Half Value Layer

Jun Hu

0963495

As one of the most important properties of X-rays, the intensity of the X-ray beam is reduced through an object as the result of absorption or scattering. There are some factors which can affect this intensity reduction, such as atomic number of the material or beam energy. When the energy of the beam is specific, the beam is more able to penetrate a material with low atomic number, and attenuate dramatically in material with higher atomic number. Moreover, X-rays can penetrate much deeper through a material when the energy of the beam increases. Thus, penetrability is a feature which can express the quality or power of an X-ray beam. In radiography, X-ray technologists use a half value layer (HVL) value to measure the quality of the X-ray beam.

Definition: HVL is the thickness of a given material required to reduce an X-ray beam's intensity by half

This definition begs the question, how can we measure the HVL of an X-ray beam? In this experiment, we want to minimize the process and necessary material because any other variation may affect our experiment’s accuracy. The necessary materials and their arrangement are demonstrated in Figure 1:

Figure 1:

In this experiment, 1 mm thick sheets of aluminum were used as an attenuating material. The beam energy used was 70 kVp. First, we recorded the exposure meter reading with no filter, and then increased the number of sheets of aluminum gradually, taking exposures and recording exposure readings. A total of seven filters (hence eight images) were used. In addition, to observe how HVL changes according to different beam energies, we repeated our experiment two more times at different kVp settings (90 and120).

Data Sheet for Half Value Layer

kVp	Added filtration mm Al	mAs	Exposure mR 1	Exposre mR 2	Average exposure mR	mR/mAs
70	0.0	40	0.185	0.184	0.1845	0.0046
70	1.0	40	0.137	0.137	0.1370	0.0034
70	2.0	40	0.104	0.104	0.1040	0.0026
70	3.0	40	0.083	0.083	0.0830	0.0021
70	4.0	40	0.068	0.068	0.0680	0.0017
70	5.0	40	0.056	0.056	0.0560	0.0014
70	6.0	40	0.047	0.047	0.0470	0.0012
70	7.0	40	0.037	0.039	0.0380	0.0010
90	0.0	40	0.325	0.325	0.3250	0.0081
90	1.0	40	0.255	0.254	0.2545	0.0064
90	2.0	40	0.207	0.206	0.2065	0.0052
90	3.0	40	0.173	0.173	0.1730	0.0043
90	4.0	40	0.146	0.146	0.1460	0.0037
90	5.0	40	0.126	0.125	0.1255	0.0031
90	6.0	40	0.109	0.109	0.1090	0.0027
90	7.0	40	0.095	0.096	0.0955	0.0024
120	0.0	40	0.561	0.561	0.5610	0.0140
120	1.0	40	0.465	0.466	0.4655	0.0116
120	2.0	40	0.396	0.396	0.3960	0.0099
120	3.0	40	0.347	0.347	0.3470	0.0087
120	4.0	40	0.304	0.304	0.3040	0.0076
120	5.0	40	0.271	0.272	0.2715	0.0068
120	6.0	40	0.243	0.243	0.2430	0.0061
120	7.0	40	0.219	0.220	0.2195	0.0055

After calculating the X-ray beam intensity (mR/mAs), a plot was constructed of beam intensity versus thickness of aluminum (Al) for three different tube voltages (see Graphs 1, 2, and 3).

Graph 1: 70 kVp

Graph 2: 90 kVp

Graph 3: 120 kVp

In these three figures, we calculated 50% of the original X-ray beam intensity (without the Al filter), then used that value on the Y-axis to draw a straight line to the curve. From this point on the curve, a line is drawn to the X-axis. This point on the curve represents the thickness of aluminum required to reduce the X-ray beam intensity to 50% of its initial value (i.e, the HVL). The HVL for each curve is estimated in Chart 1.

Chart 1:

Tube Voltage, kVp	HVL, mm Al
70	2.6
90	3.3
120	4.6

The table below shows the minimum HVL values recommended by Safety Code 35 (SC35). By comparison, the tested HVL values from our experiment are consistent with the recommendations made by SC35; therefore, corrective action is not needed.

Citation

Bushong,S. C. (2008). Chapter 9 x-ray emission. In Radiologic Sxience forTechnologists (p. 155). St. Louis: Mosby Elsevier.

Minister of Health, (2008). Safety code 35 Health Canada.