The x-ray image intensifier
The x-ray image intensifier converts the transmitted x rays into a brightened, visible light image. Within an image intensifier, the input phosphor converts the x-ray photons to light photons, which are then converted to photoelectrons within the photocathode. The electrons are accelerated and focused by a series of electrodes striking the output phosphor, which converts the accelerated electrons into light photons that may be captured by various imaging devices. Through this process, several thousand light photons are produced for each x-ray photon reaching the input phosphor. Most modern image intensifiers use cesium iodide for the input phosphor because it has a high absorption efficiency and thus decreases patient dose. Image intensifiers come in various sizes, most having more than one input image size or magnification mode. Modern image intensifiers are specified by conversion factors, which is the measure of how efficiently an image intensifier converts x rays to light. Because of design restrictions, image intensifiers are subject to inherent and induced artifacts that contribute to image degradation. Both spatial and contrast resolution gradually decrease during the lifetime of the image intensifier because the brightness gain of an image intensifier decreases with time as the phosphor ages. A well-run quality control program for the image intensifier is needed to detect the inevitable changes in settings before they appear on clinical images.
The basic principle of image intensification is identical for all different intensifier versions.An image - ultraviolet, visible light, or near infrared - is projected onto the transparent window of the vacuum tube as shown in Fig. 1. The vacuum side of this window carries a sensitive layer called the photocathode. Light radiation causes the emission of electrons from the photocathode into the vacuum which are then accelerated by an applied DC voltage towards a luminescent screen (phosphor screen) situated opposite the photocathode. The screen's phosphor in turn converts high energy electrons back to light (photons), which corresponds to the distribution of the input image radiation but with a flux amplified many times.
Fig. 1: Basic principle
The terminology "image intensifier" and "image converter" are frequently confused. In particular, image conversion refers to the transfer from an invisible to a visible spectral range, such as image converters used in night vision. On the other hand, image intensifiers which perform as the name suggests often also function as image converters.
Image intensifiers are classified in three categories: first, second, and third generation. Each generation has specific advantages and disadvantages.
Construction and Mode of Operation
The x-ray image intensifier (XII) is generally a cylindrically-shaped device containing a number of components housed in a vacuum. Figure 4.1 shows a cross-section through this cylinder. X-rays emerging from the patient enter at the input window and strike the input phosphor. The input phosphor scintillates and light photons strike the photocathode, which emits electrons. These electrons are accelerated and focussed by the electron optics onto the output phosphor which emits light. This light provides an image of the x-ray pattern that emerged from the patient which has a substantially greater intensity than when an intensifying screen is used on its own. This description of its operation is summarised in figure 4.2 .Fig 4.1
The major components inside the XII include:
The input window in older XIIs was made from glass and their performance suffered from x-ray scattering and absorption effects in this material. This limitation has been overcome in modern devices by using a relatively thin sheet (e.g. 0.25 - 0.5 mm) of aluminium or titanium where good strength is achieved for containing the vacuum with minimal x-ray attenuation.
- Input Window
The input phosphor is made from CsI, doped with Na, which is deposited on an aluminium substrate. The CsI:Na is grown in a structure of monocrystalline needles, each about 0.005 mm in diameter and up to 0.5 mm long. The aluminium substrate is about 0.5 mm thick The input phosphor is typically 15 to 40 cm in diameter, depending on the XII.
- Input Phosphor
Both Cs and I are good absorbers at diagnostic x-ray energies having K-edges at 36 and 33 keV, respectively. The CsI:Na phosphor produces a blue light when x-rays are absorbed and this light is guided along the needles in a fibre-optic fashion (i.e. without much lateral spread) to the photcathode.
An intermediate layer (less than 0.001 mm thick) is evaporated onto the inner surface of the CsI:Na phosphor and a photcathode (about 2 nm thick) is deposited on this layer .
The intermediate layer (e.g. indium oxide) has a high optical transmission and is used to chemically isolate the phosphor and photocathode materials. The photocathode typically consists of an alloy of antimony and caesium (e.g. SbCs3).
Light photons emitted by the input phosphor are absorbed via the phototelectric effect in the photocathode to release photoelectrons.
The vacuum is required so that the electrons can travel unimpeded - as in the case of the x-ray tube. A voltage of 25 to 35 kV is used to accelerate the electrons and the electon optics is used for focussing them onto the output phosphor. A current of about 10-8 to 10-7 A results and it is the acceleration and focussing of these electrons which gives rise to the image intensification.
- Vacuum & Electron Optics
Note that a cross-over point exists so that the image at the output phosphor is inverted relative to that at the input phosphor. Note also that the input phosphor and photocathode are in fact curved ( i.e. not perfectly straight as shown in figure 4.1) so as to equalise the electron path lengths and hence minimise image distortion.
Image magnification can be achieved by varying the voltages on the electrodes of the electron optics, so that a 38 cm XII can also be used to image field sizes of 26 cm and 17 cm, for instance. Three discrete field sizes are typical of many systems although XIIs with a continuous zoom feature are also available. Image brightness decreases as the field size is reduced when the input exposure rate is maintained constant.
Most XIIs also feature mechanisms for establishing and maintaining the vacuum, but this aspect of their construction is beyond the scope of the treatment here.
The output phosphor is made from ZnCdS: Ag (e.g. a P20 phosphor) deposited on the ouput window.
- Output Phosphor
This phosphor emits a green light when it absorbs the accelerated electrons, and is typically about 0.005 mm thick and 25 to 35 mm in diameter.
In addition, a thin aluminium film is placed on the inner surface of the phosphor, which serves both as the anode and to reflect light back towards the output window - so as to increase the output luminance and to prevent these light photons exciting the photocathode.
A number of designs of output window exist and include a glass window (e.g. 15 mm thick) with external anti-reflection layers, a tinted glass window and a fibre-optic window - the objective of these designs being to minimise light diffusion and reflections.
- Output Window
The resulting image is fed to an optical system to be viewed by a cine-camera, photographic camera, video camera or combinations of these cameras. Orthochromatic film is needed for the film-based cameras.
The XII envelope is made from glass or non-magnetic stainless steel, and the input window is welded to this envelope. The assembly is housed inside a metal container which contains lead, for radiation shielding, and mu-metal, to shield the electron optics from external magnetic fields. The input window is typically protected by an aluminium faceplate (e.g. 0.5 mm thick) which also serves as a safety device in case of implosion of the XII. Many systems also feature a scatter-reduction grid mounted at the faceplate. towards the bottom.In summary, consider the fate of a 50 keV x-ray photon which is totally absorbed in the input phosphor:
- The absorption will result in about 2,000 light photons, and about half of these might reach the photocathode.
- If the efficiency of the photocathode is 15%, then about 150 electrons will be released.
- If the acceleration voltage is 25 kV, the efficiency of the electron optics is 90% and each 25 keV electron releases 2,000 light photons in the output phosphor, then about 270,000 light photons will result.
- Finally, if 70% of these are transmitted through the output window, the outcome is a light pulse of about 200,000 photons produced following the absorption of one 50 keV x-ray.
II with Complete System (will be explained later in chapter 6 )
ApplicationXII coupled to:CardiologyCine + Video CameraGI StudiesSpot-Film + Video CameraAngiographyVideo CameraMobile/SurgicalVideo CameraInterventional RadiologyVideo Camera
Digital FluorographyIIX Generations
The first application for night vision was for snipers in World War II. Nicknamed the "sniperscope" and "snooperscope", they were designated the M1 and M3 infrared night sighting devices. They are simple devices that do not produce a net amplification of light, but rather allow a user to see near-infrared light. Along with beam filters, this allowed snipers to illuminate their target without their target being aware of it. However, night vision became employed by both sides, and as a result the "active" IR beams began to betray the sniper's position.
Generation 0 devices took a lot of power to use, for both the tube and the IR illuminator, had a very distorted picture due to a cone-shaped electrode design, and a short tube life due to the high electrical voltage. Generation 0 featured a photocathode made of a mixture of silver, caesium, and oxygen called S-1 which provided approximately 60 mA/lm sensitivity to light.
Generation 1 devices are also called "Starlight scopes", and were a tremendous improvement upon generation 0. They are much more power efficient, amplify light better, and produced a superior image. These devices were initially used in the Vietnam War, but were unable to function well without moonlight until heavy and bulky 3-stage tubes were deployed. Generation 1 also used a different photocathode, S-20, which provided about three times the photo sensitivity of Generation 0.
However, generation 1 devices still have a relatively short tube life, and do not amplify light much better than a dark-adjusted eye unless multiple stages are used. They still carry the benefit of being able to use a somewhat "invisible" IR illuminator, though.
Generation 1 remains one of the most popular types of night vision today. Despite its poor performance, its low cost entices people who are looking to pick up night vision as a toy.
Generation 2 was a major technological breakthrough. Although the photocathode material, S-25, wasn't much of an improvement over Generation 1's S-20, generation 2 devices introduced the microchannel plate (MCP, see: Micro-channel plate). The microchannel plate consists of a bundle of thousands of tiny glass fibres fused together in parallel, sliced transversely, and polished on both faces. Electrons impinging on one side of the plate tend to travel along the fibres, perpendicular to the plate's faces, thus preserving a coherent image. This plate is situated behind the photocathode and amplifies the number of electrons that pass through it. For every one electron that passes through the plate, another approximately 10,000 electrons are added to it. This allows there to be less charge present in the tube, as acceleration is not the principal source of light amplification, increasing battery life, tube life, and reducing distortion noticeably. As good as Generation 2 was, though, it was soon to be overshadowed by a new photocathode material.
Generation 3 is the latest "generation" and is in use by the US military and others. It is essentially generation 2 technology with a new photocathode material—gallium arsenide and a better MCP. Gallium arsenide provides far better response to near-infrared light. This is very important as the majority of starlight is in the IR spectrum. However, this comes at a cost of not being able to see blue light very well at all.
Generation 3 tubes provide significantly better resolution and sensitivity and less noise, and have better overall light amplification than generation 2. Because it can be operated entirely passively outdoors, this allows soldiers to see at great distance at night without betraying their position as generations 0 and 1 technology did with IR illuminators.
There have been recent developments in image intensification tubes, modifying generation 3 devices to be "gated and filmless". This allows for even better resolution and sensitivity, and less "blooming" in urban environments. Some manufacturers have begun calling this "generation 4", but this term has not been officially adopted by the US military.
One common misconception about night vision is that the battery life is extremely short. While this may have been true with generation 0 devices, modern generation 2 and 3 tubes can run for 40 hours or more off a single AA battery, which is far superior to a flashlight.
There are newer night vision systems available. Generation 3 Ultra and Generation 4 tubes exist. Most civilian equipment remains based on Generation 3 equipment and lower, however. Generation 3 Ultra and 4 still use MCP technology, but are designed to offer greater range, and higher resolution.