Imaging of phantom model

A known limitation of the OCT method is that it can only visualize one branch at a time. For this reason, only the model’s side branch was imaged using OCT. The reason that the side branch was chosen was because it would provide a bigger challenge to the model, because it has a curving centerline and provides a dimmer view of the stenosis (the stenosis being located on the main vessel). It should also be noted that the rapid prototyping process resulted in a rough model ‘lumen’ (Figure 4-15 and 4-19), adding another challenge to the model, as roughness was not included in the original virtual model.

Figure 4-14 The final phantom model that was created using rapid prototyping from the virtual model shown in Figure 4-12.

The material (Tangoplus) is distensible and partially transparent allowing for easy visualisation of the OCT catheter during imaging. The rapid prototyping process resulted in a smaller lumen compared to the one created in the virtual model, due to the presence of roughness which was difficult to remove.

The imaging protocol followed was the same as described in sections 4.3.1 and 4.3.2. For the OCT imaging 271 images of the model segment were acquired with a slice interval of 0.2mm, (i.e. 54.2mm length). A slice showing the presence of stenosis and the branching is shown in Figure 4-15. For the angiography data acquisition angiograms were obtained so that there were at least two unobstructed views of the model with a sufficient angle between the views to allow for good quality reconstruction, as shown in Figure 4-16.

stenosis

branch

proximal main

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4.5.4 3D Reconstruction and comparison to virtual model

The reconstruction method was the same as described in section 4.4. The angiography images and angiography-based reconstruction are shown in Figure 4-17, while the completed reconstruction compared to the original model is shown in Figure 4-18.

Figure 4-15 Top. An OCT scan slice of the phantom model, showing both the stenosis (top left) and the bifurcation. The

roughness seen on the lumen surface is a result of the rapid prototyping process, and was not present in the virtual model. Since the stenosis was in the main vessel and not in the branch, it was visualised at a distance, which has affected the contrast of the stenosis images. This was deliberately chosen to test the algorithm’s effectiveness at low contrast Bottom. A longitudinal view of the scanned model. The parameters were kept the same as in the patient protocol to facilitate comparison.

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Figure 4-16 Two angiographic views of the phantom model, injected with contrast agent. The stenosis is clearly visible in

both views. The two views form an angle of ~50 degrees, enough to provide an initial 3D reconstruction. Care was taken to include views that are routine in the cath lab.

Figure 4-17 Top row. The two angiographic views shown in Figure 4-16 were used for the initial reconstruction. The

process is the same as shown in Figure 4-2. Bottom row. The reconstruction result (shown left) and the diameter and area plots as calculated from that. The sudden dip in diameter and area near the halfway point indicates the presence of the stenosis.

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Qualitatively, from the image of the two models (original and reconstructed) superimposed it can be clearly seen that the reconstruction is in good agreement with the original geometry. Using the new reconstruction method it was possible to capture the bifurcation angle, and also trace the contour and eccentric location of the stenosis very well, despite the reduced contrast created by the dimmer view of the stenosis (Figure 4-15).

Figure 4-18 Top. The first attempt at reconstruction of the model branch.An misstep of the algorithm is visible just proximal

to the stenosis. Modifying the algorithm parameters can easily fix this. The stenosis area is shown enlarged at the inset.

Bottom. Same as the top figure, but with the original virtual model design superimposed in grey. Visual assessment

indicates that the two models (original and reconstructed) are quite close, with good agreement on the bifurcation angle and the tracing of the stenosis.

Two tests were carried out to assess quantitatively the level of agreement of the reconstructed geometry to the original one. In preparation for the tests, the 3D geometries (original and reconstructed) were divided into 2D planes parallel to the x axis (which coincides with the longitudinal axis of the main vessel) 0.2 mm apart. This simplified the 3D

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geometry into a set of 2D planes of constant x, enabling us to determine the error in the estimation of y and z coordinates only.

The first test consisted of calculating the error in the y and z direction of the reconstructed geometry’s centreline. The mean of differences between the reconstructed and original geometries was calculated for the y and z direction, respectively, and was then normalised by the diameter of the vessel. It was found that the reconstruction was accurate within 0.60% of the vessel diameter in the y direction and within 0.36% of the vessel diameter in the z direction, indicating a very high level of accuracy.

The second test consisted of calculating the correlation between the area of the reconstructed and the original cross-sections. The reconstructed cross-sectional areas were found to be 18% less than the original cross-sectional areas. This consistent underestimation is attributed to the presence of roughness in the phantom model imaged for the reconstruction, which made the effective lumen contour smaller than the originally designed one. The extent of the presence of roughness can be seen in Figure 4-19. This is a limitation of the method that was used to test the model’s accuracy. The underestimation of the cross- sectional areas is consistent with the presence of roughness in the model, suggesting that in the application of the reconstruction method on human vessels, where the interest is in detecting the true lumen, the cross-sectional areas calculated will be closer to the clinically real value.

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Figure 4-19 The OCT slice of Figure 4-15 showing the difference between the original model lumen (green marking) and

the phantom model lumen (red marking). It is obvious that the lumen of the phantom model is smaller than the model it was based on, and this is due to limitations in the rapid prototyping method resulting in a marked level of surface roughness. The reconstruction algorithm is tuned to detect the inner lumen contour (shown in red) and so resulted in a 18% underestimation of the cross-sectional area. This issue is not expected to affect application of the reconstruction method on patients.