Patients' Selection:
This prospective study included 38 patients with a total of 45 perianal fistulas and abscesses.
Inclusion criteria: Any patient with suspected perianal fistula or abscess,
eGFR ≥ 60 ml/min/1.73m2 and no contra-indication to IV gadolinium contrast or to MRI.
Exclusion criteria: Patients with contra-indication to gadolinium contrast or MRI,
e.g.
claustrophobia.
Patients with perianal abscesses who refused surgery - despite being indicated - were excluded from the study to avoid causing bias to the ADC analysis.
The decision of surgery was based solely on clinical and laboratory evaluation,
which included the following criteria: severe pain or restriction of daily activity,
restriction of sexual activity,
reddish edematous skin,
pus discharge and increased serum C-reactive protein (CRP) levels (>5 mg/L).
Fistulas which revealed pus at surgery were considered to be active,
whereas fistulas which didn’t reveal pus,
didn’t require surgery or were associated with normal CRP levels were considered non active.
Patients were classified according to the activity of fistulas into Positive Inflammatory Activity (PIA) and Negative Inflammatory Activity (NIA) groups.(2,4)
MR Imaging:
All patients were imaged on a 1.5T Philips Ingenia machine (Philips Healthcare,
Best,
the Netherlands).
Imaging sequences included routine T1W,
T2W,
fat suppressed T1W and T2W,
STIR as well as post-contrast T1W sequences in 3 orthogonal planes.
The axial plane was used for evaluation.
DWI was added to the study with the following criteria: Axial,
TR/TE = 6400/100 ms; slice thickness = 5 mm; interslice gap = 0.5 mm; number of slices = 24; matrix size = 188 × 192,
with reconstruction to 256 × 256; FOV = 385 mm × 385 mm; NEX = 4; b-values of 100,
300 and 600 s/mm2.
Image analysis:
The perianal fistula was evaluated on T2W,
DWI and post-contrast fat suppressed T1W sequences as per its visualization and extent.
T2W and DWI images were evaluated separately 2 weeks apart then both sequences were simultaneously evaluated after 2 more weeks,
to avoid recall bias.
The visibility of the fistula/abscess on T2W and DWI images was scored according to a 3-point scoring system: Score 0 for non-visualization,
Score 1 for poor visualization and Score 2 for well-visualization.(4) ADC values were recorded by placing a small ROI within the area of abnormality - on the slice where it’s best visualized - and recording the minimum ADC value.
To evaluate the performance of DWI in grading the perianal inflammation,
the extent of the perianal fistula/abscess was determined on DWI,
combined T2W & DWI and combined T2W & post-contrast images,
separately.(5) The fistula was then graded according to St.
James’s University Hospital classification using each of the DWI alone,
combined T2W & DWI and combined T2W & post-contrast images,
separately.(6) The combined T2W & post-contrast images were used as reference for grading the perianal fistula/abscess.(7) In cases of perianal abscess with non-visualization of the related fistula,
the minimum ADC value was recorded from the abscess core.
Statistical Analysis:
Statistical analyses were performed using SPSS software (version 21.0; SPSS Inc.,
Chicago,
IL,
USA).
Numerical data,
e.g.
age and ADC value,
is represented as mean ± standard deviation,
while non-parametric data as percentage.
The visibility scores on each of the DWI and the T2W images were compared to those on the combined T2W and DWI image evaluation using Chi Square test.
The same visibility scores were similarly compared between PIA and NIA groups.
All perianal abscesses belonged to the PIA group and were well visualized on both sequences,
so they were excluded from the later 2 analyses.
Independent sample T test was used to compare between ADC values of PIA and NIA groups.
Perianal abscess were excluded from this analysis because they all belonged to the PIA group.
ADC values were correlated to the CRP level and leucocytic count using Pearson’s bivariate correlation test.
Finally,
the grading of the perianal fistula/abscess (St.
James’s University Hospital grading system) on DWI and combined DWI & T2W was compared to the combined T2W & post-contrast evaluation and between PIA and NIA groups using Wilcoxon signed rank test.
Significance level is considered if p < 0.05.