Fast high-resolution electric properties tomography using three-dimensional quantitative transient-state imaging-based water fraction estimation.

In this study, we aimed to develop a fast and robust high-resolution technique for clinically feasible electrical properties tomography based on water content maps (wEPT) using Quantitative Transient-state Imaging (QTI), a multiparametric transient state-based method that is similar to MR fingerprinting. Compared with the original wEPT implementation based on standard spin-echo acquisition, QTI provides robust electrical properties quantification towards B1 + inhomogeneities and full quantitative relaxometry data. To validate the proposed approach, 3D QTI data of 12 healthy volunteers were acquired on a 1.5 T scanner. QTI-provided T1 maps were used to compute water content maps of the tissues using an empirical relationship based on literature ex-vivo measurements. Assuming that electrical properties are modulated mainly by tissue water content, the water content maps were used to derive electrical conductivity and relative permittivity maps. The proposed technique was compared with a conventional phase-only Helmholtz EPT (HH-EPT) acquisition both within whole white matter, gray matter, and cerebrospinal fluid masks, and within different white and gray matter subregions. In addition, QTI-based wEPT was retrospectively applied to four multiple sclerosis adolescent and adult patients, compared with conventional contrast-weighted imaging in terms of lesion delineation, and quantitatively assessed by measuring the variation of electrical properties in lesions. Results obtained with the proposed approach agreed well with theoretical predictions and previous in vivo findings in both white and gray matter. The reconstructed maps showed greater anatomical detail and lower variability compared with standard phase-only HH-EPT. The technique can potentially improve delineation of pathology when compared with conventional contrast-weighted imaging and was able to detect significant variations in lesions with respect to normal-appearing tissues. In conclusion, QTI can reliably measure conductivity and relative permittivity of brain tissues within a short scan time, opening the way to the study of electric properties in clinical settings.

Magnetic resonance fingerprinting-based myelin water fraction mapping for the assessment of white matter maturation and integrity in typical development and leukodystrophies.

A quantitative biomarker for myelination, such as myelin water fraction (MWF), would boost the understanding of normative and pathological neurodevelopment, improving patients' diagnosis and follow-up. We quantified the fraction of a rapidly relaxing pool identified as MW using multicomponent three-dimensional (3D) magnetic resonance fingerprinting (MRF) to evaluate white matter (WM) maturation in typically developing (TD) children and alterations in leukodystrophies (LDs). We acquired DTI and 3D MRF-based R1, R2 and MWF data of 15 TD children and 17 LD patients (9 months-12.5 years old) at 1.5 T. We computed normative maturation curves in corpus callosum and corona radiata and performed WM tract profile analysis, comparing MWF with R1, R2 and fractional anisotropy (FA). Normative maturation curves demonstrated a steep increase for all tissue parameters in the first 3 years of age, followed by slower growth for MWF while R1, R2R2 and FA reached a plateau. Unlike FA, MWF values were similar for regions of interest (ROIs) with different degrees of axonal packing, suggesting independence from fiber bundle macro-organization and higher myelin specificity. Tract profile analysis indicated a specific spatial pattern of myelination in the major fiber bundles, consistent across subjects. LD were better distinguished from TD by MWF rather than FA, showing reduced MWF with respect to age-matched controls in both ROI-based and tract analysis. In conclusion, MRF-based MWF provides myelin-specific WM maturation curves and is sensitive to alteration due to LDs, suggesting its potential as a biomarker for WM disorders. As MRF allows fast simultaneous acquisition of relaxometry and MWF, it can represent a valuable diagnostic tool to study and follow up developmental WM disorders in children.

Daytime Variation in Kidney Perfusion, Oxygenation, and Sodium Concentration Assessed by Multiparametric MRI in Healthy Volunteers.

BACKGROUND: MRI can provide information on kidney structure, perfusion, and oxygenation. Furthermore, it allows for the assessment of kidney sodium concentrations and handling, allowing multiparametric evaluation of kidney physiology. Multiparametric MRI is promising for establishing prognosis and monitoring treatment responses in kidney diseases, but its intraindividual variation during the day is unresolved. PURPOSE: To investigate the variation in multiparametric MRI measurements from the morning to the evening. STUDY TYPE: Prospective. POPULATION: Ten healthy volunteers, aged 29 ± 5 without history of kidney disease. FIELD STRENGTH/SEQUENCE: 3 T/T1 mapping, blood-oxygen level dependent imaging, arterial spin labeling perfusion imaging, diffusion weighted imaging, and sodium imaging. ASSESSMENT: A multiparametric MRI protocol, yielding T1, R2*, ADC, renal blood flow and renal sodium levels, was acquired in the morning, noon, and evening. The participants were fasting prior to the first examination. Urine biochemical analyses were performed to complement MRI data. The cortex and medulla were analyzed separately in a semi-automatic fashion, and gradients of total sodium concentration (TSC) and R2* gradients were calculated from outer cortex to inner medulla. STATISTICAL TEST: Analyses of variance and mixed-effects models to estimate differences from time of day. Coefficients of variation to assess variability within and between participants. A P-value <0.05 was considered statistically significant. RESULTS: The coefficients of variation varied from 5% to 18% for proton-based parametric sequences, while it was 38% for TSC over a day. DATA CONCLUSION: Multiparametric MRI is stable over the day. The coefficients of variation over a day were lower for proton multiparametric MRI, but higher for sodium MRI. EVIDENCE LEVEL: 2 TECHNICAL EFFICACY: Stage 2.

Simultaneous relaxometry and morphometry of human brain structures with 3D magnetic resonance fingerprinting: a multicenter, multiplatform, multifield-strength study.

Relaxation times and morphological information are fundamental magnetic resonance imaging-derived metrics of the human brain that reflect the status of the underlying tissue. Magnetic resonance fingerprinting (MRF) enables simultaneous acquisition of T1 and T2 maps inherently aligned to the anatomy, allowing whole-brain relaxometry and morphometry in a single scan. In this study, we revealed the feasibility of 3D MRF for simultaneous brain structure-wise morphometry and relaxometry. Comprehensive test-retest scan analyses using five 1.5-T and three 3.0-T systems from a single vendor including different scanner types across 3 institutions demonstrated that 3D MRF-derived morphological information and relaxation times are highly repeatable at both 1.5 T and 3.0 T. Regional cortical thickness and subcortical volume values showed high agreement and low bias across different field strengths. The ability to acquire a set of regional T1, T2, thickness, and volume measurements of neuroanatomical structures with high repeatability and reproducibility facilitates the ability of longitudinal multicenter imaging studies to quantitatively monitor changes associated with underlying pathologies, disease progression, and treatments.

Imaging gas-exchange lung function and brain tissue uptake of hyperpolarized 129 Xe using sampling density-weighted MRSI.

PURPOSE: Imaging of the different resonances of hyperpolarized 129 Xe in the brain and lungs was performed using a 3D sampling density-weighted MRSI technique in healthy volunteers. METHODS: Four volunteers underwent dissolved-phase hyperpolarized 129 Xe imaging in the lung with the MRSI technique, which was designed to improve the point-spread function while preserving SNR (1799 phase-encoding steps, 14-s breath hold, 2.1-cm isotropic resolution). A frequency-tailored RF excitation pulse was implemented to reliably excite both the 129 Xe gas and dissolved phase (tissue/blood signal) with 0.1° and 10° flip angles, respectively. Images of xenon gas in the lung airspaces and xenon dissolved in lung tissue/blood were used to generate quantitative signal ratio maps. The method was also optimized and used for imaging dissolved resonances of 129 Xe in the brain in 2 additional volunteers. RESULTS: High-quality regional spectra of hyperpolarized 129 Xe were achieved in both the lung and the brain. Ratio maps of the different xenon resonances were obtained in the lung with sufficient SNR (> 10) at both 1.5 T and 3 T, making a triple Lorentzian fit possible and enabling the measurement of relaxation times and xenon frequency shifts on a voxel-wise basis. The imaging technique was successfully adapted for brain imaging, resulting in the first demonstration of 3D xenon brain images with a 2-cm isotropic resolution. CONCLUSION: Density-weighted MRSI is an SNR and encoding-efficient way to image 129 Xe resonances in the lung and the brain, providing a valuable tool to quantify regional spectroscopic information.

Considering whole-body metabolism in hyperpolarized MRI through 13 C breath analysis-An alternative way to quantification and normalization?

PURPOSE: Hyperpolarized [1-13 C]pyruvate MRI is an emerging clinical tool for metabolic imaging. It has the potential for absolute quantitative metabolic imaging. However, the method itself is not quantitative, limiting comparison of images across both time and between individuals. Here, we propose a simple signal normalization to the whole-body oxidative metabolism to overcome this limitation. THEORY AND METHODS: A simple extension of the model-free ratiometric analysis of hyperpolarized [1-13 C]pyruvate MRI is presented, using the expired 13 CO2 in breath for normalization. The proposed framework was investigated in two porcine cohorts (N = 11) subjected to local renal hypoperfusion defects and subsequent [1-13 C]pyruvate MRI. A breath sample was taken before the [1-13 C]pyruvate injection and 5 min after. The raw MR signal from both the healthy and intervened kidney in the two cohorts was normalized using the 13 CO2 in the expired air. RESULTS: 13 CO2 content in the expired air was significantly different between the two cohorts. Normalization to this reduced the coefficients of variance in the aerobic metabolic sensitive pathways by 25% for the alanine/pyruvate ratio, and numerical changes were observed in the bicarbonate/pyruvate ratio. The lactate/pyruvate ratio was largely unaltered (<2%). CONCLUSION: Our results indicate that normalizing the hyperpolarized 13 C-signal ratios by the 13 CO2 content in expired air can reduce variation as well as improve specificity of the method by normalizing the metabolic readout to the overall metabolic status of the individual. The method is a simple and cheap extension to the hyperpolarized 13 C exam.

Assessment of focal renal ischemia-reperfusion injury in a porcine model using hyperpolarized [1-13 C]pyruvate MRI.

PURPOSE: Ischemic injury in the kidney is a common pathophysiological event associated with both acute kidney injury and chronic kidney disease; however, regional ischemia-reperfusion as seen in thromboembolic renal disease is often undetectable and thus subclinical. Here, we assessed the metabolic alterations following subclinical focal ischemia-reperfusion injury with hyperpolarized [1-13 C]pyruvate MRI in a porcine model. METHODS: Five pigs were subjected to 60 min of focal kidney ischemia. After 90 min of reperfusion, a multiparametric proton MRI protocol was performed on a clinical 3T scanner system. Metabolism was evaluated using 13 C MRI following infusion of hyperpolarized [1-13 C]pyruvate. Ratios of pyruvate to its detectable metabolites (lactate, bicarbonate, and alanine) were used to quantify metabolism. RESULTS: The focal ischemia-reperfusion injury resulted in injured areas with a mean size of 0.971 cm3 (±1.019). Compared with the contralateral kidney, the injured areas demonstrated restricted diffusion (1269 ± 83.59 × 10-6 mm2 /s vs. 1530 ± 52.73 × 10-6 mm2 /s; p = 0.006) and decreased perfusion (158.8 ± 29.4 mL/100 mL/min vs. 274 ± 63.1 mL/100 mL/min; p = 0.014). In the metabolic assessment, the injured areas displayed increased lactate/pyruvate ratios compared with the entire ipsilateral and the contralateral kidney (0.35 ± 0.13 vs. 0.27 ± 0.1 vs. 0.25 ± 0.1; p = 0.0086). Alanine/pyruvate ratio was unaltered, and we were unable to quantify bicarbonate due to low signal. CONCLUSION: MRI with hyperpolarized [1-13 C]pyruvate in a clinical setup is capable of detecting the acute, subtle, focal metabolic changes following ischemia. This may prove to be a valuable future addition to the renal MRI suite.

Initial feasibility and challenges of hyperpolarized 129 Xe MRI in neonates with bronchopulmonary dysplasia.

PURPOSE: The underlying functional and microstructural lung disease in neonates who are born preterm (bronchopulmonary dysplasia, BPD) remains poorly characterized. Moreover, there is a lack of suitable techniques to reliably assess lung function in this population. Here, we report our preliminary experience with hyperpolarized 129 Xe MRI in neonates with BPD. METHODS: Neonatal intensive care patients with established BPD were recruited (N = 9) and imaged at a corrected gestational age of median:40.7 (range:37.1, 44.4) wk using a 1.5T neonatal scanner. 2D 129 Xe ventilation and diffusion-weighted images and dissolved phase spectroscopy were acquired, alongside 1 H 3D radial UTE. 129 Xe images were acquired during a series of short apneic breath-holds (˜3 s). 1 H UTE images were acquired during tidal breathing. Ventilation defects were manually identified and qualitatively compared to lung structures on UTE. ADCs were calculated on a voxel-wise basis. The signal ratio of the 129 Xe red blood cell (RBC) and tissue membrane (M) resonances from spectroscopy was determined. RESULTS: Spiral-based 129 Xe ventilation imaging showed good image quality and sufficient sensitivity to detect mild ventilation abnormalities in patients with BPD. 129 Xe ADC values were elevated above that expected given healthy data in older children and adults (median:0.046 [range:0.041, 0.064] cm2 s-1 ); the highest value obtained from an extremely pre-term patient. 129 Xe spectroscopy revealed a low RBC/M ratio (0.14 [0.06, 0.21]). CONCLUSION: We have demonstrated initial feasibility of 129 Xe lung MRI in neonates. With further data, the technique may help guide management of infant lung diseases in the neonatal period and beyond.

Clinically feasible B1 field correction for multi-organ sodium imaging at 3 T.

Sodium MRI allows the non-invasive quantification of intra-organ sodium concentration. RF inhomogeneity introduces uncertainty in this estimated concentration. B1 field corrections can be used to overcome some of these limitations. However, the low signal-to-noise ratio in sodium MRI makes accurate B1 mapping in reasonable scan times challenging. The study aims to evaluate Bloch-Siegert off-resonance (BLOSI) B1 field correction for sodium MRI using a 3D Fermat looped, orthogonally encoded trajectories (FLORET) read-out trajectory. We propose a clinically feasible B1 field map correction method for sodium imaging at 3 T, evaluating five healthy subjects' brain, heart blood, kidneys, and thigh muscle. We scanned the subjects twice for repeatability measures and used sodium phantoms to determine organ total sodium concentration. Conventional proton scans were compared with sodium images for organ structural integrity. The BLOSI approach based on the 3D FLORET read-out trajectory was used in B1 field correction and 3D density-adapted radial acquisition for sodium imaging. Results indicate improvements in sodium imaging based on B1 field correction in a clinically feasible protocol. Improvements are determined in all organs by enhanced anatomical representation, organ homogeneity, and an increase in the total sodium concentration after applying a B1 field correction. The proposed BLOSI-based B1 field correction using a 3D FLORET read-out trajectory is clinically feasible for sodium imaging, which is shown in the brain, heart, kidney, and thigh muscle. This supports using fast B1 field mapping in the clinical setting.

Deuterium metabolic imaging and hyperpolarized 13C-MRI of the normal human brain at clinical field strength reveals differential cerebral metabolism.

Deuterium metabolic imaging (DMI) and hyperpolarized 13C-pyruvate MRI (13C-HPMRI) are two emerging methods for non-invasive and non-ionizing imaging of tissue metabolism. Imaging cerebral metabolism has potential applications in cancer, neurodegeneration, multiple sclerosis, traumatic brain injury, stroke, and inborn errors of metabolism. Here we directly compare these two non-invasive methods at 3 T for the first time in humans and show how they simultaneously probe both oxidative and non-oxidative metabolism. DMI was undertaken 1-2 h after oral administration of [6,6'-2H2]glucose, and 13C-MRI was performed immediately following intravenous injection of hyperpolarized [1-13C]pyruvate in ten and nine normal volunteers within each arm respectively. DMI was used to generate maps of deuterium-labelled water, glucose, lactate, and glutamate/glutamine (Glx) and the spectral separation demonstrated that DMI is feasible at 3 T. 13C-HPMRI generated maps of hyperpolarized carbon-13 labelled pyruvate, lactate, and bicarbonate. The ratio of 13C-lactate/13C-bicarbonate (mean 3.7 ± 1.2) acquired with 13C-HPMRI was higher than the equivalent 2H-lactate/2H-Glx ratio (mean 0.18 ± 0.09) acquired using DMI. These differences can be explained by the route of administering each probe, the timing of imaging after ingestion or injection, as well as the biological differences in cerebral uptake and cellular physiology between the two molecules. The results demonstrate these two metabolic imaging methods provide different yet complementary readouts of oxidative and reductive metabolism within a clinically feasible timescale. Furthermore, as DMI was undertaken at a clinical field strength within a ten-minute scan time, it demonstrates its potential as a routine clinical tool in the future.

Lung Abnormalities Detected with Hyperpolarized 129Xe MRI in Patients with Long COVID.

Background Post-COVID-19 condition encompasses symptoms following COVID-19 infection that linger at least 4 weeks after the end of active infection. Symptoms are wide ranging, but breathlessness is common. Purpose To determine if the previously described lung abnormalities seen on hyperpolarized (HP) pulmonary xenon 129 (129Xe) MRI scans in participants with post-COVID-19 condition who were hospitalized are also present in participants with post-COVID-19 condition who were not hospitalized. Materials and Methods In this prospective study, nonhospitalized participants with post-COVID-19 condition (NHLC) and posthospitalized participants with post-COVID-19 condition (PHC) were enrolled from June 2020 to August 2021. Participants underwent chest CT, HP 129Xe MRI, pulmonary function testing, and the 1-minute sit-to-stand test and completed breathlessness questionnaires. Control subjects underwent HP 129Xe MRI only. CT scans were analyzed for post-COVID-19 interstitial lung disease severity using a previously published scoring system and full-scale airway network (FAN) modeling. Analysis used group and pairwise comparisons between participants and control subjects and correlations between participant clinical and imaging data. Results A total of 11 NHLC participants (four men, seven women; mean age, 44 years ± 11 [SD]; 95% CI: 37, 50) and 12 PHC participants (10 men, two women; mean age, 58 years ±10; 95% CI: 52, 64) were included, with a significant difference in age between groups (P = .05). Mean time from infection was 287 days ± 79 (95% CI: 240, 334) and 143 days ± 72 (95% CI: 105, 190) in NHLC and PHC participants, respectively. NHLC and PHC participants had normal or near normal CT scans (mean, 0.3/25 ± 0.6 [95% CI: 0, 0.63] and 7/25 ± 5 [95% CI: 4, 10], respectively). Gas transfer (Dlco) was different between NHLC and PHC participants (mean Dlco, 76% ± 8 [95% CI: 73, 83] vs 86% ± 8 [95% CI: 80, 91], respectively; P = .04), but there was no evidence of other differences in lung function. Mean red blood cell-to-tissue plasma ratio was different between volunteers (mean, 0.45 ± 0.07; 95% CI: 0.43, 0.47]) and PHC participants (mean, 0.31 ± 0.10; 95% CI: 0.24, 0.37; P = .02) and between volunteers and NHLC participants (mean, 0.37 ± 0.10; 95% CI: 0.31, 0.44; P = .03) but not between NHLC and PHC participants (P = .26). FAN results did not correlate with Dlco) or HP 129Xe MRI results. Conclusion Nonhospitalized participants with post-COVID-19 condition (NHLC) and posthospitalized participants with post-COVID-19 condition (PHC) showed hyperpolarized pulmonary xenon 129 MRI and red blood cell-to-tissue plasma abnormalities, with NHLC participants demonstrating lower gas transfer than PHC participants despite having normal CT findings. © RSNA, 2022 Online supplemental material is available for this article. See also the editorial by Parraga and Matheson in this issue.

RF coil design for accurate parallel imaging on 13 C MRSI using 23 Na sensitivity profiles.

PURPOSE: To develop a coil-based method to obtain accurate sensitivity profiles in 13 C MRI at 3T from the endogenous 23 Na. An eight-channel array is designed for 13 C MR acquisitions. As application examples, the array is used for two-fold accelerated acquisitions of both hyperpolarized 13 C metabolic imaging of pig kidneys and the human brain. METHODS: A flexible coil array was tuned optimally for 13 C at 3T (32.1 MHz), with the coil coupling coefficients matched to be nearly identical at the resonance frequency of 23 Na (33.8 MHz). This is done by enforcing a high decoupling (obtained through highly mismatched preamplifiers) and adjusting the coupling frequency response. The SNR performance is compared to reference coils. RESULTS: The measured sensitivity profiles on a phantom showed high spatial similarity for 13 C and 23 Na resonances, with average noise correlation of 9 and 11%, respectively. For acceleration factors 2, 3, and 4, the obtained maximum g-factors were 1.0, 1.1, and 2.6, respectively. The 23 Na profiles obtained in vivo could be used successfully to perform two-fold acceleration of hyperpolarized 13 C 3D acquisitions of both pig kidneys and a healthy human brain. CONCLUSION: A receive array has been developed in such a way that the 13 C sensitivity profiles could be accurately obtained from measurements at the 23 Na frequency. This technique facilitates accelerated acquisitions for hyperpolarized 13 C imaging. The SNR performance obtained at the 13 C frequency, compares well to other state-of-the-art coils for the same purpose, showing slightly better superficial and central SNR.

An approach to evaluation of the point-spread function for 23 Na magnetic resonance imaging.

Despite the technical challenges that require lengthy acquisitions to overcome poor signal-to-noise ratio (SNR), sodium (23 Na) magnetic resonance imaging (MRI) is an intriguing area of research due to its essential role in human metabolism. Low SNR images can impact the measurement of the point-spread function (PSF) by adding uncertainty into the resulting quantities. Here, we present methods to calculate the PSF by using the modulation transfer function (MTF), and a 3D-printed line-pair phantom in the context of 23 Na MRI. A simulation study investigated the effect of noise on the resulting MTF curves, which were derived by direct modulation (DM) and a method utilizing Fourier harmonics (FHs). Experimental data utilized a line-pair phantom with nine spatial frequencies, filled with different concentrations (15, 30, and 60 mM) of sodium in 3% agar. MTF curves were calculated using both methods from data acquired from density-adapted 3D radial projections (DA-3DRP) and Fermat looped orthogonally encoded trajectories (FLORET). Simulations indicated that the DM method increased variability in the MTF curves at all tested noise levels over the FH method. For the experimental data, the FH method resulted in PSFs with a narrower full width half maximum with reduced variability, although the improvement in variability was not as pronounced as predicted by simulations. The DA-3DRP data indicated an improvement in the PSF over FLORET. It was concluded that a 3D-printed line-pair phantom represents a convenient method to measure the PSF experimentally. The MTFs from the noisy images in 23 Na MRI have reduced variability from a FH method over DM.

Detection of increased pyruvate dehydrogenase flux in the human heart during adenosine stress test using hyperpolarized [1-13C]pyruvate cardiovascular magnetic resonance imaging.

BACKGROUND: Hyperpolarized (HP) [1-13C]pyruvate cardiovascular magnetic resonance (CMR) imaging can visualize the uptake and intracellular conversion of [1-13C]pyruvate to either [1-13C]lactate or 13C-bicarbonate depending on the prevailing metabolic state. The aim of the present study was to combine an adenosine stress test with HP [1-13C]pyruvate CMR to detect cardiac metabolism in the healthy human heart at rest and during moderate stress. METHODS: A prospective descriptive study was performed between October 2019 and August 2020. Healthy human subjects underwent cine CMR and HP [1-13C]pyruvate CMR at rest and during adenosine stress. HP [1-13C]pyruvate CMR images were acquired at the mid-left-ventricle (LV) level. Semi-quantitative assessment of first-pass myocardial [1-13C]pyruvate perfusion and metabolism were assessed. Paired t-tests were used to compare mean values at rest and during stress. RESULTS: Six healthy subjects (two female), age 29 ± 7 years were studied and no adverse reactions occurred. Myocardial [1-13C]pyruvate perfusion was significantly increased during stress with a reduction in time-to-peak from 6.2 ± 2.8 to 2.7 ± 1.3 s, p = 0.02. This higher perfusion was accompanied by an overall increased myocardial uptake and metabolism. The conversion rate constant (kPL) for lactate increased from 11 ± 9 *10-3 to 20 ± 10 * 10-3 s-1, p = 0.04. The pyruvate oxidation rate (kPB) increased from 4 ± 4 *10-3 to 12 ± 7 *10-3 s-1, p = 0.008. This increase in carbohydrate metabolism was positively correlated with heart rate (R2 = 0.44, p = 0.02). CONCLUSIONS: Adenosine stress testing combined with HP [1-13C]pyruvate CMR is feasible and well-tolerated in healthy subjects. We observed an increased pyruvate oxidation during cardiac stress. The present study is an important step in the translation of HP [1-13C]pyruvate CMR into clinical cardiac imaging. Trial registration EUDRACT, 2018-003533-15. Registered 4th of December 2018, https://www.clinicaltrialsregister.eu/ctr-search/search?query=2018-003533-15.

Three dimensional MRF obtains highly repeatable and reproducible multi-parametric estimations in the healthy human brain at 1.5T and 3T.

Magnetic resonance fingerprinting (MRF) is highly promising as a quantitative MRI technique due to its accuracy, robustness, and efficiency. Previous studies have found high repeatability and reproducibility of 2D MRF acquisitions in the brain. Here, we have extended our investigations to 3D MRF acquisitions covering the whole brain using spiral projection k-space trajectories. Our travelling head study acquired test/retest data from the brains of 12 healthy volunteers and 8 MRI systems (3 systems at 3 T and 5 at 1.5 T, all from a single vendor), using a study design not requiring all subjects to be scanned at all sites. The pulse sequence and reconstruction algorithm were the same for all acquisitions. After registration of the MRF-derived PD T1 and T2 maps to an anatomical atlas, coefficients of variation (CVs) were computed to assess test/retest repeatability and inter-site reproducibility in each voxel, while a General Linear Model (GLM) was used to determine the voxel-wise variability between all confounders, which included test/retest, subject, field strength and site. Our analysis demonstrated a high repeatability (CVs 0.7-1.3% for T1, 2.0-7.8% for T2, 1.4-2.5% for normalized PD) and reproducibility (CVs of 2.0-5.8% for T1, 7.4-10.2% for T2, 5.2-9.2% for normalized PD) in gray and white matter. Both repeatability and reproducibility improved when compared to similar experiments using 2D acquisitions. Three-dimensional MRF obtains highly repeatable and reproducible estimations of T1 and T2, supporting the translation of MRF-based fast quantitative imaging into clinical applications.

Repeatability and reproducibility of human brain morphometry using three-dimensional magnetic resonance fingerprinting.

Three-dimensional (3D) Magnetic resonance fingerprinting (MRF) permits whole-brain volumetric quantification of T1 and T2 relaxation values, potentially replacing conventional T1-weighted structural imaging for common brain imaging analysis. The aim of this study was to evaluate the repeatability and reproducibility of 3D MRF in evaluating brain cortical thickness and subcortical volumetric analysis in healthy volunteers using conventional 3D T1-weighted images as a reference standard. Scan-rescan tests of both 3D MRF and conventional 3D fast spoiled gradient recalled echo (FSPGR) were performed. For each sequence, the regional cortical thickness and volume of the subcortical structures were measured using standard automatic brain segmentation software. Repeatability and reproducibility were assessed using the within-subject coefficient of variation (wCV), intraclass correlation coefficient (ICC), and mean percent difference and ICC, respectively. The wCV and ICC of cortical thickness were similar across all regions with both 3D MRF and FSPGR. The percent relative difference in cortical thickness between 3D MRF and FSPGR across all regions was 8.0 ± 3.2%. The wCV and ICC of the volume of subcortical structures across all structures were similar between 3D MRF and FSPGR. The percent relative difference in the volume of subcortical structures between 3D MRF and FSPGR across all structures was 7.1 ± 3.6%. 3D MRF measurements of human brain cortical thickness and subcortical volumes are highly repeatable, and consistent with measurements taken on conventional 3D T1-weighted images. A slight, consistent bias was evident between the two, and thus careful attention is required when combining data from MRF and conventional acquisitions.

Cardiac T2∗ measurement of hyperpolarized 13 C metabolites using metabolite-selective multi-echo spiral imaging.

PURPOSE: Noninvasive imaging with hyperpolarized (HP) pyruvate can capture in vivo cardiac metabolism. For proper quantification of the metabolites and optimization of imaging parameters, understanding MR characteristics such as T2∗ s of the HP signals is critical. This study is to measure in vivo cardiac T2∗ s of HP [1-13 C]pyruvate and the products in rodents and humans. METHODS: A dynamic 13 C multi-echo spiral imaging sequence that acquires [13 C]bicarbonate, [1-13 C]lactate, and [1-13 C]pyruvate images in an interleaved manner was implemented for a clinical 3 Tesla system. T2∗ of each metabolite was calculated from the multi-echo images by fitting the signal decay of each region of interest mono-exponentially. The performance of measuring T2∗ using the sequence was first validated using a 13 C phantom and then with rodents following a bolus injection of HP [1-13 C]pyruvate. In humans, T2∗ of each metabolite was calculated for left ventricle, right ventricle, and myocardium. RESULTS: Cardiac T2∗ s of HP [1-13 C]pyruvate, [1-13 C]lactate, and [13 C]bicarbonate in rodents were measured as 24.9 ± 5.0, 16.4 ± 4.7, and 16.9 ± 3.4 ms, respectively. In humans, T2∗ of [1-13 C]pyruvate was 108.7 ± 22.6 ms in left ventricle and 129.4 ± 8.9 ms in right ventricle. T2∗ of [1-13 C]lactate was 40.9 ± 8.3, 44.2 ± 5.5, and 43.7 ± 9.0 ms in left ventricle, right ventricle, and myocardium, respectively. T2∗ of [13 C]bicarbonate in myocardium was 64.4 ± 2.5 ms. The measurements were reproducible and consistent over time after the pyruvate injection. CONCLUSION: The proposed metabolite-selective multi-echo spiral imaging sequence reliably measures in vivo cardiac T2∗ s of HP [1-13 C]pyruvate and products.

Combined 23 Na and 13 C imaging at 3.0 Tesla using a single-tuned large FOV birdcage coil.

PURPOSE: An unmet need in carbon-13 (13 C)-MRI is a transmit system that provides uniform excitation across a large FOV and can accommodate patients of wide-ranging body habitus. Due to the small difference between the resonant frequencies, sodium-23 (23 Na) coil developments can inform 13 C coil design while being simpler to assess due to the higher naturally abundant 23 Na signal. Here we present a removable 23 Na birdcage, which also allows operation as a 13 C abdominal coil. METHODS: We demonstrate a quadrature-driven 4-rung 23 Na birdcage coil of 50 cm in length for both 23 Na and 13 C abdominal imaging. The coil transmit efficiencies and B1+ maps were compared to a linearly driven 13 C Helmholtz-based (clamshell) coil. SNR was investigated with 23 Na and 13 C data using an 8-channel 13 C receive array within the 23 Na birdcage. RESULTS: The 23 Na birdcage longitudinal FOV was > 40 cm, whereas the 13 C clamshell was < 32 cm. The transmit efficiency of the birdcage at the 23 Na frequency was 0.65 µT/sqrt(W), similar to the clamshell for 13 C. However, the coefficient of variation of 23 Na- B1+ was 16%, nearly half that with the 13 C clamshell. The 8-channel 13 C receive array combined with the 23 Na birdcage coil generated a greater than twofold increase in 23 Na-SNR from the central abdomen compared with the birdcage alone. DISCUSSION: This 23 Na birdcage coil has a larger FOV and improved B1+ uniformity when compared to the widely used clamshell coil design while also providing similar transmit efficiency. The coil has the potential to be used for both 23 Na and 13 C imaging.

Dissolved 129 Xe lung MRI with four-echo 3D radial spectroscopic imaging: Quantification of regional gas transfer in idiopathic pulmonary fibrosis.

PURPOSE: Imaging of the different resonances of dissolved hyperpolarized xenon-129 (129 Xe) in the lung is performed using a four-echo flyback 3D radial spectroscopic imaging technique and is evaluated in healthy volunteers (HV) and subjects with idiopathic pulmonary fibrosis (IPF). THEORY AND METHODS: 10 HV and 25 subjects with IPF underwent dissolved 129 Xe MRI at 1.5T. IPF subjects underwent same day pulmonary function tests to measure forced vital capacity and the diffusion capacity of the lung for carbon monoxide (DLCO ). A four-point echo time technique with k-space chemical-shift modeling of gas, dissolved 129 Xe in lung tissue/plasma (TP) and red blood cells (RBC) combined with a 3D radial trajectory was implemented within a 14-s breath-hold. RESULTS: Results show an excellent chemical shift separation of the dissolved 129 Xe compartments and gas contamination removal, confirmed by a strong agreement between average imaging and global spectroscopy RBC/TP ratio measurements. Subjects with IPF exhibited reduced imaging gas transfer when compared to HV. A significant increase of the amplitude of RBC signal cardiogenic oscillation was also observed. In IPF subjects, DLCO % predicted was significantly correlated with RBC/TP and RBC/GAS ratios and the correlations were stronger in the inferior and periphery sections of the lungs. CONCLUSION: Lung MRI of dissolved 129 Xe was performed with a four-echo spectroscopic imaging method. Subjects with IPF demonstrated reduced xenon imaging gas transfer and increased cardiogenic modulation of dissolved xenon signal in the RBCs when compared to HV.

Characterization and correction of center-frequency effects in X-nuclear eddy current compensations on a clinical MR system.

PURPOSE: The aim of the study was to investigate whether incorrectly compensated eddy currents are the source of persistent X-nuclear spectroscopy and imaging artifacts, as well as methods to correct this. METHODS: Pulse-acquire spectra were collected for 1 H and X-nuclei (23 Na or 31 P) using the minimum TR permitted on a 3T clinical MRI system. Data were collected in 3 orientations (axial, sagittal, and coronal) with the spoiler gradient at the end of the TR applied along the slice direction for each. Modifications to system calibration files to tailor eddy current compensation for each X-nucleus were developed and applied, and data were compared with and without these corrections for: slice-selective MRS (for 23 Na and 31 P), 2D spiral trajectories (for 13 C), and 3D cones trajectories (for 23 Na). RESULTS: Line-shape distortions characteristic of eddy currents were demonstrated for X-nuclei, which were not seen for 1 H. The severity of these correlated with the amplitude of the eddy current frequency compensation term applied by the system along the axis of the applied spoiler gradient. A proposed correction to eddy current compensation, taking account of the gyromagnetic ratio, was shown to dramatically reduce these distortions. The same correction was also shown to improve data quality of non-Cartesian imaging (2D spiral and 3D cones trajectories). CONCLUSION: A simple adaptation of the default compensation for eddy currents was shown to eliminate a range of artifacts detected on X-nuclear spectroscopy and imaging.