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The purpose of this experiment is to demonstrate how kVp and attenuating thickness affect the attenuation and transmission of an x-ray beam. A quantitative experiment was designed, exposing various thicknesses of attenuating material to a range of kVp values. An automatic exposure control (AEC) was used to determine the mAs produced by the exposure and an average of the results was calculated. The results corresponded with the hypothesis, showing that an increase in kVp reduced the mAs, and an increase in thickness increased the mAs.
During exposure, the x-ray photons in the primary beam are reduced because they are subject to interactions occurring between the photons and matter. This process is known as attenuation. The photons are reduced by a fractional amount per increment of matter they pass through, thus the relationship is exponential (Bushong, 2008). The percentage of the x-rays attenuated per unit of thickness is known as the linier attenuation coefficient (NDT, 2010).
Within the diagnostic ranges of radiography, there are two possible interactions that result in the attenuation of the x-ray beam: the photoelectric effect and Compton scattering (Fauber, 2009). The photoelectric effect results in absorption of the x-ray photons. During this interaction the photon is absorbed by an inner shell electron giving it enough energy to be ejected from its binding shell: a process known as ionisation. The vacancy left on the inner shell is occupied by an outer shell electron, returning the atom to its natural state (Bushong, 2008). During a photoelectric interaction a secondary photon is produced because the outer shell electron looses energy as it moves to the inner shell. The secondary photon does not affect the image quality since it is absorbed by adjacent tissues (Bushong, 2008).Compton scatter, occurs when the photon interacts with an outer shell electron. The incident photon does not transfer all of its energy to the electron and is deflected, continuing in a different direction with less energy (Farr, 1998). Scattered radiation may interact with the image receptor but contributes no diagnostic information to the image. Scattered radiation does affect image quality because it results in a reduction of image contrast. Photons that are neither absorbed nor subject to Compton scattering are transmitted through the matter and reach the image receptor.
Increasing the kilovoltage applied to the x-ray beam increases the energy of the photons and their penetratability (Shepherd, 2003). Increasing the kV above the appropriate level for an examination results in over-penetration and produces a low contrast image. Excessive density will also be apparent on the resultant image (Bushong, 2008). On the other hand, increasing the kVp allows for a reduction of mA and the exposure time, reducing the patient's exposure to radiation (Fauber, 2009). The radiographer should aim to produce images with optimal density and contrast, but at the same time, exposure to the patient should be kept to a minimum (IR(ME)R, 2000).
Automatic exposure controls (AEC), measure the transmitted x-rays during an examination. X-ray photons interact with the air or gas contained within an ionisation chamber, generating an electrical charge (Fauber, 2009). The AEC is programmed to allow a specific voltage to be met according to the examination being completed. When the desired voltage for the examination has been received the exposure is terminated (Shepherd, 2003). The exposure time is determined by the number of photons interacting with the AEC. When used correctly, the resultant image should demonstrate an optimal level of density (Fauber, 2009).
Hâ‚ Increasing the attenuating thickness will result in increased mAs. The mAs will be reduced if the kVp is increased.
Hâ‚€ Altering the attenuating thickness and the kVp will have no effect on the mAs produced.
A quality control test was completed before the experiment began (Appendix A). The results of the test (Table 1) showed that, when compared with previous QC results (Appendix B), the AEC performed consistently, therefore, any changes to the mAs during the experiment occurred because of changes made to the kVp value or attenuating thicknesses.
(Table 1 - Results of QC testing )
The experiment was completed in the same morning during a one hour timescale. The equipment was set up as shown in photograph 1. In addition, a cassette measuring 35cm x 43cm and a table to record the data were used during the experiment.
(Photograph 1 - X-Ray room 1 set up and equipment)
The centre of the attenuating material was centered to the cassette to ensure that the x-rays penetrated the attenuating material effectively. The collimation was adjusted, leaving a space of 1cm at each edge of the attenuating material, restricting the useful x-ray beam to the area above the ionisation chambers.
The source to image receptor distance (SID), focal spot size, mA selection, and chamber selection remained constant throughout the experiment. The SID was set at 110cm to account for the inbuilt table bucky grid, and prevented grid cut-off of the x-ray beam. A broad focal spot selection reduced the risk of potential damage to the x-ray tube. 100mA produced a sufficient number of photons and remained unchanged throughout the experiment to ensure that any changes to the mAs occurred as a result of changes made to the variable factors. For the same reason, the centre chamber selection remained constant throughout the experiment.
Before the experiment, a pilot test was completed using 6cm of attenuating material. An exposure was made using 60kVp and 100mA. The pilot indicated that 60kVp produced x-rays with sufficient energy to penetrate 6cm of attenuating material.
To begin the experiment, 6cm of attenuating material were exposed to 100mA at 60kVp. The mAs produced was recorded on the table (Appendix C), by two people to reduce the risk of error, and the procedure was repeated twice more: enabling the average mAs to be calculated. The procedure was repeated at 80kVp, 100kVp, and 120kVp, and then at each increment of attenuating thickness, removing 1cm of attenuating material at a time, until the procedure had been completed at 0 attenuating thickness.
Table 2 displays the average mAs calculated from the data collected during the experiment. The full data collected (Appendix C) demonstrated little or no deviation and has therefore not been acknowledged in the results displayed. The graph below shows a comparison of the average mAs recorded for each attenuating thickness at varying kVp factors.
Attenuating material (cm)
(Table 2 - Average mAs calculated from data collected during experiment)
The data obtained during the experiment clearly demonstrates that when the kVp value increased, the mAs reduced. The graph shows that a comparison can be made, for example, to the results obtained at 60kVp and those obtained at 120kVp. At 60Kvp, the mAs produced is significantly higher than at 120kVp. The results indicate that at 120kVp the energy of the x-ray beam was greater, allowing the photons to penetrate through the attenuating material and generate the charge required to terminate the exposure quicker than at 60kVp. The number of photons in the beam remained constant during the experiment, therefore the exposure was terminated because fewer interactions occurred at a higher kVp value and transmission of the photons increased.
The results also show that varying the attenuating thickness of the material had an effect on the mAs produced during the experiment. As the thickness of the material increased, the mAs increased. An example of this is clearly shown on the graph at 60kVp. When 0 attenuating thickness was exposed to 60kVp 0.7mAs was produced. In contrast, when 6cm of attenuating material was exposed to 60kVp the exposure produced 2.6mAs. The results indicate that attenuation of the x-ray beam increased when the thickness of the attenuating material increased. The increase in mAs fits an exponential trendline illustrating that the x-ray beam is being attenuated by a fractional amount as it interacts with the attenuating material.
In clinical practice this information is relevant because the thickness of the patient and the selection of exposure factors must be considered in order to produce diagnostic images when using an AEC. The x-ray beam must have enough energy to penetrate the anatomical area of interest in order for a sufficient charge to be generated to terminate the exposure. If the exposure is not terminated appropriately the patient may be exposed to an unnecessary amount of radiation during an examination. Increasing the energy of the x-ray beam also allows for a lower mA selection, reducing the patient's exposure to radiation. On the other hand, increasing the kVp over the appropriate limits for the examination results in a reduction of image quality as discussed in the literature review.
The results show that the validity of the experiment was sufficient as little or no deviation of the mAs produced was noted. The AEC gave consistent results throughout the experiment. The accuracy of the experiment was subject to human interaction. The position and alteration of the attenuating material was done by hand and not measured on each occasion. A marker was placed to ensure that the Perspex remained in approximately the same position; however this could have been measured more accurately if the time of the experiment had not been limited. The results obtained during this experiment prove that the hypothesis was correct.
In conclusion, the experiment undertaken set out to demonstrate that altering the kVp, and/or attenuating thickness had a direct effect on the mAs produced during an exposure. The experiment proved that the hypothesis was correct. When the kVp increased, a reduction in mAs was recorded. In addition, when the attenuating material increased, an increase in mAs was seen.