METHODS

The technique relies on the frequency response from a bSSFP acquisition to acquire relatively rapid, high-resolution images that may be reconstructed to separate contributions from different metabolites. The ability to produce images from spectrally separated metabolites was demonstrated in vitro, as well as in vivo following administration of hyperpolarized 1-13C pyruvate in mice with xenograft tumors.

RESULTS

In vivo images of pyruvate, alanine, pyruvate hydrate, and lactate were reconstructed from four images acquired in 2 s with an in-plane resolution of 1.25 × 1.25 mm(2) and 5 mm slice thickness.

CONCLUSION

The phase advance method allowed acquisition of spectroscopically selective images with high spatial and temporal resolution. This method provides an alternative approach to hyperpolarized 13C spectroscopic MRI that can be combined with other techniques such as multiecho or fluctuating equilibrium bSSFP. Magn Reson Med 76:1102-1115, 2016. © 2015 Wiley Periodicals, Inc.

MRSI via fully-refocused spatiotemporal encoding with polychromatic spectral pulses

A novel method for the rapid acquisition of quality multi-slice 2D images targeting a small number of spectroscopic resonances, is introduced and illustrated. The method exploits the robustness derived from recently proposed spatiotemporal encoding (SPEN) methods, when operating in the so-called "fully refocused" mode. Fully-refocused SPEN provides high-fidelity single-shot images thanks to its refocusing of all offset-derived effects throughout the course of the acquisition. This refocusing, however, prevents exploiting such robustness for spectroscopic imaging. We propose here a solution to this limitation, based on the use of polychromatic refocusing pulses. It is shown that if used to address a series of a priori known resonance positions, these pulses can lead to quality spectroscopic images in a small number of scans - generally equal or slightly larger than the number of targeted peaks. Such strategy is explored in combination with both fully-refocused SPEN and echo-planar-imaging (EPI) acquisitions. The expected SPEN advantages were observed in both phantom-based models, and in in vivo results of fat and water separation in mice at 7 T.

Hyperpolarized (129) Xe imaging of the rat lung using spiral IDEAL

PURPOSE

To implement and optimize a single-shot spiral encoding strategy for rapid 2D IDEAL projection imaging of hyperpolarized (Hp) (129) Xe in the gas phase, and in the pulmonary tissue (PT) and red blood cells (RBCs) compartments of the rat lung, respectively.

THEORY AND METHODS

A theoretical and experimental point spread function analysis was used to optimize the spiral k-space read-out time in a phantom. Hp (129) Xe IDEAL images from five healthy rats were used to: (i) optimize flip angles by a Bloch equation analysis using measured kinetics of gas exchange and (ii) investigate the feasibility of the approach to characterize the exchange of Hp (129) Xe.

RESULTS

A read-out time equal to approximately 1.8 × T2* was found to provide the best trade-off between spatial resolution and signal-to-noise ratio (SNR). Spiral IDEAL approaches that use the entire dissolved phase magnetization should give an SNR improvement of a factor of approximately three compared with Cartesian approaches with similar spatial resolution. The IDEAL strategy allowed imaging of gas, PT, and RBC compartments with sufficient SNR and temporal resolution to permit regional gas exchange measurements in healthy rats.

CONCLUSION

Single-shot spiral IDEAL imaging of gas, PT and RBC compartments and gas exchange is feasible in rat lung using Hp (129) Xe. Magn Reson Med 76:566-576, 2016. © 2015 Wiley Periodicals, Inc.

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.

Application of flow sensitive gradients for improved measures of metabolism using hyperpolarized (13) c MRI

PURPOSE

To develop the use of bipolar gradients to suppress partial-volume and flow-related artifacts from macrovascular, hyperpolarized spins.

THEORY AND METHODS

Digital simulations were performed over a range of spatial resolutions and gradient strengths to determine the optimal bipolar gradient strength and duration to suppress flowing spins while minimizing signal loss from static tissue. In vivo experiments were performed to determine the efficacy of this technique to suppress vascular signal in the study of hyperpolarized [1-(13)C]pyruvate renal metabolism.

RESULTS

Digital simulations showed that in the absence of bipolar gradients, partial-volume artifacts from the vasculature were still present, causing underestimation of the apparent reaction rate of pyruvate to lactate (kP). The addition of a bipolar gradient with b = 32 s/mm(2) sufficiently suppressed the vascular signal without a substantial decrease in signal from static tissue. In vivo results corroborate digital simulations, with similar peak lactate signal to noise ratio (SNR) but substantially different kP in the presence of bipolar gradients.

CONCLUSION

The proposed approach suppresses signal from flowing spins while minimizing signal loss from static tissue, removing contaminating signal from the vasculature and increasing kinetic modeling accuracy without substantially sacrificing SNR or temporal resolution.

Chemical shift encoded imaging of hyperpolarized (13) C pyruvate

PURPOSE

To demonstrate a reconstruction technique for separating signal from different hyperpolarized carbon-13 metabolites.

METHODS

A reconstruction method is described for chemical shift encoded separation of the signal from pyruvate and its downstream metabolites. This method uses consistency of the data with the signal model rather than an additional free-induction decay (FID) acquisition to estimate the B0 offset. Compressed sensing was also integrated into the reconstruction allowing reconstruction of metabolite images from undersampled datasets. The performance of the reconstruction was assessed using thermal phantoms, digital phantoms, and in vivo hyperpolarized [1-(13) C] pyruvate experiments.

RESULTS

Thermal and digital phantoms indicate that metabolite separation is feasible given Signal-to-noise ratio > 5 and an initial B0 offset estimate within -105 Hz to 90 Hz of the actual B0 offset. In vivo comparisons to an existing FID calibrated reconstruction show improved fidelity in regions with significant field map inhomogeneity provided that these field map variations are accounted for using an additional proton acquisition. Prospectively and retrospectively undersampled studies show acceleration factors of 2 are feasible using compressed sensing.

CONCLUSION

A reconstruction framework for the separation of signal from pyruvate and its downstream metabolites is shown. This reconstruction eliminates the need to acquire additional calibration FID acquisition and allows acceleration through compressed sensing.

Apparent rate constant mapping using hyperpolarized [1-(13)C]pyruvate

Hyperpolarization of [1-13C]pyruvate in solution allows real-time measurement of uptake and metabolism using MR spectroscopic methods. After injection and perfusion, pyruvate is taken up by the cells and enzymatically metabolized into downstream metabolites such as lactate, alanine, and bicarbonate. In this work, we present comprehensive methods for the quantification and interpretation of hyperpolarized 13C metabolite signals. First, a time-domain spectral fitting method is described for the decomposition of FID signals into their metabolic constituents. For this purpose, the required chemical shift frequencies are automatically estimated using a matching pursuit algorithm. Second, a time-discretized formulation of the two-site exchange kinetic model is used to quantify metabolite signal dynamics by two characteristic rate constants in the form of (i) an apparent build-up rate (quantifying the build-up of downstream metabolites from the pyruvate substrate) and (ii) an effective decay rate (summarizing signal depletion due to repetitive excitation, T1-relaxation and backward conversion). The presented spectral and kinetic quantification were experimentally verified in vitro and in vivo using hyperpolarized [1-13C]pyruvate. Using temporally resolved IDEAL spiral CSI, spatially resolved apparent rate constant maps are also extracted. In comparison to single metabolite images, apparent build-up rate constant maps provide improved contrast by emphasizing metabolically active tissues (e.g. tumors) and suppression of high perfusion regions with low conversion (e.g. blood vessels). Apparent build-up rate constant mapping provides a novel quantitative image contrast for the characterization of metabolic activity. Its possible implementation as a quantitative standard will be subject to further studies.

Direct water and fat determination in two-point Dixon imaging with flexible echo times

PURPOSE

Identifying water and fat unambiguously in multipoint Dixon imaging often requires phase correction, which can be challenging and may fail. The purpose of this work is to present a geometric interpretation of the two-point Dixon method with flexible echo times (TEs) and to investigate the conditions under which water and fat can be determined directly without phase correction.

METHODS

Geometrically, the equation for the magnitude of the acquired signal at a given TE represents an ellipse in the water-fat plane centered at the origin. Determining water and fat in two-point Dixon imaging thus amounts to finding the correct intercept between two ellipses from the signals at two TEs. At the right TE combinations, the physicality requirement that water and fat be non-negative can be used to select a unique water and fat solution. Systematic computer simulations were conducted to examine the ranges of the TEs for which this approach is feasible and how different noise levels impact the feasibility. Phantom and in vivo experiments on a 1.5-T whole-body MRI scanner were used to validate the computer simulations.

RESULTS

In simulation and phantom experiments, nearly all pixels of pure water or pure fat were reliably identified based on the physicality requirement alone for a range of practically useful TE combinations (e.g., around 3 ms/6 ms at 1.5 T) and at moderate to high SNR levels (≥ 25). At other TE combinations, finding the correct solution based on the physicality requirement alone was not feasible or became sensitive to noise. In vivo findings were in overall agreement with the simulation and phantom studies, although the percentage of pixels that were correctly determined was lower.

CONCLUSIONS

The problem of direct water and fat determination without phase correction can be understood geometrically. Using the physicality requirement, it is possible to identify the different TE combinations and imaging conditions under which water and fat imaging can be performed either completely without phase correction or by generating a first-pass solution that can be used to improve the processing reliability of a phase-correction based method.

Bolus tracking for improved metabolic imaging of hyperpolarised compounds

Dynamic nuclear polarisation has enabled real-time metabolic imaging of pyruvate and its metabolites. Conventional imaging sequences rely on predefined settings and do not account for intersubject variations in biological parameters such as perfusion. We present a fully automatic real-time bolus tracking sequence for hyperpolarised substrates which starts the imaging acquisition at a defined point on the bolus curve. This reduces artefacts due to signal change and allows for a more efficient use of hyperpolarised magnetisation. For single time point imaging methods, bolus tracking enables a more reliable and consistent quantification of metabolic activity. An RF excitation with a small flip angle is used to obtain slice-selective pyruvate tracking information in rats. Moreover, in combination with a copolarised urea and pyruvate injection, spectrally selective tracking on urea allows obtaining localised bolus tracking information without depleting the pyruvate signal. Particularly with regard to clinical application, the bolus tracking technique could provide an important step towards a routine assessment protocol which removes operator dependencies and ensures comparable results.

Hybrid multiband excitation multiecho acquisition for hyperpolarized (13) C spectroscopic imaging

PURPOSE

Fast dynamic imaging of hyperpolarized (13) C-labeled pyruvate and its downstream metabolites shows great potential for probing metabolic changes in the heart. Sequences that allow for fast encoding of the spectral and spatial information of the myocardial metabolism and optimal signal excitation are usually limited by gradient performance, especially at high magnetic fields. Here we propose a combination of a spectral-spatial multiband excitation and multiecho readout to overcome these limitations.

METHODS

By using a low-bandwidth, two-pulse excitation, a thinner slice compared with conventional spectral-spatial excitation is achieved, while at the same time allowing for low flip angle excitation on pyruvate and high flip angle excitation on bicarbonate and lactate, which optimizes signal-to-noise ratio (SNR) in cardiac metabolic imaging. The implementation was evaluated in 13 healthy female Sprague-Dawley rats at 9.4T.

RESULTS

Using a slice thickness of 4 mm, a mean (± standard deviation) peak SNR of 18.3 (±8.4), 15.2 (±6.6), and 8.6 (±2.0) was observed for pyruvate, lactate, and bicarbonate, respectively.

CONCLUSION

This approach provides high SNR in metabolic images while at the same time allowing for a thin slice selection even at high magnetic fields. This is crucial in metabolic imaging in small animal models.

Joint spatial-spectral reconstruction and k-t spirals for accelerated 2D spatial/1D spectral imaging of 13C dynamics

PURPOSE

To develop a novel imaging technique to reduce the number of excitations and required scan time for hyperpolarized (13)C imaging.

METHODS

A least-squares based optimization and reconstruction is developed to simultaneously solve for both spatial and spectral encoding. By jointly solving both domains, spectral imaging can potentially be performed with a spatially oversampled single echo spiral acquisition. Digital simulations, phantom experiments, and initial in vivo hyperpolarized [1-(13)C]pyruvate experiments were performed to assess the performance of the algorithm as compared to a multi-echo approach.

RESULTS

Simulations and phantom data indicate that accurate single echo imaging is possible when coupled with oversampling factors greater than six (corresponding to a worst case of pyruvate to metabolite ratio < 9%), even in situations of substantial T(2)* decay and B(0) heterogeneity. With lower oversampling rates, two echoes are required for similar accuracy. These results were confirmed with in vivo data experiments, showing accurate single echo spectral imaging with an oversampling factor of 7 and two echo imaging with an oversampling factor of 4.

CONCLUSION

The proposed k-t approach increases data acquisition efficiency by reducing the number of echoes required to generate spectroscopic images, thereby allowing accelerated acquisition speed, preserved polarization, and/or improved temporal or spatial resolution.

Chemical shift encoding-based water-fat separation methods

The suppression of signal from fat constitutes a basic requirement in many applications of magnetic resonance imaging. To date, this is predominantly achieved during data acquisition, using fat saturation, inversion recovery, or water excitation methods. Postponing the separation of signal from water and fat until image reconstruction holds the promise of resolving some of the problems associated with these methods, such as failure in the presence of field inhomogeneities or contrast agents. In this article, methods are reviewed that rely on the difference in chemical shift between the hydrogen atoms in water and fat to perform such a retrospective separation. The basic principle underlying these so-called Dixon methods is introduced, and some fundamental implementations of the required chemical shift encoding in the acquisition and the subsequent water-fat separation in the reconstruction are described. Practical issues, such as the selection of key parameters and the appearance of typical artifacts, are illustrated, and a broad range of applications is demonstrated, including abdominal, cardiovascular, and musculoskeletal imaging. Finally, advantages and disadvantages of these Dixon methods are summarized, and emerging opportunities arising from the availability of information on the amount and distribution of fat are discussed.

Proton-density fat fraction and simultaneous R2* estimation as an MRI tool for assessment of osteoporosis

OBJECTIVE

To investigate multi-echo chemical shift-encoded MRI-based mapping of proton density fat fraction (PDFF) and fat-corrected R2* in bone marrow as biomarkers for osteoporosis assessment.

METHODS

Fifty-one patients (28 female; mean age 69.7 ± 9.0 years) underwent dual energy X-ray absorptiometry (DXA). On the basis of the t score, 173 valid vertebrae bodies were divided into three groups (healthy, osteopenic and osteoporotic). Three echo chemical shift-encoded MRI sequences were acquired at 3 T. PDFF and R2* with correction for multiple-peak fat (R2*MP) were measured for each vertebral body. Kruskal-Wallis test and post hoc analysis were performed to evaluate differences between groups. Further, the area under the curve (AUC) for each technique was calculated using logistic regression analysis.

RESULTS

On the basis of DXA, 92 samples were normal (53 %), 47 osteopenic (27 %) and 34 osteoporotic (20 %). PDFF was increased in osteoporosis compared with healthy (P = 0.007). R2*MP showed significant differences between normal and osteopenia (P = 0.004), and between normal and osteoporosis (P < 0.001). AUC to differentiate between normal and osteoporosis was 0.698 for R2*MP, 0.656 for PDFF and 0.74 for both combined.

CONCLUSION

PDFF and R2*MP are moderate biomarkers for osteoporosis. PDFF and R2*MP combination might improve the prediction in differentiating healthy subjects from those with osteoporosis.

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.

In vivo 3D spatial/1D spectral imaging by spatiotemporal encoding: a new single-shot experimental and processing approach

A novel method for acquiring and processing quality multislice spectroscopically resolved 2D images in a single shot is introduced and illustrated. By contrast to the majority of single-scan spectroscopic imaging sequences developed so far, the method here discussed is not based on the acquisition of echo planar data in the k/t-space, but rather on the use of recently proposed spatiotemporal encoding methods. These techniques provide a robust alternative to classical techniques, as they can scan two spatial plus one spectral dimension by oscillating a single imaging gradient. This work demonstrates that the use of extended spectral/spatial super-resolution algorithms coupled to new experimental spatiotemporal encoding formulations based on swept inversions rather than on chirped excitations can lead to novel spatiotemporal encoding-based tools for resolving complex multisliced 2D images according to the chemical shifts in subsecond experiments. A number of phantom-based models were explored to clarify the relative merits of this technique and estimate its sensitivity performance. In vivo results of fat and water separation on abdominal imaging of mice at 7 T and on human breast imaging at 3 T are presented.

Chemical shift MR imaging methods for the quantification of transcatheter lipiodol delivery to the liver: preclinical feasibility studies in a rodent model

PURPOSE

To demonstrate the feasibility of using chemical shift magnetic resonance (MR) imaging fat-water separation methods for quantitative estimation of transcatheter lipiodol delivery to liver tissues.

MATERIALS AND METHODS

Studies were performed in accordance with institutional Animal Care and Use Committee guidelines. Proton nuclear MR spectroscopy was first performed to identify lipiodol spectral peaks and relative amplitudes. Next, phantoms were constructed with increasing lipiodol-water volume fractions. A multiecho chemical shift-based fat-water separation method was used to quantify lipiodol concentration within each phantom. Six rats served as controls; 18 rats underwent catheterization with digital subtraction angiography guidance for intraportal infusion of a 15%, 30%, or 50% by volume lipiodol-saline mixture. MR imaging measurements were used to quantify lipiodol delivery to each rat liver. Lipiodol concentration maps were reconstructed by using both single-peak and multipeak chemical shift models. Intraclass and Spearman correlation coefficients were calculated for statistical comparison of MR imaging-based lipiodol concentration and volume measurements to reference standards (known lipiodol phantom compositions and the infused lipiodol dose during rat studies).

RESULTS

Both single-peak and multipeak measurements were well correlated to phantom lipiodol concentrations (r(2) > 0.99). Lipiodol volume measurements were progressively and significantly higher when comparing between animals receiving different doses (P < .05 for each comparison). MR imaging-based lipiodol volume measurements strongly correlated with infused dose (intraclass correlation coefficients > 0.93, P < .001) with both single- and multipeak approaches.

CONCLUSION

Chemical shift MR imaging fat-water separation methods can be used for quantitative measurements of lipiodol delivery to liver tissues.

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.