1) If you do multiple images on a single
IP, the computer needs
3 distinct collimated boarders to recognize each data field. My
students want to know why and I can't give them a good enough answer.
Can anybody explain why *3* boarders are required?
The three margin rule is primarily for off
centered single fields, but
could apply to multiple fields also. When you do multiple fields you
need clean collimation margins between the edge of the plate and the
exposure fields and clean collimation margins between the exposure
fields. If the exposure fields extend past he edge of the plate and
if
the exposure fields over lap, the histogram is likely to contain
extraneous data (off focus radiation) and automatic rescaling will
probably produce erratic image appearance (dark or light when the
actual exposure level was correct) . There is some variation in
these
"rules" between vendors because there is no standardization how
vendors
deal with multiple fields. The "three margin rule" most often
applies
to a single field that tis not centered to the plate. If there are
three margins the software is more likely to recognize the exposure
field that if the off centered field only has two margins. The
reasons
why this is true related to the field recognition logic in the
software. Three margins is "stronger" than two and the software does
not have to "estimate" the exposure field location. I have done labs
with these rules and we get failures only part of the time. If the
plate is overexposed and there is an alignment rule violated the
failures occur more consistently. The way I try to describe these
rule
is "if you violate one of the alignment/collimation rules image
quality
will be erratic.
--
Barry Burns, MS, RT(R), DABR
Professor
Division of Radiologic Science
UNC-CH School of Medicine
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