Advances in SPECT reconstruction techniques have allowed for the use of quantitative image metrics such as SUV to be used in 99mTc-MDP bone imaging studies. Israel et al.  was the first to calculate the 99mTc-MDP in vertebra without attenuation correction,and the error was about 30–40%. Recently,with the introduction of CT-based attenuation correction,the error of calculating SUV in SPECT/CT images was less than 10% ,leading to the widespread use of SPECT/CT bone quantification in clinical studies. SPECT/CT quantification has been demonstrated as an efficient tool for evaluating the efficacy of disease  and identifying the benign and malignant bone diseases .To the best of our knowledge, this is the first large sample study to obtain SUV measurements of all 24 vertebrae based on OSCG-enhanced reconstruction algorithm of CT attenuation correction.
Cachovan et al. used SPECT/CT bone quantification to obtain the mean SUVmax of 99mTc - diphosphono - sponge propanedi - carboxylic acid (DPD) of L3-5 vertebral spongy bone in 50 females (mean ± SD = 5.91 ± 1.54). In our study, the mean SUVmax of lumbar in female participants was 7.04 ± 1.47, which was slightly higher than the reference value,because the tracer kinetics were not same༌this might lead the different results.On the other hand,in our study, the VOI contained the cortical part of the bone that high bone salt metabolism leading to the increased SUVmax. The results remind us that when comparing SUV of bones, it is important to determine whether bone cortex is included in VOI. The SUVmax of normal vertebra in our study was similar to the previously reported studies which also included bone cortex in their VOIs . And the VOI of both studies also included bone cortex. It is well known that bone lesions including tumors, inflammation and other diseases often end up with cortical hyperplasia.Thereafter,we highly recommend that to establish the normal vertebral SUVmax, cortical bone should be included when outlining the VOI.
Hounsfield Unit(HU) is a commonly used measurement index in CT images that indicates the X-ray attenuation degree in tissue (also known as bone density). Bone mineral density (BMD) obtained through dual X-ray absorptiometry (DEXA) is the gold standard and previous studies showed a significant positive correlation between HU and BMD. By analyzing the correlation between CT value and SUV, we found that SUVmax of each vertebral segment had a negatively strong correlation with the CT value (HU), which is contrary to the results of previous studies ,those only analyzed the relationship between SUVmax and CT value of lumbar vertebrae while the gravity and blood supply of vertebral bodies were consistent. Different from previous studies, we analyzed the relationship between SUVmax and CT value of all vertebral body segments.SUVmax of 99mTc-MDP in bone is often associated with blood supply and bone mineral density . Maybe due to the influence of gravity and calcium loss in elderly,the bone mineral density of lumbar vertebra was lower than the cervical vertebra in our study. The blood supply of the lumbar artery from the abdominal aorta is richer than that from the vertebral artery  and increased pressure of lumbar leads richer blood supply. So although the CT value of the lumbar spine was lower, it might have a high SUVmax. This might explain why we got the opposite resul, but the mechanism underlying this correlation need further investigation. And we separately compared the correlation of SUmax and CT in different vertebral segment (data not shown). The result was consistent with previous study in lumbar vertebrae.
It has been reported that SUVmax of the vertebra and pelvis was independent of height, however, the results are controversial. The opposite conclusions from these studies may due to the small sample size. With a large sample size, our results showed that SUVmax was positively correlated with the height,weight in men and with the weight in women which further validated the hypothesis proposed by Tomohiro et al.  that the increased pressure leads to more blood supply, thus resulting in the increasing SUVmax. In our study, the SUVmax and SUVmean were significant higher in men than those in women. This may also be due to the women’s height in our study was generally lower compared to men. In addition, the female participants were mostly postmenopausal,and it has been reported that decreased estrogen levelmay cause bone density decline. Since the number of participants in each age group was not evenly distributed by height, we did not find a correlation between SUV value and age.
Our study found no difference in the coefficient of variation between SUVmax and SUVmean. The results suggested that both SUVmax and SUVmean could be used as reference value of normal physiology. And further studies of precise grouping will be conducted to verify our results. Different from a previous study , our results showed that the SUVmax variation coefficient of T8 vertebrae of men and L2 vertebrae of women was the smallest, suggesting that they could be used as an optimal reference in the future studies.To obtain the normal value of vertebrae in each segment of men and women, we summarized the SUVmax value of the vertebrae with no significant statistical difference as reference for clinical validation.
Our study also has the following limitations. Although age distribution was wide, the uneven distribution of research object number of each age group could make it is hard to reflect the SUVmax of all ages. In addition, the SUVmax acquired in our study was all based on the weight. Since the 99mTc-MDP uptake mainly exists in bone, the standardization of bone volume can improve the accuracy of quantification. This indicated that bone volume should be included in future studies. The quantitative accuracy of bone imaging is also affected by the reconstruction parameters. Previous studies have shown that quantitative values increase with higher number of iterations, 50–100 iterations reached a steady state . Thus, in SPECT/CT, it is important to optimize the reconstruction parameters and improve the quantitative accuracy. Therefore, in future studies, we will further stratify experimental subjects according to age and height, BMI and further optimize reconstruction parameters to obtain more accurate bone quantitative standard values.