Matías de la Fuente

Statistical validation metric for accuracy assessment in medical image segmentation

AbstractObjective Validation of medical image segmentation algorithms is an open question, considering variance of individual pathologies and the related clinical requirements for accuracy. In this paper, we propose a validation metric capable to distinguish between an over and under-segmentation and account for different clinical applications. Materials and methods In this paper, we propose a validation metric representing a tradeoff between sensitivity and specificity. The metric has an advantage of differentiating between an over or under-segmentation which is an important feature for validating large sets of segmentation results, as human inspection is exhausting and time consuming. Although it is oriented to the accuracy measurement it is also closely related to the robustness of a method. Results Features of the metrics are analyzed alongside their medical impact. A set of numerical simulations is performed in order to compare the proposed metric with standardly used discrepancy measures. The metric is illustrated with a clinical case study, presenting accuracy assessment of an algorithm for calvarial tumor segmentation, validated on six patients.

Assessment of optical localizer accuracy for computer aided surgery systems

The technology for localization of surgical tools with respect to the patients reference coordinate system in three to six degrees of freedom is one of the key components in computer aided surgery. Several tracking methods are available, of which optical tracking is the most widespread in clinical use. Optical tracking technology has proven to be a reliable method for intra-operative position and orientation acquisition in many clinical applications; however, the accuracy of such localizers is still a topic of discussion. In this paper, the accuracy of three optical localizer systems, the NDI Polaris P4, the NDI Polaris Spectra (in active and passive mode) and the Stryker Navigation System II camera, is assessed and compared critically. Static tests revealed that only the Polaris P4 shows significant warm-up behavior, with a significant shift of accuracy being observed within 42 minutes of being switched on. Furthermore, the intrinsic localizer accuracy was determined for single markers as well as for tools using a volumetric measurement protocol on a coordinate measurement machine. To determine the relative distance error within the measurement volume, the Length Measurement Error (LME) was determined at 35 test lengths. As accuracy depends strongly on the marker configuration employed, the error to be expected in typical clinical setups was estimated in a simulation for different tool configurations. The two active localizer systems, the Stryker Navigation System II camera and the Polaris Spectra (active mode), showed the best results, with trueness values (mean ± standard deviation) of 0.058 ± 0.033 mm and 0.089 ± 0.061 mm, respectively. The Polaris Spectra (passive mode) showed a trueness of 0.170 ± 0.090 mm, and the Polaris P4 showed the lowest trueness at 0.272 ± 0.394 mm with a higher number of outliers than for the other cameras. The simulation of the different tool configurations in a typical clinical setup revealed that the tracking error can be estimated to be 1.02 mm for the Polaris P4, 0.64 mm for the Polaris Spectra in passive mode, 0.33 mm for the Polaris Spectra in active mode, and 0.22 mm for the Stryker Navigation System II camera.

Fluoroscopic navigation system for hip surface replacement

Metal-on-metal hip resurfacing arthroplasties represent an alternative to total hip arthroplasties for young and active patients, enabling the preservation of intact femoral bone and therefore improving the prognosis for future hip joint replacements. Follow-up studies have shown that the main reasons for early implant failure are mal-orientation of the implant stem in relation to the femoral neck axis, and notching of the femoral neck during femoral head preparation, as well as by exposed cancellous bone after implantation. A computer-assisted planning and navigation system for the implantation of femoral hip resurfacing implants has been developed which supports the surgeon during intraoperative fluoroscopy-based planning and navigation of implant positioning. This paper presents the results of a cadaver study performed to evaluate the systems functionality and accuracy.

Surface-based determination of the pelvic coordinate system

In total hip replacement (THR) one technical factor influencing the risk of dislocation is cup orientation. Computer-assisted surgery systems allow for cup navigation in anatomy-based reference frames. The pelvic coordinate system most used for cup navigation in THR is based on the mid-sagittal plane (MSP) and the anterior pelvic plane (APP). From a geometrical point of view, the MSP can be considered as a mirror plane, whereas the APP can be considered as a tangent plane comprising the anterior superior iliac spines (ASIS) and the pubic tubercles. In most systems relying on the pelvic coordinate system, the most anterior points of the ASIS and the pubic tubercles are selected manually. As manual selection of landmark points is a tedious, time-consuming and error-prone task, a surface-based approach for combined MSP and APP computation is presented in this paper: Homologous points defining the MSP and the landmark points defining the APP are selected automatically from surface patches. It is investigated how MSP computation can benefit from APP computation and vice versa, and clinical perspectives of combined MSP and APP computation are discussed. Experimental results on computed tomography data show that the surface-based approach can improve accuracy.

A-mode ultrasound-based intra-femoral bone cement detection and 3D reconstruction in RTHR

Due to the difficulty of determining the 3D boundary of the cement-bone interface in Revision Total Hip Replacement (RTHR), the removal of the distal intra-femoral bone cement can be a time-consuming and risky operation. Within the framework of computer- and robot-assisted cement removal, the principles and first results of an automatic detection and 3D surface reconstruction of the cement-bone boundary using A-mode ultrasound are described. Sound propagation time and attenuation of cement were determined considering different techniques for the preparation of bone cement, such as the use of a vacuum system (Optivac®, Biomet). A laboratory setup using a rotating, standard 5-MHz transducer was developed. The prototype enables scanning of bisected cement-prepared femur samples in a 90° rotation range along their rotation axis. For system evaluation ex vivo, the distal femur of a human cadaver was prepared with bone cement and drilled (Ø 10 mm) to simulate the prosthesis cavity in a first approximation. The sample was cut in half and CT scanned (0.24 mm resolution; 0.5 mm distance; 0.5 mm thickness), and 3D voxel models of the manually segmented bone cement were reconstructed, providing the ground truth. Afterwards, 90° segments of each ex-vivo sample were scanned by the A-mode ultrasound system. To obtain better ultrasound penetration, we used coded signal excitation and pulse compression filtering. A-mode ultrasound signal detection, filtering and segmentation were accomplished fully automatically. Subsequently, 3D voxel models of each sample were calculated. Accuracy evaluation of the measured ultrasound data was performed by ICP matching of each ultrasound dataset (∼8000 points) to the corresponding CT dataset and calculation of the residual median distance error between the corresponding datasets. Prior to each ICP matching, an initial pre-registration was calculated using prominent landmarks in the corresponding datasets. This method yielded a median distance error in the region of 0.25 mm for the cement-bone interface in both femur halves.

In-vitro cell treatment with focused shockwaves—influence of the experimental setup on the sound field and biological reaction

BackgroundTo improve understanding of shockwave therapy mechanisms, in vitro experiments are conducted and the correlation between cell reaction and shockwave parameters like the maximum pressure or energy density is studied. If the shockwave is not measured in the experimental setup used, it is usually assumed that the device’s shockwave parameters (=manufacturer’s free field measurements) are valid. But this applies only for in vitro setups which do not modify the shockwave, e.g., by reflection or refraction. We hypothesize that most setups used for in vitro shockwave experiments described in the literature influence the sound field significantly so that correlations between the physical parameters and the biological reaction are not valid.MethodsTo reveal the components of common shockwave in vitro setups which mainly influence the sound field, 32 publications with 37 setups used for focused shockwave experiments were reviewed and evaluated regarding cavitation, cell container material, focal sound field size relative to cell model size, and distance between treated cells and air. For further evaluation of the severity of those influences, experiments and calculations were conducted.ResultsIn 37 setups, 17 different combinations of coupling, cell container, and cell model are described. The setup used mainly is a transducer coupled via water to a tube filled with a cell suspension. As changes of the shockwaves’ maximum pressure of 11 % can already induce changes of the biological reaction, the sound field and biological reactions are mainly disturbed by use of standard cell containers, use of coupling gel, air within the 5 MPa focal zone, and cell model sizes which are bigger than half the −6 dB focal dimensions.ConclusionsUntil now, correct and sufficient information about the shockwave influencing cells in vitro is only provided in 1 of 32 publications. Based on these findings, guidelines for improved in vitro setups are proposed which help minimize the influence of the setup on the sound field.

Automatic extraction of the mid-sagittal plane using an ICP variant

Precise knowledge of the mid-sagittal plane is important for the assessment and correction of several deformities. Furthermore, the mid-sagittal plane can be used for the definition of standardized coordinate systems such as pelvis or skull coordinate systems. A popular approach for mid-sagittal plane computation is based on the selection of anatomical landmarks located either directly on the plane or symmetrically to it. However, the manual selection of landmarks is a tedious, time-consuming and error-prone task, which requires great care. In order to overcome this drawback, previously it was suggested to use the iterative closest point (ICP) algorithm: After an initial mirroring of the data points on a default mirror plane, the mirrored data points should be registered iteratively to the model points using rigid transforms. Finally, a reflection transform approximating the cumulative transform could be extracted. In this work, we present an ICP variant for the iterative optimization of the reflection parameters. It is based on a closed-form solution to the least-squares problem of matching data points to model points using a reflection. In experiments on CT pelvis and skull datasets our method showed a better ability to match homologous areas.

Fluoroskopische Navigation versus konventionelle manuelle Positionierung der Femurkomponente beim Oberflächenersatz der Hüfte: erste experimentelle Erprobung / Fluoroscopic navigation versus conventional manual positioning of the femoral component for hip resurfacing: first experimental trial

Zusammenfassung Hintergrund: Zu den wesentlichen Verbesserungen moderner Oberflächenersatzprothesen des Hüftgelenkes gehört neben der Einführung der Metall-Metall-Gleitpaarung die Integration eines Verfahrens zur exakten und reproduzierbaren Positionierung der femoralen Komponente durch prothesenspezifische Zielinstrumentarien. Dennoch gelten vorwiegend operationstechnische Gründe, insbesondere die Schädigung der Femurhalskortikalis („Notching des Schenkelhalses“) wie auch eine ungenaue Prothesenpositionierung während des Eingriffes, als ursächlich für ein Versagen der femoralen Komponente. Material und Methode: Im Rahmen einer In-vitro-Untersuchung wurden jeweils 6 DUROM™-Hip-Oberflächenersatzprothesen mit dem prothesenspezifischen Zielinstrumentarium wie auch computergestützt fluoroskopisch navigiert in künstliche Femora implantiert. Ziel der Untersuchungen war die Evaluation der Funktionalität und der Genauigkeit eines neuartigen computergestützten fluoroskopischen Planungs- und Navigationssystems auf Grundlage von Planungs- und Navigationsmodulen wie auch der Vergleich mit dem prothesenspezifischen Zielinstrumentarium. Ergebnisse: Der durchschnittliche Winkelunterschied zwischen geplantem und erreichtem Prothesenwinkel betrug 0,2±1,2° für die fluoroskopische Navigation vs. 6,5±4,1° für das prothesenspezifische Zielinstrumentarium, die durchschnittliche Abweichung zwischen geplantem und erreichtem anterioren Offset 1,2±1,2 mm vs. -0,83±4,1 mm. Der Zeitaufwand für die insgesamt 5 Planungs- und Navigationsschritte betrug im Durchschnitt 17±1,2 min vs. 14±0,8 min für das prothesenspezifische Zielinstrumentarium. Der durchschnittliche Abstand zwischen geplanter und erreichter Prothesenposition konnte mit 1,9±0,6 mm vs. 5,3±2,4 mm bemessen werden. Weder fluoroskopisch noch konventionell trat ein „Notching des Schenkelhalses“ auf. Schlussfolgerung: Das computergestützte fluoroskopische Planungs- und Navigationssystem zeigte im Rahmen der hier vorgestellten In-vitro-Studie vielversprechende erste Ergebnisse. So erlaubt es eine praktikable Planung und eine hohe Präzision bei der Umsetzung dieser Planung. Dennoch muss sich das System – ähnlich der hier vorgestellten In-vitro-Untersuchung – in weiteren Studien wie im klinischen Alltag noch nachweisbar bewähren und in den klinischen Workflow integriert werden. Weiterführende Untersuchungen sollen unter zusätzlicher Integration eines Pfannenmoduls eine navigiert-assistierte Bestimmung der „Range of Motion“ zur Verbesserung des postoperativen Bewegungsausmaßes beinhalten. Abstract Background: The most essential improvement of modern hip resurfacing arthroplasty is the metal-on-metal bearing as well as the integration of a procedure for the exact and repeatable positioning of the femoral component through a specific mechanical alignment instrument. Nevertheless, the main reasons for early implant failure are mal-positioning of the femoral component and notching of the femoral neck during femoral head preparation. Materials and methods: In the context of an in vitro study, in each case six DUROM™-Hip resurfacing prostheses were implanted in artificial femora with the prosthesis-specific mechanical alignment instrument, as well as under navigation control. The aim of the study was to evaluate the functionality and accuracy of a computer-assisted planning and navigation system on the basis of a navigation module library from Surgitaix AG (Aachen, Germany), as well as a comparison with the prosthesis-specific mechanical alignment instrument. Results: The main angulation error between planning and navigation of the stem-shaft angle was 0.2±1.2° for the navigation system and 6.5±4.1° for the mechanical alignment instrument, the main anterior offset error was 1.2±1.2 mm vs. -0.83±4.1 mm. The mean time for all five planning and navigation steps was 17±1.2 min vs. 14±0.8 min. The main distance error between planning and navigation was 1.9±0.6 mm for the navigation system, and 5.3±2.4 mm for the mechanical alignment instrument. Femoral notching was not observed for navigational or conventional positioning. Conclusion: The computer-assisted fluoroscopic planning and navigation system for hip resurfacing showed, within the scope of this in vitro study, first promising experiences. The system approves a practicable planning with a high accuracy in implementation. Nevertheless, the potential benefit has to be evaluated in further clinical studies, especially from the perspective of a possible integration of this navigation system into the clinical workflow. Further studies should consider a fluoroscopy-assisted range of motion assessment under consideration of an additional cup-module to enhance the postoperative range of motion after hip resurfacing procedures.

Real-time determination of skull thickness for a manually-navigated synergistic trepanation tool

Trepanation of the skull is a common procedure in neurosurgery with the problems of dural tears and wide cutting gaps. A hand-guided instrument containing a soft-tissue preserving saw whose cutting depth is automatically adapted on the basis of a-priori data (CT, MRI) is envisioned to reduce these problems.

Fluoroscopy-based computer-assisted navigation for implant placement and hip resurfacing arthroplasty in the proximal femur: the zero-dose C-arm navigation approach

Abstract In the proximal femur, a high accuracy of implant placement reduces the risk of mechanical failure. We have tested a new computer-assisted planning and navigation system based on two-dimensional fluoroscopy using the so-called zero-dose C-arm navigation approach to optimise implant placement in fracture fixation and hip resurfacing. The aim of this review is to compare the results of this system with the current literature. Use of the novel system enables a minimally invasive approach to the hip and results in enhanced accuracy of implant placement compared with conventional techniques. Its precision is comparable to navigation systems currently in the market. The new system reduces irradiation but requires more operation time in comparison with established navigation systems. We believe zero-dose C-arm navigation can effectively be used to support surgeons in modern orthopaedic and trauma surgery departments, and can sufficiently serve the demands of both sections, especially at a time focusing on saving costs.