The evaluation of bone marrow edema in sacroiliac joint in patients with ankylosing spondylitis using magnetic resonance imaging Dixon sequence

Ankylosing spondylitis (AS) affects one in two hundred individuals. AS is usually diagnosed many years after onset of symptoms [1]. It is a type of immune-mediated systemic chronic inflammatory disorder, common in adolescents, primarily involving the sacroiliac joint and spine [2]. Therefore, the clinical manifestation is chronic lumbosacral region pain. Severe AS can cause spinal deformity and joint rigidity. Sacroiliac disease or lesions are considered as an earlier indicators of AS. This helps in the early diagnosis and treatment of AS.

The damage of articular cartilage injury and edema of bone marrow under the articular surface is the main pathological manifestation of sacroiliac arthritis in the early stage. It is difficult to find sacroiliac joint lesions using the X-ray in the early period because of the deep sacroiliac joint location, twisted, close joint surface, and large individual differences. However, the introduction of magnetic resonance imaging in sacroiliac joint has improved imaging over the recent years.

X-rays remains the cornerstone of diagnosis of AS. However, MRI has the ability to visualize bone marrow edema and inflammatory lesions within bone in three dimensions. Therefore, diagnosis of AS can be made earlier than when using X-ray to find sacroiliac joint lesions with multi-parameter, multi-sequence, multi-directional imaging and high soft tissue resolution characteristics. With the development of new MRI technology, a growing number of sequences are used in the diagnosis of sacroiliac disease, such as Dixon sequence. Dixon sequence is a quantitative measure of tissue water-fat ratio imaging sequence.

Bone marrow edema (BME) is defined as an area of altered signal on the magnetic resonance imaging (MRI) of the bone. Quantitative assessment of BME using MRI is useful in determining the stage of sacroiliac arthritis. However, there are few reports on the application value of Dixon technique in the evaluation of bone marrow edema in sacroiliac arthritis. The purpose of this study was to explore the value of magnetic resonance imaging Dixon sequence in the quantitative analysis of bone marrow edema in the diagnosis of sacroiliac arthritis in patients with ankylosis spondylitis.

The bone marrow water-fat ratio in the region of interest delineated under the sacroiliac joint surface by MRI Dixon-vibe sequence was measured. The results showed that the water-fat ratio in the ankylosing spondylitis (AS) group were significantly higher than that in the control group (P < 0.05). This suggested that this sequence can be applied to evaluate bone marrow edema in the sacroiliac joint. This will help to diagnose the active phase of inflammation. Therefore, similar to previous studies by koo [6], obvious bone marrow edema exists in early AS, which is displayed using MRI Dixon. During the active phase of inflammation, fat content bone marrow edema is lower than the normal marrow region. As post-inflammatory phase, the proportion of fat stored in the bone marrow will increased accumulation [6]

AS is a chronic inflammatory disease affecting mainly young adults. AS belongs to the group of multiple seronegative spondyloarthropathy. Early and accurate detection of AS is essential for effective disease treatment [7]. Results show that bone marrow edema of the sacroiliac joint is the early imaging manifestation and the symbol of inflammatory activity of AS [8]. Therefore, the early and quantitative detection of bone marrow edema in the sacroiliac joint is crucial for the timely treatment of AS patients, as well as the evaluation of efficacy and the monitoring of pathological activity during treatment.

Currently, BASDAI, BASFI score, CPR and erythrocyte sedimentation rate are mostly used to evaluate the disease activity and severity in clinical practice [9]. However, previous studies have showed that the sensitivity of ESR and CRP to the evaluation of AS was not well [10]. The indicators above have certain limitations. For example, some patients still have pain and other uncomfortable clinical symptoms even with normal ESR and CPR. This shows that ESR and CRP do not objectively reflect the disease state at that time.

Other indicators have their own limitations except for ESR and CRP poor sensitivity. BASDAI and BASFI scores are based on subjective evaluation of condition of the patients, which is affected by many factors. Meanwhile, they are not supported by some quantitative clinical indicators. Some patients have no peripheral joint performance. Therefore, their final score result cannot objectively and accurately evaluate the disease.

The Spondyloarthritis Research Consortium of Canada (SPARCC) and Sacroiliac Joint (SIJ) Structural Score (SSS) methodologies are the most popular imaging standards of active and chronic disease activity, with high accuracy and reproducibility [11]. This SPARCC score is an internationally recognized quantitative indicator used to evaluate the degree of sacroiliac joint inflammatory edema. SPARCC score is based on the semi-quantitative score obtained from the subjective visual examination of the doctor reading the film. It does not provide accurate and objective quantitative signal value of bone marrow edema.

The Assessment in Spondylo Arthritis International Society (ASAS) introduced new criteria for the classification of axial spondyloarthropathies in 2009. MRI has become the major imaging modality for detecting sacroiliitis in the sacroiliac joints [12]. Additionally, phase maps in MRI can reflect local field variation and play a key role in many clinical applications, such as water-fat separation, susceptibility-weighted imaging, and quantitative susceptibility mapping [13].

MRI Dixon-vibe sequence is a quantitative technique based on the separation of water and fat. It makes use of the proton precession frequency difference between water and fat molecule [14]. It adjusts the echo time to do multiple collection, a breath-holding collection of 6 echoes in order to separately calculate the signal values of the water phase and the fat phase [15]. The bone marrow of adults contains about 50% fat and 50% water. When the AS patients are in the active stage of bone marrow edema, the proportion of bone marrow fat content decreases, resulting in the change of the water-fat ratio of Dixon sequence. Bone marrow edema presents with high signal on the water phase for this Dixon-vibe sequence, but low signal on the fat phase. Therefore, the content of bone marrow edema in the delineated area can be quantitatively evaluated more accurately. A quantitative and visible observation index can be provided to clinicians. The proportion of fat deposition was higher as the disease progressed [6]. This was higher than in the normal bone marrow and can be detected using Dixon sequence. In this study, the differences in water-fat ratio between study group and control group were observed. The correlations between the sacroiliac joint surface water-fat ratio in the study group, and BASDAI score, BASFI score, CPR, ESR , and SPARCC score were also analyzed. The water-fat ratio in the study group was positively correlated with BASDAI score, BASFI score, and SPARCC score (all P < 0.05). This suggested that this sequence not only diagnoses the active phase of sacroiliac arthritis, but also can be used as a reference index to evaluate the clinical inflammatory activities. This enables clinicians to obtain more accurate numerical evaluation from the previous semi-quantitative evaluation for patients with inflammatory edema. This provides a more reliable and objective basis for disease staging.

The bony changes are best identified using conventional radiographs [16]. Spinal inflammation is best assessed using MRI. MRI plays an important role in diagnosing, classifying and monitoring the progress of AS inflammation with the ability to provide water-only, in-phase, opposed-phase, and fat-only images in one acquisition [17]. Meanwhile, with the development of new MRI technology, more and more sequences are used to evaluate the therapeutic effect of the sacroiliitis.

Limitations: first, the sample size was small; second, only patients with earlier disease stage were selected in the experimental group. In the later stage, the sample size and the number of patients with late disease stage, should be increased. Intergroup comparison of Dixon sequence can also be performed in patients with advanced stable stage and advanced active stage.

MRI Dixon-vibe sequence is a relatively accurate quantitative imaging technology. It can be used to identify the disease as early as possible, accurately assess the disease and early block the course of disease development to guide the management and treatment of sacroiliac arthritis. It is expected to become a simple, accurate and effective efficacy observation index which will guide clinical treatment, evaluate efficacy and monitor disease activities. This promotes clinical application.