Effects of postprandial state and mesenteric blood flow on the repeatability of MR elastography in asymptomatic subjects

Magnetic resonance elastography in a nutshell: Tomographic imaging of soft tissue viscoelasticity for detecting and staging disease with a focus on inflammation

Magnetic resonance elastography (MRE) is an emerging clinical imaging modality for characterizing the viscoelastic properties of soft biological tissues. MRE shows great promise in the noninvasive diagnosis of various diseases, especially those associated with soft tissue changes involving the extracellular matrix, cell density, or fluid turnover including altered blood perfusion - all hallmarks of inflammation from early events to cancer development. This review covers the fundamental principles of measuring tissue viscoelasticity by MRE, which are based on the stimulation and encoding of shear waves and their conversion into parameter maps of mechanical properties by inverse problem solutions of the wave equation. Technical challenges posed by real-world biological tissue properties such as viscosity, heterogeneity, anisotropy, and nonlinear elastic behavior of tissues are discussed. Applications of MRE measurement in both humans and animal models are presented, with emphasis on the detection, characterization, and staging of diseases related to the cascade of biomechanical property changes from early to chronic inflammation in the liver and brain. Overall, MRE provides valuable insights into the biophysics of soft tissues for imaging-based detection and staging of inflammation-associated tissue changes.

Distribution of Normative Percentiles of Liver Stiffness Measurement Using Ultrasound Shear Wave Elastography in an Adult Asian Indian Population

Objective  The aim of this study was to determine the normative percentiles for liver stiffness measurement (LSM) using shear wave elastography in an adult Asian Indian population as part of the preventive radiology initiative of the Indian Radiological and Imaging Association (IRIA). Methods  LSMs were ascertained by two-dimensional (2D) shear wave elastography using the Mindray Resona series of ultrasound machines. The image quality was assessed using the motion stability index (M-STB) and reliability (RLB) map. Ten acquisitions were documented, and an interquartile range-to-median (IQR/M) ratio ≤30% kilopascal (kPa) units was considered a good-quality measurement. A subgroup of the study population without comorbidities was chosen to derive the normative percentile distribution of LSM using a generalized least squares multivariable fractional polynomial regression model that adjusted for sex and body mass index (BMI). The effectiveness of the estimated percentiles was assessed on the entire study population using the greater than 90th percentile value of the LSM as the cutoff for abnormality. Results  The study included 852 people who underwent shear wave elastography from June 2022 to July 2023. The magnitude of compensated advanced chronic liver disease (cACLD) and clinically significant portal hypertension (CSPH) was 6.81% (95% confidence interval [CI]: 5.30-8.7) and 4.91% (95% CI: 3.67-6.60), respectively. The normative percentiles were estimated from 282 persons without associated comorbidity and risk factors. The mean age (standard deviation [SD]) of the normal individuals was 40.90 ± 12.92 years, and 210 (71.47%) were males. The mean age (SD) of the 570 persons excluded from the normative percentiles analysis was 47.94 (12.49) years and 72.11% were males. The sex- and BMI-adjusted age-specific 90th percentiles of LSM were 8.76, 8.78, 8.96, 8.97, 9.25, and 9.45 kPa for 18 to 20, 21 to 30, 31 to 40, 41 to 50, 51 to 60, and 61 to 70 years, respectively. Conclusion  The sex- and BMI-adjusted age-specific 90th percentiles for LSM using shear wave elastography in Asian Indian adults are almost similar to the greater than 9 kPa cutoff proposed by the Society of Radiologists in Ultrasound Liver Elastography Consensus Statement guidelines to discriminate cACLD and CSPH from normal individuals.

Liver Fibrosis, Fat, and Iron Evaluation with MRI and Fibrosis and Fat Evaluation with US: A Practical Guide for Radiologists

Quantitative imaging biomarkers of liver disease measured by using MRI and US are emerging as important clinical tools in the management of patients with chronic liver disease (CLD). Because of their high accuracy and noninvasive nature, in many cases, these techniques have replaced liver biopsy for the diagnosis, quantitative staging, and treatment monitoring of patients with CLD. The most commonly evaluated imaging biomarkers are surrogates for liver fibrosis, fat, and iron. MR elastography is now routinely performed to evaluate for liver fibrosis and typically combined with MRI-based liver fat and iron quantification to exclude or grade hepatic steatosis and iron overload, respectively. US elastography is also widely performed to evaluate for liver fibrosis and has the advantage of lower equipment cost and greater availability compared with those of MRI. Emerging US fat quantification methods can be performed along with US elastography. The author group, consisting of members of the Society of Abdominal Radiology (SAR) Liver Fibrosis Disease-Focused Panel (DFP), the SAR Hepatic Iron Overload DFP, and the European Society of Radiology, review the basics of liver fibrosis, fat, and iron quantification with MRI and liver fibrosis and fat quantification with US. The authors cover technical requirements, typical case display, quality control and proper measurement technique and case interpretation guidelines, pitfalls, and confounding factors. The authors aim to provide a practical guide for radiologists interpreting these examinations. © RSNA, 2023 See the invited commentary by Ronot in this issue. Quiz questions for this article are available in the supplemental material.

Evaluation of liver fibrosis and cirrhosis on the basis of quantitative T1 mapping: Are acute inflammation, age and liver volume confounding factors?

PURPOSE

To evaluate potential confounding factors in the quantitative assessment of liver fibrosis and cirrhosis using T1 relaxation times.

METHODS

The study population is based on a radiology-information-system database search for abdominal MRI performed from July 2018 to April 2019 at our institution. After applying exclusion criteria 200 (59 ± 16 yrs) remaining patients were retrospectively included. 93 patients were defined as liver-healthy, 40 patients without known fibrosis or cirrhosis, and 67 subjects had a clinically or biopsy-proven liver fibrosis or cirrhosis. T1 mapping was performed using a slice based look-locker approach. A ROI based analysis of the left and the right liver was performed. Fat fraction, R2*, liver volume, laboratory parameters, sex, and age were evaluated as potential confounding factors.

RESULTS

T1 values were significantly lower in healthy subjects without known fibrotic changes (1.5 T MRI: 575 ± 56 ms; 3 T MRI: 857 ± 128 ms) compared to patients with acute liver disease (1.5 T MRI: 657 ± 73 ms, p < 0.0001; 3 T MRI: 952 ± 37 ms, p = 0.028) or known fibrosis or cirrhosis (1.5 T MRI: 644 ± 83 ms, p < 0.0001; 3 T MRI: 995 ± 150 ms, p = 0.018). T1 values correlated moderately with the Child-Pugh stage at 1.5 T (p = 0.01, ρ = 0.35).

CONCLUSION

T1 mapping is a capable predictor for detection of liver fibrosis and cirrhosis. Especially age is not a confounding factor and, hence, age-independent thresholds can be defined. Acute liver diseases are confounding factors and should be ruled out before employing T1-relaxometry based thresholds to screen for patients with liver fibrosis or cirrhosis.

The role of elastography in viral hepatitis and autoimmune hepatitis

The prognosis of chronic liver diseases, which represent a major public health problem, is mainly linked to the extent and progression of liver fibrosis and the subsequent risk of developing cirrhosis and related complications, mainly hepatocellular carcinoma. During the past decade many noninvasive methods and in particular electrographic techniques, have been developed to reduce the need for liver biopsy in staging fibrosis and to overcome whenever possible its limitations, mainly: invasiveness, costs, low reproducibility and poor acceptance by patients. The aim of this review was to provide a comprehensive review of the role of elastography techniques in viral chronic liver diseases and autoimmune hepatitis, with the focus on the possible advantages and limitations of these techniques and on their diagnostic accuracy in predicting the stage of liver fibrosis.

Interobserver and Intraobserver Reliability of Hepatic Shear Wave Elastography and the Influence of Fasted Versus Nonfasted States in Healthy Volunteers

OBJECTIVES

The primary objective of this study was to assess the effect of fasting versus ingestion of food and water on hepatic measurements by shear wave elastography (SWE) in healthy participants. The secondary objective was to assess inter- and intra-reader reliability of hepatic elastography in healthy participants.

METHODS

Twenty healthy participants were enrolled in this prospective study and underwent quantitative SWE under fasting conditions and after the ingestion of water and food and water. Two blinded sonographers each independently performed a total of 6 sessions of hepatic SWE in each participant. Sessions 1 to 3 were performed on day 0 and sessions 4 to 6 on day 7. Statistical tests used included the Wilcoxon signed ranks test, the intraclass correlation coefficient, and Bland-Altman plots.

RESULTS

There were no significant differences in hepatic SWE measurements after the ingestion of water versus the fasting state. Statistical significance was assessed as P < 0.05. The postprandial status had a statistically significant effect on hepatic SWE measurements at 1 hour (P = .04) but not at 3 hours (P = .08). By the intraclass correlation coefficient, there was poor-to-moderate inter-reader agreement and minimal-to-moderate intra-reader agreement. The median inter-reader difference in SWE measurements ranged from 0.66 to 0.96 kPa. The median intra-reader difference ranged from 0.43 to 0.55 kPa.

CONCLUSIONS

Our study shows that the ingestion of water has no effect on hepatic SWE measurements in healthy participants. The postprandial state had a significant effect on SWE measurements at 1 hour after ingestion but not at 3 hours. The inter-reader and intra-reader agreements were variable and moderate at best.

Noninvasive imaging assessment of portal hypertension

Portal hypertension (PH) is a spectrum of complications of chronic liver disease (CLD) and cirrhosis, with manifestations including ascites, gastroesophageal varices, splenomegaly, hypersplenism, hepatic hydrothorax, hepatorenal syndrome, hepatopulmonary syndrome and portopulmonary hypertension. PH can vary in severity and is diagnosed via invasive hepatic venous pressure gradient measurement (HVPG), which is considered the reference standard. Accurate diagnosis of PH and assessment of severity are highly relevant as patients with clinically significant portal hypertension (CSPH) are at higher risk for developing acute variceal bleeding and mortality. In this review, we discuss current and upcoming noninvasive imaging methods for diagnosis and assessment of severity of PH.

Reproducibility of 2 Liver 2-Dimensional Shear Wave Elastographic Techniques in the Fasting and Postprandial States

OBJECTIVES

The purpose of this study was to compare the reliability and agreement of 2 methods of 2-dimensional (2D) shear wave elastography (SWE) on liver stiffness in healthy volunteers. We also assessed effects of the prandial state and operator experience on measurements.

METHODS

Two operators, 1 experienced and 1 novice, independently examined 20 healthy volunteers with 2D SWE on 2 ultrasound machines (Aixplorer [SuperSonic Imagine, Aix-en-Provence, France] and Aplio 500 [Canon Medical Systems Corporation, Otawara, Japan]). Volunteers were scanned 8 times by the operators using both machines in fasting and postprandial states. Agreement was evaluated by a Bland-Altman analysis, and the correlation was assessed by the Pearson correlation and intraclass correlation coefficients (ICCs). An analysis of variance was conducted to determine the contribution of the machine, prandial state, and operator experience to the variability.

RESULTS

Agreement assessed by Bland-Altman plots showed no statistically significant difference in measured liver stiffness between the machines (mean difference, -0.8%; 95% confidence interval, -3.7%, 2.1%), with a critical difference of 1.36 kPa. The correlation was good to excellent for both the crude overall Pearson coefficient and the ICC, both measuring 0.88 (95% confidence interval, 0.82, 0.92). Subclass ICCs for the fasting state, postprandial state, novice operator, and experienced operator were 0.89, 0.88, 0.90, and 0.86, respectively. The 2-way mixed effect analysis of variance showed that the volunteers accounted for 86.3% of variation in median liver stiffness, with no statistically significant contribution from operator experience, the prandial state, or the machine (P = .108, .067, and .296, respectively).

CONCLUSIONS

Our study showed that the 2D SWE techniques had a high degree of reliability and agreement in measurement of liver stiffness in a healthy population. Operator experience and the prandial state did not impart significant variability to stiffness measurements.

Normative values for magnetic resonance elastography-based liver stiffness in a healthy population

INTRODUCTION Chronic liver disease resulting in fibrosis, and ultimately cirrhosis, is a significant cause of morbidity and mortality worldwide. None of the conventional imaging techniques are able to detect early fibrosis and compare its grade with the histopathologic scale. Liver biopsy, as the diagnostic standard for liver fibrosis, also has limitations and is not well accepted by patients. Magnetic resonance elastography is a well‑established technique for evaluating liver stiffness and may replace invasive procedures. Detection of liver fibrosis in its early stages, however, requires a detailed knowledge of normal liver stiffness. OBJECTIVES This study aimed to determine normal liver stiffness values in healthy volunteers. PATIENTS AND METHODS A total of 102 volunteers (mean age, 21.6 years; range, 20-28 years) with no history of gastrointestinal, hepatobiliary, or cardiovascular disease were enrolled in the study. Liver stiffness was evaluated by magnetic resonance elastography with a 1.5T clinical magnetic resonance scanner. Images of the induced transverse wave propagation were obtained and converted to tissue stiffness maps (elastograms). RESULTS The mean (SD) liver stiffness for the entire group of volunteers was 2.14 (0.28) kPa (range, 1.37-2.66 kPa). For women, the mean (SD) stiffness value was 2.14 (0.30) kPa (range, 1.37-2.66 kPa), and for men, 2.14 (0.25) kPa (range, 1.54-2.54 kPa). CONCLUSIONS Liver stiffness in a healthy adult cohort did not exceed 2.7 kPa and is not influenced by sex, body mass index, or fat content.

Magnetic resonance elastography of liver and spleen: Methods and applications

The viscoelastic properties of the liver and spleen can be assessed with magnetic resonance elastography (MRE). Several actuators, MRI acquisition sequences and reconstruction algorithms have been proposed for this purpose. Reproducible results are obtained, especially when the examination is performed in standard conditions with the patient fasting. Accurate staging of liver fibrosis can be obtained by measuring liver stiffness or elasticity with MRE. Moreover, emerging evidence shows that assessing the tissue viscous parameters with MRE is useful for characterizing liver inflammation, non-alcoholic steatohepatitis, hepatic congestion, portal hypertension, and hepatic tumors. Further advances such as multifrequency acquisitions and compression-sensitive MRE may provide novel quantitative markers of hepatic and splenic mechanical properties that may improve the diagnosis of hepatic and splenic diseases.

Quantitative Elastography Methods in Liver Disease: Current Evidence and Future Directions

Chronic liver diseases often result in the development of liver fibrosis and ultimately, cirrhosis. Treatment strategies and prognosis differ greatly depending on the severity of liver fibrosis, thus liver fibrosis staging is clinically relevant. Traditionally, liver biopsy has been the method of choice for fibrosis evaluation. Because of liver biopsy limitations, noninvasive methods have become a key research interest in the field. Elastography enables the noninvasive measurement of tissue mechanical properties through observation of shear-wave propagation in the tissue of interest. Increasing fibrosis stage is associated with increased liver stiffness, providing a discriminatory feature that can be exploited by elastographic methods. Ultrasonographic (US) and magnetic resonance (MR) imaging elastographic methods are commercially available, each with their respective strengths and limitations. Here, the authors review the technical basis, acquisition techniques, and results and limitations of US- and MR-based elastography techniques. Diagnostic performance in the most common etiologies of chronic liver disease will be presented. Reliability, reproducibility, failure rate, and emerging advances will be discussed. ^© RSNA, 2018 Online supplemental material is available for this article.

Repeatability of MR Elastography of Liver: A Meta-Analysis

Purpose To perform a meta-analysis to generate an estimate of the repeatability coefficient (RC) for magnetic resonance (MR) elastography of the liver. Materials and Methods A systematic search of databases was performed for publications on MR elastography during the 10-year period between 2006 and 2015. The identified studies were screened independently and were verified reciprocally by all authors. Two reviewers independently determined the percentage RC and effective sample size from each article. A forest plot was constructed of the percentage RC estimates from the 12 studies. Bootstrap 95% confidence intervals (CIs) were constructed for the summary percentage RCs. Results Twelve studies comprising 274 patients met the eligibility criteria and were included for analysis. A flow diagram of studies included according to Preferred Reporting Items for Systematic Reviews and Meta-Analyses guidelines was prepared for the inclusion and exclusion criteria. All studies included in the meta-analysis fulfilled four or more of the seven categories of the Quality Assessment of Diagnostic Accuracy Studies (QUADAS)-2. The estimated summary RC was 22% (95% CI: 16.1%, 28.2%). The three main sources for this heterogeneity were the trained versus untrained operator drawing contours to choose regions of interest, the time between two replicate examinations, and, finally, the field strength of the MR imaging unit. The RC estimates tended to be higher for studies that did not use a well-trained operator, those with 1.5-T field strength imaging units, and those with longer time intervals between examinations. Conclusion The meta-analysis results provide the basis for the following draft longitudinal Quantitative Imaging Biomarkers Alliance MR elastography claim: A measured change in hepatic stiffness of 22% or greater, at the same site and with use of the same equipment and acquisition sequence, indicates that a true change in stiffness has occurred with 95% confidence. ^© RSNA, 2017.

Primary liver injury and delayed resolution of liver stiffness after alcohol detoxification in heavy drinkers with the PNPLA3 variant I148M

AIM

To investigate the influence of PNPLA3 genotype in heavy drinkers on serum markers and liver stiffness (LS) during alcohol withdrawal and its association with histology.

METHODS

Caucasian heavy drinkers (n = 521) with a mean alcohol consumption of 192.1 g/d (median alcohol consumption: 169.0 g/d; 95%CI: 179.0-203.3) were enrolled at the Salem Medical Center, University of Heidelberg. LS was measured by transient elastography (Fibroscan, Echosens SA, Paris, France). LS and serum markers were prospectively studied in these patients with all stages of alcoholic liver disease (steatosis, steatohepatitis, fibrosis) prior and after alcohol detoxification with a mean observation interval of 6.2 ± 3.2 d. A liver biopsy with histological analysis including the Kleiner score was obtained in 80 patients.

RESULTS

The PNPLA3 rs738409 genotype distribution for CC, CG and GG was 39.2%, 52.6% and 8.2%. GG genotype primarily correlated with histological steatohepatitis (r = 0.404, P < 0.005), ballooning (r = 0.319, P < 0.005) and less with steatosis (r = 0.264, P < 0.05). Mean LS was lowest in CC carriers (13.1 kPa) as compared to CG and GG carriers (17.6 and 17.2 kPa). Notably, LS primarily correlated with fibrosis stage (r = 0.828, P < 0.005), ballooning (r = 0.516, P < 0.005), steatohepatitis (r = 0.319, P < 0.005) but not with steatosis. After alcohol withdrawal, LS did not change in CC carriers, significantly decreased in CG-carriers from 17.6 to 12.7 kPa but to a lesser extent in GG carriers from 17.6 to 14.5 kPa. This was due to prolonged resolution of inflammation with significantly elevated aspartate transaminase levels after alcohol withdrawal in GG carriers. Non-invasive fibrosis assessment by LS in all patients showed a significantly higher F0 rate as compared to the biopsy cohort (47% vs 6%) with 3.8% more CC carriers while 3.7% less were seen in the F4 cirrhosis group. Thus, about 20% of patients with alcoholic liver cirrhosis would be attributable to PNPLA3 G variants. The OR to develop cirrhosis corrected for age, gender and body mass index was 1.295 (95%CI: 0.787-2.131) for CG + GG carriers.

CONCLUSION

In heavy drinkers, PNPLA3 GG primarily correlates with ballooning/steatohepatitis but not steatosis resulting in a delayed inflammation-associated resolution of LS. Consequently, sustained ballooning-associated LS elevation seems to be a potential risk factor for fibrosis progression in PNPLA3 GG carriers.

Arterial pressure suffices to increase liver stiffness

Noninvasive measurement of liver stiffness (LS) has been established to screen for liver fibrosis. Since LS is also elevated in response to pressure-related conditions such as liver congestion, this study was undertaken to learn more about the role of arterial pressure on LS. LS was measured by transient elastography (μFibroscan platform, Echosens, Paris, France) during single intravenous injections of catecholamines in anesthetized rats with and without thioacetamide (TAA)-induced fibrosis. The effect of vasodilating glycerol trinitrate (GTN) on LS was also studied. Pressures in the abdominal aorta and caval and portal veins were measured in real time with the PowerLab device (AD Instruments, Dunedin, New Zealand). Baseline LS values in all rats (3.8 ± 0.5 kPa, n = 25) did not significantly differ from those in humans. Epinephrine and norepinephrine drastically increased mean arterial pressure (MAP) from 82 to 173 and 156 mmHg. Concomitantly, LS almost doubled from 4 to 8 kPa, while central venous pressure remained unchanged. Likewise, portal pressure only showed a slight and delayed increase. In the TAA-induced fibrosis model, LS increased from 9.5 ± 1.0 to 25.6 ± 14.7 kPa upon epinephrine injection and could efficiently be decreased by GTN. We finally show a direct association in humans in a physiological setting of elevated cardiac output and MAP. During continuous spinning at 200 W, MAP increased from 84 ± 8 to 99 ± 11 mmHg while LS significantly increased from 4.4 ± 1.8 to 6.7 ± 2.1 kPa. In conclusion, our data show that arterial pressure suffices to increase LS. Moreover, lowering MAP efficiently decreases LS in fibrotic livers that are predominantly supplied by arterial blood.

General review of magnetic resonance elastography

Magnetic resonance elastography (MRE) is an innovative imaging technique for the non-invasive quantification of the biomechanical properties of soft tissues via the direct visualization of propagating shear waves in vivo using a modified phase-contrast magnetic resonance imaging (MRI) sequence. Fundamentally, MRE employs the same physical property that physicians utilize when performing manual palpation - that healthy and diseased tissues can be differentiated on the basis of widely differing mechanical stiffness. By performing “virtual palpation”, MRE is able to provide information that is beyond the capabilities of conventional morphologic imaging modalities. In an era of increasing adoption of multi-parametric imaging approaches for solving complex problems, MRE can be seamlessly incorporated into a standard MRI examination to provide a rapid, reliable and comprehensive imaging evaluation at a single patient appointment. Originally described by the Mayo Clinic in 1995, the technique represents the most accurate non-invasive method for the detection and staging of liver fibrosis and is currently performed in more than 100 centers worldwide. In this general review, the mechanical properties of soft tissues, principles of MRE, clinical applications of MRE in the liver and beyond, and limitations and future directions of this discipline -are discussed. Selected diagrams and images are provided for illustration.

Inflammation-adapted liver stiffness values for improved fibrosis staging in patients with hepatitis C virus and alcoholic liver disease

BACKGROUND & AIMS

It is well known that inflammation increases liver stiffness (LS) in patients with chronic hepatitis C (HCV) and alcoholic liver disease (ALD) independent of fibrosis stage, but no inflammation-adapted cut-off values have been settled so far. An early identification of rapid fibrosers, however, is essential to decide whom to treat first with the novel but expensive antiviral drugs.

METHODS

Liver stiffness, biopsy-proven fibrosis stages F0-F4 (METAVIR or Kleiner score) and routine laboratory parameters were studied in 2068 patients with HCV (n = 1391) and ALD (n = 677).

RESULTS

Among the routine parameters for liver damage, AST correlated best with LS (HCV: r = 0.54, P < 0.0001 and ALD: r = 0.34, P < 0.0001). In the absence of elevated transaminases, cut-off values were almost identical between HCV and ALD for F1-2, F3 and F4 (HCV: 5.1, 9.0 and 11.9 kPa vs ALD: 4.9, 8.1 and 10.5 kPa). These cut-off values increased exponentially as a function of median AST level. The impact of AST on LS was higher in lobular-pronounced ALD as compared to portal tract-localized HCV. Most notably, Cohen's weighted Kappa displayed an improved agreement of the novel AST-dependent cut-off values with histological fibrosis stage both for HCV (0.68 vs 0.65) and ALD (0.80 vs 0.76).

CONCLUSIONS

The novel AST-adapted cut-off values improve non-invasive fibrosis staging in HCV and ALD and may be also applied to other liver diseases. Especially in HCV, they could help to decide whom to treat first with the novel but expensive antiviral drugs.

Magnetic resonance elastography (MRE) in cancer: Technique, analysis, and applications

Tissue mechanical properties are significantly altered with the development of cancer. Magnetic resonance elastography (MRE) is a noninvasive technique capable of quantifying tissue mechanical properties in vivo. This review describes the basic principles of MRE and introduces some of the many promising MRE methods that have been developed for the detection and characterization of cancer, evaluation of response to therapy, and investigation of the underlying mechanical mechanisms associated with malignancy.

Automated liver elasticity calculation for MR elastography

PURPOSE

MR elastography (MRE) is a phase-contrast MRI technique that is used to quantitatively assess liver stiffness for staging hepatic fibrosis. The current approach requires manual selection of a region of interest (ROI) with good wave quality from which to measure stiffness. The purpose of this work was to develop and evaluate a fully automated approach for measuring hepatic stiffness from MRE images to further reduce measurement variability.

MATERIALS AND METHODS

An automated liver elasticity calculation (ALEC) algorithm was developed to address reader stiffness measurement variability. ALEC has three stages: initial tissue estimation, segmentation, and ROI cleanup. Stiffnesses measured by the algorithm were compared with technicians and an expert radiologist in a set of 121 clinical cases acquired at 1.5 Tesla. Intra-class correlation (ICC), Bland-Altman analysis, and a noninferiority test were performed to evaluate whether the algorithm can be used in place of manual analysis by technicians.

RESULTS

The stiffness measurement difference with the expert was 1.42% ± 11.17% (mean ± standard deviation) for the algorithm and 1.82% ± 13.65% for the technicians. The ICCs were 0.981 and 0.984, respectively. Both the algorithm and technicians were equivalent to the expert within a 5% significance margin (P < 0.01). The algorithm had no failures in the 119 cases that were considered analyzable by the human readers.

CONCLUSION

The results of this study show that the newly developed automated algorithm is able to measure stiffness in clinical liver MRE exams with an accuracy that is equivalent to that of an expert radiologist. ALEC may be useful for analysis of archived data and suitable for performing multi-center studies.

Short-Term Repeatability of Magnetic Resonance Elastography at 3.0T: Effects of Motion-Encoding Gradient Direction, Slice Position, and Meal Ingestion

PURPOSE

Magnetic resonance elastography (MRE) can estimate liver stiffness (LS) noninvasively. We prospectively assessed whether motion-encoding gradient (MEG) direction, slice position, or high-caloric food intake affects the repeatability of MRE measurements of LS.

MATERIALS AND METHODS

Twenty healthy volunteers (8 women, 12 men; age, 48 ± 12 years) were imaged in a 3.0T scanner at four timepoints: twice after overnight fasting (B1 , B2 ) and twice after consuming a 1050-calorie standardized meal (A1 , A2 ; after 30 and 60 min, respectively). Each session comprised sequential MRE acquisitions in which MEG was applied in three orthogonal directions with three slices positioned over the liver for each. Between sessions, the participants were repositioned to assess test-retest reproducibility.

RESULTS

The LS measurements before/after food intake were 3.36 ± 1.31 kPa/3.22 ± 1.03 kPa, 2.04 ± 0.33 kPa/2.27 ± 0.38 kPa, and 2.47 ± 0.50 kPa/2.64 ± 0.76 kPa for MEG superimposed along the anterior-posterior (AP), foot-head (FH), and right-left (RL) directions, respectively. Before and after food intake, LS estimates were lower and more reproducible (<10% coefficient of variation) when the MEG was in the FH direction, not the AP or RL direction. Liver stiffness estimates were significantly elevated after meal consumption when the MEG was in the FH direction (P < 0.05 for B1 vs. A1 , B1 vs. A2 , B2 vs. A1 , and B2 vs. A2 ).

CONCLUSION

MRE estimates of LS were highly reproducible, particularly when MEG was applied in the FH direction, suggesting that this method could be used for long-term monitoring of antifibrotic therapy without repeated biopsies. High-caloric food intake resulted in slightly elevated LS on MRE.

Reliability of magnetic resonance elastography using multislice two-dimensional spin-echo echo-planar imaging (SE-EPI) and three-dimensional inversion reconstruction for assessing renal stiffness

BACKGROUND

To evaluate the reliability of MRE using a spin-echo echo-planar imaging (SE-EPI) renal MRE technique in healthy volunteers.

METHODS

Institutional review board approved prospective study in which all participants provided written informed consent. Sixteen healthy volunteers comprising seven males and nine females with a median age of 35 years (age range: 23 to 59 years) were included. Coronal 90 Hz and 60 Hz MRE acquisitions were performed twice within a 30-min interval between examinations. Renal MRE reliability was assessed by (i) test-retest repeatability, and (ii) inter-rater agreement between two independent readers. The MRE-measured averaged renal stiffness values were evaluated using: intraclass correlation coefficient (ICC), Bland-Altman and the within-subject coefficient of variation (COV).

RESULTS

For test-retest repeatability, Bland-Altman showed a mean stiffness difference between examinations of 0.07 kPa (95% limits of agreement: -1.41, 1.54) at 90 Hz and 0.01 kPa (95% limits of agreement: -0.51, 0.53) at 60 Hz. Coefficient of repeatability was 1.47 kPa and 0.52 kPa at 90 Hz and 60 Hz, respectively. The within-subject COV was 13.6% and 7.7% at 90 Hz and 60 Hz, respectively. ICC values were 0.922 and 0.907 for test-retest repeatability and 0.998 and 0.989 for inter-rater agreement, respectively (P < 0.001).

CONCLUSION

SE-EPI renal MRE is a reliable technique.

Hepatic MR Elastography: Clinical Performance in a Series of 1377 Consecutive Examinations

PURPOSE

To assess the technical success rate and diagnostic performance of liver magnetic resonance (MR) elastography.

MATERIALS AND METHODS

This retrospective study was approved by the institutional review board with patient informed consent. A total of 1377 consecutive MR elastography examinations performed between 2007 and 2010 in 1287 patients for clinical indications were included. Medical records were used to retrieve liver stiffness as assessed with MR elastography, histologic analysis, blood work, and other liver disease-related information. Nonparametric Kruskal-Wallis tests and analysis of covariance methods were used to evaluate the diagnostic values and relationships of the collected data.

RESULTS

Hepatic MR elastography had a success rate of 94.4% (1300 of 1377 cases) and yielded reproducible measurements (r = 0.9716, P < .0001) in the study cohort, with a complex patient profile and multiple interpreters. Body mass index had no significant effect on success rate (P = .2). In 289 patients who underwent liver biopsy within 1 year of the MR elastography date, mean liver stiffness as assessed with MR elastography was significantly higher in patients with advanced fibrosis (stages F3, F4) than in those with mild to moderate fibrosis (stages F0, F1, F2) (5.93 kPa ± 2.31 [standard deviation] vs 3.35 kPa ± 1.44, P < .0001). Liver stiffness is associated with many factors other than fibrosis extent, including cause of fibrosis (viral hepatitis C vs nonalcoholic fatty liver disease, P = .025), inflammation (severe vs mild to moderate, P = .03), and hepatic metabolic and synthetic function (no fibrosis vs intermediate fibrosis, P ≤ .01).

CONCLUSION

In a general clinical practice environment, hepatic MR elastography is a robust imaging method with a high success rate in a broad spectrum of patients. It also shows the complex association between liver stiffness and hepatic pathophysiology.

Ultrasound Elastography and MR Elastography for Assessing Liver Fibrosis: Part 2, Diagnostic Performance, Confounders, and Future Directions

OBJECTIVE

The purpose of the article is to review the diagnostic performance of ultra-sound and MR elastography techniques for detection and staging of liver fibrosis, the main current clinical applications of elastography in the abdomen.

CONCLUSION

Technical and instrument-related factors and biologic and patient-related factors may constitute potential confounders of stiffness measurements for assessment of liver fibrosis. Future developments may expand the scope of elastography for monitoring liver fibrosis and predict complications of chronic liver disease.

Magnetic resonance elastography of abdomen

Many diseases cause substantial changes in the mechanical properties of tissue, and this provides motivation for developing methods to noninvasively assess the stiffness of tissue using imaging technology. Magnetic resonance elastography (MRE) has emerged as a versatile MRI-based technique, based on direct visualization of propagating shear waves in the tissues. The most established clinical application of MRE in the abdomen is in chronic liver disease. MRE is currently regarded as the most accurate noninvasive technique for detection and staging of liver fibrosis. Increasing experience and ongoing research is leading to exploration of applications in other abdominal organs. In this review article, the current use of MRE in liver disease and the potential future applications of this technology in other parts of the abdomen are surveyed.

Latest developments in the imaging of fibrotic liver disease

According to the World Health Organization, liver cirrhosis accounted for 1.8% of all deaths in Europe, causing about 170,000 deaths per year. Approximately 29 million persons in the EU suffer from chronic liver disease and this trend is on the rise. Liver disease is the EU's fifth most common cause of death accounting for at least one in six deaths. Early detection and monitoring of fibrosis has the potential to direct management of these chronic liver diseases and avert morbidity and mortality. Although the available techniques are in their infancy and the very early stages of fibrosis are difficult to detect, there have been significant advances in imaging over the last decade that has resulted in the use of these new imaging techniques being introduced into the patient pathway. This review explores the accuracies of these imaging techniques, their role in the management of patients, and the potential for the future.

Magnetic resonance elastography of liver

Magnetic resonance elastography (MRE) assesses tissue stiffness in vivo by imaging propagating shear waves through the tissues and processing the wave information. MRE is a robust technology with excellent technical success; is applicable in almost all patients and body habitus; and has excellent reproducibility, repeatability, and interobserver agreement for assessing liver stiffness. It is currently the most accurate noninvasive technique for detection and staging of liver fibrosis and has the potential to replace liver biopsy. This article describes the principles and technique of MRE, current clinical applications, and emerging clinical indications.

Quantitative liver MRI combining phase contrast imaging, elastography, and DWI: assessment of reproducibility and postprandial effect at 3.0 T

PURPOSE

To quantify short-term reproducibility (in fasting conditions) and postprandial changes after a meal in portal vein (PV) flow parameters measured with phase contrast (PC) imaging, liver diffusion parameters measured with multiple b value diffusion-weighted imaging (DWI) and liver stiffness (LS) measured with MR elastography (MRE) in healthy volunteers and patients with liver disease at 3.0 T.

MATERIALS AND METHODS

In this IRB-approved prospective study, 30 subjects (11 healthy volunteers and 19 liver disease patients; 23 males, 7 females; mean age 46.5 y) were enrolled. Imaging included 2D PC imaging, multiple b value DWI and MRE. Subjects were initially scanned twice in fasting state to assess short-term parameter reproducibility, and then scanned 20 min. after a liquid meal. PV flow/velocity, LS, liver true diffusion coefficient (D), pseudodiffusion coefficient (D*), perfusion fraction (PF) and apparent diffusion coefficient (ADC) were measured in fasting and postprandial conditions. Short-term reproducibility was assessed in fasting conditions by measuring coefficients of variation (CV) and Bland-Altman limits of agreement. Differences in MR metrics before and after caloric intake and between healthy volunteers and liver disease patients were assessed.

RESULTS

PV flow parameters, D, ADC and LS showed good to excellent short-term reproducibility in fasting state (CV <16%), while PF and D* showed acceptable and poor reproducibility (CV = 20.4% and 51.6%, respectively). PV flow parameters and LS were significantly higher (p<0.04) in postprandial state while liver diffusion parameters showed no significant change (p>0.2). LS was significantly higher in liver disease patients compared to healthy volunteers both in fasting and postprandial conditions (p<0.001). Changes in LS were significantly correlated with changes in PV flow (Spearman rho = 0.48, p = 0.013).

CONCLUSIONS

Caloric intake had no/minimal/large impact on diffusion/stiffness/portal vein flow, respectively. PC MRI and MRE but not DWI should be performed in controlled fasting state.

Short- and midterm repeatability of magnetic resonance elastography in healthy volunteers at 3.0 T

The purpose of this study was to evaluate the short- and midterm repeatability of liver stiffness measurements with magnetic resonance elastography (MRE) in healthy subjects at 3.0T. Twenty-two healthy volunteers were enrolled in this prospective study. The stiffness measurements were obtained from three slices with three repeated acquisitions for each slice (session 1) by two independent raters. After a mean period of 7±2days (session 2) and 195±15days (session 3), each subject was scanned again using the same protocol and MR system. The liver stiffness differences were calculated between sessions or raters. The intraclass correlation coefficient (ICC) was calculated to assess interrater agreement and intersession agreement. The stiffness differences over the short- and midterm intervals was (-0.004±0.086) kPa for sessions 1-2, lower than (-0.055±0.150) kPa for sessions 1-3 and (-0.051±0.173) kPa for sessions 2-3. The liver stiffness was more repeatable for the short-term interval with the mean overall ICC of 0.96 (sessions 1-2) (95% confidence interval [CI]: 0.90-0.98) compared with 0.91 (sessions 1-3) (95% CI: 0.78-0.96) and 0.87 (sessions 2-3) (95% CI: 0.69-0.95) for the midterm intervals. The overall ICC of interrater agreement was excellent at 0.987 (95% CI: 0.983 to 0.990). These results confirm that MRE is a reproducible technique for liver stiffness quantification over short- and midterm intervals up to 6months in a healthy population at 3.0T.

Magnetic resonance elastography of liver: clinical applications

Magnetic resonance elastography (MRE) has been successfully implemented in the assessment of diffuse liver diseases. Currently, MRE is the most accurate noninvasive technique for detection and staging of liver fibrosis with a potential to replace liver biopsy. Magnetic resonance elastography is able to differentiate isolated fatty liver disease from steatohepatitis with or without fibrosis. Potential clinical applications include the differentiation of benign and malignant focal liver masses and the assessment of treatment response in diffuse liver diseases.

MR elastography of the liver: defining thresholds for detecting viscoelastic changes

PURPOSE

To define thresholds for detecting significant change in liver viscoelasticity with magnetic resonance (MR) elastography, both for whole-liver measurements and for voxel-wise measurements in relation to spatial resolution.

MATERIALS AND METHODS

This prospective study was approved by the institutional review board, and all participants provided written informed consent. Thirty participants (16 volunteers and 14 patients with hepatitis B or C; 18 men; median age, 30.4 years; age range, 18.9-58.6 years) underwent imaging twice while in the same position (intraimage reproducibility), after repositioning (within-day reproducibility), and 1-4 weeks later (between-weeks reproducibility). MR elastography parameters comprised elasticity, viscosity, attenuation parameter α, and propagation parameter β. Bland-Altman analysis was used to calculate repeatability indexes for each parameter. Analyses were performed in a region-of-interest and a voxel-by-voxel level. Voxel-wise results were calculated in relation to spatial resolution by applying Gaussian filtering to establish the optimal trade-off point between resolution and reproducibility.

RESULTS

For elasticity, α, and β, within-day and between-weeks results were significantly lower than intraimage results (P ≤ .018 for all). Within-day and between-weeks results did not differ significantly. Over-time changes of more than 22.2% for elasticity, 26.3% for viscosity, 26.8% for α, and 10.1% for β represented thresholds for significant change. The optimal trade-off between spatial resolution and reproducibility was found at a filter size of 8-mm full width at half maximum (FWHM) for elasticity and propagation parameter β and at 16-mm FWHM for viscosity and attenuation parameter α.

CONCLUSION

Repositioning causes a significant decrease in the reproducibility of MR elastography. The propagation parameter β is the most reliable parameter, with an over-time threshold for significant change of 10.1% and the ability to reproduce viscoelasticity up to a resolution of 8-mm FWHM. Online supplemental material is available for this article.

Magnetic resonance elastography of liver in healthy Asians: normal liver stiffness quantification and reproducibility assessment

PURPOSE

To determine normal liver stiffness values evaluated with magnetic resonance elastography (MRE) in healthy normal Asian volunteers and assess its reproducibility.

MATERIALS AND METHODS

Liver stiffness was evaluated with MRE in 41 healthy Asians (23 females, 18 males; mean age, 41.8 years, and mean body mass index [BMI], 23.4 kg/m(2) ) on a 1.5T clinical scanner. The correlations between mean liver stiffness and age, gender, BMI, and fat fraction percentage of the liver were studied. Another 12 volunteers underwent liver MRE exams on two separate days 4-6 weeks apart under similar conditions for reproducibility assessment. Intraclass correlation coefficient (ICC) analysis was performed and within-subject coefficient of variation (CV) of stiffness was estimated.

RESULTS

The mean ± standard deviation (SD) of liver stiffness in normal healthy Asian subjects was 2.09 ± 0.22 kPa (95% confidence interval [CI], 2.04-2.15 kPa; range 1.68-2.48 kPa). The mean liver stiffness did not significantly correlate with age, gender, BMI, or fat content of the liver. The ICC for mean liver stiffness was 0.90 (95% CI, 0.78-0.96) and CV ranged from 2.2%-11.4%.

CONCLUSION

The liver stiffness in normal Asians is not affected by age, gender, BMI, or fat content. Liver stiffness with MRE is highly reproducible.

Hepatic and splenic stiffness augmentation assessed with MR elastography in an in vivo porcine portal hypertension model

PURPOSE

To investigate the influence of portal pressure on the shear stiffness of the liver and spleen in a well-controlled in vivo porcine model with magnetic resonance elastography (MRE). A significant correlation between portal pressure and tissue stiffness could be used to noninvasively assess increased portal venous pressure (portal hypertension), which is a frequent clinical condition caused by cirrhosis of the liver and is responsible for the development of many lethal complications.

MATERIALS AND METHODS

During multiple intraarterial infusions of Dextran-40 in three adult domestic pigs in vivo, 3D abdominal MRE was performed with left ventricle and portal catheters measuring blood pressure simultaneously. Least-squares linear regressions were used to analyze the relationship between tissue stiffness and portal pressure.

RESULTS

Liver and spleen stiffness have a dynamic component that increases significantly following an increase in portal or left ventricular pressure. Correlation coefficients with the linear regressions between stiffness and pressure exceeded 0.8 in most cases.

CONCLUSION

The observed stiffness-pressure relationship of the liver and spleen could provide a promising noninvasive method for assessing portal pressure. Using MRE to study the tissue mechanics associated with portal pressure may provide new insights into the natural history and pathophysiology of hepatic diseases and may have significant diagnostic value in the future.

Magnetic resonance elastography noninvasively detects in vivo renal medullary fibrosis secondary to swine renal artery stenosis

OBJECTIVES

Magnetic resonance elastography (MRE) can noninvasively sample tissue stiffness in vivo. Renal fibrosis secondary to renal artery stenosis (RAS), which is aggravated in atherosclerotic RAS (ARAS), may increase its stiffness. An increase in cortical stiffness in vivo can be masked by intrinsic hemodynamic determinants, whereas renal medullary stiffness is less dependent on renal hemodynamics. Therefore, this study tested the hypothesis that MRE-determined medullary stiffness would correspond to the histological degree of medullary fibrosis in stenotic kidneys in RAS and detect its exacerbation in ARAS.

MATERIALS AND METHODS

Seventeen pigs were studied 10 weeks after induction of unilateral RAS (n = 6), ARAS (n = 5), or sham (n = 6). Stiffness of the cortex and the medulla was determined through 3-dimensional MRE, and renal perfusion and function were determined using multidetector computed tomography. Kidney fibrosis was subsequently assessed ex vivo using the Masson trichrome staining.

RESULTS

Renal stenotic cortex and medulla were significantly more fibrotic in RAS and ARAS compared with healthy kidney. However, MRE detected increased stiffness in RAS compared with the healthy kidney (12.7 ± 0.41 kPa vs 10.7 ± 0.18 kPa; P = 0.004) only in the medulla, which was further increased in ARAS (16.6 ± 1.3 kPa; P = 0.017 vs RAS). Magnetic resonance elastography-derived medullary, but not cortical, stiffness significantly correlated with histological degree of fibrosis, although cortical and medullary fibroses were correlated. Renal blood flow and function were similarly decreased in RAS and ARAS compared with the healthy kidney.

CONCLUSIONS

Noninvasive 3-dimensional MRE detects increased renal medullary stiffness in RAS and ARAS in vivo, which correlates with its fibrosis ex vivo and may also reflect cortical fibrosis. Hence, MRE-derived medullary stiffness can be potentially useful in detecting renal fibrosis and track disease progression.

Review of MR elastography applications and recent developments

The technique of MR elastography (MRE) has emerged as a useful modality for quantitatively imaging the mechanical properties of soft tissues in vivo. Recently, MRE has been introduced as a clinical tool for evaluating chronic liver disease, but many other potential applications are being explored. These applications include measuring tissue changes associated with diseases of the liver, breast, brain, heart, and skeletal muscle including both focal lesions (e.g., hepatic, breast, and brain tumors) and diffuse diseases (e.g., fibrosis and multiple sclerosis). The purpose of this review article is to summarize some of the recent developments of MRE and to highlight some emerging applications.

Liver stiffness measured by magnetic resonance elastography as a risk factor for hepatocellular carcinoma: a preliminary case-control study

OBJECTIVE

To examine if liver stiffness measured by magnetic resonance elastography (MRE) is a risk factor for hepatocellular carcinoma (HCC) in patients with chronic liver disease.

METHODS

By reviewing the records of magnetic resonance (MR) examinations performed at our institution, we selected 301 patients with chronic liver disease who did not have a previous medical history of HCC. All patients underwent MRE and gadoxetic acid-enhanced MR imaging. HCC was identified on MR images in 66 of the 301 patients, who were matched to controls from the remaining patients without HCC according to age. MRE images were obtained by visualising elastic waves generated in the liver by pneumatic vibration transferred via a cylindrical passive driver. Risk factors of HCC development were determined by the odds ratio with logistic regression analysis; gender and liver stiffness by MRE and serum levels of aspartate transferase, alanine transferase, alpha-fetoprotein, and protein induced by vitamin K absence-II.

RESULTS

Multivariate analysis revealed that only liver stiffness by MRE was a significant risk factor for HCC with an odds ratio (95 % confidence interval) of 1.38 (1.05-1.84).

CONCLUSION

Liver stiffness measured by MRE is an independent risk factor for HCC in patients with chronic liver disease.