[ RadSafe ] Imaging Technologies- stimulus for discussion. Corrections/additions welcomed.
GEOelectronics at netscape.com
Thu Dec 2 13:51:16 CST 2010
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 flash.
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 source
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
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