DIGITAL FLUOROSCOPY
The introduction of digital fluoroscopy, where a computer has been added to traditional fluoroscopic
equipment, has provided many advantages.
A “pulsed-progressive mode” has been added where the x-ray tube, underneath the table,
can operate at an mA measured in the hundreds like traditional x-ray overhead tubes, but
using extremely short bursts of radiation with cooling periods between them.
This is like an extreme version of pulsed fluoro mode, but it is not dependent upon
the incoming current frequency to the x-ray generator.
Compared to pulsed mode at 30 frames per second, the mA is typically increased to produce the
appearance of a continuous radiation signal.
In this mode, the dose administered between the frames is wasted, and the x-ray tube experiences
high levels of heat production, reducing the life of the tube.
With digital fluoroscopy, individual image frames allow image subtraction techniques
to be used which can be extremely advantageous for exams using intravenous contrast media
injection.
The exposure time for each pulse, called the pulse width, can be adjusted from 3 to 10
milliseconds.
Combining this feature with a standard mA station, the dose can be significantly reduced.
Newer units have the ability of combining this mode with high mA and spectral filters
made of a copper or a copper/aluminum alloy to produce higher image quality while limiting
patient exposure.
mA stations as high as 200 or 300 mA are possible, creating increased image quality, but also
with the potential to produce some excessive patient exposure.
Image intensifier tubes are being replaced by flat-panel digital detectors capable of
recording live images.
There are two types of flat panel detectors available today.
Indirect detectors utilize a cesium iodide phosphor coupled to an active matrics array
of amorophous silicon thin film transistors.
These require an additional conversion of energy compared to the other type of flat
panel detector available, the direct detector.
Direct detectors use amorphous selenium thin film transistors.
Both are capable of providing a dynamic view under fluoroscopy, and they are not to be
confused with the flat panel detectors utilized in plain film, or static radiography outside
of fluoro.
The FPD used in fluoro has larger matrix sizes, up to 2048 x 2048 with pixel sizes of 200-300
microns, compared to digital radiography detectors which are limited to 100-150 microns.
There are dual-use systems available which can perform both static images and dynamic,
with higher resolution capabilities in static mode.
Digital Flat Panel Detectors also have a much higher signal-to-noise ratio than image intensifiers.
A post-processing technique called temporal frame averaging allows for a reduction of
noise by as much as 44% according to your textbook.
This process averages each pixel value for one or two frames prior to, and one or two
frames following a specific captured frame.
Another advantage for DFPs are contrast enhancement capabilities, high DQE (detector quantum efficiency)
and improved dynamic range of the detectors.
In addition, they’re not sensitive to magnetic fields, and they have a linear response that
can be manipulated like static digital detectors are able to for post-processing techniques.
All of these features are causing the industry to quickly replace traditional image intensification
systems.