Biomedical Imaging - Principles of Biomedical Engineering - page 319

(but adjacent to) a phosphor screen in the same way that light exposes an ordinary
photograph. In this case, the regions of high X-ray absorption leave the film unex-
posed and they appear white on the film; that is, it is a negative image (bright areas
correspond to high attenuation). Ribs and the vertebral column appear white while
the lungs with little attenuation appear black. In the film, photons are absorbed in
the silver halide (e.g., AgBr) crystals, generating very small amounts of free silver.

AgBr +→ + hf Ag Br

To increase sensitivity and thus lower radiation dose, the photosensitive film
emulsions are made thicker and occasionally coated on both sides of the film, in
contrast to normal photographic film. The X-ray image itself is formed by tiny
black silver particles, just as in a normal black and white photographic negative.
During film processing, any grain with small amounts of free silver are completely
converted to metallic, nontransparent silver, while the remaining unreduced silver
halide is removed by the fixative.

2Ag + 2HCl → + 2AgCl H

2

After exposing AgCl to metallic Ag dark spots appear (negative image). Ideally,
the intensity of the image should be logarithmic with incident X-ray intensity such
that it measures the summed attenuation along a line. Improved efficiency is ob-
tained by photographing the light using a fluorescent screen made of luminescent
ceramics such as terbium-doped gadolinium oxisulfide (Gd 2 O 2 S:Tb). The screen is
incorporated in a light-tight cassette; the film is loaded into the cassette in a dark
room and the cassette is then positioned behind the object or body part to be im-
aged. The silver content of the film makes X-ray films rather expensive.
Any film has a specific range of optimal sensitivity (exposure range from com-
plete white to complete black). Although equipment is normally assisted by elec-
tronic exposure meters, the correct choice of film, exposure time, exposure cur-
rent, and high voltage is decided by the judgment of the technician. To improve
the sensitivity and thus lower radiation exposure to the patient, the film is often
brought in contact with a sheet of intensifying screen. The screen contains chemi-
cal compounds of the rare Earth elements, which emit visible blue-green light when
struck by X-rays or other ionizing radiation. This permits the use of photographic
film with thinner emulsions and more normal sensitivity to visible light. While
increasing the sensitivity, the use of the intensifying screen blurs the images as the
registration of X-ray radiation becomes an indirect process. The patients or the ob-
ject is not only the source of X-ray absorption but also of X-ray scattering, mainly
due to Compton scattering. The scattered radiation carries no direct information
about the object and thus only reduces the quality of the image. An antiscatter grid,
which suppresses radiation from directions deviating from the direct connecting
line between the X-ray source and detector element, is necessary in most cases to
suppress scattered radiation and thus improve the quality of the image.
Another subsystem in the acquisition typically includes a signal processor,
which amplifies and filters the output. Signals are amplified using photomultiplier
tubes (PMTs), also called phototubes in the past. PMTs are light detectors with