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

MRI-guided biopsy to correlate tissue specimens with MR elastography stiffness readings in liver transplants

RATIONALE AND OBJECTIVES

Magnetic resonance elastography (MRE) can noninvasively measure the stiffness of liver tissue and display this information in anatomic maps. Magnetic resonance imaging (MRI) guidance has not previously been used to biopsy segments of heterogeneous stiffness identified on MRE. Dedicated study of MRE in post-liver transplant patients is also limited. In this study, the ability of real-time MRI to guide biopsies of segments of the liver with different MRE stiffness values in the same post-transplant patient was assessed.

MATERIALS AND METHODS

MRE was performed in 9 consecutive posttransplant patients with history of hepatitis C. Segments of highest and lower stiffness on MRE served as targets for subsequent real-time MRI-guided biopsy using T2-weighted imaging. The ability of MRI-guided biopsy to successfully obtain tissue specimens was assessed. The Wilcoxon signed-rank test was used to compare mean stiffness differences for highest and lower MRE stiffness segments, with α = 0.05.

RESULTS

MRI guidance allowed successful sampling of liver tissue for all (18/18) biopsies. There was a statistically significant difference in mean MRE stiffness values between highest (4.61 ± 1.99 kPa) and lower stiffness (3.03 ± 1.75 kPa) (P = .0039) segments biopsied in the 9 posttransplant patients.

CONCLUSION

Real-time MRI can guide biopsy in patients after liver transplantation based on MRE stiffness values. This study supports the use of MRI guidance to sample tissue based on functional information.

New acquisition techniques: fields of application

Conventional MR imaging of the liver has a central role in the assessment of liver diseases. Diffusion-weighted MR imaging, MR elastography, and time-resolved dynamic contrast-enhanced MR imaging improve the anatomical information provided by conventional MR imaging and add quantitative functional information in diffuse and focal liver diseases. Particularly, accurate detection and characterization of liver fibrosis are feasible with quantitative MR elastography, detection of liver tumors is increased with diffusion-weighted MR imaging and time-resolved dynamic contrast-enhanced MR imaging, characterization of tumors can be improved with quantitative diffusion-weighted MR imaging and MR elastography. These methods also have the potential to provide adequate biomarkers for assessing the response to treatment. Currently, the main limitations of quantitative MR imaging are related to reproducibility, standardization, and/or limited clinical data. It is important to improve and standardize the quantitative MR methods and validate their role in large multicenter trials.

Gd-EOB-DTPA-enhanced MR imaging: prediction of hepatic fibrosis stages using liver contrast enhancement index and liver-to-spleen volumetric ratio

PURPOSE

To develop and evaluate a quantitative parameter for staging hepatic fibrosis by contrast enhancement signal intensity and morphological measurements from gadoxetic acid (Gd-EOB-DTPA)-enhanced MR imaging.

MATERIALS AND METHODS

MR images were obtained in 93 patients; 75 patients had histopathologically proven hepatic fibrosis and 18 patients who had healthy livers were evaluated. The liver-to-muscle signal intensity ratio (SI(post) = SIliver/SImuscle), contrast enhancement index (CEI = SIpost/SIpre), and liver-to-spleen volumetric ratio (VR = Vliver/Vspleen) were evaluated for staging hepatic fibrosis.

RESULTS

VR was most strongly correlated with fibrosis stage (7.21; r = -0.83; P < 0.001). Sensitivity, specificity, and area under the ROC curve demonstrated by linear regression formula generated by VR and CEI in predicting fibrous scores were 100%, 73%, and 0.91, respectively, for the detection of hepatic fibrosis F1 or greater (≥ F1),100%, 87%, and 0.96 for ≥ F2, 74%, 98%, and 0.93 for ≥ F3 and 91%, 100%, and 0.97 for F4.

CONCLUSION

The liver-to-spleen volumetric ratio and contrast enhancement index were reliable biomarkers for the staging of hepatic fibrosis on Gd-EOB-DTPA-enhanced MR imaging.

Separation of advanced from mild hepatic fibrosis by quantification of the hepatobiliary uptake of Gd-EOB-DTPA

OBJECTIVES

To apply dynamic contrast-enhanced (DCE) MRI on patients presenting with elevated liver enzymes without clinical signs of hepatic decompensation in order to quantitatively compare the hepatocyte-specific uptake of Gd-EOB-DTPA with histopathological fibrosis stage.

METHODS

A total of 38 patients were prospectively examined using 1.5-T MRI. Data were acquired from regions of interest in the liver and spleen by using time series of single-breath-hold symmetrically sampled two-point Dixon 3D images (non-enhanced, arterial and venous portal phase; 3, 10, 20 and 30 min) following a bolus injection of Gd-EOB-DTPA (0.025 mmol/kg). The signal intensity (SI) values were reconstructed using a phase-sensitive technique and normalised using multiscale adaptive normalising averaging (MANA). Liver-to-spleen contrast ratios (LSC_N) and the contrast uptake rate (K (Hep)) were calculated. Liver biopsy was performed and classified according to the Batts and Ludwig system.

RESULTS

Area under the receiver-operating characteristic curve (AUROC) values of 0.71, 0.80 and 0.78, respectively, were found for K (Hep), LSC_N10 and LSC_N20 with regard to severe versus mild fibrosis. Significant group differences were found for K (Hep) (borderline), LSC_N10 and LSC_N20.

CONCLUSIONS

Liver fibrosis stage strongly influences the hepatocyte-specific uptake of Gd-EOB-DTPA. Potentially the normalisation technique and K (Hep) will reduce patient and system bias, yielding a robust approach to non-invasive liver function determination.

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.

New noninvasive ultrasound techniques: can they predict liver cirrhosis?

OBJECTIVE

This study aimed to determine the threshold values and accuracy of 2 noninvasive techniques, contrast-enhanced ultrasound with maximum-intensity projection (MIP) imaging and ultrasound elastography, acoustic radiation force impulse (ARFI), to differentiate a normal from a cirrhotic liver.

MATERIALS AND METHODS

One hundred thirty-two patients were predicted clinically as having a normal liver (n = 60) or cirrhosis (n = 72). All had MIP liver vessel evaluation on an Acuson Sequoia and 90 of them had ARFI on an S2000 (Siemens, Mountain View, Calif). Two readers reviewed 4 parameters on MIP data and predicted cirrhosis (n = 65) or normal (n = 67) outcome. They were considered as having cirrhosis when more than 1 MIP parameter was positive. Acoustic radiation force impulse values above 1.36 m/s suggested cirrhosis. Receiver operating characteristic curve analysis was performed to evaluate the diagnostic accuracy of the ARFI measurement and to extract the optimal cutoff value in the differentiation of a cirrhotic liver from a noncirrhotic liver.

RESULTS

Sixty-four patients (64/65, 98.5%) with abnormal and 8 (8/59, 13.6%) with normal vessels on MIP imaging were clinically cirrhosis (P < 0.001). Forty-five patients (45/49, 91.8%) with abnormal ARFI (≥ 1.36 m/s) and 6 (6/41, 14.6%) with normal ARFI (<1.36 m/s) had a clinical cirrhotic liver (P < 0.001). Sensitivity and specificity of MIP of vessel morphology to predict cirrhosis were 89% (64/72) and 98% (59/60), respectively. Acoustic radiation force impulse showed a sensitivity of 88% (45/51) and a specificity of 90% (35/39), and the area under the receiver operating characteristic curve was 0.932, suggesting very good accuracy. The combination of ARFI and MIP showed increased sensitivity and specificity to 95.8% and 100%.

CONCLUSIONS

Liver evaluation with MIP and ARFI contributes to a noninvasive prediction of cirrhosis.

Performance of magnetic resonance elastography and diffusion-weighted imaging for the staging of hepatic fibrosis: A meta-analysis

A meta-analysis was performed to assess and compare the accuracies of magnetic resonance elastography (MRE) and diffusion-weighted imaging (DWI) for the staging of hepatic fibrosis. Online journal databases and a manual search from January 2000 to May 2011 were used. We identified 41 studies, but only 14 met the criteria to perform a meta-analysis assessing MRE (five trials) or DWI (10 trials). Fibrosis was categorized by redistribution into five stages according to histopathological description. A bivariate binomial model was used to combine the sensitivity and specificity and their 95% confidence intervals (CIs), from which diagnostic odds ratio (DOR), positive likelihood ratio (PLR), negative likelihood ratio (NLR), and summary receiver operating characteristic (sROC) were derived to indicate the diagnostic accuracy of imaging modalities. With MRE, the sensitivity, specificity, DOR, PLR, NLR, and area under sROC curve (with 95% CIs) for staging F0 ∼ F1 versus F2 ∼ F4 and F0 ∼ F2 versus F3 ∼ F4 were 0.94 (0.81-0.98), 0.95 (0.87-0.98), 20 (7-57), 0.06 (0.02-0.22), 317 (55-1,796), 0.98 (0.97-0.99) and 0.92 (0.85-0.96), 0.96 (0.91-0.98), 21 (10-45), 0.08 (0.04-0.16), 251 (103-609), and 0.98 (0.96-0.99), respectively; and with DWI, these values were 0.77 (0.71-0.82), 0.78 (0.69-0.85), 3 (2-5), 0.30 (0.22-0.40), 12 (6-21), 0.83 (0.79-0.86) and 0.72 (0.60-0.81), 0.84 (0.77-0.89), 5 (3-7), 0.34 (0.23-0.50), 13 (6-29), and 0.86 (0.83-0.89), respectively. A z test demonstrated that MRE had a significantly higher accuracy than DWI in those indicators (P < 0.05).

CONCLUSION

MRE is more reliable for staging hepatic fibrosis, compared with DWI, with a high combination of sensitivity, specificity, likelihood ratios, DOR, and area under sROC curve.

Accuracy of MR elastography and anatomic MR imaging features in the diagnosis of severe hepatic fibrosis and cirrhosis

PURPOSE

To compare the diagnostic accuracy of magnetic resonance imaging elastography (MRE) and anatomic MRI features in the diagnosis of severe hepatic fibrosis and cirrhosis.

MATERIALS AND METHODS

Three readers independently assessed presence of morphological changes associated with hepatic fibrosis in 72 patients with liver biopsy including: caudate to right lobe ratios, nodularity, portal venous hypertension (PVH) stigmata, posterior hepatic notch, expanded gallbladder fossa, and right hepatic vein caliber. Three readers measured shear stiffness values using quantitative shear stiffness maps (elastograms). Sensitivity, specificity, and diagnostic accuracy of stiffness values and each morphological feature were calculated. Interreader agreement was summarized using weighted kappa statistics. Intraclass correlation coefficient was used to assess interreader reproducibility of stiffness measurements. Binary logistic regression was used to assess interreader variability for dichotomized stiffness values and each morphological feature.

RESULTS

Using 5.9 kPa as a cutoff for differentiating F3-F4 from F0-2 stages, overall sensitivity, specificity, and diagnostic accuracy for MRE were 85.4%, 88.4%, and 87%, respectively. Overall interreader agreement for stiffness values was substantial, with an insignificant difference (P = 0.74) in the frequency of differentiating F3-4 from F0-2 fibrosis. Only hepatic nodularity and PVH stigmata showed moderately high overall accuracy of 69.4% and 72.2%. Interreader agreement was substantial only for PVH stigmata, moderate for C/R m, deep notch, and expanded gallbladder fossa. Only posterior hepatic notch (P = 0.82) showed no significant difference in reader rating.

CONCLUSION

MRE is a noninvasive, accurate, and reproducible technique compared with conventional features of detecting severe hepatic fibrosis.

Pediatric MR elastography of hepatic fibrosis: principles, technique and early clinical experience

Numerous pediatric conditions result in hepatic fibrosis. As treatments develop for the underlying disorders, a non-invasive assessment of liver fibrosis would be beneficial as an adjunct or possible replacement for the traditional gold standard, liver biopsy. Magnetic resonance elastography is a noninvasive imaging technique that has been used successfully in adults for identification and assessment of liver fibrosis. This review describes the basic principles of MR elastography as well as the technical aspects specific to children. Clinical pediatric applications, limitations and areas for future research are described.

[New developments in MRI of the liver]

Radiology has gained an exceptional position in medicine because a correct diagnosis is the most crucial issue in determining an accurate and personalized therapeutic strategy. This has a direct influence not only on the individual patient but also on the socio-economic aspects of healthcare services in terms of shortening the time interval to establish a diagnosis and to avoid risk-associated invasive diagnostic methods or long-term, cost-intensive follow-up. Magnetic resonance imaging (MRI) is an excellent example of this which due to continuous technological developments and emerging techniques allows a non-invasive diagnosis of the different hepatic diseases. In this article, we illustrate the direct correlation between the recent technical advances in MRI, such as 3.0 T, diffusion-weighted imaging, perfusion imaging, spectroscopy, texture analysis and MR elastography and obtaining a confident non-invasive diagnosis of focal and diffuse liver diseases.

Loss tangent and complex modulus estimated by acoustic radiation force creep and shear wave dispersion

Elasticity imaging methods have been used to study tissue mechanical properties and have demonstrated that tissue elasticity changes with disease state. In current shear wave elasticity imaging methods typically only shear wave speed is measured and rheological models, e.g. Kelvin-Voigt, Maxwell and Standard Linear Solid, are used to solve for tissue mechanical properties such as the shear viscoelastic complex modulus. This paper presents a method to quantify viscoelastic material properties in a model-independent way by estimating the complex shear elastic modulus over a wide frequency range using time-dependent creep response induced by acoustic radiation force. This radiation force induced creep method uses a conversion formula that is the analytic solution of a constitutive equation. The proposed method in combination with shearwave dispersion ultrasound vibrometry is used to measure the complex modulus so that knowledge of the applied radiation force magnitude is not necessary. The conversion formula is shown to be sensitive to sampling frequency and the first reliable measure in time according to numerical simulations using the Kelvin-Voigt model creep strain and compliance. Representative model-free shear complex moduli from homogeneous tissue mimicking phantoms and one excised swine kidney were obtained. This work proposes a novel model-free ultrasound-based elasticity method that does not require a rheological model with associated fitting requirements.

Transient elastography for predicting clinical outcomes in patients with chronic liver disease

There is increasing interest in developing noninvasive means to evaluate liver fibrosis in patients with chronic liver disease to determine disease severity, prognosis and optimal treatment. Transient elastography (TE) has previously been demonstrated to predict the presence or absence of advanced fibrosis. The current study was conducted to determine whether TE can identify patients with chronic liver disease at risk of clinical decompensation. A total of 667 patients underwent TE and were followed for a median of 861 days and 57 patients achieved the primary outcome, a composite of clinical endpoints including death, ascites, encephalopathy, increased Child Score ≥ 2, variceal bleed, hepatocellular carcinoma or listing for transplant. Overall, TE had an area under the receiver operating characteristic curve of 0.87 for predicting clinical outcome. Using a cut-off of 10.5 kPa, TE has a sensitivity, specificity, positive predictive value and negative predictive value (NPV) of 94.7%, 63.0%, 19.3% and 99.2%, respectively. A predictive model for clinical events was developed using generalized cross-validation for clinical endpoints considering TE, liver biopsy results and multiple other predictors. Individually, TE performed better than biopsy, or any other variable, for predicting clinical outcome [Harrell's C Statistic 0.86 for TE, 0.78 for stage]. Patients with a TE score of >12.5 kPa were found to have a relative hazard for clinical event of 18.99 compared with patients with TE score <10.5. A combined variable model including TE, aspartate aminotransferase/alanine aminotransferase ratio and model for end-stage liver disease (MELD) yielded the highest predictive accuracy with Harrell's C value of 0.93. In the subset of patients with cirrhosis, TE was not found to be independently associated with clinical outcomes in univariate or multivariate analysis although it retained a high sensitivity and NPV of 97.5% and 92.3%, respectively, at a kPa cut-off of 10.5. TE can successfully identify patients with chronic liver disease who are at low risk of clinical decompensation over a time period of 2 years.

A Review of Shearwave Dispersion Ultrasound Vibrometry (SDUV) and its Applications

Measurement of tissue elasticity has emerged as an important advance in medical imaging and tissue characterization. However, soft tissue is inherently a viscoelastic material. One way to characterize the viscoelastic material properties of a material is to measure shear wave propagation velocities within the material at different frequencies and use the dispersion of the velocities, or variation with frequency, to solve for the material properties. Shearwave Dispersion Ultrasound Vibrometry (SDUV) is an ultrasound-based technique that uses this feature to characterize the viscoelastic nature of soft tissue. This method has been used to measure the shear elasticity and viscosity in various types of soft tissues including skeletal muscle, cardiac muscle, liver, kidney, prostate, and arterial vessels. This versatile technique provides measurements of viscoelastic material properties with high spatial and temporal resolution, which can be used for assessing these properties in normal and pathologic tissues. The goals of this paper are to 1) give an overview of viscoelasticity and shear wave velocity dispersion, 2) provide a history of the development of the SDUV method, and 3) survey applications for SDUV that have been previously reported.

Diffusion-weighted magnetic resonance imaging for diagnosis of liver fibrosis and inflammation in chronic viral hepatitis: the performance of low or high B values and small or large regions of interest

OBJECTIVE

To investigate the performance of different b values and regions of interest (ROI) for diagnosing liver fibrosis in patients with chronic viral hepatitis by using diffusion-weighted (DW) magnetic resonance imaging (MRI).

METHODS

Eleven healthy participants and 33 patients with viral hepatitis B or C were enrolled. The stage of liver fibrosis and the grade of necroinflammation were determined by using a histologic activity index. Single-shot spin-echo echo-planar DW-MRI was performed in all participants at b values of 0-500, 0-700, and 0-1000 s/mm(2) by using 2 circular small and large ROIs of 100 and 200 mm(2). To evaluate the performance of different b values for determining cirrhosis, the receiver-operating characteristic curves were depicted, and the areas under the curves were compared.

RESULTS

The average values of apparent diffusion coefficients significantly decreased with increasing stage or grade categories at all the 3 b values and for both small and large ROIs. The performance at b = 500 s/mm(2) was significantly better than b = 1000 s/mm(2) for determining cirrhosis or bridging fibrosis. The cut point of 153.4 for apparent diffusion coefficient (×10(-5) mm(2)/s) at b = 500 s/mm(2) could determine cirrhosis or bridging fibrosis with a sensitivity of 96% and specificity of 82%. No difference was found between the average apparent diffusion coefficient values of large or small ROIs. Also, there was no difference in performance of large or small ROIs in the diagnosis of liver fibrosis.

CONCLUSIONS

This study provided beneficial data for clinical utilisation of DW-MRI in diagnosing liver fibrosis: b = 500 s/mm(2) is better in performance than b = 1000 s/mm(2), and a small ROI of 100 mm(2) is sufficient for determining cirrhosis or bridging fibrosis.

Validation of the FIB4 index in a Japanese nonalcoholic fatty liver disease population

BACKGROUND

A reliable and inexpensive noninvasive marker of hepatic fibrosis is required in patients with nonalcoholic fatty liver disease (NAFLD). FIB4 index (based on age, aspartate aminotransferase [AST] and alanine aminotransferase [ALT] levels, and platelet counts) is expected to be useful for evaluating hepatic fibrosis. We validated the performance of FIB4 index in a Japanese cohort with NAFLD.

METHODS

The areas under the receiver operating characteristic curves (AUROC) for FIB4 and six other markers were compared, based on data from 576 biopsy-proven NAFLD patients. Advanced fibrosis was defined as stage 3-4 fibrosis. FIB4 index was assessed as: age (yr) × AST (IU/L)/(platelet count (10(9)/L) × √ALT (IU/L)) RESULTS: Advanced fibrosis was found in 64 (11%) patients. The AUROC for FIB4 index was superior to those for the other scoring systems for differentiating between advanced and mild fibrosis. Only 6 of 308 patients with a FIB4 index below the proposed low cut-off point (< 1.45) were under-staged, giving a high negative predictive value of 98%. Twenty-eight of 59 patients with a FIB4 index above the high cut-off point (> 3.25) were over-staged, giving a low positive predictive value of 53%. Using these cutoffs, 91% of the 395 patients with FIB-4 values outside 1.45-3.25 would be correctly classified. Implementation of the FIB4 index in the Japanese population would avoid 58% of liver biopsies.

CONCLUSION

The FIB4 index was superior to other tested noninvasive markers of fibrosis in Japanese patients with NAFLD, with a high negative predictive value for excluding advanced fibrosis. The small number of cases of advanced fibrosis in this cohort meant that this study had limited power for validating the high cut-off point.

Magnetic Resonance Elastography

Often compared to the practice of manual palpation, magnetic resonance elastography is an emerging technology for quantitatively assessing the mechanical properties of tissue as a basis for characterizing disease. The potential of MRE as a diagnostic tool is rooted in the fact that normal and diseased tissues often differ significantly in terms of their intrinsic mechanical properties. MRE uses magnetic resonance imaging (MRI) in conjunction with the application of mechanical shear waves to probe tissue mechanics. This process can be broken down into three essential steps: inducing shear waves in the tissue,imaging the propagating shear waves with MRI, andanalyzing the wave data to generate quantitative images of tissue stiffness MRE has emerged as a safe, reliable and noninvasive method for staging hepatic liver fibrosis, and is now used in some locations as an alternative to biopsy. MRE is also being used in the ongoing investigations of numerous other organs and tissues, including, for example, the spleen, kidney, pancreas, brain, heart, breast, skeletal muscle, prostate, vasculature, lung, spinal cord, eye, bone, and cartilage. In the article that follows, some fundamental techniques and applications of MRE are summarized.

Comparison of transient elastography and acoustic radiation force impulse for non-invasive staging of liver fibrosis in patients with chronic hepatitis C

OBJECTIVES

Transient elastography (TE) is adequate for a diagnosis of cirrhosis, but its accuracy for milder stages of fibrosis is much less satisfactory. The objective of this study was to compare the performance and the discordance rate of acoustic radiation force impulse (ARFI) and TE with liver biopsy in a cohort of chronic hepatitis C (CHC) patients.

METHODS

One hundred thirty-nine consecutive patients with CHC were enrolled in two tertiary centers, and evaluated for histological (Metavir score) and biochemical features. All patients underwent TE and ARFI.

RESULTS

TE was unreliable in nine patients (6.5%), while in no cases (0%) were ARFI invalid measurements recorded (P=0.029). By area under receiver operating characteristic curve (AUROC), the best cutoff values for TE and ARFI for significant fibrosis (≥F2) were ≥6.5 kPa (AUROC: 0.78) and ≥1.3 m/s (AUROC: 0.86), respectively. For severe fibrosis (F3-F4), these cutoff values were 8.8 kPa (AUROC: 0.83) for TE and 1.7 m/s (AUROC: 0.94) for ARFI. For cirrhosis, TE had its best cutoff at ≥11 kPa (AUROC: 0.80) and ARFI at ≥2.0 m/s (AUROC: 0.89). By pairwise comparison of AUROC, ARFI was significantly more accurate than TE for a diagnosis of significant and severe fibrosis (P=0.024 and P=0.002, respectively), while this difference was only marginal for cirrhosis (P=0.09). By partial AUROC analysis, ARFI performance results significantly higher for all three stages of fibrosis. The average concordance rates of TE and ARFI vs. liver biopsy were 45.4 and 54.7%, respectively. By multivariate analysis, ARFI was not associated with alanine aminotransferase (ALT), body mass index, Metavir grade, and liver steatosis, while TE was significantly correlated with the ALT value (P=0.027).

CONCLUSIONS

In a cohort of patients with CHC, ARFI imaging was more accurate than TE for the non-invasive staging of both significant and severe classes of liver fibrosis.

Cross-validation of MR elastography and ultrasound transient elastography in liver stiffness measurement: discrepancy in the results of cirrhotic liver

PURPOSE

To evaluate individual differences in liver stiffness measurement using both MR elastography (MRE) and ultrasound transient elastography (UTE) in patients with chronic liver disease.

MATERIALS AND METHODS

This study included 80 patients with chronic liver disease who underwent both UTE and MRE. MRE and UTE were performed using a pneumatic driver (60 Hz) and an ultrasound probe with a vibrator (50 Hz), respectively. Liver stiffness data measured using the two techniques (μ(UTE) and μ(MRE) ) were compared with respect to shear modulus. The patients were subdivided into four quartiles on the basis of average of the μ(UTE) and μ(MRE) values for each patient.

RESULTS

The analysis of the 4 quartile groups revealed that μ(UTE) was significantly higher than μ(MRE) in the two most stiff liver groups: μ(UTE) versus μ(MRE) , 7.5 (1.2) versus 6.0 (0.72) kPa for the group with [μ(UTE) + μ(MRE) ]/2 of 5.6-8.0 kPa; 15.1(4.2) versus 6.7 (1.4) kPa for the group with >8.0 kPa. However, in the least stiff liver group (i.e., the group with [μ(UTE) + μ(MRE) ]/2 < 3.2 kPa), μ(UTE) was significantly lower than μ(MRE) .

CONCLUSION

The shear modulus measured by UTE and MRE are not equivalent, especially in patients with stiff livers.

Acoustic radiation force impulse elastography of the liver: can fat deposition in the liver affect the measurement of liver stiffness?

PURPOSE

The aim of this study was to compare acoustic radiation force impulse (ARFI) results between livers with and without fat deposition.

MATERIALS AND METHODS

We studied 200 consecutive healthy individuals who underwent health checkups at our institution. The subjects were divided into three groups according to the echogenicity of the liver on ultrasonography (US) and the liver-spleen attenuation ratio index (LSR) on computed tomography: normal liver group (n = 121, no evidence of bright liver on US and LSR > 1); fatty liver group (n = 46, bright liver on US and LSR < 1); others (n = 33, inconclusive results). Subjects in the inconclusive group and those who consumed alcohol >5 days a week (n = 18) were excluded from the analysis. The velocities measured by ARFI in the normal and fatty liver groups were compared using the two one-sided test.

RESULTS

The mean (SD) velocity measured in the normal and fatty liver groups were 1.03 (0.12) m/s and 1.02 (0.12) m/s, respectively. The ARFI results of the fatty liver group were similar to those of the normal liver group (P < 0.0001).

CONCLUSION

This study suggested that fat deposition in the liver does not affect the liver stiffness measurement determined by ARFI.

MR elastography for noninvasive assessment of hepatic fibrosis: experience from a tertiary center in Asia

PURPOSE

To determine the sensitivity and specificity of MR elastography (MRE) in the staging of hepatic fibrosis (HF) using histopathology as the reference standard in an Asian population.

MATERIALS AND METHODS

MRE was performed on 55 patients with chronic liver diseases or biliary diseases and on 5 living related liver donors (48 men and 12 women; mean age, 55.7 years). MRE was performed with modified, phase-contrast, gradient-echo sequences, and the mean stiffness values were measured on the elastograms in kilopascals(kPa). Receiver operating characteristic curve analysis was performed to determine the cutoff value and accuracy of MRE for staging HF. Histopathologic staging of HF according to the METAVIR scoring system served as the reference.

RESULTS

Liver stiffness increased systematically along with the fibrosis stage. With a shear stiffness cutoff value of 3.05 kPa, the predicted sensitivity and specificity for differentiating significant liver fibrosis (≥ F2) from mild fibrosis (F1) were 89.7% and 87.1%, respectively. In addition, MRE was able to discriminate between patients with severe fibrosis (F3) and those with liver cirrhosis (sensitivity, 100%; specificity, 92.2%), with a shear stiffness cutoff value of 5.32 kPa.

CONCLUSION

MRE could be a promising, noninvasive technique with excellent diagnostic accuracy for detecting significant HF and liver cirrhosis.

Effects of gadoxetic acid on liver elasticity measurement by using magnetic resonance elastography

The purpose of this study was to evaluate the effect of gadoxetic acid (Gd-EOB-DTPA) on measurements of liver stiffness by using magnetic resonance elastography (MRE). In this study, 104 consecutive patients (mean age, 67.7±9.4 years) underwent MRE using a 1.5-T MR scanner equipped with a cylindrical passive driver that was placed across the right chest wall for delivering vibrations. Axial gradient-echo images, which were automatically converted to elastograms that represented stiffness (kPa), were acquired using a continuous sinusoidal vibration of 60 Hz. Two raters independently placed a region of interest on the right lobe of the liver on the elastograms obtained before and after Gd-EOB-DTPA was administered. Liver stiffness was measured using these two elastograms and compared using a paired t test and correlation analysis. No significant difference was observed in liver stiffness before and after Gd-EOB-DTPA was administered (Rater 1, P=.1200; Rater 2, P=.3585). The correlation coefficients were 0.986 (Rater 1) and 0.984 (Rater 2), indicating excellent correlation between the stiffness values before and after Gd-EOB-DTPA was administered. Liver stiffness measured by MRE did not differ before and after Gd-EOB-DTPA was administered.

Is it better to use two elastographic methods for liver fibrosis assessment?

AIM: To find out if by combining 2 ultrasound based elastographic methods: acoustic radiation force impulse (ARFI) elastography and transient elastography (TE), we can improve the prediction of fibrosis in patients with chronic hepatitis C.

METHODS: Our study included 197 patients with chronic hepatitis C. In each patient, we performed, in the same session, liver stiffness (LS) measurements by means of TE and ARFI, respectively, and liver biopsy (LB), assessed according to the Metavir score. 10 LS measurements were performed both by TE and ARFI; median values were calculated and expressed in kilopascals (kPa) and meters/second (m/s), respectively. Only TE and ARFI measurements with IQR < 30% and SR ≥ 60% were considered reliable.

RESULTS: On LB 13 (6.6%) patients had F0, 32 (16.2%) had F1, 52 (26.4%) had F2, 47 (23.9%) had F3, and 53 (26.9%) had F4. A direct, strong correlation was found between TE measurements and fibrosis (r = 0.741), between ARFI and fibrosis (r = 0.730) and also between TE and ARFI (r = 0.675). For predicting significant fibrosis (F ≥ 2), for a cut-off of 6.7 kPa, TE had 77.5% sensitivity (Se) and 86.5% specificity (Sp) [area under the receiver operating characteristic curve (AUROC) 0.87] and for a cut-off of 1.2 m/s, ARFI had 76.9% Se and 86.7% Sp (AUROC 0.84). For predicting cirrhosis (F = 4), for a cut-off of 12.2 kPa, TE had 96.2% Se and 89.6% Sp (AUROC 0.97) and for a cut-off of 1.8 m/s, ARFI had 90.4% Se and 85.6% Sp (AUROC 0.91). When both elastographic methods were taken into consideration, for predicting significant fibrosis (F ≥ 2), (TE ≥ 6.7 kPa and ARFI ≥ 1.2 m/s) we obtained 60.5% Se, 93.3% Sp, 96.8% positive predictive value (PPV), 41.4% negative predictive value (NPV) and 68% accuracy, while for predicting cirrhosis (TE ≥ 12.2 kPa and ARFI ≥ 1.8 m/s) we obtained 84.9% Se, 94.4% Sp, 84.9% PPV, 94.4% NPV and 91.8% accuracy.

CONCLUSION: TE used in combination with ARFI is highly specific for predicting significant fibrosis; therefore when the two methods are concordant, liver biopsy can be avoided.

Noninvasive in vivo liver fibrosis evaluation using supersonic shear imaging: a clinical study on 113 hepatitis C virus patients

Supersonic shear imaging (SSI) has recently been demonstrated to be a repeatable and reproducible transient bidimensional elastography technique. We report a prospective clinical evaluation of the performances of SSI for liver fibrosis evaluation in 113 patients with hepatitis C virus (HCV) and a comparison with FibroScan (FS). Liver elasticity values using SSI and FS ranged from 4.50 kPa to 33.96 kPa and from 2.60 kPa to 46.50 kPa, respectively. Analysis of variance (ANOVA) shows a good agreement between fibrosis staging and elasticity assessment using SSI and FS (p < 10(-5)). The areas under receiver operating characteristic (ROC) curves for elasticity values assessed from SSI were 0.948, 0.962 and 0.968 for patients with predicted fibrosis levels F ≥ 2, F ≥ 3 and F = 4, respectively. These values are compared with FS area under the receiver operating characteristic curve (AUROC) of 0.846, 0.857 and 0.940, respectively. This comparison between ROC curves is particularly significant for mild and intermediate fibrosis levels. SSI appears to be a fast, simple and reliable method for noninvasive liver fibrosis evaluation.

Segmental difference of the hepatic fibrosis from chronic viral hepatitis due to hepatitis B versus C virus infection: comparison using dual contrast material-enhanced MRI

OBJECTIVE

We wanted to identify the geographic differences in hepatic fibrosis and their associations with the atrophy-hypertrophy complex in patients with chronic viral hepatitis using the dual-contrast material-enhanced MRI (DC-MRI) with gadopentetate dimeglumine and ferucarbotran.

MATERIALS AND METHODS

Patients with chronic C (n = 22) and B-viral hepatitis (n = 35) were enrolled for determining the subjective grade of fibrosis (the extent and thickness of fibrotic reticulations) in the right lobe (RL), the caudate lobe (CL), the medial segment (MS) and the lateral segment (LS) of the liver, with using a 5-grade scale, on the gradient echo T2(*)-weighted images of DC-MRI. The fibrosis grades of different segments were compared using the Kruskal-Wallis test followed by post-hoc analysis to establish the segment-by-segment differences. The incidences of two pre-established morphologic signs of cirrhosis were also compared with each other between the two groups of patients.

RESULTS

There were significant intersegmental differences in fibrosis grades of the C-viral group (p = 0.005), and the CL showed lower fibrosis grades as compared with the grades of the RL and MS, whereas all lobes were similarly affected in the B-viral group (p = 0.221). The presence of a right posterior hepatic notch was significantly higher in the patients with intersegmental differences of fibrosis between the RL and the CL (19 out of 25, 76%) than those without such differences (6 out of 32, 19%) (p < 0.001). An expanded gallbladder fossa showed no significant relationship (p = 0.327) with the segmental difference of the fibrosis grades between the LS and the MS.

CONCLUSION

The relative lack of fibrosis in the CL with more advanced fibrosis in the RL can be a distinguishing feature to differentiate chronic C-viral hepatitis from chronic B-viral hepatitis and this is closely related to the presence of a right posterior hepatic notch.

Calculation of shear stiffness in noise dominated magnetic resonance elastography data based on principal frequency estimation

Magnetic resonance elastography (MRE) is a non-invasive phase-contrast-based method for quantifying the shear stiffness of biological tissues. Synchronous application of a shear wave source and motion encoding gradient waveforms within the MRE pulse sequence enable visualization of the propagating shear wave throughout the medium under investigation. Encoded shear wave-induced displacements are then processed to calculate the local shear stiffness of each voxel. An important consideration in local shear stiffness estimates is that the algorithms employed typically calculate shear stiffness using relatively high signal-to-noise ratio (SNR) MRE images and have difficulties at an extremely low SNR. A new method of estimating shear stiffness based on the principal spatial frequency of the shear wave displacement map is presented. Finite element simulations were performed to assess the relative insensitivity of this approach to decreases in SNR. Additionally, ex vivo experiments were conducted on normal rat lungs to assess the robustness of this approach in low SNR biological tissue. Simulation and experimental results indicate that calculation of shear stiffness by the principal frequency method is less sensitive to extremely low SNR than previously reported MRE inversion methods but at the expense of loss of spatial information within the region of interest from which the principal frequency estimate is derived.

Staging liver fibrosis by using liver-enhancement ratio of gadoxetic acid-enhanced MR imaging: comparison with aspartate aminotransferase-to-platelet ratio index

OBJECTIVE

To compare the diagnostic ability of gadoxetic acid-enhanced hepatocyte-phase MR images with aspartate aminotransferase-to-platelet ratio index (APRI) to predict liver fibrosis stage.

MATERIALS AND METHODS

Our study included 100 patients who underwent gadoxetic acid-enhanced MRI and either liver biopsy or liver surgery. Liver fibrosis stage was histologically determined according to the METAVIR system: F0 (n=16), F1 (n=17), F2 (n=10), F3 (n=21) and F4 (n=36). Four measures were used as imaging-based fibrosis markers: liver-spleen contrast ratio, liver-enhancement ratio, corrected liver-enhancement ratio and spleen index. APRI represented a blood test-based fibrosis marker. The diagnostic ability of those fibrosis markers were compared through receiver-operating characteristic analysis.

RESULTS

The area under the curve (AUC) for APRI prediction of severe fibrosis (≥F3 and F4) was significantly greater than that of corrected liver-enhancement ratio. However, corrected liver-enhancement ratio had a greater AUC for prediction of mild fibrosis (≥F1) than APRI, although the difference was insignificant.

CONCLUSION

Corrected liver-enhancement ratio with gadoxetic acid-enhanced MRI is correlated to the stage of liver fibrosis. APRI, however, has greater reliability for predicting severe fibrosis and cirrhosis than does the imaging-based fibrosis marker tested in this study.

Using static preload with magnetic resonance elastography to estimate large strain viscoelastic properties of bovine liver

Traditional magnetic resonance elastography (MRE) applies small amplitude vibration to tissues. Thus currently MRE measures only the small deformation behaviour of tissues. MRE has the potential to estimate the strain-varying shear modulus of soft tissues, if applied at different static strains, which may allow prediction of the large-strain behaviour of tissues. This study uses MRE of bovine liver specimens under various levels of static compressive pre-strain up to 30%. Storage and loss moduli measured using MRE increased non-linearly with static compressive pre-strain, and exponential models fit well to these data to describe this relationship (R(2)>0.93). Based on these models, a 10% linear compression of liver would result in a 47% overestimate of the 'true' storage modulus of the uncompressed tissue. The results of this study have implications for MRE transducer design and interpretation of results from in vivo MRE studies.

Subzone based magnetic resonance elastography using a Rayleigh damped material model

PURPOSE

Recently, the attenuating behavior of soft tissue has been addressed in magnetic resonance elastography by the inclusion of a damping mechanism in the methods used to reconstruct the resulting mechanical property image. To date, this mechanism has been based on a viscoelastic model for material behavior. Rayleigh, or proportional, damping provides a more generalized model for elastic energy attenuation that uses two parameters to characterize contributions proportional to elastic and inertial forces. In the case of time-harmonic vibration, these two parameters lead to both the elastic modulus and the density being complex valued (as opposed to the case of pure viscoelasticity, where only the elastic modulus is complex valued).

METHODS

This article presents a description of Rayleigh damping in the time-harmonic case, discussing the differences between this model and the viscoelastic damping models. In addition, the results from a subzone based Rayleigh damped elastography study of gelatin and tofu phantoms are discussed, along with preliminary results from in vivo breast data.

RESULTS

Both the phantom and the tissue studies presented here indicate a change in the Rayleigh damping structure, described as Rayleigh composition, between different material types, with tofu and healthy tissue showing lower Rayleigh composition values than gelatin or cancerous tissue.

CONCLUSIONS

It is possible that Rayleigh damping elastography and the concomitant Rayleigh composition images provide a mechanism for differentiating tissue structure in addition to measuring elastic stiffness and attenuation. Such information could be valuable in the use of Rayleigh damped magnetic resonance elastography as a diagnostic imaging tool.

[Imaging and renal failure: from inflammation to fibrosis]

Multiple chronic renal diseases evolve to end-stage kidney disease due to progressive renal tissue fibrosis at the level of the interstitium or glomeruli. Fibrosis often results from transformation of the extracellular matrix by cytokines and chemokines released by activated cells in the setting of recurrent episodes of acute inflammation. Newer techniques to image intrarenal inflammation and fibrosis are mandatory for the non-invasive evaluation of these processes to improve follow-up and monitoring of drug therapy. These techniques are based on methods of cellular and molecular imaging, and methods of functional, such as diffusion weighted imaging, and structural, such as elastography.

Assessment of chronic hepatitis and fibrosis: comparison of MR elastography and diffusion-weighted imaging

OBJECTIVE

The purpose of our study was to compare the utility of MR elastography (MRE) and diffusion-weighted imaging (DWI) in characterizing fibrosis and chronic hepatitis in patients with chronic liver diseases.

SUBJECTS AND METHODS

Seventy-six patients with chronic liver disease underwent abdominal MRI, MRE, and DWI. Severities of liver fibrosis and chronic hepatitis were graded by histopathologic analysis according to standard disease-specific classifications. The overall predictive ability of MRE and DWI in assessment of fibrosis was compared by constructing a receiver operating characteristic (ROC) curve and calculating the area under the curve (AUC) on the basis of histopathologic analysis.

RESULTS

Using ROC analysis, MRE showed greater capability than DWI in discriminating stage 2 or greater (≥ F2), stage 3 or greater (≥ F3), and cirrhosis (≥ F4), shown as significant differences in AUC (p = 0.003, p = 0.001, and p = 0.001, respectively). Higher sensitivity and specificity were shown by MRE in predicting fibrosis scores ≥ F2 (91% and 97%), scores ≥ F3 (92% and 95%), and scores F4 (95% and 87%) compared with DWI (84% and 82%, 88% and 76%, and 85% and 68%, respectively). Although MRE had higher ability in identification of liver with fibrosis scores ≥ F1 than DWI, a significant difference was not seen (p = 0.398). Stiffness values on MRE increased in relation to increasing severity of fibrosis confirmed by histopathology scores; however, a consistent relationship between apparent diffusion coefficient (ADC) values and stage of fibrosis was not shown. In addition, liver tissue with chronic hepatitis preceding fibrosis may account for mild elevation of liver stiffness.

CONCLUSION

MRE had greater predictive ability in distinguishing the stages of liver fibrosis than DWI.

Functional magnetic resonance imaging of the liver: parametric assessments beyond morphology

There is growing interest in exploring and using functional imaging techniques to provide additional information on structural alterations in the liver, which often occur late in the disease process. This article presents a summary of the different functional MR imaging techniques currently in use, focusing on dynamic contrast-enhanced MR imaging, diffusion-weighted MR imaging, MR spectroscopy, in- and oppose-phase MR imaging, and T2*-weighted imaging. For each technique, the biologic underpinning for the technique is explained, the clinical applications surveyed, and the challenges for their application enumerated. Developing and less frequently used techniques such as MR elastography, blood oxygenation level dependent imaging, dynamic susceptibility contrast-enhanced MR imaging, and diffusion-tensor imaging are reviewed. The challenges widespread adoption of functional MR imaging and the translation of such techniques to high field strengths are also discussed.

Wide-range dynamic magnetic resonance elastography

Tissue mechanical parameters have been shown to be highly sensitive to disease by elastography. Magnetic resonance elastography (MRE) in the human body relies on the low-dynamic range of tissue mechanics <100 Hz. In contrast, MRE suited for investigations of mice or small tissue samples requires vibration frequencies 10-20 times higher than those used in human MRE. The dispersion of the complex shear modulus (G(⁎)) prevents direct comparison of elastography data at different frequency bands and, consequently, frequency-independent viscoelastic models that fit to G(*) over a wide dynamic range have to be employed. This study presents data of G(*) of samples of agarose gel, liver, brain, and muscle measured by high-resolution MRE in a 7T-animal scanner at 200-800 Hz vibration frequency. Material constants μ and α according to the springpot model and related to shear elasticity and slope of the G(*)-dispersion were determined. Both μ and α of calf brain and bovine liver were found to be similar, while a sample of fibrotic human liver (METAVIR score of 3) displayed about fifteen times higher shear elasticity, similar to μ of bovine muscle measured in muscle fiber direction. α was the highest in fibrotic liver, followed by normal brain and liver, while muscle had the lowest α-values of all biological samples investigated in this study. As expected, the least G(*)-dispersion was seen in soft gel. The proposed technique of wide-range dynamic MRE can provide baseline data for both human MRE and high-dynamic MRE for better understanding tissue mechanics of different tissue structures.

Recent advances in imaging hepatic fibrosis and steatosis

Liver disease is an increasing cause of morbidity and mortality worldwide. Currently, the gold standard for diagnosis and assessment of parenchymal disease is histopathological assessment of a percutaneous or transjugular liver biopsy. The risks and limitations of this technique are well recognized and as a result, significant effort has gone into the development of novel noninvasive methods of diagnosis and longitudinal assessment. Imaging techniques have improved significantly over the past decade and new technologies are beginning to enter clinical practice. Ultrasound, computed tomography and MRI are the main modalities currently used, but novel MRI-based techniques will have an increasing role. While there has been extensive research into the imaging of focal liver disease, the evidence base for imaging in diffuse disease has also undergone recent rapid development, particularly in the assessment of fibrosis and steatosis. Both of these abnormalities of the parenchyma can lead to cirrhosis and/or hepatocellular carcinoma and represent an important opportunity for detection of early liver disease. We discuss the recent advances in liver imaging techniques and their role in the diagnosis and monitoring of diffuse liver disease, with a focus on their current and potential clinical relevance and whether they may replace or augment liver biopsy. We also discuss techniques currently under development and their potential clinical applications in the future.

Non-invasive assessment of liver fibrosis in chronic hepatitis C

Quantification of hepatic fibrosis is of critical importance in chronic hepatitis C not only for prognosis, but also for antiviral treatment indication. Two end points are clinically relevant: detection of significant fibrosis (indication for antiviral treatment) and detection of cirrhosis (screening for eosphageal varices and hepatocellular carcinoma). Until recently, liver biopsy was considered the reference method for the evaluation of liver fibrosis. Limitations of liver biopsy (invasiveness, sampling error, and inter-observer variability) have led to the development of non-invasive methods. Currently available methods rely on two different approaches: a "biological" approach based on the dosage of serum fibrosis biomarkers; and a "physical" approach based on the measurement of liver stiffness, using transient elastography (TE). This review is aimed at discussing the advantages and limits of non-invasive methods and liver biopsy and the perspectives for their rational use in clinical practice in the management of patients with chronic hepatitis C.

Staging hepatic fibrosis: comparison of gadoxetate disodium-enhanced and diffusion-weighted MR imaging--preliminary observations

PURPOSE

To evaluate the utility of hepatocyte-phase gadoxetate disodium-enhanced magnetic resonance (MR) imaging in staging hepatic fibrosis and to compare it with diffusion-weighted imaging.

MATERIALS AND METHODS

This retrospective study had institutional review board approval, and the requirement for informed consent was waived. Gadoxetate disodium-enhanced and diffusion-weighted MR images obtained in 114 consecutive patients (70 men, 44 women; age range, 37-91 years) were evaluated. Liver-to-muscle signal intensity (SI) ratio on hepatocyte-phase images (SI(post)), contrast enhancement index calculated as SI(post) /SI(pre), where SI(pre) is liver-to-muscle SI ratio on nonenhanced images, and apparent diffusion coefficient (ADC) of the liver were measured. Necroinflammatory activity grades and hepatic fibrosis stages were histopathologically determined in 99 patients. Multiple regressions of SI(post), contrast enhancement index, ADC, serum albumin concentration, serum total bilirubin level, prothrombin time, and Child-Pugh score were examined to determine correlation with hepatic necroinflammatory activity grades and fibrosis stages.

RESULTS

Among the MR, hematologic, and clinical parameters, contrast enhancement index was most strongly correlated with fibrosis stage (r = -0.79, P < .001). Multiple regression analysis showed that the contrast enhancement index, ADC, and prothrombin time were significantly correlated (r(2) = 0.66, P < .05) with fibrosis stage and that the contrast enhancement index and serum total bilirubin level were weakly correlated (r(2) = 0.24, P < .05) with the necroinflammatory activity grade.

CONCLUSION

Gadoxetate disodium-enhanced MR imaging is more reliable for staging hepatic fibrosis than are diffusion-weighted MR imaging, hematologic, and clinical parameters.

Effects of frequency- and direction-dependent elastic materials on linearly elastic MRE image reconstructions

The mechanical model commonly used in magnetic resonance elastography (MRE) is linear elasticity. However, soft tissue may exhibit frequency- and direction-dependent (FDD) shear moduli in response to an induced excitation causing a purely linear elastic model to provide an inaccurate image reconstruction of its mechanical properties. The goal of this study was to characterize the effects of reconstructing FDD data using a linear elastic inversion (LEI) algorithm. Linear and FDD phantoms were manufactured and LEI images were obtained from time-harmonic MRE acquisitions with variations in frequency and driving signal amplitude. LEI responses to artificially imposed uniform phase shifts in the displacement data from both purely linear elastic and FDD phantoms were also evaluated. Of the variety of FDD phantoms considered, LEI appeared to tolerate viscoelastic data-model mismatch better than deviations caused by poroelastic and anisotropic mechanical properties in terms of visual image contrast. However, the estimated shear modulus values were substantially incorrect relative to independent mechanical measurements even in the successful viscoelastic cases and the variations in mean values with changes in experimental conditions associated with uniform phase shifts, driving signal frequency and amplitude were unpredictable. Overall, use of LEI to reconstruct data acquired in phantoms with FDD material properties provided biased results under the best conditions and significant artifacts in the worst cases. These findings suggest that the success with which LEI is applied to MRE data in tissue will depend on the underlying mechanical characteristics of the tissues and/or organs systems of clinical interest.

Magnetic resonance elastography of the liver: preliminary results and estimation of inter-rater reliability

PURPOSE

The aim of this study was to estimate inter-rater reliability and validate magnetic resonance elastography (MRE) as a tool to measure liver elasticity.

MATERIALS AND METHODS

The study protocol was approved by the institutional review board at our institution. In all, 10 normal volunteers and 110 patients, who provided written informed consent, were enrolled. The pathological fibrosis score was applied as a standard reference of liver fibrosis in 21 patients. MRE was performed with a 1.5-T magnetic resonance imaging scanner with a cylindrical passive longitudinal shear wave driver placed over the right chest wall to deliver vibrations. A gradient-echo MRE sequence was used to acquire axial wave images, which were automatically converted to elastograms representing elasticity (in kilopascals, or kPa). The region of interest was placed in the right lobe of the liver on elastograms by two raters independently. To evaluate interrater reliability, the intraclass correlation coefficient was calculated. The elasticity measurements correlated with the pathological fibrosis score (F1-F4) in 21 patients.

RESULTS

The intraclass correlation coefficient was almost perfect (0.993) between the elasticities measured by the two raters. The mean elasticity value for patients with F4 was 5.7 kPa; F3, 4.4 kPa; F2, 3.1 kPa; F1, 2.2 kPa; and F0, 2.1 kPa.

CONCLUSION

MRE is a reliable tool to measure liver elasticity.

Dispersion and shear modulus measurements of porcine liver

A narrow-band ultrasonic shear-wave imaging technique for estimating phase speed was applied to fresh and thermally damaged porcine liver in vitro. Two constitutive models were applied to the measurements to represent rheological behavior of the tissue and estimate the complex shear modulus at frequencies between 50 and 300 Hz. Our results were compared to similar values from the literature to assess how well models represent liver measurements over a range of shear-wave frequencies, experimental conditions and mammalian species. We found remarkable consistency in some parameters but not in others, suggesting that the Kelvin-Voigt model commonly applied in elasticity-imaging situations is representative of tissue dispersion but the description it offers is incomplete. Data are consistent with the theory that viscoelastic contrast is more likely due to changes in protein and other biomolecular-scale structures than from tissue anatomy larger than a cell. Dispersion measurements at frequencies between 0.5-1000 kHz are needed to more completely evaluate models for the viscoelastic behavior liver.

Quantification of superparamagnetic iron oxide-mediated signal intensity change in patients with liver cirrhosis using T2 and T2* mapping: a preliminary report

PURPOSE

To quantify the accumulation of superparamagnetic iron oxide (SPIO) in patients with and without liver cirrhosis using T2 and T2* mapping.

MATERIALS AND METHODS

We enrolled 10 patients without liver cirrhosis and 30 patients with liver cirrhosis (Child-Pugh class A, n = 18; and B/C, n = 12). T2 and T2* mapping were performed before and after SPIO administration. The reductions in T2 (DeltaT2) and T2* (DeltaT2*) after SPIO administration were compared between the control and liver cirrhosis groups and the control, Child-Pugh A, and Child Pugh B/C groups.

RESULTS

There were significant differences in DeltaT2 (22.2 +/- 4.4 and 15.1 +/- 7.0) and DeltaT2* values (24.3 +/- 3.3 and 16.8 +/- 8.1) (P = 0.005 and P < 0.001) between the control and the liver cirrhosis groups. There was a significant difference in the DeltaT2* between the Child-Pugh A and Child-Pugh B/C groups (P < 0.001) and in the DeltaT2 and DeltaT2* between the control and Child-Pugh B/C groups (P = 0.018 and P < 0.001).

CONCLUSION

DeltaT2 and DeltaT2* are significantly larger in patients without liver cirrhosis than those with liver cirrhosis. DeltaT2* is also significantly larger in Child-Pugh class A patients than those in Child-Pugh B/C.

MR elastography: Spleen stiffness measurements in healthy volunteers--preliminary experience

OBJECTIVE

The purpose of this article is to establish the range of normal splenic stiffness in healthy volunteers using MR elastography (MRE) and to investigate any correlation with physiologic parameters and driver position.

SUBJECTS AND METHODS

Sixteen volunteers (mean [+/- SD] age, 37 +/- 9 years) with no history of gastrointestinal, hepatobiliary, or cardiovascular disease were recruited. The MRI protocol included T2-weighted axial and gradient-echo MRE sequences using steady-state 60-Hz excitation. Two MRE acquisitions were performed, one with the driver placed on the right side of the abdomen and the other with the driver placed on the left side. Volunteers' body mass index (BMI), arterial mean blood pressure, age, spleen volume, and liver stiffness were also determined. Two radiologists independently measured the spleen stiffness on the MRE inversion images. The correlations between spleen stiffness and BMI, arterial mean blood pressure, age, spleen volume, and liver stiffness were quantified.

RESULTS

Sixteen volunteers underwent MRE. With the driver placed on the right side of the abdomen, the mean splenic stiffness was 3,565 +/- 586 Pa (range, 2,353-4,442 Pa); with the driver on the left side of the abdomen, the mean splenic stiffness was significantly (p < 0.004) different (4,255 +/- 625 Pa; range, 3,194-5,581 Pa). No significant correlation was found between spleen stiffness and BMI, arterial mean blood pressure, age, spleen volume, and liver stiffness (all p > 0.05)

CONCLUSION

These preliminary results in a small number of healthy volunteers show that spleen stiffness is not significantly correlated with BMI, arterial mean blood pressure, spleen volume, or liver stiffness. A significant difference was observed using different driver positions.

Liver fibrosis in chronic hepatitis C virus infection: differentiating minimal from intermediate fibrosis with perfusion CT

PURPOSE

To prospectively assess the utility of perfusion computed tomography (CT) for differentiating minimal from intermediate fibrosis in treatment-naïve patients with chronic hepatitis C virus (HCV) infection.

MATERIALS AND METHODS

This study was approved by the Institutional Review Board, and informed consent was obtained. Fifty-two patients with treatment-naïve HCV infection underwent perfusion CT and percutaneous liver biopsy on the same day. Portal vein, arterial, and total liver perfusion; mean transit time; and distribution volumes for the right and left liver lobes were measured. Liver samples were scored for fibrosis, and fibrosis area was determined. Differences in quantitative perfusion parameters between patients with minimal fibrosis (score of F1) and those with intermediate fibrosis (score of F2 or F3) were tested.

RESULTS

In patients with intermediate fibrosis (F2 and F3) compared with those with minimal fibrosis (F1), the portal venous perfusion (87 mL min(-1) 100 mL(-1) +/- 27 [standard deviation] vs 138 mL min(-1) 100 mL(-1) +/- 112, P = .042) and total liver perfusion (107 mL min(-1) 100 mL(-1) +/- 31 vs 169 mL min(-1) 100 mL(-1) +/- 137, P = .02) were significantly decreased, and the mean transit time was significantly increased (16 seconds +/- 4 vs 13 seconds +/- 5, P = .025). At multivariate analysis, only the mean transit time was an independent factor (odds ratio, 1.18; 95% confidence interval: 1.02, 1.37; P = .030). Receiver operating characteristic curve analysis showed that a mean transit time threshold of 13.4 seconds allowed discrimination between minimal and intermediate fibrosis with a sensitivity of 71% and a specificity of 65%.

CONCLUSION

The results of this study show that perfusion changes occur early during fibrosis in chronic HCV infection and can be detected with perfusion CT. Perfusion CT may help to discriminate minimal from intermediate fibrosis. Mean transit time appears to be the most promising perfusion parameter for differentiating between fibrosis stages, although the large amount of overlap in the measured parameters limits the clinical utility of this test at present.

Magnetic resonance elastography: a review

Magnetic resonance elastography (MRE) is a rapidly developing technology for quantitatively assessing the mechanical properties of tissue. The technology can be considered to be an imaging-based counterpart to palpation, commonly used by physicians to diagnose and characterize diseases. The success of palpation as a diagnostic method is based on the fact that the mechanical properties of tissues are often dramatically affected by the presence of disease processes, such as cancer, inflammation, and fibrosis. MRE obtains information about the stiffness of tissue by assessing the propagation of mechanical waves through the tissue with a special magnetic resonance imaging technique. The technique essentially involves three steps: (1) generating shear waves in the tissue, (2) acquiring MR images depicting the propagation of the induced shear waves, and (3) processing the images of the shear waves to generate quantitative maps of tissue stiffness, called elastograms. MRE is already being used clinically for the assessment of patients with chronic liver diseases and is emerging as a safe, reliable, and noninvasive alternative to liver biopsy for staging hepatic fibrosis. MRE is also being investigated for application to pathologies of other organs including the brain, breast, blood vessels, heart, kidneys, lungs, and skeletal muscle. The purpose of this review article is to introduce this technology to clinical anatomists and to summarize some of the current clinical applications that are being pursued.

Analysis of intrahepatic vascular morphological changes of chronic liver disease for assessment of liver fibrosis stages by micro-flow imaging with contrast-enhanced ultrasound: preliminary experience

OBJECTIVE

To assess morphological vascular changes due to an increase in liver fibrosis by using micro-flow imaging (MFI) of contrast-enhanced ultrasound.

METHODS

MFI was performed in 47 patients who underwent liver biopsy, and in 10 normal cases. For 27/57 cases, we performed MFI twice in order to assess the reproducibility of the examination, thus yielding a total of 84 examinations. Seven physicians interpreted each case individually by assigning confidence levels for the presence or absence of three imaging features that were related to alteration of portal vein morphology: angle widening, tapering/interruption and tortuosity.

RESULTS

Pearson's correlation coefficient between the average rating scores based on tortuosity and the histological fibrosis stage was 0.806 (p < 0.001). The diagnostic accuracy of the average area under the ROC curve, which was estimated by use of the confidence levels of tapering/interruption, tortuosity and angle widening, was 0.964 for F1 vs. F2-4, 0.968 for F1-2 vs. F3-4 and 0.910 for F1-3 vs. F4. The average correlation coefficient between the ratings on different images from the same patients was 0.838.

CONCLUSION

Assessment of morphological intrahepatic vascular changes on MFI may be useful for grading liver fibrosis.

Repeatability of magnetic resonance elastography for quantification of hepatic stiffness

PURPOSE

To determine the sources of variability of MRE hepatic stiffness measurements using healthy volunteers and patients and to calculate the minimum change required for statistical significance. Hepatic stiffness measured with magnetic resonance elastography (MRE) has demonstrated tremendous potential as a noninvasive surrogate of hepatic fibrosis, although the underlying repeatability of MRE for longitudinal tracking of liver disease has not been documented.

MATERIALS AND METHODS

MRE stiffness measurements from 20 healthy volunteers and 10 patients were obtained twice on the same day, and repeated 2-4 weeks later for volunteers in this institutional review board-approved study. A linear mixed effects model was used to estimate the component sources of variability in the data.

RESULTS

The standard deviation of MRE measurements of the same individual on different days is 11.9% (percent of the measured stiffness) using the same reader and 12.0% using different readers. The standard deviation of the difference between two measurements (i.e., longitudinal change in an individual) is 17.4%; the corresponding 95% confidence interval for zero change is (-27.0%, 37.0%).

CONCLUSION

MRE is a repeatable method for quantifying liver stiffness. Using the described MRE technique, changes greater than 37.0% of the smaller measured stiffness value represent meaningful changes in longitudinal liver stiffness measurements.

MR elastography of the liver at 3 T with cine-tagging and bending energy analysis: preliminary results

OBJECTIVE

To preliminarily evaluate the feasibility and usefulness of MR elastography of the liver at 3 T with cine-tagging and bending energy (BE) analysis for the evaluation of hepatic fibrosis.

MATERIALS AND METHODS

Twenty-two patients underwent MR elastography with four different cine-tagging grids on the liver (16- or 20-mm sagittal or coronal). Nine images serially obtained during 1-s of exhalation were analyzed to define coordinates of grid intersections. BE values were calculated using the thin-plate spline method. BE values were compared among patient groups with different fibrosis stage thresholds.

RESULTS

In the 22 patients, six had a fibrosis score of F0, one had F1, seven had F2, three had F3, and five had F4. Mean BE value with 16-mm sagittal grid was greater with fibrosis score F0 (1.54 ± 0.63) than with ≥F1 (0.97 ± 0.12, P = 0.013) as well as with ≤F1 (1.48 ± 0.60) than with ≥F2 (0.96 ± 0.36, P = 0.019).

CONCLUSION

Our results showed that MR elastography with 16-mm sagittal grid and BE analysis had a potential in discrimination for the patients with moderate or advanced hepatic fibrosis from those with healthy liver or slight fibrosis.

Magnetic resonance elastography: a general overview of its current and future applications in brain imaging

Magnetic resonance elastography (MRE) has been developed over the last few years as a non-invasive means of evaluating the elasticity of biological tissues. The presence of the skull has always prevented semeiotic palpation of the brain, but MRE now offers the possibility of "palpating by imaging" in order to detect brain consistency under physiological and pathological conditions. The aim of this article is to review the current state-of-the-art of MRE imaging and discuss its possible future diagnostic applications in neuroscience.