Continuous Ultrasound Speckle Tracking with Gaussian Mixtures

Speckle tracking echocardiography (STE) is now widely used for measuring strain, deformations, and motion in cardiology. STE involves three successive steps: acquisition of individual frames, speckle detection, and image registration using speckles as landmarks. This work proposes to avoid explicit detection and registration by representing dynamic ultrasound images as sparse collections of moving Gaussian elements in the continuous joint space-time space. Individual speckles or local clusters of speckles are approximated by a single multivariate Gaussian kernel with associated linear trajectory over a short time span. A hierarchical tree-structured model is fitted to sampled input data such that predicted image estimates can be retrieved by regression after reconstruction, allowing a (bias-variance) trade-off between model complexity and image resolution. The inverse image reconstruction problem is solved with an online Bayesian statistical estimation algorithm. Experiments on clinical data could estimate subtle sub-pixel accurate motion that is difficult to capture with frame-to-frame elastic image registration techniques

Image Registration for the Management of Organ Motion and Deformation in Image Guided Radiotherapy (Beeldregistratie voor het omgaan met orgaanbeweging en vervorming in beeldgebaseerde radiotherapie)

Cancer has evolved to one of the leading diseases in the Western world today. The treatment of cancer usually consists of a mix of radiotherapy, chemotherapy and surgery. Radiotherapy is therein an important part as it has been proven to be very efficient in a large number of indications. In the last decades, radiotherapy has undergone a huge evolution with treatment machines allowing the delivery of much more conformal, 3D dose distributions. Additionally, the availability of image guidance during planning, treatment and follow-up, allows to further increase conformality thereby decreasing normal tissue dose and increasing tumour dose. However, these new techniques become increasingly demanding in terms of image acquisition and image processing. Often, a prerequisite to use image guidance is the availability of accurate image registration, i.e. the establishment of point-by-point correspondence between images to allow fusion of images with different, complementary information. In this work, we investigate the challenges posed by image guided radiotherapy and the possible answers provided by using image registration techniques. First, we assess the use of nonrigid registration for multimodal planning. To accurately irradiate the target, it is important to know the location of the malignant cells. This is not always straightforward as the image modality required for dose planning, CT, doesn t always provide sufficient soft-tissue contrast to properly see the tumour. This is for example the case in rectal cancer, where other imaging modalities, such as MRI or PET, should be preferred for target delineation as they provide better soft-tissue anatomy contrast or functional information about the tissue, respectively. For dose planning, however, these additional images need to be fused with the planning CT. As the rectum is a deformable organ, susceptible to variations in rectal and bladder filling, mere rigid registration techniques will not suffice. Therefore, we explore the use ofnonrigid registration techniques to align these image modalities fortreatment planning (or additionally for treatment follow-up). We show that, while multimodal treatment planning is feasible, it is very challenging in the pelvic region due to the uncertainties that arise in the images between different acquisitions. Next, we show how to calculate treatment margins which account for deformation in rectal cancer. At treatment planning, when the target is known, it is required to take uncertainties into account which are related to either microscopic tumour invasion (invisible on conventional imaging), organ motion and deformation or machine inaccuracies. Usually, the target is enlarged by a margin to account for these uncertainties. Among these uncertainties, organ motion and deformation contribute significantly and while it has been shown how to take organ motion into account to determine margins, this has not yet been done for organ deformation. We introduce a novel methodology which calculates margins that take the deformation of the target into account. We apply this methodology to rectalcancer patients who would benefit from an additional boost to the mesorectum. We show that our non-uniform, 3D margins are likely much more suited than the commonly used uniform margins. Considering currenttechnical limitations, we simplify our 3D margins and give recommendations on the size of margins for clinical use. Next, we want to decrease the motion of the target with respect to the treatment beam by using image guidance. Initially, we look at the use of implanted fiducial markers for aligning a patient between different treatment fractions. We describe an automatic 2D/3D registration which aligns the patient based on the marker positions as automatically detected in the planning CT and on the marker positions as manually detected in orthogonal kV and MV electronic portal images. We compare our automatic registration, which also includes prostate shrinkage and prostate rotation,to manual registration in terms of translations and show that it is nearly equivalent. Furthermore, we show that prostate rotation can be significant and that prostate shrinkage occurs due to hormone therapy combined with radiotherapy. We conclude that automatic 2D/3D registration is a useful technique for replacing manual registration. Furthermore, it might be useful to perform weekly replanning of these patients to take the prostate shrinkage into account. Next, we look at the use of 3D CBCT imaging for patient alignment. As some patients are not eligible for implantation with fiducial markers, they require an alternative positioning as the initial laser positioning is known to suffer from a series of inaccuracies. We propose to use on-board CBCT images based on the soft-tissue information to align the patient with respect to the planning CT. We use a methodology based on masked CBCT images and mutual information based rigid registration and validate using fiducial markers as ground-truth. We show that our method accurately recovers left-right translation and is a strong improvement for both AP and IS translation. However, the method is currently not able to recover prostate rotations. Our method allows to reduce margins in comparison to traditional laser-based positioning. Finally, we lookat the detection of intrafraction motion using automatically detected fiducial markers. While interfraction motion has been well described, the impact of intrafraction motion is just now being investigated. Using images acquired during the delivery of a treatment beam in IMRT treatment, we are able to observe the implanted fiducial markers during treatment. We have developed a method that automatically detects these markers in 2D images using a series of image filtering and discrete optimization. We show how these detected markers lead to the actual intrafraction motion by assuming a shortest path between the treatment beams. Using this methodology, we show that intrafraction motion is largest at the start of treatment, between the initial kV/MV positioning and the first treatment beam. Furthermore, we show that intrafraction motion occurring during treatment itself (between the first and last treatment beam) is fairly small but increases significantly as the treatment duration increases. This leads us to the conclusion that treatment time should be as short as possible and that initial positioning should be as close to the first beam as possible. Overall, we have shown the use of image registration for optimizing every step of the radiotherapy treatment process. While in some cases we provided a definite answer to a certain problem, some challenges still need to be overcome in the future. Mainly in terms of image acquisition and the reduction of uncertainties by proper patient preparation, a lot of gain can be expected.Slagmolen P., ''Image registration for the management of organ motion and deformation in image guided radiotherapy'', Proefschrift voorgedragen tot het behalen van het doctoraat in de ingenieurswetenschappen, K.U.Leuven, September 2010, Leuven, Belgium.status: publishe

Strain mapping in the Achilles tendon - A systematic review

Achilles tendinopathy remains one of the most prevalent overuse injuries in elite as well as recreational athletes. Regardless of the fact that the aetiology of tendinopathy has not been fully understood, therapeutic mechanical loading programs have emerged as being the treatment of choice. In this light, mechanical properties of the tendon and their response to changes in loading or unloading have been the subject of many previous investigations. One of these properties often investigated is strain, a measure of relative deformation. By means of a systematic review, an overview was given of research in this field, with a primary objective to list the methods used and secondary aim to synthesize data on strain mapping in the Achilles tendon. Following the guidelines of the PRISMA statement, 47 articles were found appropriate for qualitative assessment. Achilles tendon strain has been investigated across a variety of contexts, including the response to exercise, walking, unloading, ageing, hormonal changes and weight. Only three studies investigated the effect of the presence of tendinopathy on strain. Ultrasound was the most often used imaging modality to measure or estimate strain. Further methodological parameters, e.g. the location of measurement, differed greatly between all different studies. Nearly all studies considered global strain. Some studies investigated the transverse strain response of the Achilles tendon. Recently, however, the role of local - intratendinous - strain distribution has been found to be of critical importance and further studies should focus on imaging modalities to investigate these local changes.publisher: Elsevier articletitle: Strain mapping in the Achilles tendon – A systematic review journaltitle: Journal of Biomechanics articlelink: http://dx.doi.org/10.1016/j.jbiomech.2016.02.057 content_type: article copyright: © 2016 Elsevier Ltd. All rights reserved.status: publishe

Image enhancement techniques allowing observation of intra-fractional motion in IMRT treatment for prostate carcinoma

status: publishe

Settlers of the brain: A board game to illustrate “Probabilistic framework for population analysis of brain MR images”

Ribbens A., Slagmolen P., Maes F., Suetens P., ''Settlers of the brain: A board game to illustrate “Probabilistic framework for population analysis of brain MR images”'', Knowledge for Growth - 9th edition of FlandersBio’s annual life sciences convention, May 30, 2013, Ghent, Belgium.status: publishe

Quantification of diffusion-weighted MRI for treatment response assessment in head and neck cancer

Van Herck H., Slagmolen P., Maes F., Suetens P., ''Quantification of diffusion-weighted MRI for treatment response assessment in head and neck cancer'', Internal report KUL/ESAT/PSI/1201, KU Leuven, ESAT, September 2012, Leuven, Belgium.status: publishe

Validation of nonrigid registration for multi-tracer PET-CT treatment planning in rectal cancer radiotherapy

Paper Abstract The goal of radiotherapy is to deliver maximal dose to the tumor and minimal dose to the surrounding tissue. This requires accurate target definition. In sites were the tumor is difficult to see on the CT images, such as for rectal cancer, PET-CT imaging can be used to better define the target. If the information from multiple PETCT images with different tracers needs to be combined, a nonrigid registration is indispensable to compensate for rectal tissue deformations. Such registration is complicated by the presence of different volumes of bowel gas in the images to be registered. In this paper, we evaluate the performance of different nonrigid registration approaches by looking at the overlap of manually delineated rectum contours after registration. Using a B-spline transformation model, the results for two similarity measures, sum of squared differences and mutual information, either calculated over the entire image or on a region of interest are compared. Finally, we also assess the effect of the registration direction. We show that the combination of MI with a region of interest is best able to cope with residual rectal contrast and differences in bowel filling. We also show that for optimal performance the registration direction should be chosen depending on the difference in bowel filling in the images to be registered.Slagmolen P., Roels S., Loeckx D., Haustermans K., Maes F., ''Validation of nonrigid registration for multi-tracer PET-CT treatment planning in rectal cancer radiotherapy'', SPIE medical imaging 2009 conference : image processing, vol. 7259, February 7-12, 2009, Orlando, Florida, USA.status: publishe

CT characteristics allow identification of patient-specific susceptibility for radiation-induced lung damage

There is a huge difference in radiosensitivity of lungs between patients. The present study aims to identify and quantify patient-specific radiosensitivity based on a single pre-treatment CT scan.Defraene G., van Elmpt W., Crijns W., Slagmolen P., De Ruysscher D., ''CT characteristics allow identification of patient-specific susceptibility for radiation-induced lung damage'', Radiotherapy and oncology, vol. 117, no. 1, pp. 29-35, October 2015.status: publishe