X-Ray Image Formation
First we must understand a bit about the formation of the
coronary artery image from X-rays.
Figure A represents the cross-section of the
contrast-filled lumen of an idealized coronary artery.
Figure B is a plot of the idealized X-ray absorption across
the lumen or the density of the X-ray image of the lumen (they
are approximately the same thing). The density is depicted both
by the height of the curve and by the darkness of the area
under the curve.
The absorption of X-rays is exponentially proportional to
the thickness of the absorbing material. As a result, the
center of the lumen absorbs very many more X-rays than the
edges. In fact, the very edge of the lumen absorbs no X-rays
at all! From this we can draw a somewhat startling conclusion:
we can never see the true edges of a coronary artery
lumen in an X-ray image!
In the real-world, the formation of X-ray images is affected
by several factors other than just the absorption of the
X-rays.
One of the complicating factors is blur introduced by the
finite focal-spot of the X-ray tube and the optics of the
image intensifier and camera. This is depicted in Figure C by
the spreading of the base of the density curve.
The other complicating factor is "noise" in the
image. Noise is caused by many things; among them are quantum
mottle, intervening structures in the body, the X-ray detector
and the image viewing device. The level of image noise is
depicted by the dotted area at the bottom of Figure C.
A structure is only visible when its density rises both
above the noise and above the perceptual threshold of the
viewer. The perceptual threshold is affected by such factors
as the settings of the viewing device (e.g., brightness) and
the visual acuity of the viewer. The perceptual threshold is
depicted by the horizontal gray line in Figure C. Only
the portion of the density curve above the horizontal gray
line is visible to the viewer.
The true diameter of the lumen is labeled D0 and is
depicted by the red lines drawn through figures A, B and C. As
you can see in Figure C, the perceived diameter of the lumen
is somewhat smaller than the true diameter and will vary as
the noise level and perceptual threshold go up and down.
Probably the most important benefit of digital detection of
the lumen edges is the reduction of the effects of noise and
unpredictable perceptual threshold on the measured size of the
lumen, resulting in consistent and predictable measurements.