Question

How are the latent and manifest images made in computed radiography?

Answer 1

In computed radiography a cassette of phosphor screen is used. Then the x-rays hit they form a latent image in the phosphor screen. The cassette is then placed into a reader with a laser shone on to it which releases the stored photons, collects the signal and digitises it to be displayed on a display screen.

Formation of Latent image:

X-ray photons are absorbed into a phosphor crystal giving rise to a high energy photoelectron. This ionises a large number of atoms along its track releasing thousands of electrons (one x-ray photon absorbed gives rise to over 100 trapped electrons). The electrons become temporarily trapped at specific sites throughout the layer of phosphor crystals producing the latent image.

Formation of Manifest Image:

After exposure, the imaging plate is placed into the computer radiography reader, where the photo stimulated phosphor screen is automatically removed. The latent image on the photo stimulated phosphor screen is changed to a manifest image as it is moved at a constant speed and scanned by a narrow high-intensity helium-neon or solid state laser to obtain the pixel data. The laser causes excitation of the electrons storing the latent image resulting in the electrons to emit a light equal to the intensity of the latent image. The challenge associated with exciting an electron is to making sure that the (wavelength) light given off by the excited electron is different from that laser light (wavelength) used to excite it.