[Cat 1] Describe how an AEC system operates in generic terms.
Principle of Operation
Automatic Exposure Control (AEC) is an x-ray exposure termination device which allows the operator to perform an exposure without having to manually set the duration of the exposure (s). It works on a basic principle of a feedback loop from the image receptor to the generator, which terminates the exposure when the receptor has received enough signal (e.g. optical density or SNR) to produce a diagnostic image quality.
Advantages
- Helps achieve consistent x-ray film densities
- More efficient
Disadvantages
AEC devices are susceptible to operator error;
- mispositioned anatomy
- incorrect AEC chamber selection
- Prosthetic devices (eg. total hip hardware) can cause the selected ionization chamber to overexpose the image receptor. Leading to increased dose and necessity for a repeated exposure.
Factors affecting AEC
| Factor | AEC |
| Increase mA | decreased duration of exposure |
| Increase kVp | decreased duration of exposure |
| Increase SID | increased duration of exposure |
| Increase density setting | increased duration of exposure |
| Decrease collimation | increased duration of exposure |
| Full field collimation | increases scatter which will be detected by the AEC as density |
Minimum response time – length of time necessary for the AEC to respond to ionisation and terminate the exposure
Backup time – maximum exposure time order to prevent overexposure or tube overload
Anatomically programmed radiography – Combines AEC with preprogrammed anatomical controls – the area of interest is selected, the system determines which AEC chamber to be used.
Radiography
In plain radiography an AEC system uses one or more thin radiation ionization chambers which is positioned between the patient and the x-ray film cassette.
In a simple AEC system a weak ionization signal from the AEC detector is integrated as a ramp shaped voltage waveform. This ramp signal rises until it matches a pre-set threshold. At this point the x-ray exposure is terminated.[4] AEC devices are calibrated to ensure that similar exams have linearity in optical density.
Mammography
As low energy x-rays are used such as in mammography the AEC detector is placed behind the image receptor to avoid creating a shadow.
Computed Tomography
Ues Tube Current (mA) Modulation
- CT AEC systems use the initial “scanogram/scout view”, a fixed angle planning view, to determine the relative size of the patient, and variation over their length.
- The tube output is then adjusted for overall size.
- The output is also typically modulated for each rotation in response to changes in attenuation over patient length. Some systems adjust output during each rotation, which is known as rotational modulation, based on measured attenuation in the previous rotation.
Fluoroscopy
Automatic BRIGHTNESS Control
- the output from fluoroscopy is LIGHT
- remember, the image is NOT terminated, as is occurring with AEC
- ABC continually adjusts factors (kvP and ma) to maintain a constant brightness or signal to noise ration, taing into account the contrast reqeuirments of the imaging and patient attenuation
Ionisation Chamber
An ionisation chamber is a gas-filled radiation detector which measures charge from the number of ion pairs created within gas due to ionising radiation.
An ionization chamber measures the charge from the number of ion pairs created within a gas caused by incident radiation.[nb 1] It consists of a gas-filled chamber with two electrodes; known as anode and cathode. The electrodes may be in the form of parallel plates (Parallel Plate Ionization Chambers: PPIC), or a cylinder arrangement with a coaxially located internal anode wire.
A voltage potential is applied between the electrodes to create an electric field in the fill gas. When gas between the electrodes is ionized by incident ionizing radiation, ion-pairs are created and the resultant positive ions and dissociated electrons move to the electrodes of the opposite polarity under the influence of the electric field. This generates an ionization current which is measured by an electrometer circuit. The electrometer must be capable of measuring the very small output current which is in the region of femtoamperes to picoamperes, depending on the chamber design, radiation dose and applied voltage.
gas caused by incident radiation. It consists of a gas-filled chamber with an anode and cathode.
When gas between the electrodes is ionised by incident ionising radiation, ion-pairs are created and the resultant positive ions and dissociated electrons move to the electrode of it’s opposite polarity, generating an ionisation current which is measured by an electrometer circuit. The electrometer must be capable of measuring the very small output current which is in the region of femtoamperes to picoamperes, depending on the chamber design, radiation dose and applied voltage.