Pulsed Fluoro & ABCs

Automatic Brightness Control (ABC)

circuit alters the kVp, mA or both according to changes in x-ray beam attenuation by the anatomic part due to patient position, thickness atomic number, or density

• the radiologist may select the brightness level desired, and the ABC will maintain it throughout the procedure

ABC

Automatic Brightness Control

AERC

Automatic Exposure Rate Control

ABC & AERC =

2 different names for the process of Automatic Stabilization of Brightness

example - as the patient rolls onto side, the increased tissue thickness must be compensated for to maintain fluoroscopic image brightness 

Two general approaches are used….

Automatic Gain Control (AGC)

Automatic Brightness Stabilization (ABS) 

Automatic Gain Control (AGC)

after the image is acquired, amplification of the electronic signal (current) is increased or decreased as needed by the circuits.

• if an insufficient number of x-ray photons are reaching the image intensifier, no amount of increased electronic amplification can overcome the mottle or noise that is produced 

Automatic Brightness Stabilization (ABS) 

more common: actual radiographic technique factors are adjusted to compensate for fluctuations in the electronic signal, the incident x-ray beam, or the measured light output.

ABS Circuits

1. Variable mA, Preset kVp:

the kVp is set by the operator and does not vary. The circuit automatically varies the mA up and down as needed to maintain image brightness

ABS Circuits

2. Variable mA with kVp following : 

The circuit automatically varies the mA up and down as needed to maintain image brightness, but as an addition circuit measures if an upper boundary set for the mA is reached - When this limit is reached, the system switches to a motor-driven variable transformer that varies the kVp up and down 

ABS Circuits

3. Variable kVp with selected mA: 

mA is set by the operator and does not vary. A motor - driven variable transformer varies the kVp up and down as needed to maintain image brightness. (high mA can be set to force lower kVp’s for iodine-based studies, low mA for GI work) 

ABS Circuits

4. Variable kVp, Variable mA: 

Both kVp and mA are varied up and down as needed to maintain constant image contrast or suppress noise. There is no operator control.

Fluoroscopic Beam-On Time 

The operator must be made aware of accumulated beam - on time by a manual reset, 5-minute timer.

Intermittent Fluoroscopy 

A series of short visual checks, rather than continuous fluoroscopy, dramatically reduces cumulative exposure to the patient 

• this is the most important way to limit patient exposure during fluoroscopic studies 

• Controlled by the radiologist

Last Image Hold Devices

Keeping the last image displayed on the monitor, has been shown to reduce cumulative patient exposure as much as 50% - 80%

Pulsed Fluoroscopy

Pulsing of the x-ray beam spares the patient radiation exposure between frames. Shorter pulse times can also be selected, further reducing dose

Used correctly - it doesnt harm image quality 

Lower frame rates than realtime, can be used in conjunction with digital memory to provide a continuous-looking image 

• should NEVER be above 20 fps because the human eye cant detect greater than

Frames per second 

Pulsed Fluoroscopy Technique

The mA must be increased from about 2 mA to 20 mA or more

• Conventional Fluoroscopy typically 0.5 - 5mA were used to reduce pateint dose and keep tube heat in control

• Digital systems are higher mA, so dose in controlled with a pulsed, intermittent exposure sequence 

  • But because it isn’t continuous it is actually less dose to the patient 

Tube heat capacities are reduced

Parts of the Pulse Waveform

Pulse Height

Pulse width 

Pulse interval 

Pulse Height

height created by the mA setting (intensity)

• Increased as more photon flux is needed 

Pulse width

how long the x-ray is on for

aka the “duty cycle”

Does not affect flux gain

Shorter widths = 10ms or less

• less blurry images, less dose, less tube heat

Pulse Interval

How long the x-ray is off for

Short “ones” allow fast moving anatomy to be better viewed

If long “ones” are used, use stationary imaging

Improve the fluoro blurred image by utilizing …

Fast moving anatomy may need these adjustments

• Short pulse widths 

• Short pulse intervals 

• Fast frame rates 

Pulse Frequency or Puse Rate

The lower the pulse number the less the patient dose

An increased pulse rate will improve temporal resolution (improve blurring or “smearing” of the image)

• But increase patient dose

Typical rates …

• 3.75

• 7.5

• 15

• 30 - no dose reduction 

• continuous - should not be used