Image Intensified Fluoroscopy



Image Intensified Fluoroscopy





Introduction


Shortly after Dr. Roentgen’s discovery of x-rays and subsequent announcement, many other scientists began experimenting with this new phenomenon. Among them was the famed American inventor Thomas Edison. Among Mr. Edison’s more notable inventions in this area was the first commercially available fluoroscope in 1896, although it was not in a form we would recognize today (Figure 14-1). His fluoroscope was a calcium tungstate screen that interacted with the remnant beam, producing a very faint image that one viewed while standing in the path of the x-ray beam as it exited the patient and screen. The practice of standing in the direct path of the x-ray beam meant that the dose to the fluoroscopist was extremely high. Additionally, because the image was very dim, the fluoroscopist had to “dark-adapt” by sitting in a darkened room for a period or by wearing adaptation goggles with red lenses before performing the fluoroscopic examination. However, fluoroscopy’s great advantage, which ensured its continued development, was that it allowed for dynamic radiographic examination. That is, the inner workings of the human body could be viewed in real time.



In the 1950s the image intensifier was introduced into the fluoroscopic system. The image intensifier improved the process in two ways. First, it brightened the image significantly, eliminating the need to dark-adapt and improving the details that could be seen. Second, it allowed for a means of indirectly viewing the fluoroscopic image, first by mirror optics and later by television monitors, greatly reducing the radiation dose to the fluoroscopist. This chapter discusses the image intensifier and its characteristics, viewing and recording systems, and finally the digital fluoroscopy process in use today.



Construction


Conventionally the fluoroscopic chain consists of an x-ray tube, an image intensifier, a recording system, and a viewing system (Figure 14-2). The integration of digital technology is changing parts of this system, as is discussed at the end of this chapter. Here the focus is on the design and function of the image intensifier, recording, and viewing systems.



Figure 14-3 illustrates the image intensifier within the fluoroscopic tower. The image intensifier is an electronic vacuum tube that converts the remnant beam to light, then to electrons, then back to light, increasing the light intensity in the process. It consists of five basic parts: the input phosphor, photocathode, electrostatic focusing lenses, accelerating anode, and output phosphor. The input phosphor is made of cesium iodide and is bonded to the curved surface of the tube itself. Cesium iodide absorbs the remnant x-ray photon energy and emits light in response. The photocathode is made of cesium and antimony compounds. These metals emit electrons in response to light stimulus in a process called photoemission. The photocathode is bonded directly to the input phosphor using a very thin adhesive layer. These layers are curved so that all of the electrons emitted from the photocathode travel the same distance to the output phosphor (Figure 14-3). The electrostatic focusing lenses are not really lenses at all, but are negatively charged plates along the length of the image intensifier tube. These negatively charged plates repel the electron stream, focusing it on the small output phosphor. To set the electron stream in motion at a constant velocity, an accelerating anode is located at the neck of the image intensifier near the output phosphor. This accelerating anode maintains a constant potential of approximately 25 kV. The output phosphor is made of silver-activated zinc cadmium sulfide and is much smaller than the input phosphor. It is located at the opposite end of the image intensifier tube, just beyond the accelerating anode. This layer absorbs electrons and emits light in response.



The entire tube is approximately 50 cm in length and 15 to 58 cm in diameter (diameter depends on manufacturer and intended use). The input phosphor faces the patient and receives the x-ray exposure that constitutes the remnant beam. The x-rays are absorbed and light is emitted in response, proportional to the percentage of x-ray absorption. This light immediately exposes the photocathode, which in turn emits electrons in proportion to the light intensity. The ratio of light to electron emission is not one-to-one. It takes many light photons to result in the emission of one electron. The resultant electrons are accelerated toward the output phosphor by the accelerating anode and “focused” on the output phosphor by the electrostatic focusing lenses. These high-energy electrons result in many light photons being emitted from the output phosphor. Each electron results in substantially more light photons than was necessary to cause its release. The end result of this process is an increase in image intensity and brightness.




Intensification Principles


The radiographer must be familiar with several principles and concepts associated with image intensification. Brightness gain is one such principle. Brightness gain is an expression of the ability of an image intensifier tube to convert x-ray energy into light energy and increase the brightness of the image in the process. Traditionally, brightness gain was found by multiplying the flux gain by the minification gain.


brightness gain=flux gain×minification gain


image

Although the term brightness gain continues to be used, it is now common practice to express this increase in brightness with the term conversion factor (discussed next). Flux gain has to do with the very concept of using an image intensifier to create a brighter image by taking a few x-ray photons and converting that energy into many light photons. Flux gain is expressed as the ratio of the number of light photons at the output phosphor to the number of light photons emitted in the input phosphor and represents the tube’s conversion efficiency. Minification gain is an expression of the degree to which the image is minified (made smaller) from input phosphor to output phosphor. This characteristic makes the image appear brighter because the same number of electrons is being concentrated on a smaller surface area. It is found by dividing the square of the diameter of the input phosphor by the square of the diameter of the output phosphor.


minification gain=input phosphor diameter2÷output phosphor diameter2


image

Generally, the input phosphors are 15 to 30 cm and the output phosphor is usually 2.5 cm.


The International Commission on Radiation Units and Measurements now recommends the use of the conversion factor to quantify the increase in brightness created by an image intensifier. Conversion factor is an expression of the luminance at the output phosphor divided by the input exposure rate, and its unit of measure is the candela per square meter per milliroentgen per second (cd/m2/mR/s). The numeric conversion factor value is roughly equal to 1% of the brightness gain value. The higher the conversion factor or brightness gain value, the greater the efficiency of the image intensifier.



Regardless of whether the term brightness gain or the term conversion factor is used to express the increase in brightness, the ability of the image intensifier to increase brightness deteriorates with the age of the tube. The radiographer should be aware that as the image intensifier ages, more and more radiation is necessary to produce the same level of output brightness, translating to an ever-increasing patient dose.



The radiographer must also be familiar with automatic brightness control (ABC), a function of the fluoroscopic unit that maintains the overall appearance of the fluoroscopic image (contrast and density) by automatically adjusting the kilovoltage peak (kVp), milliamperage (mA), or both. The ABC generally operates either by monitoring the current through the image intensifier or the output phosphor intensity and adjusting the exposure factor if the monitored value falls below preset levels. The fluoroscopic unit allows the fluoroscopist to select a desired brightness level, and this level is subsequently maintained by the ABC. The ABC is a little slow in its response to changes in patient tissue thickness and tissue density as the fluoroscopy tower is moved about over the patient. This is visible to the radiographer as a lag in the image brightness on the monitor as the tower is moved.


Another function of some image intensifiers is multifield mode or magnification mode. Most image intensifiers in use today have this capability. When operated in magnification mode, the voltage to the electrostatic focusing lenses is increased. This increase tightens the diameter of the electron stream and the focal point is shifted farther from the output phosphor (Figure 14-4). The effect is that only those electrons from the center area of the input phosphor interact with the output phosphor and contribute to the image, giving the appearance of magnification. For example, a 30/23/15–cm trifocus image intensifier can be operated in any of those three modes. When operated in the 23-cm mode, only the electrons from the center 23 cm of the input phosphor interact with the output phosphor; those about the periphery will miss and not contribute to the image. The same is true of the 15-cm mode. Selecting magnification mode automatically adjusts x-ray beam collimation to match the displayed tissue image and avoids irradiating tissue that does not appear in the image. The degree of magnification (magnification factor [MF]) may be found by dividing the full-size input diameter by the selected input diameter. For example:



MF=30÷15=2×magnification

Feb 27, 2016 | Posted by in GENERAL RADIOLOGY | Comments Off on Image Intensified Fluoroscopy
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