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Evaluation of the Kalman Filter Approach to Segmentation 43

The Kalman filter[57] approach to echocardiographic segmentation has been used in several works by for instance Orderud[86], Bersvendsen[11] and Snare[102].

Since its invention in 1960, the Kalman filter has been used for a variety of purposes, like navigation, control of manufacturing processes and prediction of the paths of celestial objects[43]. The filter does have some limitations and assumptions that are often not true.[56]. Firstly, the Kalman filter has an assumption that the next time step is solely determined by the previous, this is called the Markov property. Secondly, the noise in the system is assumed to be additive Gaussian, although modifications can be made if noise is known to be something else. Finally, the process is assumed to be either linear or locally linear.

The Markov property does not hold for a cardiac image, which is cyclical, meaning that if at time step k to k+1 there is an expansion of the ventricle, that expansion will most likely continue into k+2. Knowing how much, if any, expansion there was between previous time steps would give extra information about where in the cardiac cycle the system is in, and could be used to give more accuracy, but the Kalman filter does typically not allow this.

An assumption on the edge detection procedure developed by Orderud[86] is that the individual edge detections are independent of each other. This is not true, as edge detectors close to each other will likely detect the same objects, and so the results of one heavily correlate with the results of the others. Factors

like general contrast and quality of the image influence all edge detectors, giving a further reason why independence is not true.

Finally, the Kalman algorithm requires a good starting guess, and if the guess is too wrong, the entire algorithm can fail. In situations where there is too much variation, this can be fixed by manual input or a special initialization stage.

Despite these theoretical limitations, the model has in practical applications shown good results over a variety of uses, like the segmentation of the LV, RV and LA, the placement of the mitral valveand the estimation of standard views.

It is computationally efficient and allows for an intuitive and adaptable union of known information about shape and size of what is to be estimated, and the measurements from the image.

4.5 Future Improvements and Applications

For the sharp models, it would be an advantage if the Kalman filter could be used to determine the proper weight for any edge or vertex, instead of it being manually determined ahead of time, as was done in paper I. This could possibly be done by determining how sharp the local curvature is based on the edge detectors, and this could be used as input to the Kalman filter.

The sharp Doo-Sabin methods have the potential to be used in other applications of modeling sharp features, especially objects that have a mix of smooth and sharp edges. This could be for medical segmentation, potentially the valves could be modeled with a sharp centre, but also for non-medical applications. In addition, the sharp Doo-Sabin models could be used for the segmentation of the RV in other modalities, like MRI.

One of the advantages of the standard view algorithm constructed in the second paper is that it can run in real time-after initialization. This means it would be possible to update the standard view at every frame to make sure all features are in view. The algorithm could also be modified to find other landmarks in TEE images.

The four-chamber model Doo-Sabin could also be used on other modalities like MRI or CT. Seeing as other four chamber models have been made for those modalities, it would be an interesting point of comparison. The four-chamber model does have the potential for use in cases where the chamber walls are partially obstructed. While the accuracy would be less than if the chamber was in full view, using the placement and size of the fully visible chambers to guide the partially visible ones. Figure 4.3 shows an example of this, but a rigorous test is necessary.

It is possible to expand on the four-chamber model in order to use it in more applications. By setting further landmarks on the models, other features in the image could be identified. It would also be possible to add the aortic outflow tract or other such features to the model. The four-chamber model could also be used for interventional guidance, helping with determining what part of the heart the tools are positioned at. For a robot guidance system for use in cardiac

Future Improvements and Applications

Figure 4.3: A comparison of a single chamber and four-chamber approach in a case with partially obscured LV. The four-chamber approach is shown on the left, and the single on the right. The four-chamber version of the LV is better at determining the apex and the base, likely helped by the RV and LA placement.

intervention, a four-chamber model fitted to an image of the heart could be used to determine landmark positions that the robot could guide itself from.

Other fitting algorithms could be considered. Chen et al.[23] note several algorithms that combine deformable models and deep learning fitting algorithms, which might prove an interesting alternative.

Chapter 5

Conclusions

This thesis has been part of the INIUS project in cooperation with GE Healthcare.

The aim has been to create new solutions to practical problems, and the premise of the thesis was that this can be done by expanding on GE Healthcare’s framework for echocardiographic segmentation.

Following this idea, several improvements have been made to the framework originally described by Orderud et al. [86]. Two sharp versions of the Doo-Sabin surfaces have been created and an RV model has been made based on those surfaces. A technique for solving the initialization problem of the Kalman filter in some situations has been invented, and a four-chamber model with a new implementation of wall thickness between the chambers has been made. While these were made to solve practical problems, they are also theoretical contributions to the fields of mathematical splines, Kalman filters, and echocardiographic segmentations.

Given the aim of the thesis, these expansions of the framework were applied to several practical problems in the field of ultrasound segmentation. The sharp Doo-Sabin models were used to create more accurate segmentation of the RV, accuracy mainly being measured by the model’s ability to estimate the volume and ejection fraction. Accurate segmentation of the RV is useful to determine RV dysfunction, and RV ejection fraction is a predictor for moderate heart failure.

We have created an algorithm for the automatic determination of standard views in TEE images. The solution to the initialization problem of Kalman filters was vital in order to detect the aorta and mitral valve, the location of which was used to determine standard views in TEE images. Automatic standard views would allow for shorter time being spent on manually determining it during the examination of for instance valvular dysfunction and could be useful for guidance during surgery.

Finally, the four-chamber model was used to accurately and simultaneously segment all four chambers of the heart in ultrasound images. Again volumetric measures were used to test how well the four-chamber model worked, but this model could easily be used to extract landmarks. It could also be used to study how the chambers interact.

Through these applications, we believe we have shown that the expansions of Orderud’s framework allow it to be used in new applications, and have real value when it comes to solving practical segmentation problems, leading to an improved workflow and diagnostic accuracy.

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Papers

Paper I

Two Methods for Modified Doo-Sabin Modeling of

Non-Smooth Surfaces - Applied to Right Ventricle Modelling

Håkon Strand Bølviken, Jørn Bersvendsen, Fredrik Orderud, Sten Roar Stange, Pål Brekke, Eigil Samset

Published inJournal of Medical Imaging, December 2020, volume 7, issue 6, pp. 1–17. DOI: 10.1117/1.JMI.7.6.067001.

I

Abstract

Purpose: In recent years there has been increased clinical interest in the right ventricle(RV) of the heart. RV dysfunction is an important prognostic marker for several cardiac diseases. Accurate modeling of the RV shape is important for estimating performance. We have created computationally effective models that allow for accurate estimation of the RV shape.

Approach: Previous approaches to cardiac shape modeling, including modeling the RV geometry, has used Doo-Sabin surfaces. Doo-Sabin surfaces allow for effective computation and adapt to smooth, organic surfaces. However, they struggle with modeling sharp corners or ridges without many control nodes. This paper modifies the Doo-Sabin surface to allow for sharpness using weighting of vertices and edges instead. This was done in two different ways. For validation, we compared the standard Doo-Sabin vs. the sharp Doo-Sabin models in modeling the RV shape of 16 cardiac ultrasound images, against a ground truth manually drawn by a cardiologist. A Kalman filter fitted the models to the ultrasound images, and the difference between the volume of the model and the ground truth was measured.

Results: The two modified Doo-Sabin models both outperformed the standard Doo-Sabin model in modeling the RV. On average the regular Doo-Sabin had 8 ml error in volume, while the sharp models had 7 and 6 ml error respectively.

Conclusions: Compared to the standard Sabin, the modified Doo-Sabin models can adapt to a larger variety of surfaces while still being compact models. They were more accurate on modeling the RV shape, and could have uses elsewhere.