Table of Contents
Introduction
In the most contemporary clinical imaging situation, the most typical way of reducing scatter is by the use of radiographic grid. However several methods have been put forward to reduce the scatter effect but the easiest way of reducing scatter has been proved to be simply decreasing the exposure field (Dance, 2002). For example, if one wants to clearly capture the image of a gall bladder, a much clearer picture will be obtained if the shutters are brought down to focus on the gall bladder only instead of capturing the entire upper abdominal part of the patient. Radiographic grids therefore operate through this concept though their use is varied to a number of types. This study will evaluate the basic grid types and analyze their applicability.
Stationery Grid
The stationery grid uses the non-oscillating or non moving grid. The image projected using this technique is visible on the radiograph used. The technique incorporates the use of two strips of lead arranged in two series, parallel to each other and at right angles (Saia, 2008). However modern developments of the technology led to the use of leaner lead strips, extremely thin and separated by plastic, aluminum or any other radiolucent materials. The grids may either be in parallel form of focused to one another whereby the lead strips are focused to one centre.
The major disadvantage of the use of stationery radiography is that it leaves "blank" or white traces on the image in focus. Usually such defects on the film are referred to as lines of grid. Nevertheless, newer versions of the technology reduce these grid lines and require less patient involvement than conventional or earlier grid types.
It is also quite expensive for most patients though it is widely accepted in organ imagery (Saia, 2008).
Moving Grid
The moving grid radiography was developed as a later development in the field of radiography to eliminate scattered radiation. The technique operates when one or more strips are moved in an opaque shield but synchronous with the same number of slips in the shield between the patient and the film (Saia, 2008). The resultant strips are usually long and narrow after which they are scattered on the patient's abdomen and transmitted through the strips on the lower side below the patient. The radiation scattered by the patient is thereafter intercepted by the shield below, which is usually opaque, thereby failing to reach the film (Saia, 2008).
This technique has the main disadvantage of distortion of image because of the motions occurring when the x ray beams are scattered over the patient. Nevertheless, this disadvantage is minimized with new x-ray technologies which are fast, thereby subjecting the patient to lesser exposure time. The new technology incorporates high-milliamperage generators and uncommon earth screens that are intense (Saia, 2008).
Linear Grid
A linear grid is constructed using parallel and grid strips which are focused in line with one another. They have been the standard type of grids used in mammography. However, linear grids have been noted to reduce the scatter on only one direction which is usually perpendicular to the material with the grid absorption.
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In other words, the scatter reduction in the grid is parallel to the grid lines. However, this defect can be further enhanced using the cross hatch technique. The main advantage of linear strips is that they can be used in situations where the proper alignment of the x-ray tube is difficult to attain. In addition, it is also effective when the alignment of the grid and film cassette is difficult (Hendee, 2002).
Cross Hatch Grid
This technology involves the focus of one grid over the other with the strips existent on one grid being perpendicular to the other. The parallel or focused grids are actually placed at right angles to one another thereby providing a sequence of tiny squares. When a high KV is used in organ imagery, a lot of scatter results manifested in all directions is experienced (Hendee, 2002). This technique is usually used to better this effect. Considering the nature of this technique, the x-ray must be centered on the grids. In addition, it must be at right angles in accordance to the design of the grid.
However, situations may exist when it warrants a tilt of the beam; in such situations, a single beam must be used, failure to which the beam may fall into another grid thereby distorting the image. The distinguishing feature about this technique is the increase in exposure compared to other radiographic grids. Nevertheless in other clinical procedures such as biplane radiography, where the ray tubes may be used at maximum loading, it is advisable to sacrifice the high quality image provided by cross hatch technique for the use of a single grid (Hendee, 2002).
Typical cross hatch grids have been observed to be better used at removing scatter than linear grids even with the use of the same grid ratio. The reasoning behind it is that a linear grid is less effective because it fails to absorb scattered photons that are parallel to then grid strips. However, some of its primary disadvantages are that the rapid and slow movements of the grid can easily increase the vibrations which inevitably lead to a loss of image quality.
Secondly, the grid is usually stationery when the movements are reversed, thereby increasing the possibility of the movements being synchronized. This usually happens when the lines of the grid are in the same position for every pulse. However such risks can be minimized when a different type of grid movement is used; for instance, a vibration movement that has a varying frequency of alteration but decreasing with time (Hendee, 2002).
Focused Grid
The use of focused radiography has been a major field of contention among clinical practitioners despite its acknowledged radiographic potential. This technique employs smaller x-ray machines with radiation probes. Present Research studies are aimed at trying to device ways to reduce the effect of the burden of radiation on patients and increase the image focus on the image. Relatively, high air exposures ranging between 42,050 mR per film, for high resolution images to 3,214 mR per film for images of low resolutions, by the incorporation of a middle line radiation technique for focused radiography have been established (Hendee, 2002).
However, recent research has proposed reduced exposures of 420mR per film for high resolution images and 14mR for every film of low resolution.
Proposed modifications of the current mid line focused radiography have been noted to produce ranges that are acceptable for radiography especially on dental patients. This has led to the increased acceptability of this technique in clinical imagery among patients. This has led to the identification of focused image having a high contrast as an advantage over other radiographic techniques, though it is criticized for providing fewer details of the image (Hendee, 2002).
Conclusion
All the grid types are majorly centered on reducing the scatter in image. However, their applicability should be selective because each has its advantages and disadvantages. The linear grid has been a standard technique in clinical procedures though the cross hatch has been identified to be an improvement to it. The focused technique has also been criticized for showing less detail but preferred for its high contrast of the image. The moving grid method has been less effective because of the distortion of the image while the stationery technique has been identified to leave blank or white traces on the image. Nevertheless, all these techniques are all used to increase image quality though medical practitioners should be selective on their use.