Holger Timinger

Motion compensated coronary interventional navigation by means of diaphragm tracking and elastic motion models

Current catheter tracking in the x-ray catheter laboratory during coronary interventions is performed using 2D fluoroscopy. Although this features real-time navigation on high-resolution images, drawbacks such as overlap and foreshortening exist and hamper the diagnosis and treatment process. An alternative to fluoroscopy-based tracking is device tracking by means of a magnetic tracking system (MTS). Having measured the 3D location of the interventional device, its position can be reconstructed on 3D images or virtual roadmaps of the organ or vessel structure under examination. In this paper, a method is presented which compensates the interventional device location measured by the MTS for organ motion and thus registers it dynamically to a 3D virtual roadmap. The motion compensation is accomplished by using an elastic motion model which is driven by the ECG signal and a respiratory sensor signal derived from ultrasonic diaphragm tracking. The model is updated during the intervention itself, thus allowing for a local refinement in regions which bear a complex geometric structure, such as stenoses and bifurcations. The evaluation is done by means of a phantom-based study using a dynamic heart-phantom. The mean displacement caused by the overall motion of the heart is improved from 10.4+/-4.8 mm in the uncompensated case to 2.1+/-1.2 mm in the motion compensated case.

Motion compensation for interventional navigation on 3D static roadmaps based on an affine model and gating

Current cardiac interventions are performed under 2D fluoroscopy, which comes along with well-known burdens to patients and physicians, such as x-ray exposure and the use of contrast agent. Furthermore, the navigation on complex structures such as the coronaries is complicated by the use of 2D images in which the catheter position is only visible while the contrast agent is introduced. In this work, a new method is presented, which circumvents these drawbacks and enables the cardiac interventional navigation on motion-compensated 3D static roadmaps. For this, the catheter position is continuously reconstructed within a previously acquired 3D roadmap of the coronaries. The motion compensation makes use of an affine motion model for compensating the respiratory motion and compensates the motion due to cardiac contraction by gating the catheter position. In this process, only those positions which have been acquired during the rest phase of the heart are used for the reconstruction. The method necessitates the measurement of the catheter position, which is done by using a magnetic tracking system. Nevertheless, other techniques, such as image-based catheter tracking, can be applied. This motion compensation has been tested on a dynamic heart phantom. The evaluation shows that the algorithm can reconstruct the catheter position on the 3D static roadmap precisely with a residual motion of 1.0 mm and less.

Integration of Interactive Corrections to Model-Based Segmentation Algorithms

3D deformable shape models have become a common approach for solving complex segmentation tasks in medical image processing. Nevertheless sometimes the segmentation fails due to low image resolution or contrast, structures lying closely together or an insufficient initialization of the model. Although the error is often obvious to physicians, they have no opportunity to improve the result. This paper presents 3D tools for the correction of erroneous segmentations and provides a method which allows the integration of these corrections to deformable model-based segmentation methods. The integration is accomplished by a user deformation energy which is defined in a way that allows efficient corrections without the need to segment the complete erroneous region manually. This new approach is illustrated on the segmentation of a vertebra and a femur-head.

Fast and Accurate Automatic Registration for MR-Guided Procedures Using Active Microcoils

A fast, robust, accurate, and automatic registration technique based on magnetic resonance (MR) active microcoils (active markers) for registration of tracked medical devices to preprocedural MR-images is presented. This allows for a straightforward integration of position measurement systems into clinical procedures. The presented method is useful for guidance purposes in clinical applications with high demands on accuracy and ease-of-use (e.g., neurosurgical or orthopedic applications). The determination of the positions of the active markers is integrated into the preparation phase of the actual MR imaging scan. The technique features a generic interface using DICOM standards for communication with navigation workstations linked to an MR system. The position of the active markers is fixed with respect to a reference system of an optical positioning measurement system (OPMS) and thus the coregistration of the MR system and the OPMS is established. In a phantom study, a mean overall targeting accuracy of 0.9plusmn0.1 mm was achieved and compared favorably to results obtained from manual registration tests (1.8plusmn0.3 mm) carried out in parallel. For a test person trained for both registration methods, workflow improvements of 3-6 min per registration step were found. The need for manual interaction is entirely eliminated thus avoiding user-bias, which is advantageous for the usage in clinical routine. The method improves the ease-of-use of tracking equipment during stereotactic guidance. The method is finally demonstrated in a volunteer study using a model of a Mayfield skull clamp with integrated active and optical reference markers

DESIGN PARAMETERS FOR CRYOGENIC THERMOSYPHONS

Cryogenic thermosyphons are the thermal conductors of choice for a variety of applications such as conduction-cooled superconducting devices. They exhibit a small effective thermal resistance at small cross-sections. A careful design, however, is crucial to ensure sufficient heat transport for all possible heatloads. The aim of this work is to obtain experimental results on critical limitations and the effective thermal conductivity dependent on the length, the cross-sectional area, and the working liquid fill level of a thermosyphon for different heatloads. For the experiments, a modular thermosyphon was designed with 5 different adiabatic tubes of length [cm]/cross-sectional diameter [cm] 10/1, 10/2, 30/0.5, 30/1, 30/2, which can be mounted between condenser and evaporator. The thermosyphon was operated with different fill levels of either nitrogen or neon and different heatloads. The effective thermal conductivity between condenser and evaporator was determined, dependent on the design parameters menti...

Ultrasonic diaphragm tracking for cardiac interventional navigation on 3D motion compensated static roadmaps

In this paper, a novel approach to cardiac interventional navigation on 3D motion-compensated static roadmaps is presented. Current coronary interventions, e.g. percutaneous transluminal coronary angioplasties, are performed using 2D X-ray fluoroscopy. This comes along with well-known drawbacks like radiation exposure, use of contrast agent, and limited visualization, e.g. overlap and foreshortening, due to projection imaging. In the presented approach, the interventional device, i.e. the catheter, is tracked using an electromagnetic tracking system (MTS). Therefore, the catheters position is mapped into a static 3D image of the volume of interest (VOI) by means of an affine registration. In order to compensate for respiratory motion of the catheter with respect to the static image, a parameterized affine motion model is used which is driven by a respiratory sensor signal. This signal is derived from ultrasonic diaphragm tracking. The motion compensation for the heartbeat is done using ECG-gating. The methods are validated using a heart- and diaphragm-phantom. The mean displacement of the catheter due to the simulated organ motion decreases from approximately 9 mm to 1.3 mm. This result indicates that the proposed method is able to reconstruct the catheter position within the VOI accurately and that it can help to overcome drawbacks of current interventional procedures.

Determination of the temporal and spatial resolution in retrospectively gated cardiac cone-beam CT

In this contribution, we investigate the spatial and temporal resolution of retrospectively gated cardiac cone-beam CT. Data of a static and a dynamic resolution phantom are acquired for various heart rates, table speeds and scanner rotation times. The projection data are reconstructed in different motion states with the help of a retrospectively gated helical cardiac cone-beam reconstruction method. This multi-cycle method automatically adapts the number of heart cycles used for the reconstruction, based on the scan parameters and the ECG data. The spatial resolution is derived from a resolution phantom by multi-planar reformation (MPR) along the scan direction.