building your practice with imaging
can’t just say “oh, well” and take another image when it
doesn’t turn out the way you’d like.
To ensure that your images always are of optimal quality,
remember these three key points: positioning, technique
and collimation. When these three conditions are satisfied,
you’ll have a beautiful image every time, whether it’s through
film, computed radiography (CR) or digital radiography (DR).
These are the basics of image capture and they work in
conjunction with each other. So let’s look at each of these in
more detail.
Positioning
is the most important of the three. It’s
estimated that 85% of all repeat images are due to positioning
errors. Most modern X-ray systems come equipped with
automatic exposure controls, also known as AECs or AEDs. It’s
also sometimes referred to as phototiming. These devices are
ionization chambers that measure a preset quantity of
radiation and break the timing circuit when a dose sufficient to
produce the desired film density has been reached. When
using an AEC, it’s critical that the location of the ionization
chamber be determined and the precise positioning of tissue
over that location be achieved. Automatic exposure devices
provide a diagnostic quality density only for structures
positioned directly above the ionization chambers.1
The function of the AEC is to eliminate the need for
radiographers to set an exposure time. However, they must still
manually set the mA and kVp. Failure to position properly over
the AEC has a big impact on exposure technique, which we’ll
discuss next. And failure to position correctly over the AEC will
cause the exposure to terminate prematurely, producing an
image that’s underexposed, with poor image quality.
Technique,
for the purposes of this article, refers to the
exposure factors of kVp, mA and mAs. Think of technique as
light in a film camera. If you take a picture in low light, there’s
not enough light to expose the film and the image is
“85% of all
repeat images
are due to
positioning
errors.”
underexposed. If exposed to bright light, the image is overexposed. The same is true in film radiography. If the exposure
techniques are too low, the film is underexposed, and if they’re
too high, the film will be overexposed.
But keep in mind that this not necessarily the case with digital
imaging such as CR and DR. In digital imaging, the film density
will remain the same, regardless of the exposure techniques,
but the amount of image noise will increase or decrease,
depending on the exposure techniques. Image noise presents
with a mottled appearance on the film, like it was sprinkled
with salt and pepper. This mottling, caused by underexposure
of the image, decreases the amount of detail that can be seen
in an image and may result in a misdiagnosis. An overexposed
image will have less mottling but result in unnecessary
radiation exposure to the patient. Too high an exposure may
also cause saturation of the receptor. In this case, image
information will be lost a retake will be required – with
additional radiation exposure to the patient.
Collimation