Cross-sectional SEM photograph of a polycapillary fiber with 10 μm diameter channels.
Multifiber collimating lens with output, input, focal length (optimal distance to source) of 250 mm, length 127 mm, and transmission efficiency of 37% at 17.5 keV.
Monochromatic imaging setup.
Experimental setup for polychromatic imaging, without an optic or crystal. The 5.56 mm thick aluminum filters were placed before the phantom to avoid over-exposing the Fuji image plate. The exposure time for this source with the aluminum filter was about 30 s, which was about the minimum required for this source to come up to full voltage (this source is normally used in continuous, not pulsed, operation).
The construction of the 8 keV step phantoms. In the beam view the x-ray beam is coming straight out of the paper.
Copper rotating anode spectrum taken with a NaI detector through a 5.65 mm aluminum filter and a small pinhole.
Output of the x-rays off the crystal in two dimensions. The angle between the surface and the optic axis is set to . The angle of a particular plane to the crystal surface is . The angle of the x-ray to the optic axis is . The angle of the x-ray to the plane is then . The angle of the reflected x-ray to the crystal surface is .
Image plate data from five different polypropylene step phantoms. The top row was taken without the optic (conventional), the bottom row was taken with a monochromatic beam. Columns a/b is the phantom with a step height of 1.5 mm, columns c/d with 2.0 mm, columns e/f with 6.6 mm, columns g/f with 10.6 mm, and columns i/j with 15.5 mm. Notice that contrast is already visible in the monochromatic case even for the lowest step height.
Set-up for monochromatic imaging. The double dotted lines correspond to the image locations (a)–(c) displayed below the setup. The fiber structure of the optic is clearly visible in (a). Location of (b) is 17 cm after the optic. The divergence has blurred the fiber structure. Post crystal images are labeled for silicon (c1), for mica (c2), and for graphite (c3). Aluminum filters of varied thickness were used for (a) through (c) except for imaging with silicon and mica crystals.
A magnified picture after the silicon crystal clearly indicates the extra shadow of the doublet separation.
Rocking curves of three crystals: Silicon (a) with full width at half maximum (FWHM) 4.0 mrad, mica (b) with FWHM 4.4 mrad, and graphite (c) with FWHM 42.5 mrad. Note the change in scale for graphite rocking curve.
Knife-edge resolution profiles obtained with silicon (a), mica (b) and graphite (c) crystals. The edge shows a double step for silicon and mica due to the doublet separation.
Optic parameters. The second optic has a shorter focal distance appropriate for use with a compact source.
Crystals used for monochromatizing the beam, with their orientation (which set of crystal planes are parallel to the surface) and the spacing, , between the atomic planes (Refs. 22 and 23). Equation (1) is used to compute the Bragg angle which is different for different wavelengths (energies). For silicon (and graphite) the Bragg angle is computed for the 4th (2nd) order diffracted beam, since the theoretical intensity from the lower order diffractions is zero due to destructive interference (Ref. 24). The mica was supplied by XOS, Inc. and Ted Pella, Inc. and the pyrolytic graphite, graded ZYH, by Advanced Ceramics Corporation.
Comparison of contrast for step phantoms using 8 keV monochromatic beams obtained with silicon, mica, and graphite crystals to the polychromatic contrast of conventional imaging, taken without crystals or the optic. The monochromatic contrast is independent of the crystal used, because even the widest crystal angular bandwidth gives an energy spread of only 1.4 keV. The polychromatic contrast is independent of whether or not an optic is used or the optic to phantom distance. Collimation does not affect the contrast.
Contrast enhancement, monochromatic contrast divided by the polychromatic contrast, for the step phantoms at 8 keV using the silicon crystal.
Contrast between polypropylene and polyvinyl chloride plastics were compared for the 8 keV monochromatic and polychromatic beams.
Contrast at 17.5 keV with 45 mm lucite phantom.
Rocking curves and resolution measurements using three different crystals. The rocking curve widths are due to the combined effects of the angular bandwidth of the crystal and the 4 mrad output divergence of the optic. The effect of the detector has been subtracted. The angular resolution calculations are given by Eq. (9). The energy width for graphite is taken to include the whole doublet.
The angular resolution obtained for the molybdenum and peaks. The first column is the measured angular width of the knife edge image, after subtracting the detector contribution. The theory is computed from Eq. (9). The resolution is computed given the theoretical angular width, for a 50 mm thick patient with an ideal detector.
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