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Orthopaedic disorders are a leading cause of disability in the U.S., with arthritis and/or spine problems adversely affecting quality of life for more than 20% of adults. While advances in diagnostic imaging have improved our ability to detect structural changes in musculoskeletal tissues, they typically reveal little about joint function. There is evidence that abnormal mechanical joint function contributes significantly to the development and progression of many types of joint disease. Notably, joint translations of only a few millimeters are critical to estimating tissue stress or joint impingement during loaded functional movements. Dynamic Stereo X-ray (DSX) is the only currently available technology that can achieve sub-millimeter bone pose (position and orientation) estimation accuracy during a wide variety of functional movements.
Note: there are many names coined for DSX, include Röntgen Stereometric Analysis, Biplanar Videoradiography and X-ray Reconstruction of Moving Morphology (XROMM).
A typical custom biplane videoradiography system. For those interested, contact Marty Kulis at Imaging Systems
The X-ray tubes generate X-ray beams which pass through the joint of interest and enter the image intensifiers.
Each image intensifier projects onto a phosphor screen an image that is subsequently captured by a camera mounted on the intensifier.
The two cameras collect the images in a time synchronized manner.
The purpose of the DSX software is to identify the pose (position and orientation) of the bones from the 2D X-ray images and subject specific model of the bones.
Our objective was to develop commercial software for rapid, robust, and reliable bone pose estimation from radiographic image sequences with minimal operator intervention. The research applications of DSX are clearly established, and are leading to breakthroughs in nearly all orthopaedic specialties. The potential of DSX as a clinical tool for diagnosing joint and tissue disorders and directly improving patient treatment and outcomes has yet to be realized. The greatest challenge is robustness and the minimization of manual labor. Just as cine-angiography has revolutionized diagnosis and treatment of cardiovascular disorders, widespread availability of DSX could significantly improve treatment for a wide variety of orthopaedic disorders. We hope that the release of DSX will open the door for large clinical studies and the development of a myriad of diagnostic imaging applications.
Mitigating concerns over the limited availability of appropriate imaging hardware and radiation exposure. The cost of a dynamic imaging system is actually similar to or less than that of many current diagnostic imaging systems (biplane cine-angiography, CT, MRI). The evolution of DSX should expand the applications to simpler, lower-cost equipment by removing the requirement for synchronous imaging. The system provides dynamic data during functional activities that is unavailable from any other modality (including MRI), and should have unique clinical applications for evaluating movement-related musculoskeletal disorders.
|Summary of Markerless Pose Estimation Algorithm|
The 3D pose of a bone is quantified by a position (3 degrees of freedom) plus an orientation (3 degrees of freedom). A pose map is a series of contiguous poses for a bone represented as either discrete poses at each X-ray frame or by a spline across all frames. Given a 3D representation of a bone extracted from a high-resolution CT scan of the subject, a local reference frame assigned to the bone, and a time series of X-ray images containing the bone, a pose map is the solution of the DSX across all frames. The DSX algorithm solves for the 3D pose by registering two non-coplanar X-ray images of a bone to two digitally reconstructed radiographs (DRRs) (Figure). Given the position and orientation of an X-ray source, an X-ray image plane, and volumetric CT bone, a DRR is the projection of the CT bone onto a virtual X-ray image using a simplified X-ray generation model. In other words, rays from the X-ray source are cast through the bone to generate a simulated X-ray with the same size and resolution of the actual X-ray.
Figure: 3D representation of a biplane X-ray configuration when the two X-rays are synchronized. The distal femur, reconstructed from the CT data, is shown in the middle. The inline X-ray image (in line with the X-axis of the lab reference frame) is shown in the red frame; the red line is the perpendicular from the center of the X-ray image plane to the X-ray source. The offset X-ray image (offset from the X-axis of the lab reference frame) is shown in the green frame; the green line is the perpendicular from the center of the X-ray image plane to the X-ray source. For illustration, the inline X-ray image is shown after processing (smoothing and edge detection) and the offset image is shown unprocessed.
|Summary of Marker-Based (Implanted Beads) Pose Estimation Algorithm|
The gold standard for Biplanar Videoragiography is based on tracking beads implanted onto the bones. Locate3D can be used for tracking the beads, and Visual3D can be used to estimate the pose. Unlike Marker-based optical motion capture the beads do not move relative to the bones (i.e. there is no soft tissue artifact), and the resulting pose estimates are accurate to less than 1 mm.
If beads are implanted, all of the flexibility and power of Visual3D comes into play to analyze and report the biomechanical results.
Orthopaedic disorders are a leading cause of disability in the U.S., with arthritis and/or spine problems adversely affecting quality of life for more than 20% of adults. With an aging population, the rate of disability from orthopaedic disorders has been increasing steadily. While the majority of tools for clinical assessment of orthopaedic conditions rely upon static measures, joints must function properly in a range of postures and complex loading conditions. Pain and functional limitations are often activity-specific, and can defy reliable diagnosis with conventional clinical tests. Abnormal structural findings on MRI can be poorly correlated with clinical symptoms (Carragee et al, 2006; Djurasovic et al 2012). Joint instability is a common diagnosis for a variety of joint disorders, but there are no diagnostic tests that actually evaluate functional joint stability. Widely-used clinical laxity tests for assessing the knee after ACL injury are insufficient for identifying functional status and ineffective for determining which individuals might be able to cope with the injury without surgery (Eastlack et al, 1999)). By evaluating joint function during activities that are important to the patient (e.g., lifting for the factory worker with low back pain, running/jumping for the athlete with a knee injury, or climbing stairs for an individual with patello-femoral pain or osteoarthritis) the relationship between structural abnormalities and dysfunction can be assessed directly. By combining high-accuracy bone motion with patient-specific soft-tissue geometry from MRI, it is possible to characterize soft-tissue behavior directly.
Requirements for a dynamic imaging system with these capabilities include sample rates high enough to capture dynamic movements and sub-millimeter spatial accuracy to characterize tissue deformation, identify joint instability or impingement, and/or identify early signs of tissue degeneration. DSX is the only currently available technology that can achieve this requisite level of performance during a wide variety of functional movements [Tashman and Anderst, 2003; Bey et al, 2006; Bey et al, 2008; Anderst et al, 2009; Martin et al, 2011; Li et al, 2012; Aiyangar at al, 2014). Thus, it is not surprising that the popularity of dynamic radiographic imaging has grown rapidly over the last decade. DSX has been used to characterize a variety of joint disorders, including changes in joint contact kinematics with knee injuries (ACL,PCL, meniscus) (Tashman et al., 2004; Gill et al., 2009; Van de Velde et al, 2009; Hoshino et al., 2013; Goyal et al., 2012; Marsh et al., 2014), dynamic aspects of patello-femoral disorders (Fernandez et al., 2008; Bey et al., 2008), femoro-acetabular impingement of the hip (Martin et al., 2011; Kapron et al., 2014), shoulder function after rotator cuff injury (Bey et al, 2011) and arthroplasty (Massimini et al., 2010), changes in intervertebral kinematics with lumbar disc degeneration (Anderst et al., 2008; Li et al, 2011), and deformation of the joint capsule and intervertebral discs with cervical spine disc fusion (Anderst et al., 2013; Anderst et al., 2014).
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Anderst WJ, Vaidya R, Tashman S (2008). A technique to measure three-dimensional in vivo rotation of fused and adjacent lumbar vertebrae. Spine J. 2008; vol 8: 991-7. PMID: 17919983.
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Gill TJ, Van de Velde SK, Wing DW, Oh LS, Hosseini A, Li G (2009). Tibiofemoral and Patellofemoral Kinematics Following Reconstruction of an Isolated Posterior Cruciate Ligament Injury: In Vivo Analysis During Lunge. Am J Sports Med; vol 37(12): 2388-85. PMID: 19726621.
Goyal K, Tashman S, Wang JH, Li K, Zhang X, Harner C (2012). In vivo analysis of the isolated posterior cruciate ligament-deficient knee during functional activities. Am J Sports Med; vol 40(4): 777-85. PMID: 22328708.
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Tashman S, Collon D, Anderson K, Kolowich P, Anderst W (2004). Abnormal rotational knee motion during running after anterior cruciate ligament reconstruction. Am J Sports Med; vol 32(4): 975-83. PMID: 15150046.
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Zhang X, Aiyangar A, Zheng L, Tashman S, Anderst W (2013). Capturing Three-dimensional In Vivo Lumbar Intervertebral Joint Kinematics Using Dynamic Stereo-X-ray Imaging. J Biomech Eng; vol 136(1): 011004. PMID: 24149991.
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The DSX workflow is complicated given the number of files and programs that are used. The graphic below shows the flow of data between the different programs in the DSX suite. Some of the data (e.g. bead and landmark locations) are stored in the subject file, other data (e.g., large image files, and data required for processing in Visual3D) are stored in separate files. All DSX programs store the file path of the files they create in the subject file.
It is not required to use all programs in the DSX suite. A user may prefer to use a third party application to generate a surface model from the subject's scan data. In this situation the user needs to manually update the subject file in xManager with the path to the generated bone surface model (obj file), the labeled (and potentially cropped) ct data, and with landmark locations.
DSX Suite of Applications
DSX Suite is designed to process data from biplanar videoradiography. The suite of applications allows processing of the X-ray data from collection through analysis and reporting. At the heart of the application is the ability to track 3D objects (bones, implants, etc.) in X-ray images. DSX is based on a 3D-to-2D approach to markerless motion capture that generates digitally reconstructed radiographs (DRRs) of the objects and matches them to the X-ray images. It calculates the 3D X-ray configuration parameters of the X-ray hardware and distortion-corrects the X-ray images. Tools for analyzing the tracking results are in Visual3D.
In the DSX Suite documentation there are some technical terms that are specific to the program and technology. Definitions of these terms can be found here.
There are seven applications in the DSX suite, plus Visual3D:
|XManager is the central program....|
XManager is the central program that lets you manage a subject and all of the system configurations, trials, and data files associated with it.
The 6 data processing programs can be launched from the toolbar.
The most important function of XManager is to create, store, and load the data associated with a subject.
|CalibrateDSX calibrates the system.|
CalibrateDSX calculates the 3D configuration of the X-ray hardware (the pose of the X-ray sources and image planes) from images of the calibration object.
The primary purpose of the CalibrateDSX program is to correct the uniformity, correct any distortions, and resize the X-ray images. CalibrateDSX also allows the following functions:
Note: The transform matrix between the X-ray frame and the motion capture reference frame is needed if you want to use motion capture data to seed the pose optimization in X4D.
|Surface3D creates tracking bones and surface models.|
Surface3D creates tracking bones (RAW files or stacked TIFFs) and surface models (OBJs) from CT and MRI data. Surface3D (S3D) is used to create triangulated surface models from 3D image data using the Marching Cubes algorithm. Currently, Surface3D can only use CT data. Surface3D can also export CT data that has been cropped to an individual object of interest. This cropping is required to enable the creation of Digitally Reconstructed Radiographs (DRRs) for each individual object. This in turn enables the optimization of the 3D position and orientation for each object individually.
|Orient3D works with the surface models created|
|Locate3D tracks beads in x-ray trials.|
Locate3D tracks beads in X-ray trials.
|X4D tracks bones in X-ray trials, using single-frame or 4D optimization.|
X4D tracks bones in X-ray trials, using single-frame or 4D optimization.
|PlanDSX can be used to design X-ray configurations when planning a new study.|
PlanDSX can be used to design X-ray configurations when planning a new study.
|Visual3D can be used for viewing the kinematic analysis results|
Visual3D' is not a part of the DSX suite of applications but can be used for: