How Automatic Exposure Rate Control Works in Fluoroscopic Systems

What the automatic control loop actually does

During fluoroscopy the detector or image receptor reports the signal level in a small measuring field. The control system compares it with a target value and adjusts the exposure parameters until the target is met. On older image-intensifier systems this was called automatic brightness control (ABC); on digital flat-panel systems the same principle is usually described as automatic exposure rate control (AERC) or automatic dose rate control.

The loop works on a few levers at once: tube voltage (kV), tube current (mA), pulse width in pulsed mode, and the choice of added copper or aluminium filtration. How these are traded against each other is called the dose-rate curve or the technique curve, and it is one of the most important differences between systems.

Technique curves and high level control

A technique curve defines which combination of kV and mA is used as patient thickness grows. A curve that favours higher kV lowers skin dose and tube load but reduces contrast, which is acceptable in abdominal work and less so when iodinated contrast or small structures must be seen. A curve that favours lower kV gives more contrast at the cost of more dose.

High level control (HLC), sometimes called boost, is a separate operator-selected mode that allows a higher dose rate for a limited time when a difficult anatomical region needs better image quality. Regulatory and standards bodies generally expect the dose-rate limit of this mode to be higher than normal, to need a deliberate action and to give an audible or visible indication, so ask your supplier how HLC is implemented and limited.

• Normal mode: standard dose-rate curve for routine work
• Low-dose mode: reduced pulse rate and lower target signal for simple tracking tasks
• High level control: temporary higher dose rate with a clear indicator

Fluoroscopy controls and the control panel

The fluoroscopy control panel should let the operator choose pulse rate, dose level, magnification and region of interest without searching through menus, and show the current dose-area product (DAP) and cumulative air kerma in real time. The foot switch must give a dead-man behaviour, so that radiation stops the moment the pedal is released.

Several modern R/F tables provide touchscreen consoles with anatomical presets. For example, Villa's Chronos has a 12 inch touchscreen console, a touchscreen collimator with joysticks, virtual collimation and real-time DAP monitoring, which supports dose awareness during the examination. Apollo DRF from the same manufacturer handles continuous and pulsed fluoroscopy on a 43 × 43 cm dynamic detector and recognises the projection type to set collimation, filtration and grid automatically.

Pulsed fluoroscopy and its relation to dose

In pulsed fluoroscopy the beam is switched on in short pulses rather than continuously, and a lower pulse rate usually means a lower dose per second of screening time. The operator should be able to reduce the rate for slow-moving anatomy and raise it for fast motion such as cardiac or paediatric studies. Image processing, including temporal filtering (see flat panel detectors in fluoroscopy), compensates for the noise of a lower dose.

Dose-reducing options are not additive in a simple way. Reducing the field with collimation, using last-image hold instead of continued screening, and removing the grid for small patients all matter as much as the dose-rate curve itself. A system with automatic controls is only as good as the habits and training of the people using it.

Questions to ask a supplier before you buy

Ask for the dose-rate curve settings available per examination type, the maximum air kerma rate for normal and high level modes, and the way the system reports DAP. Ask how acceptance tests are done in accordance with the relevant parts of IEC 60601-1 and IEC 60601-2-54, and how the curves can be adjusted after installation by a service engineer and a medical physicist.

Also check that the exposure data are exported to your PACS or dose-management system through DICOM, and that your physicist is involved in the commissioning stage. For R/F rooms in the UAE, confirm the current radiation-protection licensing requirements of the authorities that apply to your facility. When you are ready to compare configurations, request a consultation from Beckfinity's specialists.