MR elastography of the liver: preliminary results

In vivo visualization of abdominal malignancies with acoustic radiation force elastography

The utility of acoustic radiation force impulse (ARFI) imaging for real-time visualization of abdominal malignancies was investigated. Nine patients presenting with suspicious masses in the liver (n = 7) or kidney (n = 2) underwent combined sonography/ARFI imaging. Images were acquired of a total of 12 tumors in the nine patients. In all cases, boundary definition in ARFI images was improved or equivalent to boundary definition in B-mode images. Displacement contrast in ARFI images was superior to echo contrast in B-mode images for each tumor. The mean contrast for suspected hepatocellular carcinomas (HCCs) in B-mode images was 2.9 dB (range: 1.5-4.2) versus 7.5 dB (range: 3.1-11.9) in ARFI images, with all HCCs appearing more compliant than regional cirrhotic liver parenchyma. The mean contrast for metastases in B-mode images was 3.1 dB (range: 1.2-5.2) versus 9.3 dB (range: 5.7-13.9) in ARFI images, with all masses appearing less compliant than regional non-cirrhotic liver parenchyma. ARFI image contrast (10.4 dB) was superior to B-mode contrast (0.9 dB) for a renal mass. To our knowledge, we present the first in vivo images of abdominal malignancies in humans acquired with the ARFI method or any other technique of imaging tissue elasticity.

Increased stiffness of the rat liver precedes matrix deposition: implications for fibrosis

Liver fibrosis, the response to chronic liver injury, results from the activation of mesenchymal cells to fibrogenic myofibroblasts. We have recently shown that two key myofibroblast precursor populations, hepatic stellate cells and portal fibroblasts, undergo activation in culture in response to increasing substrate stiffness. We therefore hypothesized that alterations in liver stiffness precede myofibroblast activation and fibrosis in vivo as well. To test this hypothesis, we induced fibrosis in rats by twice weekly injections of carbon tetrachloride (CCl(4)) and then killed the animals at various time points ranging from 3 to 70 days after the initiation of injury. The shear storage modulus of the whole liver was measured on fresh tissue; fixed and frozen tissue from the same livers was used to quantify fibrosis. We observed that liver stiffness increased immediately and continued to increase, leveling out by day 28. Fibrosis, measured histologically by trichrome staining as well as by quantitative sirius red staining, increased with time, although these increases were delayed relative to changes in stiffness. There was no direct correlation between stiffness and fibrosis at early or late time points. Treatment of a second cohort of rats with the lysyl oxidase inhibitor, beta-aminopropionitrile (BAPN), partially prevented early increases in liver stiffness. We concluded that increases in liver stiffness precede fibrosis and potentially myofibroblast activation. Liver stiffness appears to result from matrix cross-linking and possibly other unknown variables in addition to matrix quantity. We suggest that increased liver stiffness may play an important role in initiating the early stages of fibrosis.

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

PURPOSE

To prospectively compare the sensitivity and specificity of magnetic resonance (MR) elastography with those of the routinely available aspartate aminotransferase-to-platelet ratio index (APRI) test for staging hepatic fibrosis in patients who have undergone liver biopsy for suspicion of chronic liver disease, with histopathologic examination as the reference standard.

MATERIALS AND METHODS

The study was approved by the ethics committee. All patients gave written informed consent. Eighty-eight patients (37 men, 51 women; mean age, 54.0 years +/- 13.1 [standard deviation]) who underwent liver biopsy for suspicion of chronic liver disease underwent MR elastography and APRI testing within 2 days after liver biopsy. At histopathologic examination, the fibrosis stage was assessed according to METAVIR scores (fibrosis scores F0 [no fibrosis] to F4 [cirrhosis]). MR elastography was performed by transmitting mechanical waves within the liver and measuring the small cyclic displacement of the liver spins with a phase-contrast spin-echo sequence. The performances of MR elastography and APRI testing were assessed, and the optimal cutoff values for fibrosis stage were determined with receiver operating characteristic (ROC) curve analysis.

RESULTS

At MR elastography, areas under the ROC curves (A(z)) for elasticity and viscosity, respectively, were 0.999 and 0.863 at fibrosis scores greater than or equal to F2, 0.997 and 0.962 at scores greater than or equal to F3, and 1.000 and 0.986 at score F4. A(z) values for elasticity at MR were significantly larger than those for the APRI (0.854 at scores > or = F2, P < .001; 0.886 at scores > or = F3, P = .003; and 0.851 at score F4, P = .004). Optimal cutoff values of elasticity were 2.5 kPa for fibrosis scores greater than or equal to F2, 3.1 kPa for scores greater than or equal to F3, and 4.3 kPa for score F4.

CONCLUSION

Large A(z) values for elasticity (>0.990 for scores > or = F2, > or = F3, and F4) show that MR elastography was accurate in liver fibrosis staging and superior to biochemical testing with APRIs.

Quantitative assessment of hepatic fibrosis in an animal model with magnetic resonance elastography

Chronic liver disease is a world-wide problem that causes progressive hepatic fibrosis as a hallmark of progressive injury. At present, the gold standard for diagnosing hepatic fibrosis is liver biopsy, which is an invasive method with many limitations, including questionable accuracy and risks of complications. MR elastography (MRE), a phase-contrast MRI technique for quantitatively assessing the mechanical properties of soft tissues, is a potential noninvasive diagnostic method to assess hepatic fibrosis. In this work, MRE was evaluated as a quantitative method to assess the in vivo mechanical properties of the liver tissues in a knockout animal model of liver fibrosis. This work demonstrates that the shear stiffness of liver tissue increases systematically with the extent of hepatic fibrosis, as measured by histology. A linear correlation between liver stiffness and fibrosis extent was well-defined in this animal model. An additional finding of the study was that fat infiltration, commonly present in chronic liver disease, does not significantly correlate with liver stiffness at each fibrosis stage and thus does not appear to interfere with the ability of MRE to assess fibrosis extent. In conclusion, MRE has the potential not only for assessing liver stiffness, but also for monitoring potential therapies for hepatic fibrosis.

Diffusion-weighted MRI for quantification of liver fibrosis: preliminary experience

OBJECTIVE

The purpose of this study was to evaluate our preliminary experience using diffusion-weighted MRI for quantification of liver fibrosis.

SUBJECTS AND METHODS

Diffusion-weighted MRI with single-shot echo-planar technique at b values of 50, 300, 500, 700, and 1,000 s/mm2 was prospectively performed on 23 patients with chronic hepatitis and on seven healthy volunteers. The apparent diffusion coefficient (ADC) was measured in four locations in the liver. Liver biopsy results (n = 19) were retrospectively reviewed by two hepatopathologists in consensus to determine stage of fibrosis and grade of inflammation. A Mann-Whitney test was used to compare the ADCs between patients classified with respect to having stage 2 or greater versus stage 1 or less fibrosis and stage 3 or greater versus stage or less 2 fibrosis. Receiver operating characteristics analysis was used to assess the performance of ADC in prediction of the presence of stage 2 or greater and stage 3 or greater fibrosis.

RESULTS

Using a b value of 500 s/mm2 and all combined b values, we found significantly lower hepatic ADCs in stage 2 or greater versus stage 1 or less fibrosis and stage 3 or greater versus stage 2 or less fibrosis. The mean ADCs (x 10(-3) mm2/s) with all b values were 1.47 +/- 0.11 (SD) versus 1.65 +/- 0.10 for stage 2 or greater versus stage 1 or less fibrosis (p < 0.001) and 1.44 +/- 0.07 versus 1.66 +/- 0.10 for stage 3 or greater versus stage 2 or less fibrosis (p <0.001). Hepatic ADC was a significant predictor of stage 2 or greater and stage 3 or greater fibrosis, with areas under the curve of 0.896 and 0.896, sensitivity of 83.3% and 88.9%, and specificity of 83.3% and 80.0% (ADC with all b values, 1.54-1.53 x 10(-3) mm2/s or less).

CONCLUSION

Diffusion-weighted MRI can be used for prediction of the presence of moderate and advanced liver fibrosis.

Congruence of imaging estimators and mechanical measurements of viscoelastic properties of soft tissues

Biomechanical properties of soft tissues are important for a wide range of medical applications, such as surgical simulation and planning and detection of lesions by elasticity imaging modalities. Currently, the data in the literature is limited and conflicting. Furthermore, to assess the biomechanical properties of living tissue in vivo, reliable imaging-based estimators must be developed and verified. For these reasons, we developed and compared two independent quantitative methods--crawling wave estimator (CRE) and mechanical measurement (MM) for soft tissue characterization. The CRE method images shear wave interference patterns from which the shear wave velocity can be determined and hence the Young's modulus can be obtained. The MM method provides the complex Young's modulus of the soft tissue from which both elastic and viscous behavior can be extracted. This article presents the systematic comparison between these two techniques on the measurement of gelatin phantom, veal liver, thermal-treated veal liver and human prostate. It was observed that the Young's moduli of liver and prostate tissues slightly increase with frequency. The experimental results of the two methods are highly congruent, suggesting CRE and MM methods can be reliably used to investigate viscoelastic properties of other soft tissues, with CRE having the advantages of operating in nearly real time and in situ.

Assessment of hepatic fibrosis with magnetic resonance elastography

BACKGROUND & AIMS

Accurate detection of hepatic fibrosis is crucial for assessing prognosis and candidacy for treatment in patients with chronic liver disease. Magnetic resonance (MR) elastography, a technique for quantitatively assessing the mechanical properties of soft tissues, has been shown previously to have potential for noninvasively detecting liver fibrosis. The goal of this work was to obtain preliminary estimates of the sensitivity and specificity of the technique in diagnosing liver fibrosis, and to assess its potential for identifying patients who potentially can avoid a biopsy procedure.

METHODS

MR elastography was performed in 35 normal volunteers and 50 patients with chronic liver disease. MR imaging measurements of hepatic fat to water ratios were obtained to assess the potential for fat infiltration to affect stiffness-based detection of fibrosis.

RESULTS

Liver stiffness increased systematically with fibrosis stage. Receiver operating curve analysis showed that, with a shear stiffness cut-off value of 2.93 kilopascals, the predicted sensitivity and specificity for detecting all grades of liver fibrosis is 98% and 99%, respectively. Receiver operating curve analysis also provided evidence that MR elastography can discriminate between patients with moderate and severe fibrosis (grades 2-4) and those with mild fibrosis (sensitivity, 86%; specificity, 85%). Hepatic stiffness does not appear to be influenced by the degree of steatosis.

CONCLUSIONS

MR elastography is a safe, noninvasive technique with excellent diagnostic accuracy for assessing hepatic fibrosis. Based on the high negative predictive value of MR elastography, an initial clinical application may be to triage patients who are under consideration for biopsy examination to assess possible hepatic fibrosis.

Preliminary assessment of one-dimensional MR elastography for use in monitoring focused ultrasound therapy

The purpose of this work is to assess a fast technique that measures tissue stiffness and temperature during focused ultrasound thermal therapy (FUS). A one-dimensional (1D) MR elastography (MRE) pulse sequence was evaluated for the purpose of obtaining rapid measurements of thermally induced changes in tissue stiffness and temperature for monitoring FUS treatments. The accuracy of the 1D measurement was studied by comparing tissue displacements measured by 1D MRE with those measured by the well-established 2D MRE pulse sequence. The reproducibility of the 1D MRE measurement was assessed, in gel phantoms and ex vivo porcine tissue, for varied FUS intensity levels (31.5-199.9 W cm(-2)) and over a range of displacements at the focus (0.1-1 microm). Temperature elevations in agarose gel phantoms were measured using 1D MRE and calibrated using fiberoptic-thermometer-based measurements. The 1D MRE displacement measurements are highly correlated with those obtained with the 2D technique (R(2) = 0.88-0.93), indicating that 1D MRE can successfully measure tissue displacement. Ten repeated trials at each FUS power level yielded a minimum detectable displacement change of 0.2 microm in phantoms and 0.4 microm in tissue (at 95% confidence level). The 1D MRE temperature measurements correlated well with temperature changes measured simultaneously with fiberoptic thermometers (R(2) = 0.97). The 1D MRE technique is capable of detecting tissue displacements as low as 0.4 microm, which is an order of magnitude smaller than 5 microm displacements expected during FUS therapy (Le et al 2005 AIP Conf. Proc.: Ther. Ultrasound 829 186-90). Additionally, 1D MRE was shown to provide adequate measurements of temperature elevations in tissue. These findings indicate that 1D MRE may be an effective tool for monitoring FUS treatments.

Diffusion-weighted magnetic resonance imaging for the assessment of fibrosis in chronic hepatitis C

Liver biopsy is the gold standard for assessing fibrosis but has several limitations. We evaluated a noninvasive method, so-called diffusion-weighted magnetic resonance imaging (DWMRI), which measures the apparent diffusion coefficient (ADC) of water, for the diagnosis of liver fibrosis in patients with chronic hepatitis C virus (HCV). We analyzed 20 healthy volunteers and 54 patients with chronic HCV (METAVIR: F0, n = 1; F1, n = 30; F2, n = 8; F3, n = 5; and F4, n = 10) prospectively included. Patients with moderate-to-severe fibrosis (F2-F3-F4) had hepatic ADC values lower than those without or with mild fibrosis (F0-F1; mean: 1.10 +/- 0.11 versus 1.30 +/- 0.12 x 10(-3) mm2/s) and healthy volunteers (mean: 1.44 +/- 0.02 x 10(-3) mm2/s). In discriminating patients staged F3-F4, the areas under the receiving operating characteristic curves (AUCs) were 0.92 (+/-0.04) for magnetic resonance imaging (MRI), 0.92 (+/-0.05) for elastography, 0.79 (+/-0.08) for FibroTest, 0.87 (+/-0.06) for the aspartate aminotransferase to platelets ratio index (APRI), 0.86 (+/-0.06) for the Forns index, and 0.87 (+/-0.06) for hyaluronate. In these patients, the sensitivity, specificity, positive predictive value, and negative predictive value were 87%, 87%, 72%, and 94%, respectively, with an ADC cutoff level of 1.21 x 10(-3) mm2/s. In discriminating patients staged F2-F3-F4, the AUC values were 0.79 (+/-0.07) for MRI, 0.87 (+/-0.05) for elastography, 0.68 (+/-0.09) for FibroTest, 0.81 (+/-0.06) for APRI, 0.72 (+/-0.08) for the Forns index, and 0.77 (+/-0.06) for hyaluronate.

CONCLUSION

This preliminary study suggests that DWMRI compares favorably with other noninvasive tests for the presence of significant liver fibrosis.

Portal-systemic shunting in patients with fibrosis or cirrhosis due to chronic hepatitis C: the minimal model for measuring cholate clearances and shunt

BACKGROUND

Measurement of portal inflow and portal-systemic shunt using cholate clearances could be useful in monitoring patients with liver disease.

AIM

To examine relationships of cholate clearances and shunt to cirrhosis and varices and to define minimal sampling requirements.

METHODS

Five hundred forty-eight studies were performed in 282 patients enrolled in the Hepatitis C Antiviral Long-term Treatment to prevent Cirrhosis (HALT-C) trial. Stable, non-radioactive isotopes of cholate were administered intravenously and orally, clearances (Cl(iv) and Cl(oral)) were calculated from [dose/area under curve (AUC)] and cholate shunt from [(AUC(oral):AUC(iv)) x (Dose(iv):Dose(oral)) x 100%].

RESULTS

Cholate Cl(oral) and cholate shunt correlated with prevalences of both cirrhosis and varices (P < 0.0001 for all). Peripheral venous sampling at 5, 20, 45, 60 and 90 min defined the minimal model. Linear regression of cholate shunt determined from five points within 90 min vs. the standard method of 14 points over 3 h yielded slope of 1.0 and intercept 0.5% (r(2) = 0.98, P < 0.0001). Results were identical in the 189 validation studies (slope 1.0, intercept 0.5%, r(2) = 0.99, P < 0.0001).

CONCLUSIONS

Cholate Cl(oral) and cholate shunt may be useful in monitoring patients with liver disease. The 5-point model enhances application of cholate Cl(oral) and cholate shunt in the non-invasive assessment of the portal circulation.

Hepatic viscoelastic parameters measured with MR elastography: Correlations with quantitative analysis of liver fibrosis in the rat

PURPOSE

To determine the correlations between the viscoelastic parameters of the liver measured with in vivo MR elastography and quantitative analysis of liver fibrosis.

MATERIALS AND METHODS

MR elastography of the liver was performed in 10 rats with hepatic fibrosis induced by intraperitoneal carbon tetrachloride (CCl(4)) injections and five normal rats. Longitudinal waves of 200 MHz were transmitted into the liver with a mechanical transducer. Wave propagation into the liver was analyzed with a phase-locked spin-echo sequence at 1.5 T. The viscoelastic parameters, obtained with the Voigt model, were correlated with automatic image analysis of the fibrotic areas and with analysis of the hydroxyproline content of the liver.

RESULTS

Substantial correlations were observed between the shear viscoelastic parameters and the percentage of fibrosis at automatic image analysis (r = 0.7, P = 0.005 for the elasticity, and r = 0.8, P = 0.001 for the viscosity) and moderate correlations were seen between the shear viscoelastic parameters and the hydroxyproline content (r = 0.6, P = 0.016 for the elasticity and r = 0.5, P = 0.041 for the viscosity).

CONCLUSION

The viscoelastic parameters of the liver measured with in vivo MR elastography correlate with quantitative analysis of liver fibrosis. These results suggest that MR elastography is a promising noninvasive method to quantify liver fibrosis.