Sacroiliitis is the inflammation of the one or both sacroiliac joint (SIJ) that causes lower back pain. Rheumatic inflammatory and noninflammatory diseases, infectious diseases and other diseases such as hyperparathyroidism, lymphoma and other malignencies might manifest as sacroiliitis. Therefore, SIJ involvement is nonspecific . Clinical examination of sacroiliitis includes tenderness of the involved SIJ with palpation, positive SIJ pain provocation test namely FABER (Flexion, ABduction, and External Rotation). In addition to that, intense SIJ pain with night worsening, morning stiffness might also be seen in both acute and chronic sacroiliitis. However, these clinical findings are not specific for sacroiliitis. [2,3]. Therefore diagnostic imaging modalities have important role for early and accurate diagnosis of sacroiliitis .
Conventional radiography is firstly applied, easy-accessible and cheap method in case of lower back pain to investigate SIJ . The SIJs have oblique position in the skeletal system and they are not easy assessible like the peripheral joints due to their deep placement in the body. Therefore, accurate evaluation of SIJ might be difficult with X-ray roentgenography . Furthermore, SIJ pathologies are not visible on direct radiography in the early stage of the disease. X-ray roentgenography might be more helpful in the late stage of the disease when anatomical changes happen . Computed tomography (CT) is superior to conventional radiography with regard to detailed anatomical information. Structural changes in bone such as erosion, sclerosis, and ankylosis can be shown with CT. However, these changes appear during late stage of the disease. CT, might not be preferable in the early stage of sacroiliitis. Furthermore, it has high radiation risk .
Magnetic resonance imaging (MRI) can demonstrate inflammation and bone marrow edema (BME) on SIJ in early stage of sacroiliitis thanks to its high resolution. However, minor BME on MRI images are not specific for sacroiliitis. Many benign diseases can bee seen as BME on MRI images. Because of the possible false positive (FP) results of MRI, MRI should not be utilized as only diagnostic modality for diagnosis of sacroiliitis [1,6].
Bone scintigraphy is a sensitive and crucial method for imaging skeletal and joint abnormalities including imaging sacroiliitis especially in early stage of disease before roentgenographic images become visible. However, its specificity is low [7,12]. Nuclear medicine physician should know patient’s clinical history, physical examination findings, other laboratory and imaging method results. Evaluation of normal distribution and symmetry of the radiopharmecutical agent is important before interpreting the scintigraphic images. In contrast to radiologic methods, scintigraphic images show metabolic changes on bone in early stage of sacroiliitis [7,11].
Bone scan is interpreted as visually, therefore this imaging method has remained as subjective evaluation of symmetrical uptake of radioactive imaging agent on both SIJs and sacrum. Quantitative bone scintigraphy (QBS) that augmentes diagnostic utility of bone scintigraphy has been used since 1970s . QBS provides comparison of the uptake in both SIJs with the uptake in the sacrum and as a result of that SIJ index is acquired for both SIJs [13,14]. QBS has been studied for detection of sacroiliitis in patients with ankylosing spondylitis, Reiter’s syndrome, psoriatic arthritis, ulcerative colitis, rheumatoid arthritis, osteoarthrosis [12,15,16]. This method has high accuracy, interobserver agreement, and repeatability [11,17,18]. High SIJ uptake were reported in patients with mechanical-type back pain, metabolic bone disease and structural abnormalities in the low back [11,19]. Therefore, similar to bone scintigraphy, QBS has lack of specificity for sacroiliitis . In conrast to various authors, others reported that bone scintigraphy was not found helpful for early diagnosis of sacroiliitis [19-21]. We found different and controversial results in the literature.
Various methods were used to calculate SIJ index with QBS . These methods are, irregular region of interest (ROI) method [19,22] , slice method through the both SIJ using iliac wings instead of sacrum that is commonly used as a reference point , single pixel thickness slice method through the both SIJ and sacrum , horizontal slice method with unspecified thickness across middle of both SIJ [11,25] , substraction method , horizontal rectangular method . Therefore, we investigated three different methods; 1) irregular ROI method 2) rectangular ROI method and 3) profile peak counts method. We aimed to compare these three methods in different age groups including children and adults, taking gender and laterality into consideration. We also wanted to develop normal range of SIJ index value in healthy population. This study might provide essential informative data for future work.