Magnetic Resonance Elastography (MRE) of the liver is a novel noninvasive clinical diagnostic tool to stage fibrosis based on measured stiffness. The purpose of this study is to design, evaluate and validate a rapid MRE acquisition technique for noninvasively quantitating liver stiffness which reduces by half the scan time, thereby decreasing image registration errors between four MRE phase offsets. In vivo liver MRE was performed on 16 healthy volunteers and 14 patients with biopsy-proven liver fibrosis using the standard clinical Gradient Recalled Echo (GRE) MRE sequence (MREs) and a developed rapid GRE MRE sequence (MREr) to obtain the mean stiffness in an axial slice. The mean stiffness values obtained from the entire group using MREs and MREr were 2.72±0.85kPa and 2.7±0.85kPa, respectively, representing an insignificant difference. A linear correlation of R2=0.99 was determined between stiffness values obtained using MREs and MREr. Therefore, we can conclude that MREr can replace MREs, which reduces the scan time to half of that of the current standard acquisition (MREs), which will facilitate MRE imaging in patients with inability to hold their breath for long periods.

Purpose To determine alteration in left ventricular (LV) myocardial stiffness (MS) with hypertension (HTN). Cardiac magnetic resonance elastography (MRE) was used to estimate MS in HTN induced pigs and MRE-derived MS measurements were compared against LV pressure, thickness and circumferential strain. Materials and Methods Renal-wrapping surgery was performed to induce HTN in 8 pigs. LV catheterization (to measure pressure) and cardiac MRI (1.5T; gradient echo-MRE and tagging) was performed pre-surgery at baseline (Bx), and post-surgery at month 1 (M1) and month 2 (M2). Images were analyzed to estimate LV-MS, thickness and circumferential strain across the cardiac cycle. The associations between end-diastolic (ED) and end-systolic (ES) MS and i) mean LV pressure; ii) ED and ES thickness respectively; and iii) circumferential strain were evaluated using Spearman’s correlation method. Results From Bx to M2, mean pressure, MRE-derived stiffness, and thickness increased while circumferential strain decreased significantly (slope test, p≤0.05). Both ED and ES MS had significant positive correlation with i) mean pressure (ED MS: ρ =0.56; p=0.005 and ES MS: ρ =0.45; p=0.03); ii) ED thickness (ρ =0.73; p<0.0001) and ES thickness (ρ =0.84; p<0.0001) respectively; but demonstrated a negative trend with circumferential strain (ED MS: ρ =0.31 and ES MS: ρ =0.37). Conclusion This study demonstrated that in HTN porcine model, MRE-derived MS increased with increase in pressure and thickness.

Purpose Aortic stiffness plays an important role in evaluating and predicting the progression of systemic arterial hypertension (SAH). The aim of this study is to determine the stiffness of aortic wall using magnetic resonance elastography (MRE) in a hypertensive porcine model and compare it against invasive aortic pressure measurements. Methods Renal wrapping surgery was performed on eight pigs to induce SAH. Aortic MRE was performed at baseline and two months post-surgery using a retrospectively pulse-gated gradient-echo MRE sequence on a 1.5T scanner. Mechanical waves of 70Hz were introduced into the aorta. Invasive central aortic pressure measurements were obtained prior to each scan to calculate mean arterial pressure (MAP). MRE data were analyzed to obtain effective aortic stiffness. Spearman’s rank correlation analysis was performed to assess the relationship between MAP and MRE-derived aortic stiffness. Results Significant increase in effective aortic stiffness was observed between baseline and two months post-surgery measurements (paired t-test; p=0.004). The average MAP by pooling all animals was 65.24±9.42mmHg at baseline and 92.57±11.80mmHg two months post-surgery with p<0.0001. Moderate linear correlation was observed between MAP and effective aortic stiffness (ρ=0.52; p=0.046). Conclusion This study demonstrated that in SAH porcine model, MRE-derived aortic stiffness increased with increase in MAP.

Purpose To estimate change in left ventricular (LV) end-systolic and end-diastolic myocardial stiffness (MS) in pigs induced with myocardial infarction (MI) with disease progression using cardiac magnetic resonance elastography (MRE) and to compare it against ex-vivo mechanical testing, LV circumferential strain and MRI relaxometry parameters (T1, T2, and extracellular volume fraction (ECV)). Methods MRI (1.5T) was performed on 7 pigs, before surgery (Bx), and 10 (D10) and 21 (D21) days after creating MI. cardiac MRE-derived MS was measured in infarcted region (MIR) and remote region (RR), and validated using mechanical testing-derived MS obtained post-sacrifice on D21. Circumferential strain and MRI relaxometry parameters (T2, T1, and ECV) were also obtained. Multi-parametric analysis was performed to determine correlation between cardiac MRE-derived MS and i) strain, ii) relaxometry parameters, and iii) mechanical testing. Results Mean diastolic (D10:5.09±0.6kPa; D21:5.45±0.7kPa) and systolic (D10:5.72±0.8kPa; D21:6.34±1.0kPa) MS in MIR were significantly higher (p<0.01) compared to mean diastolic (D10:3.97±0.4kPa; D21:4.12±0.2kPa) and systolic (D10:5.08±0.6kPa; and D21:5.16±0.6kPa) MS in RR. The increase in cardiac MRE-derived MS at D21 (MIR) was consistent and correlated strongly with mechanical testing-derived MS (r(diastolic)=0.86; r(systolic)=0.89). Diastolic MS in MIR demonstrated a negative correlation with strain (r=0.58). Additionally, cardiac MRE-derived MS demonstrated good correlations with post-contrast T1 (r(diastolic)= −0.549; r(systolic)= −0.741) and ECV (r(diastolic)=0.548; r(systolic)=0.703), and no correlation with T2. Conclusion As MI progressed, cardiac MRE-derived MS increased in MIR compared to RR, which significantly correlated with mechanical testing-derived MS, T1 and ECV.

Background Due to complexities of in-vivo cardiac diffusion tensor imaging (DTI), ex-vivo formalin-fixed specimens are used to investigate cardiac remodeling in diseases, and reported results have shown conflicting trends. This study investigates the impact of formalin-fixation on diffusion properties and optimizes tracking parameters based on controls to understand remodeling in myocardial-infarction (MI). Methods DTI was performed on 4 healthy (controls) and 4 MI induced formalin-fixed (PoMI) ex-vivo porcine hearts. Controls were scanned pre-fixation (PrCtrl) and re-scanned (PoCtrl) after formalin-fixation. Fractional anisotropy (FA) and apparent diffusion coefficient (ADC) were estimated in all hearts. Tracking parameters (FA, tract termination angle (TTA), fiber-length) were optimized in controls and then used to investigate structural remodeling in PoMI hearts. Results Fixation increased ADC and decreased FA. PoMI showed increased ADC but decreased FA in infarcted zone compared to remote zone. TTA showed sharp increase in slope from 5°–10°, which flattened after 25° in all groups. Mean fiber-length for different tracking length range showed that PoCtrl had shorter fibers compared to PrCtrl. Fibers around infarction were shorter in length and disarrayed compared to PoCtrl group. Conclusion Formalin-fixation affects diffusion properties and hence DTI parametric trends observed in pathology may be influenced by the fixation process which can cause contradictory findings.

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