Bernhard Brendel

Registration of CT and Intraoperative 3-D Ultrasound Images of the Spine Using Evolutionary and Gradient-Based Methods

A system for the registration of computed tomography and 3-D intraoperative ultrasound images is presented. Three gradient-based methods and one evolutionary algorithm are compared with regard to their suitability to solve this image registration problem. The system has been developed for pedicle screw insertion during spinal surgery. With clinical preoperative and intraoperative data, it is demonstrated that precise registration is possible within a realistic range of initial misalignment. Significant differences can be observed between the optimization methods. The covariance matrix adaptation evolution strategy shows the best overall performance, only four of 12 000 registration trials with patient data failed to register correctly.

Vibrography During Tumor Neurosurgery

The aim of this study was to determine whether elastography, a sonographically based real‐time strain imaging method for registering the elastic properties of tissue, can be used in brain tumor surgery.

Bone registration with 3D CT and ultrasound data sets

Abstract For many navigated surgical procedures, the precise registration of preoperative data sets with bones of the patient is an important requisite. Conventional navigation systems use paired point registration based on anatomical landmarks or fiducial markers. This approach increases the invasiveness, since landmarks must be exposed and fiducial markers must be connected to the bone. Intraoperative imaging modalities can overcome this disadvantage. Ultrasound seems to be ideal because of the easy data acquisition. The problem, however, is the low imaging quality regarding bones. The proposed algorithm for the registration of CT and ultrasound data sets considers the ultrasound imaging properties. That part of the bone surface, which should be visible in the ultrasound data is estimated from the CT data. The ultrasound data is preprocessed to emphasize bone surfaces. Thus, the ultrasound data contains a bright shape that is formed like the surface estimated from the CT data. A surface–volume registration tries to correlate the estimated surface with this bright shape. The algorithm was validated using an ex vivo preparation of a human lumbar spine. The algorithm was shown to cope with initial misalignments of about 30 mm and 15°. Successful registration of in vivo data of lumbar spine, tibia and shoulder indicate the feasibility of the approach.

Optimization of apodizations based on the sidelobe pressure energy in simulated ultrasound fields

High sidelobe levels of ultrasound beams can generally be reduced by apodizing the sound emitting aperture. The integrated sidelobe energy could be a criterion for the optimization of such an apodization. Thus, we developed an algorithm, which generates an apodization that minimizes the sidelobe energy for a given width of the ultrasound beam. The performance of the algorithm was evaluated on simulated ultrasound fields.

Fast, extended velocity range flow imaging based on nonuniform sampling using adaptive wall filtering and cross correlation

Pulsed Wave Doppler (PWD) systems acquire an ensemble of N echoes per beam line at a constant pulse repetition frequency f/sub prf/. The total time span determines the velocity resolution, and f/sub prf/ the unambiguous velocity range. The ensemble size N is by approximation inversely proportional to the frame rate, assuming that the system performs interleaving. For a given frame rate, a tradeoff can only be made between velocity resolution and velocity range. We propose a pulse sequence, a velocity estimation algorithm and an adaptive wall filter for nonuniform sampling, i.e. sampling with varying sampling intervals. This approach allows to increase the frame rate by increasing the velocity estimation error for high flow velocities.

In vivo Evaluierung und in vitro Genauigkeitsmessung für einen Algorithmus zur Registrierung von Ultraschall- und CT-Datensätzen

Ein Algorithmus zur Registrierung von Knochen in dreidimensionalen CT- und Ultraschalldatensatzen, der auf einem Oberflachen-Volumen-Matching basiert, wurde bezuglich seiner Genauigkeit in vitro getestet. Als Vergleichsverfahren diente eine Punktregistrierung anhand kunstlicher Mar-ken. Der Vergleich ergab, dass fur einen Wirbel Abweichungen bis 1.7 mm zwischen den Registrierungen auftraten, wobei der systematische Anteil der Abweichung vernachlassigbar klein war. Weiterhin wurde die Ultraschallregistrierung an in vivo Daten evaluiert, wobei sich ergab, dass der Algorithmus fur unterschiedliche Knochenstrukturen geeignet ist.

Registrierung von Knochen in 3D-Ultraschall- und CT-Daten: Vergleich verschiedener Optimierungsverfahren

Ein zentrales Problem in der bildgestutzten, navigierten Chirurgie ist die exakte Registrierung praoperativer CT/MR-Daten im Koordinatensystem des Operationssaales. Um die Nachteile einer landmarkenbasierten Registrierung zu umgehen, dient bei unserem Ansatz 3-dimensionaler, intraoperativer Ultraschall zur Registrierung. Ein automatischer Registrieralgorithmus leistet die Abbildung der Knochenstrukturen aus einem praoperativen CT in den Ultraschalldatensatz. Gegenstand unserer Untersuchungen ist die Umsetzung der Registrierung, die Robustheit des Registrieralgorithmus und die intraoperativ benotigte Rechenzeit. Das Zielgebiet ist exemplarisch die Lendenwirbelsaule; die Datensatze stammen von einem Phantom mit 3 Plastikwirbeln und von einem Patienten.

Optimized filters for dynamic RF echo blending in multiple focal zone imaging

For several practical and technical reasons related to e.g. computational cost, hardware complexity, sensitivity to motion artifacts, frame rate and SNR most medical ultrasound machines use a fixed focus on transmit and dynamic focusing and filtering on receive. To further improve image resolution, multiple focal zones may be used. The resulting A-lines are superimposed (blended) after envelope detection by means of depth dependent weighting. Due to this incoherent superposition, the spatial resolution of the resulting image can only reach the resolution of the images that were superimposed. To overcome the shortcomings of incoherent superposition we have developed a technique for coherent filtered blending of the echoes. In addition to echoes representing different transmit foci, echoes from neighboring beam lines may also be considered.

Parametrisierung evolutionärer Strategien für die Registrierung von Wirbelknochen in Ultraschall und CT-Daten

Ein zentrales Problem der bildgestutzten, navigierten Chirurgie stellt die Registrierung praoperativer Bilddaten mit dem Koordinatensystem des Operationssaals dar. Bei der rigiden Registrierung werden sechs Parameter benotigt, um die korrekte Transformation eines Koordinatensystems in ein anderes zu bestimmen. Diese Parameter werden mit einem Optimierungsverfahren ermittelt. Das Ergebnis einer Optimierung hangt von der Parametrisierung der Optimierungsstrategie ab und die Wahl dieser Hyperparameter muss in Abhangigkeit von dem zu losenden Problem getroffen werden. Anhand zweier reprasentativer realer Datensatze (jeweils 3-dimensionale CT- und Ultraschalldatensatze) wurden verschiedene Einstellungen der Hyperparameter getestet.

A modified synthetic aperture focusing technique for the correction of geometric artefacts in intravascular ultrasound elastography

Various elastography methods have been developed for imaging the elastic properties of coronary plaques with intravascular ultrasound (IVUS). In these methods the vessel tissue is compressed due to changes of the intraluminal pressure, i.e. by a force originating from the lumen center. If the transducer is not centered in the lumen, the ultrasound beams are not parallel to the direction of force, which leads to errors in the strain estimate. In this work a modified synthetic aperture focusing technique (SAFT) is presented, that allows the reconstruction of ultrasound beams parallel to the force direction. To verify this approach, IVUS data and a cylindrical phantom are simulated with various eccentric catheter positions.