Automatic Exposure Control; How Aec Works; Adaptation To Anatomy - GE Revolution CT User Manual

2.4 Automatic Exposure Control

Patients come in all shapes and sizes. For the purpose of achieving a desirable image quality
with a scan technique that reflects the patient's size and shape, there are several approaches to
employing automatic and manual mA setting modes of CT operation. These approaches are
designed to adjust the X-ray output of the system according to the X-ray attenuation presented
by a patient's anatomy. For example, the patient's weight or Body Mass Index (BMI) may be
used as a guide to set a fixed mAs for the acquisition. Alternatively, some measure of patient
thickness or girth, such as anterior-posterior (AP) thickness, lateral width, or patient
circumference can be used as a basis to choose an appropriate fixed mAs value, i.e., a value
that yields an image adequate for diagnosis with a patient dose as low as reasonably
achievable. However, these methods have at least two inherent limitations. First, as they
produce a fixed mAs value, they do not adjust for differences in body-region thickness and
associated variation in X-ray attenuation along the patient length and/or around the patient
circumference. Second, the use of weight, thickness or circumference is an incomplete
surrogate for X-ray attenuation, which is one of the most relevant physical parameters affecting
image quality and which depends on the elemental composition and density of human tissue as
well as on its shape and thickness.
Automatic Exposure Control (AEC), on the other hand, is designed to adjust the scanner
radiation output to meet a desired, pre-set level of image quality/noise criterion by empirically
assessing the patient's attenuation and automatically modulating the mA accordingly. AEC can
provide a desired level of image quality/noise at a lower patient dose than would be possible
with a fixed scanner radiation output. In general, CT systems may accomplish AEC in two ways:
1. Modulating the mA dynamically during scanning in the X-Y and/or Z dimensions to adapt to
variations in the patient's attenuation.
2. Adjusting the mAs to a fixed value based on measurement and calculation of the patient's
overall attenuation: the mAs is constant during scanning, but its value has been
quantitatively determined so as to yield an average pre-set level of image noise.
Most AEC systems operate as described in list item #1 above. Discussion of AEC, hereafter,
applies to these types of systems unless otherwise indicated.

2.4.1 How AEC works

On the basis of a patient's attenuation, AEC sets mA values as the X-ray tube rotates around
the patient. The technology uses knowledge about the scanner's imaging chain and the
measured attenuation of the patient to appropriately adjust mA values in order to achieve the
desired, constant image noise/quality criterion.
Larger patients typically require scanning at a higher mAs than the mAs used for smaller
patients. Similarly, thicker projections (e.g., laterally through the shoulders versus AP through
the shoulders) typically require more mAs to achieve the same resultant image noise/quality
criterion. Finally, anatomy with greater attenuation (e.g., abdomen or pelvis compared to the
lungs) requires more mAs to achieve the same image noise/quality criterion.

2.4.2 Adaptation to anatomy

As patient attenuation changes throughout the course of the scan, either rotationally around the
patient or along the length of the patient, AEC is designed to adjust dynamically the mA for each
body part and projection. If the attenuation does not change, AEC sets the mA at a constant
Chapter 11 Scan
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