By definition the Half Value Layer (HVL) is the thickness of any given material where 50% of the incident beam has been attenuated by the time it reaches the detector, (Bushong, 2013) i.e. the level at which the beam intensity has halved, usually in millimeters (mm) or centimeters (cm). This occurs because the beam is attenuated or scattered as it passes through the object being irradiated. The degree of attenuation is directly affected by the atomic number of the object that it passes through. With a higher atomic number more attenuation occurs, conversely more penetration with a lower atomic number (Bushong, 2013). Essentially that means the penetrability of a beam is directly linked to the quality or energy of the x-ray. This is why quality control procedures are carried out to measure the quality and energy of the x-ray beam. Simply put this exam was preformed to verify that the machine's x-ray beam was up to standard according to HARP (1990) and SC35 (2008). To perform this experiment, an exposure meter, lead and aluminium strips and tape were gathered. The table was set up and lead strips were placed across the detector to ensure the detector was not exposed. The exposure meter was placed on top of the lead strips. The machine was turned on and set to zero. Once the set up was complete, the technical factors were set to 70kVp at 40mAs. The first exposure was taken and the value on the machine was recorded, a second exposure was also taken for assurance. Following that exposure a 1mm strip of aluminum was taped to the collimator and another two exposures were taken. This was repeated until there was 7mm of filtration taped on to the collimator. Once that was complete the procedure was repeated two more times, first at 90kVp and then at 120kVp. By doing this the HVL changes according to different beam energies and levels of filtration were able to be observed. Every exposure was done at a constant 40mAs and done twice at each filtration thickness. A total of 48 exposures were taken. Once this was complete the room was cleaned up and everything was put away. The average of the two exposures was taken which was then divided by 40. This simple calculation helped us find the x-ray beam intensity at that level of filtration. A graph was made which compared the x-ray intensity to the thickness of the aluminum filtration. With the help of the graph the half value layers of the x-ray beam at the three different energy levels were estimated, which are stated below. The half value layer values were then compared to HARP and SC35. The minimum requirements were met and are stated below. It is noted that the half value layers for 70, 90 and 120 kVp are 2.6, 3.4 and 4.4. All the stated half value layers are within HARP and SC35 standards, thus no corrective actions need to take place. |