MRI Study On The Rectum Canal Around Abscess

ObjectiveTo compare the imaging of anorectal abscess by different MRI sequences,to explore the better sequences and scan orientation of the internal opening of abscess and fistula. To evaluate DWI performance of anorectal abscess, and the value of DWI and ADC value in differential diagnosis of anorectal abscess and fluid.Materials and Methods32 cases which were confirmed as anorectal abscess by surgery. A main clinical manifestation of 30 cases is anal sore. There are 30 males ages from 20 to 66 years old, and the average age is 43.97±11.13 years old. Their weights are from 55kg to103kg, the average weight is 76.50±12.99kg. There are 2 females, their age are 22 and 37 years old and the weights are 55 and 66kg. 18 cases with pus or anal secretions, 5 patients with high fever and chills. There were 3 patients who had anal fistula or anorectal abscess surgery, and 1 patient with operations twice in the same location because of perianal abscess. Preoperative MRI examination and a rectal examination were carried out for all cases, there were 4 cases enhanced. Magnetic resonance imaging system is Siemens Avanto 1.5T SQ, using 8-channel phased array surface coil. Conducting sagittal T2WI imaging through the body midline, determining the relative position of the anal canal, the axial and coronal imaging is parallel and perpendicular to the long axis of the anal canal. Scanning sequences included sagittal T2WI, coronal T2WI, coronal T2 fat suppression, axial T1WI, axial T2WI, axial T2 fat suppression and dynamic contrast-enhanced sequence, with additional diffusion-weighted imaging sequence. The ADC values of anorectal abscess are measured, obtaining average of multi-point measurements.FOV is based on the patient's height, weight, etc. and FOV size is 230mm×230mm ~ 400mmx400mm and it should cover the region above the levator ani muscle to 6 cm, and the space behind rectum and the perineum.SPSS15.0 statistical software were used. The data was analyzed byχ2test and t test. The result would be statistically significant when p is less than 0.05.Results Abscess showed long T2 and long T1 signal intensity around the anal sphincter gap or intersphincteric space and high signal in imaging of fat suppression sequence and high signal in DWI. There are intersphincteric abscess in 17 cases, ischial anal fossa abscess in 22 cases, pelvic rectal space abscess in 5 cases, abscess around the anus in 5 cases, abscess under the rectal mucosa in 4 cases, abscess appears like latter half of horseshoe in 1 case.Results of identifying the abscess and effusion or edema: the ADC values of abscesses in 32 cases, ADC values of fluid in 23 cases, t test, t=3.472, p=0.003.In this study, locations of internal opening are 6 o'clock in 15 cases, 12 o'clock in 4 cases, 2 o'clock in 5 cases, 8 o'clock position in 2 cases, 9 o'clock position in 2 cases. Axial T2WI shows more internal openings compared with axial T1WI(χ2=16.409, p=0.000), axial T2WI-FS shows more internal openings compared with axial T1WI (χ2=20.046, p=0.000), axial T2WI shows more internal openings compared with coronal T2WI (χ2=16.056, p=0.000), axial T2WI-FS shows more internal openings compared with coronal T2WI (χ2=18.050, p=0.000).Fistula is long T1 and long T2 signal tubular from the abscess parts to the anal canal or perianal skin and is high signal in imaging of fat suppression sequence and high signal in DWI.There are fistulas between the internal and external sphincter in 9 cases, fistulas transverse the sphincter in 6 cases, sphincter exterior in 1 case, 5 cases above the sphincter, fistulas appear like latter half of horseshoe in 3 cases. The proportions of intersphincteric fistula, transsphincteric fistula, suprasphincteric fistula, and extrasphincteric fistula are 45%, 30%, 20%, 5%, respectively.Coronal T2WI shows more fistulas compared with axial T1WI (χ2=5.143, p=0.023).Axial T1WI compared and axial T2WI comparison, axial T2WI and coronal T2WI comparison, axial T2WI-FS and coronal T2WI comparison, axial T2WI and axial T2WI-FS comparison, the results of the fistula showed no difference (p> 0.05).ConclusionMRI can accurately diagnose anorectal abscess and have high diagnostic accuracy rate about the abscess location, scope and availability of fistula.Transverse position has high diagnostic accuracy rate about fistula internal opening especially axial T2WI (axial T2WI-FS). Coronal position displays abscess area and if the levator ani muscle is involved better. The coronal position has same value with the axial position in fistula display.DWI can provide important supplementary information for the diagnosis of anorectal abscess and differential diagnosis. DWI and ADC values can help identify anorectal abscess and fluid.The study can provide more detailed image data for the choice of surgical methods, can significantly optimize surgical approach and the probing scope of surgery, and has important value in guiding surgery.