value that is appropriate for the overall patient thickness and that achieves the desired image
noise/quality criterion.
2.4.3 When to use AEC
AEC technology has the greatest impact when the portion of the patient being scanned has non-
uniform size, shape, or density. In these cases, AEC adjusts scanner radiation output to the
changing anatomy and modulates the mA in the Z-direction (along the patient) and/or in the XY-
direction (around the patient). Even though AEC is used, before scanning the operator must still
select scan parameters, including AEC parameters, which provide a desired image noise/quality
criterion. Scan parameters including AEC parameters must be chosen to carefully balance
patient radiation dose and image performance.
Even when the patient’s anatomy has consistent size, shape, and density throughout the
planned scan range, AEC technology chooses the appropriate exposure settings to achieve the
image noise/quality criterion requested by the user.
When bismuth or other shields are considered for use in the planned scanned range, consult
the system user manual for specific information.
2.4.4 When not to use AEC
AEC might not be available for all scanning modes or on all scanners. When AEC is available, if
users do not understand the relationship between AEC parameters, image noise, and dose,
AEC should not be used. Also, if the patient cannot be centered in the scanner, AEC is not
recommended because the attenuation calculations used for AEC are designed with the
assumption that the patient is centered in the gantry. Finally, if there is any question, radiologic
technologists should always consult their medical physicist and radiologist to ensure that proper
exposure techniques are used.
2.4.5 AEC does not guarantee reduction of radiation doses in all patients
The use of AEC does not always result in dose reduction, especially when compared to a fixed
mA/mAs protocol. For example, when providing the desired image noise/quality criterion setting
for a large patient, AEC might appropriately increase the scanner radiation output as compared
to that for an average-sized patient. For most examinations of average-sized or small patients,
and for the same image noise/quality criterion settings, AEC use will result in the same or lower
CTDI
vol
as that of a fixed mA/mAs protocol. (However, a larger patient would appropriately
require more fixed mA than for a smaller patient.)
NOTE:
Radiologic technologists must be fully aware that proper patient centering is critical for
accurate AEC function. Improper patient centering can result in an exposure that is
either too high or too low to achieve the desired image noise/quality criterion. Note
that proper patient centering can be more challenging for smaller pediatric patients,
and so special care should be taken.
2.4.6 Effect of AEC control setting
For a given patient, changing the image noise/quality criterion setting in AEC will affect the
patient dose: asking for lower image noise/higher image quality criterion will result in more dose
to the patient as the Noise Index value is decreased (made smaller). In contrast, asking for
higher image noise/lower image quality criterion by increasing (make larger) the Noise Index
value will result in less dose to the patient.
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2 Scan Theory
