Liver is usually the nidus where malignant tumours are more likely to metastasise and is a good site for metastatic cancer, cause malignant problems. Liver is also a target organ for many malignant diseases and is associated with a large number of benign lesions[13]. Early diagnosis is therefore very important. Currently, the diagnosis of liver-occupying lesions is mainly based on imaging modalities such as ultrasound, CT and MRI. Conventional imaging modalities have their limitations and are easily influenced by anatomical location and cannot assess systemic tumour invasion and biological characteristics[14]. PET/CT can provide functional imaging from a molecular biology perspective, and the organic combination of metabolic imaging and anatomical imaging can provide in vivo metabolic information non-invasively[15].18F-FDG indirectly reflects intracellular glucose metabolism by monitoring deoxyglucose and has become an important tool for the qualitative diagnosis, staging, prognosis and efficacy assessment of tumours[16][17]. 18F-FDG PET/CT reflects the glucose metabolism of tumour tissue, and the differential diagnosis between benign and malignant tumours is based on the difference in glucose metabolism activity between tumour cells and normal tissue cells. 18F-FDG is an derivative of glucose and is involved in the process of glucose metabolism. Because it is deoxygenated, it cannot form hexose diphosphate, preventing the next step of metabolism and remaining in the cell. In tumour cells, 18F-FDG uptake is increased by a combination of high expression of glucose transporter mRNA, increased levels of glucose transporter proteins Glut-1 and Glut-3, increased expression of hexokinase and downregulation of glucose-6-phosphatase levels.In highly and moderately differentiated HCC, 18F-FDG uptake levels are similar to normal liver parenchyma due to low GLUT-1 and GLUT-2 expression and high glucose-6-phosphatase activity, whereas in hypofractionated HCC, 18F-FDG affinity is usually higher[18].
In this study, SUVmax and Kimax, TBRSUV and TBRKi were significantly different between the benign liver group and the malignant liver group, and the difference was statistically significant (P < 0.05), and SUVmax and Kimax, TBRSUV and TBRKi were higher in the liver malignancy group than in the benign liver lesion group. The results show that dynamic whole-body 18F-FDG PET/CT parametric imaging provides highly sensitive imaging of whole-body uptake changes and that 18F-FDG uptake in benign liver lesions can be distinguished from uptake in malignant liver lesions by dynamic imaging to assess uptake changes. Therefore, whole-body dynamic PET/CT has a high diagnostic value in the differential diagnosis of benign and malignant liver lesions.In the case of focal nodular hyperplastic lesions where 18F-FDG uptake levels are identical to normal liver parenchyma, visual analysis of metabolic imaging findings may be confused with malignant lesions of highly differentiated hepatocellular carcinoma (HCC), and further analysis requires quantitative assessment of 18F-FDG uptake to differentiate between malignant and benign lesions. It has been suggested that this quantitative analysis may have similar value to conventional early delayed PET scans[19]. During the delineation of the HCC, some lesions showed no 18F-FDG uptake and only hypointense shadows on the CT images (Fig. 5), which may be due to the fact that the better differentiated HCC behaved negatively on images similar to the surrounding normal liver tissue. Since the degree of 18F-FDG uptake in HCC is related to the degree of differentiation of the tumour tissue, the higher the degree of differentiation, the lower the FDG uptake. A proportion of more differentiated hepatocellular carcinomas have higher levels of dephosphorylation, allowing lower FDG uptake by cancer cells and predisposing to false-negative results.
In this study, by analysing box plots, it was found that TBRKi was higher than TBRSUV for different pathological types of liver malignancy. MRFDG images were found to have better tumour-liver contrast than SUV images[20]. On visual analysis, most of the different pathological types of liver malignancy were detected in both modalities, with the exception of one HCC nodule, which showed non-hypermetabolism in both modalities. Thus, TBRSUV was largely sufficient to detect lesions in most cases. Correlation analysis showed positive correlations between SUVmax and Kimax (r = 0.882, P < 0.01) and TBRSUV and TBRKi (r = 0.848, P < 0.01), which were stronger when different pathological types of liver malignancies were considered separately.This suggests that MRFDG images can identify hypermetabolic lesions[21]. TBR may demonstrate the ability to differentiate focal lesions from background and map appropriate regions of interest for further quantitative evaluation. On visual analysis, TBRSUV value of2.34 and TBRKi value of 3.20 in a liver lesion that was negative on both SUV and MRFDG images(Fig. 5).TBRSUV value of 0.33 and TBRKi value of 0.51 in a liver lesion that was negative on both SUV and MRFDG images(Fig. 6).The ratio of TBR in both cases had a greater TBRKi value than TBRSUV, suggesting that the Patlak dynamic parameter Kimax is slightly more sensitive in detecting hypermetabolic lesions than the static parameter SUVmax. In addition, most malignant lesions can show a gradual increase in FDG uptake, indicating higher metabolism at the lesion site, while benign lesions and areas of physiological uptake show little change or decrease. Recent reports suggest that the TBR ratio of Ki in Patlak analysis is higher than the commonly used SUV values, indicating that the contrast of parametric PET images is higher than that of static images[22][23][24]. In addition, there are relevant findings suggesting that Patlak parametric imaging can provide improved TBR as well as an additional set of highly quantitative tumour features beyond the static features currently supported by the respective SUV image metrics[25][26].
In this study, the sensitivity and specificity of whole-body static PET/CT SUVmax parametric imaging for the diagnosis of primary liver cancer (including hepatocellular carcinoma and intrahepatic cholangiocarcinoma) with liver metastases were 84.0% and 65.7%, respectively, with an area under the curve of 0.805. The sensitivity and specificity of TBRSUV in the differential diagnosis of primary liver cancer and liver metastases were 64.0% and 85.7%, respectively, with an area under the curve of 0.834. The sensitivity and specificity of Kimax parameters in the differential diagnosis of primary liver cancer and liver metastases were 68.0% and 88.6%, respectively, and the area under the curve was 0.853. The sensitivity and specificity of TBRKi for the differential diagnosis of primary liver cancer and liver metastases were 92.0% and 68.6%, respectively. The area under the curve was 0.843. The ROC curve fit of Patlak parametric imaging was better than that of static whole-body PET/CT parametric imaging in the differential diagnosis of primary liver cancer versus liver metastases. The use of dynamic imaging to assess temporal changes in FDG uptake has several clinical advantages over static imaging. Continuous assessment of changes in FDG uptake, even over short periods of time, can help distinguish pathological uptake, particularly in the abdominal region[27].
MRFDG images have been shown to have good visual quality and lesion contrast. In addition, MRFDG images complement standard SUV images by providing better quantification and improve image reading, which also reduces the number of false positives. Patlak parametric imaging has been shown to provide better quantitation and specificity for the detection of hepatocellular carcinoma lesions[28]. It has been suggested that Patlak parametric images can complement conventional standardised uptake value (SUV) images and improve current applications or enable new ones. The high sensitivity of our PET/CT system in continuous bed motion mode is capable of providing image quality for continuous short-interval whole body dynamic scanning for routine image interpretation[29][30].
The main limitation of this study is the relatively small number of patients. Nevertheless, we also found the value of whole-body dynamic Ki-parametric imaging in the differential diagnosis of benign and malignant liver lesions and in the differential diagnosis of different pathological types of liver malignancy to provide useful conclusions for the clinical performance assessment of whole-body dynamic 18F-FDG PET Ki-parametric imaging.In addition, manual ROI mapping for IDIF calculation is subjective to the outliner's subjective factors and may be affected by variability. We plan to continue this work in a larger cohort of prospective controlled studies for specific tumour types. Therefore, future studies should increase sample size and minimise selection bias to better assess the diagnostic value of whole-body dynamic 18F-FDG PET/CT for liver occupancy.