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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 181 records · Page 10
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

No evidence of top–down effects by ants on litter decomposition in a temperate grassland

Ants play multiple roles in ecosystems, but their ability to affect decomposition processes in temperate grasslands is relatively unknown. We investigated whether the suppression of ant populations influenced litter decomposition in grasslands via predation of some decomposers (e.g., mites and springtails) and/or microbial activity and composition. We performed two successful ant suppression treatments (seven weeks, 37% suppression, year 1, 10 weeks, 70% suppression, year 2) over the course of a 59-week experiment. We then assayed the effects of ant suppression using coarse- and fine-mesh litterbags and evaluated litter chemistry, microbial and arthropod communities, and microbial enzyme activity. Ant suppression efforts reduced ant abundance and altered ant, arthropod decomposer, and non-ant predator community composition. However, ant suppression did not affect decomposer arthropod abundance, litter mass loss, microbial composition, or enzyme activity in litterbags. Litterbag mesh size did not alter microbial composition, perhaps due to a failure to exclude decomposers, as mites and springtails were more or equally abundant in fine-mesh bags. Nevertheless, mesh size did change litter chemistry, suggesting that mesh size-mediated microenvironments affect decomposition environment regardless of invertebrate exclusion. Coarse-mesh litterbags had higher concentrations of microbial sugars, lignin, and N and lower concentrations of litter C and crystalline cellulose than fine-mesh litterbags. Litterbag mesh size may alter decomposition processes irrespective of invertebrate abundance. We found no evidence that ant predation was an important driver of decomposer populations or decomposition in these systems, and we suspect redundancy at the top of the detrital food web dilutes the role of ants.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2016

Contribution of lianas to plant area index and canopy structure in a Panamanian forest

Abstract Lianas are an important component of tropical forests, where they reduce tree growth, fecundity, and survival. Competition for light from lianas may be intense; however, the amount of light that lianas intercept is poorly understood. We used a large‐scale liana‐removal experiment to quantify light interception by lianas in a Panamanian secondary forest. We measured the change in plant area index ( PAI ) and forest structure before and after cutting lianas (for 4 yr) in eight 80 m × 80 m plots and eight control plots (16 plots total). We used ground‐based Li DAR to measure the 3‐dimensional canopy structure before cutting lianas, and then annually for 2 yr afterwards. Six weeks after cutting lianas, mean plot PAI was 20% higher in control vs. liana removal plots. One yr after cutting lianas, mean plot PAI was ~17% higher in control plots. The differences between treatments diminished significantly 2 yr after liana cutting and, after 4 yr, trees had fully compensated for liana removal. Ground‐based Li DAR revealed that lianas attenuated light in the upper‐ and middle‐forest canopy layers, and not only in the upper canopy as was previously suspected. Thus, lianas compete with trees by intercepting light in the upper‐ and mid‐canopy of this forest.

Rodríguez‐Ronderos, M. Elizabeth↗
DOE OSTI2017

Effect of Aluminum Titanate (Al 2 TiO 5 ) Doping on the Mechanical Performance of Solid Oxide Fuel Cell Ni-YSZ Anode

The mechanical behavior of un-doped and 1–10 wt.% aluminum titanate doped NiO-YSZ anodes was evaluated in the oxidized and reduced state of the material. Sample bars 25 × 5 × 2 mm were fabricated and tested in a three-point bending apparatus and statistical analyzes of the collected data were performed. A remarkable enhancement of the flexural strength was found both for the reduced and oxidized state of the material when compared to the un-doped samples. In both cases, the development of a secondary phase was observed proportional to the doping amount of aluminum titanate. Morphological analysis along with preliminary phase identification and suspected mechanisms for strength enhancement are presented and discussed.

25 ENERGY STORAGE↗
DOE OSTI2020

Thermal Stability of a Eutectic Mixture of Bis(2,2-dinitropropyl) Acetal and Formal: Part C. Kinetic Compensation Effect

Here, the aging behavior of a eutectic mixture of bis(2,2-dinitropropyl) acetal and formal [called NP here] has been studied in various atmospheres [dry (air or nitrogen) versus wet] at temperatures 70 °C and below. The properties of aged samples were analyzed using Fourier transform infrared (FTIR) spectroscopy, Karl Fischer (KF) titration, liquid chromatography/mass spectrometry (LC/MS), and thermogravimetric analysis (TGA) over a period of three years. The results indicate that at aging temperatures up to 55 °C, the initial rates of water production from nitrous acid (HONO) formation and decomposition into the water, NO, and NO 2 follows a 1st order rate law and the rate constants follow an Arrhenius law as a function of temperature. The activation energies and pre-factors for water and volatiles production yield a single linear kinetic compensation plot, suggesting a common degradation pathway between NP and the various combinations of its constituents. Within a narrow temperature range, around 55 °C, a trace amount of water in NP stabilizes its properties by preventing HONO elimination. When the aging temperature is substantially higher than 55 °C, the nature of the degradation mechanism changes. It is suspected that the degradation products of NO x , water, and HNO 3 serve as catalysts to auto-catalyze (kinetics beyond the 1st order) and further degrade NP. The effect of headspace volume on this auto-catalytic process will be discussed.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

An Investigation into the Surface Skidding Response of PBX 9501 and PBX 9502

While polymer-bonded explosives, PBX, have reduced sensitivity to ignition from mechanical shock or heating compared to conventional explosives, the investigation of the mechanical ignition mechanisms for PBX remains vital to assessing the safety during machining and general handling. Under frictional heating, hot spot generation due to high melting point contaminants in the interface is a suspected source for increased probability of detonation. Here, to investigate such frictional heating, samples of PBX 9501 and PBX 9502 were impacted and skidded against a tangentially moving target and thermally imaged. Temporally resolved temperature and forces were simultaneously measured with and without 300 µm silica grit at the frictional interface. A trend of increasing temperature was observed as the speed and tangential force on the sample increased. Grit particles in the frictional interface were found to act as frictional heat concentration spots owing to the grit's protrusion from the surface and lesser susceptibility to melting compared to the PBX. The coefficient of friction between PBX 9501 and the skidding surface was observed to be largely constant for forces and speeds at the lower end of the test range. In contrast, at high tangential speeds, the coefficient was found to be significantly lower.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Coupling between radiative flux divergence and turbulence near the surface

Near‐surface turbulent flux and radiation divergence field observations are analysed over a grass‐covered surface located at the Wageningen observatory, the Netherlands. Net radiative flux divergence appears to be a large component of the energy budget near the surface, accounting for a cooling rate of several tens of degrees per day. Long‐wave radiation divergence dominates this radiation divergence flux. We show here that long‐wave flux divergence near the surface is strongly coupled to sensible heat flux, except in high relative humidity (>90%) and foggy conditions. The net long‐wave radiative flux divergence exhibits a sharp gradient within the first 20 m above the surface. This flux divergence is itself strongly coupled to sensible heat flux through adjustments in surface‐layer profiles. Nonetheless, no systematic effect of radiation is witnessed on the turbulent temperature spectrum so that the main effect of near‐surface radiation is on the mean heat budget. As typical flux‐gradient relationships are derived based on observations taken in the near‐surface boundary‐layer region where sharp long‐wave divergence is present, we suspect that those relationships implicitly represent some of the long‐wave divergence term. A modification of Monin–Obukhov Similarity Theory to include the effect of radiative divergence is proposed and discussed. This calls for independent measurements of turbulent fluxes and radiative flux divergence near the surface to re‐derive turbulent flux‐gradient relationships independently of radiative effects.

Gentine, Pierre↗
DOE OSTI2014

SIMS analysis of high-performance accelerator niobium

Niobium is used to fabricate superconducting radio frequency accelerator modules because of its high critical temperature, high critical magnetic field, and easy formability. Recent experiments have shown a very significant improvement in performance (over 100%) after a high-temperature bake at 1400 degrees C for 3h. SIMS analysis of this material showed the oxygen profile was significantly deeper than the native oxide with a shape that is indicative of diffusion. Positive secondary ion mass spectra showed the presence of Ti with a depth profile similar to that of O. It is suspected that Ti is associated with the performance improvement. The source of Ti contamination in the anneal furnace has been identified, and a new furnace was constructed without Ti. Initial results from the new furnace do not show the yield improvement. Further analyses should determine the relationship of Ti to cavity performance.

Maheshwari, P.↗
DOE OSTI2023

On the black hole/string transition

We discuss aspects of the possible transition between small black holes and highly excited fundamental strings. We focus on the connection between black holes and the self gravitating string solution of Horowitz and Polchinski. This solution is interesting because it has non-zero entropy at the classical level and it is natural to suspect that it might be continuously connected to the black hole. Surprisingly, we find a different behavior for heterotic and type II cases. For the type II case we find an obstruction to the idea that the two are connected as classical solutions of string theory, while no such obstruction exists for the heterotic case. We further provide a linear sigma model analysis that suggests a continuous connection for the heterotic case. We also describe a solution generating transformation that produces a charged version of the self gravitating string. This provides a fuzzball-like construction of near extremal configurations carrying fundamental string momentum and winding charges. We provide formulas which are exact in α' relating the thermodynamic properties of the charged and the uncharged solutions.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2019

UV cancellations in gravity loop integrands

In this work we explore the properties of four-dimensional gravity integrands at large loop momenta. This analysis can not be done directly for the full off-shell integrand but only becomes well-defined on cuts that allow us to unambiguously specify labels for the loop variables. The ultraviolet region of scattering amplitudes originates from poles at infinity of the loop integrands and we show that in gravity these integrands conceal a number of surprising features. In particular, certain poles at infinity are absent which requires a conspiracy between individual Feynman integrals contributing to the amplitude. We suspect that this non-trivial behavior is a consequence of yet-to-be found symmetry or a hidden property of gravity amplitudes.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2020

Transverse momentum broadening of a jet in quark-gluon plasma: an open quantum system EFT

We utilize the technology of open quantum systems in conjunction with the recently developed effective field theory for forward scattering to address the question of massless jet propagation through a weakly-coupled quark-gluon plasma in thermal equilibrium. We discuss various possible hierarchies of scales that may appear in this problem, by comparing thermal scales of the plasma with relevant scales in the effective field theory. Starting from the Lindblad equation, we derive and solve a master equation for the trans- verse momentum distribution of a massless quark jet, at leading orders both in the strong coupling and in the power counting of the effective field theory. Markovian approximation is justified in the weak coupling limit. Using the solution to the master equation, we study the transverse momentum broadening of a jet as a function of the plasma temperature and the time of propagation. We discuss the physical origin of infrared sensitivity that arises in the solution and a way to handle it in the effective field theory formulation. We suspect that the final measurement constraint can only cut-off leading infrared singularities and the solution to the Markovian master equation resums a logarithmic series. This work is a stepping stone towards understanding jet quenching and jet substructure observables on both light and heavy quark jets as probes of the quark-gluon plasma.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2020

The UV fate of anomalous U(1)s and the Swampland

Massive U(1) gauge theories featuring parametrically light vectors are suspected to belong in the Swampland of consistent EFTs that cannot be embedded into a theory of quantum gravity. We study four-dimensional, chiral U(1) gauge theories that appear anomalous over a range of energies up to the scale of anomaly-cancelling massive chiral fermions. We show that such theories must be UV-completed at a finite cutoff below which a radial mode must appear, and cannot be decoupled — a Stückelberg limit does not exist. When the infrared fermion spectrum contains a mixed U(1)-gravitational anomaly, this class of theories provides a toy model of a boundary into the Swampland, for sufficiently small values of the vector mass. In this context, we show that the limit of a parametrically light vector comes at the cost of a quantum gravity scale that lies parametrically below MP1, and our result provides field theoretic evidence for the existence of a Swampland of EFTs that is disconnected from the subset of theories compatible with a gravitational UV-completion. Moreover, when the low energy theory also contains a U(1) 3 anomaly, the Weak Gravity Conjecture scale makes an appearance in the form of a quantum gravity cutoff for values of the gauge coupling above a certain critical size.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2020

False positive PSMA PET for tumor remnants in the irradiated prostate and other interpretation pitfalls in a prospective multi-center trial

Readers need to be informed about potential pitfalls of [ 68 Ga]Ga-PSMA-11 PET interpretation. Here we report [ 68 Ga]Ga-PSMA-11 PET findings discordant with the histopathology/composite reference standard in a recently published prospective trial on 635 patients with biochemically recurrent prostate cancer. Consensus reads were false positive in 20 regions of 17/217 (8%) patients with lesion validation. Majority of the false positive interpretations (13 of 20, 65%) occurred in the context of suspected prostate (bed) relapse (T) after radiotherapy ( n = 11); other false positive findings were noted for prostate bed post prostatectomy (T, n = 2), pelvic nodes (N, n = 2), or extra pelvic lesions (M, n = 5). Major sources of false positive findings were PSMA-expressing residual adenocarcinoma with marked post-radiotherapy treatment effect. False negative interpretation occurred in 8 regions of 6/79 (8%) patients with histopathology validation, including prostate (bed) ( n = 5), pelvic nodes ( n = 1), and extra pelvic lesions ( n = 2). Lesions were missed mostly due to small metastases or adjacent bladder/urine uptake.[ 68 Ga]Ga-PSMA-11 PET at biochemical recurrence resulted in less than 10% false positive interpretations. Post-radiotherapy prostate uptake was a major source of [ 68 Ga]Ga-PSMA-11 PET false positivity. In few cases, PET correctly detects residual PSMA expression post-radiotherapy, originating however from treated, benign tissue or potentially indolent tumor remnants.

Radiology, Nuclear Medicine & Medical Imaging↗
DOE OSTI2019

Double contrast-enhanced ultrasound improves the detection and localization of occult lesions in the pancreatic tail: a initial experience report

Purpose: The aim of this study is to review our initial experience of Double contrast-enhanced ultrasonography (DCEUS) in the preoperative detection of pancreatic tail occult tumors. Methods: Thirty-five patients were recruited to undergo DCEUS of the pancreas suspected by occult lesions of pancreatic with MRI or clinical evidence. The radiologists assessed the images (conventional US, DCEUS, and MRI) for enhancement phases, tumor presence, location, enhancement characteristics, and tumor conspicuity. The differences in the onset times of the phases between DCEUS and MRI were noted. Tumor conspicuity was graded on a four-point scale for conspicuity comparison among three imaging modalities. Results: Pancreatic tail tumors were missed on conventional transabdominal US in 11 of 35 patients. DCEUS revealed 12 lesions of 35 patients. Pancreatic tail tumors were identified on MRI in 10 of 35 patients. The sensitivity and specificity of DCEUS for depicting occult lesions of ≤ 2.2 cm were 92% and 95%, respectively. In contrast, the sensitivity and specificity of conventional US were 67% and 66%, respectively. The sensitivity and specificity of MRI were 91% and 88%, respectively. The conspicuity ratings of the three phases did not significantly differ between the DCEUS and MRI groups (P > 0.05). The DCEUS phases started much earlier than the corresponding MRI phases. Conclusion: DCEUS is a promising technique in the detection of occult pancreatic tail tumors and is possibly superior to dynamic enhanced MRI in the case of some peripheral lesions.

62 RADIOLOGY AND NUCLEAR MEDICINE↗
DOE OSTI2019

Assessing liver function: diagnostic efficacy of parenchymal enhancement and liver volume ratio of Gd-EOB-DTPA-enhanced MRI study during interstitial and hepatobiliary phase

Aim: To assess the efficacy of signal intensity in interstitial and hepatobiliary phase normalized for liver volume, on gadolinium-ethoxybenzyl-diethylenetriaminepentaacetic acid (Gd-EOB-DTPA)-enhanced magnetic resonance imaging (MRI) study, for the evaluation of liver function through the comparison with Child–Pugh (CP), model for end-stage liver disease (MELD), and biochemical tests. Methods: All dynamic Gd-EOB-DTPA MRI studies performed in patients with suspected liver lesions were retrospectively reviewed. The rate of liver-to-muscle ratio on T1 sequence 70 s (interstitial phase) and 20 min (hepatobiliary phase) after injection of Gd-EOB-DTPA was calculated for each MRI study and then normalized for liver volume (irINTnorm and irHEPnorm). Pearson correlation coefficient was computed to assess the correlation among these values and CP and MELD scores, and biochemical tests. Results: A total of 303 MRI studies, performed on 221 patients, were included. Mean age was 63.8 years ± 12.9 with a majority of male patients (186; 61.4%). A total of 186 out of 303 (61.4%) were cirrhotic patients. The irHEPnorm was significantly lower in cirrhotic than non-cirrhotic patients (0.0004 ± 0.0002 to 0.0005 ± 0.0003, p = 0.010). This value had a moderate, significant correlation with Child–Pugh and MELD scores (R = − 0.292, p < 0.0001 and R = − 0.192, p = 0.010, respectively). In particular, irHEPnorm progressively decreased from Child–Pugh A to C (0.0004–0.0002, p < 0.0001) and from MELD ≤ 10 to 19–24 (0.0004–0.0003, p = 0.018). Among biochemical parameters, total bilirubin, GOT, and albumin had the strongest correlation with irHEPnorm (R = − 0.258, − 0.291, and 0.262, p < 0.0001, respectively). No correlations were found between irINTnorm and CP and MELD scores. Conclusion: irHEPnorm value derived from Gd-EOB-DTPA-enhanced MRI is a reliable, non-invasive, useful tool to quantify liver function and to assess the degree of cirrhosis, offering a strict relationship with clinical scores and biochemical parameters. This could help surgeons in clinical decision-making, allowing them to choose the more suitable surgical approach for cirrhotic patients.

62 RADIOLOGY AND NUCLEAR MEDICINE↗
DOE OSTI2019

Magnetic resonance elastography SE-EPI vs GRE sequences at 3T in a pediatric population with liver disease

Purpose: The goal of our study is to compare hepatic stiffness measures using gradient-recalled echo (GRE) versus spin-echo echo planar imaging (SE-EPI)-based MR Elastography (MRE) at 3T used to measure hepatic stiffness in a patients with suspected liver diseases. Materials and methods: This retrospective study included 52 patients with liver disease who underwent a 3T MRE exam including both an investigational SE-EPI-based technique and a product GRE-based technique. Regions of interest (ROI) were placed on the elastograms to measure elastography-derived liver stiffness as well as the area included within the ROIs. The mean liver stiffness values and area of ROIs were compared. Results: The mean liver stiffness was 3.72 kilopascal (kPa) ± 1.29 using GRE MRE and 3.78 kPa ± 1.13 using SE-EPI MRE. Measurement of liver stiffness showed excellent agreement between the two pulse sequences with a mean bias of − 0.1 kPa (range − 1.8 to 1.7 kPa) between sequences. The mean measurable ROI area was higher with SE-EPI (313.8 cm{sup 2} ± 213.8) than with the GRE technique (208.6 cm{sup 2} ± 114.8), and the difference was statistically significant (P < 0.05). Conclusions: Our data shows excellent agreement of measured liver stiffness between GRE and SE-EPI-based sequences at 3T. Our results show the advantage of a SE-EPI MRE sequence in terms of image quality, ROI size and acquisition time with equivalent liver stiffness measurements as compared to GRE-MRE sequence.

62 RADIOLOGY AND NUCLEAR MEDICINE↗
DOE OSTI2019

MR elastography of liver at 3 Tesla: comparison of gradient-recalled echo (GRE) and spin-echo (SE) echo-planar imaging (EPI) sequences and agreement across stiffness measurements

Purpose: To compare 2D gradient-recalled echo (GRE) and 2D spin-echo (SE) echo-planar imaging (EPI) MR elastography (MRE) for measurement of hepatic stiffness in adult patients with known or suspected liver disease at 3 Tesla. Materials and methods: Three hundred and eighty-seven consecutive patients underwent MRE of the liver at 3 Tesla with 2D-GRE and 2D-SE-EPI sequences. ‘Mean liver stiffness (LS)’ calculated by averaging 3 ROIs in the right lobe, ‘Maximum LS’ calculated by an ROI in the right lobe; and ‘Freehand LS’ calculated by an ROI in the entire liver were measured by two independent readers. Inter-observer and inter-class variability in stiffness measurements were assessed. Stiffness values were correlated with degree of liver fibrosis (METAVIR scores) in 97 patients who underwent biopsy. The diagnostic performance was compared by a receiver-operating characteristic analysis. Results: The technical failure rate was 2.8% for 2D-SE-EPI (11/387) and 4.1% for 2D-GRE (16/387, 9 had R{sub 2}* > 80 s{sup −1} indicating iron overload). There is high reproducibility for both GRE and SE-EPI variants (ICC = 0.84–0.94 for both GRE and SE-EPI MRE). The highest sensitivity, specificity, and accuracy of differentiating mild fibrosis (F0–F2) from advanced fibrosis (F3–F4) are 0.84 (GRE Freehand measurement), 0.92 (GRE Maximum stiffness measurement), and 0.88 (GRE Freehand measurement), respectively. Conclusions: High intra-class correlation and intra-reader correlation are seen on measured hepatic stiffness for both 2D-GRE and 2D-SE-EPI MRE. 2D-SE-EPI has lower failure rate. Diagnostic performance of both sequences is equivalent, with highest sensitivity for 2D-GRE Freehand stiffness measurement, and highest specificity 2D-GRE Maximum stiffness measurement.

62 RADIOLOGY AND NUCLEAR MEDICINE↗
DOE OSTI2019

Differentiating IgG4-related sclerosing cholangiopathy from cholangiocarcinoma using CT and MRI: experience from a tertiary referring center

Purpose: To compare the cross-sectional imaging findings of immunoglobulin G4-related sclerosing cholangiopathy (IgG4-SC) and cholangiocarcinoma (CCA). Methods: Retrospective search of radiology and pathology databases identified 24 patients with IgG4-SC and over 500 patients with CCA from January 2009 to December 2016. Patients with no pre-treatment imaging studies available on PACS, non-contrasted imaging only, presence of mass lesions, metastatic disease or biliary stents were excluded. 17 patients with IgG4-SC and a selected group of 20 (age and gender matched) patients with CCA were obtained. Images were blinded and independently reviewed by two radiologists. Differences in proportions and means between groups were analyzed using Fishers and Mann–Whitney tests, respectively. Results: Both readers identified a statistically significant difference in the presence of abrupt common bile duct narrowing between IgG4-SC and CCA (6.7% vs. 68.4%, p < 0.001; 33.3% vs. 75%, p = 0.019). No difference was seen in biliary wall thickening, wall enhancement, extrahepatic exclusive location of disease, or pancreatic duct dilation. Inter-observer variability was κ = 0.52. Total bilirubin and CA 19-9 were unable to differentiate between IgG4-SC and CCA. Serum IgG4 was positive in two of six IgG4-SC patients who were tested. Conclusion: IgG4-SC and CCA share many clinical and imaging findings on CT and MRI. Abrupt bile duct cut sign strongly favors CCA. In the absence of this finding, IgG4-SC should be considered in the differential diagnosis in all cases of suspected extrahepatic CCA.

62 RADIOLOGY AND NUCLEAR MEDICINE↗
DOE OSTI2019

Comparison of navigator-gated and breath-held image acquisition techniques for multi-echo quantitative dixon imaging of the liver in children and young adults

Purpose: Acquired over a breath hold, multi-echo Dixon (mDixon) magnetic resonance imaging (MRI) of the liver can be used to quantify proton density fat fraction (PDFF) and iron-related signal decay. However, young, obese, and co-morbid patients may have limited breath holding capacity and could benefit from a motion-robust mDixon acquisition. The purpose of this study was to compare hepatic PDFF and R2* values between navigator-gated and breath-held mDixon MRI acquisition techniques in children and young adults with suspected liver disease. Materials and methods: This retrospective study was institutional review board-approved with a waiver of informed consent. Patients who underwent liver MRI with breath-held and navigator-gated mDixon sequences between January 2017 and July 2018 were included. One reviewer, blinded to sequence, measured PDFF and R2* on four images from each sequence. Another blinded reviewer graded respiratory motion (5-point Likert scale). Pearson correlation (r), Lin’s concordance coefficients (r{sub c}), and Bland–Altman analyses were used to assess agreement between techniques. Frequency of clinically limiting motion (score ≥ 3) was compared with Fisher’s exact test. Results: Forty-two patients were included (15 female, 27 male; mean age: 15.7 ± 4.6 years). Mean PDFF and R2* were 16.6 ± 13.1% and 29.3 ± 4.7 s{sup −1} (breath-held) versus 17.0 ± 13.2% and 29.6 ± 5.2 s{sup −1} (navigator-gated). PDFF agreed almost perfectly between sequences (r{sub c} = 0.997, 95% CI 0.994–0.998; mean bias: 0.3%; 95% limits of agreement: − 2.4 to +1.7%), while R2* values correlated very strongly but with poor agreement (r = 0.837, r{sub c} = 0.832, 95% CI 0.716–0.910). Navigator-gated images exhibited significantly higher frequency of clinically limiting respiratory motion (88% vs. 48%, p = 0.0001). Conclusion: Despite greater respiratory motion artifact, a free-breathing navigator-gated mDixon sequence produces PDFF values with almost perfect agreement to a breath-held sequence.

62 RADIOLOGY AND NUCLEAR MEDICINE↗