MR imaging of suspected perianal inflammatory pathologies is an established method for diagnosis and defining the extent of this inflammation. The standard MRI sequences used for those patients include basic anatomical sequences with pre- and post-contrast images. The principle role of MRI in the setting of perianal fistulas is to define the extension of the tract, side branches, and whether there are any deep abscesses, especially at supra-levator level. Post-contrast sequences are an essential component of this study to better reveal hidden extension, branching tracts, and suspected abscesses. Active granulation tissue in the edges of the fistula enhances diffusely, while its lumen does not. Inflammation that surrounds the fistulous tract also enhances diffusely [8, 9]. An abscess gives the characteristic ring enhancement which confirms its diagnosis and outlines its extent. Few authors tried to correlate the enhancement of the fistula to its activity [8,9,10,11,12,13,14]. One author suggested that rapid and maximum enhancement during dynamic MRI scanning correlated well to the disease activity. However, the applicability of this dynamic imaging is limited by the poor spatial coverage (to improve the temporal resolution) which limits the evaluation of the extent of inflammation .
DWI is recently being studied by some researchers to compare its performance in visualizing and grading perianal fistulas and abscesses. Extra-cranially, oncologic applications of DWI are the most common. However, the evaluation of abscesses is a very important application for DWI, owing to the maximum contrast between the abscess cavity and the surrounding inflammation on the DWI image [1,2,3,4, 16,17,18,19].
Some authors suggested that DWI was more sensitive than T2W sequence in the visibility of perianal fistulas. Dohan suggested that DWI had a sensitivity of 100% in detecting perianal fistulas, whereas T2W had a sensitivity of 91.2 % . Other authors suggested that the visibility of perianal fistulas was significantly higher with combined DWI and T2W than with T2W alone [3, 4, 7]. In our study, we were unable to reproduce the former results. In our sample of patients, DWI was equally effective as T2W sequence in visualizing perianal fistulas and abscesses (p = 0.14), although DWI has detected less number of fistulas than T2W in all visibility grades. But we did agree with Cavusoglu et al., Hori et al., and Bakan et al. that the visibility of perianal fistulas was higher using combined DWI and T2W evaluation, where we detected 96.7 % of perianal fistulas [3, 4, 7]. Only 1 fistula could not be visualized on both DWI and T2W; this was visualized on post-contrast images, and this patient had a grade 1 perianal fistula with normal CRP; the patient was in the NIA group and did not require surgery. Visibility of perianal fistulas on DWI images was not significantly different for PIA and NIA groups. Perianal abscesses, on the other hand, were equally well visualized on both sequences.
Perianal fistulas generally had higher ADC values than perianal abscesses. The average ADC value for perianal fistulas was 1.39 ± 0.4 × 10−3 mm2/s. For perianal abscesses, the average ADC value was 0.8 ± 0.66 × 10−3 mm2/s. This comes in agreement with other authors, reflecting the viscid nature of the abscess core, which is not seen in fistulous tracts [1, 2].
The ADC of perianal fistulas did not show any correlation to the disease activity as determined by CRP and leucocytic count (p = 0.38 and p = 0.59, respectively). It also did not vary between PIA and NIA groups (p = 0.45). This came in contradiction to some authors who suggested that ADC values were lower in PIA patients . The reason for the contradiction between our results and Yoshizako et al. may be related to the method of ADC measurement, where in their study, they used a free ROI that encompassed the entire lesion and calculated the mean ADC value, whereas in our study, we used a small ROI within the center of the lesion and calculated the minimum ADC value . It was also postulated by some authors that small abscesses or pus collection may give an artifactual high ADC value than large abscesses which is probably related to the difficult positioning of the ROI in small abscesses . This was noted in one of our patients, where she had a small abscess with ADC value of 0.9 × 10−3 mm2/s. Another explanation for the overlap of ADC values of PIA and NIA groups may be related to the different concentrations of inflammatory cells in small and large abscesses, different immune response of the patient, and age of the abscess; all of these factors influence the viscosity of the pus and accordingly alter the ADC value [2, 20, 21]. Some authors suggested that there was a significant difference between ADC values of PIA and NIA groups in cases of fistulas associated with abscess, but not for fistulas without abscesses. However, in their study, the authors used a large ROI that encompassed a large area of the lesion, so the viscid nature of the abscess probably dominated the ADC measurement, rather than the fistula itself . In our study, all perianal abscesses belonged to the PIA group and the ADC of abscesses were significantly lower than that of perianal fistulas without abscesses, which comes in agreement with Bakan et al. .
Using the St. James’s University Hospital classification, DWI accurately classified 84.4% of the perianal fistulas and abscesses. Only 1 case (2.2%) was misclassified by DWI alone; this patient had a very small collection with high ADC value, so it was considered grade 1 on DWI images, while on post-contrast images, it was considered grade 2. DWI alone was significantly less than post-contrast images in the accurate classification of perianal disease, mainly in the NIA group, but not in the PIA group, who are more likely to have surgeries. Using combined DWI and T2W evaluation, 97.8% of the perianal fistulas and abscesses were accurately classified and this was not significantly different between PIA and NIA groups. Our results agree with the results of Cavusoglu et al., where he states that the combined DWI and T2W evaluation had a high diagnostic performance that is not significantly different from the combined T2W and post-contrast images evaluation .