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).

Biplane Videoradiography

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.

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.

DSX_Workflow

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. You may prefer to use a third party application to generate a surface model from the subject's scan data. In this situation you need 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 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. The documentation contains some technical terms that are specific to the program and technology. Definitions of these terms can be found here. The release notes for DSX, with a list of enhancements and bug fixes in every release since the first can be found here.

There are seven applications in the DSX suite, plus Visual3D:

**XManager** is the graphical interface to a subject file, which contains all of the information about one DSX subject, including links to all of the data files. This information is read and written by the other DSX applications (except PlanDSX).

**PlanDSX** is a design tool that helps you determine the appropriate configuration of the X-ray equipment to capture high-quality images of the joint[s] of interest when planning a new study. It is essentially a CAD tool that creates a virtual lab containing the subject, X-ray equipment, motion capture cameras, and other equipment such as treadmills and force plates. The subject skeleton can be animated with motion capture data, and the bones generate simulated X-rays as they pass in front of the virtual X-ray image planes.

**Surface3D** segments objects (bones, implants, etc.) from CT data for use in model-based tracking, and it creates surface models of these objects for subsequent kinematic analysis of the tracking results. It works by labelling and segmenting CT data for each object, then saving the segmented objects to RAW or TIFF files. Then it produces polygonal surface models from the segmented objects using a marching cubes algorithm, and saves them to OBJ files. The segmented objects are used by X4D to generate DRRs for model-based tracking. The surface models are used by Orient3D to define anatomical reference frames and regions of interest (for distance map calculations), and by Visual3D for kinematic analysis. Surface3D also allows you to identify landmarks (e.g., ligament attachments) and points of interest (e.g., implanted beads) in the image data.

**Orient3D** prepares surface models created by Surface3D (or third-party software such as Mimics and ScanIP) for use in kinematic analyses after they have been tracked in X4D or Locate3D. With it you can define a local (anatomical) coordinate system (LCS) for each object, landmarks, such as ligament attachment sites, and regions of interest (ROIs), which are used in the calculation of distance maps.

**CalibrateDSX** prepares the X-ray images and calculates the configuration of the X-ray hardware. It performs three important tasks:

1. correcting the X-ray images (uniformity, distortion, smoothing, resizing)
2. calculating the 3D configuration of the X-ray hardware (the pose of the X-ray sources and camera image planes)
3. calculating the transformation between the X-ray lab frame and the motion capture frame

**Locate3D** tracks radiopaque beads in X-ray trials. It is most often used to track bones with implanted beads. It is similar to X4D, except that it uses multiple beads to determine the pose of an object, instead of the contours of the object itself. Locate3D requires that the configuration of the X-ray equipment has been calculated (with CalibrateDSX), and that the locations of the beads in the object's CT frame have been determined (with Surface3D).

**X4D** tracks 3D objects (bones, implants, etc.) in X-ray images by generating digitally reconstructed radiographs (DRRs) of the objects and matching them to the contours in the X-ray images. It requires that the configuration of the X-ray equipment has been calculated (with CalibrateDSX). It is recommended that the LCSs of the objects to be tracked have been determined (with Orient3D), so there is no need to apply an additional transform, which could have rounding errors. For a validation study, it is required that the implanted beads have been tracked (with Locate3D).

**Visual3D** is not part of the DSX suite of applications but can be used for:

1. defining the kinematic models used to process the motion capture data.
2. viewing the tracking results.
3. performing kinematic analyses including: joint animations, distance maps, and ligament lengths.