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. J. Magn. Reson. Imaging 2012;35:607‐610. © 2011 Wiley Periodicals, Inc.     

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. J. Magn. Reson. Imaging 2012;35:1356–1364. © 2012 Wiley Periodicals, Inc.     

Magnetic resonance elastography in the liver at 3 Tesla using a second harmonic approach

Magnetic resonance elastography (MRE) using mechanical stimulation has demonstrated diagnostic value and clinical promise in breast, liver, and kidney at 1.5 Tesla (T). However, MRE at 1.5T suffers from long imaging times and would benefit from greater signal‐to‐noise for more robust postprocessing. We present an MRE sequence modified for liver imaging at 3.0T. To avoid artifacts in the phase images, the sequence maintains a short TE by using a second harmonic approach, including stronger motion encoding gradients, shorter radio frequency pulses and an echo‐planar readout. Scan time was decreased by a factor of ～2 relative to 1.5T by using an EPI readout and a higher density sampling of the phase waveform was used to calculate shear stiffness and viscosity. Localized (small region of interest) and global (whole‐liver region of interest) measurements in normal healthy subjects compared very favorably with previously published results at 1.5T. There was no significant difference between global and localized measures. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Wideband MR elastography for viscoelasticity model identification

The growing clinical use of MR elastography requires the development of new quantitative standards for measuring tissue stiffness. Here, we examine a soft tissue mimicking phantom material (Ecoflex) over a wide frequency range (200 Hz to 7.75 kHz). The recorded data are fit to a cohort of viscoelastic models of varying complexity (integer and fractional order). This was accomplished using multiple sample sizes by employing geometric focusing of the shear wave front to compensate for the changes in wavelength and attenuation over this broad range of frequencies. The simple axisymmetric geometry and shear wave front of this experiment allows us to calculate the frequency‐dependent complex‐valued shear modulus of the material. The data were fit to several common models of linear viscoelasticity, including those with fractional derivative operators, and we identified the best possible matches over both a limited frequency band (often used in clinical studies) and over the entire frequency span considered. In addition to demonstrating the superior capability of the fractional order viscoelastic models, this study highlights the advantages of measuring the complex‐valued shear modulus over as wide a range of frequencies as possible. Magn Reson Med 70:479–489, 2013. © 2012 Wiley Periodicals, Inc.