Comparison of entrance exposure and signal-to-noise ratio between an SBDX prototype and a wide-beam cardiac angiographic system
SBDX geometry, on right, compared to a conventional wide-beam system with large detector (image intensifier or flat panel). With SBDX only a small volume is irradiated at any instant, and the scatter generated has low probability of striking the detector. The inverse geometry spreads source x-rays over a larger area at the patient entrance.
Comparison of x-ray beam solid angle, anode geometry, electron beam spatial distribution, and exposure time for a point fixed to the anode.
Comparison of target self-filtration. A reflective anode source uses x-rays emerging at a right angle to the electron beam. SBDX uses x-rays transmitted through the thin tungsten target and substrate layers.
Isocentric imaging geometries. Noise equivalent quanta and entrance exposure were evaluated with acrylic phantoms, each positioned with midplane at mechanical isocenter. To measure contrast, a disk tray was inserted into a small gap created at the midplane (not shown). The scatter fraction was measured with a lead beam-stop array at the phantom entrance and the disk removed (tray still in place).
(a) Off-focus radiation emerging from collimator holes adjacent to the targeted hole. The SBDX geometry is exaggerated in this figure. (b) Measuring the magnitude of off-focus radiation vs the distance from the focal spot. (c) An “image” of the off-focus radiation relative to the focal spot position. To facilitate display, the central pixel is clipped and the remaining values are logged.
SBDX contrast measured with the 1-mm deep disk ( iodine equivalent).
SBDX zero-frequency NEQ expressed in units of equivalent quanta per at mechanical isocenter.
Measured large-area iodine SNR of the II/CCD system (open squares) and the SBDX prototype (solid curves, for 70, 80, 90,100, 110, and operation). Open circles represent SBDX operating at the II/CCD system kVp.
Exposure rate at the phantom entrance during measurement of SNR, for the II/CCD system (open squares), SBDX prototype at (closed squares), and SBDX operating at the II/CCD system kVp (open circles).
Measured SBDX zero-frequency DQE, shown versus kVp. Bars indicate the range of measurements across phantom thickness; squares represent the average.
Measured SBDX scatter plus off-focus radiation fraction (SF). Bars indicate the range of measurements across source kVp; squares represent the average.
(a) SBDX to II/CCD ratios evaluated for equal-kVp imaging of 23.3 to phantoms (: entrance area, 1-SF: scatter degradation factor, DQE: detective quantum efficiency. : geometric detection efficiency or grid primary transmission, entrance exposure-to-fluence conversion factor, : patient transmission, : detected primary contrast). (b) Relative SBDX entrance exposure at equal SNR (RX), measured and modeled.
(a) SBDX to II/CCD ratios in the modeled relative SNR, for the SBDX prototype operating at equal-kVp and maximum power at each kVp. (: total beam-on time per frame, : x-ray production efficiency, : beam current, : x-ray beam solid angle divided by object-plane field area.) (b) Comparison of measured and modeled relative SBDX SNR.
SBDX SNR measurements rescaled by to reflect projected design improvements. Circles represent SBDX operating at the II/CCD system kVp.
Selected thin phantoms, used to simulate acrylic phantom attenuation during SBDX DQE measurements.
II/CCD imaging conditions and measurements of contrast and NEQ.
SBDX SNR and entrance exposure (also shown as percent of the corresponding II/CCD value) when operating at II/CCD kVp.
Measured SBDX and II/CCD entrance exposure efficiency. Relative SBDX entrance exposure at equal SNR (RX).
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