Comparison of sonographic hepatorenal ratio and the degree of hepatic steatosis in magnetic resonance imaging-proton density fat fraction

Hepatic steatosis is a condition of excessive intrahepatocytic accumulation of fat. Steatosis is the hallmark of non-alcoholic fatty liver disease (NAFLD) and also occurs in patients with alcohol abuse, viral hepatitis, HIV and those receiving certain pharmacological treatment^(1,2). NAFLD is the most common chronic liver disease affecting approximately 30% of the general population^(2,3). Steatosis can progress to liver fibrosis and, eventually, cirrhosis, and it can also worsen the course of disease in patients with viral hepatitis^(4,5).

A quantitative tool for assessment and monitoring of hepatic steatosis is needed in clinical practice. Liver biopsy still remains the gold standard for the assessment of liver fat⁽⁶⁾. However, its routine clinical use is limited due to its invasiveness with a small potential risk of life-threatening complications, sampling errors and inter-observer variability^(7–9).

Noninvasive imaging techniques, including MR spectroscopy (¹H-MRS) and magnetic resonance imaging-proton density fat fraction (MRI-PDFF), can accurately measure hepatic fat. ¹H MRS is accepted as the noninvasive reference standard for liver fat quantification⁽¹⁰⁾.

PDFF measured by MRI is a direct measure of the fraction of mobile triglyceride protons relative to those in water⁽¹⁰⁾. MRI-PDFF proved to have excellent agreement with ¹H-MRS^(11–15) and to be closely correlated with liver histology^(14,16,17), as well as has been recently considered as a standardized MR biomarker of steatosis used in clinical practice and research. Despite the accuracy of the MR techniques, their use is restricted due to high operational cost and considerable time consumption.

Ultrasonography (US) is often the first imaging modality used to detect hepatic steatosis. However, conventional ultrasonography can provide only qualitative or semiquantitative assessment based on liver echogenicity, vessel visibility, and beam attenuation. Overall sensitivity and specificity of ultrasonography previously reported were about 60–94% and 66–97%, respectively⁽¹⁸⁾. Prior studies have investigated sonographic parameters in an attempt to overcome subjectivity and qualitative limitations, such as hepatic-renal differences⁽¹⁹⁾ and hepatorenal (HR) ratio^(20–25). The diagnostic value of the HR ratio varies among studies depending on different standard references and study populations; therefore further validation for clinical application is still needed. Moreover, a direct comparison between sonographic HR ratio and MRI-PDFF has not been performed yet.

The aim of this study was to evaluate the sonographic HR ratio for the diagnosis and quantification of hepatic steatosis using MRI-PDFF as reference.

The study was approved by the Institutional Review Board. No informed consent was required due to the retrospective nature of the study.

Ultrasonographic examination

All participants underwent upper abdominal examination with ultrasonography in B-mode (GE logiq E9 with a 1–6 MHz Broad-spectrum convex transducer) by board-certified radiologists. Various image parameters were adjusted for each examination. All images were transmitted to a Picture Archiving and Communication System (PACS; Fujifilm Synapse, Tokyo, Japan).

Ultrasound image analysis

All ultrasound image analyses were performed by two independent operators, using a region of interest (ROI) tool on standard workstation from DICOM images on PACS without using extra-image analysis software.

The first operator was a radiologist with more than 8 years of experience in advance abdominal imaging, and the second operator was a radiology resident. Both of them were blinded to patients’ clinical details. The hepatic and right renal ROIs were selected at the same depth along the focusing area near the center of the image to avoid image distortion effects and beam attenuation. An ROI in the liver parenchyma of 200–250 mm² was selected, excluding blood vessels, bile ducts or focal liver lesions. An ROI of the right renal cortex of 20–30 mm² was selected, excluding large vessels, renal sinus, masses or cysts. Portions of ROIs affected by artifacts were also excluded (Fig. 1).

Fig. 1.

The hepatic and right renal ROIs were selected at the same depth along the focusing area near the center of the image to avoid image distortion effects and beam attenuation

The sonographic HR ratio was calculated by dividing the mean echo intensity of the pixels within the selected hepatic ROI by those within the selected ROIs of the right renal cortex.

MRI examinations were performed using 3T GE Discovery MR750w. All participants underwent PDFF examination using complex-based, chemical shift–based, water and fat separation technique with low-flip angle, T1-weighted, 2D multiple-echo, spoiled gradient-echo sequence. The images were processed by using the software provided by the manufacturer (IDEAL-IQ; GE Healthcare) which created water, fat, R2* maps and PDFF parametric map representing fat mapping of the entire liver (Fig. 2). This technique provided correction of confounding factors on MR signal intensity, such as T1 bias, T2* decay, spectral complexity of fat, noise bias, and eddy currents⁽⁶⁾. (Detailed imaging parameters are as follows: TR7.6/TE3.6, flip angle 4°, bandwidth 90.91 Hz/pixel, number of excitations, 0.5; field of view 42 cm; section thickness, 8 mm; intersection gap, 0, matrix 192×160. We acquired data sets with eight different echo times that ranged from 1.25 to 11.83 msec.)

Fig. 2.

MR elastography (MRE) was also performed in all individuals with 60-Hz vibrations applied to the abdominal wall using a pneumatic actuator (MR-Touch; GE Healthcare), with two-dimensional gradient-echo MR elastography sequence and the direct inversion algorithm, previously described elsewhere⁽²⁶⁾. (Detailed imaging parameters were as follows: TR/TE, 1000.3/63 ms; bandwidth 250 Hz/pixel; field-of-view (FOV), 38 cm; matrix, 64 × 64; NEX, 2; slice thickness, 7.0 mm with a 2.5-mm intersection gap.)

A total of 179 patients were included in this study (82 males and 97 females; mean age of 53 years; range: 23–77 years). The indications for abdominal MR imaging included nonalcoholic fatty liver disease (n = 97), hepatitis C virus (HCV) infection (n = 59), psoriasis on methotrexate treatment (n = 4), HBV and HCV coinfection (n = 2), post liver transplantation with HCV infection (n = 2), post liver transplantation (n = 2), patients treated with chemotherapy (n = 1), unexplained elevation of liver enzymes (n = 1) and unknown etiology (n = 11).

Hepatic fat content by MRI-PDFF ranged from 1.5to 35.0%. Based on the diagnostic criteria for steatosis, 75 patients had no steatosis (75/179, 42%) and 104 patients had steatosis (104/179, 58%). Grade 1 (≥5.56% and <16.3), grade 2 (≥16.3% and <21.7%) and grade 3 (≥21.7%) steatosis was found in 71 (40%), 20 (11%) and 13 (7%) patients, respectively. The median age of patients without steatosis was 58 years, and the median age of patients with steatosis was 51 years; the difference was statistically significant (p = 0.007). BMI was statistically significantly higher in the steatosis group than non-steatosis group (26.9 ± 3.4 vs. 23.7 ± 3.5 kg/m²; p-value <0.001). The patients in the steatosis group had elevated triglyceride and SGPT levels compared with the non-steatosis group (p <0.05). There was no significant difference in total cholesterol, low density lipoprotein (LDL), albumin, TB or ALP between these two groups. Demographic data of the 179 patients in the study sample are summarized in Tab. 1.

The HR ratio was significantly highly correlated with MRI-PDFF (Spearman’s correlation coefficient = 0.83) (p <0.001) (Fig. 3). In subgroup analysis, there was a higher correlation between HR ratio and MRI-PDFF in patients with normal liver stiffness (n = 105) than those with increased liver stiffness (n = 74) (Spearman’s correlation coefficient = 0.87 vs. 0.69, respectively, p = 0.0017). High correlation of HR ratio with MRI-PDFF was observed in patients with less than stage 2 fibrosis, whereas moderate correlation of HR ratio with MRI-PDFF was found in patients with stage 2 fibrosis or higher (Tab. 2).

Fig. 3.

The median of HR ratio was 1.31 (1.10–1.55), and the range was 0.78–2.87. There was a significant difference in the median of HR ratio between patients with steatosis and those without, i.e. 1.49 (1.34–1.68) vs. 1.11 (1.01–1.18) (p <0.001), respectively. Higher HR ratio corresponded with higher PDFF. There were also significant differences in the median HR ratio among sub-groups (p <0.001) except for between grade 2 and 3 steatosis, which was marginally significant (p = 0.05) (Fig. 4).

Fig. 4.

ROC analysis and the HR ratio cutoff points

The appropriate HR ratio cutoff points were selected on the basis of the sensitivity and specificity, using the ROC analysis. The optimal cutoff for the HR ratio to define steatosis at MRI-PFFF ≥5.56% was a value equal to or greater than 1.18, with a sensitivity of 90.0% (95% CI, 83.0–95.3%), specificity of 80.0% (95% CI, 69.2–88.4%) and AUC 92.6 (95% CI, 88.8– 96.4%), PPV 86.2% (95% CI, 79.9–90.8%) and NPV 85.7% (95% CI, 76.7– 91.6%).

The optimal HR ratio cutoff point for prediction of grade 2 hepatic steatosis ≥16.3% was 1.55, with sensitivity of 90.9% (95% CI, 75.7–98.1%) and specificity of 89.7% (95% CI, 83.6– 94.1%), AUC 95% (95% CI, 92.4–98.2%), PPV 66.7% (95% CI 55.0–76.6%) and NPV 97.7% (95% CI, 93.7–99.2%). The optimal HR ratio cutoff point for the prediction of grade 3 hepatic steatosis ≥21.7% was 1.60, with sensitivity of 92.3% (95% CI, 64.0–99.8%) and specificity of 83.1% (95% CI, 76.6–88.5%), AUC 95% (95% CI, 91.1– 99.2%), PPV 30.0% (95% CI, 22.8–38.4%), NPV 99.3% (95% CI, 95.5–99.9%) (Fig. 5).

Fig. 5.

Interobserver reliability

The reproducibility of HR ratios was assessed by intraclass correlation coefficient (ICC), showing a high correlation between two independent operators (ICC = 0.882, p <0.001) (Fig. 6).

Fig. 6.

Nowadays, ultrasonography is usually the most initial modality to assess hepatic steatosis. The diagnosis of hepatic steatosis on ultrasonography involves subjective evaluation of increased liver echogenicity, portal vein blurring, beam attenuation and poor visualization of diaphragm. Prior studies found that abnormal hepatorenal echogenicity was the most sensitive among other sonographic signs in the diagnosis of hepatic steatosis^(18,28). Sonographic parameters for quantitative assessment of hepatic steatosis, including HR ratio, have been previously investigated. Prior studies have reported high correlation between sonographic HR ratio and histologic steatosis among patients with various liver diseases (r = 0.71–0.82)^(20,22) and also in patients with NAFLD (r = 0.80)⁽²³⁾. There was also a high correlation between sonographic HR ratio and ¹H-MRS in patients with NAFLD (r = 0.89–0.95)^(24,25). Therefore, HR ratio may offer a potentially useful quantitative tool for hepatic steatosis.

The previously reported HR cutoff ratios are varied, depending on the characteristics of the target population, standard reference and the selected threshold. For example, Webb et al.⁽²⁰⁾ and Marshall et al.⁽²²⁾ reported HR ratio cutoff value using specialized software at 1.49 and 1.28, respectively, for identifying more than 5% of fat from biopsy in patients with various liver diseases. Shiralkar⁽²⁹⁾ investigated the same patients included in the study by Marshall et al., and found that the HR ratio can be accurately calculated from PACS without specialized software yielding a cutoff point at 1.34. Borges et al.⁽²³⁾ reported a cutoff ratio at 1.24 in patients with only NAFLD using the same threshold from biopsy. Mancini et al.⁽²²⁾ and Martin-Rodriguez⁽²⁴⁾ investigated HR ratio in patients with NAFLD for the prediction of ¹H-MRS of at least 5%, yielding the best HR ratio cutoff point at 2.2 and 1.28, respectively. However, there is no perfect agreement in the percentage of fat from liver biopsy and MR techniques since they measure different aspects of hepatic steatosis. Also, a direct comparison between sonographic HR ratio and MRI-PDFF has not been performed yet.

In this study, we assessed sonographic HR ratio to predict the degree of fat content measured by MRI-PDFF in patients with liver disease of varying etiology. We calculated HR ratio using standard workstation without additional software because we attempted to keep the method simple so as to be easily applied and repeated. In this study, we measured PDFF on parametric maps at hepatic segment VI because the sonographic HR ratios were calculated in this segment.

Our study revealed a high correlation between sonographic HR ratio and percentage fat fraction measured by MRI-PDFF. We found that HR ratio could provide diagnosis of small fat content as low as PDFF of 5.56% with good accuracy. In our population, the HR cutoff ratio for predicting hepatic steatosis at 1.18 yielded sensitivity of 90.0% and specificity of 80.0%. Our study shared similar findings with the study by Kim et al. ⁽²⁸⁾ which correlated conventional ultrasonography signs with the MRI-PDFF. They found that PDFF was 2.6% (S.D. ± 1.22) in subjects without any signs of hepatic steatosis, while PDFF was 5.4% (S.D. ± 3.35) when abnormal hepatorenal echoes were present. Abnormal hepatorenal echoes yielded 96.6% sensitivity, 74.8% specificity and 87.5% AUC for detecting 6.4% PDFF.

Interference of liver fibrosis in liver parenchymal echogenicity had been previously investigated. Some studies^(18,30) showed that bright liver echogenicity significantly correlated with steatosis, but not fibrosis. Borges et al.⁽²³⁾ also reported that HR ratio was more strongly correlated with steatosis than fibrosis on liver biopsy. On the other hand, Perez et al. showed that US was inaccurate for diagnosing hepatic steatosis in patients with chronic liver disease due to the higher probability of hepatic fibrosis or inflammation in these patients, causing echogenic alteration seen in US⁽³¹⁾. Stiffness value measured by MRE is now the most accurate noninvasive method for the detection of liver fibrosis⁽²⁶⁾. In our study, we found a reduced correlation between HR ratio and MRI-PDFF in patients with ≥ stage 2 fibrosis. Therefore, the HR ratio should be used carefully in this group of patients.

Nowadays, liver fibrosis can be assessed not only with MRE, but also with ultrasound elastography (UE), providing comprehensive one-time assessment of liver steatosis and fibrosis. However, confounding of steatosis grade on the fibrosis estimation of UE is controversial and still under investigated^(32,33).

Limitations of this study include its retrospective design. Also, this study had no histologic evaluation, which is the gold standard of quantification of hepatic steatosis. Furthermore, the result may be affected by selection bias since the study population who underwent MRI and US at our institution were not representative of the general population. Also, sonographic HR ratio could be assessed only in patients who had adequate sample area of each organ on ultrasonographic image and without focal lesions or diseases of the liver and the right kidney, which may alter their echogenicity. Due to the nature of heterogeneous distribution of hepatic steatosis, it might lead to sampling errors, which can occur whatever method is used. The population in our study was heterogeneous due to the broad etiologies of liver disease that can be responsible for liver steatosis. However, we believe that the population might be more representative of the general population rather than focused on patients with NAFLD only.

Further validation is needed, including future study in larger populations and evaluation of HR ratio in the detection of subtle change of fat content for follow-up study.

In conclusion, the sonographic HR ratio was highly correlated with PDFF measured by MRI, easily applicable using standard workstation without additional software, and also reproducible. It could become an effective tool for quantitative evaluation of hepatic steatosis alternative to MRI-PDFF.