Laura LeBlanc, 1204502
The intensity of an x-ray beam is an important property in radiography and can be reduced as it penetrates an object by absorption or scattering. Reduction in the intensity of the beam can be affected by the atomic number of the absorbing material or beam energy. In radiography, technologists use the half value layer (HVL) to measure the quality or intensity of the beam. The HVL of an x-ray beam is defined as the amount of absorbing material that is needed to reduce the beam to half of its original potential. HVL is an indirect measure of photon energy or beam hardness. HVL is an important quality control test as it is used to measure whether or not there is sufficient filtration in the x-ray beam to remove low energy radiation, which can be damaging. It also helps to determine the type and thickness of shielding required in the facility.
In this experiment, the purpose is to measure the quality of the x-ray beam. The materials necessary and the setup are outlined below in Figure 1.
Figure 1:
In this experiment, the purpose is to measure the quality of the x-ray beam. The materials necessary and the setup are outlined below in Figure 1.
Figure 1:
In this experiment, 1mm thick aluminum sheets were used as attenuating material. The beam energy was tested at 70 kVp, 90 kVp and 120 kVp with a range from 0-7mm of aluminum thickness. First, the exposure was measured at 70 kVp with 0mm of added filtration and with each subsequent exposure, 1mm of aluminum was added until an exposure with 7mm was achieved. This was repeated at 90 kVp and 120 kVp. The exposure was measured using the Cobia Smart X-ray Meter in milligray and microgray and the results are included in the table below (Table 1).
Table 1
Data Sheet for Half Value Layer
Table 1
Data Sheet for Half Value Layer
After calculating the mR/mAs, another table was constructed with the percentage transmission. This can be seen in Table 2 below.
Table 2
Percentage Transmission Table
Table 2
Percentage Transmission Table
A plot (Figure 2) was constructed using the percentage transmission values versus the thickness of aluminum (in mm) used for the 3 tube voltages. Next, using the plot, the HVL for each curve was calculated. This can be seen in figures 3, 4 & 5. Using the percentage transmission that was calculated above, a straight-line was drawn from 50% transmission to the curve. This point represents the amount or thickness of aluminum required to reduce the x-ray beam to half of it's original intensity. This step was repeated for 90 & 120 kVp. The results are summarized in the table below (Figure 6). |
Below is a table set out by Safety Code 35 (S.C. 35). The table shows the minimum HVL values that the safety code recommends.
As outlined by Safety Code 35, there must be radiation absorbing filters in place that provide a certain degree of attenuation. This means that the HVL of aluminum must not be less than the values stated in Table 8 for a specific kVp. Comparing the values from Table 3: HVL Results table to Table 8 from Safety Code 35, it is clear that no corrective action is required. The slight differences seen between Table 3 and Table 8 of the Safety Code could have occurred due to: too much added or inherent filtration resulting in a hardened beam that filtered more than what was necessary of the damaging low energy photons. |
On the right, are the standards listed in Table 8 by H.A.R.P.. According to H.A.R.P., the HVL of aluminum must not be less than the values stated. Comparing the values from Table 3: HVL results table to Table 7 from H.A.R.P., no corrective action is required meaning the values obtained in the lab fall within specified standards. |
According to Papp, a filtration check is performed before new equipment is used and then annually, unless service has been performed on the x-ray tube or collimator. The best way to determine if adequate filtration exists is to measure HVL as it is not possible to measure inherent filtration. The filament evaporates as the tube is used, adding a layer of tungsten to the inside of the x-ray tube window. So by measuring the HVL, it doesn't matter what material is in the path of the beam, as long as sufficient beam quality is obtained. Another test can be performed to determine if adequate filtration is present, if HVL cannot be determined. However, this test only determines if adequate filtration is present and doesn't account for the total amount of filtration. This test is done using a 2.3mm thick piece of aluminum and a dosimeter and should not replace the HVL measurement during quality control testing. According to Table 7-1 in Papp and Table 3: HVL results, no corrective action is required.
Determining HVL is important and failure to comply to standards defined by S.C. 35, H.A.R.P. and Papp would result in added dose to patient. The low energy photons will not be filtered out and will add entrance skin exposure to patients, increasing their overall dose.
Determining HVL is important and failure to comply to standards defined by S.C. 35, H.A.R.P. and Papp would result in added dose to patient. The low energy photons will not be filtered out and will add entrance skin exposure to patients, increasing their overall dose.
References:
Bushong, S. (2008). Radiologic science for technologists: Physics, biology, and protection (9th ed.). St.
Louis, Mo.: Mosby/Elsevier.
Gray, J. (1983). Quality control in diagnostic imaging: A quality control cookbook. Baltimore: University
Park Press.
Health Canada. (2008, January 1). Safety code 35: Safety procedures for the installation, use and
control of x-ray equipment in large medical radiological facilities. Retrieved January 22, 2015,
from http://www.hc-sc.gc.ca.
Papp, J. (2011). Quality management in the imaging sciences. St. Louis, Mo.: Mosby Elsevier.
Service Ontario. (2011). Healing Arts Radiation Protection Act. Retrieved from http://www.e-
laws.gov.on.ca/html/regs/english/elaws_regs_900543_e.htm.