C. Welp

Investigation of the influence of blood flow rate on large vessel cooling in hepatic radiofrequency ablation / Untersuchung des Einflusses der Blutflussgeschwindigkeit auf die Gefäßkühlung bei der Radiofrequenzablation von Lebertumoren

Abstract Radiofrequency (RF) ablation using high-frequency current has become an important treatment method for patients with non-resectable liver tumors. Tumor recurrence is associated with tissue cooling in the proximity of large blood vessels. This study investigated the influence of blood flow rate on tissue temperature and lesion size during monopolar RF ablation at a distance of 10 mm from single 4- and 6-mm vessels using two different approaches: 1) an ex vivo blood perfusion circuit including an artificial vessel inserted into porcine liver tissue was developed; and 2) a finite element method (FEM) model was created using a novel simplified modeling technique for large blood vessels. Blood temperatures at the inflow/outflow of the vessel and tissue temperatures at 10 and 20 mm from the electrode tip were measured in the ex vivo set-up. Tissue temperature, blood temperature and lesion size were analyzed under physiological, increased and reduced blood-flow conditions. The results show that changes in blood flow rate in large vessels do not significantly affect tissue temperature and lesion size far away from the vessel. Monopolar ablation could not produce lesions surrounding the vessel due to the strong heat-sink effect. Simulated tissue temperatures correlated well with ex vivo measurements, supporting the FEM model.

Evaluation of ultrasonic texture and spectral parameters for coagulated tissue characterization

Radiofrequency ablation is a well established, minimally invasive approach for the treatment of tumors. However, at present there is a lack of suitable imaging modalities for accurate online monitoring of the coagulation process. The aim of this work is to evaluate the potential of various tissue characterizing ultrasonic parameters from spectral and spatial domain to differentiate between coagulated and noncoagulated tissue. The calculated parameters include first and second order texture parameters, estimates of attenuation coefficients, spectral parameters (slope, intercept and midband value) and coefficients of autoregressive spectral estimates. As a measure of selectivity of each parameter the area under the receiver operating characteristic (ROC) curve was utilized. The best performing parameters can be used to be processed by a classification system.

[Ultrasound-based imaging modalities for thermal therapy monitoring].

Thermal therapy has been established as an alternative and minimally invasive approach for the treatment of tumors. During a thermal therapy tissue is heated locally up to above 60 degrees C. Cancerous tissue can thus be destroyed by coagulation. At present there are no suitable imaging modalities available for an accurate real-time monitoring of the coagulation process. A subproject of the Ruhr Center of Competence for Medical Engineering (KMR Bochum) aims at developing an ultrasound-based, real-time capable monitoring system for thermal therapy. Therefore several tissue characterizing imaging modalities will be combined in a new multimodal concept. Initial experiments with porcine liver in vitro have shown that real-time monitoring of a thermal therapy using various imaging methods simultaneously will be possible.

Classification of Thermally Ablated Tissue Using Diagnostic Ultrasound

In this work the application of an ultrasound based classification system for the detection of thermally ablated tissue is presented. In-vitro and in-vivo experiments have been carried out using an RF ablation device. A conventional diagnostic ultrasound device was used for measurements. Several features from spectral and spatial domain were extracted from ultrasound RF data and processed by the classifier. Classification results are presented in binary coagulation maps

A classification system for monitoring thermal therapies

In this work a classification system for monitoring of thermal therapies is presented. Various features from spectral and spatial domain are processed by the system using the maximum likelihood measure as a classifier. First and second order texture features, estimates of attenuation coefficients, and measures of backscatter (slope, intercept and midband value) are used as tissue characterizing features. For evaluation of the system measurements were performed on bovine liver samples in vitro. Coagulation was induced using a commercial RF-ablation device. Acquisition of radio frequency data was done immediately after finishing the ablation process and additionally after cooling down of the liver samples to evaluate the systems ability for online monitoring as well as for follow up examinations. Classification was done by total cross validation over cases. The best feature combination found by sequential forward selection was processed by the classifier. Classification results were visualized in binary coagulation maps. Superimposing the coagulation maps to conventional B-mode images can advance the visualization of the coagulated zone. For each coagulation map, sensitivity and specificity were determined as a quality measure of the classification results. Classification of datasets acquired immediately after RF ablation yielded sensitivities and specificities of 0.91±0.15 and 0.85±0.09, respectively. Classification of datasets acquired after cooling down the samples yielded sensitivities and specificities of 0.90±0.11 and 0.79±0.12, respectively.

Sonographic Classification of Thermally Coagulated Tissue

Thermal ablation is well accepted for the treatment of tumors in cases where established therapeutic methods such as surgical resection are either inapplicable or ineffective. However, at present there is a lack of suitable imaging modalities for accurate online monitoring of the coagulation process. The aim of this work is to combine various tissue characterizing ultrasonic parameters in a classification system to differentiate between coagulated and noncoagulated tissue. As a measure of selectivity of each parameter, the area under the receiver operating characteristic (ROC) curve was estimated. The best parameter combination is processed by a classification system using linear classifiers. Classification results are presented in binary coagulation maps.

Ultrasound Based Methods for Monitoring of Thermal Therapies

Thermal therapy has been established as a non- or minimally invasive approach for the treatment of tumors alternative to surgical resection. Techniques used for thermal treatment include RF, laser, therapeutic ultrasound and microwaves. During a thermal therapy tissue is heated locally up to above 60° C. Cancerous tissue can thus be destroyed by coagulation.

Investigation of cardiac and cardio-therapeutical phenomena using a pulsatile circulatory model / Untersuchung kardiologischer und kardiotherapeutischer Phänomene anhand eines pulsatilen Herz/Kreislauf-Modells

Abstract For many years computer models of the cardiovascular system have existed with which the complex physiological interrelations of regulation processes may be studied. However, one major disadvantage of these models is the simplified modeling of the heart kinematics whereby the short term regulation effects of the heart itself on varying physical strains may only be simulated rudimentarily. The developed pulsatile model emphasizes the detailed description of the systolic wall tension progression subject to the Hill/Frank-Starling mechanisms and the simulation of the cardiac valves. Moreover, to be able to describe long term effects the pulsatile model was coupled to the established cardio vascular model of Guyton. Using the coupled system the influences of impaired valve function and ventricular relaxation disorders on the cardiac output as well as the need for an implementation of a physical strain dependent av-delay in existing pacemakertypes were investigated. Both the dynamics and the steadystate values of a subset of variables (pressure, volume, flow) were successfully validated by comparing them to published experimental and clinical recordings. Seit mehreren Jahren existieren Computermodelle des Herzkreislaufsystems, mit denen die komplexen physiologischen Zusammenhänge der Kreislaufregulation untersucht werden können. Ein Nachteil dieser Modelle ist jedoch die stark vereinfachte Modellierung der Herzkinematik, wodurch der Einfluss kurzfristiger Regelmechanismen des Herzens auf variierende körperliche Belastungen nur ansatzweise simuliert werden kann. In dem entwickelten pulsatilen Modell wurde neben einer detaillierten Beschreibung der systolischen Wandspannungsentwicklung in Abhängigkeit des Hill- und Frank-Starling-Mechanismus ein Schwerpunkt auf die Nachbildung der Herzklappen gelegt. Um auch langfristige Regelmechanismen beschreiben zu können, wurde das pulsatile Modell anschließend mit dem etablierten Kreislaufmodell von Guyton gekoppelt. Dadurch konnten die Auswirkungen einer Herzklappenfehlfunktion und einer ventrikulären Relaxationsstörung auf die Auswurfleistung des Herzens, als auch die Notwendigkeit der Implementierung einer belastungsabhängigen AVÜberleitungszeit in bestehende Herzschrittmachersysteme untersucht werden. Sowohl die Dynamik als auch die stationären Werte ausgewählter Variablen (Druck, Volumen, Fluss) wurden mithilfe veröffentlichter Messdaten erfolgreich validiert.

FE-MODELL DER RF-THERMOABLATION BEI LEBERTUMOREN ZUR UNTERSUCHUNG PERFUSIONSBEDINGTER APPLIKATIONSSTÖRUNGEN

S l ! \ l M k . rhermal ablation using high frequency i i i n v n i Kvnme an important treatment method forpatients w i t h h \cr tuinors vvhich are not suitable for the employment of chemothcrapy or resection. Thereby an alternating ok-ctrual tiekl hetween an active electrode placed inside he tumor and a neutral electrode placed on the patients Kk*k heats up the tnnior tissue. Temperatiires above 54 °C e.ui.sc cell necrosis through protein coagulation. Due to the stiong hver pertiision, large vessels have the capacity to u u »l down adjacent tissue areas. The shape of the produced /one of necrosis is therefore dependent on the location of the so-cülled thermal signiiicant vessels. To investigate the physienl effccts of thermal ablation and the effects of large vcssel cooling a finite element niodel was developed.

Ein pulsatiles Herz/Kreislauf-Modell für die Herzschrittmachertechnik (A Pulsatile Circulatory Model for the Pacemaker Technology)

Das unter Matlab/Simulink entwickelte Computermodell des Herz/Kreislaufsystems legt den Schwerpunkt auf eine exakte mathematische Beschreibung der Herzkinematik, um pulsatile Effekte und damit die Blutkreislaufdynamik zu reproduzieren. Dadurch und weiterhin durch die Kopplung des pulsatilen Modells mit dem validierten Modell der Kreislaufregulation von Guyton können kardiologische und kardiotherapeutische Phänomene untersucht werden. Die Simulationsergebnisse wurden mit publizierten Referenzwerten verglichen und Übereinstimmungen sowohl im Wertebereich als auch in der Dynamik der Druck-, Fluss-, und Volumenkennlinien nachgewiesen. Darüber hinaus wurden das Systemverhalten bei Schrittmacherstimulation, die Auswirkungen von Relaxationsstörungen des Herzmuskels, sowie verschiedene physiologische Steuergrößen für frequenzadaptive Herzschrittmachersysteme evaluiert.