QUANTUM NOISE (CR Image Grain)
Quantum noise appears as
grain on CR images. Quantum noise is a direct result of the exposure factors
used and scatter radiation (Compton Effect). There is a distinct difference
between film-screen and CR radiographic receptors with respect to quantum
noise. Noise level in film-screen radiography is determined by the design
characteristics of the intensifying screens and film used (speed). When using a
film-screen receptor the exposure must be set to match the sensitivity of the
receptor or the results will be either and underexposed (light film) or
overexposed (dark film) image. The noise level can only be changed by
changing the receptor, typically by changing the film to one with a different
sensitivity (speed).
CR radiographic receptors
do not have a fixed sensitivity like film-screen receptors. One of the
valuable characteristics of CR is a wide exposure
dynamic range as illustrated below. This means that images with
good contrast characteristics can be produced over a wide range of exposure
values. It is not like conventional radiography where any deviation from
the correct exposure results in under or over exposed films.
There are definite
advantages of this wide dynamic exposure range; exposure errors do not result
in a loss of contrast as with film. Also, the ability to capture the full range
of exposure (increased grayscale) from anatomy with large variations in density
such as the lumbar spine and chest produces an image with more diagnostic
information. When the full dynamic range is captured, digital processing is
used to enhance and optimize the contrast. This is the normal procedure
in CR imaging.
Excessive quantum noise
(image grain) is a potential factor in CR because it is possible to produce
images with improper exposures and/or high levels of scatter radiation that
will look adequate as far as contrast (window and level) is concerned, but will
be quite grainy in appearance. This condition is illustrated in the phantom
lumbar spine/pelvis images below.
In CR radiography it is
important to use proper exposure factors and to collimate to the region of
interest (ROI) for each projection. Posterior lead masking for lateral
spines is especially beneficial to CR image quality. An optimum exposure is one
that produces an image with an acceptable noise level without excessive
exposure to the patient from the primary beam or scatter radiation.
Quantum Noise Examples
Exposure 1 (under exposed):
300 MA, 3/10 Sec 80 KV. Note the decrease in quantum noise (image grain) in the
image on the right from collimation and lead masking.
Exposure 2 (optimum exposure): 300 MA, 3/5 Sec, 85KV. Note the decrease in quantum noise (image grain) in the image on the right from collimation and lead masking.
Exposure 3 (over exposed): 200 MA, 2 Sec, 85 KV. Note the decrease in quantum noise (image grain) in the image on the right from collimation and lead masking.
Excerpted and edited from the
web-based edition of
The Physical Principles of Medical
Imaging, 2nd Ed. Perry
Sprawls, Ph.D.
The lumbar spine/pelvis phantom
images were taken at the iCRco factory QC department in Torrance.
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