| ObjectiveTo observe the performances of main structures in anus region on CT images divided into normal group and anal diseases group.By the means of imaging and anatomic observation and measurement on the structures,including local skin,sciatic anal diaphragm,deep fat,blood vessels,muscles,anal canal,rectum,and their positional relationships,so as to explore the diagnostic values of CT in common anal region diseases.Materials and MethodsCT imaging data of 172 cases of non anal diseases(referred to as the control group)and 118 cases of anal diseases(referred to as the test group)from April 1,2015 to December 10,2016 were retrospectively analyzed..Using SIEMENS AW4.2post-processing workstation software and PHILPS EBW(4.5.6 version)workstation AVA software,from shallow to deep,one by one to observe and measure the local skin,septal anal canal,deep fat,blood vessels,muscles,anal canal and rectum.The t-test or one-way analysis of variance were used to compare the imaging differences in and between the control group and the test group,in order to evaluate the values and methods in the diagnosis of common anal diseases.Results(1)In the control group,the margin of the anal epidermis and dermis were clear,and the thickness of the male(2.06±0.47~2.38±0.51mm)was slightly larger than that of the female(1.63±0.49~1.92±0.44 mm).In the test group,some CT signs such as thickening of the epidermis and dermis,hazy outline of the local dermis were observed.,The CT sign of thickness was significant than the control group in the perianal abscess,anal fistula and hemorrhoids(P<0.001).The CT sign of hazy outline was found in the perianal abscess,anal fistula,hemorrhoids,rectal cancer patients,especially in the perianal abscess patients(P<0.001).(2)Line shadows of soft tissue density and the superficial superficial fascia transverse were easily displayed on CT.The soft tissue density stripes was obviouslymore in male than that of female,and the rate of visualization of subcutaneous superficial was significantly higher than that of female as well(P<0.05).The CT value of superficial layer fat in male subcutaneous fascia was significantly higher than that in female(P<0.05),The thickness of female subcutaneous fascia is obviously larger than that of male(P<0.05).In patients with perianal abscess,the superficial fascia of the subcutaneous fascia can be seen as patchy and nodular lesions,usually accompanied by subcutaneous fat,superficial soft tissue density,stripe shadow increase,thickening change,and local CT value will be significantly increased(P<0.001).(3)On the CT images,the Milligan's septum might be symmetrically obsvered as a continuous,or interrupted.or branching soft tissue density shadow,which extended from the external anal sphincter to the calcaneal nodules.The detection rate of Milligan's septum was 23.3% in the control group,the average thickness of the left and right sides were 2.50±0.46 mm,2.89±0.78 mm,respectively.In the control group,The density difference between the upper and lower adipose tissue could be identified by the naked eye in 88.4% of men and 38.4% of women,with significant differences between sexes in the incidence(c 2=46.313,P <0.001).In the patients with perianal abscess,proctitis,the density difference of the adipose tissue above and below the Milligan's septum tended to be more obviously.(4)The posterior diameter and the transverse diameter of the anorectal space and ischorectal space were all consistent in both sides or between men and women(P>0.05).In the control group,developing fog occurred in 6 cases of pelvic rectal space,1 case of ischial rectum and 30 cases of posterior rectal space.The percentage of clear imaging of the three gaps was 97.67%,99.42% and 82.56% respectively.The mesenteric fascia of27 cases was not developed,the development rate was 84.30%.The mean CT values of adipose tissue in the pelvic rectal space,ischial rectal space and posterior rectal space were-100.62±8.84 Hu,-112.06±5.26 Hu,-87.66±11.62 Hu,respectively.The difference of CT values between every two groups was significant(P<0.05),and there was no significant difference in CT value between the left and right sides of the same gap(P>0.05).In the test group,the development of the above spaces and mesorectal fascia is most obvious in the rectal cancer patients group(c2=13.810,P=0.003,P<0.05).In theperiosteal abscess group,the CT values of adipose tissue of the ischial rectal space and the posterior rectal space was significantly higher than that in the control group(t1=3.366,p<0.001.t2=4.119,P<0.001).In rectal cancer group,the CT value of adipose tissue of the pelvic rectal space and the posterior rectum of the rectum were also significantly higher than those in the control group(t1=4.127,P<0.001.t2=3.228,P<0.001).(5)In the control group,the suspicious vascular development rate in the subcutaneous region of anus was 15.12%(26/172),and the rate of vascular development was 100% in ischial anus fossa and pelvic rectal space.There were no significant differences in the left and right sides of the vessels in diameter,length and between the male and the female(P>0.05).The rate of vascular development in the posterior rectal space was 53.48%.In the patients with high perianal abscess and rectal cancer,the rates of vascular development in the pelvic rectal space and ischial rectal space were significantly reduced because of the lesion concealment(P <0.001).(6)The anal muscle groups colud be clearly displayed on MPR reconstructed CT images.The levator ani muscle was appeared as Chinese inverted "?" word in shape,located on both sides of the rectum.On coronal position,it might be presented as funnel-shaped(6.54%),basin(8.46%)or dome(85.00%),and the average thickness of male was less than that of female(P<0.05).The levator ani muscle might be invaded when perianal abscess occured,,resulting in thickening of the levator ani muscle,especially in high abscess patients,but this change were not happened in the residual patients.The pubic muscles constitute the medial wall of the anal fossa and appear as a "U" letter shape surrounded by the vagina(or prostate in male),the urethra and the anal canal.The obturator inner muscle is also appeared as Chinese inverted "?" word in shape,and it is one side of the triangular fossa of the rectum,the other two sides are the pubic rectal muscles and the posterior gluteus maximus.The edges of the muscles were clear,and no significant differences of the muscles thickness were found between male and female in the control group.In the test group,the local puborectalis muscle swelling could only be seen in the perianal abscess petients because of the invasion.The deep part of the external anal sphincter showed as the shape of "V".The superficial part of the sphincter is oval,located in the upper,middle and lower parts of the anal canal,andthe tail of the superficial muscle extended to be the posterior part of the anal tail ligament.In the middle of the coronal position,the superficial part of the anal sphincter is inverted as "J" shape,just located below the anal canal,and the average widths of the deep part,the superficial part and the subcutaneous part of the muscle on the left and the right were(7.02±1.63,7.40±1.50)mm,(6.60±1.38,7.27±1.27)mm and(7.13±1.65,7.32±1.81)mm with no significantly difference between male and female.In the test group,the size and morphology of anal sphincter spacing were almost same in the test group with the control group,but the anal tube,the involvement of the order from deep and shallow,,the intersphincteric groove and the Minor triangle always showed unclearly than that in the control group(P< 0.001),especially in the perianal abscess and hemorrhoids patients.(7)The developed unclear ratio of the upper anal canal and the distance between the internal and external sphincter was 5.23% and 2.33%,respectively.The development rate of sphincter,Minor triangle and anal ligament were 100%.Of the 172 cases,the anococcygeal ligament by external anal sphincter(superficial),composed of the levator ani muscle,composed of levator ani muscle and external anal sphincter(superficial)together constitute of number of cases were 21 cases,49 cases and 102 cases,and the proportion of the three muscle constructs was 12.21%,28.48%,59.88%,which the length and width were about 28.28±10.19 mm and 9.77±2.89 mm respectively.The anteroposterior diameter of the upper and lower anal canal of male was larger than that of female,however its left and right diameter was smaller than that of female,the former of difference was statistically significant(P<0.05).The latter was not statistically significant(P>0.05).The mean values of sphincter spacing were3.66±1.29 mm and 3.33±1.43 mm in male and female,The average diameter of the sphincter of the male and female were 2.81±1.01 mm and 3.56±3.72 mm respectively.The anteroposterior diameter and width of the anal ligament were about 28.28±10.19 mm and 9.77±2.89 mm,there was no significant difference between them.In experimental group,the size of anal canal,distance between internal and external sphincter,sphincter groove and so on were similar to those of control group,but there was significant difference between anal canal,internal and external sphincter spacing,sphincter sulcus and Minor triangle(P<0.001),especially perianal abscesses,hemorrhoids patients.(8)On the coronal MPR images,the most common way of the side bends of therectum from top to bottom was the "right-left-right" mode(27.90%)and the rectal serosal surfaces were usually clear(93.60%,162/172 cases.Rectal gas were observed in152 cases(87.79%).The pararectal lymph nodes were found in 30 cases(17.4%)with the average size of(3.6±1.27)×(3.87±1.87)mm.In the natural state,comparison between male and female on the maximum anterior and posterior diameters of the upper,middle and lower rectal canal showed no statistically significant difference(P>0.05).There were significant differences between the test group and the control group,whether the rectal wall were clear or not,and the lymph nodes presented or not(P<0.05).Whether the lymph nodes presented or not,and the numbers and size were valuable in the diagnosis of the perianal diseases.Conclusion(1)On CT,the anatomical observation fo skin,fascia,fatty tissues,muscles,rectum,anal canal could be done accurately.From the point of technology,the messurement of upper anal canal,deep anal sphincter,puborectal rectum,blood vessels in pelvis rectal space were difficult.(2)Constant morphological performances existed in skin,fat density,obturator muscle,gluteus maximus,levator ani muscle,external anal sphincter,anal canal internal and external sphincter spacing,intersphincteric groove and minor triangle.Other indicators such as the anal ligament,rectum wall,the way of the rectum side bends were diffcult to analyze due to their varieties of performances.(3)In normal subjects,significant differences of the CT value and thickness of subcutaneous adipose tissue were found between male and female.No differences of CT value and size of deep adipose tissue occurred between male and female.The CT values of the adipose tissue in the pelvic space were similar to that of the subcutaneous space.It was probably associated with the presence of the mesorectal fascia and less fat content in the posterior space of the rectum.(4)As a boundary between ischial rectal space and anal subcutaneous gap,ischial anal canal septum could be identified from the difference of CT density in the two spaces,and some of them could develop directly on the CT..(5)The subcutaneous fascia vessels in the anal region were difficult to develop,and the vascular development rate was 53.48% in the posterior pelvic space.The vesselswere 100% displayed in the space of the anorectal space and the ischiorectal space.In the perianal abscess and rectal cancer,the development of blood vessels might be helpful for the diagnosis of these diseases,but the specificity is not strong.(6)On the coronal position,the levator ani muscle in normal persons showed three forms(fornix,basin,funnel).Thickness of the muscle in women was significantly greater than that of men.Three muscles(Pubic rectal muscle,obturator muscle and gluteus maximus)formed the ischioanal fossa which looked like a triangular shape and with little difference of the size between men and wemen.The anal canal wsa surrounded by the external anal sphincter which composed of three parts(deep,shallow,subcutaneous),and the deep part looked like "V" shape,the superficial was "oval",and the subcutaneous part was varused like "J" shape.The potential gaps(such as the internal and external sphincter spaces,the sphincter gap,and the Minor triangle)between the anal sphincter and the anal tube could be observed on CT.(7)In the perianal abscess,when the lower anal subcutaneous gap were involved,the regional skin usually showed thickening significantly,and the vague sign frequently occurred in the anal sphincter spacing,the sphincter groove and the Minor triangle.There subcutaneous fascia might increase obviously or not,and the density difference between the upper and lower regions of the septum might be changed.When abscess occurred in the anorectal area or the ischorectal area,the CT density of fatty tissue in these areas might turn to be higher.Sometimes,the lesion might invade the fascia of the rectum,the levator muscle,the puborectal rectum,external anal sphincter or anal canal.(8)When suffering from proctitis,the corresponding pelvic rectal space might turn blurred,the ischial anal canal regional density might turn different,even the Minor triangle might turn blurred,but all of these specificities were not high.(9)In hemorrhoids,especially mixed hemorrhoids with mucosal prolapsed,signs of blurred sphincter groove and blurred Minor's triangle were very common and only next to the perianal abscess disease.(10)In rectal cancer,some specific characteristics could be showed on CT,such as thickening,roughness,and greater inhomogeneous enhancement of the rectal wall.Other manifestations,such as increased lymph nodes near the rectum,vague of the anorectal space and posterior pelvic space,and increased CT values of adipose tissue inthe gap,were also common.(11)In anal fistula,constipation and anal fissure,few special performances were found on CT imaging. |
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