Energy Spectrum CT Differential Diagnosis of Idiopathic Calcication and Ossication of Adult Intervertebral Disc: A Report of three Cases

Calcication of adult intervertebral disc is a rare clinical disease, and its imaging diagnosis directly affects the formulation of the operation plan. In this study, three cases are used to illustrate that the application of energy spectrum CT as the preoperative diagnosis can better reect the pathological situation and structure of the focus, further helping with preoperative judgment. We report two cases of idiopathic calcication of adult thoracic intervertebral disc. In addition to Routine CT and MRI before the operation, energy spectrum CT was taken to produce analytical diagnosis of the lesions. According to the results, the surgical resection mode was determined. The postoperative conrmation was highly consistent with the image diagnosis. Likewise, a case of posterior margin of vertebral body and ossication of intervertebral disc was selected for the same diagnosis procedure, and the two were compared for differentiation.

Calci cation of adult intervertebral disc is a rare clinical disease, and its imaging diagnosis directly affects the formulation of the operation plan. In this study, three cases are used to illustrate that the application of energy spectrum CT as the preoperative diagnosis can better re ect the pathological situation and structure of the focus, further helping with preoperative judgment.

Case presentation
We report two cases of idiopathic calci cation of adult thoracic intervertebral disc. In addition to Routine CT and MRI before the operation, energy spectrum CT was taken to produce analytical diagnosis of the lesions. According to the results, the surgical resection mode was determined. The postoperative con rmation was highly consistent with the image diagnosis. Likewise, a case of posterior margin of vertebral body and ossi cation of intervertebral disc was selected for the same diagnosis procedure, and the two were compared for differentiation.

Conclusion
In the illustration that the texture of the vertebral canal presser being uncertain, the energy spectrum can identify the calcium type of the presser, as well as the atomic number histogram being able to x the absolute content value and the distribution rule of calcium. Meanwhile, the calcium-water astigmatism map can resolve the calcium deposition mode, with better estimation of the focus presser hardness.

Background
The calci cation of intervertebral disc is a sort of degenerative disease with abnormal deposition of calcium salt in cartilage, which was primarily discovered by Von Luschka [1] . Its pathology is mainly characterized by the amorphous deposition of calcium salt without the role of chondrocytes. The composition analysis of in vitro samples is mostly hydroxyapatite crystal (Ca 10 (PO 4 ) 6 (OH) 2 , HAP) [2] .
Due to the lack of effective direct clinical imaging diagnosis and the low incidence rate of the disease, the clinical features are frequently confused with ossi cation of the posterior longitudinal ligament (OPLL), together with rim ossi cation of the intervertebral disc. Regular preoperative CT is not sensitive to the identi cation of calcium salt deposits. Despite of calci cation and ossi cation being completely different pathological processes, it is demanding to predict the texture and possible components of the lesion. It often falls into the situation of problematic differential diagnosis, which affects the planning of operation and delays the operation time. Dual energy CT is a non-invasive examination method that can distinguish the material components and structures. For its suitability and handiness in examination and analysis, it could provide su cient diagnostic basis for doctors. In this group of cases, preoperative energy spectrum CT imaging analysis was used, and single energy CT energy spectrum curve, calcium distribution histogram, material density scatter diagram were obtained successively, and parallel to liver, brain, coronary artery and aortic calcium. The hardness characteristics of calcium deposition lesions were attained by contrast differentiation of calci cation, edge of vertebral body / ossi cation of intervertebral disc. It was con rmed that the preoperative diagnosis method was an effective method to detect such intraspinal space occupying lesions.

Case Presentation
Case 1 Male, 37 years old. who had been having Lumbosacral pain with numbness and weakness of both lower limbs was admitted to the hospital in January. One month ago, the hospital accepted "T12/L1 nucleus pulposus removal radiofrequency ablation minimally invasive surgery". After the operation, the chest and waist pain gradually appeared, accompanied by numbness and weakness of both lower limbs. Physical examination: on T11-12 plane, there were postoperative scars, T10-L2 spinous process and paravertebral tenderness, skin pain on both sides of the lateral leg and the back of the foot decreased. The muscle strength of extensor dorsalis of the left ankle was grade II, the muscle strength of the extensor dorsalis of the right toe was grade II, the tendon re ex of both sides was weakened, the clonus of the right ankle was (+), the Babbinski sign of both sides was (-). CT and MRI ( Fig. 1) of thoracic vertebrae were performed in the hospital, which indicated that "the T12/L1 space was decreased after operation, and the T11/12 intervertebral disc in front of thoracic canal was hard occupying lesion?" In the energy spectrum CT scan, we considered "idiopathic calci cation of adult intervertebral disc" and "resection of calci cation focus after decompression of thoracic vertebral canal". When the focus was removed, it was relatively tough and easy to be removed, and the content of adhesion with the brous ring was in the shape of "toothpaste". The postoperative pathology suggested that brocartilage calci cation ( Fig. 3-a) was formed.

Case 2
Male, 65 years old, who had been having chest and waist pain for 8 months, and came for treatment after aggravating and numbness in the lower extremities for 2 month. There was radiation pain to both buttocks at night, accompanied by chest and waist "band sense", physical examination: T10/L1 paravertebral tenderness, decreased skin pain below the groin, normal feeling in the saddle area, left hip extension muscle strength grade IV, normal bilateral tendon re ex, bilateral Babinski sign (-). Routine Xray, CT and MRI were performed in the hospital, indicating "hard space occupying lesion at T10/L1 disc level in front of thoracic vertebral canal?" According to the analysis of energy spectrum CT scan ( Fig. 4-b), we considered "idiopathic calci cation of adult intervertebral disc" and performed "resection of calci cation focus after decompression of thoracic vertebral canal". The focus was soft and closely related to the brous ring, and the content was in the shape of "toothpaste". The pathology after the operation indicated "formation of broken cartilage and calci cation focus" (Fig. 3-b).

Case 3
Female, 48 years old, two lower limbs with acid swelling for 8 months, aggravating with weakness, admitted in April. Physical examination: skin hypoesthesia in bilateral inguinal area and perineum, bilateral exion hip muscle strength grade IV, bilateral knee tendon re ex (+ +), bilateral Babinski sign (-). Routine X-ray, CT and MRI examination (Fig. 2) were performed in the hospital, which indicated that "osteophyte formation at T11/12 disc level and hard space occupying lesions at corresponding level?" While the energy spectrum CT scan ( Fig. 4-a) considered "T11/12 posterior ossi cation formation". In spite of the performance of posterior decompression of thoracic spinal canal, some lesions were still taken out in the front of the side for pathological examination. It was hard to remove the lesions, the texture was rigid, and it was not easy to take out the lesions in blocks. The postoperative pathological examination suggested " brous ring ossi cation formation" (Fig. 3-c).

Discussion And Conclusions
Calci cation and ossi cation are the outcomes of histopathological changes. The abnormal deposition of calcium in tissues caused by diverse reasons leads to sclerosis. In terms of calci cation, the pathological process is the amorphous deposition of calcium salt on the non-bone like matrix, while the ossi cation is the stereotyped deposition on the bone like matrix. The process of the two pathological changes and the degree of hardening of lesions are unalike. The calci cation of adult idiopathic intervertebral disc is mostly seen in the "marginal type" calci cation of intervertebral disc. Meanwhile, the tissue changes generally from the brous ring, which may result from external factors such as trauma and in ammation acting on the local brous ring, resulting in the amorphous deposition of calcium salt in the local brous ring tissue, and then the calci cation result after a series of complex absorption process [3] . The pathological picture shows that the calci cation focuses on the edge of the brous ring, the structure of calci cation is loose and independent of normal brous chondrocyte nests, which are arranged in "concentric ring" with scattered chondrocyte nests in the lesions ( Fig. 3-a, b). The ossi cation of intervertebral disc edge and the posterior vertebral body edge are common in degenerative bone and cartilage lesions. The tissue sources are different, except calcium salt is deposited in the corresponding tissues. Bone cells participate in the formation of ossi cation, which makes calcium salt form bone structure deposition. The trigger and formation process are still unspeci ed at present [4] . Pathological pictures demonstrate that the ossi cation extends far away based on the calci ed cartilage / bone cell area. The ossi cation focus is uniform and presents a "beam structure", the inner surface does not contain bone cells, there is no crack or connective tissue lling in the ossi cation focus, the ossi cation focus is dense and has obvious boundary with the surrounding tissue (Fig. 3-c).
In the process of spine surgery, there are usually vertebrate tissues protruding into the spinal canal from the level of intervertebral disc. Most of these lesions characterized by calcium deposition are ossi cation and calci cation, which have no effect on the light ones and affect the nervous function on the heavy ones. Surgical resection and decompression becomes the main hand segment to solve this kind of disease. Nevertheless, when choosing the surgical approach, spine surgeons normally give the X-ray system of the focus CT and MRI are used to determine the nature of the lesions, so as to develop a less risky way of neurode compression. This experience is habitually determined by the texture of the lesions and the location of the contents of the spinal canal. That is, the lesion is soft and easy to be removed, and it is easy to choose posterior surgery. If the lesion is hard, the posterior surgery is di cult, and the lateral surgery is easy to choose.Therefore, it is particularly imperative to decide the nature of the compression products. This principle was also followed in the three cases of adult idiopathic intervertebral disc calci cation and ossi cation before operation, but it is challenging for ordinary CT to judge the presser materials. The calci cation and ossi cation of the intervertebral disc showed highdensity structure on conventional CT, but the texture of the lesion was as soft as "toothpaste" after removal. Although the MRI signal of calci cation was different from that of ossi cation [5] , with poor evidence, it often resulted in misjudgment.
Energy spectrum CT is a kind of CT technology, which uses double kinds of substances to absorb different and speci c X-rays with distinctive energy [6] . Compared with conventional single energy CT imaging, the greatest advantage of energy spectrum CT is that it can use dissimilar means and methods of attenuation characteristics to distinguish similar substances, which are di cult to realize simply with CT value [7] .
Additionally Healthcare, USA), all organ unit scans are routine diagnosis and treatment processes of scanning main pathological unit, which have been approved and led by family members and patients themselves with informed consent and hospital ethics approval.There was no side effect and additional injury to the examination and diagnosis. It processed by data blind method, and automatically generated single energy value (keV) -CT value (HU) energy spectrum attenuation curve, calcium content atomic number histogram and calcium water-based scatter diagram.
Compared with the energy spectrum curve of calci cation and ossi cation, it can be seen that the curve of calci cation is below the curve of ossi cation, indicating that the energy spectrum of calci cation and ossi cation is different. Temporarily, the material properties are different. By contrast, the energy spectrum curve trend of calci cation in liver, brain, coronary artery and aorta is similar, (Fig. 5) indicating that the material types in the focus are similar, but the nature of the focus is different, and the average CT value of calci cation is low under the condition of 70 Kev (Fig. 4). The density of ossi cation focus, that is to say, the density of ossi cation focus, which represents the composition of similar substances, is higher than that of calci cation focus.
The density distribution histogram of calcium content under the condition of 70 Kev, i.e. the effective atomic number histogram, shows that most of the voxels in calci ed foci contain between 60 mg / cm 3 and 100 mg / cm 3 of calcium. While most of the voxels in ossi ed foci contain between 180 mg / cm 3 and 280 mg / cm 3 of calcium, which means that the average distribution of calcium content in calci ed foci is also signi cantly lower than that in ossi ed foci. What is more, the atomic number distribution of ossi ed foci with bone structure is relatively balanced. The distribution of atomic number of calci ed foci is close to the partial distribution, indicating that the structure of ossi ed foci is more regular and orderly than calci ed foci (Fig. 6).
The calcium content of calci ed foci is 50-100 mg / cm 3 , which is lower than the average density of calci ed foci 170-260 mg / cm 3 . That is to say, the average density of calcium salt in the whole ossi cation focus is higher (Fig. 7). The scatter plot of the density of calci cation focus in different tissues is generated. The tting curve of each group of scatter plot is drawn as gure (Fig. 8) according to the principle of energy spectrum CT to determine substances; two "basic substances" can express any kind of substance attenuation [8] . The formula is µ substance = D calcium *µ calcium + D water *µ water .(under µ substance : 70 Kev, the average energy attenuation value of the measured substance, D calcium : the weight value of the average density of the measured calcium, mg / cm 3 , D water : the weight value of the average density of the measured water, mg / cm 3 , µ water : 70 Kev, the average energy attenuation value of the measured level, µ calcium and µ water are known, which are obtained by looking up the table). From the tting curve, the tting curve of the substance is linear. The slope k can be expressed as K = D calcium / D water , then µ Substance = D calcium * (µ calcium + 1 / K * µ water ). Thus, the larger the K is, the higher the weight of the measured calcium is, and the smaller the K is, the lower the weight of the calcium is. Compared with the ossi cation focus, the calci cation focus K calci cation > ossi cation focus K ossi cation , so the energy spectrum curve of the calci cation focus is slower than that of the ossi cation focus (Fig. 4, 7). That is, the curve is slower, which also explains why the CT values of the two similar calcium containing lesions in conventional multi energy CT are similar. Consequently, it is reasonable to take K as the deposition e ciency of calcium salt, and verify that the rest of calci cation lesions are consistent with the results (Fig. 8), it is worth noting that the measured density here is not the absolute content value, which needs to be distinguished from the absolute value of calcium content [9] .
In summary, the key point of surgical differentiation of calcium salt related focuses lies in its hardness estimation. Under the same circumstances, several vital factors related to the hardness of lesions include: calcium salt type, absolute calcium salt content, calcium salt deposition mode (arrangement and structure), which is hard to distinguish as conventional CT, energy spectrum CT energy spectrum to identify calcium salt type, column atomic number determination to estimate the absolute calcium salt content and row. The calcium water astigmatism can get the information of calcium salt deposition mode, and the lesions can be determined by the combination of the three. Hence, the operation plan is more accurate.  energy spectrum curve of calci cation focus of liver, brain, coronary artery and aorta