Matthew D. Robson

Purpose: Assess the impact of incomplete spoiling and spatial saturation on mapping using the variable flip angle (VFA) spoiled gradient recalled echo sequence (SPGR). Develop a correction for incomplete spoiling that results in an accurate estimate of , essential for quantitative MRI applications. Methods: A correction factor, derived from extended phase graph simulations, was applied to the measured SPGR signal to account for deviations from the theoretical steady‐state signal. An iterative fitting process was used, refining the incomplete spoiling correction factor at each step based on the updated estimate. The incomplete spoiling correction method was validated against inversion recovery spin echo in phantom experiments and applied to in vivo data at 3T. Additionally, the hypothesis that spatial saturation disrupts the steady‐state was also tested and validated in a phantom. Results: Without a correction, the median error was +7.4% (IQR [4.4, 9.6]%) in both simulations and phantom experiments. After applying the correction, the median error decreased to −0.3% (IQR [−2.5, 2.1]%). In vivo, incomplete spoiling caused overestimation of 10–50 ms depending on inhomogeneity. Spatial saturation induced on average a −10% bias. Conclusion: A correction for incomplete spoiling is essential to achieve an accurate measure of using the VFA SPGR method, addressing systematic overestimation when using this mapping technique. Furthermore, other mechanisms that may disrupt the steady‐state signal, such as spatial saturation, should be considered.

The escalating global incidence of obesity, cardiometabolic disease and sarcopenia necessitates reliable body composition measurement tools. MRI-based assessment is the gold standard, with utility in both clinical and drug trial settings. This study aims to validate a new automated volumetric MRI method by comparing with manual ground truth, prior volumetric measurements, and against a new method for semi-automated single-slice area measurements. 4905 individuals from the UK Biobank with repeat whole-body Dixon MRI scans were selected. MRI data were processed automatically to derive new (1) volumetric and (2) single-slice area measurements at L3 vertebral level for visceral adipose tissue (VAT), subcutaneous adipose tissue (SAT), and abdominal skeletal muscle (SM). For comparison, prior volumetric measurements of VAT and SAT were included. A separate set of scans from 100 subjects was randomly selected and body composition volumes and areas were manually segmented as ground truth. The new automated volumetric measurements were found to have excellent agreement with manual ground truth with no substantial bias (ICC ≥ 0.96, CoV ≤ 3.8%). In the cohort of 4905 individuals (49% male, mean age 62 years ± 8, BMI 26 kg/m² ± 4), we confirmed that prior and new volumetric methods of VAT and SAT measurement were very strongly correlated (VAT: ρ = 0.99; SAT: ρ = 1; both p < 0.001). Single-slice L3 area measurements demonstrated very strong correlations with corresponding volumes (for VAT ρ = 0.97, for SAT ρ = 0.94, for SM ρ = 0.95, all p < 0.001), and remained excellent across sex, age, and cardiometabolic characteristics (median ρ = 0.95). We found robust correlations between manually segmented, automated volumetric, and semi-automated single-slice body composition methods. The interchangeability of these methods suggests that for each application the method should be selected according to practical considerations, operational differences and measurement granularity, rather than technical performance.

Purpose: To develop an accurate and precise liver 3D mapping method using only scanner‐agnostic sequences. Methods: While the spoiled gradient‐recalled echo sequence is widely available on clinical scanners, variable flip angle mapping methods based on this sequence provide biased estimates, with the largest systematic error arising from inhomogeneities. To correct for this, the flip angle was mapped using a 2D gradient‐echo double‐angle method approach. To correct for the confounding effect of fat on liver and , Dixon and fat saturation techniques were used in combination with the variable flip angle and the map acquisitions, respectively. The and mapping methods were validated with a ‐phantom against gold standard methods. An intra‐ and inter‐repeatability study was conducted at 3T in 10 healthy individuals' livers. Results: The developed 3D mapping method achieved an excellent agreement with the gold standard, with a weighted root mean squared normalized error below 2.8%. In vivo, a median standard deviation of 31 ms and an interquartile range of [27, 39] ms was achieved across all measurements, including the intra‐ and inter‐repeatability study data. A within‐subject standard deviation for of 21 ± 5 ms had a corresponding repeatability coefficient of 60 ms. The measured values agree well with MOLLI and SASHA mapping methods, with average differences of 5%. Conclusion: Accurate and precise 3D liver measurements can lead the way to the wider adoption of a clinically feasible measurement as a marker of hepatic fibro‐inflammation.

Background: The global rise in kidney diseases underscores the need for reliable, noninvasive imaging biomarkers. Among these, renal cortical T1 has shown promise but further technical validation is still required.Purpose: To evaluate the repeatability, reproducibility, and observer variability of kidney cortical T1 mapping in human volunteers without known renal disease.Study Type: Prospective.Subjects: Three cohorts without renal disease: 1) 25 volunteers (median age 38 [interquartile range, IQR: 28–42] years, female N = 11) for scan–rescan assessments on GE 1.5 T and Siemens 1.5 T; 2) 29 volunteers (median age 29 [IQR: 24–40] years, female N = 15) for scan–rescan assessments on Siemens 3 T; and 3) 16 volunteers (median age 34 [IQR: 31–42] years, female N = 8) for cross‐scanner reproducibility.Field Strength/Sequences: 1.5 T and 3 T, a modified Look‐Locker imaging (MOLLI) sequence with a balanced steady‐state free precession (bSSFP) readout.Assessment: Kidney cortical T1 data was acquired on GE 1.5 T scanner, Siemens 1.5 T and 3 T scanners. Within‐scanner repeatability and inter/intra‐observer variability: GE 1.5 T and Siemens 1.5 T, and cross‐scanner manufacturer reproducibility: Siemens 1.5 T–GE 1.5 T.Statistical Tests: Bland Altman analysis, coefficient of variation (CoV), intra‐class coefficient (ICC), and repeatability coefficient (RC).Results: Renal cortical T1 mapping showed high repeatability and reliability across scanner field strengths and manufacturers (repeatability: CoV 1.9%–2.8%, ICC 0.79–0.88, pooled RC 73 msec; reproducibility: CoV 3.0%, ICC 0.75, RC 90 msec). The method also showed robust observer variability (CoV 0.6%–1.4%, ICC 0.93–0.98, RC 22–48 msec).Data Conclusion: Kidney cortical T1 mapping is a highly repeatable and reproducible method across MRI manufacturers, field strengths, and observer conditions.Evidence Level: 2Technical Efficacy: Stage 2

Purpose: To show that variations through slice and slice profile effects are two major confounders affecting 2D dual angle maps using gradient‐echo signals and thus need to be corrected to obtain accurate maps.Methods: The 2D gradient‐echo transverse complex signal was Bloch‐simulated and integrated across the slice dimension including nonlinear variations in inhomogeneities through slice. A nonlinear least squares fit was used to find the factor corresponding to the best match between the two gradient‐echo signals experimental ratio and the Bloch‐simulated ratio. The correction was validated in phantom and in vivo at 3T.Results: For our RF excitation pulse, the error in the factor scales by approximately 3.8% for every 10 Hz/cm variation in along the slice direction. Higher accuracy phantom maps were obtained after applying the proposed correction; the root mean square error relative to the gold standard decreased from 6.4% to 2.6%. In vivo whole‐liver maps using the corrected map registered a significant decrease in gradient through slice.Conclusion: inhomogeneities varying through slice were seen to have an impact on the accuracy of 2D double angle maps using gradient‐echo sequences. Consideration of this confounder is crucial for research relying on accurate knowledge of the true excitation flip angles, as is the case of mapping using a spoiled gradient recalled echo sequence.

Novel low-energy defibrillation therapies are thought to be driven by virtual-electrodes (VEs), due to the interaction of applied monophasic electric shocks with fine-scale anatomical structures within the heart. Significant inter-species differences in the cardiac (micro)-anatomy exist, however, particularly with respect to the degree of endocardial trabeculations, which may underlie important differences in response to low-energy defibrillation protocols. Understanding the interaction of monophasic electric fields with the specific human micro-anatomy is therefore imperative in facilitating the translation and optimisation of these promising experimental therapies to the clinic. In this study, we sought to investigate how electric fields from implanted devices interact with the highly trabeculated human endocardial surface to better understand shock success in order to help optimise future clinical protocols. A bi-ventricular human computational model was constructed from high resolution (350 μm) ex-vivo MR data, including anatomically accurate endocardial structures. Monophasic shocks were applied between a basal right ventricular catheter and an exterior ground. Shocks of varying strengths were applied with both anodal [positive right ventricle (RV) electrode] and cathodal (negative RV electrode) polarities at different states of tissue refractoriness and during induced arrhythmias. Anodal shocks induced isolated positive VEs at the distal side of “detached” trabeculations, which rapidly spread into hyperpolarised tissue on the surrounding endocardial surfaces following the shock. Anodal shocks thus depolarised more tissue 10 ms after the shock than cathodal shocks where the propagation of activation from VEs induced on the proximal side of “detached” trabeculations was prevented due to refractory endocardium. Anodal shocks increased arrhythmia complexity more than cathodal shocks during failed anti-arrhythmia shocks. In conclusion, multiple detached trabeculations in the human ventricle interact with anodal stimuli to induce multiple secondary sources from VEs, facilitating more rapid shock-induced ventricular excitation compared to cathodal shocks. Such a mechanism may help explain inter-species differences in response to shocks and help to develop novel defibrillation strategies.

Purpose: Phosphorus (³¹P) metabolites are emerging liver disease biomarkers. Of particular interest are phosphomonoester and phosphodiester (PDE) “peaks” that comprise multiple overlapping resonances in ³¹P spectra. This study investigates the effect of improved spectral resolution at 7 Tesla (T) on quantifying hepatic metabolites in cirrhosis.Methods: Five volunteers were scanned to determine metabolite T₁s. Ten volunteers and 11 patients with liver cirrhosis were scanned at 7T. Liver spectra were acquired in 28 min using a 16‐channel ³¹P array and 3D chemical shift imaging. Concentrations were calculated using γ‐adenosine‐triphosphate (γ‐ATP) = 2.65 mmol/L wet tissue.Results: T₁ means ± standard deviations: phosphatidylcholine 1.05 ± 0.28 s, nicotinamide‐adenine‐dinucleotide (NAD⁺) 2.0 ± 1.0 s, uridine‐diphosphoglucose (UDPG) 3.3 ± 1.4 s. Concentrations in healthy volunteers: α‐ATP 2.74 ± 0.11 mmol/L wet tissue, inorganic phosphate 2.23 ± 0.20 mmol/L wet tissue, glycerophosphocholine 2.34 ± 0.46 mmol/L wet tissue, glycerophosphoethanolamine 1.50 ± 0.28 mmol/L wet tissue, phosphocholine 1.06 ± 0.16 mmol/L wet tissue, phosphoethanolamine 0.77 ± 0.14 mmol/L wet tissue, NAD⁺ 2.37 ± 0.14 mmol/L wet tissue, UDPG 2.00 ± 0.22 mmol/L wet tissue, phosphatidylcholine 1.38 ± 0.31 mmol/L wet tissue. Inorganic phosphate and phosphatidylcholine concentrations were significantly lower in patients; glycerophosphoethanolamine concentrations were significantly higher (P < 0.05).Conclusion: We report human in vivo hepatic T₁s for phosphatidylcholine, NAD⁺, and UDPG for the first time at 7T. Our protocol allows high signal‐to‐noise, repeatable measurement of metabolite concentrations in human liver. The splitting of PDE into its constituent peaks at 7T may allow more insight into changes in metabolism. Magn Reson Med 78:2095–2105, 2017. © 2017 The Authors Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.