In this study, the diagnostic results of CDFI and SMI were significantly different in the identification of suspicious LNs. The correlation between the diagnosis results of SMI and pathological results was better than that of CDFI, and the diagnostic efficiency of SMI was also better than that of CDFI [15–16]. The CDFI was mainly formed by the Doppler signal generated by the movement of red blood cells in the blood vessels, and it was often easy to display for vessels with large diameter and large blood flow [17]. However, early tumors have small vascular diameter and slow blood flow. The Doppler signal intensity of red blood cells in the neovascularization was low, which was easy to be covered by noise, and it was difficult to show clearly on CDFI [10, 18]. As an innovative technology with high sensitivity to tiny blood flow, SMI could effectively collect low-speed blood flow signals, enhance the display rate of peripheral and central small blood vessels in LNs, and improve the diagnostic accuracy of suspicious LNs [11–12, 19]. In this study, compared with CDFI, the diagnostic accuracy of SMI for suspicious LNs increased by 13.5%. In addition, the gender and age in this study were also different in different pathological results of LNs, and the risk of metastatic LNs in young patients was higher than that in the elderly, which was consistent with previous research results [20]. However, the correlation between these two factors and the pathological results was not satisfactory.
As we all know, most cervical metastatic LNs from PTC will have abnormal distribution of feeding vessels, which is often one of the important indicators to predict metastatic LNs [13, 21]. In this study, CDFI and SMI in benign LNs were dominated by central portal type, while peripheral type was rare. However, the blood flow distribution characteristics of metastatic LNs were often different, and there were many tiny blood vessels around the LNs, which was mainly related to the way the tumor invaded the LNs. Tumor invasion of LNs is usually centripetal progression, and tumor growth is often accompanied by the proliferation of new blood vessels, so there are often abnormal feeding vessels in the peripheral cortex of LNs [13, 22]. In metastatic LNs, the peripheral type was predominant in CDFI, while the mixed type was predominant in SMI. This indicated that CDFI showed insufficient central portal blood flow in some abnormal LNs, which might be related to abnormal portal structure or decreased portal blood flow caused by tumor invasion [23]. In addition, CDFI had a limited ability to display peripheral microscopic vessels, and partial mixed type was often considered to be central portal type in CDFI, leading to certain false negative results [18, 24]. Similarly, the avascular type shown by CDFI often required vigilance against the possibility of tiny vessels being overlooked [10, 18, 25]. Therefore, the ability of SMI to display small vessels was significantly higher than that of CDFI, which was also confirmed in this study. The number of vessels in LNs displayed by SMI was always higher than that of CDFI in both the LNs groups and the overall sample.
There are some potential limitations to this study. First, this study was a single-center study, and there was a certain bias in the selection of enrolled cases. Second, only two sonographers interpreted the ultrasound images, and the consistency of interpretation results was not evaluated in the study. Finally, the included sample size of this study was relatively small, and the research results needed to be further verified by larger research samples.