Understanding the Contrast Between II and FPD Images

 In a recent article, we were able to show the benefits of fluoroscopy in the veterinary field. Now, we can highlight in a bit more detail, the differences between the two methods of obtaining fluoroscopy images and explain when each one may be appropriate. 

Just like standard radiography where there are two methods for image capture - computed radiography (CR) or direct digital radiography (DDR) - in the field of fluoroscopy, there are also two methods - via an Image Intensifier (II) or a Flat Panel Detector (FPD).

For ease of understanding and by relating to familiar X-ray technologies, similarities can also be drawn between the standard X-ray capture options and the fluoroscopy image capture options with both the CR and II methods relying on an intermediate system between the point of image capture, and the viewing of the image, but the DDR and FPD methods connecting directly to a viewing computer. 

How the image is obtained

Image intensified images: 

The X-ray image intensifier is a unit that utilizes a series of photochemical reactions to achieve a viewable image.

At the front of the unit, exposed to the X-ray beam, sits an input phosphor (a solid material that emits light when exposed to radiation – the same technique as in an old-style intensifying screen in a film-based or CR X-ray setup). Immediately adjacent to this layer is a photocathode.

When the radiation beam strikes the input phosphor, causing visible light to be released, this visible light in turn strikes the photocathode. The photocathode releases electrons in direct proportion to the visible light, which is directed through the tube by electron lenses onto an output phosphor.

The output phosphor emits light the same way as the input phosphor does, so the image is obtained. This image is only around 1” in diameter, so to be diagnostic it needs to be magnified by attaching a video camera and then digitized in a computer. 

Flat Panel Detector images: 

In the FPD system, the X-ray image intensifier unit and video camera are replaced by a single digital assembly where the X-ray photons are converted directly to electrical charge and displayed on a monitor. 

Benefits of each system

Image intensified images: 

Being an older system, II fluoroscopy is a cheaper option than the more modern FPD systems, making it more accessible to the majority of veterinary clinics. There may also be a degree of familiarity in their use and maintenance is likely to be more easily obtained and more affordable than for FPD systems. 

Flat panel detector images: 

The images obtained in this way are much more detailed, of higher quality, and of a higher resolution than those captured by an image intensifier, therefore making them much more likely to result in an accurate diagnosis.

As well as a higher resolution image, the C-arms used in FPD units range from 12” x 12” to 17” x 17” which is a much larger field of view than those used with II units.

This means a wider area of the patient can be examined in a single capture. The whole process is also much faster than with an image intensifier, so the patient is exposed to a shorter duration of radiation. 

Downsides of each system

Image intensified images: 

Just as the image quality is the main benefit of the FPD system, the lack of image quality is the main downside to the II system - the images obtained tend to be of low resolution which can make diagnoses challenging.

The C-arms used also cover a much smaller field of view – typically around 9” x 9” or 12” x 12” meaning the anatomical area examined is also much smaller.

The slower processing of the image means a longer duration of exposure to radiation for the patient. As with many older technological systems, the II units are typically larger and take up more space within a practice than the more modern FPD units. 

Flat panel detector images: 

The main drawback to an FPD system is the cost – being a newer and much more advanced technique, the set-up costs are understandably much higher than for an image intensifier system.

This advanced equipment is also more delicate, and great care and training are needed to be able to use it safely. Some maintenance costs will also reflect this higher value. 

 

When choosing which system is right for a hospital, all the above factors need to be taken into consideration. There will not be one-size-fits-all. In the future, just like what happened when digital radiography took over from film radiography, we may all move more towards flat panel detectors but for the moment, clinics have the two possible options.

Whichever system is chosen, having fluoroscopy within a practice will greatly enhance the services they can offer and aid in increasing the diagnostic toolkit available to the doctors. 

 

References

1. https://newvetequipment.com/blog/veterinary-fluoroscopy

2. Gingold, E. (n.d.). Modern Fluoroscopy Imaging Systems. Image Wisely. https://www.imagewisely.org/Imaging-Modalities/Fluoroscopy/Modern-Imaging-Systems Figure 1, figure 2.

3. https://youtu.be/rex_N_H4zxU