The fluoroscopic image

Chapter 35 The fluoroscopic image



Chapter contents



35.1 Aim 259


35.2 Fluoroscopic principles 259


35.3 The image intensifier 260


35.4 Solid-state image intensifiers 261


35.5 Fluoroscopic image quality 262


35.6 Dose reduction in fluoroscopy 263


Further reading 264



35.1 Aim


Fluoroscopy is a technique that is widely used in radiography to produce images of moving structures. These can be used for real-time diagnostic imaging and treatment guidance. The aim of this chapter is to explain how fluoroscopic images are formed, concluding with a consideration of image quality and radiation dose issues. Traditionally the term fluoroscopy has referred to real-time imaging used for guiding procedures, while fluorography produces higher-quality images for organ investigations.



35.2 Fluoroscopic principles


In the process of fluorescence, materials called phosphors absorb high-energy photons such as X-rays and emit short bursts of visible light photons. The brief nature of the light emissions is useful in radiography, where there is a need to produce sharp images free from ‘lag’ or blur. It was light from fluorescent crystals of barium platinocyanide that led Roentgen to realise in 1895 that hidden ‘X-rays’ were being produced from an electrified vacuum tube in a darkened laboratory. The rest, of course, is history.


Phosphor materials are very widely used in radiography and need to have two key characteristics – the ability to strongly absorb X-rays and the ability to convert a percentage of this absorbed energy to visible light. X-ray absorption (by the photoelectric absorption process) is improved by the presence of high atomic number elements like caesium, barium, iodine and the ‘rare earths’. These all typically have K-shell absorption edges at about 30–35 keV, which are well placed to absorb X-rays produced at about 70–100 kVp (see Ch. 23).



Insight


The greatest intensity of X-rays produced within the continuous X-ray spectrum by Bremsstrahlung is at about one-third to one-half of the peak beam energy (which is determined by the kVp across an X-ray tube). Thus it is useful to match the K-edge energies of absorbing materials to this point of greatest intensity, not to the peak beam energy (as very few X-rays are produced at the peak energy).


The overall luminescent radiant efficiency (conversion rate of X-ray energy to light energy) is normally only about 10–20% for phosphor materials. This means that considerable intensities of X-rays are needed to produce enough light for a glowing fluoroscopic image to be viewed directly (without any amplification), even in a darkened room. In the first half of the twentieth century, imaging department staff had to do exactly this; view a phosphor screen directly during fluoroscopic procedures, with patient and staff receiving large radiation doses in the process.



35.3 The image intensifier


In radiography, an image intensifier is simply a device which amplifies the visible light resulting from the fluoroscopic process. Such devices were introduced in the 1950s and permitted fluoroscopy to take place in normal room lighting conditions, as well as greatly reducing radiation doses to patients and staff. This section will describe a traditional X-ray image intensifier, which is based around a cylindrical evacuated tube designed to accelerate and focus electrons. These devices have been partly replaced by more modern ‘solid-state’ devices, but are still widely used in radiography.


The traditional X-ray image intensifier involves various energy changes between the input and output phosphors of the device, as shown in Figure 35.1.


image

Figure 35.1 The conversion processes that take place within an image intensifier.


Note: Although X-ray energy is transferred to light energy within the device, it should be noted that light photons are not turned into electrons. At the photocathode, light energy is used to promote the energy of existing electrons within the material so that they are emitted from it.


Table 35.1 indicates the relative numbers of photons or electrons arising at each stage of the process of image intensification.


Table 35.1 Approximate relative numbers of photons or electrons arising at each stage of the image intensification process















TRANSFORMATION NUMBERS OF RESULTANT PHOTONS OR ELECTRONS
1 X-ray photon absorbed in the input phosphor ≈2×103 light photons are emitted by the input phosphor – of these, about half are absorbed in the phosphor material
≈1×103 light photons strike the photocathode ≈2×102 electrons are emitted by the photocathode
≈2×102 electrons are accelerated and strike the output phosphor ≈1×105 light photons are emitted by the output phosphor

The flux gain of an image intensifier refers to the relative numbers of incident X-ray photons striking the input phosphor to emitted light photons leaving the output phosphor. This is in the general region of 1:104 to 105

Related posts:

Principles of radiography Rectification Units of measurement The radiographic image Electromagnetic radiation Radionuclide imaging

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Mar 6, 2016 | Posted by in GENERAL RADIOLOGY | Comments Off on The fluoroscopic image

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