Liver fibrosis: noninvasive assessment with MR elastography versus aspartate aminotransferase-to-platelet ratio index

Post-operative imaging in liver transplantation: State-of-the-art and future perspectives

Orthotopic liver transplantation (OLT) represents a major treatment for end-stage chronic liver disease, as well as selected cases of hepatocellular carcinoma and acute liver failure. The ever-increasing development of imaging modalities significantly contributed, over the last decades, to the management of recipients both in the pre-operative and post-operative period, thus impacting on graft and patients survival. When properly used, imaging modalities such as ultrasound, multidetector computed tomography, magnetic resonance imaging (MRI) and procedures of direct cholangiography are capable to provide rapid and reliable recognition and treatment of vascular and biliary complications occurring after OLT. Less defined is the role for imaging in assessing primary graft dysfunction (including rejection) or chronic allograft disease after OLT, e.g., hepatitis C virus (HCV) recurrence. This paper: (1) describes specific characteristic of the above imaging modalities and the rationale for their use in clinical practice; (2) illustrates main imaging findings related to post-OLT complications in adult patients; and (3) reviews future perspectives emerging in the surveillance of recipients with HCV recurrence, with special emphasis on MRI.

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.

Biomarkers of liver fibrosis

Liver fibrosis is the final common pathway for almost all causes of chronic liver injury. Liver fibrosis is now known to be a dynamic process having significant potential for resolution. Therefore, fibrosis prediction is an essential part of the assessment and management of patients with chronic liver disease. As such, there is strong demand for reliable liver biomarkers that provide insight into disease etiology, diagnosis, therapy, and prognosis in lieu of more invasive approaches such as liver biopsy. Current diagnostic strategies range from use of serum biomarkers to more advanced imaging techniques including transient elastography and magnetic resonance imaging. In addition to these modalities, there are other approaches including the use of novel, but yet to be validated, biomarkers. In this chapter, we discuss the biomarkers of liver fibrosis including the use of invasive and noninvasive biomarkers and disease-specific biomarkers in various chronic liver diseases.

Histogram analysis of hepatobiliary phase MR imaging as a quantitative value for liver cirrhosis: preliminary observations

PURPOSE

To investigate whether histogram analysis of the hepatobiliary phase on gadoxetate enhanced-MRI could be used as a quantitative index for determination of liver cirrhosis.

MATERIALS AND METHODS

A total of 63 patients [26 in a normal liver function (NLF) group and 37 in a cirrhotic group] underwent gadoxetate-enhanced MRI, and hepatobiliary phase images were obtained at 20 minutes after contrast injection. The signal intensity of the hepatic parenchyma was measured at four different regions of interest (ROI) of the liver, avoiding vessels and bile ducts. Standard deviation (SD), coefficient of variation (CV), and corrected CV were calculated on the histograms at the ROIs. The distributions of CVs calculated from the ROI histogram were examined and statistical analysis was carried out.

RESULTS

The CV value was 0.041±0.009 (mean CV±SD) in the NLF group, while that of cirrhotic group was 0.071±0.020. There were statistically significant differences in the CVs and corrected CV values between the NLF and cirrhotic groups (p<0.001). The most accurate cut-off value among CVs for distinguishing normal from cirrhotic group was 0.052 (sensitivity 83.8% and specificity 88.5%). There was no statistically significant differences in SD between NLF and cirrhotic groups (p=0.307).

CONCLUSION

The CV of histograms of the hepatobiliary phase on gadoxetate-enhanced MRI may be useful as a quantitative value for determining the presence of liver cirrhosis.

Optimization of scanning parameters for MR elastography at 3.0 T clinical unit: volunteer study

PURPOSE

We sought to optimize scanning parameters for MR elastography at 3.0 T clinical unit.

MATERIALS AND METHODS

10 volunteers were scanned with various magnetization encoding gradient (MEG) frequencies from 60 to 120 Hz at every 10 Hz, with otherwise fixed parameters (external driver frequency/amplitude = 60 Hz/50 %, 10 mm slice thickness, etc.). Images were qualitatively assessed for the degree of image defects, and also quantitatively for the areas without cross-hatching. After determining optimal MEG frequency, external driver amplitudes of 70 % (vs 50 %) and slice thickness of 8 mm (vs 10 mm) were also tested. With the optimized parameters, scans were repeated 1 week after the initial scan, and the repeatability of the liver stiffness measurement was validated.

RESULTS

80 or 90 Hz was shown to be the best MEG frequency. There were no significant differences in the qualitative and quantitative assessment between the two amplitudes and two slice thicknesses; however, 70 % amplitude resulted in discomfort at the chest wall beneath the external acoustic driver. Thus, MEG 80 (or 90) Hz, amplitude 50 %, and thickness 10 (or 8) mm were considered optimal. Repeatability of the liver stiffness measurement was ±10 % (95 % confidence interval).

CONCLUSIONS

With the optimized parameters, repeatability of ±10 % in liver stiffness measurement was obtained.

Viscoelastic parameters for quantifying liver fibrosis: three-dimensional multifrequency MR elastography study on thin liver rat slices

Objective

To assess in a high-resolution model of thin liver rat slices which viscoelastic parameter at three-dimensional multifrequency MR elastography has the best diagnostic performance for quantifying liver fibrosis.

Materials and Methods

The study was approved by the ethics committee for animal care of our institution. Eight normal rats and 42 rats with carbon tetrachloride induced liver fibrosis were used in the study. The rats were sacrificed, their livers were resected and three-dimensional MR elastography of 5±2 mm liver slices was performed at 7T with mechanical frequencies of 500, 600 and 700 Hz. The complex shear, storage and loss moduli, and the coefficient of the frequency power law were calculated. At histopathology, fibrosis and inflammation were assessed with METAVIR score, fibrosis was further quantified with morphometry. The diagnostic value of the viscoelastic parameters for assessing fibrosis severity was evaluated with simple and multiple linear regressions, receiver operating characteristic analysis and Obuchowski measures.

Results

At simple regression, the shear, storage and loss moduli were associated with the severity of fibrosis. At multiple regression, the storage modulus at 600 Hz was the only parameter associated with fibrosis severity (r = 0.86, p<0.0001). This parameter had an Obuchowski measure of 0.89+/−0.03. This measure was significantly larger than that of the loss modulus (0.78+/−0.04, p = 0.028), but not than that of the complex shear modulus (0.88+/−0.03, p = 0.84).

Conclusion

Our high resolution, three-dimensional multifrequency MR elastography study of thin liver slices shows that the storage modulus is the viscoelastic parameter that has the best association with the severity of liver fibrosis. However, its diagnostic performance does not differ significantly from that of the complex shear modulus.

Parameter identification in a generalized time-harmonic Rayleigh damping model for elastography

The identifiability of the two damping components of a Generalized Rayleigh Damping model is investigated through analysis of the continuum equilibrium equations as well as a simple spring-mass system. Generalized Rayleigh Damping provides a more diversified attenuation model than pure Viscoelasticity, with two parameters to describe attenuation effects and account for the complex damping behavior found in biological tissue. For heterogeneous Rayleigh Damped materials, there is no equivalent Viscoelastic system to describe the observed motions. For homogeneous systems, the inverse problem to determine the two Rayleigh Damping components is seen to be uniquely posed, in the sense that the inverse matrix for parameter identification is full rank, with certain conditions: when either multi-frequency data is available or when both shear and dilatational wave propagation is taken into account. For the multi-frequency case, the frequency dependency of the elastic parameters adds a level of complexity to the reconstruction problem that must be addressed for reasonable solutions. For the dilatational wave case, the accuracy of compressional wave measurement in fluid saturated soft tissues becomes an issue for qualitative parameter identification. These issues can be addressed with reasonable assumptions on the negligible damping levels of dilatational waves in soft tissue. In general, the parameters of a Generalized Rayleigh Damping model are identifiable for the elastography inverse problem, although with more complex conditions than the simpler Viscoelastic damping model. The value of this approach is the additional structural information provided by the Generalized Rayleigh Damping model, which can be linked to tissue composition as well as rheological interpretations.

Evaluation of hepatic fibrosis using intravoxel incoherent motion in diffusion-weighted liver MRI

OBJECTIVES

To determine whether intravoxel incoherent motion (IVIM)-diffusion-weighted image (DWI)-derived parameters showed better diagnostic performance than the apparent diffusion coefficient (ADC(total)) for the evaluation of hepatic fibrosis (HF).

METHODS

This retrospective study was approved by institutional review board, and informed consent was waived. Fifty-five patients with chronic liver disease who had undergone IVIM-DWI using 8 b-values at 3 T were included. True diffusion coefficient (Dt), pseudo-diffusion coefficient (Dp), perfusion fraction (f), and ADC(total) were calculated. Receiver operating characteristic analysis was performed for all parameters for the HF staging.

RESULTS

All parameters showed a significant correlation with the HF stages (-0.31 to -0.72, P < 0.05). All parameters were significantly higher in F0 to F1 than in F4 (P < 0.05). The Dp showed better performance than the ADC(total) in differentiating significant HF (≥F2) from F0 to F1.

CONCLUSIONS

The IVIM-derived parameters and ADC(total) showed significant correlation with HF. The D p showed better diagnostic performance for differentiating significant HF than did ADC(total).

Noninvasive assessment of hepatic fibrosis in patients with chronic hepatitis B viral infection using magnetic resonance elastography

Objective

To evaluate the diagnostic performance of magnetic resonance elastography (MRE) for staging hepatic fibrosis in patients with chronic hepatitis B virus (HBV) infection.

Materials and Methods

Patients with chronic HBV infection who were suspected of having focal or diffuse liver diseases (n = 195) and living donor candidates (n = 166) underwent MRE as part of the routine liver MRI examination. We measured liver stiffness (LS) values on quantitative shear stiffness maps. The technical success rate of MRE was then determined. Liver cell necroinflammatory activity and fibrosis were assessed using histopathologic examinations as the reference. Areas under the receiver operating characteristic curve (Az) were calculated in order to predict the liver fibrosis stage.

Results

The technical success rate of MRE was 92.5% (334/361). The causes of technical failure were poor wave propagation (n = 12), severe respiratory motion (n = 3), or the presence of iron deposits in the liver (n = 12). The mean LS values, as measured by MRE, increased significantly along with an increase in the fibrosis stage (r = 0.901, p < 0.001); however, the mean LS values did not increase significantly along with the degree of necroinflammatory activity. The cutoff values of LS for ≥ F1, ≥ F2, ≥ F3, and F4 were 2.45 kPa, 2.69 kPa, 3.0 kPa, and 3.94 kPa, respectively, and with Az values of 0.987-0.988.

Conclusion

MRE has a high technical success rate and excellent diagnostic accuracy for staging hepatic fibrosis in patients with chronic HBV infection.

Shear wave elastography for liver stiffness measurement in clinical sonographic examinations: evaluation of intraobserver reproducibility, technical failure, and unreliable stiffness measurements

OBJECTIVES

The purpose of this study was to determine the optimal minimum number of liver stiffness measurements on shear wave elastography (SWE) and to evaluate the frequency of technical failures and unreliable stiffness measurements and the intraobserver reproducibility of SWE.

METHODS

This retrospective study was approved by our Institutional Review Board, and informed consent was waived. From August 2011 to January 2013, 540 patients underwent abdominal sonography, including SWE. In 86 patients (group 1), the minimum number of examinations was determined by comparing the intraclass correlation coefficient (ICC) of subsets of the first 2 to 14 measurements with that from 15 measurements. In 454 patients (group 2), 2 SWE sessions were performed in the right lobe within 1 day. Technical failure was defined as when no or little signal was obtained in the elastogram during the first 5 acquisitions; unreliable SWE results were defined as when the interquartile range/median liver stiffness value exceeded 30%. Intraobserver reproducibility was assessed using ICCs and Bland-Altman plots.

RESULTS

In group 1, the ICCs did not significantly increase after the first 6 measurements. In group 2, there were technical failures and unreliable results in 47 patients (10.35%) and 74 patients (16.29%), respectively. In 407 patients, after excluding technical failures, there was no significant difference in the median liver stiffness values between the 2 sessions (6.95 versus 6.86 kPa; P > .05). The overall intraobserver reproducibility was excellent (ICC, 0.95).

CONCLUSIONS

In this study, the optimal minimum number of SWE measurements was 6, and SWE using 6 measurements showed excellent intraobserver reproducibility.

Evaluation of liver fibrosis and hepatic venous pressure gradient with MR elastography in a novel swine model of cirrhosis

PURPOSE

To assess the correlation among MR elastography (MRE) measured liver stiffness (LS), liver fibrosis, and hepatic venous pressure gradient (HVPG) in a swine model of cirrhosis.

MATERIALS AND METHODS

Three swine served as controls, and liver fibrosis was induced in eight swine by transarterial embolization. LS and HVPG were obtained at baseline and 4 weeks (prenecropsy) following induction of liver fibrosis.

RESULTS

Four weeks following the induction of liver cirrhosis, experimental animals developed an increase in HVPG of 8.0±6.4 mmHg compared with 0.3±1.2 mmHg for controls (P=0.08). Over the same timeframe, mean MRE-measured LS increased 0.82±0.39 kPa for experimental swine and 0.1±0.05 kPa for controls (P=0.01). A positive correlation was observed between increases in HVPG and LS (ρ=0.682; P=0.02). Liver fibrosis was measured on explanted livers at 4 weeks and yielded mean fibrosis scores of 2.8 for experimental animals and 0 for controls (P=0.0016). A positive correlation was observed between higher LS and liver fibrosis (ρ=0.884; P=0.0003).

CONCLUSION

MRE is a reliable noninvasive technique to measure LS in a swine model of cirrhosis. Significant positive correlations were observed between LS and HVPG as well as LS and fibrosis.

Determination of normal hepatic elasticity by using real-time shear-wave elastography

PURPOSE

To determine normal reference values of liver elasticity and measurement reliability by using real-time shear-wave elastography (SWE) in patients with a range of ages and body mass index (BMI) measurements, with presence or absence of hepatic steatosis.

MATERIALS AND METHODS

The institutional review board approved this study, and informed consent was waived because of the retrospective nature of the study. Two hundred thirty-eight patients who underwent SWE and ultrasonography-guided liver biopsies on the same day were identified retrospectively. The median kilopascal value of three consecutive measurements was used as a representative value for each subject. One hundred ninety-six patients who were potential donors for living-donor liver transplantation and had biopsy-proven normal (123 nonsteatotic and 73 steatotic) livers as the only histologic abnormality were included in the study. Reference ranges of normal hepatic elasticity were calculated by using lower and upper limits at the 2.5 and 97.5 percentiles. With the upper value of the reference range as a cutoff value, the sensitivity and specificity for the diagnosis of hepatic fibrosis were calculated. Measurement reliability was evaluated by using the intraclass correlation coefficient (ICC). To investigate the effects of potential confounding factors (age, hepatic steatosis, and BMI) on liver elasticity, the Pearson correlation test and the Student t test were performed.

RESULTS

The reference range of normal hepatic elasticity was 2.6-6.2 kPa. With 6.2 kPa as a cutoff value, the sensitivity and specificity for the diagnosis of hepatic fibrosis were 91% (20 of 22 subjects) and 95.9% (188 of 196 subjects), respectively. The overall ICC for the elasticity measurements was 0.924. The potential confounding factors that we considered had negligible effects on the elasticity values.

CONCLUSION

Hepatic elasticity values measured with SWE in histologically proven normal livers ranged from 2.6 to 6.2 kPa, with high measurement reliability. The effect of the potential confounding factors on liver elasticity was negligible.

Comparison of magnetic resonance elastography and gadoxetate disodium-enhanced magnetic resonance imaging for the evaluation of hepatic fibrosis

OBJECTIVES

The objective of this study was to compare the diagnostic performance of magnetic resonance elastography (MRE) and gadoxetate disodium-enhanced magnetic resonance imaging (MRI) in the staging of hepatic fibrosis (HF) in patients with liver diseases.

MATERIALS AND METHODS

This retrospective study was approved by our institutional review board, and the informed consent was waived. One hundred and sixty-eight patients with chronic liver disease or suspected focal hepatic lesions underwent MRE and gadoxetate disodium-enhanced MRI at 1.5 T. Liver stiffness values were measured on quantitative shear-stiffness maps. The contrast enhancement index (CEI) was calculated as SIpost / SIpre, where SIpost and SIpre are, respectively, the liver-to-muscle signal intensity (SI) ratio on hepatobiliary phase images and on unenhanced images. The diagnostic performance of MRE and CEI for staging HF was compared using the receiver operating characteristic curve analysis on the basis of the histopathologic analysis of HF.

RESULTS

The liver stiffness values measured on MRE (r = 0.802; P < 0.0001) were more strongly correlated with the HF stage than with the CEI (r = -0.378; P < 0.0001). The areas under the receiver operating characteristic curve values of the liver stiffness values were significantly larger than those of CEI were for discriminating all stages of HF (P < 0.001 for ≥ F1, ≥ F2, ≥ F3, and ≥ F4). Magnetic resonance elastography showed higher sensitivity and specificity for predicting HF ≥ F1 (91% and 87%), ≥ F2 (87% and 91%), ≥ F3 (80% and 89%), and F4 (81% and 85%) compared with CEI (46% and 85%, 46% and 82%, 63% and 68%, and 76% and 65%, respectively).

CONCLUSIONS

Magnetic resonance elastography was superior to the gadoxetate disodium-enhancement MRI for HF staging.

Non-invasive diagnosis of hepatitis B virus-related cirrhosis

Chronic hepatitis B (CHB) infection is a major public health problem associated with significant morbidity and mortality worldwide. Twenty-three percent of patients with CHB progress naturally to liver cirrhosis, which was earlier thought to be irreversible. However, it is now known that cirrhosis can in fact be reversed by treatment with oral anti-nucleotide drugs. Thus, early and accurate diagnosis of cirrhosis is important to allow an appropriate treatment strategy to be chosen and to predict the prognosis of patients with CHB. Liver biopsy is the reference standard for assessment of liver fibrosis. However, the method is invasive, and is associated with pain and complications that can be fatal. In addition, intra- and inter-observer variability compromises the accuracy of liver biopsy data. Only small tissue samples are obtained and fibrosis is heterogeneous in such samples. This confounds the two types of observer variability mentioned above. Such limitations have encouraged development of non-invasive methods for assessment of fibrosis. These include measurements of serum biomarkers of fibrosis; and assessment of liver stiffness via transient elastography, acoustic radiation force impulse imaging, real-time elastography, or magnetic resonance elastography. Although significant advances have been made, most work to date has addressed the diagnostic utility of these techniques in the context of cirrhosis caused by chronic hepatitis C infection. In the present review, we examine the advantages afforded by use of non-invasive methods to diagnose cirrhosis in patients with CHB infections and the utility of such methods in clinical practice.

Magnetic resonance elastography for the detection and staging of liver fibrosis in chronic hepatitis B

OBJECTIVES

We measured the accuracy of magnetic resonance elastography (MRE) for the detection and staging of liver fibrosis in chronic hepatitis B (CHB) and compared it with serum fibrosis markers.

METHODS

Prospective comparison of MRE and routine serum fibrosis markers, namely serum alanine aminotransferase (ALT), serum aspartate aminotransferase (AST), ALT/AST ratio (AAR), AST to platelet ratio index (APRI) and prothrombin index (PI), was performed in 63 consecutive CHB patients who underwent MRE and histological confirmation of liver fibrosis within a 6-month interval. Diagnostic performance of MRE and serum markers for staging fibrosis (≥F1), significant fibrosis (≥F2), advanced fibrosis (≥F3) and cirrhosis (F4) was compared.

RESULTS

The study group comprised 63 patients (19 female; mean age ± SD, 50 ± 11.9 years). MRE (ρ = 0.94, P < 0.0001), APRI (ρ = 0.42, P = 0.0006), PI (ρ = 0.42, P = 0.0006) and AST (ρ = 0.28, P = 0.028) results correlated significantly with fibrosis stage. MRE was significantly more accurate than serum fibrosis markers for the detection of significant fibrosis (0.99 vs. 0.55-0.73) and cirrhosis (0.98 vs. 0.53-0.77). Sensitivity, specificity, positive predictive and negative predictive values for MRE for significant fibrosis and cirrhosis were 97.4 %, 100 %, 100 % and 96 %, and 100 %, 95.2 %, 91.3 % and 100 %, respectively.

CONCLUSION

MRE is an accurate non-invasive technique for the detection and staging of liver fibrosis in CHB.

KEY POINTS

• Magnetic resonance elastography is accurate for liver fibrosis detection and staging. • MR elastography is more accurate than serum tests for staging liver fibrosis. • MR elastography can potentially replace liver biopsy in chronic hepatitis B.

Sample interval modulation for the simultaneous acquisition of displacement vector data in magnetic resonance elastography: theory and application

SampLe Interval Modulation-magnetic resonance elastography (SLIM-MRE) is introduced for simultaneously encoding all three displacement projections of a monofrequency vibration into the MR signal phase. In SLIM-MRE, the individual displacement components are observed using different sample intervals. In doing so, the components are modulated with different apparent frequencies in the MR signal phase expressed as a harmonic function of the start time of the motion encoding gradients and can thus be decomposed by applying a Fourier transform to the sampled multidirectional MR phases. In this work, the theoretical foundations of SLIM-MRE are presented and the new idea is implemented using a high field (11.7 T) vertical bore magnetic resonance imaging system on an inhomogeneous agarose gel phantom sample. The local frequency estimation-derived stiffness values were the same within the error margins for both the new SLIM-MRE method and for conventional MRE, while the number of temporally-resolved MRE experiments needed for each study was reduced from three to one. In this work, we present for the first time, monofrequency displacement data along three sensitization directions that were acquired simultaneously and stored in the same k-space.

Association of splenic MR elastographic findings with gastroesophageal varices in patients with chronic liver disease

PURPOSE

To identify magnetic resonance imaging (MRI)-based parameters associated with gastroesophageal varices (GEVs) in patients with chronic liver disease.

MATERIALS AND METHODS

Ninety-three patients were divided into three groups based on endoscopic findings: group 1 with no GEVs (n = 49), group 2 with mild GEVs (n = 30), and group 3 with severe GEVs (n = 14). We used a multivariate logistic regression analysis to assess liver stiffness, aspartate aminotransferase-to-platelet ratio index, spleen stiffness and volume, portal vein velocity, cross-sectional area, and flow volumes potential independent associators of any (mild and severe) GEVs or severe GEVs.

RESULTS

The analysis showed that spleen and liver stiffness and spleen volume were independently associated with any GEVs (spleen stiffness, odds ratio [95% confidence interval], 1.25 [1.04-1.68], P = 0.018; liver stiffness, 1.52 [1.13-2.17], P = 0.006; spleen volume, 1.01 [1.00-1.01], P = 0.016), whereas spleen stiffness was associated with severe GEVs (1.82 [1.25-2.95]; P = 0.005).

CONCLUSION

Liver and spleen stiffness and spleen volume are associated with GEVs in patients with chronic liver disease. Compared with liver stiffness and spleen volume, spleen stiffness is more strongly associated with severe GEVs.

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.

Quantification of liver viscoelasticity with acoustic radiation force: a study of hepatic fibrosis in a rat model

Ultrasound elastography, based on shear wave propagation, enables the quantitative and non-invasive assessment of liver mechanical properties such as stiffness and has been found to be feasible for and useful in the diagnosis of hepatic fibrosis. Most ultrasound elastographic methods use a purely elastic model to describe liver mechanical properties. However, to describe tissue that is dispersive and to obtain an accurate measure of tissue elasticity, the viscoelasticity of the tissue should be examined. The objective of this study was to investigate the shear viscoelastic characteristics, as measured by ultrasound elastography, of liver fibrosis in a rat model and to evaluate the diagnostic accuracy of viscoelasticity for staging liver fibrosis. Liver fibrosis was induced in 37 rats using carbon tetrachloride (CCl4); 6 rats served as controls. Liver viscoelasticity was measured in vitro using shear waves induced by acoustic radiation force. The measured mean values of liver elasticity and viscosity ranged from 0.84 to 3.45 kPa and from 1.12 to 2.06 Pa·s for fibrosis stages F0-F4, respectively. Spearman correlation coefficients indicated that stage of fibrosis was well correlated with elasticity (0.88) and moderately correlated with viscosity (0.66). The areas under receiver operating characteristic curves were 0.97 (≥F2), 0.91 (≥F3) and 1.00 (F4) for elasticity and 0.91 (≥F2), 0.79 (≥F3) and 0.74 (F4) for viscosity, respectively. The results confirmed that shear wave velocity was dispersive in frequency, suggesting a viscoelastic model to describe liver fibrosis. The study finds that although viscosity is not as good as elasticity for staging fibrosis, it is important to consider viscosity to make an accurate estimation of elasticity; it may also provide other mechanical insights into liver tissues.

Correlation of MR elastography with morphometric quantification of liver fibrosis (Fibro-C-Index) in chronic hepatitis B

PURPOSE

We evaluated the correlation of MR Elastography (MRE) with morphometric assessment of liver fibrosis in chronic hepatitis B (CHB).

METHODS

Thirty-two patients with CHB underwent both MRE and a liver biopsy within a 6-month interval. MRE was performed using standard MRE sequence on a 1.5 Tesla clinical scanner. The liver stiffness (LS) was measured on automatically generated stiffness maps. Morphometric quantification of fibrosis of liver biopsies was performed using a semi-automated image analysis program and expressed as percentage area (Fibro-C-Index). Correlations between MRE, Fibro-C-Index, and histologic fibrosis stages were evaluated. Receiver operating curve (ROC) analysis of MRE and Fibro-C-index for differentiating fibrosis (≥F1), significant fibrosis (≥F2), advanced fibrosis (≥F3), and cirrhosis (F4) was performed.

RESULTS

MRE showed excellent correlation with both Fibro-C-Index (r = 0.78, 95% confidence interval [CI], 0.59-0.88, P < 0.001) and histologic staging (rho = 0.87, 95% CI, 0.72-0.94, P < 0.0001). Significant differences in MRE (P = 0.0001) and Fibro-C-Index (P = 0.003) among different stages of liver fibrosis was found. MRE and Fibro-C-Index had similar accuracies for differentiating fibrosis stages: ≥F1 (0.87 versus 0.81, P = 0.6), ≥F2 (0.95 versus 0.94, P = 0.78), ≥F3 (0.98 versus 0.96, P = 0.76), and F4 (1.00 versus 0.92, P = 0.10).

CONCLUSION

MRE is an excellent noninvasive indicator of liver fibrosis burden in CHB.

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.

Liver elastography, comments on EFSUMB elastography guidelines 2013

Recently the European Federation of Societies for Ultrasound in Medicine and Biology Guidelines and Recommendations have been published assessing the clinical use of ultrasound elastography. The document is intended to form a reference and to guide clinical users in a practical way. They give practical advice for the use and interpretation. Liver disease forms the largest section, reflecting published experience to date including evidence from meta-analyses with shear wave and strain elastography. In this review comments and illustrations on the guidelines are given.

Rapid acquisition of multifrequency, multislice and multidirectional MR elastography data with a fractionally encoded gradient echo sequence

In MR elastography (MRE), periodic tissue motion is phase encoded using motion-encoding gradients synchronized to an externally applied periodic mechanical excitation. Conventional methods result in extended scan time for quality phase images, thus limiting the broad application of MRE in the clinic. For practical scan times, researchers have been relying on one-dimensional or two-dimensional motion-encoding, low-phase sampling and a limited number of slices, and artifact-prone, single-shot, echo planar imaging (EPI) readout. Here, we introduce a rapid multislice pulse sequence capable of three-dimensional motion encoding that is also suitable for simultaneously encoding motion with multiple frequency components. This sequence is based on a gradient-recalled echo (GRE) sequence and exploits the principles of fractional encoding. This GRE MRE pulse sequence was validated as capable of acquiring full three-dimensional motion encoding of isotropic voxels in a large volume within less than a minute. This sequence is suitable for monofrequency and multifrequency MRE experiments. In homogeneous paraffin phantoms, the eXpresso sequence yielded similar storage modulus values as those obtained with conventional methods, although with markedly reduced variances (7.11 ± 0.26 kPa for GRE MRE versus 7.16 ± 1.33 kPa for the conventional spin-echo EPI sequence). The GRE MRE sequence obtained better phase-to-noise ratios than the equivalent spin-echo EPI sequence (matched for identical acquisition time) in both paraffin phantoms and in vivo data in the liver (59.62 ± 11.89 versus 27.86 ± 3.81, 61.49 ± 14.16 versus 24.78 ± 2.48 and 58.23 ± 10.39 versus 23.48 ± 2.91 in the X, Y and Z components, respectively, in the case of liver experiments). Phase-to-noise ratios were similar between GRE MRE used in monofrequency or multifrequency experiments (75.39 ± 14.93 versus 86.13 ± 18.25 at 28 Hz, 71.52 ± 24.74 versus 86.96 ± 30.53 at 56 Hz and 95.60 ± 36.96 versus 61.35 ± 26.25 at 84Hz, respectively).

Usefulness of MR elastography for predicting esophageal varices in cirrhotic patients

PURPOSE

To evaluate the usefulness of magnetic resonance elastography (MRE) as a noninvasive tool for predicting esophageal varices and identifying high-risk varices.

MATERIALS AND METHODS

In all, 126 patients with liver cirrhosis, and who underwent both MRI including MRE of the liver as well as upper gastrointestinal endoscopy for variceal screening within 1 month before or after the MRI, were included in this study. The relationship between the liver stiffness values measured by MRE and the degree of esophageal varices was assessed using Spearman's correlation analysis. In addition, the diagnostic performance of MRE for predicting the presence of varices or high-risk varices (grade≥II) was evaluated using the receiver-operating characteristics (ROC) curves.

RESULTS

The mean stiffness values of liver parenchyma measured on MRE were well correlated with the grade of esophageal varices (r=0.63). In addition, the MRE-based liver stiffness values were significantly lower in the lower-risk group than in the higher-risk group (P<0.0001). The area under the ROC curve values of MRE for predicting the presence of varices or high-risk varices (grade≥II) were 0.859 and 0.810, respectively. Using a liver stiffness cutoff value of 5.803 kPa, the sensitivity, specificity, positive predictive value, and negative predictive value for predicting high-grade (≥II) esophageal varices were 96%, 60%, 36%, and 98%, respectively.

CONCLUSION

The MRE-based liver stiffness value may be useful for noninvasively predicting esophageal varices and identifying high-risk varices in cirrhotic patients.

Assessment of in vivo laser ablation using MR elastography with an inertial driver

PURPOSE

To evaluate the feasibility of using MR Elastography (MRE) to monitor tissue coagulation extent during in vivo percutaneous laser ablation of the liver.

METHODS

A novel inertial acoustic driver was developed to apply mechanical waves via the ablation instrument. Ablation testing was performed in live juvenile female pigs under anesthesia in a 1.5-T whole-body MRI scanner.

RESULTS

The inertial driver produced suitable mechanical wave fields in the liver before, during, and after the laser ablation. During 2-min ablations using 4.5-, 7.5- and 15-W laser power, the stiffness of the lesions changed substantially in response to laser heating, indicative of protein denaturation. After a lethal thermal dose (2-min, 15-W) ablation, lesion stiffness was significantly greater than the baseline values (P < 0.007) and became stiffer over time; the mean stiffness increments from baseline were significantly greater than those after lower dose (2-min, 7.5-W) ablations (64.4% vs. 22.5%, P = 0.009).

CONCLUSION

MRE was shown capable of measuring tissue stiffness changes due to in vivo laser ablation. If confirmed through additional studies, this technology may be useful in clinical tumor ablation to monitor the spatial extent of tissue coagulation.

Magnetic resonance elastography for staging liver fibrosis in chronic hepatitis C

PURPOSE

We evaluated the use of magnetic resonance (MR) elastography (MRE) for staging liver fibrosis in patients with chronic hepatitis C and compared the ability of MRE and serum fibrosis markers for discriminating each stage of fibrosis.

METHODS

We evaluated 114 patients with chronic hepatitis C in whom the pathological fibrosis stage was determined (fibrosis stage 0 [F0], 3; F1, 15; F2, 28; F3, 25; and F4, 43). All patients underwent MRE using a 1.5-tesla MR system and pneumatic driver system. We measured stiffness values (kPa) of the liver in a circular region of interest placed on elastograms. We determined the optimal cutoff value and diagnostic ability for discriminating each stage of fibrosis using receiver operating characteristic (ROC) curve analysis and compared the discriminative ability of MRE with that of serum fibrosis markers.

RESULTS

The mean stiffness values of the liver increased with stage of fibrosis: F0, 2.10±0.10 kPa; F1, 2.42±0.29 kPa; F2, 3.16±0.32 kPa; F3, 4.21±0.78 kPa; and F4, 6.20±1.08 kPa. The mean area under the ROC curve (Az) values for discriminating liver fibrosis stages were: ≥F1, 0.984 (95% confidence interval, 0.933-0.996); ≥F2, 0.986 (0.956-0.996); ≥F3, 0.973 (0.935-0.989); and ≥F4, 0.976 (0.945-0.990). The Az values for discriminating fibrosis stages were significantly higher for MRE than serum fibrosis markers.

CONCLUSION

MRE is a reliable technique for staging liver fibrosis and discriminating liver fibrosis stages in patients with chronic hepatitis C.

Liver fibrosis: histopathologic and biochemical influences on diagnostic efficacy of hepatobiliary contrast-enhanced MR imaging in staging

PURPOSE

To evaluate the diagnostic performance of gadoxetic acid-enhanced magnetic resonance (MR) imaging in the staging of liver fibrosis in patients with diffuse chronic liver diseases (CLDs) and to investigate the factors that may influence the results.

MATERIALS AND METHODS

With the approval of the Hospital Ethics Committee and waiver of the informed consent requirement, data in 102 patients with histologically proven liver fibrosis (classified according to the METAVIR system) of various underlying causes were retrospectively analyzed. Patients underwent 3.0-T MR imaging with gadoxetic acid. The signal intensity of the liver was defined by using region of interest measurements before contrast material injection and in the hepatobiliary phase (20 minutes after contrast material administration), and relative enhancement was calculated. Univariate and multivariate regression analyses were applied to identify variables associated with relative enhancement measurements, and the performance of relative enhancement measurements in the staging of liver fibrosis was assessed by using area under the receiver operating characteristic curve (AUC) analysis.

RESULTS

At analysis of the relationship between enhancement measurements and histologic parameters, the relative enhancement values correlated strongly with liver fibrosis stage (r = -0.65, P < .0001) and moderately with necroinflammatory activity grades (r = -0.41, P = .002) and the presence of iron load (r = -0.21, P = .05). In multivariate analysis, only liver fibrosis stage independently influenced relative enhancement values (P < .001). The measurements performed well in the staging of liver fibrosis, with an AUC of 0.81 for stages of F1 or greater, 0.82 for stages of F2 or greater, 0.85 for stages of F3 or greater, and 0.83 for stage F4. Increased aspartate aminotransferase, gammaglutamyl transpeptidase, and alkaline phosphatase levels were independent predictors of false-negative results.

CONCLUSION

The presence of hepatic fibrosis can be assessed with good discrimination by using gadoxetic acid-enhanced MR imaging, but assessment can be confounded in the setting of abnormal aspartate aminotransferase, gammaglutamyl transpeptidase, and alkaline phosphatase levels.

Measurement of Tissue interstitial volume in healthy patients and those with amyloidosis with equilibrium contrast-enhanced MR imaging

PURPOSE

To investigate equilibrium contrast material-enhanced magnetic resonance (MR) imaging measurement of extracellular volume (ECV) fraction within healthy abdominal tissues and to test the hypotheses that tissue ECV in systemic amyloid light-chain (AL) amyloidosis is greater than in healthy patients and show that this increase correlates with organ amyloid burden.

MATERIALS AND METHODS

A local ethics committee approved the study and all patients gave written informed consent. Forty healthy volunteers (18 men, 22 women; median age, 43 years; age range, 24-88 years) and 67 patients with AL amyloidosis (43 men, 24 women; median age, 65 years; age range, 38-81 years) underwent equilibrium MR imaging of the upper abdomen. ECV was measured in the liver, spleen, and paravertebral muscle. Patients with amyloidosis also underwent serum amyloid P (SAP) component scintigraphy so that specific organ involvement by amyloid could be scored. Variation in ECV between tissues was assessed by using a Friedman Test. Tissue ECV in healthy and amyloidosis groups were compared by using a Mann-Whitney U test. Spearman correlation was used to test for an association between the organ SAP score and ECV.

RESULTS

ECV measured at equilibrium MR imaging varied significantly between organs in healthy volunteers (χ(2) = 31.0; P < .001). ECV was highest in the spleen (0.34), followed by liver (0.29) and muscle (0.09). ECVs measured within the spleen (0.39; P< .001), liver (0.31; P = .005), and muscle (0.16; P< .001) were significantly higher in patients with amyloidosis than in healthy control subjects. ECV measured in the liver and spleen showed increasing organ amyloid burden assessed at SAP scintigraphy (liver, rs = 0.54; spleen, rs = 0.57).

CONCLUSION

Equilibrium MR imaging can be used to define ECV within healthy tissues. ECV is increased in amyloidosis compared with healthy tissues, and this increase correlates with rising tissue amyloid burden.

MR-based measurements of portal vein flow and liver stiffness for predicting gastroesophageal varices

OBJECTIVES

We evaluated flow parameters measured by phase-contrast magnetic resonance (MR) imaging (PC-MRI) of the portal venous system and liver stiffness measured by MR elastography (MRE) to determine the usefulness of these methods in predicting gastroesophageal varices (GEV) in patients with chronic liver disease (CLD).

METHODS

In patients with CLD and controls, we performed PC-MRI on the portal (PV) and superior mesenteric veins; calculated mean velocity (V, cm/s), cross-sectional area (S, mm²), and flow volume (Q, mL/min); and determined markers of liver fibrosis (liver stiffness [kPa]) and aspartate aminotransferase (AST) platelet ratio index [APRI]). We visually assessed GEV and development of collateral pathways of the PV on routine contrast-enhanced dynamic MR imaging and compared patient characteristics, flow parameters, liver stiffness markers, and visual analysis among 3GEV groups, those with mild, severe, or no GEV with reference to endoscopic findings.

RESULTS

Child-Pugh grade, VPV, SPV, liver stiffness, APRI, and visually identified GEV (visible GEV) differed significantly among the 3 groups (P<0.05). We investigated VPV, SPV, liver stiffness, and visible GEV as independent markers to distinguish patients with and without GEV and examined VPV and visible GEV to predict severe GEV. Visible GEV showed low sensitivity (14 to 30%) and high specificity (98%) for predicting GEV in patients with CLD. A subgroup analysis that excluded cases with collateral pathway demonstrated slightly improved diagnostic performance of VPV and liver stiffness.

CONCLUSIONS

Portal vein flow parameters and liver stiffness can be useful markers for predicting GEV in patients with CLD.

Measurement consistency of MR elastography at 3.0 T: comparison among three different region-of-interest placement methods

PURPOSE

To test inter- and intraobserver consistency of liver stiffness measurement on MR elastography (MRE) at 3.0 T.

MATERIALS AND METHODS

Two abdominal radiologists independently measured stiffness of the liver on MRE in three volunteers and seven patients with chronic liver diseases using three different region-of-interest (ROI) placement methods. Methods 1 and 2 involved placing circular and free-hand-drawn ROIs, respectively, visually referring to anatomical (three-dimensional T1-weighted) and wave images. Method 3 involved placing ROIs on the fused images of MRE and anatomical images developed on a work station, visually referring to wave images. The inter- and intraobserver consistency was assessed with regression and Bland-Altman analysis.

RESULTS

Thirty-eight images were available for measurement in total. As for interobserver consistency, method 3 showed the best regression coefficient, correlation coefficient, and y intercept. The absolute values of the interobserver differences for method 3 were significantly smaller than those of method 1 or method 2 (p < 0.05, each). Intraobserver consistency of method 3 was excellent for both observers.

CONCLUSION

Stiffness measurement of the liver on MRE performed with the fusion method at 3.0 T provides the highest inter- and intraobserver consistency.

Magnetic resonance elastography of liver: technique, analysis, and clinical applications

Many pathological processes cause marked changes in the mechanical properties of tissue. MR elastography (MRE) is a noninvasive MRI based technique for quantitatively assessing the mechanical properties of tissues in vivo. MRE is performed by using a vibration source to generate low frequency mechanical waves in tissue, imaging the propagating waves using a phase contrast MRI technique, and then processing the wave information to generate quantitative images showing mechanical properties such as tissue stiffness. Since its first description in 1995, published studies have explored many potential clinical applications including brain, thyroid, lung, heart, breast, and skeletal muscle imaging. However, the best-documented application to emerge has been the use of MRE to assess liver disease. Multiple studies have demonstrated that there is a strong correlation between MRE-measured hepatic stiffness and the stage of fibrosis at histology. The emerging literature indicates that MRE can serve as a safer, less expensive, and potentially more accurate alternative to invasive liver biopsy which is currently the gold standard for diagnosis and staging of liver fibrosis. This review describes the basic principles, technique of performing a liver MRE, analysis and calculation of stiffness, clinical applications, limitations, and potential future applications.

Modern imaging evaluation of the liver: emerging MR imaging techniques and indications

Modern MR imaging evaluation of the liver allows for a comprehensive morphologic and functional assessment of the liver parenchyma, hepatic vessels, and biliary tree, thus aiding in the diagnosis of both focal and diffuse liver diseases.

Staging of hepatic fibrosis: comparison of magnetic resonance elastography and shear wave elastography in the same individuals

Objective

To cross-validate liver stiffness (LS) measured on shear wave elastography (SWE) and on magnetic resonance elastography (MRE) in the same individuals.

Materials and Methods

We included 94 liver transplantation (LT) recipients and 114 liver donors who underwent either MRE or SWE before surgery or biopsy. We determined the technical success rates and the incidence of unreliable LS measurements (LSM) of SWE and MRE. Among the 69 patients who underwent both MRE and SWE, the median and coefficient of variation (CV) of the LSM from each examination were compared and correlated. Areas under the receiver operating characteristic curve in both examinations were calculated in order to exclude the presence of hepatic fibrosis (HF).

Results

The technical success rates of MRE and SWE were 96.4% and 92.2%, respectively (p = 0.17), and all of the technical failures occurred in LT recipients. SWE showed 13.1% unreliable LSM, whereas MRE showed no such case (p < 0.05). There was moderate correlation in the LSM in both examinations (r = 0.67). SWE showed a significantly larger median LSM and CV than MRE. Both examinations showed similar diagnostic performance for excluding HF (Az; 0.989, 1.000, respectively).

Conclusion

MRE and SWE show moderate correlation in their LSMs, although SWE shows higher incidence of unreliable LSMs in cirrhotic liver.

Functional imaging of the liver

Anatomical-based imaging is used widely for the evaluation of diffuse and focal liver, including detection, characterization, and therapy response assessment. However, a limitation of anatomical-based imaging is that structural changes may occur relatively late in a disease process. By applying conventional anatomical-imaging methods in a more functional manner, specific pathophysiologic alterations of the liver may be assessed and quantified. There has been an increasing interest in both the clinical and research settings, with the expectation that functional-imaging techniques may help solve common diagnostic dilemmas that conventional imaging alone cannot. This review considers the most common functional magnetic resonance imaging, computed tomography, and ultrasound imaging techniques that may be applied to the liver.

Magnetic resonance elastography in the diagnosis of hepatic fibrosis

Liver fibrosis is a common feature of many chronic liver diseases, and can ultimately progress to cirrhosis. Conventional imaging is insensitive to liver fibrosis, necessitating a liver biopsy for diagnosis and monitoring of progression. However, liver biopsy risks complications, and is an imperfect gold standard in view of sampling error and intraobserver or interobserver variation. Magnetic resonance elastography (MRE) is a noninvasive method for assessing the mechanical properties of tissues and is gaining credence as a method of assessment for hepatic fibrosis. The aim of this review is to describe how MRE is performed, to review the present literature on the subject, to compare MRE with other noninvasive techniques used to assess for liver fibrosis, and to highlight areas of future research.

[Magnetic resonance elastography of the liver]

CLINICAL/METHODICAL ISSUE

The early detection of liver fibrosis remains a major challenge in medical imaging.

STANDARD RADIOLOGICAL METHODS

Nowadays staging of liver fibrosis is not a task for radiological examinations and the gold standard is liver puncture.

METHODICAL INNOVATIONS

Elastography is sensitive to the mechanical properties of soft tissues and in the liver stiffness is highly correlated to the degree of fibrosis. In magnetic resonance imaging elastography (MRE) time-harmonic vibrations are induced in the liver and encoded by motion-sensitive phase-contrast sequences. Viscoelastic constants are recovered from the obtained wave images and displayed by so-called elastograms.

PERFORMANCE

The MRE procedure is able to discriminate low grades of fibrosis (F0-F1) from medium and severe fibrosis (F2-F4) with a diagnostic accuracy (AUROC) of 0.92.

ACHIEVEMENTS

Currently, MRE is the most sensitive imaging modality for the noninvasive staging of liver fibrosis.

PRACTICAL RECOMMENDATIONS

Current technical developments of MRE may further improve the accuracy of the method towards a new gold standard for noninvasive staging of fibrosis by radiologists.

MR elastography of healthy liver parenchyma: Normal value and reliability of the liver stiffness value measurement

PURPOSE

To investigate the normal liver stiffness value using MR elastography (MRE) and to compare the repeatability and reproducibility of three measurement methods.

MATERIALS AND METHODS

Forty-nine subjects who underwent liver MR imaging including elastography using a 1.5 Tesla scanner, who had normal laboratory test results, and who underwent liver donation, were included in this study. Two, independent readers measured the liver stiffness value of the hepatic parenchyma using three methods, including evaluating various liver parenchyma portions using different-sized, regions-of-interest (ROIs): 2cm-ROI-per-slice in the right lobe (2cm-per-slice); 1cm-ROI-per-segment (1cm-S); and 70%-of-the-surface area (70% S). The mean liver stiffness values were compared between gender using the t-test and among the age groups using one-way analysis of variance. The reproducibility and repeatability of the three liver stiffness value measurement methods were determined using intraclass correlation coefficients (ICCs) and the 95% Bland-Altman limits of agreement.

RESULTS

The liver stiffness values in living liver donors ranged from 1.54 to 2.87 kPa. The mean stiffness value was 2.05 kilopascal using the 2cm-per-slice method, 2.01 kilopascal with the 1cm-S method and 2.12 kilopascal using the 70% S method. There was no significant difference in the liver stiffness value according to the gender or age factors. For reproducibility, the ICCs were 0.416 with the 2cm-per-slice method, 0.800 using the 1cm-S method, and 0.845 with the 70% S method. In terms of repeatability, the ICCs were 0.238 using the 2cm-per-slice method, 0.914 with the 1cm-S method, and 0.852 using the 70% S method. The ICCs determined using the 2cm-per-slice method were significantly lower than those of the 1cm-S or 70% S method for assessing both reproducibility and repeatability. The 95% limit of agreement was 54.0% with the 2cm-per-slice method, 24.0% using the 1cm-S method, and 18.8% with the 70% S method, in terms of reproducibility. To assess the repeatability, the 95% limit of agreement was 63.3% using the 2cm-per-slice method, 20.6% with the 1cm-S method, and 17.4% using the 70% S method.

CONCLUSION

The mean liver stiffness values in living donors ranged from 2.05 to 2.12 kilopascals and did not differ significantly for either gender or age. The 1cm-S and 70% S methods were significantly more reliable compared with the 2cm-per-slice method.

Cutoff values for alcoholic liver fibrosis using magnetic resonance elastography technique

BACKGROUND

Due to the lack of cutoff values validated for specific liver diseases, the purpose of this study was to set up specific magnetic resonance elastography (MRE) cutoff values for asymptomatic liver fibrosis in alcoholic patients.

METHODS

Ninety patients underwent 3 clinical exams. The liver stiffness was measured locally with the Fibroscan, and globally through cartographies of shear modulus generated with MRE. The Fibroscan method was chosen as the gold standard to classify the fibrosis. The liver score was also obtained with the Fibrometer A, and the diagnostic performance of the methods was analyzed with receiver-operating characteristic (ROC) curves and cutoff values were calculated.

RESULTS

Spearman correlation and area under the ROC curve revealed that MRE is a better diagnostic method than the Fibrometer A, to identify various levels of fibrosis. The results showed that the Fibrometer A was adapted for severe fibrosis. The MRE cutoff values are F1:2.20 kPa, F2:2.57 kPa, F3:3.31 kPa, and F4:4 kPa and were not influenced by the glutamic oxaloacetic transaminase level. By using the ultrasound cutoff values attributed for alcoholism, 66% of patients had a similar liver fibrosis diagnosis as the MRE cutoffs. However, both imaging techniques did not provide the same distribution for minor fibrosis.

CONCLUSIONS

None of the imaging techniques (Fibroscan, MRE) could replace the gold standard of the biopsy. However, due to the risk and the unnecessary procedure for the present recruited alcoholic patients, the Fibroscan method was chosen as the reference. Since MRE is currently being used as a clinical exam, the present MRE cutoffs could aid clinicians with their diagnosis of liver fibrosis for alcoholism disease.

Assessment of liver viscoelasticity by using shear waves induced by ultrasound radiation force

PURPOSE

To investigate the value of viscosity measured with ultrasonographic (US) elastography in liver fibrosis staging and to determine whether the use of a viscoelastic model to estimate liver elasticity can improve its accuracy in fibrosis staging.

MATERIALS AND METHODS

The study, which was performed from February 2010 to March 2011, was compliant with HIPAA and approved by the institutional review board. Written informed consent was obtained from each subject. Ten healthy volunteers (eight women and two men aged 27-55 years) and 35 patients with liver disease (17 women and 18 men aged 19-74 years) were studied by using US elasticity measurements of the liver (within 6 months of liver biopsy). US data were analyzed with the shear wave dispersion ultrasound vibrometry (SDUV) method, in which elasticity and viscosity are measured by evaluating dispersion of shear wave propagation speed, as well as with the time-to-peak (TTP) method, where tissue viscosity was neglected and only elasticity was estimated from the effective shear wave speed. The hepatic fibrosis stage was assessed histologically by using the METAVIR scoring system. The correlation of elasticity and viscosity was assessed with the Pearson correlation coefficient. The performances of SDUV and TTP were evaluated with receiver operating characteristic (ROC) curve analysis.

RESULTS

The authors found significant correlations between elasticity and viscosity measured with SDUV (r = 0.80) and elasticity measured with SDUV and TTP (r = 0.94). The area under the ROC curve for differentiating between grade F0-F1 fibrosis and grade F2-F4 fibrosis was 0.98 for elasticity measured with SDUV, 0.86 for viscosity measured with SDUV, and 0.95 for elasticity measured with TTP.

CONCLUSION

The results suggest that elasticity and viscosity measured between 95 Hz and 380 Hz by using SDUV are correlated and that elasticity measurements from SDUV and TTP showed substantially similar performance in liver fibrosis staging, although elasticity calculated from SDUV provided a better area under the ROC curve.

MR elastography of the head and neck: driver design and initial results

PURPOSE

The purpose was to describe the design and fabrication of a driver suitable for magnetic resonance elastography (MRE) of the head and neck and to assess its performance in evaluating human parotid gland, lymph nodes and thyroid at 3.0 T.

MATERIALS AND METHODS

A head and neck driver was fabricated using a commercial transducer, headrest mould and piston extension. Driver performance was tested using a motion-sensitized spin-echo MRE pulse sequence. Six healthy volunteers and three patients (two metastatic nodes and one papillary carcinoma) were evaluated using MRE. Viscoelastic maps were computed to obtain storage modulus (G') and loss modulus (G") of the normal parotid and thyroid, metastatic node and thyroid cancer. Reproducibility was assessed by coefficient of variation.

RESULTS

All subjects completed MRE examination without discomfort. Initial G' and G" values were as follows: normal parotid gland, 1.12 kPa and 0.48 kPa; thyroid, 0.58 kPa and 0.42 kPa; metastatic node, 0.66 kPa and 0.58 kPa; and thyroid cancer, 0.17 kPa and 0.28 kPa. Based on parotid data, the coefficient of variation for G' and G" was 4.7% and 9.8%.

CONCLUSION

A new MRE driver for head and neck was successfully implemented, and our initial results suggested the device was suitable for the mechanical assessment of tissues in the head and neck.

Magnetic resonance elastography reveals altered brain viscoelasticity in experimental autoimmune encephalomyelitis

Cerebral magnetic resonance elastography (MRE) measures the viscoelastic properties of brain tissues in vivo. It was recently shown that brain viscoelasticity is reduced in patients with multiple sclerosis (MS), highlighting the potential of cerebral MRE to detect tissue pathology during neuroinflammation. To further investigate the relationship between inflammation and brain viscoelasticity, we applied MRE to a mouse model of MS, experimental autoimmune encephalomyelitis (EAE). EAE was induced and monitored by MRE in a 7-tesla animal MRI scanner over 4 weeks. At the peak of the disease (day 14 after immunization), we detected a significant decrease in both the storage modulus (G′) and the loss modulus (G″), indicating that both the elasticity and the viscosity of the brain are reduced during acute inflammation. Interestingly, these parameters normalized at a later time point (day 28) corresponding to the clinical recovery phase. Consistent with this, we observed a clear correlation between viscoelastic tissue alteration and the magnitude of perivascular T cell infiltration at both day 14 and day 28. Hence, acute neuroinflammation is associated with reduced mechanical cohesion of brain tissues. Moreover, the reduction of brain viscoelasticity appears to be a reversible process, which is restored when inflammation resolves. For the first time, our study has demonstrated the applicability of cerebral MRE in EAE, and showed that this novel imaging technology is highly sensitive to early tissue alterations resulting from the inflammatory processes. Thus, MRE may serve to monitor early stages of perivascular immune infiltration during neuroinflammation.

New paradigm for management of hepatocellular carcinoma by imaging

Based on recent clinical practice guidelines, imaging is largely replacing pathology as the preferred diagnostic method for determination of hepatocellular carcinoma (HCC). A variety of imaging modalities, including ultrasound (US), computed tomography (CT), magnetic resonance imaging (MRI), nuclear medicine, and angiography, are currently used to examine patients with chronic liver disease and suspected HCC. Advancements in imaging techniques such as perfusion imaging, diffusion imaging, and elastography along with the development of new contrast media will further improve the ability to detect and characterize HCC. Early diagnosis of HCC is essential for prompt treatment, which may in turn improve prognosis. Considering the process of hepatocarcinogenesis, it is important to evaluate sequential changes via imaging which would help to differentiate HCC from premalignant or benign lesions. Recent innovations including multiphasic examinations, high-resolution imaging, and the increased functional capabilities available with contrast-enhanced US, multidetector row CT, and MRI have raised the standards for HCC diagnosis. Although hemodynamic features of nodules in the cirrhotic liver remain the main diagnostic criterion, newly developed cellspecific contrast agents have shown great possibilities for improved HCC diagnosis and may overcome the diagnostic dilemma associated with small or borderline hepatocellular lesions. In the 20th century paradigm of medical imaging, radiological diagnosis was based on morphological characteristics, but in the 21st century, a paradigm shift to include biomedical, physiological, functional, and genetic imaging is needed. A multidisciplinary team approach is necessary to foster an integrated approach to HCC imaging. By developing and combining new imaging modalities, all phases of HCC patient care, including screening, diagnosis, treatment, and therapy, can be dramatically improved.