Biomedical Engineering Reference
In-Depth Information
Body armor
Scintillator Mirror
X-ray tube
X-rays
High-speed
camera
FIGURE 6.20
Typical multi-anode system configuration.
used to image the event from different directions without overlap of the
projection onto storage phosphor screens or x-ray film, from which still
video images were obtained. This approach creates parallax and geometry
issues, thus motivating the current approach in which successive frames
are captured on a single area location on a scintillator screen. A framing
camera with an image intensifier is synchronized with the x-ray pulses to
capture snapshots over events of duration from tens of microseconds to a
few milliseconds.
The choice between 150 and 450 keV x-rays represents a tradeoff between
material penetration and contrast ratio. The attenuation coefficient for clay,
for example, as reported by NIST [17] and Aberdeen, is 0.2 cm −1 at 450 keV and
0.3 cm −1 at 150 keV. For 10 in. of clay, the 150 keV flux is attenuated by about
15 X with respect to a 450 keV pulse. It is possible, using 450 keV x-rays and
a storage phosphor screen, to image a 0.5 in. depression under the described
angular alignment. The attenuation differential with x-ray energy implies
a contrast ratio difference. As it is desirable to image depressions smaller
than 0.5 in. (1.27 cm), with optimal resolution, the improved contrast ratio
that obtains for 150 keV x-rays is an advantage—if the higher flux require-
ment can be accomodated.
6.13.4 Scintillators for Flash X-Ray
Six commercially available fast scintillators, from two manufacturers and one
distributor, were characterized for efficiency and speed at 150 and 450 keV,
as efficiency differences impact the increased flux requirements for imag-
ing at 150 keV. No information on uniformity or sensitivity was available,
and the speed of the samples deviated from data published in the literature.
Availability, pricing, and timely delivery of suitable scintillators is expected
to be a challenge associated with assembly of multiple systems.
Search WWH ::




Custom Search