Dynamic Spatial Reconstructor"

The Dynamic Spatial Reconstructor
The seminal scanner for dynamic volumetric imaging is the x-ray CT scanner known as the Dynamic Spatial Reconstructor (DSR) designed and installed at the Mayo Clinic.[1, 2] The DSR has the ability to obtain up to 240 contiguous 0.9mm thick sections in a short a time period as 1/60 second and to repeat this acquisition rate 60 times per second. In practice, the data rate is somewhat reduced from these numbers. The DSR consists of a gantry weighing approximately 17 US tons with a length of 20.5 feet and a diameter of 15 feet. 14 x-ray guns reside in a hemicylindrical configuration and aim at a juxtaposed hemicylindrical fluorescent screen. The images produced on the fluorescent screen by the firing of the x-ray guns are recorded by a bank of 14 television cameras which, until recently, were image isocons which sent analogue signals to be recorded on a bank of 8 video disc recorders. Since american television is comprised of 240 usable lines, and video rates are 60 per second, it is possible to reconstruct a cross sectional image of the body by digitizing each of the 240 television line for each of the 14 cameras and produce 240 cross sections representing 1/60 second resolution. Each stop action stack of slices in actuality represents approximately 0.1second which is the time in which all 14 x-ray guns are sequentially pulsed on. Because there were only 8 video disc recorders, every two television line were averaged to reduce the lines per camera to 120, and the data from two cameras were recorded interlaced on a single video channel. In addition, as the images are produced the gantry rotates at 15 rotations per minute. Thus, in 1/60 second, the gantry moves a degree and a half such that if the organ system of interest moves slow enough to allow for 2/60 second of scanning, it is possible to generate 28 angles of view to use in the reconstruction process. Up to a point, the more angles of view used in the reconstruction process, the better the images. Typically, at least 4/60 seconds of scanning are used to generate a good quality reconstruction. This can not only be accomplished by utilizing contiguous 1/60 second data sets, but it is possible to retrospectively gate the data together by selecting the same time point from within several physiologic cycles such as the cardiac or respiratory cycle. Physiologic signals are recorded in with the video signals to allow for this retrospective gating process. The designers of the DSR, to achieve the ability to obtain dynamic, volumetric image data sets made compromises in the image resolution such that grey scale resolution was sacrificed. To improve the image quality, the video imaging chains have been converted from image isocon cameras to charged coupled device (CCD) cameras. [76] Much larger lenses were ground by Old-Delf and tapered fiberoptics were pulled to take the images from the lens to a microchannel plate intensifier which then transmits the image to the CCD chips. The process of pulling the tapered fiberoptics introduces some twisting of the fiberoptics and therefor custom warping algorithms had to be developed for each camera. The images are digitized on each, camera, the images are unwarped, and the data is sent now to digital tape running at video rates. Although the DSR has remained a one of a kind system and represents a true tour de force, much of the current image manipulation and display associated with the massive data sets generated have served as the vanguard for data handling of images coming off of the currently commercially available scanners.

1. Ritman, E.L., R.A. Robb, and L.D. Harris. Imaging Physiological Functions: Experience with the DSR (Philadelphia: Praeger, 1985)

2.Robb, R.A., E.A. Hoffman, L.J. Sinak, L.D. Harris, and E.L. Ritman: High-speed three-dimensional x-ray computed tomography: The Dynamic Spatial Reconstructor. Proceedings of the IEEE 71 (3): 308-319, (March) 1983.

3. Jorgensen, S.M., S.V. Whitlock, P.J. Thomas, R.W. Roessler and E.L. Ritman: A programmable real-time digital recording CCD based video camera. Electronic Imaging East `89, (International Electronic Imaging Exposition and Conference) 2: 738-740, 1989.

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