For the method to evaluate the severity of acute pancreatitis in the past there are also many, commonly used have Ranson, Glasgow, APACHE Ⅱ etc., but there are some difficulties in the actual clinical applications [18].CECT examination has good specificity and sensitivity for the evaluation of acute pancreatitis, but there are some problems such as timing and cost. Therefore, it is still necessary to find a convenient, non-invasive, sensitive and specific early evaluation method[19].With the development of ultrasonography and the constant update of ultrasound examination equipment, the rate of diagnosis and diagnosis of acute pancreatitis by US has reached a high level, and it is harmless, economical, convenient and easy to be accepted by patients, so it has become a routine auxiliary diagnosis and dynamic monitoring method for acute pancreatitis[20, 21].
CEUS provides a dynamic view of the “vascular pattern” through the exclusively intravascular flow of the microbubbles, assessing the aspect of increased echogenicity through nonlinear signals from the microbubbles[22]. The contrast agent is removed from the body by breathing through the lungs, so it can be used in patients with SAP associated renal failure. Unlike the contrast agent used in ultrasonography, the contrast agent used in CT and MRI has a short intravascular retention time, being rapidly cleared from the circulation and reaching the extracellular space, and then excreted by the kidneys [23].
Vitali F et al. considered that the performance of CEUS in diagnosing SAP to be very good compared to CT (89% specificity, 95% sensitivity) [24]. Our study compared the detection rate of the areas of pancreatic necrosis in SAP through CECT (gold standard diagnostic) and CEUS, and there was no statistically significant difference in diagnose accordance rate.
Analyzing the current data from literature regarding the use of CEUS in acute pancreatitis, most of them are qualitative. Till now, there are few published studies on the quantitative analysis of pancreatic tissue perfusion and determination of diagnostic thresholds for AP and SAP [25, 26].
TIC analysis of grey scale signal changes after contrast agent injection is one of the current methods for quantitative ultrasonic study of blood perfusion [27]. Among the quantitative parameters, PI has a good correlation with tissue perfusion amount, which refers to the most significant enhancement intensity of contrast agent on the local section, reflecting the maximum dose of contrast agent reaching this section per unit time. AUC is related to the distribution volume, blood flow velocity and perfusion time of the contrast agent. When the dose and instrument conditions are controlled at a constant rate, the AUC is affected by the flow rate and distribution volume, and is linearly related to the blood flow of the substance. In other words, AUC reflects the change of blood flow volume in the blood vessel, which is one of the most valuable parameters in TIC. The PPT is the time from the beginning of the contrast agent entering the tissue to the maximum strength of the enhancement. The change of the upward slope of the curve can reflect the perfusion rate of the contrast agent entering the tissue[28].
The results of TIC analysis in our study showed that TIC in control group was characterized by a sharp arc curve with a steep ascending branch. After reaching the peak, the descending branch decreased gently compared with the ascending branch. In the AP group, TIC presented a relatively flat arc curve, with the ascending branch slowly rising and the descending branch even more slowly extending. Compared with control group, the AT, TTP and WT in AP group were higher than that in control group. The PI and AUC of the contrast agent in AP group were smaller than those in control group, because the small blood vessels and microcirculation were damaged, the blood flow was slow, the perfusion volume was reduced, and the blood flow volume in the blood vessels was reduced. And by analyzing the TIC parameters, we can draw ROC curves for statistically significant parameters. Then we can determine the best cut-off point of AP and SAP.
A limitation of this study is the relatively small number of patients and the diversity in the ultrasonographically assessed structural changes. Our study provides a more objective, rapid, and accurate diagnostic method for the diagnosis of AP and SAP. Together with other clinical and biohumoral information, these ultrasonographic parameters and their statistical analysis can assist in early clinical evaluation of the severity of AP.