The role of MRI in evaluation of ankylosing spondylitis related posterior pelvic articular changes

Introduction

Spondyloarthropathy comprises a group of chronic inflammatory rheumatic diseases, including ankylosing spondylitis, reactive arthritis (Reiter's syndrome), arthritis, or spondylitis associated with inflammatory bowel disease, psoriatic arthritis, and undifferentiated spondyloarthritis. These afflictions predominantly affect the axial skeleton, causing pain and stiffness are seronegative for rheumatoid factor, and are often associated with the presence of human lymphocyte antigen (HLA)–B27 ^[1].

Sacroiliitis is an inflammatory disease induced by an immune-mediated mecha-nism and is the most common initial feature, The usual presenting symptom is inflammatory low back pain^{[2, 3].}

Imaging of sacroiliitis provides critical objective evidence for diagnosis. Plain radiography,  computed tomography (CT), magnetic resonance imaging (MRI) and quantitative SI scintigraphy is the available imaging modalities to evaluate sacroiliitis ^[4].

Ankylosing spondylitis develops through complex interactions between genetic background and environmental factors as an autoimmune disease ^[5].

Materials and methods

Patient populations

This observational analytic study was conducted in the MRI unit, department of Diagnostic Radiology, Faculty of Medicine, Minia University, during the period from June 2021 to Jan 2022, after being approved by the Medical Ethics Committee of the department.

Forty patients diagnosed with AS were referred from Rheumatology Clinic to the MRI unit to be recruited in the study through counseling and a written informed consent were obtained from each patient prior to participating in the study.

All recruited patients were submitted to the following:

• Thorough medical history taking and examination using Ankylosing spondylitis Disease Activity Score (ASDAS) ^{[6, 7]}.
• Underwent MRI examination.
• All patients underwent CT scan.

MRI study of the SIJs using Ingenia 1.5 T Philips closed MR scanner using body phased-array coil.

Image analysis:

• Adequate preparations are done to all patient acquiring an optimal MRI examination.
• Psychological preparation was done for the patient about scanner environment and informing the patient about each step to make the patient felt comfortable and safe, which are critical and essential for avoiding patient movement, that may decrease image quality to great extent.
• The patient lied in a supine position, head first for MRI exami-nation of the sacroiliac joints.
• All metallic substances were removed prior to examination.

Results

The study entrolled 40 patients, 27 patients are male (67.5 %) and 13 patients  (13 %) are females. Mean age of entrolled patients was 34+/- 11 years, Table 1.

Table 1: Demographic data

The current study showed that sensitivity and specificity for Backfill on MRI Dixon  is higher than sensitivity and specificity for Backfill on MRI T1 in comparison with erosions on CT, Table 2.

Table 2: Sensitivity and specificity for Backfill on MRI Dixon/ MRI T1 in comparison with erosions on CT

PPV (positive predictive value), NPV (Negative predictive value).

Discussion

Bone erosions seen on T1W images and in-phase T1W based 3D Dixon images, our results show that number of erosions on Dixons images is higher than number of erosions on T1WI in same patients using CT study as a reference. This is in agreement with (Huang, H. et al., 2020) ^[8].

Our results showed that both in-phase and fat-only T1W based 3D Dixon images are considered superior in detecting subchon-dral sclerosis in sacroiliac joints in comparison with conventional T1-weigh-ted images. This is in agreement with (Özgen, A., 2017). ^[9]

Water-only T1w based 3D Dixon images couldnot show active subchondral bone marrow changes of sacroiliitis unlike Water-only  T2-weighted multipoint Dixon images is considered superior for the detection of the active changes ^[9]