Computed tomography (CT) imaging of the thorax is a common application of CT in radiology. Most of these scans are performed with a helical scan protocol. A significant number of images suffer from motion artefacts due to the inability of the patients to hold their breath or due to hiccups or coughing. Some images become nondiagnostic while others are simply degraded in quality. In order to correct for these artefacts a motion compensated reconstruction for non-periodic motion is required.
For helical CT scans with a pitch smaller or equal to one the redundancy in the helical projection data can be used to generate images at the identical spatial position for multiple time points. As the scanner moves across the thorax during the scan, these images do not have a fixed time point, but a well-defined temporal distance inbetween the images. Using image based registration a motion vector field can be estimated based on these images. The motion artefacts are corrected in a subsequent motion compensated reconstruction. The method is tested on mathematical phantom data (reconstruction) and clinical lung scans (motion estimation and reconstruction).
Biomechanical modelling enables large deformation simulations of breast tissues under different loading conditions to be performed. Such simulations can be utilised to transform prone Magnetic Resonance (MR) images into a different patient position, such as upright or supine. We present a novel integration of biomechanical modelling with a surface registration algorithm which optimises the unknown material parameters of a biomechanical model and performs a subsequent regularised surface alignment. This allows deformations induced by effects other than gravity, such as those due to contact of the breast and MR coil, to be reversed. Correction displacements are applied to the biomechanical model enabling transformation of the original pre-surgical images to the corresponding target position.
The algorithm is evaluated for the prone-to-supine case using prone MR images and the skin outline of supine Computed Tomography (CT) scans for three patients. A mean target registration error (TRE) of 10:9 mm for internal structures is achieved. For the prone-to-upright scenario, an optical 3D surface scan of one patient is used as a registration target and the nipple distances after alignment between the transformed MRI and the surface are 10:1 mm and 6:3 mm respectively.
KEYWORDS: Breast, Magnetic resonance imaging, Modeling, Computer simulations, Image segmentation, Protactinium, Medical imaging, In vivo imaging, Data modeling, Chest
In biomechanical simulations of the human breast, the analysed geometry is often reconstructed from in vivo medical imaging procedures. For example in dynamic contrast enhanced magnetic resonance imaging, the acquired geometry of the patient's breast when lying in the prone position represents a deformed configuration that is pre-stressed by typical in vivo conditions and gravity. Thus, physically realistic simulations require consideration of this loading and, hence, establishing the undeformed configuration is an important task for accurate and reliable biomechanical modelling of the breast. We compare three different numerical approaches to recover the unloaded configuration from the loaded geometry given patient-specific biomechanical models built from prone and supine MR images. The algorithms compared are:(i) the simple inversion of gravity without the consideration of pre-stresses, (ii) an inversefinite deformation approach and (iii) afixed point type iterative approach which uses only forward simulations. It is shown that the iterative and the inverse approach produce similar zero-gravity estimates, where as the simple inversion of gravity is only appropriate for small or highly constrained deformations.
In the context of dynamic contrast enhanced breast MR imaging we analyzed the effect of motion compensating
registration on the characterization of lesions. Two registration techniques were applied: 1) rigid registration
and 2) elastic registration based on the Navier-Lam´e equation. Interpreting voxels that exhibit a decline in
image intensity after contrast injection (compared to the non-contrasted native image) as motion outliers, it
can be shown that the rate of motion outliers can be largely reduced by both rigid and elastic registration.
The performance of lesion features, including maximal signal enhancement ratio and variance of the signal
enhancement ratio, was measured by area under the ROC curve as well as Cohen's κ and showed significant
improvement for elastic registration, whereas features derived from rigidly registered images did not in general
exhibit a significant improvement over the level of unregistered data.
Automated segmentation of lung lobes in thoracic CT images has relevance for various diagnostic purposes like
localization of tumors within the lung or quantification of emphysema. Since emphysema is a known risk factor for lung
cancer, both purposes are even related to each other. The main steps of the segmentation pipeline described in this paper
are the lung detector and the lung segmentation based on a watershed algorithm, and the lung lobe segmentation based
on mesh model adaptation. The segmentation procedure was applied to data sets of the data base of the Image Database
Resource Initiative (IDRI) that currently contains over 500 thoracic CT scans with delineated lung nodule annotations.
We visually assessed the reliability of the single segmentation steps, with a success rate of 98% for the lung detection
and 90% for lung delineation. For about 20% of the cases we found the lobe segmentation not to be anatomically
plausible. A modeling confidence measure is introduced that gives a quantitative indication of the segmentation quality.
For a demonstration of the segmentation method we studied the correlation between emphysema score and malignancy
on a per-lobe basis.
Early response assessment of cancer therapy is a crucial component towards a more effective and patient individualized
cancer therapy. Integrated PET/CT systems provide the opportunity to combine morphologic with
functional information. We have developed algorithms which allow the user to track both tumor volume and
standardized uptake value (SUV) measurements during the therapy from series of CT and PET images, respectively.
To prepare for tumor volume estimation we have developed a new technique for a fast, flexible, and
intuitive 3D definition of meshes. This initial surface is then automatically adapted by means of a model-based
segmentation algorithm and propagated to each follow-up scan. If necessary, manual corrections can be added by
the user. To determine SUV measurements a prioritized region growing algorithm is employed. For an improved
workflow all algorithms are embedded in a PET/CT therapy monitoring software suite giving the clinician a
unified and immediate access to all data sets. Whenever the user clicks on a tumor in a base-line scan, the
courses of segmented tumor volumes and SUV measurements are automatically identified and displayed to the
user as a graph plot. According to each course, the therapy progress can be classified as complete or partial
response or as progressive or stable disease. We have tested our methods with series of PET/CT data from 9
lung cancer patients acquired at Princess Margaret Hospital in Toronto. Each patient underwent three PET/CT
scans during a radiation therapy. Our results indicate that a combination of mean metabolic activity in the
tumor with the PET-based tumor volume can lead to an earlier response detection than a purely volume based
(CT diameter) or purely functional based (e.g. SUV max or SUV mean) response measures. The new software
seems applicable for easy, faster, and reproducible quantification to routinely monitor tumor therapy.
We have analyzed 3000 nodule delineations and malignancy ratings of pulmonary nodules made by expert observers in the IDRI CT lung image database. The agreement between nodule volume from automatic segmentation and expert delineations is almost as high as inter-observer agreement. For the experts' malignancy rating the inter-observer agreement is quite modest. Linear and support vector regression models have been tested to emulate the expert
malignancy rating from a small number of automatically computed numerical image features. Machine-observer and inter-observer agreement have been evaluated using linear correlation and weighted kappa coefficient. The results suggest that the numerical computed malignancy - if considered as an additional observer - cannot be distinguished from the expert ratings.
A real-time matching algorithm for follow-up chest CT scans can significantly reduce the workload on radiologists by
automatically finding the corresponding location in the first or second scan, respectively. The objective of this study was
to assess the accuracy of a fast and versatile single-point registration algorithm for thoracic CT scans.
The matching algorithm is based on automatic lung segmentations in both CT scans, individually for left and right lung.
Whenever the user clicks on an arbitrary structure in the lung, the coarse position of the corresponding point in the other
scan is identified by comparing the volume percentiles of the lungs. Then the position is refined by optimizing the gray
value cross-correlation of a local volume of interest. The algorithm is able to register any structure in or near the lungs,
but is of clinical interest in particular with respect to lung nodules and airways.
For validation, CT scan pairs were used in which the patients were scanned twice in one session, using low-dose non-contrast-enhanced chest CT scans (0.75 mm collimation). Between these scans, patients got off and on the table to
simulate a follow-up scan. 291 nodules were evaluated. Average nodule diameter was 9.5 mm (range 2.9 - 74.1 mm).
Automatic registration succeeded in 95.2% of all cases (277 / 291). In successful registered nodules, average registration
consistency was 1.1 mm. The real-time matching proved to be an accurate and useful tool for radiologists evaluating
follow-up chest CT scans to assess possible nodule growth.
Computer aided characterization aims to support the differential diagnosis of indeterminate pulmonary nodules. A
number of published studies have correlated automatically computed features from image processing with clinical
diagnoses of malignancy vs. benignity. Often, however, a high number of features was trained on a relatively small
number of diagnosed nodules, raising a certain skepticism as to how salient and numerically robust the various features
really are. On the way towards computer aided diagnosis which is trusted in clinical practice, the credibility of the
individual numerical features has to be carefully established.
Nodule volume is the most crucial parameter for nodule characterization, and a number of studies are testing its
repeatability. Apart from functional parameters (such as dynamic CT enhancement and PET uptake values), the next
most widely used parameter is the surface characteristic (vascularization, spicularity, lobulation, smoothness). In this
study, we test the repeatability of two simple surface smoothness features which can discriminate between smoothly
delineated nodules and those with a high degree of surface irregularity.
Robustness of the completely automatically computed features was tested with respect to the following aspects: (a)
repeated CT scan of the same patient with equal dose, (b) repeated CT scan with much lower dose and much higher
noise, (c) repeated automatic segmentation of the nodules using varying segmentation parameters, resulting in differing
nodule surfaces. The tested nodules (81) were all solid or partially solid and included a high number of sub- and juxtapleural
nodules. We found that both tested surface characterization features correlated reasonably well with each other
(80%), and that in particular the mean-surface-shape-index showed an excellent repeatability: 98% correlation between
equal dose CT scans, 93% between standard-dose and low-dose scan (without systematic shift), and 97% between
varying HU-threshold of the automatic segmentation, which makes it a reliable feature to be used in computer aided
diagnosis.
We present an effective and intuitive visualization of the macro-vasculature of a selected nodule or tumor in three-dimensional
image data (e.g. CT, MR, US). For the differential diagnosis of nodules the possible distortion of adjacent
vessels is one important clinical criterion.
Surface renderings of vessel- and tumor-segmentations depend critically on the chosen parameter- and threshold-values
for the underlying segmentation. Therefore we use rotating Maximum Intensity Projections (MIPs) of a volume of
interests (VOI) around the selected tumor. The MIP does not require specific parameters, and allows much quicker
visual inspection in comparison to slicewise navigation, while the rotation gives depth cues to the viewer. Of the vessel
network within the VOI, however, not all vessels are connected to the selected tumor, and it is tedious to sort out which
adjacent vessels are in fact connected and which are overlaid only by projection. Therefore we suggest a simple
transformation of the original image values into connectivity values. In the derived connectedness-image each voxel
value corresponds to the lowest image value encountered on the highest possible pathway from the tumor to the voxel.
The advantage of the visualization is that no implicit binary decision is made whether a certain vessel is connected to
the tumor or not, but rather the degree of connectedness is visualized as the brightness of the vessel. Non-connected
structures disappear, feebly connected structures appear faint, and strongly connected structures remain in their original
brightness. The visualization does not depend on delicate threshold values. Promising results have been achieved for
pulmonary nodules in CT.
Accurate image registration is a necessary prerequisite for many diagnostic and therapy planning procedures
where complementary information from different images has to be combined. The design of robust and reliable
non-parametric registration schemes is currently a very active research area. Modern approaches combine
the pure registration scheme with other image processing routines such that both ingredients may benefit from
each other. One of the new approaches is the combination of segmentation and registration ("segistration").
Here, the segmentation part guides the registration to its desired configuration, whereas on the other hand
the registration leads to an automatic segmentation. By joining these image processing methods it is possible
to overcome some of the pitfalls of the individual methods. Here, we focus on the benefits for the registration task.
In the current work, we present a novel unified framework for non-parametric registration combined with energy-based
segmentation through active contours. In the literature, one may find various ways to combine these image
processing routines. Here, we present the most promising approaches within the general framework. It is based
on a single variational formulation of both the registration and the segmentation part. The performance tests
are carried out for magnetic resonance (MR) images of the brain, and they demonstrate the potential of the
proposed methods.
We have compared and validated image registration methods with respect to the clinically relevant use-case
of lung CT max-inhale to max-exhale registration. Four fundamentally different algorithms representing main
approaches for image registration were compared using clinical images. Each algorithm was assigned to a different
person with extensive working knowledge of its usage. Quantitative and qualitative evaluation is performed.
Whereas the methods achieve similar results in target registration error, characteristic differences come to show
by closer analysis of the displacement fields.
During medical imaging and therapeutic interventions, pulmonary structures are in general subject to cardiac
and respiratory motion. This motion leads potentially to artefacts and blurring in the resulting image material
and to uncertainties during interventions. This paper presents a new automatic approach for surface based
motion tracking of pulmonary structures and reports on the results for cardiac and respiratory induced motion.
The method applies an active shape approach to ad-hoc generated surface representations of the pulmonary
structures for phase to phase surface tracking. Input of the method are multi-phase CT data, either cardiac or
respiratory gated. The iso-surface representing the transition between air or lung parenchyma to soft tissue,
is triangulated for a selected phase p0. An active shape procedure is initialised in the image of phase p1 using
the generated surface in p0. The used internal energy term penalizes shape deformation as compared to p0.
The process is iterated for all phases pi to pi+1 of the complete cycle. Since the mesh topology is the same for
all phases, the vertices of the triangular mesh can be treated as pseudo-landmarks defining tissue trajectories.
A dense motion field is interpolated. The motion field was especially designed to estimate the error margins
for radiotherapy. In the case of respiratory motion extraction, a validation on ten biphasic thorax CT images
(2.5mm slice distance) was performed with expert landmarks placed at vessel bifurcations. The mean error on
landmark position was below 2.6mm. We further applied the method to ECG gated images and estimated the
influence of the heart beat on lung tissue displacement.
Registration of medical images, i.e. the integration of two or more images into a common geometrical system of reference so that corresponding image structures correctly align, is an active field of current research. Registration algorithms in general are composed of three main building blocks: a geometrical transformation is applied in order to transform the images into the geometrical system of reference, a similarity measure puts the comparison of the images into quantifiable terms, and an optimization algorithm searches for that transformation that leads to optimal similarity between the images. Whereas in the literature fixed configurations of registration algorithms are investigated, here we present a modular toolbox containing several similarity measures, transformation classes and optimization strategies. Derivative-free optimization is applicable for any similarity measure, but is not fast enough in clinical practice. Hence we consider much faster derivative-based Gauss-Newton and Levenberg-Marquardt optimization algorithms that can be used in conjunction with frequently needed similarity measures for which derivatives can be easily obtained. The implemented similarity measures, geometrical transformations and optimization methods can be freely combined in order to configure a registration algorithm matching the requirements of a particular clinical application. Test examples show that particular algorithm configurations out of this toolbox allow e.g. for an improved lesion identification and localization in PET-CT or MR registration applications.
B-splines are a well-known approach for non-rigid image registration. Though successfully applied to various medical applications they exhibit a high computational complexity mainly because of the lack of dedicated optimization methods. In this work we focus on a Levenberg-Marquardt type optimization routine. As a similarity measure we use least-squares functionals such as the sum of squared differences, the cross-correlation and the local correlation measure, respectively. The latter is used for multi-modality registration tasks. The proposed registration algorithm consists of three main parts. In each iteration step one has to (a) build a linear system of equations, (b) solve this system and compute an update, (c) determine the step length for the following iteration step. Appropriate stopping criteria ensure the termination of the registration task. A standard approach for (c) and several modifications are investigated. Using a quadratic model we are able to avoid additional execution of (b) during the step length adaption. Several solvers (Cholesky, CG, pre-conditioning) for (b) have been evaluated. Also, modifications on the most time consuming task (a) are investigated, leading to a speed-up by a factor up to 30. Finally, the algorithm is embedded in a multi-scale framework (both on image and on parameter level) providing additional regularization, an increased capture range and speed-up. Convergence tests have been successfully applied for a priori known transformations. Feasibility of the proposed approach is also shown for clinical applications including PET-CT registrations (19 data sets) and MR mammography.
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