SynaFlexer™ X-ray Positioning Frame
and Phantom
Radiographic joint-space width as an indirect measure of articular cartilage thickness is currently the primary structural endpoint used in clinical trials of osteoarthritis of the knee and hip. Most of the variation associated with serial joint-space width measurements in clinical trials is introduced during image acquisition. Minimizing this source of variation is therefore the key to effective multi-center radiography.
The principal sources of variability in radiography of the knee and hip are:
- Joint positioning (flexion, rotation)
- Beam centering and angulation
- Geometric magnification
- Parallax error
These potential sources of measurement error are controlled and corrected with the SynaFlexer™ positioning frame and phantom. SynaFlexer™ is a specialized image-acquisition aid developed by our experts Dr. Charles Peterfy and David White, Ph.D. More than 6,000 knee radiographs have been evaluated using the SynaFlexer™ system.
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During knee imaging, the SynaFlexer™ frame positions the patient's feet reproducibly in 10° external rotation, and fixes the degree of flexion of both the tibia and femur to ensure that the exact same regions of articular cartilage and joint space are projected every time. Unless both the tibia and femur positions are fixed, different regions of their cartilage surfaces can articulate on follow-up examinations, leading to inaccurate and imprecise serial measurements of cartilage thickness. SynaFlexer™ minimizes this problem and also eliminates the need for personalized foot maps, which are cumbersome and time consuming to use, possible to confuse among patients, and difficult to replace if lost. SynaFlexer™ is also useful for standardizing radiography of the hip with the feet positioned in 10° internal rotation.
The SynaFlexer™ frame is equipped with a specially designed projection phantom that is used by our SynaVu™ image analysis software to automatically verify the beam angle that was used during radiography and which knee (right / left) was imaged at each visit. Radiology technicians occasionally mislabel knee side, and this can be very difficult to detect during image analysis or verify in retrospect. The phantom built into the SynaFlexer™ frame unambiguously identifies knee side on every examination, so this can never happen. The SynaFlexer™ system also corrects any magnification differences caused by equipment changes during the course of a study, and corrects parallax errors introduced by beam angulation.
SynaVu™ Digital X-ray
Reading System
Radiographic assessment of joint-space narrowing in clinical trials of osteoarthritis has become a highly sophisticated process. It begins with careful radiography of the knee or hips using standardized techniques, positioning frames and calibration phantoms designed to ensure reproducible projection of the joints on serially acquired images from multiple centers involved in a study. The analog images from these multi-center examinations are then centrally digitized to allow safer and more economical archival, rapid recovery and distribution to multiple readers, and electronic submission to regulatory agencies.
These electronic images are analyzed with our specialized reading workstations (SynaVu™) equipped with high-resolution monitors.
The SynaVu™ system edge-enhances the digital images using unsharp masking to maximize the delineation of cortical and trabecular bone. It then strips the images of all identifying information to ensure reader blinding, and presents the images side-by-side as serial sets for each subject in order to maximize the reader's ability to detect the smallest changes during semi-quantitative scoring (Kellgren-Lawrence or OARS(I)) or ensure anatomical consistency in serial joint-space width measurements. Examination order is usually randomized to maintain chronological blinding. The SynaVu™ software provides our readers with tools to optimize image quality further and aid analysis. These include image window and level adjustment, zoom and panning functions. It also provides a robust edge-detection algorithm to delineate the weightbearing articular surfaces and automated minimum joint space width measurement. The SynaVu™ software also provides validated electronic score sheets with reader sign-off. Results are automatically encrypted and input into a database.
The software stores all steps involved in the calculation of joint-space width, including:
- Raw data for computation of magnification and beam angle
- Automatically generated femur and tibia edges
- Reader-corrected femur and tibia edges
- Location of medial and lateral minimum joint-space width
The availability of this information makes it possible to audit any step in the process of computing the joint-space width.
Coupling specialized image acquisition aids with computer-assisted analysis makes clinical-trials radiology an integrated process and maximizes precision, speed and regulatory compliance.
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