With all of the new curriculum material being added to the Registry in the coming years, it is required for us to have good working knowledge of how the photostimulable phosphor plates work with CR cassettes. Most curriculums are encorporating this into study now, and I won't bother going into those details, but I would like to post about an observation I made when reading up on the topic and preparing for some lab sessions with my current students.
According to the Carlton and Addler textbook that our students are required to purchase, the active layer is the photostimulable phosphor layer, which is responsible for retaining a latent image after x-ray exposure. This happens when x-rays ionize the plate, releasing electrons to be collected by an "electron trap" in the conductive layer of the phosphor screen.
This image, taken from the FUJI website (an excellent source for information by the way), depicts the electrons collected in the trap, and representing a stored charge that is now our latent image.
During development of the cassette, a helium-neon laser (a really intense light source), sweeps the cassette during the forward motion of plate travel through the reader, and that charge is released in the form of light photons (following picture also from FUJI).
As you can see, there is a "light guide" which collects the visible light emitted from the plate and transfers it to a photomultiplier tube, which amplifies the light signal. Later it goes to an analog to digital converter to be interpreted into an electrical signal that the monitor can display as our radiograph at our QC station, and so on and so forth.
If you have ever opened one of the CR cassettes, even after a radiographic exposure has been made on it, you might notice that the room light does not fog the image plate. So knowing that an intense light (helium-neon laser) is used to release the charge on the plate during image processing, I wondered if I could reproduce that effect outside of the image processor myself.
Here's what I decided to do... I received two laser pens for my birthday recently (green in luminance) that my wife and I jokingly use to point out things when we watch TV, i.e., physical flaws possessed by contestants on the television show "The Bachelorette," crooked nostrils, stains on clothing, you get the picture (way beside the point, but fun).
I wondered if the laser pens were intense enough to cause a change in the radiographic density on the image plate.
First, I took an unexposed plate and removed it from the cassette and turned out the room lights. I thought "it's photostimulable, so what would happen if I simply shine my laser light on it? Would it emit a visible light? Would it produce a density on my developed image?" Uh... no. After trying this in an unexposed image plate, there was no visible light produced except what was caused by my own laser pen, and no visible density was recorded on the developed image after processing.
So next, I tried a relatively small exposure (about a finger technique) on a cassette, then tried removing the image plate from the cassette to see if I could observe visible light when shining my pen on it. I was not able to perceive visible light when I tried this... why? Probably because the light would be very dim, and the brightness of my laser pen interfered with any that I might have been able to see. There is, after all, a photomultiplier tube which enhances the light emitted by the screen inside the reader. However, when I developed the image, this was the result:
It appears as if my laser light, in fact did have enough intensity to release the trapped electrons from their place in the conductive band. We have an area of decreased density on the image where the my laser pen interacted with the plate and released the electrons. The reader was not able to detect any light emitted from these regions with the helium-neon laser because the electrons had already been released, and no density was provided in that area.
Does this help us in our formation of the image? Probably not, but it does help to understand the process of turning our latent image into the manifest image using CR. I highly reccommend purchasing a laser-pen, even if you never use it for radiography-related purposes, it's still good fun for television. A special shout to my wife - thanks for my laser-pens!
According to the Carlton and Addler textbook that our students are required to purchase, the active layer is the photostimulable phosphor layer, which is responsible for retaining a latent image after x-ray exposure. This happens when x-rays ionize the plate, releasing electrons to be collected by an "electron trap" in the conductive layer of the phosphor screen.
This image, taken from the FUJI website (an excellent source for information by the way), depicts the electrons collected in the trap, and representing a stored charge that is now our latent image.
During development of the cassette, a helium-neon laser (a really intense light source), sweeps the cassette during the forward motion of plate travel through the reader, and that charge is released in the form of light photons (following picture also from FUJI).
As you can see, there is a "light guide" which collects the visible light emitted from the plate and transfers it to a photomultiplier tube, which amplifies the light signal. Later it goes to an analog to digital converter to be interpreted into an electrical signal that the monitor can display as our radiograph at our QC station, and so on and so forth.
If you have ever opened one of the CR cassettes, even after a radiographic exposure has been made on it, you might notice that the room light does not fog the image plate. So knowing that an intense light (helium-neon laser) is used to release the charge on the plate during image processing, I wondered if I could reproduce that effect outside of the image processor myself.
Here's what I decided to do... I received two laser pens for my birthday recently (green in luminance) that my wife and I jokingly use to point out things when we watch TV, i.e., physical flaws possessed by contestants on the television show "The Bachelorette," crooked nostrils, stains on clothing, you get the picture (way beside the point, but fun).
I wondered if the laser pens were intense enough to cause a change in the radiographic density on the image plate.
First, I took an unexposed plate and removed it from the cassette and turned out the room lights. I thought "it's photostimulable, so what would happen if I simply shine my laser light on it? Would it emit a visible light? Would it produce a density on my developed image?" Uh... no. After trying this in an unexposed image plate, there was no visible light produced except what was caused by my own laser pen, and no visible density was recorded on the developed image after processing.
So next, I tried a relatively small exposure (about a finger technique) on a cassette, then tried removing the image plate from the cassette to see if I could observe visible light when shining my pen on it. I was not able to perceive visible light when I tried this... why? Probably because the light would be very dim, and the brightness of my laser pen interfered with any that I might have been able to see. There is, after all, a photomultiplier tube which enhances the light emitted by the screen inside the reader. However, when I developed the image, this was the result:
It appears as if my laser light, in fact did have enough intensity to release the trapped electrons from their place in the conductive band. We have an area of decreased density on the image where the my laser pen interacted with the plate and released the electrons. The reader was not able to detect any light emitted from these regions with the helium-neon laser because the electrons had already been released, and no density was provided in that area.
Does this help us in our formation of the image? Probably not, but it does help to understand the process of turning our latent image into the manifest image using CR. I highly reccommend purchasing a laser-pen, even if you never use it for radiography-related purposes, it's still good fun for television. A special shout to my wife - thanks for my laser-pens!
You are correct in that the light emitted is faint, it is also blue, at least it is for Agfa CR plates.
ReplyDeleteIf you can capture the plate before it is erased by the reader, the bright white light after the read cycle, you can read the plate again and get the same image again. Reduced in quality and likely not diagnostic. The same image will creap back to the surface and produce a faint artifact if the plate is not used for several days.
You are correct in that the light emitted is faint, it is also blue, at least it is for Agfa CR plates.
ReplyDeleteIf you can capture the plate before it is erased by the reader, the bright white light after the read cycle, you can read the plate again and get the same image again. Reduced in quality and likely not diagnostic. The same image will creap back to the surface and produce a faint artifact if the plate is not used for several days.