Patient-Specific Coronary Artery 3D Printing Based on Intravascular Optical Coherence Tomography and Coronary Angiography

School of Informatics, Xiamen University, Xiamen, China Department of Computer Science and Technology, Tongji University, Shanghai, China School of Computer and Information, Anhui Normal University, Wuhu, China Faculty of Computer Science, University of Sunderland, Sunderland, UK School of Mechanical and Aerospace Engineering, Nanyang Technological University, 639798, Singapore School of Engineering and Computer Science, University of Hull, Hull HU6 7RX, UK Department of Instrumental and Electrical Engineering, Xiamen University, Xiamen, China School of Software, Tongji University, Shanghai, China Department of Cardiology, Shanghai Tenth People’s Hospital, Tongji University School of Medicine, Shanghai, China


Introduction
With the fast development of the society, China's basic medical care has been well improved.However, the prevalence and mortality of cardiovascular diseases (CVD) in residents is still rising, and which in recent years rapidly grow for the low-aged and low-income citizens.Since 2004, the rate of hospitalization of cardiovascular diseases is much faster than the growth rate of GDP, greatly increasing the social burden [1][2][3].CVD mainly include cerebrovascular disease, coronary heart disease, arrhythmia, and heart failure.CVD has become the largest proportion of residents suffering from all disease; 2 out of every 5 deaths are due to cardiovascular diseases.Since 2009, CVD mortality rate in rural regions has exceeded and continues to be above urban levels.In 2015, the CVD mortality rate of rural residents was 298.42/100,000, of which the heart disease mortality rate was 144.79/100,000, the cerebrovascular disease mortality rate was 153.63/100,000, and the urban residents' CVD mortality rate was 264.84/100,000, including heart disease death.e rate was 13.661/100,000, and the mortality rate of cerebrovascular disease was 128.23/100,000.In 2015, the proportion of CVD deaths in rural and urban residents accounted for 45.01% and 42.61%, respectively.us, it is imperative to early diagnose cardiovascular disease [1].
ere are two methods being widely used in the detection of cardiovascular diseases, namely, OCT technology and coronary angiography.Optical coherence tomography (OCT) technology is an imaging technique that uses nearinfrared light to display the associated tissue structure in the blood vessels without radiation, high resolution, and highdetection sensitivity [4,5].OCTcan perform high-resolution cross section tomography images of biological tissue endometrium and provide a resolution ten times higher than intravascular ultrasound, so it is also called optical biopsy.On the basis of the first-generation time domain OCT (TD-OCT) system, scientists developed frequency domains with faster scanning speeds by varying the frequency of light to obtain different depth tissue imaging OCT (frequency domain OCT, FD-OCT) system [6].X-ray coronary angiography is an arterial angiography method using an angiography machine.e procedures are as follows: first, through a special cardiac catheter percutaneous puncture, a special catheter is inserted through the right iliac artery or the lower extremity femoral artery, and the descending aorta is retrogradely to the ascending aorta root.en, the left or right coronary artery is inserted and injected with the contrast agent.Eventually, the coronary artery and morphological are visible.X-ray coronary angiography can clearly distinguish the location and morphological characteristics of vascular lesions, quantitatively determine and classify narrow vascular segments, and determine the circulation state of lateral branches [7].However, the technology itself has certain limitations.Firstly, it can only display the projection of the lumen of the blood vessel and not the cross-sectional information of the tube wall, which may cause the doctor to miss the diagnosis; secondly, it cannot provide information on the morphology and properties of plaques, and it is thus impossible to develop interventional procedures for specific plaque information; lastly, because of the uneven filling of the contrast agent in the blood vessels, it is easy for the doctor to underestimate the degree of lumen narrowing.
In response to the limitations of the above two technologies, we introduced 3D printing technology.ree-dimensional (3D) printing technology, also known as additive manufacturing technology, has advantages of fast, directness, and digitalize.[8] Since its appearance in the 1980s, 3D printing technology has developed rapidly in this century and has been widely used in the fields of automobile, aerospace, and medical.It can be applied to enhance the diagnosis and treatment of complex cardiovascular diseases, allowing doctors to visually evaluate the spatial geometry of the entire vessel [9].e appearance of rapid prototyping techniques (RPT) greatly made the computer technology move forward, and 3D printing is one type of the RPT [10].A rapid 3D prototyping process consists of many steps.is technique needs to obtain a series of figures, such as computed tomography (CT) images, transform the figures into a format compatible with a 3D printer, and then print the model [11].Some surgeries can be planned using threedimensional computer aided design and manufacturing (3D CAD/CAM) software.e 3D virtual planning is performed in PROPLAN software and exported into 3-matic software to design and then use the biocompatible material to print [12].Shaheen et al. and Rengier and Mehndiratta used dedicated postprocessing algorithm to extract a spatial model from image data sets and export to machine-readable data [12,13].
At present, there are few research studies on 3D printing of coronary arteries.is paper explores the 3D printing of blood vessels based on optical coherence tomography and coronary angiography.Angiographic images and optical coherence tomography images of 8 patients with coronary artery stenosis before and after implantation of stents were collected.e vascular model of the eight patient models were modeled with the proposed 3D reconstruction technique by merging the morphological details of the contour view of the vessel lumen and the curvature and length information provided by the coronary angiography.After outputting the STL file format, the built-in coronary STL format file is then imported into the 3D printer and the STL format file is converted into a machine code Gcode that can be recognized by the 3D printer.Finally, the 3D printer performs coronary artery model printing operation.e main contribution of the paper is to originally complete the 3D printing of specific blood vessel models with high correlations between the diameters of the vascular stenosis segments extracted from the 3D reconstruction model.
Compared with the traditional scheme, our new approach has advantages of high-quality simulation, applicability in clinical usages, and the convenience of preoperative planning.

Methods
e overall flowchart of our proposed framework with 5 steps is shown in Figure 1, where 5 steps are mainly contained.e original images are preprocessed and the used for reconstructing the 3D solid model of the blood vessel.And then the 3D point clouds are extracted from biodegradable stent, which are introduced to the reconstruction of the 3D solid model of the bioabsorbable stent.Finally, the 3D solid model of the blood vessel after implantation of the bioabsorbable stent is reduced through feature subtraction.

Individualized ree-Dimensional Reconstruction of Coronary Arteries.
e individualized 3D reconstruction of the coronary arteries is performed by merging the 2 Complexity morphological details of the contour view of the vessel lumen and the curvature and length information provided by the coronary angiography.We first performed a three-dimensional reconstruction with coronary angiography containing information on vessel curvature and high-resolution intravascular optical coherence tomography.e reconstruction steps include blood vessel image acquisition and preprocessing, reconstruction of a 3D vascular solid model, extraction of a 3D point cloud of a biodegradable stent, reconstruction of a 3D solid model of the biodegradable stent, [14] and reduction of the feature after subtraction of the implanted bioabsorbable stent of the 3D solid model of blood vessels [15].e following are a specific description of the various steps of the reconstruction.

Step 1: Acquire and Preprocess the Vascular Image.
e central line of vascular structure was extracted according to the coronary angiography image.According to the number of framing pictures of the intravascular OCT image, evenly divide the center line to obtain a series of points and set a plane perpendicular to the center line at each point.

Step 2: Rebuild the ree-Dimensional Vascular Solid
Model.Each frame is placed in the corresponding plane in order from the IVOCT image sequence, the contour of the lumen is identified, and the image is removed after the contour is obtained.Repeat the above steps for each image until the skeleton corresponding to the contour of the entire vessel lumen is achieved, select the contours of each vascular cavity, perform a sweep operation along the centerline of the coronary angiography, and smooth the surface of the 3D blood vessels to eventually produce a vascular solid model [16].Figures 2(a)-2(c)represent the process of modeling a 3D model of blood vessels.

Step 3: Extract the ree-Dimensional Point Cloud of Biological Absorbable Stent.
For the vessel segment implanted with the stent column, firstly, according to the automatic stent extraction algorithm proposed in Figure 3, each stent column point is displayed at the position of the IVOCT per frame.When all the scaffold points are automatically marked, a discrete 3D scaffold point cloud is obtained.e position of the point cloud is the position of the scaffolding column in the IVOCT chart of each frame.
e geometrical space structure of the support can be approximately obtained by the support point cloud.

Step 4:
ree-Dimensional Solid Model for Reconstruction of Biological Absorbable Stents.After building the bracket point cloud, in order to restore the mesh structure of the bracket, we also need to manually connect the bracket columns to construct the spatial structure of the biological absorbable stent in the vascular cavity.According to the prior information, we know that the bracket is made up of annular structures, each of which is connected by a scaffold [17,18].According to the observation, we found that the distance of every 3 to 4 OCTframes is the thickness of a mesh bracket ring, and the structure of the stent-ring can be generated by sequentially connecting the point cloud of the stent column in the adjacent 3 to 4 frames of IVOCT by a curve.Since the bracket is thick, we first calculate the diameter of the bracket column according to the OCT picture.
en, a new plane is established on the plane perpendicular to the ring curve of the bracket, centered on the intersection of the surface and the plane, and a circle is drawn in diameter with the thickness of the precalculated bracket column.For each annular structure of the stent, the circle is swept along the curve of the stent ring to create a ring-shaped stent body having a thickness.e three-dimensional point cloud and its 3D model of the bracket are illustrated in Figures 4(a) and 4(b).

Step 5:
ree-Dimensional Solid Model of Blood Vessels after Reduction and Implantation of Biologically Absorbable Stents upon Feature Subtraction.For the vessel segment implanted in the stent, we subtract the feature of the generated vascular entity from the bioabsorbable stent entity, forming a stent groove on the modeled vascular entity to highlight the stent structure, and restore the real 3D vascular cavity structure with the stent placed [19].A 3D vascular model of an implanted biological absorbable stent with a corrugated surface is shown in Figures 5(a) and 5(b).

Exploration of ree-Dimensional Printing Specific Vascular Model.
e application of 3D printing in medicine has changed the limitation that traditional 3D models can only be virtual displayed on computers.Doctors can make more accurate preoperative planning and improve the success rate of surgery by analyzing the anatomical structure characteristics of lesions through personalized medical model entity.At present, clinicians mainly design the operation plan based on CT, MRI, angiography, or 2D vascular lumen images such as optical coherence tomography, vascular ultrasound images, and clinical experience.However, these imaging techniques can only present two-dimensional field of vision on the screen and cannot allow doctors to visualize the spatial geometry of the whole blood vessel evaluation [20]    Complexity intuitively touch and measure the pathological vascular segments, so as to determine the key steps of operation, select the appropriate size of vascular branches, and formulate the best surgical plan, which can reduce the possibility of uneven stent adherence and hence greatly improve the effectiveness of interventional surgery [21,22].e individual geometry of lesions in each branch of coronary artery varies greatly and the treatment scheme is more complex, especially in the selection of stents and implantation location of interventional surgery.In this paper, we attempt to explore the spatial geometry of the coronary artery by using 3D printing technology.rough individualized 3D reconstruction of the coronary artery, we have successfully obtained 3D vascular model structure.e structure of the 3D model is expected to change the situation in which clinicians perceive coronary artery lesions based on their own experience in the past.Quantitative experimental results also show that the 3D printing technology has high visualisation quality and the effectiveness of the 3D model of the coronary artery in individualized coronary intervention surgery and preoperative planning.

Experiment
In this study, we collected angiographic images and optical coherence tomography images of 8 patients with coronary artery stenosis before and after implantation of stents.3D modeling of the vascular model of 8 patients was performed using the proposed 3D reconstruction technique and the STL file format was output.
By analyzing the advantages and disadvantages of common 3D printing technology, this study selects a 3D printer based on melting deposition molding technology to carry out the research on the geometrical morphological structure of the coronary artery.Considering that the coronary artery vascular model is very small, in order to be able to observe the structural differences of different vascular models more intuitively by the printing vascular model, we first performed a 2x scale morphological enlargement of the vessel 3D model.In this work, we selected the fused deposition 3D printer of EasyArts's model areas and picked polylactic acid as the printing material [4].e parameters of the device are X, Y, and Z in the direction of the accuracy of 0.02 mm and the maximum printable size of 160 * 160 * 160 mm.When printing, the thickness of each layer is 0.05-0.3mm, the nozzle size is 0.3 mm, and the nozzle temperature heating range is 170-280 degrees Celsius.
After importing the coronary STL format file into the 3D printer, the first step is to check the STL format file.e original model may have some minor flaws in modeling, and in order to make the model more complete, we need to check the integrity of the STL model and then fill, smooth, rotate, zoom, and so on as needed.Convert the STL format file to a machine code Gcode that can be recognized by a 3D printer, which generates a print backplane, print support, and other 6 Complexity operations based on the model structure.3D printer performs coronary artery model printing operations.Under the control of the computer, the nozzle moves along the xy plane and the molten wire in the semiflowing state is squeezed out of the nozzle, coated on the workbench, and cooled to form a cross section.After a layer of molding, the nozzle moves up a layer of height and is stacked according to this reciprocating operation to form a 3D entity until the end of the print.

Results
e postprocessing step includes the removal of support, surface polishing, and other operations.
Finally, a smooth 3D printing vascular model entity was achieved as shown in Figure 6.
e diameter of the printing coronary artery solid stenosis section and the diameter of the normal segment were measured using a vernier caliper (Figure 7) and compared with the diameter of the narrow section and the diameter of the normal section in the 3D model of the coronary artery in the software.
Comparison of 8 cases of the coronary artery CAD model and 3D printing physical model is shown in Figure 8.
Figure 8 shows that the diameter of the printing coronary artery solid stenosis section and the diameter of the normal segment were measured using a vernier caliper and compared with the diameter of the narrow section and the diameter of the normal section in CAD models.And a series of measurements are included from Figures 9-11.It can be seen that the diameter of the stenosis segment measured in the 3D printing model is highly correlated with the diameter of the stenosis segment extracted from the CAD models.Table 1 shows that 3D printing models and CAD models of 3D measurements in stenosis and nonstenosis coronary artery segment.
e source of vascular data is rich, including coronary angiography, using X-ray acquisition, intravascular ultrasound images acquired with ultrasound techniques, intravascular optical coherence tomography images acquired via optical correlation tomography, and magnetic resonance angiography and computed tomography images.Vascular data not only preserves the information on the surface of

CAD models
CAD models 3D printing physical models 3D printing physical models Complexity blood vessels but also contains abundant information on the internal structure of blood vessels, which is bene cial to the imaging diagnosis of related cardiovascular diseases in clinic [23,24].However, the above imaging technology can only present a two-dimensional eld of vision on the screen and cannot permit doctors to intuitively evaluate the spatial geometry of the whole blood vessel [25].3D printing of vascular technology in the diagnosis and treatment of cardiovascular diseases has a very important application value, we believe that this technology has the following two main advantages.(1) High quality of simulation: experimental studies have shown that the 3D printing vascular technology has high simulation ability and can accurately model the speci c blood vessels of patients.e printing solid model is su cient for general clinical application needs and can help doctors better understand the details of blood vessels.(2) Preoperative planning: the 3D model of coronary arteries facilitates the planning of preoperative planning for vascular implantation [26].e manifested vascular model can accommodate the visual touch of the cardiologist and measure the vascular segment of the lesion, so as to determine the key steps of the operation, select the appropriate size of the blood vessel branch, develop the best surgical scheme, reduce the    Complexity possibility of stent adherence unevenness, and greatly improve the effectiveness of interventional surgery [27].At the same time, due to the stability of the printing material, the solid model can also be viewed and carried by doctors at any time [28].

Conclusion
In this paper, a new framework is proposed to print a 3D vascular model with a high simulation value.Experimental results conclude that the 3D printing vascular model based on OCT and angiography has a high correlation with the vascular 3D reconstruction model.e 3D vascular model we printed has the characteristics of high simulation, applicability in clinical application, and the convenience of preoperative planning, which has obvious advantages as compared with the traditional scheme.It also gives a new scheme for the treatment of cardiovascular diseases, which has extremely high practical significance and good application prospects.In the future, the following 3 aspects need to be improved further: (1) the currently vascular model is a solid entity and it is not possible to print a cavity vessel, and it is impossible to perform a simulated interventional procedure.(2) e currently printing material is polylactic acid, which is quite hard to read compared to vascular tissue.(3) e time of making the blood vessel model is still relatively long, and the manual deburring is required in the later stage, where the automaticity needs to be improved.

Figure 1 :Figure 3 :
Figure 1: e flowchart of our proposed framework on patient-specific coronary artery 3D printing.

Figure 5 :
Figure 5: A three-dimensional model of a bioabsorbable stent with a corrugated surface (a) obtained by subtracting the three-dimensional model of the vessel from the stent (b).

Figure 6 :
Figure 6: e process of 3D printing a specific vascular model.

Figure 8 :
Figure 8: Comparison of 8 selected coronary artery CAD models and 3D printing physical models.

Figure 9 :
Figure 9: Measurement results of 3D printing models and CAD models.

Figure 10 :
Figure 10: Correlation of the diameter of the stenosis segment measured in the 16 3D printing models and CAD models.

Figure 11 :
Figure 11: Bland-Altman plot for the 3D printing models and CAD models.

8
. 3D printing technology can be applied to the diagnosis and treatment of complex cardiovascular diseases.e 3D model of coronary artery can facilitate physicians to

Table 1 :
3D printing physical models vs CAD models of 3D measurements of blood vessels in stenosis and nonstenosis.