Nenad Mihajlovic

Analysis of friction-induced limit cycling in an experimental drill-string system

In this paper, we aim for an improved understanding of the causes for torsional vibrations that appear in rotary drilling systems used for the exploration of oil and gas. For this purpose, an experimental drill-string setup is considered. In that system, torsional vibrations with and without stick-slip are observed in steady state. In order to obtain a predictive model, a discontinuous static friction model is proposed. The steady-state behavior of the drill-string system is analyzed both numerically and experimentally. A comparison of numerical and experimental bifurcation diagrams indicates the predictive quality of the model. Moreover, specific friction model characteristics can be linked to the existence of torsional vibrations with and without stick-slip.

High-resolution resonant and nonresonant fiber-scanning confocal microscope

We present a novel, hand-held microscope probe for acquiring confocal images of biological tissue. This probe generates images by scanning a fiber-lens combination with a miniature electromagnetic actuator, which allows it to be operated in resonant and nonresonant scanning modes. In the resonant scanning mode, a circular field of view with a diameter of 190 μm and an angular frequency of 127 Hz can be achieved. In the nonresonant scanning mode, a maximum field of view with a width of 69 μm can be achieved. The measured transverse and axial resolutions are 0.60 and 7.4 μm, respectively. Images of biological tissue acquired in the resonant mode are presented, which demonstrate its potential for real-time tissue differentiation. With an outer diameter of 3 mm, the microscope probe could be utilized to visualize cellular microstructures in vivo across a broad range of minimally-invasive procedures.

Near-Field Ultrasound Imaging During Radiofrequency Catheter Ablation: Tissue Thickness and Epicardial Wall Visualization and Assessment of Radiofrequency Ablation Lesion Formation and Depth

Background Safe and successful radiofrequency catheter ablation depends on creation of transmural lesions without collateral injury to contiguous structures. Near-field ultrasound (NFUS) imaging through transducers in the tip of an ablation catheter may provide important information about catheter contact, wall thickness, and ablation lesion formation. Methods and Results NFUS imaging was performed using a specially designed open-irrigated radiofrequency ablation catheter incorporating 4 ultrasound transducers. Tissue/phantom thickness was measured in vitro with varying contact angles. In vivo testing was performed in 19 dogs with NFUS catheters positioned in 4 chambers. Wall thickness measurements were made at 222 sites (excluding the left ventricle) and compared with measurements from intracardiac echocardiography. Imaging was used to identify the epicardium with saline infusion into the pericardial space at 39 sites. In vitro, the measured exceeded actual tissue/phantom thickness by 13% to 20%. In vivo, NFUS reliably visualized electrode-tissue contact, but sensitivity of epicardial imaging was 92%. The chamber wall thickness measured by NFUS correlated well with intracardiac echocardiography (r=0.86; P<0.0001). Sensitivity of lesion identification by NFUS was 94% for atrial and 95% for ventricular ablations. NFUS was the best parameter to predict lesion depth in right and left ventricle (r=0.47; P<0.0001; multiple regression P=0.0025). Lesion transmurality was correctly identified in 87% of atrial lesions. Conclusions NFUS catheter imaging reliably assesses electrode-tissue contact and wall thickness. Its use during radiofrequency catheter ablation may allow the operator to assess the depth of ablation required for transmural lesion formation to optimize power delivery.

Gabor-based needle detection and tracking in three-dimensional ultrasound data volumes

During needle interventions for e.g. regional anaesthesia or biopsy, it is very important to visualize the needle and its tip with respect to important structures in the body. In this work, we propose a novel image-based needle detection technique in a 3D ultrasound volume dataset, which can improve the intervention. We present a novel application of the 3D Gabor transformation, which exploits needle-like structures with appropriate designs. Furthermore, we introduce a needle tracking algorithm based on Gradient Descent and show that it limits the computational complexity and detection error. Finally, we visualize the needle on 2D cross-sections of the volume in order to be presented to the physician. Evaluation of our system in challenging cases shows a high detection score (up to 100% but needs larger sets) and accurate visualization.

Observer design for an experimental rotor system with discontinuous friction

In this paper we present an experimental observer design for a rotor system with friction. The model of the system exhibits set-valued static friction law with the Stribeck effect. As the model of the setup is non-smooth and non-Lipschitz, observer has to be designed using mathematical tools from convex analysis and the theory of differential inclusions. The designed observer guarantees that there exists a unique solution to the observer dynamics and that the estimated state converges to the true state of the system. Simulation and experimental results illustrate the design and performance of the observer in practice

Frequency Tuning of Collapse-Mode Capacitive Micromachined Ultrasonic Transducer

HighlightsFrequency tuning of CMUT operated in (deep‐)collapse mode is experimentally quantified.The CMUT is operated at bias voltages up to three times higher than the collapse voltage.Fabricated CMUT operates reliably for single‐use imaging catheter application.Images are constructed based on combinations of driving frequency and bias voltage.Reciprocity measurements indicate the same bias voltage can be used for transmit and receive. &NA; The information in an ultrasound image depends on the frequency that is used. In a clinical examination it may therefore be beneficial to generate ultrasound images acquired at multiple frequencies, which is difficult to achieve with conventional transducers. Capacitive micromachined ultrasonic transducers (CMUTs) offer a frequency response that is tunable by the bias voltage. In this study we investigate this frequency tunability for ultrasonic imaging. We characterized a CMUT array operated at bias voltages up to three times higher than the collapse‐voltage. All elements of the array were connected to a single transmit and receive channel through a bias circuit. We quantified the transmit‐receive and transmit sensitivity as a function of frequency for a range of bias voltages. Impulse response measurements show that the center frequency is modifiable between 8.7 MHz and 15.3 MHz with an applied bias voltage of −50 V to −170 V. The maximum transmit sensitivity is 52 kPa/V at a center frequency of 9.0 MHz with an applied bias voltage of −105 V. The −3 dB transmit range in center frequency accessible with the variable bias voltage is 6.7–15.5 MHz. This study shows that a collapse‐mode CMUT can operate efficiently at multiple center frequencies when the driving pulse and the bias voltage are optimized. We demonstrate the usefulness of frequency tuning by comparing images at different optimal combinations of driving frequency and bias voltage, acquired by linearly moving the transducer across a tissue mimicking phantom.

Medical Instrument Detection in 3-Dimensional Ultrasound Data Volumes

Ultrasound-guided medical interventions are broadly applied in diagnostics and therapy, e.g., regional anesthesia or ablation. A guided intervention using 2-D ultrasound is challenging due to the poor instrument visibility, limited field of view, and the multi-fold coordination of the medical instrument and ultrasound plane. Recent 3-D ultrasound transducers can improve the quality of the image-guided intervention if an automated detection of the needle is used. In this paper, we present a novel method for detecting medical instruments in 3-D ultrasound data that is solely based on image processing techniques and validated on various ex vivo and in vivo data sets. In the proposed procedure, the physician is placing the 3-D transducer at the desired position, and the image processing will automatically detect the best instrument view, so that the physician can entirely focus on the intervention. Our method is based on the classification of instrument voxels using volumetric structure directions and robust approximation of the primary tool axis. A novel normalization method is proposed for the shape and intensity consistency of instruments to improve the detection. Moreover, a novel 3-D Gabor wavelet transformation is introduced and optimally designed for revealing the instrument voxels in the volume, while remaining generic to several medical instruments and transducer types. Experiments on diverse data sets, including in vivo data from patients, show that for a given transducer and an instrument type, high detection accuracies are achieved with position errors smaller than the instrument diameter in the 0.5–1.5-mm range on average.

Improving needle tip identification during ultrasound-guided procedures in anaesthetic practice

Ultrasound guidance is becoming standard practice for needle‐based interventions in anaesthetic practice, such as vascular access and peripheral nerve blocks. However, difficulties in aligning the needle and the transducer can lead to incorrect identification of the needle tip, possibly damaging structures not visible on the ultrasound screen. Additional techniques specifically developed to aid alignment of needle and probe or identification of the needle tip are now available. In this scoping review, advantages and limitations of the following categories of those solutions are presented: needle guides; alterations to needle or needle tip; three‐ and four‐dimensional ultrasound; magnetism, electromagnetic or GPS systems; optical tracking; augmented (virtual) reality; robotic assistance; and automated (computerised) needle detection. Most evidence originates from phantom studies, case reports and series, with few randomised clinical trials. Improved first‐pass success and reduced performance time are the most frequently cited benefits, whereas the need for additional and often expensive hardware is the greatest limitation to widespread adoption. Novice ultrasound users seem to benefit most and great potential lies in education. Future research should focus on reporting relevant clinical parameters to learn which technique will benefit patients most in terms of success and safety.

Automated in-plane visualization of steep needles from 3D ultrasound data volumes

During ultrasound-guided needle interventions, low signal-to-noise ratio and poor needle visibility limit the performance of automated detection systems. This becomes even more challenging when the needle is inserted at higher angles with respect to the ultrasound probe. For very large insertion angles, the needle becomes virtually invisible in the ultrasound data and medical specialists need to find the needle indirectly either from out-of-plane or in-plane views. In this paper, we propose a novel method to automatically detect steep needles in 3D ultrasound data and visualize its 2D in-plane view to the medical specialist. Our method exploits indirect information regarding the presence of a needle in the volume by examining the shadow traces of structures. The proposed algorithm successfully detects the needle plane with high accuracy for all the ten measured datasets. Furthermore, the full-length needle and its tip are always visible in the extracted scan planes. The proposed method is efficient and robust to noise and artifacts, thereby strongly supporting the clinical intervention and eliminating the need for external tracking devices.

Benchmarking of state-of-the-art needle detection algorithms in 3D ultrasound data volumes

Ultrasound-guided needle interventions are widely practiced in medical diagnostics and therapy, i.e. for biopsy guidance, regional anesthesia or for brachytherapy. Needle guidance using 2D ultrasound can be very challenging due to the poor needle visibility and the limited field of view. Since 3D ultrasound transducers are becoming more widely used, needle guidance can be improved and simplified with appropriate computer-aided analyses. In this paper, we compare two state-of-the-art 3D needle detection techniques: a technique based on line filtering from literature and a system employing Gabor transformation. Both algorithms utilize supervised classification to pre-select candidate needle voxels in the volume and then fit a model of the needle on the selected voxels. The major differences between the two approaches are in extracting the feature vectors for classification and selecting the criterion for fitting. We evaluate the performance of the two techniques using manually-annotated ground truth in several ex-vivo situations of different complexities, containing three different needle types with various insertion angles. This extensive evaluation provides better understanding on the limitations and advantages of each technique under different acquisition conditions, which is leading to the development of improved techniques for more reliable and accurate localization. Benchmarking results that the Gabor features are better capable of distinguishing the needle voxels in all datasets. Moreover, it is shown that the complete processing chain of the Gabor-based method outperforms the line filtering in accuracy and stability of the detection results.