| Background and Objective:Acute mesenteric ischemia(AMI)is caused by the abrupt decrease in arterial blood supply to small intestine and mesentery or obstruction of drainage vein,which can not meet the physiological metabolic needs of small intestine,leading to ischemic injury or even ischemic necrosis for small intestine and mesentery.With the aging of the population,the incidence rate of AMI in the elderly population is increasing,emergency physicians,vascular surgeons and radiologists start to focus on it in clinic.AMI belongs to the category of acute abdomen,asymmetry of abdominal pain symptoms to physical signs,no timely and accurate diagnosis and treatment will lead to high mortality.Although with the rapid development of modern medical diagnosis and treatment technology,the mortality rate of AMI has not been significantly improved in the past ten years.The etilogy of AMI can be divided into arterial occlusion,venous occlusion and nonocclusive mesenteric ischemia.Among them,arterial occlusion is the main cause of AMI.The superior mesenteric artery(SMA)supplies blood to small intestine and mesentery,and its stenosis or occlusion can cause ischemic injury of small intestine and mesentery.The opening of aorta at the origin of SMA is large,the angle between SMA and aorta is sharp,the direction of blood flow in SMA is almost the same as that of the aorta.The cardiac emboli travel with the blood flow and pass through the opening of the aorta to block the distal trunk and branches of SMA.In addition,the shape of SMA is slightly curved,showing a certain radian.The increased shear stress of blood flow on the wall of SMA can cause vascular endothelial injury,and thrombosis is formed on the basis of endothelial injury.Based on the above anatomic characteristics,SMA is prone to thromboembolism under pathological conditions,resulting in mesenteric ischemia injury.SMA originates from the aorta about 1cm below the celiac axis(CA).The anatomical position of SMA and CA are adjacent and functionally related.SMA and CA are derived from the primitive visceral vascular branches originating from the ventral side of abdominal aorta in early embryonic development.During embryonic development,the anatomic positions of SMA,CA and their branches may vary.However,patients with arterial occlusion of AMI need laparotomy or vascular intervention,the anatomic variation of mesenteric artery must be evaluated before operation,which is of great clinical significance for making operation plan and avoiding iatrogenic vascular injury.The incidence of arterial occlusion of AMI is higher in the elderly.Elderly patients with AMI are often complicated with cardiovascular and cerebrovascular diseases,pulmonary diseases,peripheral vascular diseases and other coexisting diseases.Coexisting diseases can affect patients with AMI on the prognosis and the choice of treatment.Elderly patients with AMI complicated with severe coexisting diseases have higher risk of irreversible intestinal ischemia necrosis,surgical treatment and postoperative complications,and postoperative mortality.Early diagnosis,and timely and accurate treatment of AMI can reduce the probability of intestinal necrosis and improve the prognosis of patients.Multidetector computed tomography(MDCT)has the characteristics of fast imaging,simultaneous display of intestine and mesenteric vessels,and powerful post-processing function.It has high sensitivity and specificity in the diagnosis of AMI,and is recommended as the preferred imaging examination method in clinic.Enhanced MDCT can show the extent and severity of SMA stenosis,whether there is anatomical variation,intestinal ischemia injury and follow-up after treatment,providing valuable help for the diagnosis,treatment and prognosis of AMI.MDCT findings of mesenteric ischemia injury include specific and non-specific signs.It is a hot and difficult point to differentiate reversible intestinal ischemic injury from irreversible intestinal ischemic necrosis on MDCT.In clinic,only the irreversible intestinal ischemic necrosis needs to be resected for further histopathological evaluation.The relation between MDCT signs,ischemic duration and histopathological grading of intestinal ischemic injury can not be comprehensively understood.Therefore,MDCT diagnostic criteria based on histopathological grading and duration of intestinal ischemic injury can be established through animal experiments,which can provide reference for clinical practice.Based on the above,this study focused on the anatomy of SMA and the imaging features of SMA obstruction of AMI on MDCT angiography,with the following purposes:(1)To systematically describe the anatomical configurations of SMA on MDCT and explain the embryonic development mechanisms of SMA anatomical configurations;(2)To evaluate the clinical application value of quantitative and qualitative imaging signs of intestinal ischemic injury in patients with arterial occlusion of AMI by combining comorbidities and other clinical baseline characteristics;(3)To analyze the relationship between MDCT signs of arterial occlusion of AMI and histopathological scores of the severity of intestinal ischemia injury by establishing animal models of SMA occlusion of AMI,in order to understand the pathological basis of MDCT signs of intestinal ischemia injury caused by arterial occlusion.Materials and methodsPart Ⅰ:MDCT imaging of anatomical variations of SMA and their embryological mechanisms1.Study samples:The imaging data of 5580 patients who underwent abdominal enhanced MDCT and angiography in the Daping Hospital of Army Medical University from February 2008 to April 2018 were retrospectively analyzed.2.MDCT examination methods:All image data were acquisited on a 64 row(LightSpeed VCT 64,GE healthcare,Unite states)or 256 slice(Brilliance iCT 256,Philips healthcare,Cleveland)MDCT.Scanning range:from the top of diaphragm to the lower edge of right kidney or pubic symphysis.The scanning methods included plain scanning and dual phase(arterial and venous phases)dynamic contrast-enhanced scanning at 20 seconds and 50 seconds after injection of contrast agent(Ultravist;Bayer Schering Pharma).3.Image analysis:The arterial phase image data were transferred to the image post-processing workstation(ADW 4.3,GE health)for multiplanar reformation(MPR),volume rendering(VR)and maximum intensity projection(MIP)image reformations.Three abdominal radiologists with more than 10 years’ experience independently analyzed the anatomical origins and configurations of SMA,CA,common hepatic artery(CHA),left gastric artery(LGA)and splenic artery(SA).If there is a dispute,an agreement is reached among observers.4.Nomenclature of anatomical configurations:As the anatomical variations of LGA,CHA and SA,we redefined CHA and CA:regardless of the origin and anatomical course,an arterial trunk including at least one hepatic artery and gastroduodenal artery can be considered as CHA;an arterial trunk including at least two branches of CHA,LGA and SA can be considered as CA.Therefore,a common trunk originated from abdominal aorta,including SMA and at least two major branches of the CA,can be defined as CMT.In order to facilitate the description,SMA,LGA,CHA and SA are represented by the letters M,G,H and S respectively.Nomenclature of anatomical configurations:the common trunk giving off branches of vessels+other branching vessels directly originating from abdominal aorta.Common trunk is represented by abbreviation combination of branching vessels plus word"trunk".For example,SMA and CHA originate from a common trunk,but LGA and SA originate directly from abdominal aorta,named MH trunk+LG A+S A.5.Embryological mechanisms:From the point of view of embryonic development and origin of abdominal vessels,the embryological mechanisms of anatomic variations were elaborated in the discussion part.6.Statistical analysis:The incidence of anatomic variation and the percentage of various anatomic configurations of SMA were calculated.Part Ⅱ:Risk factors of MDCT findings for predicting mortality of acute SMA thromboembolic mesenteric ischemia1.Study samples:From February 2013 to December 2018,33 admitted patients with AMI due to SMA thromboembolism,who were clinically verified and underwent abdominal enhanced MDCT and angiography,were retrospectively analyzed in the Daping Hospital of Army Medical University.2.MDCT examination methods:The same as the first part.3.Image analysis and comorbidity evaluation:After reviewing and recording the basic clinical characteristics,comorbidities and prognosis,the 33 patients with AMI were divided into a death group and a survival group.The signs of intestinal ischemia injury,the extent and stenosis degree of SMA thromboembolism in the 33 patients were analyzed on MDCT transection and reconstruction images.Comorbidites are divided into 1-4 classes according to the Geriatrics Index of Comorbidity(GIC).4.Statistical analysis:Pearson χ2 test or independent sample t test was used to analyze the differences in clinical basic features,MDCT signs,comorbidities and GIC classification between the death group and survival group.Receiver operating characteristic curve(ROC)was used to analyze the diagnostic efficacy of risk factors associated with mortality.P value<0.05 was considered as statistical significance.Part Ⅲ:Relation of density of thrombus on MDCT to transmural intestinal necrosis in acute arterial thromboembolic mesenteric ischemia1.Study samples:From February 2013 to December 2019,33 admitted patients with AMI due to SMA thromboembolism,who were clinically verified and underwent abdominal enhanced MDCT and angiography,were retrospectively analyzed in the Daping Hospital of Army Medical University.2.MDCT examination methods:The same as the first part.3.Image analysis and quantification of thrombus Hounsfield units(HU):According to the intraoperative findings,postoperative pathological results and prognosis,33 patients with AMI were divided into an irreversible transmural intestinal necrosis(TIN)group and a reversible intestinal ischemic injury group.Two abdominal radiologists with more than 10 years of experience analyzed the imaging signs of intestinal ischemia injury,the location and extent of SMA trunk thromboembolism,the involvement of SMA branches,and the stenosis degree of SMA trunk.Based on the most proximal filling defect of thrombus on axial MDCT images in arterial phase,the density of thrombus(HU)was measured in each section on non-contrast MDCT images.The final HU value of each thrombus was calculated by summing the HU value of the ROIs in each section of thrombus and dividing by the number of sections.Two raters determined the HU values independently,and their results were averaged.4.Statistical analysis:Univariate analysis was used to analyze the differences of clinical features,MDCT signs and density of SMA thrombus(HU)between the irreversible TIN group and the reversible intestinal ischemic injury group.Factors significantly associated with irreversible TIN were selected for multiple regression analysis and receiver operating characteristic curve(ROC)analysis.P value<0.05 was considered as statistical significance.Part Ⅳ:Radiologic-histopathologic correlation of acute arterial occlusive mesenteric ischemia:an animal experimental study1.Experimental subject:18 New Zealand white rabbits were randomly divided into 1 control group(3 rabbits)and 5 ischemia groups(3 rabbits in each group).The animal model of AMI was established by ligating the root of SMA in rabbits.A simple laparotomy was preformed without SMA ligation for control groups.2.MDCT examination methods:After ligation of SMA for 0.5 h,1.0 h,2.0 h,4.0 h and 6.0 h in ischemia group,plain scanning and dual phase(arterial phase and venous phase)enhanced scanning were performed on 64 row MDCT(LightSpeed VCT 64,GE healthcare,Unite states).The control group underwent abdominal MDCT plain scanning and dual phase(arterial phase and venous phase)enhanced scanning 6 hours after sham operation.3.Histopathological examination:After MDCT examination,all rabbits were sacrificed and a segment of jejunum was removed.The tissue specimens were stained with HE according to the standard procedure and made into paraffin sections,which were observed under light microscope for the severity of intestinal ischemic injury according to the Park/Chiu score system.4.Image analysis:MDCT signs of intestinal ischemia injury in the control group and ischemia group were analyzed.Two corresponding ROIs were randomly placed on the thin section images to measure the density of intestinal wall(HU)in the control group and ischemia group.5.Statistical analysis:The correlation of MDCT signs and density of ischemic intestinal wall(HU)with Park/Chiu score and ischemic duration were analyzed.P<0.05 was considered as statistical significance.Results:Part Ⅰ:MDCT imaging of anatomical variations of SMA and their embryological mechanisms1.In the 5580 samples of abdominal MDCT angiography,549 samples(9.84%)had SMA or CA anatomical variation.Among them,486 samples(8.71%)had anatomic variation of SMA,and 63 samples(1.13%)had other anatomic variations(SMA had no anatomic variation,but the branches of CA had anatomic variation).The anatomic configurations of SMA were found on MDCT as the following 4 types:normal(5031,90.16%),HM trunk(248,4.44%),SM trunk(67,1.2%)and CMT(171,3.06%).2.248 samples of HM trunk included 2 types:HM trunk+GS trunk(230,4.12%),HM trunk+LGA+SA(18,0.32%);67 samples of SM included 2 types:SM trunk+HG trunk(60,1.08%),SM trunk+CHA+LGA(7,0.12%).3.171 samples of CMT included 5 types:type Ⅰ,HGSM trunk(96,56.1%);type Ⅱ,HSM trunk+LGA(57,33.33%);type Ⅲ,GSM trunk+CHA(4,2.34%);type Ⅳ,HGM trunk+SA(3,1.75%);type Ⅴ,the other type complying with CMT(8,4.68%).According to the origin of LGA,CMT can be further divided into four subtypes:subtype a,LGA originated from CA(92,53.8%);subtype b,LGA originated from abdominal aorta(57,33.33%);subtype c,LGA originated from single trunk(11,6.43%);subtype d,LGA originated from other branches(8,4.68%).Part Ⅱ:Risk factors of MDCT findings for predicting mortality of acute SMA thromboembolic mesenteric ischemia1.The mortality was 54.5%(18/33)in the 33 patients with AMI.Three risk factors signifiantly associated with mortality were identified,including pneumatosis and/or portomesenteric venous gas(PPMVG)(p=0.017),four regions of SMA involved by thromboembolism(region Ⅰ+Ⅱ+Ⅲ+Ⅳ)(p=0.036),and class 3+4 of comorbidities(p=0.001).2.The sensitivity and specifiity of PPMVG,region Ⅰ+Ⅱ+Ⅲ+Ⅳ,class 3+4 of comorbidities,and the three risk factors combined for diagnosing mortality were 33.3%and 100%,27.8%and 100%,83.3%and 73.3%,and 88.9%and 73.3%,respectively.The AUC of the three risk factors combined(0.88)and class 3+4 of comorbidities(0.78)were larger than that of PPMVG(0.67)and region Ⅰ+Ⅱ+Ⅲ+Ⅳ(0.64).3.The mortality rate rose from 15.4%in patients without risk factor to 66.7%,100%,and 100%in patients with one,two,and three factors,respectively.Part Ⅲ:Relation of density of thrombus on MDCT to transmural intestinal necrosis in acute arterial thromboembolic mesenteric ischemia 1.33.3%(11/33)of AMI patients were diagnosed with TIN.Peritonitis(p=0.042),bowel wall thinning(p=0.033),PPMVG(p=0.010)were significantly associated with TIN.AMI with TIN had higher density of SMA thrombus compared with AMI without TIN(41.2±6.1 HU versus 34.2±3.0 HU,p=0.003).2.Multivariate analysis showed density of SMA thrombus was an independent predictor of TIN(p=0.044,HR＝1.82,95%CI:1.02-3.25).3.For diagnosing AMI with TIN,the sensitivity and specificity for peritonitis,bowel wall thinning,PPMVG,and density of SMA thrombus were 54.5%and 81.8%,36.4%and 95.5%,45.5%and 95.5%,and 72.7%and 86.4%(cutoff value,36.2 HU),respectively,the AUC of density of SMA thrombus(0.83)was larger than that of peritonitis(0.68),bowel wall thinning(0.66)and PPMVG(0.71).Part Ⅳ:Radiologic-histopathologic correlation of acute arterial occlusive mesenteric ischemia:an animal experimental study1.While the ischemia time lasted no more than 2 hours and the Park/Chiu score ranged from 0 to 5 score,absent or reduced bowel wall(BW)enhancement and mild effusion and dilation of bowel lumen could be found on MDCT.While the ischemia time lasted more than 4 hours and the Park/Chiu score ranged from 4 to 7 scores,absent or reduced BW enhancement,effusion and dilation of bowel lumen,bowel wall thinning,increased bowel wall attenuation,and pneumatosis intestinalis could be found on MDCT.2.With the prolongation of SMA ligation,the mean attenuation at BW on MDCT decreased,the attenuation value at BW on un-enhanced CT(un-ECT)(r=-0.56)and contrast enhanced CT(CECT)(r=-0.85),and the subtraction values at BW between CECT and un-ECT(r=-0.80)were inversely correlated with the Park/Chiu score.Conclusion:1.The anatomical configuration of SMA was systematically elaborated on MDCT.Dislocation interruption,incomplete interruption and permanent longitudinal anastomosis were the embryological mechanisms of various types of anatomical variation.2.Three risk factors for mortality were identified in patients with AMI due to SMA thromboembolism,including PPMVG and four regions of SMA involved by thromboembolism on MDCT images,and class 3+4 of comorbidities.Close monitoring of these risk factors could possibly lower the mortality.3.Erythrocyte-rich thrombi with higher density on MDCT were prone to occurrence of TIN in patients with AMI compared with erythrocyte-scarce thrombi with lower density.The density of SMA thrombus could be an independent risk factor of TIN in patients with AMI due to SMA thromboembolism.4.With the prolongation of ischemia time and the increasing Park/Chiu score,more obvious signs of intestinal ischemic injury were found and the attenuation at BW decreased on MDCT.The attenuation at BW on CECT,and subtraction values of attenuation at BW between CECT and un-ECT are superior to the attenuation at BW on un-ECT in quantifying the degree of intestinal ischemic injury. |
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