[ RadSafe ] Imaging Technologies- stimulus for discussion. Corrections/additions welcomed.
Dixon, John E. (CDC/ONDIEH/NCEH)
gyf7 at cdc.gov
Thu Dec 2 14:49:08 CST 2010
Do these airport scanners "scan" in the same way as the port cargo
scanner (CAARs - NCRP Commentary 20)?
From: radsafe-bounces at health.phys.iit.edu
[mailto:radsafe-bounces at health.phys.iit.edu] On Behalf Of Geo>K0FF
Sent: Thursday, December 02, 2010 2:51 PM
Subject: [ RadSafe ] Imaging Technologies- stimulus for discussion.
In this method the entire picture is imaged in the same instant.
Flash photography is the best everyday example of this method. Inside
the camera the film is exposed to the image all at one time, with the
total amount of exposure determined by the intensity and duration of the
Conventional X-Ray machines operate using this principle. On one side of
the subject is the X-Ray source which when activated transilluminates
the entire subject at once. On the other side of the subject is the
sensor (X-Ray film) which receives the full image in one burst. X-Ray
sources can be pulsed or continuous, the film averages out the entire
total energy into a single exposure.
Dosimetry. The X-Ray field is homogenous across the entire field.
Measuring methodology would have to take into account the frequency
issue if a pulsed beam is used, and the time duration in any case. Pulse
dependant detection technology (i.e. GM tube) is insufficient to
measure this type flux as it will produce only one "click" per episode
no matter how many actual pulses are present and independent of their
intensity. TLDs, film badges, Ion-chambers, PICs etc would certainly be
appropriate as the beam is large and all encompassing.
One small spot is scanned horizontally from one edge to the other, at
the end of each line being quickly retracted to the starting point,
incremented down by one line and repeat the horizontal scan. This is
repeated until the spot reaches the farthest corner of the image from
where it started where it is quickly retracted back to the starting
position. The resulting image is called a "frame". On the next pass, the
spot may be directed to start at a position in- between the first two
lines of the previous frame. In this type system called "interlaced" the
resolution is approximately doubled as the second scan fills in-between
the first scan, producing an overall image called a "Picture". The
"Picture' is made up of thousands of individual spots we call "Pixels".
CRT displays, TV sets and TV cameras work on this principle, in the USA
we see 60 frames and 30 full pictures each second.
The X-Ray analogy to the "Flying Spot" is the airport backscatter
machine. A "pencil beam" ( 28 keV is mentioned in literature, no
mention found of the mA, spot size, speed) is scanned across the body
from one side to the other, incremented down one line and the process
repeated ( per info on patent)..
There is really no need to go all the way back to the starting side
since the subsequent line can be scanned in a backwards configuration
and turned around in software. Since the X-Ray beam bounces off rather
than penetrate the body, two scanners are implemented, one on the front
side and one on the back side, providing continuous coverage.
Interlacing in my opinion would not be desired or required.
Dosimetry: This to me is problematic. I can see where TLDs; film badges
etc would be insufficient because the beam is small and moving. In
addition, in the case of equipment failure, the spot might be located in
one position for an extended period of time. Naturally if it were to
stall anywhere except right over the TLD, the dose would not be recorded
properly Similarly how would one measure the flux with instrumentation
even in a properly operating machine, how does the total energy
deposited over the whole surface relate to a single reading taken with a
relatively small detector ( i.e. ion-chamber)?
Similar in concept but different in detail compared to the "Flying
Spot". In the "Line Scanner", a whole line at a time is imaged. The next
line is sequentially scanned all at once and so on until a full picture
is imaged. Copy machines and FAX machines work on this principle. In the
case of a copy machine, the paper to be copied stays still and the light
is moved. In a FAX machine, the paper to be copied moves across a fixed
line of sensors.
Airport carry-on luggage scanners work using this principle. Inside is a
beam of X-Rays forming a line the full width of the conveyor belt.
Physical movement of the conveyor belt provides the "scan" into the line
of fixed sensors. It would be quite possible to build the "line" up from
a moving X-Ray beam, the principle remains the same.
New London Nucleonics Lab
GEOelectronics at netscape.com
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