1. Introduction
Hepatocellular carcinoma (HCC) and intrahepatic cholangiocarcinoma (ICC) are the most common types of primary liver cancer, with the former accounting for approximately 75%-85% [
1,
2,
3], and their morbidity rates are increasing [
4,
5,
6,
7,
8]. The treatment strategies for and prognosis of patients with HCC and ICC are very different [
2,
3,
9,
10,
11,
12,
13,
14,
15,
16]. Therefore, accurate preoperative discrimination between HCC and ICC is essential.
At present, the noninvasive differentiation of HCC from ICC remains challenging. The sensitivity and specificity of serum tumor markers, including alpha-fetoprotein (AFP) and carbohydrate antigen 19-9 (CA19-9), are unsatisfactory [
17,
18,
19,
20]. The presentation of HCC and ICC on dynamic contrast-enhanced computed tomography (CT) or magnetic resonance imaging (MRI) is mostly typical [
21,
22,
23,
24]. However, both HCC and ICC may occur in patients with chronic hepatitis, and imaging enhancement patterns may tend to be similar in some patients with both HCC and ICC [
3,
25,
26,
27,
28,
29]. In addition, the enhancement may be unremarkable or atypical in some HCC cases (especially cases of small, hypovascular, or sclerosing HCC lesions) [
30,
31,
32]. Traditional medical imaging analysis relies heavily on the physician's subjective judgment and is thus prone to misdiagnosis [
33]. Liver biopsy remains the gold standard for the final diagnosis, but this invasive procedure is refused by some patients [
34]. Therefore, a preoperative, noninvasive method for distinguishing HCC from ICC is urgently needed.
The rapid development of artificial intelligence in recent years has led to it playing an important role in personalized precision medicine. Based on existing medical imaging modalities such as CT and MRI, an emerging technique known as radiomics [
35] can be used to convert intrinsic pathophysiological information that is invisible to the human eye into high-dimensional quantitative image features, which can then be used to perform tumor classification via an analysis of the relationship between these features and clinical/genetic data [
35,
36,
37]. Studies have shown that radiomics exhibits unique advantages in classifying the disease and predicting the prognosis of patients with liver cancer [
35,
38,
39,
40,
41,
42,
43,
44,
45,
46,
47,
48,
49,
50,
51,
52]. However, there have been very few studies on the differentiation of HCC from ICC based on multisequence MRI radiomics to date. In this paper, the efficacy of a radiomic model based on preoperative fat suppression T
2-weighted imaging (FS-T
2WI) and dynamic contrast-enhanced MRI features in the arterial phase (A) and portal venous phase (P) for the differentiation of HCC from ICC was investigated.
4. Discussion
MRI is characterized by high soft-tissue contrast, multiparametric and multidirectional imaging and a lack of radiation, making it the preferred imaging method for the identification and diagnosis of liver nodules [
53,
54]. Dynamic contrast-enhanced MRI (DCE-MRI) is superior to dynamic contrast-enhanced CT in the detection and diagnosis of small HCC lesions (maximum diameter ≤2.0 cm) [
55,
56]. Typical HCC displays significant heterogeneous enhancement in the arterial phase on DCE-MRI and reduced enhancement in the portal and/or parenchymal phase that is lower than that of normal liver parenchyma, resulting in a "fast-in and fast-out" enhancement pattern [
21,
24]. In contrast, ICC shows less obvious enhancement or heterogeneous mild enhancement in the arterial phase on DCE-MRI that gradually increases with time [
22,
23]. However, it is still difficult to differentiate HCC from ICC in clinical practice. Studies [
26,
30,
31,
32] have shown that small ICC lesions (diameter<3 cm) and some ICC lesions in the setting of cirrhosis (approximately 7%) show the same enhancement pattern as typical HCC lesions, and approximately 10%-20% of HCC lesions (especially small, hypovascular, or sclerosing HCC lesions) show less obvious enhancement on imaging.
Choi et al. [
57] conducted gadoxetic acid-enhanced MR and dynamic CT scans to identify HCC and ICC. The results showed
that portal venous phase (PVP) washout instead of
conventional washout in gadoxetic acid-enhanced MRI can prevent misidentification of HCC as ICC in patients with cirrhosis; however, it reduces the sensitivity of the method for identifying HCC. Diffusion-weighted imaging (DWI) reflects the diffusion of water molecules in tissues by measuring the apparent diffusion coefficient (ADC). Wei et al. [
58] and Lewis et al. [
59] found that the ADC can help differentiate HCC from ICC. However, ICC has multiple cellular origins and shares similar biological behaviors to some extent with HCC; thus, the ADC of ICC can partially overlap with that of HCC. Additionally, DWI does not display small lesions well because of the limited spatial resolution, and conventional DWI is based on a monoexponential model that cannot differentiate between water molecule diffusion and blood perfusion [
60]. Intravoxel incoherent motion-DWI (IVIM-DWI) can simultaneously quantify the diffusion of water molecules and microcirculatory perfusion in living tissues. A previous study [
61] showed that both the ADC and Dslow values were significantly lower in HCC than in ICC, but the Dfast value was significantly higher in HCC than in ICC; furthermore, Dfast was more efficient in the differential diagnosis of HCC and ICC, and there was no significant difference in the f value between Dfast and Dslow. The value of IVIM-DWI in identifying HCC and ICC has also been reported by other scholars [
58,
62,
63]. However, the conclusion regarding D
fast and f in distinguishing HCC from ICC remains inconsistent or controversial; thus, further research is needed. As an effective tumor imaging tool, positron emission tomography (PET)-MRI can play a role in patient management. Çelebi et al. [
64] argued that PET-MRI using
18F-fluorodeoxyglucos (
18F-FDG) as a tracer agent can help differentiate between HCC and ICC. However, there is a need to deeply explore whether there are great differences in FDG uptake between HCC and ICC, the accuracy of identification in some challenging cases (e.g., specific subgroups of patients in which the standard uptake value (SUV) is not a determining factor), and the optimal imaging sequence and model.
To date, few studies have investigated the differentiation of HCC from ICC based on MRI radiomics [
38,
40,
59,
65]. Liu et al. [
40] adopted machine learning-based CT and MR image features in the identification of cHCC-CC, ICC and HCC. The results showed that MRI features had the highest efficacy in differentiating between cHCC-CC and non-cHCC-CC, while CT features were less valuable. Moreover,
precontrast- and portal-phase CT features were superior to enhanced MRI features in differentiating between HCC and non-HCC (AUC=0.79-0.81 for MRI, 0.81 for
precontrast-phase CT and 0.71
for portal-phase CT). Wang et al. [
65] used MRI radiomics to preoperatively identify cHCC-CC, HCC and ICC and found that the performance of the higher-order feature-based model was better than that of the lower-order feature-based model by approximately 10% and that the former performed better in identifying HCC in the delayed phase. Lewis et al. [
59] extracted first-order radiomic features from ADC data and evaluated the ability of these features and the Liver Imaging Reporting and Data System (LI-RADS) classification to differentiate HCC, ICC and cHCC-CC. The results revealed that the AUC of the combination of sex, LI-RADS grade and the fifth percentile of the ADC in the diagnosis of HCC was 0.90 and 0.89 for two independent observers, respectively,. T
2∗WI can reflect the magnetic susceptibility variation in tissues and thus be used to assess the biological properties of tumor tissues [
66]. Huang et al. [
38] extracted radiomic features from T
2∗W images and then established radiomic nomogram models combined with clinical risk factors to distinguish between HCC and ICC. The results showed that the AUC of the radiomic model was 0.90 and 0.91 in the training and validation groups, respectively; the AUC of clinical features was 0.88 in the training group and 0.83 in the validation group, and the AUC of the radiomic nomogram was 0.97 and 0.95 in the training and validation groups, respectively. Similar results were obtained by Zhou et al. [
39]. However, the efficacy of a joint model incorporating multiple sequence features was not investigated in these studies.
Different kinds of information related to tumor structure can be revealed by different sequences: T
2WI exhibits the underlying tumor morphology and heterogeneity, and enhancement scans can reflect differences in the tumor blood supply. In this work, enhancements in the arterial and venous phases were combined based on T
2WI to explore the efficacy of a joint model according to the blood supply status and enhancement patterns of HCC and ICC. The results showed that while each model had the potential to identify HCC and ICC in both the training and validation groups, the joint model incorporating multiple sequence features showed the highest efficacy [
40,
65]. The AUC of the T2WI model was relatively low in this study, consistent with the findings of Liu et al. [
67]. Therefore, the value of FS-T
2WI-based radiomics in distinguishing between HCC and ICC remains to be properly determined pending further research.
The radiomic features selected in this study were mainly GLCM and GLRLM features, textural features used to quantify tumor heterogeneity by reflecting the relationship between adjacent voxels/pixels [
68], which is consistent with the results of related studies [
38,
40,
69,
70,
71,
72,
73,
74]. Histogram features show the global distribution of grayscale values in the image and can also be used to assess tumor heterogeneity [
75]; Lewis et al. [
59] found that the 5
th/10
th/95
th percentiles of the ADC could significantly differentiate HCC from ICC and cHCC-CC. Shape features reflect the geometric characteristics of tumors [
68]; Zhao et al. [
76] confirmed that HCC tends to be more spherical than ICC in terms of morphology.
This study had the following limitations. (1) In this retrospective study, many HCC and ICC patients who did not undergo preoperative MRI scans were excluded, so there may be a potential selection bias. (2) The sample was small and from a single center, and cHCC-CC and ICC types other than the mass-forming type were not included in the study. In the future, the sample size should be expanded to multiple centers for further model validation. (3) Other relevant MRI sequences were not analyzed, so their potential contributions might have been ignored.