High resolution biplanar X-ray video fluoroscopy
The high speed biplanar C-arm fluoroscope was custom built in collaboration with Siemens AG, Erlangen based on a standard angiography system (Neurostar?) with 400 mm diameter image intensifiers. The original cameras were replaced by high speed video cameras (SpeedCamVisario g2 with Nikon Nikkor 50 mm lenses, Weinberger GmbH) operating with a maximum spatial resolution of 1.536 x 1.024 pixels and a maximum temporal resolution of 2.000 frames per second. To obtain such high temporal resolution, the fluoroscopes produce constant radiation instead of pulsed radiation. However, the summarized dose of radiation is low for our animals because of their small size and generally low exposure times. The mobile C-arms can be rotated and translated in various positions to each other. The minimum distance between X-ray source and image intensifier is 520 mm, the maximum distance is 750 mm. With the two image intensifiers placed perpendicular to each other, the volume of intersection of the two X-ray beams is more than 28,000 cm3 at the minimum zoom. With the custom built control unit, voltage (kV), X-ray current intensity (mA), focus of the X-ray beam, zoom of the image intensifier, scene time and iris can be adjusted to the current set-up. Optimized presetting of the control parameters for certain animals and certain experimental conditions can be saved and preselected in later experiments. A low-dose mode is available to adjust the positions of the cameras to the set-up prior to the experiments.
Left: Experimental laboratory with the high speed biplanar C-arm fluoroscope. Right: Control room with the PC and the custom built control unit.
Temporal resolution of the SpeedCam Visario high-speed cameras can be selected from 50 to 2.000 frames per second, spatial resolution can be 512 x 192, 768 x 512, 1024 x 768 or 1536 x 1024 pixels. Maximum possible sequence duration is limited by the memory capacity of the high-speed cameras and depends on the selected temporal and spatial resolution. A maximum of 16 seconds is possible at the lowest resolution. A shutter can be used to reduce motion blur. Two additional monochrome high-speed video cameras (SpeedCam MiniVis, Weinberger GmbH) are externally synchronized with those behind the image intensifiers and can be placed in the scene to collect additional perspectives (overview, specific details, e.g. the moment of touchdown on a force plate). Their spatial resolution is fixed to 512 x 512 pixels. All cameras are controlled by the software Visart (HS Vision) which further allows the post-processing of the radiographs (optimization of sharpness, contrast, cutting, saving as uncompressed or compressed files).
Synchronized recordings of the four cameras imaging a chameleon climbing a rope. X-ray radiographs in lateral and ventrodorsal
perspective, close-up and overview of the scene with normal-light recordings.
To remove the nonlinear pincushion and S-shaped distortions of the radiographs, we utilize a method developed by our colleagues from Brow text-justifyn University (Elizabeth Brainerd): a standardized sheet of perforated metal is fixed flat against the front of the image intensifiers and an image is collected. Using the idealized geometry of the perforated sheet, the radiographs are corrected by a distortion correction algorithm in MATLAB. For the calibration of the 3D space, we designed an acrylic calibration cuboid (120 x 100 mm) with 1 mm steel spheres set into the surface at a distance of 10 mm in horizontal and vertical direction. Finally, using an external trigger (a 5 volt signal), the cameras can by synchronized to other data sets such as force plate recording, electromyographs, electrocardiographs or sound recordings. All radiographs in the database are saved at high-resolution .avi files without any post-processing or correction of the distortions. The images of the calibration cube and the perforated metal sheet are available for each project.
