Severe Acute Pancreatitis Secondary Infection: Functional Imaging Diagnosis And Comparative Study

Infection of pancreatic or peripancreatic necroses occurs in 30-70% of patients with severe acute pancreatitis (SAP). This disease is often accompanied by a late deterioration of organ function or generalized systemic illness which are the leading causes of SAP-related deaths, with mortality rates of more than 30%. These conditions are also accepted as the most important prognostic factors for the determination of the severity and outcome of SAP. However, until recently, the differential diagnosis of sterile and infectious SAP has remained a challenging issue for clinicians. Ultrasonography (US), computed tomography (CT) and magnetic resonance imaging (MRI) have been applied widely to the evaluation of pancreatitis, but they have limitations in detecting secondary infections in cases of SAP i.e. unusual gas bubbles and typical manifestations of an abscess on the images. Radiolabeled white blood cell imaging (WBC imaging) also cannot distinguish between infective and sterile inflammatory conditions. Imaging-guided fine-needle aspiration has contributed to making a definitive diagnosis of infected pancreatic necrosis, but is an invasive procedure and there are difficulties in applying this technique to critically ill patients.It is therefore essential to develop a new imaging methodology that will non-invasively detect secondary infections in SAP patients. ^99mTc-ciprofloxacin (Infecton) has become one of the most promising agents in the field of nuclear medicine, which has a higher sensitivity and specificity for bacterial infections than WBC scans. In our current study, a novel SAP secondary infection model was developed in the swine using the"two-step"method. The features and effectiveness of ^99mTc-ciprofloxacin scintigraphy in the diagnosis of secondary bacterial infection in this infective SAP animal model of swine were then evaluated and compared with contrast enhanced CT, MRI and ¹⁸F-FDG PET, and with histopathological and biological testing.Part 1Study of the CT and MRI features of normal pancreas in swineObjective:To study the CT and MRI features and the optimal protocol of multi-phase enhanced CT of normal swine pancreas and compare them with human beings.Methods:CT plain scanning and thin-slice dynamic enhanced scanning were performed respectively in 10 healthy swine. Six of them underwent MRI at the same time,including T1WI,T2WI,DWI and MRCP sequences. All of the swine were paunched to observe swine pancreas'shape and position with naked eyes. The CT and MRI findings were compared with surgery.Results:(1) The pancreas of swine which liked"clover"was divided into three parts, pancreas head, left lobe and right lobe.The left lobe was the main part of swine's pancreas.It was resemble considerably to human being'pancreas ,such as dissection location,general morphologic,the per- pancreatic vessels,CT density,MRI signals of T1WI,T2WI and diffusion-weighted imaging (DWI) and ADC value. (2) The images of pancreasticobiliary of normal swine on MRCP were different from those of human being. It's bile duct and pancreatic duct converged at the duodenum respectively.The bile duct was comparatively wider with diameter of 5-6 mm, while the pancreatic duct was narrow with diameter of 1-1.5 mm and it was difficult to be shown by MRCP. (3) Multi-phase enhanced CT protocal: The results of enhanced CT scanning indicated that the time- density curve was similar with that of human beings .The optimal scanning protocal was 15 s for arterial phase, 35 s for the pancreatic parenchyma and 45 s for the peripancreatic portal vein phase.Conclusions:The CT and MRI features and multi-phase enhanced CT scanning protocals of pancreas of swine are considerable similar with that of human beings. Therefore, the swine may be an ideal model of big animal in imaging research of pancreas diseases.Part 2Establishment of a secondary infection model of severe acute pancreatitis in swineObjective: Develop a swine model of secondary infection in severe acute pancreatitis (SAP).Methods: Twenty-seven female swine were divided into 3 groups (I-III). In the first experiment, a SAP model was developed by retrograde injection of sodium taurocholate and trypsin into the pancreatic duct. In the second experiment, the SAP model was used to develop a secondary infection model. In groups of I-III, 10⁸/ml or 10⁴/ml E. coli or saline were respectively used to inoculate necrotic areas of the pancreas using CT guidance. Biochemical, histopathological, and imaging analyses were used to characterize disease presentation.Results: The survival rate was 85.2% (23/27) during the course of the 9-d experiment. The secondary infection rates in groups of I-III were 100% (8/8), 37.5% (3/8), and 14.3% (1/7), respectively. In group I, the infection rate was significantly higher in comparison to the other 2 groups (χ²=4.66 and 8.14, respectively, and both P<0.05). The biochemical and histopathological parameters and CT images indicated successful development of the SAP secondary infection model.Conclusions: The swine model of SAP secondary infection was successful developed using a two-step method, which could serve as a platform for SAP studies that need complex experimental manipulations over longer time spans, especially for imaging research.Part 3Detection of Secondary Infection of Severe Acute Pancreatitis: ^99mTc-Ciprofloxacin Scintigraphy Compared with CTObjective: Secondary infection is one of the most challenging problems in the diagnosis and treatment of severe acute pancreatitis (SAP). The aim of this work was to (1) To reveal the characteristics of a novel inflammation tracer: ^99mTc-Ciprofloxacin (Infecton); (2) To evaluate Infecton scintigraphy for detecting secondary infection of SAP in swine, in comparison with CT.Methods: (1) Ciprofloxacin was labeled with ^99mTc and the radiochemical purity and labeling rate were measured. The biodistribution in normal swine was studied. (2) The young swine were assigned into three groups. Six normal swine were assigned into the normal control group, 6 with non -infected SAP into the non-infected group, 16 swine with infected SAP into the infected group. At 7 d after inoculation, CT scan and a serie of SPECT imaging (at 0.5, 1, 2, 3 , 4, 6 h after administration of 370 MBq ^99mTc-ciprofloxacin iv) were performed. The scintigrams were visually evaluated and semi-quantitative analysed through ROIs assignments. The differences of ^99mTc-ciprofloxacin uptake and change tendency of lesion-background radioactive count ratio among different groups were recorded and compared. The image findings were compared with histopathological and bacterial culture and smear results.Results: (1) Both the radiochemical purity and labeling rate of Infecton were over 90% within 6h. Organs with rich blood supply, such as kidney , liver and spleen were target organs for the accumulation of Infecton, while no significant uptake in neither gastrointestinal tract, normal pancreas nor tissue of SAP. Rapid plasma clearance and renal excretion were observed. (2) Based on histopathological and biological results, the sensitivity, specificity and accuracy of the Infecton method were 93.8% (15/16), 91.7% (11/12), and 92.9% (26/28), whereas these values for CT were 12.5% (2/16), 100.0% (12/12), 50% (14/28), respectively. The changes of lesion-background ratio for the diseased animals were significantly different from those of the non-infected and normal swine (P<0.001). At 3 h, the radioactive counts (2350.25±602.35 k) and the mean lesion-background ratio (3.36±1.48) of the infectious foci was significantly higher than at all detection timepoints (P <0.001).Conclusions: Infecton scintigraphy may be an effective procedure for detecting secondary infections associated with SAP with more higher sensitivity and accuracy than CT. The optimal acquisition time for detecting infectious foci is 3 h after administration. It might become a useful tool to diagnose infectious SAP in human. Part 4Experimental study of comparison of functional imaging modalities for detecting SAP secondary infectionObjective: The aim of this part was to investigate the value of three functional imaging techniques (^99mTc-ciprofloxacin scintigraphy, ¹⁸F-FDG PET and diffusion-weighted imaging) for detecting secondary infection of SAP in swine.Methods: Six normal swine were assigned into the normal control group, 6 with non-infected SAP into the non-infected group, 16 swine with infected SAP into the infected group. Animal models were induced in accordance with the method we reported. At 7 d after inoculation, a serier of Infecton imaging, ¹⁸F-FDG PET scintigraphy, MRI and CT scan were performed. The images were visually evaluated and semi-quantitative analysed through ROIs assignments. Semi-quantitative indicators included lesion-background radioactive counts ratio, the apparent diffusion coefficient (ADC) and the maximum standard uptake value (SUVmax). The image findings were compared with histopathological and bacterial culture and smear results and calculated the sensitivity, specificity and accuracy. U test was used to compare rates, Bonferroni test was used to compare the differences of ADC values and SUVmax in 2 groups, and paired t test was used to compare the differences of early and delayed imaging of PET. P value less than 0.05 was considered statistically significant.Results: (1) According to visually evaluation, the sensitivity, specificity and accuracy of the Infecton method were 93.8% (15/16), 83.3% (5/6) and 90.9% (20/22), whereas these values for ¹⁸F-FDG PET were 81.3% (13/16), 33.3% (2/6) and 68.2% (15/22), for MRI were 15.4% (2/13), 83.3% (5/6) and 36.8%(7/19), and for CT were 12.5% (2/16), 100.0% (6/6) and 36.4% (8/22), respectively. The sensitivity and accuracy of Infecton and ¹⁸F-FDG PET were higher than those of MRI and CT (p were all <0.05). Infecton scintigraphy had higher accuracy than that of ¹⁸F-FDG PET and MRI (p<0.05). (2) Quantitative analysis. On Infecton images, the changes of lesion-background ratio for the infected SAP swine were significantly different from those of the non-infected swine (p<0.001). On ¹⁸F-FDG PET images of early scintigraphy, SUVmax values of infectious SAP group (2.61±1.07) and non-infected group (1.87±0.76) were no statistically significant difference (p>0.05), while on delayed scintigraphy, SUVmax values of infectious SAP group (2.27±0.75) was higher than non-infected group (1.40±0.64) (p<0.05). On DWI images, the difference of ADC values between infected foci [(2.70±0.90)*10-3mm2/s] and non-infected necrosis [(3.61±0.77)*10^-3mm²/s] was not significant (p>0.05). However, the incidence of ring wall in -infected SAP group (76.9%, 10/13) was higher than non-infected SAP group (16.7%, 1/6) (p <0.05).Conclusions: Infecton scintigraphy may be an effective procedure for detecting secondary infections associated with SAP with more higher accuracy than DWI and CT. ¹⁸F-FDG PET imaging has a high sensitivity but a low specificity. ADC values difficult distinguished infectious foci from aseptic necrosis, but DWI could display the ring wall of abscess well. Therefore, it would has potential value for determinating SAP secondary infection.Part 5Combination of ^99mTc-ciprofloxacin scintigraphy and CT for assessing the severity of SAPObjective: The aim of this part was to investigate the value and feasibility of combination of ^99mTc-ciprofloxacin scintigraphy and CT for assessing the severity of SAP secondary infection.Methods: The young swine were assigned into 2 groups. Six swine with non-infected SAP were assigned into the non-infected group, 16 swine with infected SAP into the infected group. Animal models were induced in accordance with the previous method we reported. At 7 d after inoculation, CT scan Infecton imaging were performed for assessing the severity of SAP secondary infection. Assessment methods are as follows: (1) CT severity index (CTSI) scored according to Balthazar CT criteria. (2) Modified CT severity index (MCTSI) scored according to the criteria of Mortele KJ. (3) United severity index scored according to combination of CT and ^99mTc-ciprofloxacin imaging. Compared differences between united severity index and CTSI, united modified severity index and MCTSI, respectively, when P <0.05 was considered statistically significant.Results: Seven days after inoculation, in infected SAP group, the united severity index (6.69±1.19) was higher than CTSI score (5.69±1.71) (t=-4.90, p<0.001), and the united modified severity index (7.06±1.44) was higher than MCTSI score (6.38±1.54) (t=-3.48, p<0.001). In non-infected SAP group, the differences between the united severity index and CTSI, and the differences between the united modified severity index and MCTSI were no significance (both p>0.05).Conclusions: When SAP secondary infection is suspected, the severity evaluated by CT alone, regardless of CTSI or MCTSI, would be underestimated, and united severity index, combinating the advantages of ^99mTc-ciprofloxacin imaging and CT, would improve the accuracy.