X-Ray Image Intensifier

The x-ray image intensifier has been used for almost fifty years to produce sequences of x-ray images. Its origins lie in low light level imaging, for example, night vision devices, to which an x-ray intensifying screen has been added.

The construction, mode of operation and performance characteristics of x-ray image intensifiers are considered on this page, under the following headings:

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

Fig 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 4.2

The major components inside the XII include:

• Input Window

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 Phosphor

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 (figure 4.3 - note that dimensions in the figure are not to scale). The input phosphor is typically 15 to 40 cm in diameter, depending on the XII.

Fig 4.3

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.

A photograph of a glass-envelope XII which has been cut in half to expose the inner side of the input components is shown in figure 4.4.

• Photocathode

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 (figure 4.3).

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.

• Vacuum & Electron Optics

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.

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 (figure 4.5 - note that dimensions, once again, are not to scale).

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.

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. A 15 cm XII assembly is shown in figure 4.6, with the faceplate at the top of the photograph, and the optical system and a video camera towards the bottom.

Performance Characteristics

·  Brightness Gain

The gain in image brightness results from the combined effects of image minification and the acceleration of the electrons:

• Minification Gain

This results because electons from a relatively large photocathode are focussed down to the smaller area of the output phosphor which gives rise to an increase in the number of electrons/mm². The gain is given by the ratio of the areas of the input and output phosphors and can be expressed as:

• Flux Gain

This results from the acceleration given to the electrons as they are attracted from the photocathode to the output phosphor. It is dependent on the applied voltage and is typically between 50 and 100.
 

The overall brightness gain is the product of the minification gain and the flux gain, i.e.

·  Conversion Factor

The brightness gain is not easily measured and serves simply to illustrate the performance of an XII. A more readily measured parameter is the conversion factor, which relates what the XII delivers (i.e. luminance) relative to the input (i.e. radiation exposure), and is useful for comparing the performance of XIIs as well as that of a given XII over time. The output luminance is measured using a photometer, the radiation exposure with a ionization chamber and the conversion factor is expressed as:

This factor is typically 7.5 - 15 Cd m^-2/µGy s^-1 and higher.

Note that the resulting image is relatively dim. For example the luminance of a standard domestic light bulb is about 10⁶ Cd m^-2. The green output image therefore needs a darkened room and dark-adapted eyes for direct viewing, or a sensitive video camera for remote viewing.

·  Contrast Ratio

This parameter expresses the broad-area, high contrast performance of an XII. Various scattering effects inside the XII result in a radio-opaque object not being completely opaque in the image. The contrast ratio can be measured by imaging a lead disk and expressing the luminance of its image relative to that of an open field image. For standardization purposes, the size of the disk is typically 10% of the field size and it is placed centrally in the field of view. It is expressed as follows:

Typical values are 20:1 to 30:1 or greater.

The contrast is affected by factors which include:

• X-ray scattering in the input window
• X-ray scattering in the input phosphor
• Light scattering in the input phsophor
• Electron scattering in the electron optics
• Light scattering in the output phosphor
• Light scattering in the output window

and these scattering effects are collectively referred to as veiling glare. The output phosphor has generally been regarded as the major source of veiling glare  although recent work has indicated that the input components may also contribute significantly.

·  Limiting Spatial Resolution

This parameter can be assessed using a Pb bar test pattern by determining the highest spatial frequency - in line pairs per mm (lp/mm) - that can be resolved. Images of such a test pattern are shown in figure 4.7, where a 23 cm XII is operated in a 23 cm (left), a 15 cm (middle) and an 11 cm (right) mode. The parameter is generally expressed for the centre of the field of view, since it decreases towards the image periphery depending on the quality of the electron optics.

The performance is dependent on the field size and the type of imaging camera used, and the table below shows some typical results. Note that each of the cameras degrades resolution to some extent and that the XII itself has a lower resolution than screen/film devices.

·  Spatial Non-Uniformity

XII images of a uniform object are generally brighter in the centre than in the periphery due to an unequal brightness gain in different regions of the field of view.

This effect is also called vignetting and is illustrated in figure 4.8. The image is of a uniform object acquired with an XII coupled to a video camera, and the graph on the right shows the brightness profile for a horizontal line through the centre of the image. The vignetting effect is quite apparent.

Note that this parameter is not widely assessed for XII systems - in contrast to nuclear medicine where image uniformity of gamma cameras is rigorously controlled - and a standardized measurement technique is not in widespread use. Note also that the image data in the figure reflects the combined effects of the spatial uniformity of the detected radiation beam, the coupling optics and the video camera, and not solely the XII.

 Spatial Distortion

The final performance characteristic to be considered is spatial distortion. All XIIs suffer from this effect, where images do not faithfully reproduce the spatial relationships in an object because of unequal magnification in different regions of the field of view.

The effect is illustrated in figure 4.9, where images of a regular wire matrix acquired with a modern (on the right) and an older XII are shown. The distortion is typically 'pincushion' in nature - as readily seen in the image on the left. Notice, for example, that straight lines are reasonably straight in the centre of this image and change to curves towards the peripheral regions. Notice, also, that image areas measured in the centre of the field will be less than those measured in the periphery.

One approach to assessing this characteristic is to determine the integral distortion, which is expressed as follows:

where:

• D₁: diagonal length of the central square in the image of the matrix
• D₂: diagonal length of the largest square in the image
• n: a factor to account for the relative sizes of these squares in the object.

The distortion expressed using this approach is 8.5% for the image on the right in figure 4.9, and 3.5% for the image on the right.

©Kieran Maher Last updated: 26 Jan '98

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