Obesity has emerged as a chronic health condition that has garnered significant attention from the international community. The World Health Organization's latest report indicates a staggering prevalence of over 1.9 billion overweight adults globally, with over 650 million individuals classified as obese. The gravity of this epidemic demands immediate and sustained action to mitigate the social, economic, and health consequences associated with obesity (1). Obesity is associated with numerous comorbidities, such as metabolism-associated fatty liver disease (MAFLD), cardiovascular diseases, type 2 diabetes, hypertension, and dyslipidemia. Hepatic steatosis is commonly associated with obesity, and its prevalence can be as high as 80% in extremely obese individuals (2). In a study with 1,000 morbidly obese patients, more than 95% had MAFLD (3). The contribution of obesity and chronic low-grade systemic inflammation to the progression from hepatic steatosis to fibrosis is a matter of considerable significance. These factors are known to exert significant influence on the development of liver disease, thus warranting further scrutiny in this context (4). The recalcitrance of obesity portends a surge in the incidence of MAFLD, and as this cohort advances in age, a considerable proportion will encounter hepatic cirrhosis and end-stage liver maladies.

Severe obesity presents as a chronic condition that is resistant to treatment. While it is commonly believed that a combination of dietary adjustments and heightened physical exertion may yield considerable long-term advantages, such efforts prove challenging to uphold. Regrettably, over 95% of patients who attempt conservative interventions fail to attain desired outcomes. Bariatric surgery may be more effective than medical and lifestyle interventions for the most severe form of obesity to have major and sustainable weight loss, with RYGB being the most popular surgical procedure (5). Patients undergoing this surgery not only lose a substantial amount of weight, but also are clearly improved in clusters of the metabolic syndrome.

The widespread employment of BMI as a metric for assessing the postoperative efficacy of clinical bariatric surgery is an established practice. However, this measure fails to discriminate between subcutaneous adipose tissue and visceral fat as well as muscle mass (6). The changes of laboratory values (like ALT, AST, HDL-c and LDL-c) after surgery are also the point of assessment of surgical effect (7), but lack of standardization. While liver biopsy remains the gold standard for diagnosing and grading hepatic steatosis, its invasiveness, lack of reproducibility, and potential complications preclude its use in ongoing monitoring efforts. The accurate grading of hepatic steatosis is crucial for the appropriate management of patients. While MRI-PDFF estimation offers a noninvasive alternative to liver biopsy, its high cost and limited availability constrain its global implementation (8–10). As an alternate noninvasive option, ultrasound (US) holds promise due to its widespread access and cost-effectiveness. B-mode US imaging based on the amplitude of the envelope of beamformed radiofrequency signals is routinely employed in clinics but suffers from operator-dependency, subjectivity, and low sensitivity to mild steatosis (11). While conventional US shows sensitivity and specificity ranges of 60%–94% and 66%–95%, respectively, it does not offer an adequate diagnostic tool for quantifying hepatic steatosis. Nonetheless, recent efforts have been made to develop QUS, which has the potential to enhance diagnostic accuracy and enable liver fat quantification (12, 13). QUS represents a promising application of this modality that may improve the diagnosis and management of hepatic steatosis.

In recent years, several QUS techniques have emerged as promising non-invasive approaches for assessing hepatic steatosis. These techniques employ non-image-based parameters such as the speed of sound, ultrasound attenuation, backscatter coefficient, and shear-wave dispersion to evaluate liver fat content, and have demonstrated a strong correlation with hepatic steatosis in previous studies (11, 14, 15). Despite their potential benefits, however, these QUS techniques also have certain limitations. For example, the hepatorenal index, which measures the ratio of liver to kidney echointensities on B-mode ultrasound, can quantitatively assess hepatic steatosis but may not be suitable for patients with renal disease or those with uneven distribution of steatosis (16). Controlled attenuation parameter(CAP) is only available on FibroScan, and this technique does not provide B-mode image during examing. To overcome these issues, researchers have proposed using quantitative US parameters derived from RF data analysis. Two such parameters include the attenuation coefficient (AC) based on TAI (AC-TAI) and scatter-distribution coefficient (SC) based on TSI (SC-TSI). AC-TAI indicates the slope of the US center frequency downshift with a depth that can be used to estimate acoustic attenuation (17). SC-TSI, on the other hand, reflects the local concentration and arrangement of US scatterers by indicating the average Nakagami parameters of the region of interest (ROI) (18). Unlike B-mode images, raw RF data contain frequency-dependent information of the US signal and offer additional diagnostic value. These data can be obtained by activating a specific acquisition mode. By using these parameters, researchers have demonstrated improved accuracy in diagnosing steatosis grades compared to qualitative visual scores on B-mode ultrasound (Figure 1).

The protocol is aimed to provide a method to assess hepatic steatosis with QUS techniques (TAI, TSI) in morbidly obese individuals before and after bariatric surgery, and to compare with anthropometric measurements, laboratory assessments and other imaging methods. Thereby, the application value of QUS techniques (TAI, TSI) in quantitatively assessing hepatic steatosis after bariatric surgery will be clarified.

Prior to the fomal study, we recruited 21 Chinese participants who satisfied: (1) age between 18 and 85 years old; (2) with suspected NAFLD, and (3) agreed to receive QUS, and MRI examination. They received ultrasonic examination (TAI, TSI, CAP) and MRI-PDFF (Figure 3).

The average BMI of all participants was 26.43, which meets the diagnostic criteria for overweight. More than 50% of patients (15/21, 71.43%) presented with symptom of right upper abdominal distension. For the value of MRI-PDFF, the average was 13.86%. MRI results from eight patients were below 10%. For MRI-PDFF ≥10%, a total of 13 patients were in this category (13, 62.9%).

All the data were analyzed in R (v4.3.1; R Core Team). A two-sided P < 0.05 was considered statistically significant. AC-TAI showed moderate correlations with MRI-PDFF (adjusted r = 0.632; P < 0.05). Linear correlation between SC-TSI and MRI-PDFF was not significant (P = 0.982) (Table 1). For MRI-PDFF ≥10%, SC-TSI showed moderate correlations with MRI-PDFF (adjusted r = 0.677; P < 0.05) (Figure 4). In a way, the results signified the feasibility of using QUS techniques for postoperative evaluation of bariatric surgery.

The rising prevalence of obesity has emerged as a significant public health challenge across the world, affecting both developed and developing nations. Among the numerous adverse consequences of obesity, an important correlation has been established between its degree and the incidence and severity of MAFLD. In fact, obesity has been found to increase the likelihood of developing MAFLD by 4.6 times (22). The natural course of MAFLD indicates that fibrosis manifests in around 32–37% of patients within 3–6 years, and approximately 12% of patients tend to progress to cirrhosis over 8–10 years (23).

Therapeutic interventions such as bariatric surgery have demonstrated potential in ameliorating steatosis, hepatic fibrosis, and necroinflammatory activity, especially for those with morbid obesity and NASH. Weiner et al. (24) conducted a study to evaluate the impact of bariatric surgery on the histological development of NASH. They discovered, through liver biopsy results, that 82.8% of patients exhibited complete regression of MAFLD. Dixon et al. (25) investigated the outcomes of weight loss following Roux-en-Y gastric bypass (RYGB) on liver histology and plasma aminotransferase levels (AST, ALT, and GGT) in 60 severely obese individuals. Results indicated that the mean weight loss was 31.5 ± 18 kg, and there was notable improvement in AST, ALT, GGT, lobular steatosis, inflammation, and fibrosis. Furuya et al. (26) enrolled a cohort of MAFLD patients undergoing RNYGB, and procured a wedge liver biopsy during the operation. The initial evaluation revealed that 67% of the patients had NASH, while 33% had steatosis, and cirrhosis was present in 5.5% of those with NASH. Following an average excess weight loss of 60%, steatosis and fibrosis disappeared in 84% and 75% of the patients respectively.

Continuous surveillance of hepatic steatosis following bariatric surgery is crucial for effective disease management and prompt therapeutic intervention, ultimately leading to improved health outcomes and reduced healthcare costs (10). Recently, several QUS approaches have emerged for the quantification of hepatic steatosis. The advantages of a US-based hepatic fat quantification system are numerous, including widespread availability, real-time functionality, and low cost. Recent studies have demonstrated that certain quantitative parameters derived from RF data analysis, which reflect the backscatter or attenuation of US beams, exhibit strong correlation with hepatic steatosis grades (27–29). Jeon et al. (30) investigated the diagnostic potential of QUS parameters in assessing hepatic steatosis among MAFLD patients, using MRI-PDFF as the reference standard. They found that both AC-TAI and SC-TSI were significantly correlated with MRI-PDFF (r = 0.659 and 0.727, respectively). For detecting hepatic fat contents ≥ 5%, the areas under the receiver operating characteristic (ROC) curves of AC-TAI and SC-TSI were 0.861 and 0.964, respectively. Bae et al. (31) on the other hand, evaluated the diagnostic performance of the normalized local variance ultrasound technique in detecting fatty liver, using histopathology as a reference standard. The areas under the ROC curve for detecting fatty livers of varying degrees were 0.911 for ≥S1, 0.974 for ≥S2, and 0.954 for ≥S3. This study aims to provide evidence supporting the feasibility of utilizing QUS techniques for liver fat quantification. Furthermore, we aim to prospectively evaluate the application value of QUS techniques in quantitatively assessing hepatic steatosis after bariatric surgery (32, 33).

Pereira et al. (34) conducted a comparison of noninvasive biochemical methods and FibroScan with histopathologic analysis to evaluate hepatic fibrosis in patients who underwent bariatric surgery one year prior. The results indicate that noninvasive liver fibrosis evaluation correlates highly with liver biopsy findings after one year of bariatric surgery. While QUS techniques (TAI, TSI) are more advanced ultrasound technologies, they can theoretically assess the degree of hepatic steatosis accurately after bariatric surgery. In our pre-experiment study, we preliminarily verified the feasibility of the QUS techniques in Chinese participants who suspected NAFLD. The results of the pre-experiment signified the feasibility of using QUS techniques for postoperative evaluation of bariatric surgery, making our study design reasonable.

Our study inevitably has some limitations. Specifically, we utilized MRI-PDFF rather than biopsy as the gold standard to correlate QUS parameters. Although previous research suggests that MRI-PDFF exhibits relatively high sensitivity and specificity in detecting fatty liver disease, there remains potential for incorrect diagnoses. In addition, although we conducted health education for participants before surgery, it was difficult to avoid that confounding factors (like life-style) may influence on results. Furthermore, confounding factors such as increased subcutaneous fat in individuals with higher BMI may influence QUS parameter results. With a larger patient population, we will summarize the imaging experience in hepatic steatosis evaluation by QUS techniques in obese people. We will also analyze the influence of scanning methods, imaging location and other influencing factors on the results, and explore how to improve the results.

In conclusion, this article has described methods for using QUS techniques (TAI, TSI) to assess hepatic steatosis and its changes in obese individuals before and after bariatric surgery. In addition, we compare QUS techniques to anthropometric measurements, laboratory assessments and other imaging methods. By this study, QUS techniques will be signed a widespread availability, real-time functionality, and low cost approach for assessing hepatic steatosis before and after bariatric surgery in obese individuals. Our pre-experiment results signified that using QUS techniques for postoperative evaluation of bariatric surgery is promising. Our study is the first one in China to investigate the application value of QUS techniques (TAI, TSI) in quantitatively assessing hepatic steatosis after bariatric surgery. Our study will provide strong evidence regarding whether obese people can truly benefit from QUS techniques in liver fat quantification.