A comparison of calibration methods for stereo fluoroscopic imaging systems

Stereo (biplane) fluoroscopic imaging systems are considered the most accurate and precise systems to study joint kinematics in vivo. Calibration of a biplane fluoroscopy system consists of three steps: (1) correction for spatial image distortion; (2) calculation of the focus position; and (3) calculation of the relative position and orientation of the two fluoroscopy systems with respect to each other. In this study we compared 6 methods for calibrating a biplane fluoroscopy system including a new method using a novel nested-optimization technique. To quantify bias and precision, an electronic digital caliper instrumented with two tantalum markers on radiolucent posts was imaged in three configurations, and for each configuration placed in ten static poses distributed throughout the viewing volume. Bias and precision were calculated as the mean and standard deviation of the displacement of the markers measured between the three caliper configurations. The data demonstrated that it is essential to correct for image distortion when sub-millimeter accuracy is required. We recommend calibrating a stereo fluoroscopic imaging system using an accurately machined plate and a calibration cube, which improved accuracy 2-3 times compared to the other calibration methods. Once image distortion is properly corrected, the focus position should be determined using the Direct Linear Transformation (DLT) method for its increased speed and equivalent accuracy compared to the novel nested-optimization method. The DLT method also automatically provides the 3D fluoroscopy configuration. Using the recommended calibration methodology, bias and precision of 0.09 and 0.05 mm or better can be expected for measuring inter-marker distances.     

A new type of model-based Roentgen stereophotogrammetric analysis for solving the occluded marker problem

Roentgen stereophotogrammetric analysis (RSA) measures micromotion of an orthopaedic implant with respect to its surrounding bone. A problem in RSA is that the markers are sometimes overprojected by the implant itself. This study describes the so-called Marker Configuration Model-based RSA (MCM-based RSA) that is able to measure the pose of a rigid body in situations where less than three markers could be detected in both images of an RSA radiograph. MCM-based RSA is based on fitting a Marker Configuration model (MC-model) to the projection lines from the marker projection positions in the image to their corresponding Roentgen foci. An MC-model describes the positions of markers relative to each other and is obtained using conventional RSA. We used data from 15 double examinations of a clinical study of total knee prostheses and removed projections of the three tibial component markers, simulating occlusion of markers. The migration of the tibial component with respect to the bone, which should be zero, for the double examination is a measure of the accuracy of algorithm. With the new algorithm, it is possible to estimate the pose of a rigid body of which one or two markers are occluded in one of the images of the RSA radiograph with high accuracy as long as a proper MC-model of the markers in the rigid body is available. The new algorithm makes RSA more robust for occlusion of markers. This improves the results of clinical RSA studies because the number of lost RSA follow-up moments is reduced.     

Validation of a low-dose hybrid RSA and fluoroscopy technique: Determination of accuracy, bias and precision

Analyzing skeletal kinematics with radiostereometric analysis (RSA) following corrective orthopedic surgery allows the quantitative comparison of different implant designs. The purpose of this study was to validate a technique for dynamically estimating the relative position and orientation of skeletal segments using RSA and single plane X-ray fluoroscopy. Two micrometer-based in vitro phantom models of the skeletal segments in the hip and knee joints were used. The spatial positions of tantalum markers that were implanted into each skeletal segment were reconstructed using RSA. The position and orientation of each segment were determined in fluoroscopy images by minimizing the difference between the markers measured and projected in the image plane. Accuracy was determined in terms of bias and precision by analyzing the deviation between the applied displacement protocol and measured pose estimates. Measured translational accuracy was less than 100 microm parallel to the image plane and less than 700 microm in the direction orthogonal to the image plane. The measured rotational error was less than 1 degrees . Measured translational and rotational bias was not statistically significant at the 95% level of confidence. The technique allows real-time kinematic skeletal measurements to be performed on human subjects implanted with tantalum markers for quantitatively measuring the motion of normal joints and different implant designs.     

Validation of a non-invasive fluoroscopic imaging technique for the measurement of dynamic knee joint motion

The accurate measurement of the in vivo knee joint kinematics in six degrees-of-freedom (6DOF) remains a challenge in biomedical engineering. We have adapted a dual fluoroscopic imaging system (DFIS) to investigate the various in vivo dynamic knee joint motions. This paper presents a thorough validation of the accuracy and repeatability of the DFIS system when used to measure 6DOF dynamic knee kinematics. First, the validation utilized standard geometric spheres made from different materials to demonstrate the capability of the DFIS technique to determine the object positions under changing speeds. The translational pose of the spheres could be recreated to less than 0.15±0.09 mm for velocities below 300 mm/s. Next, tantalum beads were inserted into the femur and tibia of two fresh frozen cadaver knees to compare the dynamic kinematics measured by matching knee models to the kinematics from the tantalum bead matching—a technique similar to Roentgen stereophotogrammetric analysis (RSA). Each cadaveric knee was attached to the crosshead of a tensile testing machine and vertically translated at a rate of 16.66 mm/s while images were captured with the DFIS. Subsequently, the tibia was held fixed and the femur manually flexed from full extension to 90° of flexion, as the DFIS acquired images. In vitro translation of the cadaver knee using the tensile testing machine deviated from predicted values by 0.08±0.14 mm for the matched knee models. The difference between matching the knee and tantalum bead models during the dynamic flexion–extension motion of the knee was 0.1±0.65°/s in flexion speed; 0.24±0.16 mm in posterior femoral translation; and 0.16±0.61° in internal–external tibial rotation. Finally, we applied the method to investigate the knee kinematics of a living subject during a step ascent and treadmill gait. High repeatability was demonstrated for the in vivo application. Thus, the DFIS provides an easy and powerful tool for accurately determining 6DOF positions of the knee when performing daily functional activities.     

A new model-based RSA method validated using CAD models and models from reversed engineering

Roentgen stereophotogrammetric analysis (RSA) was developed to measure micromotion of an orthopaedic implant with respect to its surrounding bone. A disadvantage of conventional RSA is that it requires the implant to be marked with tantalum beads. This disadvantage can potentially be resolved with model-based RSA, whereby a 3D model of the implant is used for matching with the actual images and the assessment of position and rotation of the implant. In this study, a model-based RSA algorithm is presented and validated in phantom experiments. To investigate the influence of the accuracy of the implant models that were used for model-based RSA, we studied both computer aided design (CAD) models as well as models obtained by means of reversed engineering (RE) of the actual implant. The results demonstrate that the RE models provide more accurate results than the CAD models. If these RE models are derived from the very same implant, it is possible to achieve a maximum standard deviation of the error in the migration calculation of 0.06 mm for translations in x- and y-direction and 0.14 mm for the out of plane z-direction, respectively. For rotations about the y-axis, the standard deviation was about 0.1 degrees and for rotations about the x- and z-axis 0.05 degrees. Studies with clinical RSA-radiographs must prove that these results can also be reached in a clinical setting, making model-based RSA a possible alternative for marker-based RSA.     

An automatic 2D-3D image matching method for reproducing spatial knee joint positions using single or dual fluoroscopic images

Fluoroscopic image technique, using either a single image or dual images, has been widely applied to measure in vivo human knee joint kinematics. However, few studies have compared the advantages of using single and dual fluoroscopic images. Furthermore, due to the size limitation of the image intensifiers, it is possible that only a portion of the knee joint could be captured by the fluoroscopy during dynamic knee joint motion. In this paper, we presented a systematic evaluation of an automatic 2D-3D image matching method in reproducing spatial knee joint positions using either single or dual fluoroscopic image techniques. The data indicated that for the femur and tibia, their spatial positions could be determined with an accuracy and precision less than 0.2?mm in translation and less than 0.4° in orientation when dual fluoroscopic images were used. Using single fluoroscopic images, the method could produce satisfactory accuracy in joint positions in the imaging plane (in average up to 0.5?mm in translation and 1.3° in rotation), but large variations along the out-plane direction (in average up to 4.0?mm in translation and 2.2° in rotation). The precision of using single fluoroscopic images to determine the actual knee positions was worse than its accuracy obtained. The data also indicated that when using dual fluoroscopic image technique, if the knee joint outlines in one image were incomplete by 80%, the algorithm could still reproduce the joint positions with high precisions.     

An automatic 2D–3D image matching method for reproducing spatial knee joint positions using single or dual fluoroscopic images

Fluoroscopic image technique, using either a single image or dual images, has been widely applied to measure in vivo human knee joint kinematics. However, few studies have compared the advantages of using single and dual fluoroscopic images. Furthermore, due to the size limitation of the image intensifiers, it is possible that only a portion of the knee joint could be captured by the fluoroscopy during dynamic knee joint motion. In this paper, we presented a systematic evaluation of an automatic 2D–3D image matching method in reproducing spatial knee joint positions using either single or dual fluoroscopic image techniques. The data indicated that for the femur and tibia, their spatial positions could be determined with an accuracy and precision less than 0.2 mm in translation and less than 0.4° in orientation when dual fluoroscopic images were used. Using single fluoroscopic images, the method could produce satisfactory accuracy in joint positions in the imaging plane (in average up to 0.5 mm in translation and 1.3° in rotation), but large variations along the out-plane direction (in average up to 4.0 mm in translation and 2.2° in rotation). The precision of using single fluoroscopic images to determine the actual knee positions was worse than its accuracy obtained. The data also indicated that when using dual fluoroscopic image technique, if the knee joint outlines in one image were incomplete by 80%, the algorithm could still reproduce the joint positions with high precisions.     

A method for measuring joint kinematics designed for accurate registration of kinematic data to models constructed from CT data

A method for measuring three-dimensional kinematics that incorporates the direct cross-registration of experimental kinematics with anatomic geometry from Computed Tomography (CT) data has been developed. Plexiglas registration blocks were attached to the bones of interest and the specimen was CT scanned. Computer models of the bone surface were developed from the CT image data. Determination of discrete kinematics was accomplished by digitizing three pre-selected contiguous surfaces of each registration block using a three-dimensional point digitization system. Cross-registration of bone surface models from the CT data was accomplished by identifying the registration block surfaces within the CT images. Kinematics measured during a biomechanical experiment were applied to the computer models of the bone surface. The overall accuracy of the method was shown to be at or below the accuracy of the digitization system used. For this experimental application, the accuracy was better than +/-0.1mm for position and 0.1 degrees for orientation for linkage digitization and better than +/-0.2mm and +/-0.2 degrees for CT digitization. Surface models of the radius and ulna were constructed from CT data, as an example application. Kinematics of the bones were measured for simulated forearm rotation. Screw-displacement axis analysis showed 0.1mm (proximal) translation of the radius (with respect to the ulna) from supination to neutral (85.2 degrees rotation) and 1.4mm (proximal) translation from neutral to pronation (65.3 degrees rotation). The motion of the radius with respect to the ulna was displayed using the surface models. This methodology is a useful tool for the measurement and application of rigid-body kinematics to computer models.     

Model-based Roentgen stereophotogrammetry of orthopaedic implants

Attaching tantalum markers to prostheses for Roentgen stereophotogrammetry (RSA) may be difficult and is sometimes even impossible. In this study, a model-based RSA method that avoids the attachment of markers to prostheses is presented and validated. This model-based RSA method uses a triangulated surface model of the implant. A projected contour of this model is calculated and this calculated model contour is matched onto the detected contour of the actual implant in the RSA radiograph. The difference between the two contours is minimized by variation of the position and orientation of the model. When a minimal difference between the contours is found, an optimal position and orientation of the model has been obtained. The method was validated by means of a phantom experiment. Three prosthesis components were used in this experiment: the femoral and tibial component of an Interax total knee prosthesis (Stryker Howmedica Osteonics Corp., Rutherfort, USA) and the femoral component of a Profix total knee prosthesis (Smith & Nephew, Memphis, USA). For the prosthesis components used in this study, the accuracy of the model-based method is lower than the accuracy of traditional RSA. For the Interax femoral and tibial components, significant dimensional tolerances were found that were probably caused by the casting process and manual polishing of the components surfaces. The largest standard deviation for any translation was 0.19mm and for any rotation it was 0.52 degrees. For the Profix femoral component that had no large dimensional tolerances, the largest standard deviation for any translation was 0.22mm and for any rotation it was 0.22 degrees. From this study we may conclude that the accuracy of the current model-based RSA method is sensitive to dimensional tolerances of the implant. Research is now being conducted to make model-based RSA less sensitive to dimensional tolerances and thereby improving its accuracy.     

In phantom evaluation of targeting accuracy in MRI-based brain radiosurgery

PurposeTo determine the targeting accuracy of brain radiosurgery when planning procedures employing different MRI and MRI + CT combinations are adopted.Materials and methodA new phantom, the BrainTool, has been designed and realized to test image co-registration and targeting accuracy in a realistic anatomical situation. The phantom was created with a 3D printer and materials that mimic realistic brain MRI and CT contrast using a model extracted from a synthetic MRI study of a human brain. Eight markers distributed within the BrainTool provide for assessment of the accuracy of image registrations while two cavities that host an ionization chamber are used to perform targeting accuracy measurements with an iterative cross-scan method. Two procedures employing 1.5 T MRI-only or a combination of MRI (taken with 1.5 T or 3 T scanners) and CT to carry out Gamma Knife treatments were investigated. As distortions can impact targeting accuracy, MR images were preliminary evaluated to assess image deformation extent using GammaTool phantom.ResultsMR images taken with both scanners showed average and maximum distortion of 0.3 mm and 1 mm respectively. The marker distances in co-registered images resulted below 0.5 mm for both MRI scans. The targeting mismatches obtained were 0.8 mm, 1.0 mm and 1.2 mm for MRI-only and MRI + CT (1,5T and 3 T), respectively.ConclusionsProcedures using a combination of MR and CT images provide targeting accuracies comparable to those of MRI-only procedures. The BrainTool proved to be a suitable tool for carrying out co-registration and targeting accuracy of Gamma Knife brain radiosurgery treatments.     

Detecting femur–insert collisions to improve precision of fluoroscopic knee arthroplasty analysis

Fluoroscopic analysis is an important tool for assessing in vivo kinematics of knee prostheses. Most commonly, a single-plane fluoroscopic setup is used to capture the motion of prostheses during a particular task. Unfortunately, single-plane fluoroscopic analysis is imprecise in the out-of-plane direction. This can result in reconstructing physically impossible poses, in which—for example—the femoral component intersects with the insert, as the normal pose estimation process does not take into account the relation between the components. In the proposed method, the poses of both components are estimated simultaneously, while preventing femur–insert collisions. In a phantom study, the accuracy and precision of the new method in estimating the relative pose of the femoral component were compared to those of the original method. With reverse engineered models, the errors in estimating the out-of-plane position decreased from 2.0±0.7 to 0.1±0.1 mm, without effects on the errors in rotations and the in-plane positions. With CAD models, the errors in estimating the out-of-plane position decreased from 5.3±0.7 mm (mean±SD) to 0.0±0.4 mm, at the expense of a decreased precision for the other position or orientation parameters. In conclusion, collision detection can prevent reconstructing impossible poses and it improves the position and motion estimation in the out-of-plane direction.     

Simultaneous measurement of three-dimensional joint kinematics and ligament strains with optical methods

The objective of this study was to assess the precision and accuracy of a nonproprietary, optical three-dimensional (3D) motion analysis system for the simultaneous measurement of soft tissue strains and joint kinematics. The system consisted of two high-resolution digital cameras and software for calculating the 3D coordinates of contrast markers. System precision was assessed by examining the variation in the coordinates of static markers over time. Three-dimensional strain measurement accuracy was assessed by moving contrast markers fixed distances in the field of view and calculating the error in predicted strain. Three-dimensional accuracy for kinematic measurements was assessed by simulating the measurements that are required for recording knee kinematics. The field of view (190 mm) was chosen to allow simultaneous recording of markers for soft tissue strain measurement and knee joint kinematics. Average system precision was between +/-0.004 mm and +/-0.035 mm, depending on marker size and camera angle. Absolute error in strain measurement varied from a minimum of +/-0.025% to a maximum of +/-0.142%, depending on the angle between cameras and the direction of strain with respect to the camera axes. Kinematic accuracy for translations was between +/-0.008 mm and +/-0.034 mm, while rotational accuracy was +/-0.082 deg to +/-0.160 deg. These results demonstrate that simultaneous optical measurement of 3D soft tissue strain and 3D joint kinematics can be performed while achieving excellent accuracy for both sets of measurements.     

Theoretical accuracy of model-based shape matching for measuring natural knee kinematics with single-plane fluoroscopy

Quantification of knee motion under dynamic, in vivo loaded conditions is necessary to understand how knee kinematics influence joint injury, disease, and rehabilitation. Though recent studies have measured three-dimensional knee kinematics by matching geometric bone models to single-plane fluoroscopic images, factors limiting the accuracy of this approach have not been thoroughly investigated. This study used a three-step computational approach to evaluate theoretical accuracy limitations due to the shape matching process alone. First, cortical bone models of the femur tibia/fibula, and patella were created from CT data. Next, synthetic (i.e., computer generated) fluoroscopic images were created by ray tracing the bone models in known poses. Finally, an automated matching algorithm utilizing edge detection methods was developed to align flat-shaded bone models to the synthetic images. Accuracy of the recovered pose parameters was assessed in terms of measurement bias and precision. Under these ideal conditions where other sources of error were eliminated, tibiofemoral poses were within 2 mm for sagittal plane translations and 1.5 deg for all rotations while patellofemoral poses were within 2 mm and 3 deg. However, statistically significant bias was found in most relative pose parameters. Bias disappeared and precision improved by a factor of two when the synthetic images were regenerated using flat shading (i.e., sharp bone edges) instead of ray tracing (i.e., attenuated bone edges). Analysis of absolute pose parameter errors revealed that the automated matching algorithm systematically pushed the flat-shaded bone models too far into the image plane to match the attenuated edges of the synthetic ray-traced images. These results suggest that biased edge detection is the primary factor limiting the theoretical accuracy of this single-plane shape matching procedure.     

An optimized image matching method for determining in-vivo TKA kinematics with a dual-orthogonal fluoroscopic imaging system

This study presents an optimized matching algorithm for a dual-orthogonal fluoroscopic image system used to determine six degrees-of-freedom total knee arthroplasty (TKA) kinematics in-vivo. The algorithm was evaluated using controlled conditions and standard geometries. Results of the validation demonstrate the algorithm's robustness and capability of realizing a pose from a variety of initial poses. Under idealized conditions, poses of a TKA system were recreated to within 0.02+/-0.01 mm and 0.02+/-0.03 deg for the femoral component and 0.07+/-0.09 mm and 0.16+/-0.18 deg for the tibial component. By employing a standardized geometry with spheres, the translational accuracy and repeatability under actual conditions was found to be 0.01+/-0.06 mm. Application of the optimized matching algorithm to a TKA patient showed that the pose of in-vivo TKA components can be repeatedly located, with standard deviations less than +/-0.12 mm and +/-0.12 deg for the femoral component and +/-0.29 mm and +/-0.25 deg for the tibial component. This methodology presents a useful tool that can be readily applied to the investigation of in-vivo motion of TKA kinematics.     

The robustness and accuracy of in vivo linear wear measurements for knee prostheses based on model-based RSA

Accurate in vivo measurements methods of wear in total knee arthroplasty are required for a timely detection of excessive wear and to assess new implant designs. Component separation measurements based on model-based Roentgen stereophotogrammetric analysis (RSA), in which 3-dimensional reconstruction methods are used, have shown promising results, yet the robustness of these measurements is unknown. In this study, the accuracy and robustness of this measurement for clinical usage was assessed. The validation experiments were conducted in an RSA setup with a phantom setup of a knee in a vertical orientation. 72 RSA images were created using different variables for knee orientations, two prosthesis types (fixed-bearing Duracon knee and fixed-bearing Triathlon knee) and accuracies of the reconstruction models. The measurement error was determined for absolute and relative measurements and the effect of knee positioning and true seperation distance was determined. The measurement method overestimated the separation distance with 0.1mm on average. The precision of the method was 0.10mm (2*SD) for the Duracon prosthesis and 0.20mm for the Triathlon prosthesis. A slight difference in error was found between the measurements with 0° and 10° anterior tilt. (difference=0.08mm, p=0.04). The accuracy of 0.1mm and precision of 0.2mm can be achieved for linear wear measurements based on model-based RSA, which is more than adequate for clinical applications. The measurement is robust in clinical settings. Although anterior tilt seems to influence the measurement, the size of this influence is low and clinically irrelevant.     

Accuracy of mobile biplane X-ray imaging in measuring 6-degree-of-freedom patellofemoral kinematics during overground gait

The aim of this study was to evaluate the accuracy with which mobile biplane X-ray imaging can be used to measure patellofemoral kinematics of the intact knee during overground gait. A unique mobile X-ray imaging system tracked and recorded biplane fluoroscopic images of two human cadaver knees during simulated overground walking at a speed of 0.7 m/s. Six-degree-of-freedom patellofemoral kinematics were calculated using a bone volumetric model-based method and the results then compared against those derived from a gold-standard bead-based method. RMS errors for patellar anterior translation, superior translation and lateral shift were 0.19 mm, 0.34 mm and 0.37 mm, respectively. RMS errors for patellar flexion, lateral tilt and lateral rotation were 1.08°, 1.15° and 1.46°, respectively. The maximum RMS error for patellofemoral translations was approximately one-half that reported previously for tibiofemoral translations using the same mobile X-ray imaging system while the maximum RMS error for patellofemoral rotations was nearly two times larger than corresponding errors reported for tibiofemoral rotations. The lower accuracy in measuring patellofemoral rotational motion is likely explained by the symmetric nature of the patellar geometry and the smaller size of the patella compared to the tibia.     

Comparative accuracy of radiostereometric and optical tracking systems

This study aims to quantify and compare the accuracy of traditional radiostereometric analysis (RSA), fluoroscopic RSA (fRSA), and optical tracking systems. Three phantoms were constructed, each having three stainless steel spheres and three reflective markers. One phantom was mounted to the base of a precision cross-slide table, one to the base of a precision rotation table, and the third was mounted to each moveable tabletop. Two dial-gauges, rigidly mounted to the cross-slide table and rotation table, quantified translations and rotations. Two fluoroscopy units placed orthogonally tracked the steel spheres while a four-camera optical motion capture system tracked the reflective markers in three-dimensional space. RSA was performed with both digital radiography and fluoroscopy. Three axes of translation were tested: parallel to one fluoroscopy image, parallel to the other fluoroscopy image, and at approximately 45° to each image. One axis of rotation was tested. Intraclass correlation coefficients indicated excellent agreement between the actual (dial-gauge) and measured translations for all modalities (ICCs>0.99) and excellent agreement between actual and measured rotations for RSA and fRSA (ICCs>0.99). Standard errors of measurement ranged from 0.032 mm and 0.121° for RSA, to 0.040 mm and 0.229° for fRSA, and to 0.109 mm and 0.613° for optical tracking. Differences between actual and measured translations along the 45° axis were significantly smaller than the two parallel axes. These findings suggest that under ideal conditions, accuracy of fRSA is comparable to traditional RSA, and superior to optical tracking. Accuracy is highest when measured at 45° to the fluoroscopy units.     

A method for measurement of joint kinematics in vivo by registration of 3-D geometric models with cine phase contrast magnetic resonance imaging data

A new method is presented for measuring joint kinematics by optimally matching modeled trajectories of geometric surface models of bones with cine phase contrast (cine-PC) magnetic resonance imaging data. The incorporation of the geometric bone models (GBMs) allows computation of kinematics based on coordinate systems placed relative to full 3-D anatomy, as well as quantification of changes in articular contact locations and relative velocities during dynamic motion. These capabilities are additional to those of cine-PC based techniques that have been used previously to measure joint kinematics during activity. Cine-PC magnitude and velocity data are collected on a fixed image plane prescribed through a repetitively moved skeletal joint. The intersection of each GBM with a simulated image plane is calculated as the model moves along a computed trajectory, and cine-PC velocity data are sampled from the regions of the velocity images within the area of this intersection. From the sampled velocity data, the instantaneous linear and angular velocities of a coordinate system fixed to the GBM are estimated, and integration of the linear and angular velocities is used to predict updated trajectories. A moving validation phantom that produces motions and velocity data similar to those observed in an experiment on human knee kinematics was designed. This phantom was used to assess cine-PC rigid body tracking performance by comparing the kinematics of the phantom measured by this method to similar measurements made using a magnetic tracking system. Average differences between the two methods were measured as 2.82 mm rms for anterior/posterior tibial position, and 2.63 deg rms for axial rotation. An intertrial repeatability study of human knee kinematics using the new method produced rms differences in anterior/posterior tibial position and axial rotation of 1.44 mm and 2.35 deg. The performance of the method is concluded to be sufficient for the effective study of kinematic changes caused to knees by soft tissue injuries.     

Image-based RSA: Roentgen stereophotogrammetric analysis based on 2D-3D image registration

Image-based Roentgen stereophotogrammetric analysis (IBRSA) integrates 2D-3D image registration and conventional RSA. Instead of radiopaque RSA bone markers, IBRSA uses 3D CT data, from which digitally reconstructed radiographs (DRRs) are generated. Using 2D-3D image registration, the 3D pose of the CT is iteratively adjusted such that the generated DRRs resemble the 2D RSA images as closely as possible, according to an image matching metric. Effectively, by registering all 2D follow-up moments to the same 3D CT, the CT volume functions as common ground. In two experiments, using RSA and using a micromanipulator as gold standard, IBRSA has been validated on cadaveric and sawbone scapula radiographs, and good matching results have been achieved. The accuracy was: |mu |< 0.083 mm for translations and |mu| < 0.023 degrees for rotations. The precision sigma in x-, y-, and z-direction was 0.090, 0.077, and 0.220 mm for translations and 0.155 degrees , 0.243 degrees , and 0.074 degrees for rotations. Our results show that the accuracy and precision of in vitro IBRSA, performed under ideal laboratory conditions, are lower than in vitro standard RSA but higher than in vivo standard RSA. Because IBRSA does not require radiopaque markers, it adds functionality to the RSA method by opening new directions and possibilities for research, such as dynamic analyses using fluoroscopy on subjects without markers and computer navigation applications.     

A model-based approach to measure the minimum joint space width of total knee replacements in standard radiographs

Excessive wear in total knee arthroplasty is detected by measuring the minimum joint space width (mJSW) in anterioposterior radiographs. The accuracy of conventional measurement methods is limited and can be improved using model-based techniques. In this study, the model-based wear measurement (MBWM) is introduced. Its accuracy and reproducibility are assessed and compared to the conventional measurement. Forty anterioposterior radiographs were obtained of a knee prosthesis using a phantom setup. Both measurement methods were applied and the accuracy and precision were compared. The reproducibility was calculated with inter- and intra-observer experiments. Three observers measured the mJSW in 30 clinical radiographs with both the conventional measurement and the MBWM and repeated this after 6 weeks. The experiments were conducted with a NexGen mobile bearing and fixed bearing prostheses. In the phantom experiment, the accuracy (mean of the absolute error) was significantly higher (t-test, p<0.01) for the MBWM as for the conventional measurement (0.15 mm versus 0.43 mm, 0.14 mm versus 0.35 mm for the mobile and fixed bearing, respectively). The standard deviation of the measurements is the smallest for the MBWM measurement for both prosthesis types (0.16 mm versus 0.47 mm, Levene's test, p<0.01). In the reproducibility experiment, both the intra- and inter-observer agreements were higher for the MBWM than for the conventional method. The results show that the MBWM is superior to the conventional measurement in both accuracy and reproducibility. Although the use of a phantom experiment poses some limitations in conveying the findings to clinical practice, this improved mJSW measurement can lead to better wear detection for surgery decisions and research purposes.