MR elastography of the liver: preliminary results

Magnetic Resonance Elastography for Liver Fibrosis in Methotrexate Treatment

INTRODUCTION

Hepatic magnetic resonance elastography (MRE) allows for noninvasive assessment of liver fibrosis. The purpose of this study was to evaluate the usefulness of MRE in detecting and quantifying liver fibrosis in patients with rheumatoid arthritis (RA) who have received methotrexate (MTX).

METHODS

The association between mean liver stiffness value as determined by MRE and variables of interest was determined. The decision for a liver biopsy in participants with an abnormal liver stiffness was made based on clinical judgment.

RESULTS

Sixty-five RA patients were enrolled. Mean liver stiffness value by MRE was abnormal in 7 patients, suggestive of hepatic injury. As a result of findings from the MRE, biopsies were performed in 5 patients and all correlated with elevated liver stiffness values. Elevated mean liver stiffness values were associated with body mass index (BMI) (OR= 1.18 per 1 kg/m²; 95% CI: 1.03, 1.36; p=0.017). Neither the total MTX dose nor the duration of MTX treatment was associated with mean liver stiffness value (p=0.51 and P=0.20, respectively).

CONCLUSIONS

MRE provides a reliable, non-invasive assessment of liver fibrosis in patients with RA receiving MTX. Patients with RA receiving MTX who have an elevated BMI may be at increased risk for chronic hepatic injury, regardless of MTX cumulative dose or duration of treatment.

AN OVERVIEW OF ELASTOGRAPHY - AN EMERGING BRANCH OF MEDICAL IMAGING

From times immemorial manual palpation served as a source of information on the state of soft tissues and allowed detection of various diseases accompanied by changes in tissue elasticity. During the last two decades, the ancient art of palpation gained new life due to numerous emerging elasticity imaging (EI) methods. Areas of applications of EI in medical diagnostics and treatment monitoring are steadily expanding. Elasticity imaging methods are emerging as commercial applications, a true testament to the progress and importance of the field.In this paper we present a brief history and theoretical basis of EI, describe various techniques of EI and, analyze their advantages and limitations, and overview main clinical applications. We present a classification of elasticity measurement and imaging techniques based on the methods used for generating a stress in the tissue (external mechanical force, internal ultrasound radiation force, or an internal endogenous force), and measurement of the tissue response. The measurement method can be performed using differing physical principles including magnetic resonance imaging (MRI), ultrasound imaging, X-ray imaging, optical and acoustic signals.Until recently, EI was largely a research method used by a few select institutions having the special equipment needed to perform the studies. Since 2005 however, increasing numbers of mainstream manufacturers have added EI to their ultrasound systems so that today the majority of manufacturers offer some sort of Elastography or tissue stiffness imaging on their clinical systems. Now it is safe to say that some sort of elasticity imaging may be performed on virtually all types of focal and diffuse disease. Most of the new applications are still in the early stages of research, but a few are becoming common applications in clinical practice.

Magnetic resonance elastography: feasibility of liver stiffness measurements in healthy volunteers at 3T

AIM

To demonstrate the feasibility of obtaining liver stiffness measurements with magnetic resonance elastography (MRE) at 3T in normal healthy volunteers using the same technique that has been successfully applied at 1.5 T.

METHODS AND MATERIALS

The study was approved by the local ethics committee and written informed consent was obtained from all volunteers. Eleven volunteers (mean age 35 ± 9 years) with no history of gastrointestinal, hepatobiliary, or cardiovascular disease were recruited. The magnetic resonance imaging (MRI) protocol included a gradient echo-based MRE sequence using a 60 Hz pneumatic excitation. The MRE images were processed using a local frequency estimation inversion algorithm to provide quantitative stiffness maps. Adequate image quality was assessed subjectively by demonstrating the presence of visible propagating waves within the liver parenchyma underlying the driver location. Liver stiffness values were obtained using manually placed regions of interest (ROI) outlining the liver margins on the gradient echo wave images, which were then mapped onto the corresponding stiffness image. The mean stiffness values from two adjacent sections were recorded.

RESULTS

Eleven volunteers underwent MRE. The quality of the MRE images was adequate in all the volunteers. The mean liver stiffness for the group was 2.3 ± 0.38 kPa (ranging from 1.7-2.8 kPa).

CONCLUSIONS

This preliminary work using MRE at 3T in healthy volunteers demonstrates the feasibility of liver stiffness evaluation at 3T without modification of the approach used at 1.5 T. Adequate image quality and normal MRE values were obtained in all volunteers. The obtained stiffness values were in the range of those reported for healthy volunteers in previous studies at 1.5 T. There was good interobserver reproducibility in the stiffness measurements.

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.

Utility of diffusion-weighted imaging in the evaluation of liver fibrosis

OBJECTIVES

To evaluate the usefulness of diffusion- weighted MRI (DWI) in the detection and staging of liver fibrosis and inflammation.

METHODS

DWI was performed with b-factors of 0, 500 and 1000 s/mm². ADC values were obtained by placing circular regions of interest in four segments of the liver. Differences between the study (n = 34) and control groups' (n = 25) ADC values were examined. Further, this study investigated if and how ADC values were related to fibrosis stages and histological activity index (HAI) scores.

RESULTS

The mean ADC value of the liver was smaller in the study group compared with the control group (P < 0.001). Spearman rho correlation analyses showed lower ADC values were associated with higher fibrosis and HAI scores (P < 0.01). There were statistically significant differences in liver ADC values between each combination of fibrosis stages (e.g. stages 0 and 1, 0 and 2) except for stages 1 and 2.

CONCLUSIONS

ADC values prove to be a valuable technique for the diagnosis of liver fibrosis and inflammation. They can also be useful in fibrosis staging, particularly in distinguishing later stages of fibrosis from intermediate and early stages.

KEY POINTS

Diffusion Weighted MRI is a promising technique for diagnosis of liver fibrosis. Apparent Diffusion Coefficients provide valuable information for staging of liver fibrosis. DWI may offer alternative to biopsy for assessing liver fibrosis.

Liver stiffness measurement using transient elastography and hepatocellular carcinoma

Liver fibrosis has been gaining noticeable attention because it may lead to end-stage liver cirrhosis and ultimately to hepatocellular carcinoma. Thus, a precise estimation of the degree of liver fibrosis is crucial for predicting prognosis and deciding management of patients with chronic liver diseases. Many non-invasive approaches for the evaluation of liver fibrosis have been developed. Among these procedures, transient elastography has recently drawn great attention. Transient elastography has been reported to be well correlated with the degree of liver fibrosis by many investigators and various institutions. Since the degree of liver fibrosis is considered as a strong predictor of risk for hepatocellular carcinoma development, several trials have been performed to verify the usefulness of measurement of liver stiffness to predict the emergence of hepatocellular carcinoma. From these studies, transient elastography seems to be a promising procedure to predict the risk of hepatocellular carcinoma; however, further cohorts with long-term monitoring of liver stiffness are needed to confirm the usefulness of this method.

Magnetic resonance elastography of the kidneys: feasibility and reproducibility in young healthy adults

PURPOSE

To evaluate the feasibility and reproducibility of renal magnetic resonance elastography (MRE) in young healthy volunteers.

MATERIALS AND METHODS

Ten volunteers underwent renal MRE twice at a 4-5 week interval. The vibrations (45 and 76 Hz) were generated by a speaker positioned beneath the volunteers' back and centered on their left kidney. For each frequency, three sagittal slices were acquired (eight phase offsets per cycle, motion-encoding gradients successively positioned along the three directions of space). Shear velocity images were reconstructed using the curl operator combined with the local frequency estimation (LFE) algorithm.

RESULTS

The mean shear velocities measured in the renal parenchyma during the two examinations were not significantly different and exhibited a mean variation of 6% at 45 Hz and 76 Hz. The mean shear velocities in renal parenchyma were 2.21 ± 0.14 m/s at 45 Hz (shear modulus of 4.9 ± 0.5 kPa) and 3.07 ± 0.17 m/s at 76 Hz (9.4 ± 0.8 kPa, P < 0.01). The mean shear velocities in the renal cortex and medulla were respectively 2.19 ± 0.13 m/s and 2.32 ± 0.16 m/s at 45 Hz (P = 0.002) and 3.06 ± 0.16 m/s and 3.10 ± 0.22 m/s at 76 Hz (P = 0.13).

CONCLUSION

Renal MRE was feasible and reproducible. Two independent measurements of shear velocities in the renal parenchyma of the same subjects showed an average variability of 6%.

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.

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.

Noninvasive evaluation of hepatic fibrosis using acoustic radiation force-based shear stiffness in patients with nonalcoholic fatty liver disease

BACKGROUND & AIMS

Nonalcoholic fatty liver disease (NAFLD), the most common form of chronic liver disease in developed countries, may progress to nonalcoholic steatohepatitis (NASH) in a minority of people. Those with NASH are at increased risk for cirrhosis and hepatocellular carcinoma. The potential risk and economic burden of utilizing liver biopsy to stage NAFLD in an overwhelmingly large at-risk population are enormous; thus, the discovery of sensitive, inexpensive, and reliable noninvasive diagnostic modalities is essential for population-based screening.

METHODS

Acoustic Radiation Force Impulse (ARFI) shear wave imaging, a noninvasive method of assessing tissue stiffness, was used to evaluate liver fibrosis in 172 patients diagnosed with NAFLD. Liver shear stiffness measures in three different imaging locations were reconstructed and compared to the histologic features of NAFLD and AST-to-platelet ratio indices (APRI).

RESULTS

Reconstructed shear stiffnesses were not associated with ballooned hepatocytes (p=0.11), inflammation (p=0.69), nor imaging location (p=0.11). Using a predictive shear stiffness threshold of 4.24kPa, shear stiffness distinguished low (fibrosis stage 0-2) from high (fibrosis stage 3-4) fibrosis stages with a sensitivity of 90% and a specificity of 90% (AUC of 0.90). Shear stiffness had a mild correlation with APRI (R(2)=0.22). BMI>40kg/m(2) was not a limiting factor for ARFI imaging, and no correlation was noted between BMI and shear stiffness (R(2)=0.05).

CONCLUSIONS

ARFI imaging is a promising imaging modality for assessing the presence or absence of advanced fibrosis in patients with obesity-related liver disease.

Non-invasive markers for hepatic fibrosis

With great advancements in the therapeutic modalities used for the treatment of chronic liver diseases, the accurate assessment of liver fibrosis is a vital need for successful individualized management of disease activity in patients. The lack of accurate, reproducible and easily applied methods for fibrosis assessment has been the major limitation in both the clinical management and for research in liver diseases. However, the problem of the development of biomarkers capable of non-invasive staging of fibrosis in the liver is difficult due to the fact that the process of fibrogenesis is a component of the normal healing response to injury, invasion by pathogens, and many other etiologic factors. Current non-invasive methods range from serum biomarker assays to advanced imaging techniques such as transient elastography and magnetic resonance imaging (MRI). Among non-invasive methods that gain strongest clinical foothold are FibroScan elastometry and serum-based APRI and FibroTest. There are many other tests that are not yet widely validated, but are none the less, promising. The rate of adoption of non-invasive diagnostic tests for liver fibrosis differs from country to country, but remains limited. At the present time, use of non-invasive procedures could be recommended as pre-screening that may allow physicians to narrow down the patients' population before definitive testing of liver fibrosis by biopsy of the liver. This review provides a systematic overview of these techniques, as well as both direct and indirect biomarkers based approaches used to stage fibrosis and covers recent developments in this rapidly advancing area.

Noninvasive In vivo assessment of renal tissue elasticity during graded renal ischemia using MR elastography

OBJECTIVES

: Magnetic resonance elastography (MRE) allows noninvasive assessment of tissue stiffness in vivo. Renal arterial stenosis (RAS), a narrowing of the renal artery, promotes irreversible tissue fibrosis that threatens kidney viability and may elevate tissue stiffness. However, kidney stiffness may also be affected by hemodynamic factors. This study tested the hypothesis that renal blood flow (RBF) is an important determinant of renal stiffness as measured by MRE.

MATERIAL AND METHODS

: In 6 anesthetized pigs MRE studies were performed to determine cortical and medullary elasticity during acute graded decreases in RBF (by 20%, 40%, 60%, 80%, and 100% of baseline) achieved by a vascular occluder. Three sham-operated swine served as time control. Additional pigs were studied with MRE 6 weeks after induction of chronic unilateral RAS (n = 6) or control (n = 3). Kidney fibrosis was subsequently evaluated histologically by trichrome staining.

RESULTS

: During acute RAS the stenotic cortex stiffness decreased (from 7.4 ± 0.3 to 4.8 ± 0.6 kPa, P = 0.02 vs. baseline) as RBF decreased. Furthermore, in pigs with chronic RAS (80% ± 5.4% stenosis) in which RBF was decreased by 60% ± 14% compared with controls, cortical stiffness was not significantly different from normal (7.4 ± 0.3 vs. 7.6 ± 0.3 kPa, P = 0.3), despite histologic evidence of renal tissue fibrosis.

CONCLUSION

: Hemodynamic variables modulate kidney stiffness measured by MRE and may mask the presence of fibrosis. These results suggest that kidney turgor should be considered during interpretation of elasticity assessments.

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.

Comparison of two methods for the generation of spatially modulated ultrasound radiation force

Spatially modulated ultrasound radiation force (SMURF) imaging is an elastographic technique that involves generating a radiation force beam with a lateral intensity variation of a defined spatial frequency. This results in a shear wave of known wavelength. By using the displacements induced by the shear wave and standard Doppler or speckle-tracking methods, the shear wave frequency, and thus material shear modulus, is estimated. In addition to generating a pushing beam pattern with a specified lateral intensity variation, it is generally desirable to induce larger displacements so that the displacement data signal-to-noise ratio is higher. We provide an analysis of two beam forming methods for generating SMURF in an elastic material: the focal Fraunhofer and intersecting plane wave methods. Both techniques generate beams with a defined spatial frequency. However, as a result of the trade-offs associated with each technique, the peak acoustic intensity outputs in the region of interest differs for the same combinations of parameters (e.g., the focal depth, the width of the area of interest, and ultrasonic attenuation coefficient). Assuming limited transducer drive voltage, we provide a decision plot to determine which of the two techniques yields the greater pushing force for a specific configuration.

A three-dimensional quality-guided phase unwrapping method for MR elastography

Magnetic resonance elastography (MRE) uses accumulated phases that are acquired at multiple, uniformly spaced relative phase offsets, to estimate harmonic motion information. Heavily wrapped phase occurs when the motion is large and unwrapping procedures are necessary to estimate the displacements required by MRE. Two unwrapping methods were developed and compared in this paper. The first method is a sequentially applied approach. The three-dimensional MRE phase image block for each slice was processed by two-dimensional unwrapping followed by a one-dimensional phase unwrapping approach along the phase-offset direction. This unwrapping approach generally works well for low noise data. However, there are still cases where the two-dimensional unwrapping method fails when noise is high. In this case, the baseline of the corrupted regions within an unwrapped image will not be consistent. Instead of separating the two-dimensional and one-dimensional unwrapping in a sequential approach, an interleaved three-dimensional quality-guided unwrapping method was developed to combine both the two-dimensional phase image continuity and one-dimensional harmonic motion information. The quality of one-dimensional harmonic motion unwrapping was used to guide the three-dimensional unwrapping procedures and it resulted in stronger guidance than in the sequential method. In this work, in vivo results generated by the two methods were compared.

Clinical feasibility of liver elastography by acoustic radiation force impulse imaging (ARFI)

BACKGROUND

Transient elastography is increasingly used for assessment of liver fibrosis. Acoustic radiation force impulse imaging (ARFI) is a new technology to perform liver elastography.

AIMS

We evaluated the clinical feasibility, validity and accuracy of the ARFI method and compared it to Fibroscan(®) and liver histology.

METHODS

Ultrasonographic elastography of the liver using ARFI was performed in 29 patients with liver cirrhosis, 70 patients with liver disease and 23 healthy controls.

RESULTS

ARFI was feasible in all patients providing a mean propagation velocity of 1.65±0.93 m/s. ARFI results of the right and left liver lobes were comparable (p<0.001). In cirrhotic patients, ARFI gave significantly higher values than in the other patients (p<0.001). Rate of invalid measurements was lower in ARFI than in Fibroscan(®) (p<0.04). Both elastography methods were highly correlated to each other (p<0.001). Furthermore, ARFI correlated to histological grading of liver fibrosis (p<0.001) and to inflammatory activity (p<0.05). Liver steatosis had no statistical influence on ARFI results (p=0.2) in contrast to Fibroscan(®) (p<0.05).

CONCLUSIONS

The new ultrasonographic method of ARFI elastography allows valid, accurate and flexible evaluation of liver stiffness. It seems more feasible in patients with liver cirrhosis than Fibroscan(®). ARFI elastography of the left liver lobe is also possible. Liver steatosis does not seem to influence ARFI elastography.

In vivo assessment of MR elastography-derived effective end-diastolic myocardial stiffness under different loading conditions

PURPOSE

To compare magnetic resonance elastography (MRE) effective stiffness to end-diastolic pressure at different loading conditions to demonstrate a relationship between myocardial MRE effective stiffness and end-diastolic left ventricular (LV) pressure.

MATERIALS AND METHODS

MRE was performed on four pigs to measure the end-diastolic effective stiffness under different loading conditions. End-diastolic pressure was increased by infusing Dextran-40 (20% of blood volume). For each infusion of Dextran-40, end-diastolic pressure was recorded and end-diastolic effective stiffness was measured using MRE. In each pig, least-square linear regression was performed to determine the correlation between end-diastolic effective stiffness and end-diastolic LV pressure.

RESULTS

A linear correlation was found between end-diastolic LV pressure and end-diastolic effective stiffness with R(2) ranging from 0.73-0.9. A linear correlation with R(2) = 0.26 was found between end-diastolic LV pressure and end-diastolic effective stiffness when pooling data points from all pigs.

CONCLUSION

End-diastolic effective myocardial stiffness increases linearly with end-diastolic LV pressure.

Frequency-dependent viscoelastic parameters of mouse brain tissue estimated by MR elastography

Viscoelastic properties of mouse brain tissue were estimated non-invasively, in vivo, using magnetic resonance elastography (MRE) at 4.7 T to measure the dispersive properties of induced shear waves. Key features of this study include (i) the development and application of a novel MR-compatible actuation system which transmits vibratory motion into the brain through an incisor bar, and (ii) the investigation of the mechanical properties of brain tissue over a 1200 Hz bandwidth from 600-1800 Hz. Displacement fields due to propagating shear waves were measured during continuous, harmonic excitation of the skull. This protocol enabled characterization of the true steady-state patterns of shear wave propagation. Analysis of displacement fields obtained at different frequencies indicates that the viscoelastic properties of mouse brain tissue depend strongly on frequency. The average storage modulus (G') increased from approximately 1.6 to 8 kPa over this range; average loss modulus (G″) increased from approximately 1 to 3 kPa. Both moduli were well approximated by a power-law relationship over this frequency range. MRE may be a valuable addition to studies of disease in murine models, and to pre-clinical evaluations of therapies. Quantitative measurements of the viscoelastic parameters of brain tissue at high frequencies are also valuable for modeling and simulation of traumatic brain injury.

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.

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

PURPOSE

To determine the repeatability of stiffness measurements in the liver using MR elastography (MRE) during the fasted and fed states. MRE has gained increased recognition as a noninvasive method to quantify fibrotic changes in the liver. It is well known that eating increases splanchnic blood flow, and fasting status of patients has been recognized as a factor that may affect hepatic stiffness measured with MRE.

MATERIALS AND METHODS

Hepatic MRE stiffness and flow through the superior mesenteric vein (SMV) were measured in 12 healthy subjects in fasted and fed states, and measurements were repeated 5 weeks later. A linear mixed effects model was used to estimate the sources of variability in the data, which included day (exams on different days) and subject. Sources were combined to calculate the overall standard deviation of a single MRE measurement.

RESULTS

The total within-subject standard deviation of an MRE exam is 8.5% (standard error [SE] = 1.7%) or 9.0% (SE = 1.8%) for fasted and fed states, respectively. No significant differences between fasted/fed state stiffness and no correlation between hepatic stiffness and SMV flow were observed.

CONCLUSION

As seen in this smaller population, healthy subjects scanned in a known fasted or fed state provide repeatable stiffness estimates with no relationship to SMV flow.

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.

Liver stiffness assessment by tagged MRI of cardiac-induced liver motion

Cirrhosis is an important and growing public health problem, affecting millions of Americans and many more people internationally. A pathological hallmark of the progression to cirrhosis is the development of liver fibrosis, so that monitoring the appearance and progression of liver fibrosis can be used to guide therapy. Here, we report a method to use magnetization-tagged magnetic resonance imaging to measure the cardiac-induced motion and deformation in the liver, as a means for noninvasively assessing liver stiffness, which is related to fibrosis. The initial results show statistically significant differences between healthy and cirrhotic subjects in the direct comparisons of the maximum displacement (mm), and the maximum (P1) and minimum (P2) two-dimensional strains, through the cardiac cycle (3.514 ± 0.793, 2.184 ± 0.611; 0.116 ± 0.043, 0.048 ± 0.011; -0.094 ± 0.020, -0.041 ± 0.015; healthy, cirrhosis, respectively; P < 0.005 for all). There are also significant differences in the displacement-normalized P1 and P2 strains (mm(-1) ) (0.030 ± 0.008, 0.017 ± 0.007; -0.024 ± 0.006, -0.013 ± 0.004; healthy, cirrhosis, respectively; P < 0.005 for all). Therefore, this noninvasive imaging-based method is a promising means to assess liver stiffness using clinically available imaging tools.

Diffusion tensor imaging of liver fibrosis in an experimental model

PURPOSE

To characterize changes in diffusion properties of liver using diffusion tensor imaging (DTI) in an experimental model of liver fibrosis.

MATERIALS AND METHODS

Liver fibrosis was induced in Sprague-Dawley rats (n = 12) by repetitive dosing of carbon tetrachloride (CCl(4)). The animals were examined with a respiratory-gated single-shot spin-echo echo-planar DTI protocol at 7 T before, 2 weeks after, and 4 weeks after CCl(4) insult. Apparent diffusion coefficient (ADC), directional diffusivities (ADC(//) and ADC(⊥)), and fractional anisotropy (FA) were measured. Liver histology was performed with hematoxylin-eosin staining and Masson's trichrome staining.

RESULTS

Significant decrease (P < 0.01) in ADC was found at 2 weeks (0.86 ± 0.09 × 10(-3) mm(2)/s) and 4 weeks (0.74 ± 0.09 × 10(-3) mm(2)/s) following CCl(4) insult, as compared with that before insult (0.97 ± 0.08 × 10(-3) mm(2)/s). Meanwhile, FA at 2 weeks (0.18 ± 0.03) after CCl(4) insult was significantly lower (P < 0.01) than that before insult (0.26 ± 0.05), and subsequently normalized at 4 weeks (0.26 ± 0.07) after the insult. Histology showed collagen deposition, presence of intracellular fat vacuoles, and cell necrosis/apoptosis in livers with CCl(4) insult.

CONCLUSION

DTI detected the progressive changes in water diffusivities and diffusion anisotropy of liver tissue in this liver fibrosis model. ADC and FA are potentially valuable in detecting liver fibrosis at early stages and monitoring its progression. Future human studies are warranted to further verify the applicability of DTI in characterizing liver fibrosis and to determine its role in clinical settings.

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.

In vivo magnetic resonance elastography of human brain at 7 T and 1.5 T

PURPOSE

To investigate the feasibility of quantitative in vivo ultrahigh field magnetic resonance elastography (MRE) of the human brain in a broad range of low-frequency mechanical vibrations.

MATERIALS AND METHODS

Mechanical vibrations were coupled into the brain of a healthy volunteer using a coil-driven actuator that either oscillated harmonically at single frequencies between 25 and 62.5 Hz or performed a superimposed motion consisting of multiple harmonics. Using a motion sensitive single-shot spin-echo echo planar imaging sequence shear wave displacements in the brain were measured at 1.5 and 7 T in whole-body MR scanners. Spatially averaged complex shear moduli were calculated applying Helmholtz inversion.

RESULTS

Viscoelastic properties of brain tissue could be reliably determined in vivo at 1.5 and 7 T using both single-frequency and multifrequency wave excitation. The deduced dispersion of the complex modulus was consistent within different experimental settings of this study for the measured frequency range and agreed well with literature data.

CONCLUSION

MRE of the human brain is feasible at 7 T. Superposition of multiple harmonics yields consistent results as compared to standard single-frequency based MRE. As such, MRE is a system-independent modality for measuring the complex shear modulus of in vivo human brain in a wide dynamic range.

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.

Acoustic radiation force impulse elastography for chronic liver disease: comparison with ultrasound-based scores of experienced radiologists, Child-Pugh scores and liver function tests

The purpose of our study was to investigate whether acoustic radiation force impulse (ARFI) elastography provides better diagnostic performance for diagnosis of chronic liver disease and correlates better with Child-Pugh scores and liver function tests, compared with an ultrasound (US) scoring system based on visual assessment of conventional B-mode US images by experienced radiologists. Five hundred and twenty-one patients with clinically proven chronic liver disease (n = 293), fatty liver (n = 95) or normal liver (n = 133) were included in this study. B-mode liver US and ARFI elastography were performed in all patients. ARFI elastography was performed at least five times, with each measurement obtained at a different area of the right hepatic lobe; mean shear wave velocity (SWV) was calculated for each patient. The mean SWV was compared with US-based scores from two radiologists (based on liver surface nodularity, parenchyma echotexture and hepatic vein contour), Child-Pugh scores and liver function tests. The mean SWV of the normal liver group was 1.08 m/s ± 0.15; of the fatty liver group, 1.02 m/s ± 0.16; and of the chronic liver disease group, 1.66 m/s ± 0.60 (p < 0.001). The area under the receiver operating characteristics curve of the mean SWV in ARFI elastography was significantly higher than that of the conventional B-mode US-based scores by two radiologists (0.89 vs. 0.74 and 0.77, p < 0.05), with a sensitivity of 75.4% and a specificity of 89.5% at the cut-off value of 1.22 m/s. The sensitivity of the mean SWV was significantly higher than the US-based scores (p < 0.001), although the specificity was not (p > 0.05). The mean SWV was better correlated with Child-Pugh scores and all liver function tests (except total protein) than the US-based scores from two radiologists. In conclusion, ARFI elastography showed better diagnostic performance than visual assessment of experienced radiologists for diagnosis of chronic liver disease, as well as for evaluation of the severity of chronic liver disease.

Contrast detection in fluid-saturated media with magnetic resonance poroelastography

PURPOSE

Recent interest in the poroelastic behavior of tissues has led to the development of magnetic resonance poroelastography (MRPE) as an alternative to single-phase MR elastographic image reconstruction. In addition to the elastic parameters (i.e., Lamé's constants) commonly associated with magnetic resonance elastography (MRE), MRPE enables estimation of the time-harmonic pore-pressure field induced by external mechanical vibration.

METHODS

This study presents numerical simulations that demonstrate the sensitivity of the computed displacement and pore-pressure fields to a priori estimates of the experimentally derived model parameters. In addition, experimental data collected in three poroelastic phantoms are used to assess the quantitative accuracy of MR poroelastographic imaging through comparisons with both quasistatic and dynamic mechanical tests.

RESULTS

The results indicate hydraulic conductivity to be the dominant parameter influencing the deformation behavior of poroelastic media under conditions applied during MRE. MRPE estimation of the matrix shear modulus was bracketed by the values determined from independent quasistatic and dynamic mechanical measurements as expected, whereas the contrast ratios for embedded inclusions were quantitatively similar (10%-15% difference between the reconstructed images and the mechanical tests).

CONCLUSIONS

The findings suggest that the addition of hydraulic conductivity and a viscoelastic solid component as parameters in the reconstruction may be warranted.

The Effect of Glisson’s Capsule on the Superficial Elasticity Measurements of the Liver

In the past decade, novel tools for surgical planning and disease diagnosis have been developed to detect the liver’s mechanical properties. Some tools utilize superficial indentation type measurements to determine the elasticity of the liver parenchyma and to assume material homogeneity. In fact, the liver is a soft tissue covered with a connective sheathing that is called Glisson’s capsule. This article quantifies the effect of this capsule on the measured or “effective” elastic modulus obtained by indentation with a spherical geometry. Two sets of parametric computational studies in which the Glisson capsule thickness and elasticity were varied, demonstrated the relationship between the measured elastic response and the underlying parenchymal elastic response. Previously reported in vivo indentation data on the human liver were utilized to determine the elasticity of its parenchyma. The results indicated a linear relationship between the effective (measured) elastic response and the underlying parenchyma for the Glisson capsule thicknesses considered. When previously published human liver indentation data were analyzed, the measured elastic modulus was nearly 6.9% greater than the underlying parenchyma elastic modulus. Although the analyzed data were obtained from a single liver and yet displayed a significant variation, the Glisson capsule elasticity induced a significant but systematic error as well. The Glisson capsule thickness error was negligible for capsule parameters associated with a normal liver. Based on this work, an emphasis on the Glisson capsule’s contribution to the mechanical response of the liver would enhance the clinical potential of indentation-based novel tools for liver care.

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.

Convertible pneumatic actuator for magnetic resonance elastography of the brain

Here we present a novel pneumatic actuator design for brain magnetic resonance elastography (MRE). Magnetic resonance elastography is a phase contrast technique capable of tracing strain wave propagation and utilizing this information for the calculation of mechanical properties of materials and living tissues. In MRE experiments, the acoustic waves are generated in a synchronized way with respect to image acquisition, using various types of mechanical actuators. The unique feature of the design is its simplicity and flexibility, which allows reconfiguration of the actuator for different applications ranging from in vivo brain MRE to experiments with phantoms. Phantom and in vivo data are presented to demonstrate actuator performance.

MR imaging in liver cirrhosis: classical and new approaches

Objective

The typical histological features of chronic hepatitis and cirrhosis are variable degrees of hepatocellular necrosis and inflammation (activity or grade of disease), fibrosis (stage of disease), and associated fat and iron deposition. These features influence the liver’s appearance and must be assessed separately by imaging biomarkers in order to be clinically useful. Hepatic morphologic alterations and features of portal hypertension identify most cases of established cirrhosis. Nowadays, research is focused on developing ways to improve detection of early and intermediate stages of fibrosis as well as hepatocyte dysfunction. Even more, most imaging-related measurements are subject to complex interactions and are influenced by different pathologic processes, such as fatty infiltration, edema, necrosis and iron accumulation.

Methods and results

MR experience throughout the last 15 years at the Dr Peset University Hospital is reviewed.

Conclusion

Nowadays, several biomarkers have been developed to grade the liver status in cirrhosis. This review will focus on these topics.

Viscoelasticity-based staging of hepatic fibrosis with multifrequency MR elastography

PURPOSE

To analyze the dynamics of the shear modulus of the liver to assess the optimal driving frequency and to determine the diagnostic accuracy of generalized frequency-independent elasticity cutoff values for staging hepatic fibrosis.

MATERIALS AND METHODS

This institutional review board-approved prospective study included 16 healthy volunteers and 72 patients with biopsy-proved liver fibrosis. After obtaining written informed consent, imaging was performed at 1.5-T by using a motion-sensitized echo-planar imaging sequence. Wave excitation was performed by an actuator introducing a superposition of four frequencies (25.0, 37.5, 50.0, 62.5 Hz) of shear waves. The elasticity µ value and the structure geometry parameter α were calculated by using the two-parameter springpot model. The performance of magnetic resonance (MR) elastography in staging liver fibrosis was assessed with area under the receiver operating characteristic curve (AUROC) analysis and Spearman correlation analysis.

RESULTS

Elasticity increased with stage of fibrosis, with mean values as follows: for volunteers, 2.25 kPa ± 0.43 (standard deviation); stage F1, 2.61 kPa ± 0.43; stage F2, 3.00 kPa ± 0.63; stage F3, 3.86 kPa ± 0.61; and stage F4, 5.86 kPa ± 1.22. Frequency-independent cutoff values derived for fibrosis and AUROC values, respectively, were as follows: stage F1 or higher, 2.84 kPa and 0.9128; stage F2 or higher, 3.18 kPa and 0.9244; stage F3 or higher, 3.32 kPa and 0.9744; and equivalent to stage F4, 4.21 kPa and 0.9931. The geometry of the tissue (α value) did not correlate with fibrosis. Frequencies of 50.0 Hz and 62.5 Hz displayed the highest diagnostic accuracy.

CONCLUSION

The diagnostic performance of multifrequency MR elastography in determining the degree of hepatic fibrosis increases with stage of fibrosis. Metrics obtained at the higher frequencies provide better diagnostic performance compared with the lower frequencies. Results of the AUROC analysis demonstrate the high accuracy of frequency-independent cutoff values for staging higher grades of hepatic fibrosis.

Time-harmonic magnetic resonance elastography of the normal feline brain

Imaging of the mechanical properties of in vivo brain tissue could eventually lead to non-invasive diagnosis of hydrocephalus, Alzheimer's disease and other pathologies known to alter the intracranial environment. The purpose of this work is to (1) use time-harmonic magnetic resonance elastography (MRE) to estimate the mechanical property distribution of cerebral tissue in the normal feline brain and (2) compare the recovered properties of grey and white matter. Various in vivo and ex vivo brain tissue property measurement strategies have led to the highly variable results that have been reported in the literature. MR elastography is an imaging technique that can estimate mechanical properties of tissue non-invasively and in vivo. Data was acquired in 14 felines and elastic parameters were estimated using a globo-regional nonlinear image reconstruction algorithm. Results fell within the range of values reported in the literature and showed a mean shear modulus across the subject group of 7-8 kPa with all but one animal falling within 5-15 kPa. White matter was statistically stiffer (p<0.01) than grey matter by about 1 kPa on a per subject basis. To the best of our knowledge, the results reported represent the most extensive set of estimates in the in vivo brain which have been based on MRE acquisition of the three-dimensional displacement field coupled to volumetric shear modulus image reconstruction achieved through nonlinear parameter estimation. However, the inter-subject variation in mean shear modulus indicates the need for further study, including the possibility of applying more advanced models to estimate the relevant tissue mechanical properties from the data.

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.

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.

Liver stiffness: a novel parameter for the diagnosis of liver disease

The noninvasive quantitation of liver stiffness (LS) by ultrasound based transient elastography using FibroScan® has revolutionized the diagnosis of liver diseases, namely liver cirrhosis. Alternative techniques such as acoustic radiation impulse frequency imaging or magnetic resonance elastography are currently under investigation. LS is an excellent surrogate marker of advanced fibrosis (F3) and cirrhosis (F4) outscoring all previous noninvasive approaches to detect cirrhosis. LS values below 6 kPa are considered as normal and exclude ongoing liver disease. LS of 8 and 12.5 kPa represent generally accepted cut-off values for F3 and F4 fibrosis. LS highly correlates with portal pressure, and esophageal varices are likely at values >20 kPa. Many other factors may also increase LS such as hepatic infiltration with tumor cells, mast cells (mastocytosis), inflammatory cells (all forms of hepatitis) or amyloidosis. In addition, LS is directly correlated with the venous pressure (eg, during liver congestion) and is increased during mechanic cholestasis. Thus, LS should always be interpreted in the context of clinical, imaging and laboratory findings. Finally, LS has helped to better understand the molecular mechanisms underlying liver fibrosis. The novel pressure-stiffness-fibrosis sequence hypothesis is introduced.

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.

Can preoperative diffusion-weighted MRI predict postoperative hepatic insufficiency after curative resection of HBV-related hepatocellular carcinoma? A pilot study

Liver fibrosis determines the functional liver reserve. Several studies have reported that the apparent diffusion coefficient (ADC) values of diffusion-weighted magnetic resonance imaging (DW-MRI) can assess liver fibrosis. We investigated whether DW-MRI predicts postoperative hepatic insufficiency and liver fibrosis in patients with hepatitis B virus (HBV)-related hepatocellular carcinoma (HCC). Twenty-six patients with HBV-related HCC who received preoperative DW-MRI on a 3-T MRI system were enrolled between July and December 2008. ADC values were measured twice by two observers. Three "b values" were used: 50, 400 and 800 s/mm(2). Postoperative hepatic insufficiency was defined as persistent hyperbilirubinemia (total bilirubin level >5 mg/dl for more than 5 days after surgery) or postoperative death without other causes. The mean age (21 men and 5 women) was 51.4 years. Three patients experienced postoperative hepatic insufficiency. liver stiffness measurement predicted postoperative hepatic insufficiency, advanced fibrosis (F3-4), and cirrhosis significantly [area under the receiving operator characteristic curve (AUROC)=0.942, 0.771 and 0.818, respectively, with P=.047, 0.048 and 0.006, respectively]; ADC values of DW-MRI, however, did not (AUROC=0.797, 0.648 and 0.491, respectively, with P=.100, 0.313 and 0.938, respectively). Reliability of ADC values between right and left hepatic lobes (rho=0.868 and rho=0.910 in the first and second measures of Observer A; rho=0.865 and rho=0.831 in the first and second measures of Observer B) was high and the intra- and interobserver reliability (rho=0.958 in observer A and rho=0.977 in observer B; rho=0.929 in the first measure and rho=0.978 in the second measure between the two observers) were high. All reliability was significant (P<.001). Our results suggest that DW-MRI on a 3-T MRI system is not suitable for predicting postoperative hepatic insufficiency, advanced liver fibrosis, and cirrhosis in patients with HBV-related HCC, despite significantly high reliability.

Proteomic approaches in the search for biomarkers of liver fibrosis

Chronic liver diseases (CLDs) can cause progressive hepatic fibrosis culminating in cirrhosis. Fibrosis staging requires liver biopsy, which is invasive, expensive and frequently poorly tolerated by patients. Serum-based panels of fibrosis biomarkers have been developed as alternatives to biopsy. Recent advances in high-throughput proteomic methods have the potential to optimise combinations of biomarkers for the diagnosis of liver fibrosis. Here, we review the key recent developments in the field of proteomics and their application to this important clinical question. We critically discuss the challenges and priorities for future research that are of critical importance to clinical hepatology.

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.

Abdominal magnetic resonance elastography

Magnetic resonance elastography (MRE) is a magnetic resonance imaging-based technique for quantitatively assessing the mechanical properties of tissues based on the propagation of shear waves. Multiple studies have described many potential applications of MRE, from characterizing tumors to detecting diffuse disease processes. Studies have shown that MRE can be successfully implemented to assess abdominal organs. The first clinical application of MRE to be well documented is the detection and characterization of hepatic fibrosis, which systematically increases the stiffness of liver tissue. In this diagnostic role, it offers a safer, less expensive, and potentially more accurate alternative to invasive liver biopsy. Emerging results suggest that measurements of liver and spleen stiffness may provide an indirect way to assess portal hypertension. Preliminary studies have demonstrated that it is possible to use MRE to evaluate the mechanical properties of other abdominal structures, such as the pancreas and kidneys. Steady technical progress in developing practical protocols for applying MRE in the abdomen and the pelvis provides opportunities to explore many other potential applications of this emerging technology.

MR imaging of liver fibrosis: current state of the art

Chronic liver disease is a major public health problem worldwide. Liver fibrosis, a common feature of almost all causes of chronic liver disease, involves the accumulation of collagen, proteoglycans, and other macromolecules within the extracellular matrix. Fibrosis tends to progress, leading to hepatic dysfunction, portal hypertension, and ultimately cirrhosis. Liver biopsy, the standard of reference for diagnosing liver fibrosis, is invasive, costly, and subject to complications and sampling variability. These limitations make it unsuitable for diagnosis and longitudinal monitoring in the general population. Thus, development of a noninvasive, accurate, and reproducible test for diagnosis and monitoring of liver fibrosis would be of great value. Conventional cross-sectional imaging techniques have limited capability to demonstrate liver fibrosis. In clinical practice, imaging studies are usually reserved for evaluation of the presence of portal hypertension or hepatocellular carcinoma in cases that have progressed to cirrhosis. In response to the rising prevalence of chronic liver diseases in Western nations, a number of imaging-based methods including ultrasonography-based transient elastography, computed tomography-based texture analysis, and diverse magnetic resonance (MR) imaging-based techniques have been proposed for noninvasive diagnosis and grading of hepatic fibrosis across its entire spectrum of severity. State-of-the-art MR imaging-based techniques in current practice and in development for noninvasive assessment of liver fibrosis include conventional contrast material-enhanced MR imaging, double contrast-enhanced MR imaging, MR elastography, diffusion-weighted imaging, and MR perfusion imaging.

MR imaging of liver fibrosis: current state of the art

Chronic liver disease is a major public health problem worldwide. Liver fibrosis, a common feature of almost all causes of chronic liver disease, involves the accumulation of collagen, proteoglycans, and other macromolecules within the extracellular matrix. Fibrosis tends to progress, leading to hepatic dysfunction, portal hypertension, and ultimately cirrhosis. Liver biopsy, the standard of reference for diagnosing liver fibrosis, is invasive, costly, and subject to complications and sampling variability. These limitations make it unsuitable for diagnosis and longitudinal monitoring in the general population. Thus, development of a noninvasive, accurate, and reproducible test for diagnosis and monitoring of liver fibrosis would be of great value. Conventional cross-sectional imaging techniques have limited capability to demonstrate liver fibrosis. In clinical practice, imaging studies are usually reserved for evaluation of the presence of portal hypertension or hepatocellular carcinoma in cases that have progressed to cirrhosis. In response to the rising prevalence of chronic liver diseases in Western nations, a number of imaging-based methods including ultrasonography-based transient elastography, computed tomography-based texture analysis, and diverse magnetic resonance (MR) imaging-based techniques have been proposed for noninvasive diagnosis and grading of hepatic fibrosis across its entire spectrum of severity. State-of-the-art MR imaging-based techniques in current practice and in development for noninvasive assessment of liver fibrosis include conventional contrast material-enhanced MR imaging, double contrast-enhanced MR imaging, MR elastography, diffusion-weighted imaging, and MR perfusion imaging.