Repeatability of MR Elastography of Liver: A Meta-Analysis

MR Elastography of the Abdomen: Experimental Protocols

Application of MRE for noninvasive evaluation of renal fibrosis has great potential for noninvasive assessment in patients with chronic kidney disease (CKD). CKD leads to severe complications, which require dialysis or kidney transplant and could even result in death. CKD in native kidneys and interstitial fibrosis in allograft kidneys are the two major kidney fibrotic pathologies where MRE may be clinically useful. Both these conditions can lead to extensive morbidity, mortality, and high health care costs. Currently, biopsy is the standard method for renal fibrosis staging. This method of diagnosis is painful, invasive, limited by sampling bias, exhibits inter- and intraobserver variability, requires prolonged hospitalization, poses risk of complications and significant bleeding, and could even lead to death. MRE based methods can potentially be useful to noninvasively detect, stage, and monitor renal fibrosis, reducing the need for renal biopsy. In this chapter, we describe experimental procedure and step by step instructions to run MRE along with some illustrative applications. We also includes sections on how to perform data quality check and analysis methods.This publication is based upon work from the COST Action PARENCHIMA, a community-driven network funded by the European Cooperation in Science and Technology (COST) program of the European Union, which aims to improve the reproducibility and standardization of renal MRI biomarkers.

MR Elastography of the Abdomen: Basic Concepts

Magnetic resonance elastography (MRE) is an emerging imaging modality that maps the elastic properties of tissue such as the shear modulus. It allows for noninvasive assessment of stiffness, which is a surrogate for fibrosis. MRE has been shown to accurately distinguish absent or low stage fibrosis from high stage fibrosis, primarily in the liver. Like other elasticity imaging modalities, it follows the general steps of elastography: (1) apply a known cyclic mechanical vibration to the tissue; (2) measure the internal tissue displacements caused by the mechanical wave using magnetic resonance phase encoding method; and (3) infer the mechanical properties from the measured mechanical response (displacement), by generating a simplified displacement map. The generated map is called an elastogram.While the key interest of MRE has traditionally been in its application to liver, where in humans it is FDA approved and commercially available for clinical use to noninvasively assess degree of fibrosis, this is an area of active research and there are novel upcoming applications in brain, kidney, pancreas, spleen, heart, lungs, and so on. A detailed review of all the efforts is beyond the scope of this chapter, but a few specific examples are provided. Recent application of MRE for noninvasive evaluation of renal fibrosis has great potential for noninvasive assessment in patients with chronic kidney diseases. Development and applications of MRE in preclinical models is necessary primarily to validate the measurement against "gold-standard" invasive methods, to better understand physiology and pathophysiology, and to evaluate novel interventions. Application of MRE acquisitions in preclinical settings involves challenges in terms of available hardware, logistics, and data acquisition. This chapter will introduce the concepts of MRE and provide some illustrative applications.This publication is based upon work from the COST Action PARENCHIMA, a community-driven network funded by the European Cooperation in Science and Technology (COST) program of the European Union, which aims to improve the reproducibility and standardization of renal MRI biomarkers. This introduction chapter is complemented by another separate chapter describing the experimental protocol and data analysis.

Value of shear wave elastography with maximal elasticity in differentiating benign and malignant solid focal liver lesions

BACKGROUND

It is important to differentiate benign and malignant focal liver lesions (FLLs) accurately. Despite the wide use and acceptance of shear wave elastography (SWE), its value for assessing the elasticity of FLLs and differentiating benign and malignant FLLs is still investigational. Previous studies of SWE for FLLs used mean elasticity as the parameter to reflect the stiffness of FLLs. Considering the inhomogeneity of tumor stiffness, maximal elasticity (Emax) might be the suitable parameter to reflect the stiffness of FLLs and to differentiate malignant FLLs from benign ones.

AIM

To explore the value of SWE with Emax in differential diagnosis of solid FLLs.

METHODS

We included 104 solid FLLs in 95 patients and 50 healthy volunteers. All the subjects were examined using conventional ultrasound (US) and virtual touch tissue quantification(VTQ) imaging. A diagnosis of benign or malignant FLL was made using conventional US. Ten VTQ values were acquired after 10 consecutive measurements for each FLL and each normal liver, and the largest value was recorded as Emax.

RESULTS

There were 56 cases of malignant FLLs and 48 cases of benign FLLs in this study. Emax of malignant FLLs (3.29 ± 0.88 m/s) was significantly higher than that of benign FLLs (1.30 ± 0.46 m/s, P < 0.01) and that of livers in healthy volunteers (1.15 ± 0.17 m/s, P < 0.01). The cut-off point of Emax was 1.945, and the area under the curve was 0.978. The sensitivity and specificity of Emax were 92.9% and 91.7%, respectively, higher (but not significantly) than those of conventional US (80.4% for sensitivity and 81.3% for specificity). Combined diagnosis of conventional US and Emax using parallel testing improved the sensitivity to 100% with specificity of 75%.

CONCLUSION

SWE is a convenient and easy method to obtain accurate stiffness information of solid FLLs. Emax is useful for differential diagnosis of FLLs, especially in combination with conventional US.

MR elastography: Principles, guidelines, and terminology

Magnetic resonance elastography (MRE) is a phase contrast-based MRI technique that can measure displacement due to propagating mechanical waves, from which material properties such as shear modulus can be calculated. Magnetic resonance elastography can be thought of as quantitative, noninvasive palpation. It is increasing in clinical importance, has become widespread in the diagnosis and staging of liver fibrosis, and additional clinical applications are being explored. However, publications have reported MRE results using many different parameters, acquisition techniques, processing methods, and varied nomenclature. The diversity of terminology can lead to confusion (particularly among clinicians) about the meaning of and interpretation of MRE results. This paper was written by the MRE Guidelines Committee, a group formalized at the first meeting of the ISMRM MRE Study Group, to clarify and move toward standardization of MRE nomenclature. The purpose of this paper is to (1) explain MRE terminology and concepts to those not familiar with them, (2) define "good practices" for practitioners of MRE, and (3) identify opportunities to standardize terminology, to avoid confusion.

Successful response of primary sclerosing cholangitis and associated ulcerative colitis to oral vancomycin may depend on brand and personalized dose: report in an adolescent

Primary sclerosing cholangitis (PSC) is a rare, progressive liver disease characterized by cholestasis and bile duct fibrosis that has no accepted therapy known to delay or arrest its progression. We report a 23-year-old female patient who at age 14 was diagnosed with moderate pancolonic ulcerative colitis (UC) and at age 15 with small-duct PSC unresponsive to conventional therapy. The patient began single drug therapy with the antibiotic oral vancomycin (OVT) and achieved normalization of liver enzymes and resolution of UC symptoms with colonic mucosal healing. These improvements have persisted over 8 years. There has been no colon dysplasia, liver fibrosis or failure, bile duct stricture, or cancer. Of note, the patient's response was dependent on the brand of oral vancomycin capsule, as well as dose. This raised the questions of possible differences in bioequivalence of different commercial versions of the drug and whether this factor might play into the variability of efficacy seen in published trials. Evidence suggests that oral vancomycin both alters the intestinal microbiome and has immunomodulatory effects. Its striking effectiveness in this and other patients supports further investigation in randomized trials, with careful attention to its bioavailability profile in the gut.

A study of spleen shear-wave elastography in indirect prediction of liver fibrosis in patients with chronic hepatitis B

PURPOSE

To evaluate the value of spleen shear-wave elastography (sound touch elastography [STE], sound touch quantification [STQ]) in indirect prediction of liver fibrosis in patients with chronic hepatitis B (CHB).

METHODS

The Young's modulus (kPa) of spleen STE, STQ and liver FibroScan were measured in 112 patients with CHB. The final diagnosis was according to histological results from liver biopsy based on Scheure G/S scoring system and liver FibroScan was considered as a reference index of prediction efficiency. Grouped by the stage of liver fibrosis, data were analyzed by Spearman correlation analysis, Mann-Whitney test and receiver operating characteristic curve (ROC).

RESULTS

Spleen STE was positively correlated with the degree of liver fibrosis, but spleen STQ was not. STEmean and STEmax of spleen were statistically different between the groups categorized by S = 2, S = 3 and S = 4 (all p < 0.05), respectively. Spleen STEmean had the best predicting performance on staging liver fibrosis. The areas under the ROC (AUC) for spleen STEmean were 0.66 (95% confidence interval [CI], 0.56-0.76) for stage S = 2 or higher, 0.72 (95% CI, 0.60-0.83) for S = 3 or higher, 0.83 (95% CI, 0.74-0.92) for S = 4 (all P < 0.01). The differences between the AUC for spleen STEmean and liver FibroScan in liver fibrosis staging were not statistically significant when the groups categorized by S = 2 and S = 4 (P = 0.146 and P = 0.052). But when categorized by S = 3, the evaluating performance of liver FibroScan was better (P = 0.004).

CONCLUSION

STEmean of spleen is applicable in indirect predicting fibrosis stage in patients with CHB.

Deep Learning Algorithm for Automated Segmentation and Volume Measurement of the Liver and Spleen Using Portal Venous Phase Computed Tomography Images

Objective

Measurement of the liver and spleen volumes has clinical implications. Although computed tomography (CT) volumetry is considered to be the most reliable noninvasive method for liver and spleen volume measurement, it has limited application in clinical practice due to its time-consuming segmentation process. We aimed to develop and validate a deep learning algorithm (DLA) for fully automated liver and spleen segmentation using portal venous phase CT images in various liver conditions.

Materials and Methods

A DLA for liver and spleen segmentation was trained using a development dataset of portal venous CT images from 813 patients. Performance of the DLA was evaluated in two separate test datasets: dataset-1 which included 150 CT examinations in patients with various liver conditions (i.e., healthy liver, fatty liver, chronic liver disease, cirrhosis, and post-hepatectomy) and dataset-2 which included 50 pairs of CT examinations performed at ours and other institutions. The performance of the DLA was evaluated using the dice similarity score (DSS) for segmentation and Bland-Altman 95% limits of agreement (LOA) for measurement of the volumetric indices, which was compared with that of ground truth manual segmentation.

Results

In test dataset-1, the DLA achieved a mean DSS of 0.973 and 0.974 for liver and spleen segmentation, respectively, with no significant difference in DSS across different liver conditions (p = 0.60 and 0.26 for the liver and spleen, respectively). For the measurement of volumetric indices, the Bland-Altman 95% LOA was −0.17 ± 3.07% for liver volume and −0.56 ± 3.78% for spleen volume. In test dataset-2, DLA performance using CT images obtained at outside institutions and our institution was comparable for liver (DSS, 0.982 vs. 0.983; p = 0.28) and spleen (DSS, 0.969 vs. 0.968; p = 0.41) segmentation.

Conclusion

The DLA enabled highly accurate segmentation and volume measurement of the liver and spleen using portal venous phase CT images of patients with various liver conditions.

MR elastography of liver: current status and future perspectives

Non-invasive evaluation of liver fibrosis has evolved over the last couple of decades. Currently, elastography techniques are the most widely used non-invasive methods for clinical evaluation of chronic liver disease (CLD). MR elastography (MRE) of the liver has been used in the clinical practice for nearly a decade and continues to be widely accepted for detection and staging of liver fibrosis. With MRE, one can directly visualize propagating shear waves through the liver and an inversion algorithm in the scanner automatically converts the shear wave properties into an elastogram (stiffness map) on which liver stiffness can be calculated. The commonly used MRE method, two-dimensional gradient recalled echo (2D-GRE) sequence has produced excellent results in the evaluation of liver fibrosis in CLD from various etiologies and newer clinical indications continue to emerge. Advances in MRE technique, including 3D MRE, automated liver elasticity calculation, improvements in shear wave delivery and patient experience, are promising to provide a faster and more reliable MRE of liver. Innovations, including evaluation of mechanical parameters, such as loss modulus, displacement, and volumetric strain, are promising for comprehensive evaluation of CLD as well as understanding pathophysiology, and in differentiating various etiologies of CLD. In this review, the current status of the MRE of liver in CLD are outlined and followed by a brief description of advanced techniques and innovations in MRE of liver.

Multiparametric MR mapping in clinical decision-making for diffuse liver disease

Accurate diagnosis, monitoring and treatment decisions in patients with chronic liver disease currently rely on biopsy as the diagnostic gold standard, and this has constrained early detection and management of diseases that are both varied and can be concurrent. Recent developments in multiparametric magnetic resonance imaging (mpMRI) suggest real potential to bridge the diagnostic gap between non-specific blood-based biomarkers and invasive and variable histological diagnosis. This has implications for the clinical care and treatment pathway in a number of chronic liver diseases, such as haemochromatosis, steatohepatitis and autoimmune or viral hepatitis. Here we review the relevant MRI techniques in clinical use and their limitations and describe recent potential applications in various liver diseases. We exemplify case studies that highlight how these techniques can improve clinical practice. These techniques could allow clinicians to increase their arsenals available to utilise on patients and direct appropriate treatments.

Quantitative Imaging in Pediatric Hepatobiliary Disease

Pediatric hepatobiliary imaging is important for evaluation of not only congenital or structural disease but also metabolic or diffuse parenchymal disease and tumors. A variety of ultrasonography and magnetic resonance imaging (MRI) techniques can be used for these assessments. In ultrasonography, conventional ultrasound imaging as well as vascular imaging, elastography, and contrast-enhanced ultrasonography can be used, while in MRI, fat quantification, T2/T2^* mapping, diffusion-weighted imaging, magnetic resonance elastography, and dynamic contrast-enhanced MRI can be performed. These techniques may be helpful for evaluation of biliary atresia, hepatic fibrosis, nonalcoholic fatty liver disease, sinusoidal obstruction syndrome, and hepatic masses in children. In this review, we discuss each tool in the context of management of hepatobiliary disease in children, and cover various imaging techniques in the context of the relevant physics and their clinical applications for patient care.

Evaluation of MR elastography as a response parameter for transarterial chemoembolization of colorectal liver metastases

OBJECTIVE

The aim of this study was to evaluate magnetic resonance elastography (MRE) as a response parameter in patients who received transarterial chemoembolization (TACE) for the treatment of colorectal liver metastases.

MATERIALS AND METHODS

Forty-two patients (29 male patients; mean age, 61.5 years; range, 41-84 years) with repeated TACE therapy of colorectal liver metastases underwent on average 2 repetitive magnetic resonance imaging (MRI) and MRE exams in 4- to 6-week intervals using a 1.5-T scanner. MRE-based liver stiffness measurements were performed in normal liver parenchyma and in metastatic lesions. Moreover, the size of the liver metastases was assessed during treatment and compared with the results of the MRE analysis.

RESULTS

Liver metastases showed a significantly higher degree of stiffness compared with the normal liver parenchyma (p < 0.001). However, only a weak correlation was found between the lesion size and stiffness (r = - 0.32, p = 0.1). MRE analysis revealed an increase in stiffness of the colorectal liver metastases from 4.4 to 7.1 kPa after three cycles of TACE (p < 0.001). Also, the mean size of the metastases decreased from 17.0 to 11.3 cm2 (p < 0.001). Finally, the entire liver stiffness increased from 2.9 to 3.1 kPa over the three cycles of TACE therapy.

CONCLUSION

In conclusion, MRE showed a significant change in stiffness and size of liver metastases. Therefore, MRE may provide an added value for an evaluation of treatment response in patients with colorectal liver metastases undergoing TACE.

KEY POINTS

• MRE showed an increase in stiffness of the colorectal liver metastases during TACE therapy. • Liver metastases showed a significantly higher degree of stiffness compared with the normal liver parenchyma. • However, only a weak correlation was found between the lesion size and stiffness.

Advances in Magnetic Resonance Elastography of Liver

Magnetic resonance elastography (MRE) is the most accurate noninvasive technique in diagnosing fibrosis and cirrhosis in patients with chronic liver disease (CLD). The accuracy of hepatic MRE in distinguishing the severity of disease has been validated in studies of patients with various CLDs. Advanced hepatic MRE is a reliable, comfortable, and inexpensive alternative to liver biopsy for disease diagnosing, progression monitoring, and clinical decision making in patients with CLDs. This article summarizes current knowledge of the technical advances and innovations in hepatic MRE, and the clinical applications in various hepatic diseases.

Noninvasive Evaluation of Nonalcoholic Fatty Liver Disease

Nonalcoholic fatty liver disease (NAFLD) is the most prevalent liver diseases and can progress to advanced fibrosis and end-stage liver disease. Thus, intensive research has been performed to develop noninvasive methods for the diagnosis of nonalcoholic steatohepatitis (NASH) and fibrosis. Currently, no single noninvasive tool covers all of the stages of pathologies and conditions of NAFLD, and the cost and feasibility of known techniques are also important issues. Blood biomarkers for NAFLD may be useful to select subjects who need ultrasonography (US) screening for NAFLD, and noninvasive tools for assessing fibrosis may be helpful to exclude the probability of significant fibrosis and to predict advanced fibrosis, thus guiding the decision of whether to perform liver biopsy in patients with NAFLD. Among various methods, magnetic resonance-based methods have been shown to perform better than other methods in assessing steatosis as well as in detecting hepatic fibrosis. Many genetic markers are associated with the development and progression of NAFLD. Further well-designed studies are needed to determine which biomarker panels, imaging studies, genetic marker panels, or combinations thereof perform well for diagnosing NAFLD, differentiating NASH and fibrosis, and following-up NAFLD, respectively.

Diagnostic problems in two-dimensional shear wave elastography of the liver

Two-dimensional shear wave elastography (2D-SWE) is used in the clinical setting for observation of the liver. Unfortunately, a wide spectrum of artifactual images are frequently encountered in 2D-SWE, the precise mechanisms of which remain incompletely understood. This review was designed to present many of the artifactual images seen in 2D-SWE of the liver and to analyze them by computer simulation models that support clinical observations. Our computer simulations yielded the following suggestions: (1) When performing 2D-SWE in patients with chronic hepatic disease, especially liver cirrhosis, it is recommended to measure shear wave values through the least irregular hepatic surface; (2) The most useful 2D-SWE in patients with focal lesion will detect lesions that are poorly visible on B-mode ultrasound and will differentiate true tumors from pseudo-tumors (e.g., irregular fatty change); and (3) Measurement of shear wave values in the area posterior to a focal lesion must be avoided.

Magnetic Resonance Elastography of the Liver in Children and Adolescents: Assessment of Regional Variations in Stiffness

RATIONALE AND OBJECTIVES

We describe our experience in measuring parenchyma stiffness across the liver Couinaud segments in lieu of the conventional practice of using a single slice-wise "global" region-of-interest. We hypothesize that the heterogeneous nature of fibrosis can lead to regional stiffness within the organ, and that it can be reflected by Couinaud segment-based magnetic resonance elastography measurements.

MATERIALS AND METHODS

This retrospective study involved from 173 patients (116 males, 57 females, 1.0-22.5 years, 14.7 ± 3.5 years) who underwent exams between June 2017 and September 2018. Liver stiffness across the eight Couinaud segments was measured in addition to a single-slice global measurement by two analysts. Inter- and intrarater analysis was performed in a subset of 20 cases. Individual segment stiffness values, the average across the segments, and the coefficients of variation (CoV) were compared to global single-slice-derived values using linear and Lin's concordance correlation coefficients. Linear correlations between stiffness values versus age, gender, and body-mass-index (BMI) were also evaluated.

RESULTS

We observed CoVs ranging from 3.1%-79.2%, 17.2 ± 7.2%. The CoV was not correlated with age or BMI (r² < 0.01, p = 0.99 for both). The CoV did not differ between males (17.1 ± 5.6%) and females (17.3 ± 9.8%) (p = 0.88). There were no correlations between global stiffness versus age (r² = 0.02, p = 0.84) or BMI (r² = 0.03, p = 0.68). A range of 0.58-0.86 was observed for Lin's concordance correlation coefficient between segmental stiffness, the average stiffness across segments, and global stiffness. Segments II and VII had the highest frequency of being the stiffest Couinaud segment. The average stiffness across the segments correlated strongly with the single-slice global measurement (r² = 0.88, p< 0.01).

CONCLUSION

There exists potential variations in parenchyma stiffness across the liver Couinaud segments, which may reflect the heterogeneous nature of fibrosis. This variation can potentially provide additional diagnostic and clinical information.

Burden of Disease due to Nonalcoholic Fatty Liver Disease

Nonalcoholic fatty liver disease (NAFLD) is rapidly becoming the most common liver disease in both Western populations and other parts of the world. This review discusses the prevalence and incidence of NAFLD in various regions around the world. The methodology used to identify the epidemiology and classify the stages of the disease is described. The impact of the disease on individuals, looking at both liver-related and extrahepatic consequences of the disease, is then discussed. Finally, the economic and societal impact of the disease is discussed.

Clinical Utility of an Increase in Magnetic Resonance Elastography in Predicting Fibrosis Progression in Nonalcoholic Fatty Liver Disease

BACKGROUND AND AIMS

Cross-sectional studies have shown that magnetic resonance elastography (MRE) is accurate in the noninvasive detection of advanced fibrosis in nonalcoholic fatty liver disease (NAFLD). However, there are limited data on the longitudinal association between an increase in liver stiffness on MRE and fibrosis progression in NAFLD. Therefore, using a well-characterized prospective cohort of patients with biopsy-proven NAFLD, we aimed to examine the longitudinal association between a 15% increase in liver stiffness on MRE and fibrosis progression in NAFLD.

APPROACH AND RESULTS

This prospective cohort study included 102 patients (62.7% women) with biopsy-proven NAFLD who underwent contemporaneous MRE and liver biopsy at baseline followed by a repeat paired liver biopsy and MRE assessment. The primary outcome was odds of fibrosis progression by one or more stage as assessed by the Nonalcoholic Steatohepatitis Clinical Research Network histologic scoring system. The mean (±SD) of age and body mass index (BMI) were 52 (±14) years and 32.6 (±5.3) kg/m² , respectively. The median time interval between the two paired assessments was 1.4 years (interquartile range 2.15 years). The number of patients with fibrosis stages 0, 1, 2, 3, and 4 was 27, 36, 12, 17, and 10, respectively. In unadjusted analysis, a 15% increase in MRE was associated with increased odds of histologic fibrosis progression (odds ratio [OR], 3.56; 95% confidence interval [CI], 1.17-10.76; P = 0.0248). These findings remained clinically and statistically significant even after multivariable adjustment for age, sex, and BMI (adjusted OR, 3.36; 95% CI, 1.10-10.31; P = 0.0339). A 15% increase in MRE was the strongest predictor of progression to advanced fibrosis (OR, 4.90; 95% CI, 1.35-17.84; P = 0.0159).

CONCLUSIONS

A 15% increase in liver stiffness on MRE may be associated with histologic fibrosis progression and progression from early fibrosis to advanced fibrosis.

Newly Developed Methods for Reducing Motion Artifacts in Pediatric Abdominal MRI: Tips and Pearls

OBJECTIVE. The purpose of this article is to review established and emerging methods for reducing motion artifacts in pediatric abdominal MRI. CONCLUSION. Clearly understanding the strengths and limitations of motion reduction methods can enable practitioners of pediatric abdominal MRI to select and combine the appropriate techniques and potentially reduce the need for sedation and anesthesia.

Reproducibility of hepatic MR elastography across field strengths, pulse sequences, scan intervals, and readers

PURPOSE

To evaluate the reproducibility of hepatic MRE under various combinations of settings of field strength, pulse sequence, scan interval, and reader in non-alcoholic fatty liver disease (NAFLD) patients.

METHODS

Adult NAFLD patients were prospectively enrolled for serial hepatic MRE with 1.5 T using 2D GRE sequence and 3.0 T using 2D SE-EPI sequence on the same day and after 2 weeks, resulting a total of four MRE examinations per patient. Three readers with various levels of background knowledge in MRE technique and liver anatomy measured liver stiffness after a training session. Linear regression, Bland-Altman analysis, within-subject coefficient of variation, and reproducibility coefficient (RDC) were used to determine reproducibility of hepatic MRE measurement.

RESULTS

Twenty patients completed the MRE sessions. Liver stiffness through MRE showed pooled RDC of 26% (upper 95% CI 30.6%) and corresponding limits of agreement (LOA) within 0.55 kPa across field strengths, MRE sequences, and 2-week interscan interval in three readers. Small mean biases and narrow LOA were observed among readers (0.05-0.19 kPa ± 0.53).

CONCLUSION

The magnitude of change across combinations of scan parameters is within acceptable clinical range, rendering liver stiffness through MRE a reproducible quantitative imaging biomarker. A lower reproducibility was observed for measurements under different field strengths/MRE sequences at a longer (2 weeks) interscan interval. Operators should be trained to acquire region of interest consistently in repeat examinations.

Liver fibrosis imaging: A clinical review of ultrasound and magnetic resonance elastography

Liver fibrosis is a histological hallmark of most chronic liver diseases, which can progress to cirrhosis and liver failure, and predisposes to hepatocellular carcinoma. Accurate diagnosis of liver fibrosis is necessary for prognosis, risk stratification, and treatment decision-making. Liver biopsy, the reference standard for assessing liver fibrosis, is invasive, costly, and impractical for surveillance and treatment response monitoring. Elastography offers a noninvasive, objective, and quantitative alternative to liver biopsy. This article discusses the need for noninvasive assessment of liver fibrosis and reviews the comparative advantages and limitations of ultrasound and magnetic resonance elastography techniques with respect to their basic concepts, acquisition, processing, and diagnostic performance. Variations in clinical contexts of use and common pitfalls associated with each technique are considered. In addition, current challenges and future directions to improve the diagnostic accuracy and clinical utility of elastography techniques are discussed. Level of Evidence: 5 Technical Efficacy Stage: 2 J. Magn. Reson. Imaging 2020;51:25-42.

Evaluation of Quantitative Imaging Biomarkers for Early-phase Clinical Trials of Steatohepatitis in Adolescents

OBJECTIVES

Early-phase pediatric nonalcoholic fatty liver disease (NAFLD) clinical trials are designed with noninvasive parameters to assess potential efficacy. Increasingly, these parameters include magnetic resonance imaging (MRI)-derived proton density fat fraction (PDFF) and MR elastography (MRE)-derived shear stiffness as biomarkers of hepatic steatosis and fibrosis, respectively. Understanding fluctuations in these measures is essential for calculating trial sample sizes, interpreting results, and planning clinical drug trials in children with NAFLD. Lack of such data in children constitutes a critical knowledge gap. Therefore, the primary aim of this study was to assess whole-liver MRI-PDFF change in adolescents with nonalcoholic steatohepatitis (NASH) over 12 weeks.

METHODS

Adolescents 12 to 19 years with biopsy-proven NASH undergoing standard-of-care treatment were enrolled. Baseline and week-12 assessments of anthropometrics, transaminases, MRI-PDFF, and MRE stiffness were obtained.

RESULTS

Fifteen adolescents were included (mean age 15.7 [SD 2.9] years). Hepatic MRI-PDFF was stable over 12 weeks (mean absolute change -0.8%, P = 0.24). Correlation between baseline and week-12 values of MRI-PDFF was high (ICC = 0.97, 95% CI 0.90-0.99). MRE stiffness was stable (mean percentage change 2.7%, P = 0.44); correlation between baseline and week-12 values was moderate (ICC = 0.47; 95% CI 0-0.79). Changes in weight, BMI, and aminotransferases were not statistically significant.

CONCLUSION

In adolescents with NASH, fluctuations in hepatic MRI-PDFF and MRE stiffness over 12 weeks of standard-of-care were small. These data on the natural fluctuations in quantitative imaging biomarkers can serve as a reference for interventional trials in pediatric NASH and inform the interpretation and planning of clinical trials.

Magnetic Resonance Elastography in Primary Sclerosing Cholangitis: Interobserver Agreement for Liver Stiffness Measurement with Manual and Automated Methods

RATIONALE AND OBJECTIVE

Primary sclerosing cholangitis, a chronic liver disease causes heterogeneous parenchymal changes and fibrosis. Liver stiffness measurement (LSM) with magnetic resonance Elastography (MRE) may be affected by this heterogeneous distribution. We evaluated interobserver agreement of LSM in primary sclerosing cholangitis (PSC) with manual and automated methods to study the influence of heterogeneous changes.

MATERIALS AND METHODS

A total of 79 consecutive patients with PSC who had a liver MRI and MRE formed the study group. Three readers with 1-3 years' experience in MRE and a MRE expert (11 years' experience) independently performed LSM. Each reader manually drew free hand (fROI) and average (aROI) on stiffness maps. Automatic liver elasticity calculation (ALEC) was used to generate automated LSM. The expert fROI was the reference standard. Correlation analysis and absolute intra-class correlation coefficient (ICC) analysis was performed.

RESULTS

LSM data of 79 livers and 315 sections were evaluated. There was excellent ICC between expert and reader fROIs (0.989, 95% confidence interval, and 0.985-0.993) and aROIs (0.971, 95% confidence interval, and 0.953-0.983) and ALEC (0.972, 0.957-0.982) with fROI performing better. The areas measured with fROIs and ALEC had moderate ICC with Expert fROI (0.64 and 0.56, respectively) whereas aROI area had a poor ICC of 0.12. Comparison of multiple methods showed significant differences in LSM between expert fROI and aROI of two readers and no significant differences for fROIs of all three readers.

CONCLUSION

LSM with MRE in PSC patients shows excellent interobserver agreement with both fROI and aROI methods with better performance with fROI. fROI may therefore be preferred for LSM measurements in PSC.

Magnetic resonance imaging of obesity and metabolic disorders: Summary from the 2019 ISMRM Workshop

More than 100 attendees from Australia, Austria, Belgium, Canada, China, Germany, Hong Kong, Indonesia, Japan, Malaysia, the Netherlands, the Philippines, Republic of Korea, Singapore, Sweden, Switzerland, the United Kingdom, and the United States convened in Singapore for the 2019 ISMRM-sponsored workshop on MRI of Obesity and Metabolic Disorders. The scientific program brought together a multidisciplinary group of researchers, trainees, and clinicians and included sessions in diabetes and insulin resistance; an update on recent advances in water-fat MRI acquisition and reconstruction methods; with applications in skeletal muscle, bone marrow, and adipose tissue quantification; a summary of recent findings in brown adipose tissue; new developments in imaging fat in the fetus, placenta, and neonates; the utility of liver elastography in obesity studies; and the emerging role of radiomics in population-based "big data" studies. The workshop featured keynote presentations on nutrition, epidemiology, genetics, and exercise physiology. Forty-four proffered scientific abstracts were also presented, covering the topics of brown adipose tissue, quantitative liver analysis from multiparametric data, disease prevalence and population health, technical and methodological developments in data acquisition and reconstruction, newfound applications of machine learning and neural networks, standardization of proton density fat fraction measurements, and X-nuclei applications. The purpose of this article is to summarize the scientific highlights from the workshop and identify future directions of work.

Validated imaging biomarkers as decision-making tools in clinical trials and routine practice: current status and recommendations from the EIBALL* subcommittee of the European Society of Radiology (ESR)

Observer-driven pattern recognition is the standard for interpretation of medical images. To achieve global parity in interpretation, semi-quantitative scoring systems have been developed based on observer assessments; these are widely used in scoring coronary artery disease, the arthritides and neurological conditions and for indicating the likelihood of malignancy. However, in an era of machine learning and artificial intelligence, it is increasingly desirable that we extract quantitative biomarkers from medical images that inform on disease detection, characterisation, monitoring and assessment of response to treatment. Quantitation has the potential to provide objective decision-support tools in the management pathway of patients. Despite this, the quantitative potential of imaging remains under-exploited because of variability of the measurement, lack of harmonised systems for data acquisition and analysis, and crucially, a paucity of evidence on how such quantitation potentially affects clinical decision-making and patient outcome. This article reviews the current evidence for the use of semi-quantitative and quantitative biomarkers in clinical settings at various stages of the disease pathway including diagnosis, staging and prognosis, as well as predicting and detecting treatment response. It critically appraises current practice and sets out recommendations for using imaging objectively to drive patient management decisions.

Current Imaging Techniques for Noninvasive Staging of Hepatic Fibrosis

OBJECTIVE. The purpose of this article is to discuss quantitative methods of CT, MRI, and ultrasound (US) for noninvasive staging of hepatic fibrosis. Hepatic fibrosis is the hallmark of chronic liver disease (CLD), and staging by random liver biopsy is invasive and prone to sampling errors and subjectivity. Several noninvasive quantitative imaging methods are under development or in clinical use. The accuracy, precision, technical aspects, advantages, and disadvantages of each method are discussed. CONCLUSION. The most promising methods are the liver surface nodularity score using CT and measurement of liver stiffness using MR elastography or US elastography.

Soft Tissue Sarcoma Stiffness and Perfusion Evaluation by MRE and DCE-MRI for Radiation Therapy Response Assessment: A Technical Feasibility Study

Soft tissue sarcomas are a rare and heterogeneous group of malignancies that present significant diagnostic and therapeutic challenges. Patient stratification based on tumor aggressiveness and early therapeutic response based on quantitative imaging may improve prediction of treatment response and the evaluation of new treatment strategies in clinical trials. The purpose of this pilot study was to determine the technical feasibility of magnetic resonance elastography (MRE) and dynamic contrast-enhanced (DCE) MRI for the evaluation of sarcoma stiffness and perfusion in 9 patients with histologically confirmed sarcoma. Additionally, we assessed the feasibility of utilizing MRE and DCE-MRI for the early evaluation of response to radiation therapy in 4 patients to determine the utility of further evaluation in a larger cohort study. Tumor size, stiffness, and perfusion parameters all decreased from baseline at the time of the pre-surgery or follow-up MRI, and results were compared to pathology or conventional imaging. MRE and DCE-MRI may be useful for the quantitative evaluation of tumor stiffness and perfusion, and therapy response assessment in soft tissue sarcomas.

Machine Learning Prediction of Liver Stiffness Using Clinical and T2-Weighted MRI Radiomic Data

OBJECTIVE. The purpose of this study is to develop a machine learning model to categorically classify MR elastography (MRE)-derived liver stiffness using clinical and nonelastographic MRI radiomic features in pediatric and young adult patients with known or suspected liver disease. MATERIALS AND METHODS. Clinical data (27 demographic, anthropomorphic, medical history, and laboratory features), MRI presence of liver fat and chemical shift-encoded fat fraction, and MRE mean liver stiffness measurements were retrieved from electronic medical records. MRI radiomic data (105 features) were extracted from T2-weighted fast spin-echo images. Patients were categorized by mean liver stiffness (< 3 vs ≥ 3 kPa). Support vector machine (SVM) models were used to perform two-class classification using clinical features, radiomic features, and both clinical and radiomic features. Our proposed model was internally evaluated in 225 patients (mean age, 14.1 years) and externally evaluated in an independent cohort of 84 patients (mean age, 13.7 years). Diagnostic performance was assessed using ROC AUC values. RESULTS. In our internal cross-validation model, the combination of clinical and radiomic features produced the best performance (AUC = 0.84), compared with clinical (AUC = 0.77) or radiomic (AUC = 0.70) features alone. Using both clinical and radiomic features, the SVM model was able to correctly classify patients with accuracy of 81.8%, sensitivity of 72.2%, and specificity of 87.0%. In our external validation experiment, this SVM model achieved an accuracy of 75.0%, sensitivity of 63.6%, specificity of 82.4%, and AUC of 0.80. CONCLUSION. An SVM learning model incorporating clinical and T2-weighted radiomic features has fair-to-good diagnostic performance for categorically classifying liver stiffness.

3D MR elastography of the pancreas in children

PURPOSE

Early diagnosis of chronic pancreatitis (CP) remains elusive. Preliminary data suggest that MR elastography (MRE) may have diagnostic value in the identification of CP. We sought to measure pancreas stiffness by MRE in healthy children and to compare measured values to stiffness values in pediatric patients with acute recurrent pancreatitis (ARP) or CP.

METHODS

Under IRB approval, 49 healthy controls volunteered to be included, and 14 patients with ARP or CP that underwent 3D MRE on a 1.5T MR scanner were included in the study. A soft passive driver was utilized and vibrated at 40 Hz. Regions of interest for measurement of pancreatic stiffness were drawn by two blinded readers and statistical analysis were performed for comparisons between the two groups.

RESULTS

Mean age of the healthy controls was 11 ± 2.7 years and mean pancreas stiffness was 1.7 ± 0.3 (Reader 1) and 1.7 ± 0.3 (Reader 2) kPa. For patients with ARP or CP, mean age was 12.6 ± 4.4 years and mean pancreas stiffness was 0.9 ± 0.2 (Reader 1) and 1.1 ± 0.3 (Reader 2) kPa. Pancreas stiffness was significantly lower in patients with ARP and CP as compared to healthy controls (p < 0.001). Between readers, there was a strong and statistically significant agreement on measured pancreas stiffness (r = 0.81; p < 0.001). Bland-Altman difference analysis showed a mean bias of only 0.05 kPa (95% limits of agreement: - 0.49 to + 0.58) CONCLUSION: MRE of the pancreas can be performed in pediatric patients. Through this study, we have defined normal pancreas stiffness for children and have shown decreases in measured stiffness in patients with ARP or CP compared to healthy controls.

CLINICAL RELEVANCE

3D MRE of the pancreas offers a novel approach for detecting pancreatic disease based on changes in tissue mechanical properties.

Prediction of nonalcoholic fatty liver disease (NAFLD) activity score (NAS) with multiparametric hepatic magnetic resonance imaging and elastography

OBJECTIVES

To investigate the use of MR elastography (MRE)-derived mechanical properties (shear stiffness (|G*|) and loss modulus (G″)) and MRI-derived fat fraction (FF) to predict the nonalcoholic fatty liver disease (NAFLD) activity score (NAS) in a NAFLD mouse model.

METHODS

Eighty-nine male mice were studied, including 64 training and 25 independent testing animals. An MRI/MRE exam and histologic evaluation were performed. Pairwise, nonparametric comparisons and multivariate analyses were used to evaluate the relationships between the three imaging parameters (FF, |G*|, and G″) and histologic features. A virtual NAS score (vNAS) was generated by combining three imaging parameters with an ordinal logistic model (OLM) and a generalized linear model (GLM). The prediction accuracy was evaluated by ROC analyses.

RESULTS

The combination of FF, |G*|, and G″ predicted NAS > 1 with excellent accuracy in both training and testing sets (AUROC > 0.84). OLM and GLM predictive models misclassified 3/54 and 6/54 mice in the training, and 1/25 and 1/25 in the testing cohort respectively, in distinguishing between "not-NASH" and "definite-NASH." "Borderline-NASH" prediction was poorer in the training set, and no borderline-NASH mice were available in the testing set.

CONCLUSION

This preliminary study shows that multiparametric MRI/MRE can be used to accurately predict the NAS score in a NAFLD animal model, representing a promising alternative to liver biopsy for assessing NASH severity and treatment response.

KEY POINTS

• MRE-derived liver stiffness and loss modulus and MRI-assessed fat fraction can be used to predict NAFLD activity score (NAS) in our preclinical mouse model (AUROC > 0.84 for all NAS levels greater than 1). • The overall agreement between the histological-determined NASH diagnosis and the imaging-predicted NASH diagnosis is 80-92%. • The multiparametric hepatic MRI/MRE has great potential for noninvasively assessing liver disease severity and treatment efficacy.

Magnetic resonance elastography: basic principles, technique, and clinical applications in the liver

Magnetic resonance elastography (MRE) is a constantly advancing technique for assessment of stiffness of tissues with newer technology and sequences. It is being increasingly used for the assessment of liver fibrosis. In this article, we discuss the advantages of MRE over biopsy and noninvasive methods such as US elastography in the assessment of liver fibrosis. Image acquisition and interpretation of liver MRE is also discussed.

Does gadoxetate disodium affect MRE measurements in the delayed hepatobiliary phase?

OBJECTIVES

To assess if the administration of gadoxetate disodium (Gd-EOB-DTPA) significantly affects hepatic magnetic resonance elastography (MRE) measurements in the delayed hepatobiliary phase (DHBP).

METHODS

A total of 47 patients (15 females, 32 males; age range 23-78 years, mean 54.28 years) were assigned to standard hepatic magnetic resonance imaging (MRI) with application of Gd-EOB-DTPA and hepatic MRE. MRE was performed before injection of Gd-EOB-DTPA and after 40-50 min in the DHBP. Liver stiffness values were obtained before and after contrast media application and differences between pre- and post-Gd-EOB-DTPA values were evaluated using a Bland-Altman plot and the Mann-Whitney-Wilcoxon test. In addition, the data were compared with regard to the resulting fibrosis classification.

RESULTS

Mean hepatic stiffness for pre-Gd-EOB-DTPA measurements was 4.01 kPa and post-Gd-EOB-DTPA measurements yielded 3.95 kPa. We found a highly significant individual correlation between pre- and post-Gd-EOB-DTPA stiffness values (Pearson correlation coefficient of r = 0.95 (p < 0.001) with no significant difference between the two measurements (p =0.49)). Bland-Altman plot did not show a systematic effect for the difference between pre- and post-stiffness measurements (mean difference: 0.06 kPa, SD 0.81). Regarding the classification of fibrosis stages, the overall agreement was 87.23% and the intraclass correlation coefficient was 96.4%, indicating excellent agreement.

CONCLUSIONS

Administration of Gd-EOB-DTPA does not significantly influence MRE stiffness measurements of the liver in the DHBP. Therefore, MRE can be performed in the DHBP.

KEY POINTS

• MRE of the liver can reliably be performed in the delayed hepatobiliary phase. • Gd-EOB-DTPA does not significantly influence MRE stiffness measurements of the liver. • MRE performed in the delayed hepatobiliary-phase is reasonable in patients with reduced liver function.

Magnetic Resonance Elastography of Liver: Current Update

The first clinical application of magnetic resonance elastography (MRE) was in the evaluation of chronic liver disease (CLD) for detection and staging of liver fibrosis. In the past 10 years, MRE has been incorporated seamlessly into a standard magnetic resonance imaging (MRI) liver protocol worldwide. Liver MRE is a robust technique for evaluation of liver stiffness and is currently the most accurate noninvasive imaging technology for evaluation of liver fibrosis. Newer MRE sequences including spin-echo MRE and 3 dimensional MRE have helped in reducing the technical limitations of clinical liver MRE that is performed with 2D gradient recalled echo (GRE) MRE. Advances in MRE technology have led to understanding of newer mechanical parameters such as dispersion, attenuation, and viscoelasticity that may be useful in evaluating pathological processes in CLD and may prove useful in their management.This review article will describe the changes in CLD that cause an increase in stiffness followed by principle and technique of liver MRE. In the later part of the review, we will briefly discuss the advances in liver MRE.

Can MR elastography be used to measure liver stiffness in patients with iron overload?

Untreated hepatic iron overload causes hepatic fibrosis and cirrhosis and can predispose to hepatocellular carcinoma. MR elastography (MRE) provides a non-invasive means to measure liver stiffness, which correlates with liver fibrosis but standard gradient recalled echo (GRE)-based MRE techniques fail in patients with high iron due to very low hepatic signal. Short echo time (TE) 2D spin echo echoplanar imaging (SE-EPI)-based MRE may allow measurement of stiffness in the iron loaded liver. The purpose of this study was to describe the use of such an MRE sequence in patients undergoing liver iron quantification by MRI. In our preliminary study of 43 patients with mean LIC of 9.3 mg/g (range 1.8-21.5 mg/g), liver stiffness measurements could be made in 77% (33/43) of patients with a short TE, SE-EPI based MRE sequence. On average, mean LIC in patients with failed MRE was higher than in those with successful MRE (15.9 mg/g dry weight vs. 7.3 mg/g), but a cut-off value for successful MRE could not be established. Seven patients (21% of those with successful MRE) had liver stiffness values suggestive of the presence of significant fibrosis (> 2.49 kPa). A short TE, SE-EPI based MR elastography sequence allows successful measurement of liver stiffness in a majority of patients with liver iron loading, potentially allowing non-invasive screening for fibrosis.

Longitudinal correlations between MRE, MRI-PDFF, and liver histology in patients with non-alcoholic steatohepatitis: Analysis of data from a phase II trial of selonsertib

BACKGROUND & AIMS

Non-invasive tools for monitoring treatment response and disease progression in non-alcoholic steatohepatitis (NASH) are needed. Our objective was to evaluate the utility of magnetic resonance (MR)-based hepatic imaging measures for the assessment of liver histology in patients with NASH.

METHODS

We analyzed data from patients with NASH and stage 2 or 3 fibrosis enrolled in a phase II study of selonsertib. Pre- and post-treatment assessments included centrally read MR elastography (MRE)-estimated liver stiffness, MR imaging-estimated proton density fat fraction (MRI-PDFF), and liver biopsies evaluated according to the NASH Clinical Research Network classification and the non-alcoholic fatty liver disease activity score (NAS).

RESULTS

Among 54 patients with MRE and biopsies at baseline and week 24, 18 (33%) had fibrosis improvement (≥1-stage reduction) after undergoing 24 weeks of treatment with the study drug. The area under the receiver operating characteristic curve (AUROC) of MRE-stiffness to predict fibrosis improvement was 0.62 (95% CI 0.46-0.78) and the optimal threshold was a ≥0% relative reduction. At this threshold, MRE had 67% sensitivity, 64% specificity, 48% positive predictive value, 79% negative predictive value. Among 65 patients with MRI-PDFF and biopsies at baseline and week 24, a ≥1-grade reduction in steatosis was observed in 18 (28%). The AUROC of MRI-PDFF to predict steatosis response was 0.70 (95% CI 0.57-0.83) and the optimal threshold was a ≥0% relative reduction. At this threshold, MRI-PDFF had 89% sensitivity and 47% specificity, 39% positive predictive value, and 92% negative predictive value.

CONCLUSIONS

These preliminary data support the further evaluation of MRE-stiffness and MRI-PDFF for the longitudinal assessment of histologic response in patients with NASH.

LAY SUMMARY

Liver biopsy is a potentially painful and risky method to assess damage to the liver due to non-alcoholic steatohepatitis (NASH). We analyzed data from a clinical trial to determine if 2 methods of magnetic resonance imaging - 1 to measure liver fat and 1 to measure liver fibrosis (scarring) - could potentially replace liver biopsy in evaluating NASH-related liver injury. Both imaging methods were correlated with biopsy in showing the effects of NASH on the liver.

Acetyl-CoA Carboxylase Inhibitor GS-0976 for 12 Weeks Reduces Hepatic De Novo Lipogenesis and Steatosis in Patients With Nonalcoholic Steatohepatitis

BACKGROUND & AIMS

Increased de novo lipogenesis (DNL) contributes to the pathogenesis of nonalcoholic steatohepatitis (NASH). Acetyl-CoA carboxylase catalyzes the rate-limiting step in DNL. We evaluated the safety and efficacy of GS-0976, a small molecule inhibitor of acetyl-CoA carboxylase, in patients with NASH.

METHODS

In an open-label prospective study, patients with NASH (n = 10) received GS-0976 20 mg orally once daily for 12 weeks. NASH was diagnosed based on a proton density fat fraction estimated by magnetic resonance imaging (MRI-PDFF) ≥10% and liver stiffness by magnetic resonance elastography (MRE) ≥2.88 kPa. The contribution from hepatic DNL to plasma palmitate was measured by 14 days of heavy water labeling before and at the end of treatment. We performed the same labelling protocol in an analysis of healthy volunteers who were not given DNL (controls, n = 10). MRI-PDFF and MRE at baseline, and at weeks 4 and 12 of GS-0976 administration, were measured. We analyzed markers of liver injury and serum markers of fibrosis.

RESULTS

The contribution of hepatic DNL to plasma palmitate was significantly greater in patients with NASH compared with controls (43% vs 18%) (P = .003). After 12 weeks administration of GS-0976, the median hepatic DNL was reduced 22% from baseline in patients with NASH (P = .004). Compared with baseline, reductions in MRI-PDFF at week 12 (15.7% vs 9.1% at baseline; P = .006), liver stiffness by MRE (3.4 kPa vs 3.1 kPa at baseline; P = .049), TIMP metallopeptidase inhibitor 1 (275 ng/mL vs 244 ng/mL at baseline; P = .049), and serum level of alanine aminotransferase (101 U/L vs 57 U/L at baseline; P = .23) were consistent with decreased hepatic lipid content and liver injury. At week 12, 7 patients (70%) had a ≥30% decrease in MRI-PDFF.

CONCLUSION

In an open-label study, patients with NASH given GS-0976 for 12 weeks had reduced hepatic DNL, steatosis, and markers of liver injury. ClinicalTrials.gov no: NCT02856555.

Technical Note: In vivo Young's modulus mapping of pancreatic ductal adenocarcinoma during HIFU ablation using harmonic motion elastography (HME)

PURPOSE

Noninvasive quantitative assessment of coagulated tissue during high-intensity focused ultrasound (HIFU) ablation is one of the essential steps for tumor treatment, especially in such cases as the Pancreatic Ductal Adenocarcinoma (PDA) that has low probability of diagnosis at the early stages and high probability of forming solid carcinomas resistant to chemotherapy treatment at the late stages.

METHODS

Harmonic motion elastography (HME) is a technique for the localized estimation of tumor stiffness. This harmonic motion imaging (HMI)-based technique is designed to map the tissue Young's modulus or stiffness noninvasively. A focused ultrasound (FUS) transducer generates an oscillating, acoustic radiation force in its focal region. The two-dimensional (2D) shear wave speed, and consequently the Young's modulus maps, is generated by tracking the radio frequency (RF) signals acquired at high frame rates. By prolonging the sonication for more than 50 s using the same methodology, the 2D Young's modulus maps are reconstructed while HIFU is applied and ablation is formed on PDA murine tumors.

RESULTS

The feasibility of this technique in measuring the regional Young's modulus was first assessed in tissue-mimicking phantoms. The contrast-to-noise ratio (CNR) was found to be higher than 11.7 dB for each 2D reconstructed Young's modulus map. The mean error in this validation study was found to be equal to less than 19%. Then HME was applied on two transgenic mice with pancreatic ductal adenocarcinoma tumors. The Young's modulus median value of this tumor at the start of the HIFU application was equal to 2.1 kPa while after 45 s of sonication it was found to be approximately three times stiffer (6.7 kPa).

CONCLUSIONS

The HME was described herein and showed its capability of measuring tissue stiffness noninvasively by measuring the shear wave speed propagation inside the tissue and reconstructing a 2D Young's modulus map. Application of the methodology in vivo and during HIFU were thus reported here for the first time.

Assessment of Nonalcoholic Fatty Liver Disease Progression in Children Using Magnetic Resonance Imaging

OBJECTIVE

To assess liver disease progression using paired magnetic resonance imaging (MRI) measurements of liver fat fraction (FF) and stiffness.

STUDY DESIGN

Retrospective cohort study including patients with nonalcoholic fatty liver disease who had undergone repeat MRI studies. Descriptive statistics were used, as well as Pearson or Spearman correlation when appropriate. Mixed model analyses were used to determine relationships between liver FF/stiffness and predictor variables.

RESULTS

Sixty-five patients (80% non-Hispanic, mean age 14 ± 3 years) were included. Time from first to last MRI was 27 ± 14 months. Over time, body mass index z score remained stable, and there were no significant differences in mean serum aminotransferases, insulin, glucose, triglycerides, low-density lipoprotein, and high-density lipoprotein (HDL) levels. However, the proportion of patients with alanine aminotransferase (ALT) < 50 U/L increased. MRI FF and stiffness decreased in 29% and 20% of patients, respectively, and increased in 25% and 22% of patients, respectively. There was a weak positive correlation between FF change and ALT change (r = 0.41, P = .053) and a moderate negative correlation between change in FF and change in serum HDL levels (r = -0.58, P = .004). After adjusting for HDL, increase in serum insulin was the only variable predictive of increase in FF (P = .061). There was no correlation between change in liver stiffness and change in ALT (r = .02, P = .910).

CONCLUSIONS

MRI-determined hepatic FF and stiffness improved in a minority of patients overtime. ALT levels were not reflective of the change in FF or stiffness. MRI-based imaging is complementary in the assessment of NAFLD progression.

Putting it all together: established and emerging MRI techniques for detecting and measuring liver fibrosis

Chronic injury to the liver leads to inflammation and hepatocyte necrosis, which when untreated can lead to myofibroblast activation and fibrogenesis with deposition of fibrous tissue. Over time, liver fibrosis can accumulate and lead to cirrhosis and end-stage liver disease with associated portal hypertension and liver failure. Detection and accurate measurement of the severity of liver fibrosis are important for assessing disease severity and progression, directing patient management, and establishing prognosis. Liver biopsy, generally considered the clinical standard of reference for detecting and measuring liver fibrosis, is invasive and has limitations, including sampling error, relatively high cost, and possible complications. For these reasons, liver biopsy is suboptimal for fibrosis screening, longitudinal monitoring, and assessing therapeutic efficacy. A variety of established and emerging qualitative and quantitative noninvasive MRI methods for detecting and staging liver fibrosis might ultimately serve these purposes. In this article, we review multiple MRI methods for detecting and measuring liver fibrosis and discuss the diagnostic performance and specific strengths and limitations of the various techniques.

Characterization of Small Renal Tumors With Magnetic Resonance Elastography: A Feasibility Study

OBJECTIVES

The aim of this study was to explore the feasibility of magnetic resonance elastography (MRE) for characterizing indeterminate small renal tumors (SRTs) as part of a multiparametric magnetic resonance (MR) imaging protocol.

MATERIALS AND METHODS

After institutional review board approval and informed consent were obtained, 21 prospective adults (15 men; median age, 55 years; age range, 25-72 years) with SRT were enrolled. Tumors (2-5 cm Ø) were imaged using 3-directional, gradient echo MRE. Viscoelastic parametric maps (shear wave velocity [c] and attenuation [α]) were analyzed by 2 independent radiologists. Interobserver agreement (Bland-Altman statistics and intraclass correlation coefficients) was assessed. Anatomical T2-weighted, dynamic contrast-enhanced (DCE) and diffusion sequences completed the acquisition protocol. Imaging parameters were compared between groups (Mann-Whitney U test).

RESULTS

Quality of MRE was good in 18 cases (mean nonlinearity <50%), including 1 papillary renal cell carcinoma and 1 metanephric adenoma. A cohort of 5 oncocytomas and 11 clear-cell renal cell carcinomas (ccRCCs) was analyzed for statistical differences. The MRE viscoelastic parameters were the strongest imaging discriminators: oncocytomas displayed significantly lower shear velocity c (median, 0.77 m/s; interquartile range [IQR], 0.76-0.79) (P = 0.007) and higher shear attenuation α (median, 0.087 mm; IQR, 0.082-0.087) (P = 0.008) than ccRCC (medians, 0.92 m/s and 0.066 mm; IQR, 0.84-0.97 and 0.054-0.074, respectively). T2 signal intensity ratio (tumor/renal cortex) was lower in oncocytomas (P = 0.02). The DCE and diffusion MR parameters overlapped substantially (P ≥ 0.1). Oncocytomas displayed a consistent MRE viscoelastic profile, corresponding to data point clustering in a bidimensional scatter plot. Values for MRE intraclass correlation coefficient were 0.982 for c and 0.984 for α, indicating excellent interobserver agreement.

CONCLUSIONS

Magnetic resonance elastography is feasible for SRT characterization; MRE viscoelastic parameters were stronger discriminators between oncocytoma and ccRCC than anatomical, DCE and diffusion MR imaging parameters.

Quantitative Elastography Methods in Liver Disease: Current Evidence and Future Directions

Chronic liver diseases often result in the development of liver fibrosis and ultimately, cirrhosis. Treatment strategies and prognosis differ greatly depending on the severity of liver fibrosis, thus liver fibrosis staging is clinically relevant. Traditionally, liver biopsy has been the method of choice for fibrosis evaluation. Because of liver biopsy limitations, noninvasive methods have become a key research interest in the field. Elastography enables the noninvasive measurement of tissue mechanical properties through observation of shear-wave propagation in the tissue of interest. Increasing fibrosis stage is associated with increased liver stiffness, providing a discriminatory feature that can be exploited by elastographic methods. Ultrasonographic (US) and magnetic resonance (MR) imaging elastographic methods are commercially available, each with their respective strengths and limitations. Here, the authors review the technical basis, acquisition techniques, and results and limitations of US- and MR-based elastography techniques. Diagnostic performance in the most common etiologies of chronic liver disease will be presented. Reliability, reproducibility, failure rate, and emerging advances will be discussed. ^© RSNA, 2018 Online supplemental material is available for this article.

Computational Modeling in Liver Surgery

The need for extended liver resection is increasing due to the growing incidence of liver tumors in aging societies. Individualized surgical planning is the key for identifying the optimal resection strategy and to minimize the risk of postoperative liver failure and tumor recurrence. Current computational tools provide virtual planning of liver resection by taking into account the spatial relationship between the tumor and the hepatic vascular trees, as well as the size of the future liver remnant. However, size and function of the liver are not necessarily equivalent. Hence, determining the future liver volume might misestimate the future liver function, especially in cases of hepatic comorbidities such as hepatic steatosis. A systems medicine approach could be applied, including biological, medical, and surgical aspects, by integrating all available anatomical and functional information of the individual patient. Such an approach holds promise for better prediction of postoperative liver function and hence improved risk assessment. This review provides an overview of mathematical models related to the liver and its function and explores their potential relevance for computational liver surgery. We first summarize key facts of hepatic anatomy, physiology, and pathology relevant for hepatic surgery, followed by a description of the computational tools currently used in liver surgical planning. Then we present selected state-of-the-art computational liver models potentially useful to support liver surgery. Finally, we discuss the main challenges that will need to be addressed when developing advanced computational planning tools in the context of liver surgery.