Randy E. Ellis

Medical Image Computing and Computer-Assisted Intervention - MICCAI 2003

Although there have been significant advances in the development of virtual reality based surgical simulations, there still remain fundamental questions concerning the fidelity required for effective surgical training. A dual station experimental platform was built for the purpose of investigating these fidelity requirements. Analogous laparoscopic surgical tasks were implemented in a virtual and a real station, with the virtual station modeling the real environment to various degrees of fidelity. After measuring subjects’ initial performance in the real station, different groups of subjects were trained on the virtual station under a variety of conditions and tested finally at the real station. Experiments involved bimanual pushing and cutting tasks on a nonlinear elastic object. The results showed that force feedback results in a significantly improved training transfer compared to training without force feedback. The training effectiveness of a linear approximation model was comparable to the effectiveness of a more accurate nonlinear model.

Patellofemoral Joint Kinematics in Individuals with and without Patellofemoral Pain Syndrome

BACKGROUND Patellofemoral pain syndrome is a prevalent condition in young people. While it is widely believed that abnormal patellar tracking plays a role in the development of patellofemoral pain syndrome, this link has not been established. The purpose of this cross-sectional case-control study was to test the hypothesis that patterns of patellar spin, tilt, and lateral translation make it possible to distinguish individuals with patellofemoral pain syndrome and clinical evidence of patellar malalignment from those with patellofemoral pain syndrome and no clinical evidence of malalignment and from individuals with no knee problems. METHODS Three-dimensional patellofemoral joint kinematics in one knee of each of sixty volunteers (twenty in each group described above) were assessed with use of a new, validated magnetic resonance imaging-based method. Static low-resolution scans of the loaded knee were acquired at five different angles of knee flexion (ranging between -4 degrees and 60 degrees). High-resolution geometric models of the patella, femur, and tibia and associated coordinate axes were registered to the bone positions on the low-resolution scans to determine the patellar motion as a function of knee flexion angle. Hierarchical modeling was used to identify group differences in patterns of patellar spin, tilt, and lateral translation. RESULTS No differences in the overall pattern of patellar motion were observed among groups (p>0.08 for all global maximum likelihood ratio tests). Features of patellar spin and tilt patterns varied greatly between subjects across all three groups, and no significant group differences were detected. At 19 degrees of knee flexion, the patellae in the group with patellofemoral pain and clinical evidence of malalignment were positioned an average of 2.25 mm more laterally than the patellae in the control group, and this difference was marginally significant (p=0.049). Other features of the pattern of lateral translation did not differ, and large overlaps in values were observed across all groups. CONCLUSIONS It cannot be determined from our cross-sectional study whether the more lateral position of the patella in the group with clinical evidence of malalignment preceded or followed the onset of symptoms. It is clear from the data that an individual with patellofemoral pain syndrome cannot be distinguished from a control subject by examining patterns of spin, tilt, or lateral translation of the patella, even when clinical evidence of mechanical abnormality was observed.

A surgical planning and guidance system for high tibial osteotomy

Objective: To develop a three-dimensional pre-surgical planner and an intraoperative guidance system for high tibial osteotomy. The parameters that describe the placement and orientation of the osteotomy resection planes were to be transmitted to an accompanying guidance system that allowed the surgeon to reproducibly perform the planned procedureMaterials and Methods: The planning system and guidance system were coded using OpenGL on UNIX workstations. In vitro tests were performed to compare the reproducibility of the computer-enhanced technique to that of the traditional technique, and an in vivo pilot study was initiatedResults: In vim, the computer-enhanced technique produced a significant reduction, by one half, in both the maximum error of correction and the standard deviation of the correction error. Preliminary in vivo results on six patients suggest that similar error diminution will occur during regular clinical application of the techniqueConclusions: Both studies showed that the computer syst...

Design and evaluation of a high-performance haptic interface

A haptic interface is a computer-controlled mechanism designed to detect motion of a human operator without impeding that motion, and to feed back forces from a teleoperated robot or virtual environment. Design of such a device is not trivial, because of the many conflicting constraints the designer must face. As part of our research into haptics, we have developed a prototype planar mechanism. It has low apparent mass and damping, high structural stiffness, high force bandwidth, high force dynamic range, and an absence of mechanical singularities within its workspace. We present an analysis of the human-operator and mechanical constraints that apply to any such device, and propose methods for the evaluation of haptic interfaces. Our evaluation criteria are derived from the original task analysis, and are a first step towards a replicable methodology for comparing the performance of different devices.

A Real-Time Freehand Ultrasound Calibration System with Automatic Accuracy Feedback and Control

This article describes a fully automatic, real-time, freehand ultrasound calibration system. The system was designed to be simple and sterilizable, intended for operating-room usage. The calibration system employed an automatic-error-retrieval and accuracy-control mechanism based on a set of ground-truth data. Extensive validations were conducted on a data set of 10,000 images in 50 independent calibration trials to thoroughly investigate the accuracy, robustness, and performance of the calibration system. On average, the calibration accuracy (measured in three-dimensional reconstruction error against a known ground truth) of all 50 trials was 0.66 mm. In addition, the calibration errors converged to submillimeter in 98% of all trials within 12.5 s on average. Overall, the calibration system was able to consistently, efficiently and robustly achieve high calibration accuracy with real-time performance.

Robust registration for computer-integrated orthopedic surgery: Laboratory validation and clinical experience

In order to provide navigational guidance during computer-integrated orthopedic surgery, the anatomy of the patient must first be registered to a medical image or model. A common registration approach is to digitize points from the surface of a bone and then find the rigid transformation that best matches the points to the model by constrained optimization. Many optimization criteria, including a least-squares objective function, perform poorly if the data include spurious data points (outliers). This paper describes a statistically robust, surface-based registration algorithm that we have developed for orthopedic surgery. To find an initial estimate, the user digitizes points from predefined regions of bone that are large enough to reliably locate even in the absence of anatomic landmarks. Outliers are automatically detected and managed by integrating a statistically robust M-estimator with the iterative-closest-point algorithm. Our in vitro validation method simulated the registration process by drawing registration data points from several sets of densely digitized surface points. The method has been used clinically in computer-integrated surgery for high tibial osteotomy, distal radius osteotomy, and excision of osteoid osteoma.

Computer-Assisted Hip Resurfacing Using Individualized Drill Templates

The goal of this study was to investigate whether individualized templates can provide an accurate and reliable computer-assisted system for femoral component placement during hip resurfacing. A consecutive series of 45 patients were examined. Using a 3-dimensional computer model of the femur, the drill trajectory for the central pin of the stem was planned. A surface-matched plastic drilling template was created using a rapid prototyping machine. This patient-specific drill guide was intraoperatively positioned on the patient anatomy, the central pin was drilled into the femoral neck, and the accuracy of the placement with respect to the planned central pin alignment was measured. With mean deviation between planned and actual central pin alignment of 1.14 degrees in varus and 4.49 degrees in retroversion, individualized templates were as accurate as conventional computer-assisted hip resurfacing.

Validation of bone segmentation and improved 3-D registration using contour coherency in CT data

A method is presented to validate the segmentation of computed tomography (CT) image sequences, and improve the accuracy and efficiency of the subsequent registration of the three-dimensional surfaces that are reconstructed from the segmented slices. The method compares the shapes of contours extracted from neighborhoods of slices in CT stacks of tibias. The bone is first segmented by an automatic segmentation technique, and the bone contour for each slice is parameterized as a one-dimensional function of normalized arc length versus inscribed angle. These functions are represented as vectors within a K-dimensional space comprising the first K amplitude coefficients of their Fourier Descriptors. The similarity or coherency of neighboring contours is measured by comparing statistical properties of their vector representations within this space. Experimentation has demonstrated this technique to be very effective at identifying low-coherency segmentations. Compared with experienced human operators, in a set of 23 CT stacks (1,633 slices), the method correctly detected 87.5% and 80% of the low-coherency and 97.7% and 95.5% of the high coherency segmentations, respectively from two different automatic segmentation techniques. Removal of the automatically detected low-coherency segmentations also significantly improved the accuracy and time efficiency of the registration of 3-D bone surface models. The registration error was reduced by over 500% (i.e., a factor of 5) and 280%, and the computational performance was improved by 540% and 791% for the two respective segmentation methods.

Numerical Methods for the Force Reflection of Contact

An important problem in the field of force-reflecting systems and telerobotics is poor rendering of contact, particularly of contact with stiff surfaces. There are numerous possible sources of poor performance, including poor contact models, sampling errors, and delays due to computation or data transmission. In this paper we examine effects due to sample-and-hold, which is a fundamental property of both the discrete domain and also of the sensors and power amplifiers used in a force-reflecting system. We propose sample-and-hold be generalized to sample-estimate-hold. We show why ordinary sample-and-hold generates an active contact interface, and provide ways of improving the feeling of the interface. We have developed a suite of numerical methods for improving the performance of rendering of surfaces by force reflection. We have conducted both simulations and experiments to demonstrate the efficacy of the proposed scheme. Our contributions are a new method of digitally processing force data, and a systematic method for coupling force-processing systems that run at different rates.

Enhancing Depth Perception in Translucent Volumes

We present empirical studies that consider the effects of stereopsis and simulated aerial perspective on depth perception in translucent volumes. We consider a purely absorptive lighting model, in which light is not scattered or reflected, but is simply absorbed as it passes through the volume. A purely absorptive lighting model is used, for example, when rendering digitally reconstructed radiographs (DRRs), which are synthetic X-ray images reconstructed from CT volumes. Surgeons make use of DRRs in planning and performing operations, so an improvement of depth perception in DRRs may help diagnosis and surgical planning