Normative values for magnetic resonance elastography–based liver stiffness in a healthy population

Introduction

Chronic liver disease is an important problem in modern medicine. In most cases, chronic liver disease progresses from the stage of fibrosis to liver cirrhosis, ultimately resulting in hepatic failure. Risk factors for the development of cirrhosis include hepatitis B and C virus infection, chronic alcohol abuse, and nonalcoholic fatty liver disease, which is closely related to the rapidly growing obesity epidemic.^1-3 Despite the implementation of new forms of therapy for some chronic liver diseases such as hepatitis C, the number of deaths from cirrhosis and its complications remains high.^4,5 The search for new drugs that could inhibit and reverse the ongoing process of fibrosis in the liver tissue requires reliable, sensitive, and repeatable tools that allow a frequent and safe assessment of the fibrotic process in hepatic tissue. Available imaging and laboratory methods enable a diagnosis of cirrhosis, but their diagnostic performance in detecting and assessing earlier stages of fibrosis is unsatisfactory. Liver biopsy as a diagnostic standard in hepatology is of limited value in the monitoring of the fibrotic process, owing to its invasive nature and potentially serious complications. For these reasons, patients are reluctant to undergo liver biopsy and repeat it frequently.^6-13 Therefore, many centers are searching for new diagnostic methods that could reliably replace liver biopsy.

Magnetic resonance elastography (MRE) is a dynamically developing noninvasive method that allows an assessment of liver fibrosis stages.^14-16 This test enables a quantitative evaluation of the mechanical properties of tissues by mechanical wave propagation analysis. The images are obtained during breath holding lasting about 15 seconds. The images reveal the propagation of the generated low-frequency shear waves (60 Hz) in the liver tissue and are automatically processed to create quantitative tissue stiffness maps called elastograms. Chronic liver damage causes progressive fibrosis of the organ, which increases its stiffness. The stiffness measurement closely parallels the progression of the fibrotic process. Multiple studies have demonstrated that MRE is a reliable, safe, and noninvasive diagnostic tool that can be used to monitor the extent of liver damage in patients with chronic liver disease.^14-16 However, a credible assessment of an elastogram in a sick person is possible only by referring to normal liver stiffness values obtained by elastography in healthy individuals.

Given the substantial existing literature on MRE, there are surprisingly few published studies of normative liver stiffness values in healthy populations. A study of liver tissue stiffness in healthy Asian volunteers was reported.¹⁷ However, published data on liver stiffness in healthy non–Asian populations are based only on small study groups.¹⁸ Therefore, we aimed to assess the MRE-based liver tissue stiffness in a healthy European population.

Patients and methods

Study population

A total of 102 white healthy volunteers (mean [SD] age, 21.5 [1.66] years; range, 20–28; 66 women [64.7%] and 36 men [35.3%]) were included in the study. The inclusion criteria were as follows: no contraindications to magnetic resonance imaging (MRI), no history of liver disease, no family history of chronic liver disease, normal liver enzyme levels, and no medication. All participants were on a normal diet and declared an alcohol consumption of less than 140 g/wk (men) and 70 g/wk (women). All volunteers provided written informed consent to participate in the study.

Magnetic resonance elastography tests were performed in the MRI laboratory of the Centre for Innovative Research in Medical and Natural Sciences (Medical Faculty of the University of Rzeszów, Poland) from December 2017 to May 2018.

Magnetic resonance elastography

A 1.5T whole-body magnetic resonance scanner (Optima, GE Healthcare, Milwaukee, Wisconsin, United States) was used to perform the MRE scans. Elastography examinations were performed on an empty stomach or 6 hours after the last meal. Volunteers were allowed to drink plain clean water up to 2 hours before the examination. The test was performed with patients in a supine position.

The MRE system includes special acquisition and processing software, as well as hardware consisting of an active and passive driver. The passive driver is a small plastic drum-like device that is placed against the body to transmit mechanical waves into the tissue. In this study, the passive driver was placed on the upper abdomen overlying the right lobe of the liver, with the center of the driver at the level of the xiphisternum. The passive driver was held in place with an elastic abdominal binder strap. The position of the passive driver was chosen so that the largest part of the liver was directly under the driver. Axial MRE slices were acquired (Figure 1A); slices were planned on the axial and coronal scout and T2-weighted images such that slices were obtained from the largest cross-section of the liver. For consistency, all sequences were acquired in breath-hold and in end-expiration. Volunteers were instructed to hold their breath in end-expiration for reproducibility of the position of the slice. No intravenous contrast was given.

The passive driver was connected via flexible tubing to the active driver unit, which was located outside the MRI room. The active driver unit generates pressure pulses (at a 60-Hz frequency in this study) that are conducted to the passive driver via flexible tubing.

The MRE images were acquired using a 16-channel abdominal phase array coil. Shear-wave imaging was conducted using a modified 2-dimensional gradient-recalled echo-based pulse sequence with the following parameters: matrix size, 256×256; slice thickness, 10 mm; repetition time, 33 ms; echo time, 20 ms; flip angle, 30°; and time steps, 4. The resulting wave images (Figure 1B) were then automatically processed by the scanner, using a manufacturer-provided 2-dimensional direct inversion algorithm to generate quantitative images depicting tissue stiffness (elastograms) (Figure 1C).^14,19

Liver stiffness was measured by placing the regions of interest (on magnitude stiffness maps) at 4 anatomically different slices of the liver. Three regions of interest were placed in each slice, avoiding large vessels, gallbladder, liver edge, and motion artefacts areas, and the stiffness measurement was recorded (Figure 1C). The mean liver stiffness value was calculated automatically and given in kPa.

Fat fraction was measured using a special technique provided by the manufacturer, called “IDEAL-IQ”, which slices through the entire liver during a single breath-hold at 6 different echo times. The data are automatically processed to generate quantitative maps showing fat fraction.

Statistical analysis

Liver stiffness values were presented as mean (SD). The Pearson correlation test was performed to assess the correlation of liver stiffness with age, estimated fat fraction percent of the liver, and body mass index (BMI). The χ² test was performed to identify significant differences between male and female volunteers. Statistical significance was assumed at a P value of less than 0.05. Statistical analysis was performed using the MedCalc software (version 18.9.1; Mariakerke, Belgium).

Ethics

The aim of the study was explained to all volunteers, and written informed consent was obtained from every individual participating in the study. The study protocol was approved by the Ethics Committee at the Medical Faculty of the University of Rzeszów (No. 8/10/2016) and conformed to the ethical guidelines of the 1975 Declaration of Helsinki (6th revision, 2008).

Results

All MRE exams were successfully completed. None of the volunteers reported any discomfort during the procedure.

The liver stiffness value in healthy volunteers ranged from 1.37 to 2.66 kPa, with a mean (SD) of 2.14 (0.28) kPa (95% CI, 2.09–2.20) (Figure 2). Liver stiffness values were similar between female and male volunteers (mean [SD], 2.14 [0.3] kPa; 95% CI, 2.07–2.22 and mean [SD], 2.13 [0.25] kPa; 95% CI, 2.05–2.22; respectively; P = 0.22) (Figure 3).

Liver stiffness values did not correlate with age (r = –0.03, P = 0.76) (Figure 4). The estimated fat fraction in the population ranged from 1.5% to 6.5%, and there was no correlation between the estimated fat content in the liver and mean liver stiffness (r = –0.02; 95% CI, –0.21 to 0.18; P = 0.85) (Figure 5). Similarly, liver stiffness did not correlate with BMI (r = 0.001; 95% CI, –0.19 to 0.20; P = 0.99) (Figure 6).

Discussion

In order to correctly interpret liver stiffness values in patients, it is important to determine the reference range in a healthy population, taking into account potential population differences. Ultrasound elastography studies have suggested that normal liver stiffness values may be slightly lower in Asian populations compared with white populations.^20-23

The present study of 102 volunteers is the largest study performed to date assessing liver stiffness in a healthy white European population. Liver stiffness values in our healthy volunteers ranged from 1.37 to 2.66 kPa, with a mean value of 2.14 kPa. In studies from the United States, the values ranged from 2.1 to 2.44 kPa.^14,19,24,25 Hines et al^24,25 reported higher liver stiffness in an American population (mean [SD], 2.44 [0.06] kPa) than that observed in our study. However, Rouviere et al,²⁶ who investigated 12 healthy volunteers, reported a mean (SD) liver stiffness value of 2.0 (0.3) kPa, which is consistent with our results.

Liver stiffness examinations in Asian volunteers were performed in small groups.^27,28 In a study performed in 5 living liver transplant donors, the stiffness was 2.20 kPa.²⁸ This value is comparable to that obtained in the present study. Venkatesh et al¹⁷ reported a slightly lower mean liver stiffness value in Asians (2.09 kPa; range, 1.68–2.48 kPa). In another study, which included 10 Asian volunteers, no stiffness value was reported.²⁷

Corpechot et al²⁹ demonstrated lower ultrasound elastography liver stiffness in women. Lee et al³⁰ examined a group of 49 liver donors and found no correlation between liver stiffness and sex. In our study, we found no significant differences in liver stiffness between men and women. We also found no correlation between liver stiffness and age, which is consistent with other reports.^24,25,31

A limitation of our study is that the volunteers were within a narrow age range (20–28 years). Although we found no correlation between liver stiffness and age, future studies should include individuals within a wider age range to confirm the results.

The strength of the study is its objectivity. Normal liver enzyme values were part of the inclusion criteria, which significantly contributes to the reliability of our results. In addition, we included the largest study group compared with all other reports available in the literature.

In conclusion, liver stiffness in a healthy European population did not exceed 2.66 kPa and is not influenced by sex, BMI, or fat content. We believe that our results will contribute to the establishment of normative liver stiffness values, which will translate into a wider application of MRE in clinical practice.