Physics Case of the Day - Sunday
During routine QA testing of a radiographic room with an indirect flat panel digital
image receptor, the collimator was adjusted to cone the beam down to about 6cm x
6cm at the 35cm x 43cm image receptor. The x-ray tube anode/cathode axis runs
parallel to long axis of the detector. A lead plate was placed over the exit of the
collimator, and a 3mm hole in the plate was centered on the x-ray beam. An
exposure was taken, and the image below was recorded.
What is the cause of the light
stripe that extends above
and below the intense spot
at the center of the image?
Author: David M. Gauntt, Ph.D.
Page 1
Physics Case of the Day - Sunday
During routine QA testing of a radiographic room with an indirect flat panel digital
image receptor, the collimator was adjusted to cone the beam down to about 6cm x
6cm at the 35cm x 43cm image receptor. The x-ray tube anode/cathode axis runs
parallel to long axis of the detector. A lead plate was placed over the exit of the
collimator, and a 3mm hole in the plate was centered on the x-ray beam. An
exposure was taken, and the image below was recorded.
The light band above and
below the intense spot is
caused by off-focus radiation
at the x-ray tube anode.
Page 2
Physics Case of the Day - Sunday
The anode of a typical
general radiographic tube
has a molybdenum body,
and a tungsten track onto
which high-energy electrons
are focused.
The deceleration of these
electrons when they strike
the tungsten at the focal spot
produces x-rays.
Page 3
Physics Case of the Day - Sunday
The pinhole in the lead plate
produces an image of the
focal spot on the image
receptor, similar to a pinhole
camera.
Page 4
Physics Case of the Day - Sunday
Some of these electrons will
scatter back from the
tungsten, and then return to
other parts of the anode.
Page 5
Physics Case of the Day - Sunday
These backscattered
electrons produce x-rays
when they land back on the
anode; these x-rays are
called off-focus radiation
because the originate away
from the focal spot.
The lead pinhole then
produces a faint x-ray image
of the entire anode at the
image receptor.
Page 6
Physics Case of the Day - Sunday
A precollimator close to the
focal spot stops most of the
off-focus radiation from
reaching the image receptor.
The outline of the
precollimator appears as a
shadow in the image of the
off-focus radiation.
Page 7
Physics Case of the Day - Sunday
The electrons that hit the
tungsten target track on the
anode produce more
electrons than those that hit
the molybdenum body of the
anode. This produces the
band above and below the
focal spot.
The image of the target track
ends at the edge of the
precollimator.
Page 8