Bone scintigraphy is a highly sensitive diagnostic nuclear medicine imaging technique that uses a radiotracer to evaluate the distribution of active bone formation in the skeleton related to malignant and benign disease, as well as, physiological uptake . The criterion for a positive diagnostic study is an increase of the activity accumulation in sacroiliac joints compared to sacrum activity as a background . The addition of quantitative methods (comparing the sacroiliac joint activity with background activity like sacrum) to bone scintigraphy has revealed an increase in the sensitivity of the bone scintigraphy .
Sieper et al.  stated that: “Since there is physiological uptake of Tc99MDP in the sacroiliac joints so, every institution should establish its own normal range.” From this point, we did this research as the first trial in Egypt to assess the SII in our institute.
Bijaynath et al.  found that the overall SII ranged from 1.06 to 1.36 in the study population of 100 normal subjects encompassing all age groups. The values of SII differ at different age groups as follows: in patients aged 2–20 years, SII ranged from 1.22 to 1.36. In patients aged 21–40 years, it ranged from 1.07 to 1.19, and for patients aged 41–60 years, it ranged from 1.08 to 1.19. In patients aged 61 years and older, SII values were slightly lower than in other groups and ranged from 1.06 to 1.13. In our study, we included close age ranges to avoid the influence of age on SII as a bias in our research. The age of our patients ranged from 23 to 55 years with overall SII range was 0.9–1.14 (mean 1.04 ± 0.09) and these values were in agreement with the aforementioned study .
In group I, we assessed the SII in both SIJ in each patient and found no significant difference between the two sides (P > 0.92). This was matching with many previous studies [5, 6]stating that there is no significant difference in SII between the right and the left SIJs with SII in the RT side 1.23 ± 0.26 vs.1.25 ± 0.25 in the left side (P value > 0.66.) .
Kacar et al.  compared the SII in healthy individuals (included 47 subjects) and sacroiliitis patients (13 cases) with also no significant difference in SII between the two sides of the sacroiliac joint in both healthy and diseased sides. However, they found a significant difference between males and females (P < 0.05) with higher SII in males and also a significant SII decrease in aged women. This may reflect the influence of a wide age range included in their study and explain also why we exclude extremities of age in our study.
In our results, we found that the mean SII in female was 1.03 ± 0.08 (at the right) and 1.04 ± 0.09 (at the left) and the mean SII in males was 1.04 ± 0.09 (at the right) and 1.04 ± 0.09 (at the left) with no significant difference regarding the gender. This was disagreeing with Vyas et al.  who found a significant difference between male and female with the SIIs in females ranged from 0.92 to 1.48 (right) and 0.88–1.32 (left) vs. in male 0.87–1.67 (right) and 0.82–1.62 (left); however, they disagree with us and a previous study  as they found that the age did not influence the SII.
We think that the different SII in normal subjects regarding the gender in our study and other studies [17, 19] may be attributed to the racial differences between different studied populations.
In a pediatric study , females had higher SII than males with a significant difference in SII when they used sacrum as a background reference in index calculation but when they repeated the index calculation using L1 vertebra as a background, they found no difference between the two genders. This may reflect influence of bone marrow composition upon radioactive materials’ uptake. This highlights the importance of fixing the method of measuring the SII in each institute.
Davis et al.  found in their study on a family that has members with Crohn’s disease and others with AS that, diagnosis of ankylosing spondylitis in the patient with Crohn’s disease could not be confirmed owing to the absence of radiological change. However, quantitative sacroiliac scintigraphy confirmed the presence of inflammatory disease at the sacroiliac joints, with SII 1.79 at the LT side and 1.62 at the RT.
The range of SII among our patients with AS (group II) was 1.2–1.5, mean 1.37 ± 0.1, with a highly significant difference with the first groups (P < 0.001). This is agreeing with Ozdogan et al.  who found that there was a significant difference between the control group and patients with ankylosing spondylitis. Also, they reported that the right SII in AS was 1.48 ± 0.22 vs. 1.29 ± 0.17 for the control group and left SII was 1.47 ± 0.20 for the patient group vs. 1.32 ± 0.18 for the control group.
On the other hand, Kim et al.  stated that there is no significant difference in SII between the control group (SII = 1.10 ± 0.21) and patients with AS (SII = 1.12 ± 0.17) by planner bone scan; however, there was a significant difference (P = 0.014) when they used SPECT/CT. This reflects the high advantage of using SII as a quantitative method, also avoids the proceeding for further coast imaging modalities.
Our third group included patients with increased uptake at one or both SIJs and proved to have SIJ deposits by MSCT bone window. The range of SII was 1.2–1.6, mean 1.39 ± 0.14. We found an overlap in SII among groups II and III with no significant difference between the two groups (P value was 0.1); it may be due to a small number of included cases, so more studies are needed to confirm the importance of this index.
Few studies tried to determine the cutoff value for SII [20, 22]. In our study, the SII at a cutoff less than 1.17 was considered normal. However, a higher cutoff value (SII < 1.3) was reported by , also, another study used SPECT/CT, determined a cutoff at 1.50 to differentiate between normal and AS subjects . This may reflect lower mean of SII in Egyptians than other races.
As we know bone scintigraphy is a routine investigation in follow-up of all cancer patients, so our research could be the beginning for further larger and wider studies to assess the SII by many institutes to settle a uniform index which could help in daily work.
The limitations of our study were the small number of included subjects as we limited the study to certain age groups to avoid the influence of age. Also, we used a manual method to calculate SII due to limited equipment and Gamma Camera software in our institute.