Correlation between qualitative and semi-quantitative ultrasound assessment of diffuse fatty liver disease: A case-control study

Abstract

Objective:

To study the relationship between qualitative and semi-quantitative assessment of diffuse liver steatosis in ultrasound.

Patients and Methods:

This was a case-control study, conducted in the Campus University Hospital Centre of Lome (Togo) over a 3-month period. It included 40 patients showing ultrasonographic signs of diffuse hepatic steatosis and 40 volunteers (healthy) whose echostructure and echogenicity of the hepatic parenchyma were normal. The B-mode sonographic grade of steatosis was compared with the hepatorenal echogenicity gradient and the ultrasound attenuation coefficient.

Results:

The average body mass index in patients was 30.87 ± 4.65 kg/m² versus 24.25 ± 4.30 kg/m² in the healthy group (p < 0.00001). Hepatomegaly was observed in 57.5% of the patients versus 17.5% in the healthy group (p = 0.0005). The average hepatorenal echogenicity ratio was 1.18 ± 0.07 in patients versus 1.01 ± 0.03 in the healthy group (p < 0.00001). The average difference in hepatorenal echogenicity was 9.30 ± 3.41 dB in patients versus 1.52 ± 1.07 dB in the healthy group (p < 0.00001). The attenuation of ultrasound waves increased with the grade of steatosis, averaging 0.08 ± 0.23 dB/cm/MHz (ranging from −0.33 to 0.61 dB/cm/MHz) in patients versus −0.24 ± 0.21 (ranging from −0.69 to 0.19 dB/cm/MHz) in the healthy group (p < 0.00001).

Conclusion:

Despite the advancements in new ultrasound technologies today, qualitative methods continue to be effective for the detection of hepatic steatosis and could prove useful in monitoring the effectiveness of hepatic steatosis treatment.

Introduction

Hepatic steatosis has become the most common form of chronic liver disease over the last decade. It is defined as the excessive accumulation of fat, in the form of triglycerides, within hepatocytes. ¹ This accumulation can have a diffuse or focal distribution within the liver parenchyma. Typically, it results from chronic alcohol intoxication (alcoholic liver steatosis), but it can exist independently of alcoholism, in which case it is termed non-alcoholic liver steatosis. ¹ The overall prevalence of non-alcoholic hepatic steatosis is 25% worldwide, with the lowest prevalence in Africa (13.5%) and the highest in the Middle East (31.8%) and South America (30.4%).^2–4

The frequency of this disease and the high risk of its progression to cirrhosis make liver steatosis a major public health issue. ⁵ Therefore, it is necessary to have simple tools for screening and quantification. Histology, once considered the gold standard, is no longer considered as such for detecting and quantifying diffuse fatty liver disease. ⁶ In fact, apart from its invasive nature and the morbidity risks associated with biopsy, it only samples a fraction of the liver, disregarding the condition of the rest of the parenchyma. As such, there is increasing recourse to medical imaging methods, which allow complete liver analysis. Among these, magnetic resonance imaging (MRI) is the most sensitive modality, followed by computed tomography (CT) and ultrasound. ⁷ However, MRI is not always available and CT is highly radiating, making it unsuitable for surveillance. Thus, ultrasound is the most widely available modality, even practised in the most remote regions. Continuous advances in ultrasound technology now allow the detection and determination of the evolutionary stages of liver disease, including diffuse steatosis.^8–10

High-end machines capable of a quantitative evaluation of liver steatosis, through attenuation imaging and liver hardness measurement using elastography, are scarcely available. Therefore, qualitative evaluation is the most commonly practised method. The purpose of this study was to investigate the relationship between the qualitative and quantitative evaluation of diffuse liver steatosis by ultrasound.

Patients and methods

This study was a case-control study conducted at the Campus University Hospital Centre of Lome (Togo). The recruitment took place over a 3-month period from March 01 to May 29, 2022, and included consenting adults. We selected 40 patients with diffuse liver steatosis based on ultrasound findings and 40 volunteers without known liver disease, with normal hepatic parenchyma echogenicity and echotexture. Volunteers were chosen from subjects who underwent another ultrasound examination and had normal liver function tests. Adults were defined as individuals aged 18 years or older. The study received approval from the hospital’s medical advisory committee and was conducted following ethical standards in accordance with the Helsinki Declaration. Pregnant women, patients with ultrasound signs of liver cirrhosis (morphological distortions, irregular contours), ascites, ultrasound and/or biochemical signs of nephropathy, and ultrasound signs of any other hepatic parenchymal lesions or intrahepatic bile duct lesions were excluded from the study.

We used a GE Healthcare Logiq E ultrasound machine, which was put into service in March 2021, equipped with a multifrequency convex probe (4 C). For this study, the probe frequency was set at 4 MHz for all measurements. The examinations were performed by a single radiologist with more than 10 years of experience. The evaluation was conducted in two stages. First, a qualitative assessment was performed in real time. Echo-level measurements were retrospectively conducted by the same radiologist who was blinded to the results of the qualitative assessment.

To measure the echogenicity of the liver and renal parenchyma, we used a circular region of interest (ROI) with a circumference of 1–2 cm, avoiding bile ducts and intrahepatic vessels (for the liver) and pyramids and sinus (for the kidney). Measurement was taken at the same depth for both the liver and the right renal cortex (Figure 1). Subsequently, we calculated the hepatorenal echogenicity ratio (HRR) and difference based on these measurements. To calculate the HRR, the recorded echo level values (in dB) were converted to greyscale values using the formula: Y = 255 + (255/99) × NE (8), where Y is the greyscale value and NE is the echo level. For the calculation of ultrasound attenuation, we measured the liver’s echogenicity at 2 and 7 cm from the hepatic capsule along the same vertical line in the right midclavicular line (Figure 2). The ultrasound attenuation was calculated using the following formula: (echogenicity of the liver at 2 cm–echogenicity of the liver at 7 cm)/5 cm/4 MHz. On this same vertical line, we measured the thickness of the anterior abdominal wall from the skin to the hepatic capsule. The measurement of the anterior abdominal wall thickness was performed by gently pressing the ultrasound probe to limit extrinsic compression.

Figure 1.

Ultrasound image showing the technique for measuring echogenicity of the liver and renal cortex using the 1–2 cm ROI.

Figure 2.

Ultrasound image showing the method to measure liver echogenicity at 2 and 7 cm from the liver capsule (a). Image (b) illustrates the method of measuring the anterior abdominal wall thickness and the hepatic span.

The variables studied were age, sex, body mass index (BMI), anterior abdominal wall thickness, liver size, ultrasound grade of steatosis, HRR, hepatorenal echogenicity difference (HRD), and ultrasound attenuation in depth. We defined hepatomegaly as a liver with a span greater than 15 cm, measured on a sagittal section passing through the right renal kidney.^11,12 The distance between the lower edge and the highest point of the hepatic dome was measured, as illustrated in Figure 2. Based on qualitative analysis, hepatic steatosis was classified into three grades: ¹³ mild steatosis (grade 1) as a slight and diffuse increase in parenchymal echogenicity with normal visualisation of the diaphragmatic and portal vein borders; moderate steatosis (grade 2) as a moderate and diffuse increase in parenchymal echogenicity with slightly impaired visualisation of the portal vein wall and the diaphragm; and severe steatosis (grade 3) as a major and diffuse increase in parenchymal echogenicity with poor or no visualisation of the portal vein borders, diaphragm, and the posterior part of the right lobe.

Data were collected and entered using the EPI data software. Analysis was performed using the SPSS software, version 21. Qualitative variables were expressed as frequency and percentage, and quantitative variables were expressed as mean ± standard deviation. Analysis of variance or the Student’s t-test was used to compare data from groups of quantitative variables, and linear regression was employed to demonstrate the correlation between these quantitative variables. Data from groups of qualitative variables were compared using the Chi-square test. A significance level of 5% was established for the statistical tests.

Results

General characteristics of the population

We recorded 40 cases of steatosis (patients) and 40 subjects served as controls. The average age of all participants was 44.90 ± 14.09 years, ranging from 19 to 80 years. The average age among the patients was 46.78 ± 12.31 years (ranging from 26 to 80 years) versus 43.03 ± 15.60 years (ranging from 19 to 76 years) for the control group (p = 0.16). There was no significant difference in gender distribution between the two groups (p = 0.82). In the patient group, there were 22 females (55%) and 18 males (45%). The control group consisted of 20 females (50%) and 20 males (50%). The average BMI among the patients was 30.87 ± 4.65 versus 24.25 ± 4.30 kg/m² among the control group (p < 0.00001).

Ultrasound data

Based on the ultrasound appearance of the liver, we recorded normal liver (n = 40), diffuse hepatic steatosis grade 1 (n = 7), diffuse hepatic steatosis grade 2 (n = 24), and diffuse hepatic steatosis grade 3 (n = 9) (Figure 3).

Figure 3.

Ultrasound images showing the different grades of diffuse hepatic steatosis. No steatosis (a), steatosis grade 1 (b), steatosis grade 2 (c), and steatosis grade 3 (d).

The average liver size was 12.89 ± 4.75 cm in patients versus 11.78 ± 4.17 cm in the control group. Hepatomegaly was noted in 23 patients (57.5%) versus 7 (17.5%) in the control group (p = 0.0005). The average of the anterior abdominal wall thickness was 2.21 ± 0.54 cm in patients versus 1.68 ± 0.38 cm in the control group. This thickness increased with the grade of steatosis (p < 0.00001) as shown in Table 1. There was also a positive linear correlation between the anterior abdominal wall thickness and BMI (r = 0.97; p = 0.009).

Table 1.

Anterior abdominal wall thickness according to steatosis grade.

	Anterior abdominal wall thickness	p-Value
Control group (n = 40)	1.68 ± 0.38	<0.00001
Patients (n = 40)	2.21 ± 0.54
Steatosis grade 1 (n = 7)	2.01 ± 0.49
Steatosis grade 2 (n = 24)	2.20 ± 0.50
Steatosis grade 3 (n = 9)	2.41 ± 0.65

The average HRR was 1.18 ± 0.07 in patients versus 1.01 ± 0.03 in the healthy subjects (p < 0.00001). The average HRD was 9.30 ± 3.41 dB in patients versus 1.52 ± 1.07 dB in the control group (p < 0.00001) (Table 2).

Table 2.

Hepatorenal echogenicity gradient values by group.

	Mean ± SD	Minimum	75th percentile	95th percentile	Maximum
HRR
Patients	1.18 ± 0.07	1.06	1.21	1.33	1.39
Control group	1.01 ± 0.03	0.94	1.02	1.06	1.07
HRD (dB)
Patients	9.30 ± 3.41	3	11.1	15.91	17.2
Control group	1.52 ± 1.07	0	2	3.4	3.7

HRR: hepatorenal echogenicity ratio; HRD: hepatorenal echogenicity difference.

The average ultrasound wave attenuation in patients was 0.08 ± 0.23 dB/cm/MHz (ranging from −0.33 to 0.61 dB/cm/MHz) versus −0.24 ± 0.21 (ranging from −0.69 to 0.19 dB/cm/MHz) in the control group, with a statistically significant difference (p < 0.00001). This attenuation increased with the grade of steatosis, but this relationship was not statistically significant (p = 0.13) (Table 3).

Table 3.

Attenuation coefficient values by steatosis grade.

Steatosis grade	Ultrasound wave attenuation	p-value
	Mean ± SD (dB/cm/MHz)
Grade 1	0.03 ± 0.15	0.13
Grade 2	0.04 ± 0.20
Grade 3	0.24 ± 0.32

SD: standard deviation.

Discussion

In this study, the average liver size was 12.89 ± 4.75 cm in patients versus 11.78 ± 4.17 cm in the control group. Patients were more likely to exhibit hepatomegaly. Previous studies have made similar observations.^8,14 Also, the anterior abdominal wall thickness was greater in patients compared with the healthy subjects and increased significantly with the grade of steatosis. An abnormally thick anterior abdominal wall can indicate obesity. Indeed, other authors have reported this correlation between the thickness of the anterior abdominal wall and BMI.^15,16 Others have been more stringent, measuring only the subcutaneous adipose layer, and have also found this correlation.^17,18 In this study, the average BMI in patients was significantly higher than that in the healthy subjects, highlighting the significant association between hepatic steatosis and obesity. Most studies have also reported this significant association between hepatic steatosis and obesity.^19–21

Ultrasound diagnosis of hepatic steatosis is based on the increased absorption and reflection of ultrasound waves caused by fat overload. The hyperechoic appearance of the hepatic parenchyma is the marker of steatosis. Steatosis is qualitatively suggested by comparing the echogenicity of the liver with that of the renal cortex. This conventional method is subjective and based on the operator’s experience. Indeed, studies have reported that among patients with hepatic steatosis, the intra- and inter-reader correlation varied from 0.5 to 0.6 and from 0.4 to 0.5, respectively. ²² Moreover, they are not optimal for detecting mild steatosis. ²³ Qualitative assessment can be made more sensitive by calculating the ratio of signal intensity measured at the same depth on the liver and the right renal cortex. In this study, we found that the hepatorenal echogenicity gradient was significantly higher in patients than in the healthy subjects, at 1.18 versus 1.01 for HRR and 9.30 versus 1.52 dB for HRD. Von Volkmann et al. ⁸ found a similar result, at 6.2 versus 1.9 dB for HRD and 1.01 versus 0.9 for HRR. Osawa and Mori ²⁴ also reported that HRD was significantly higher in the group with hepatic steatosis than in the group with a normal-appearing liver (8.9 ± 2.0 dB vs. 2.5 ± 4.5 dB). They also established an HRD greater than 7.0 dB as a diagnostic criterion. According to our study, a threshold of 1.06 if using HRR or 3.4 dB if using HRD can be established to define hepatic steatosis. The threshold values for the HRR to affirm steatosis vary among authors. Webb et al. ²⁵ reported a threshold of 1.49 in a study correlating HRRs with histological grades. Marshall et al. ²⁶ reported that a hepatorenal index of 1.28 or more had a sensitivity of 100%, a specificity of 54%, a positive predictive value of 0.57, and a negative predictive value of 1.0 for identifying hepatic steatosis. Shiralkar et al. found a threshold of 1.34 by re-measuring the same sample from Marshall et al. ²⁶ using DICOM (Digital Imaging and Communications in Medicine) images. ²⁷ These observed differences between studies could be due to the differences related to the parameters of the ultrasound machines. The statistically significant difference between the gradients of the patients and healthy subjects suggests that they can be used to confirm diffuse liver steatosis.

Attenuation refers to the loss of energy when an ultrasound wave passes through a tissue and depends on the properties of the tissue and the frequency of the ultrasound. ²⁸ There are two main approaches to the assessment of liver steatosis using ultrasound attenuation measurement: ²⁹ the controlled attenuation parameter obtained with the impulse elastography device, using ultrasound guided by A-mode and imaging guided by B-mode ultrasound attenuation. In this study, the attenuation of ultrasound waves in patients was higher than that in the healthy subjects, with a statistically significant difference. This attenuation increased with the degree of steatosis, but this relationship was not statistically significant. Von Volkmann et al. ⁸ also found a statistically significant difference between the average deep ultrasound attenuation of patients and healthy subjects, at 3.9 (0.0−8 dB/cm/MHz) versus −1.0 (−4.0−4.2) dB/cm/MHz, respectively. Dioguardi Burgio et al. ³⁰ found that patients with steatosis of any grade had a higher attenuation coefficient than those without steatosis (mean 0.77 ± 0.13 vs. 0.63 ± 0.09 dB/cm/MHz, respectively). Other studies have also shown that ultrasound attenuation increases with fat infiltration of the liver.^29,31–34

Attenuation imaging and measurements of changes in backscatter and speed of sound differences encountered in a steatotic liver are novel tools designed to complement older technologies dedicated to liver exploration, with innovations to detect hepatic steatosis at an early stage. ²³ Although these new applications are used to quantify steatosis on ultrasound, it must be recognised that quantitative methods for measuring echogenicity are laborious and not always reliable. ³⁵ Some authors have even reported that in certain situations, the qualitative assessment of steatosis on ultrasound is more precise than quantitative estimation.^36,37 In addition, quantitative information from B-mode ultrasound images is sometimes limited, as ultrasound images are highly dependent on the device’s parameters.

Study limitations

The primary limitation of this study is that it did not assess the intra- and inter-operator variability in the evaluation of hepatic steatosis using ultrasound.

Conclusion

This study demonstrated a perfect correlation between qualitative and quantitative evaluations of hepatic steatosis on ultrasound. Quantitative methods for measuring echogenicity are laborious and require sophisticated machines. Qualitative assessment of liver steatosis is an easily accessible method, simple to perform, and cost-effective despite its operator-dependent nature. These conventional methods continue to be effective for the screening of hepatic steatosis and could prove useful in monitoring the effectiveness of hepatic steatosis treatment.