A Novel Computerized Method for Measuring the Length of the Aorta in Patients with Severe Kyphotic Deformity

Objective: This study aimed to explore a new method for measuring the length of the aorta in patients with severe kyphotic deformity. Methods: The computed tomography (CT) scan data of one patient with severe kyphotic deformity were retrospectively collected. The data were saved as Digital Imaging and Communications in Medicine (DICOM) format, and were imported into MIMICS software for processing. Then, the MASK function of the MIMICS software was used to mark the aorta in each slice of CT, and a three-dimensional (3D) reconstruction model of the aorta was established. After that, the length of the aorta was dened as the length of the centerline, which was calculated by the MIMICS. Besides, two points were xed as anchor, and the length of aorta was acquired by measuring the distance between the two points. The proximal one was the origin of the left subclavian artery, and the fork was the distal of iliac artery. The length of the aorta was measured preoperatively and postoperatively as well. Results: The 3D reconstruction model of the aorta was successfully established. It was revealed that the length of aorta was 418.9 mm preoperatively, and 435.4 mm postoperatively. The patient also underwent pedicle subtraction osteotomy (PSO). After orthopedic surgery, the length of the aorta was stretched by 16.5 mm. Conclusion: In the present research, a 3D reconstruction model of the aorta was successfully established, and the length of the aorta was accurately measured without any invasive procedure. Using MIMICS software, the length of aorta in patients with severe kyphotic deformity could be effectively and precisely measured.


Introduction
The spine kyphosis deformity is commonly complicated by the thoracolumbar kyphosis and/or lumbar lordosis or lumbar kyphosis, seriously in uencing a patient's life quality [1,2]. To our knowledge, osteotomy surgery is highly essential for such patients by lengthening the anterior column or/and shortening the posterior column, which is inevitable to stretch the aorta [3][4][5]. It is of great importance to study the changes in the aorta length in patients with severe kyphotic deformity with minor invasive procedures. However, previously conducted studies typically measured the aorta by two-dimensional (2D) images. Therefore, this study aimed to propose a new computerized method for measuring the length of the aorta in patients with severe kyphotic deformity.

Patients And Methods
This retrospective study was approved by the Institutional Review Board (IRB) of hospital. (Grant No: 51772328) All patients involved in the study consent to participate in the study, including the photographys. And the written consent has been obtained from all the patients. Computed tomography (CT) data of the chest and abdomen of one patient with severe kyphotic deformity were collected. The data were collected by a CT machine (GE Healthcare, Chicago, IL, USA) available in Peking University People's Hospital (Beijing, China) for the thoracic and lumbar spine. The patient was placed in a supine position and remained neutral, and underwent CT scan that ranged from the T1 vertebral body to the lumbar vertebra L5.
Scanning conditions were as follows: bulb voltage: 140 kV, bulb current: 200 mA, layer thickness: 0.625 mm, image matrix: 512 × 512. In addition, the gray scale of the CT image was adjusted. The contrast was changed, and the image details were processed to achieve a clear CT image. The CT scan raw data were saved as Digital Imaging and Communications in Medicine (DICOM) format.
Additionally, those data were imported into MIMICS 19.0 software for processing. The MIMICS software was run and the view direction was set to de ne the sagittal, coronal, and cross sections to save multiple DICOM data in an orderly manner (Fig. 1). With pre-processing the image, the resolution and smoothness can be improved, and the software also contains a selection tool for regularization. According to the different gray values of the tissue on different images, we attempted to set the corresponding gray threshold interval.
The CT window position was adjusted to make the density of the large blood vessels in image relatively obvious (Fig. 2), and the aortic shadow was calibrated using MASK function of the MIMICS software ( Fig. 3). The aortic shadows of each layer were sequentially calibrated as well.
CT scan data of each layer were further precisely labeled with the aorta. As shown in Fig. 4 The centerline of the aortic model was estimated using the "centerline" of the "analysis objects" in the MIMICS, and the length of the centerline was measured to de ne the length of the aorta.

The Length Of Aorta
The length of the aorta increased from 418.9 mm preoperatively to 435.4 mm postoperatively. After the surgery, the aorta was stretched by 16.5 mm (Figs. 7-8).
Regardless of the type of osteotomy surgery, the general principle is to lengthen the anterior column or/and shorten posterior column [3][4][5]. In order to achieve a high level of correction of deformity, lengthening the anterior column is of great importance, which may result in injury. The complication of aorta injury is rare in the procedure of spinal osteotomy for the correction of Pott's thoracolumbar angular kyphotic deformity.
Numerous previous studies have explored the changes of aorta after undergoing PSO. Weatherley et al. [9] reported patients with severe kyphotic deformity who underwent SPO. The corrected kyphosis angle was 45°, and the length of the aorta was stretched by 2 cm. Chang et al. [10] applied osteotomy in the treatment of kyphosis, and it was demonstrated that the aorta was lengthened by 2.8 cm (1.7-3.5). Ji et al. [11] reported that in case of osteotomy after the treatment of kyphosis, the length of the aorta was increased by 2.2 cm and the diameter of the aorta was decreased by 0.41 cm. Bourghli et al. [12] adapted a new surgical method of osteotomy to treat the angular kyphosis caused by fracture, and it was found that after surgery, the length of the aorta was increased by 2.3 cm. The above-mentioned studies indicated that due to elongation of the anterior column, the aorta may be stretched and vulnerable to injuries, especially in the elderly patients with reduced elasticity of the aortic wall. Once that is stretched seriously, the incidence of aortic injury increases. Besides, there is a relationship between the extension of the length of the aorta and the decrease of diameter of aorta, which may in uence the hemodynamic.
Therefore, vascular complications are required to be highly considered by surgeons.
Several scholars presented methods for measuring the length of the aorta. Chang et al. [10] used an atherosclerotic plaque to measure the change of the length of the aorta. Since no obvious calci cation was observed in many cases, it therefore was not appropriate for all patients. Ji et al. [11] and Bourghli et al. [12] measured the length of the aorta between the instrumented vertebrate, and reported the changes in aorta. Although the aorta was xed with aortic hiatus and branch vessel, the position of the aorta shifted with the change of body, indicating that their method is inaccurate. Besides, for the patient with angular kyphosis, the pathway of aorta is irregular and tortuosus, therefore, it is di cult and inaccurate to measure the length of the aorta using the above-mentioned methods. Hence, in the present study, we adapted a new method for measuring the length of the aorta in patients with angular kyphosis.
Firstly, we xed two points as anchors and the length of aorta was obtained by measuring the distance between the two points. The proximal one was the origin of the left subclavian artery, and the distal was the fork of the iliac artery. As a result, the effects of modi ed aorta position on outcomes could be eliminated. Additionally, it is essential to acquire 3D image to accurately measure the length of the aorta.
The most common approach in clinic is angiography, requiring injection of radio-opaque contrast agents. As performing angiography was dangerous for our patients with severe kyphotic deformity, we refused carrying out that examination. Alternatively, we used MIMICS software to establish a 3D reconstruction model of the aorta. This accurate method could measure the length of the aorta by measuring the length of the aortic diameter, without application of angiography, which could re ect the changes in the length of aorta.
Compared with previous methods described in the literature, the proposed method possesses a number of advantages as follows: rstly, for patients with severe kyphotic deformity, especially for patients with Pott's thoracolumbar angular kyphotic deformity, the aorta ran in different directions, and it was hence incorrect to measure each segment of the aorta by 2D images. However, we can determine the 3D reconstruction model of the aorta by using MIMICS software, resulting in a precise measurement. Secondly, patients with severe kyphotic deformity are commonly complicated by ischemia, therefore, angiography for such patients may be dangerous. With the help of MIMICS, the risk of ischemia is signi cantly reduced. In addition, the xed two points can eliminate the effects of movement of the aorta caused by the correction of deformity.
Although the proposed method could be used in an invasive manner to measure the length of aorta, it still has some limitations. Firstly, the proposed method requires calibration of location of aorta for several times manually, demonstrating that the mentioned method is time-and energy-consuming. Furthermore, due to anatomical variations of patients with deformity, an experienced spine surgeon should measure its length to reduce the error.

Conclusions
For patients with severe kyphotic deformity, it is dangerous to undergo angiography to achieve a 3D image, and measuring the length of the aorta by a 2D image is an inaccurate method as well. In the present study, we successfully established a 3D reconstruction model of the aorta, and the length of the aorta was accurately measured without requiring an invasive procedure. The proposed method appeared as effective and safe to measure the length of the aorta in patients with severe kyphotic deformity.

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