After the exclusion of 5 cases where the image was deteriorated (Fig. 2), the participants comprised 136 patients (93 men and 43 women) who underwent Gd-EOB-DTPA-enhanced MRI and 99mTc-GSA liver scintigraphy at our hospital within 2 weeks between 15 October 2012 and 3 February 2021. The patient age was 69.3 ± 9.6 years (mean ± standard deviation) (range: 46-93 years). Patient backgrounds are shown in Table 1. All patient data were collected retrospectively and anonymized prior to analysis. In accordance with the provision of the “Ethical guidelines for medical and health researches involving human subjects” (Ministry of Education, Culture, Sports, Science; and Technology and Ministry of Health, Labour and Welfare, Japan, Bulletin No. 3, 2014), information was posted on a notice board in our hospital requesting consent for the secondary use of medical care information. Patient information and consent were requested by disclosing the research contents to the public on the home page of the website of Japan Community Healthcare Organization Hokkaido Hospital. Informed consent was obtained from all individual participants included in the study. This study design was approved by the appropriate ethics review boards of Japan Community Healthcare Organization Hokkaido Hospital (Research Reference No. 2013-32 and No. 2018-21).
Contrast agents and radiopharmaceuticals
For Gd-EOB-DTPA-enhanced MRI, 0.025 mmol/kg of body weight of Gd-EOB-DTPA (PrimovistÒ 0.25 mmol/mL; Bayer Yakuhin, Osaka, Japan) was injected intravenously. For 99mTc-GSA liver scintigraphy, 1.0 mL of 99mTc-GSA (AsialosynchisÒ 185 MBq; Nihon Medi-Physics, Tokyo, Japan) was injected intravenously regardless of body weight.
Imaging data acquisition
The device used for MRI was Achieva 1.5T A-series R.2.6 (Philips Medical Systems Japan, Tokyo, Japan) with a quadrature body coil, which has the best uniformity of images in coils within the field of view (FOV) (Fig. 1c). Phased array coils were not used because the image non-uniformity correction, which is indispensable for using these coils, causes image unevenness and a decrease in contrasts (Fig. 1d) . The breath-hold T1-weighted two-dimensional images were acquired using fast field echo [FFE] of multi slice at 20.7 sec. In addition, the fat suppression on the images was carried out using the principle of selective excitation technique [ProSet], and its pulse type was 121. The pulse sequence parameters were as follows: echo time was 5.1 ms, repetition time was 126 ms, flip angle was 80°, number of excitations was 1, and band width was 313.8 Hz/pixel. The imaging parameters were as follows: FOV was 420 mm, matrix was 256×160 (frequency×phase), k-space trajectory was linear, scan percentage was 62.5 %, and phase FOV was 100 %. The other parameters on slices were as follows: slice thickness was 6.5 mm, slice gap was 3.5 mm, slice scan order was interleaved, and slice number was 7. Hepatobiliary-phase images were acquired, not at 20 min, but at 60 min after intravenous injection .
The device used for scintigraphy was INFINIA Functional Imaging Scanner (General Electric Healthcare, Tokyo, Japan) with a low-energy high-resolution collimator. The scan parameters were as follows: frame rate was 30 s/frame, frame number was 40 frame, matrix was 128×128, energy level was 140 keV, window width was ± 10 %, and continuous scan time was 20 min.
The measurement of LSC was performed using Basic Viewing (Philips Medical Systems Japan, Tokyo, Japan). The region of interest (ROI) was set with a rectangle of approximately 50 pixels in order to suppress statistical fluctuations . To minimize the signal change inside the ROIs, the ROIs of the SIL and SIS were placed on the areas with the smallest standard deviation, avoiding large vessels, masses, and artefacts . The LSC was calculated using the SIL and SIS, according to the following equation [11,17]:
LSC = (SIL – SIS) / (SIL + SIS) (1),
which is referred to as the Michelson contrast. In this study, the LSC was determined from five axial images acquired in the hepatobiliary phase, and the final measurement of the LSC was the average of five LSC values calculated from these five images.
The analyses of the HH15 and LHL15 were performed on Xeleris 3.0 Functional Imaging Workstation (General Electric Healthcare, Tokyo, Japan). The HH15 was calculated using cardiac counts at 3 and 15 min (H3 and H15, respectively) after the intravenous injection of 99mTc-GSA, according to the following equation :
HH15 = H15 / H3 (2).
The ROI used to measure the cardiac counts was set to surround both ventricles, in order to obtain the largest possible measurement area [5,20]. The LHL15 was calculated using the H15 and the liver counts at 15 min (L15) after the intravenous injection of 99mTc-GSA, according to the following equation :
LHL15 = L15 / (H15 + L15) (3).
The ROI used to measure the liver counts was set to surround the entire liver .
The biochemical tests of serum bilirubin and albumin were assessed within 2 weeks before and after MRI scanning. The value of indirect bilirubin was computed to be the difference between total and direct bilirubin. The ALBI score is calculated using the total bilirubin [μmol/L] and albumin [g/L], according to the following equation:
ALBI score = (0.66 ∙ log10 total bilirubin) – (0.085 ∙ albumin) (4),
which is a new objective index that enables the quantitative evaluation of liver function . The ALBI Grade is the grading system for determining the hepatic function in HCC patients, and its cut points classified by ALBI scores are as follows: -2.60 or less (Grade 1), more than -2.60 to -1.39 or less (Grade 2), and more than -1.39 (Grade 3) . In this study, the group of patients who were at Grade 1 but had neither chronic liver disease nor hepatocellular carcinoma were classified as ‘Normal’ on the comparison with the LSC or LHL15.
Imaging-based clinical stage classification
There is a clinical stage classification based on the severity of chronic liver diseases (currently known as liver damage classification for making decisions concerning the treatment of Japanese HCC patients) . The criteria of LHL15 corresponding to this clinical stages exists as follows: 0.942 ± 0.017 (Normal), 0.909 ± 0.044 (Mild [Stage Ⅰ]), 0.844 ± 0.066 (Moderate [Stage Ⅱ]), and 0.706 ± 0.112 (Severe [Stage Ⅲ]) . These criteria are used as one of the evaluation standards of 99mTc-GSA liver scintigraphy and have been supported for many years . In this study, the eLHL15 and LHL15 were divided into these four groups using the threshold levels of 0.936, 0.880, and 0.790, and were compared using contingency tables and images.
The LSC was compared to the LHL15 and HH15 using Pearson's correlation coefficients. Additionally, tests of no correlation were performed. Equally, their indices were compared with the ALBI score and its related laboratory parameters. The comparison of the LSC or LHL15 among ALBI grades was performed using analysis of variance (ANOVA) and Tukey's multiple comparison test. The linear regression analysis wherein the LSC is a variable was performed, and the accuracy of the eLHL15 was evaluated by the R2 and standard error (SE). The contingency table analysis between the eLHL15 and LHL15 was performed using chi-square (χ2) test and Cramer's coefficient of association (Cramer's V), and the degree of agreement was tested on imaging-based clinical stage classification. In all statistical tests, a P-value of <0.05 (two-tailed) was considered as statistically significant. For interval estimation, the 95% confidence interval (CI) was calculated. All statistical analyses were carried out using statistical software (MicrosoftÒ Excel 2010; Microsoft, Redmond, WA).