Analysis of Univest-X Apron Lead-Equivalent Thickness

I.A. Cunningham, PhD, FCCPM, FAAPM
Professor, The University of Western Ontario
March 23, 2006

Executive Summary
The protective value of Univest-X apron lead for radiation protection in diagnostic radiology was measured using x-ray spectra from 60 to 150 kV. The exposure protection factor was compared with the protection offered by pure lead and expressed in terms of an equivalent lead thickness.

Two samples were received from Univest-X Inc for analysis.

Sample #1 consisted of two layers with a total nominal lead-equivalent thickness of 0.5 mm.

Sample #2 consisted of nominal lead-equivalent thickness of 0.25 mm in one layer.

Results are summarized in the following table. In all cases, the measured exposure protection factor was consistent with the stated lead-equivalent thickness within the accuracy of the measurement. It is concluded that sample #1 has a lead-equivalent
thickness of 0.5 mm and sample #2 has a lead-equivalent thickness of 0.25 mm as claimed by the manufacturer over the energy range of 60 to 150 kV.

Stated Lead-Equivalent Measured Lead-Equivlent
Thickness (mm) Thickness (mm)
Sample #1 (two layers) 0.5 0.5
Sample #2 (one layers) 0.25 0.25

Introduction

Federal and provincial legislations impose minimum requirements on the effectiveness of x-ray protective devices in terms of a thickness of lead in mm. Some protective aprons and other garments are made of materials other than lead. This report summarizes a calculation in which the protective value of non-lead materials is determined and expressed as a “lead-equivalent” thickness. With this testing, it is known that a material specified as having a certain Pb-equivalent thickness truly provides the same protection as the stated thickness of pure lead.

The protection factor, P, is the ratio of Xo, exposure (in mR) received without the protective device, to X, exposure with the protective device. Thus, the exposure received by someone wearing the protection, X, is given by X = Xo / P.

Figure 1 shows a theoretical calculation of the protection factor offered by 0.5 mm of pure lead as a function of x-ray energy. Because of this strong energy dependence, the lead-equivalent thickness of a material can only be determined for an x-ray spectrum of known shape.

Protection Factor of 0.5 mm of Lead

Protection
Factor

100

10

1

0 50 100 150

X-Ray Energy (keV)

Figure 1. Protection factor of 0.5 mm of pure lead as a function of x-ray energy.

Methods

Evaluation of lead-equivalent thickness is completed with the following steps.

1. Characterize spectra covering a range of energies in the diagnostic spectrum in terms of kV and half-value layer.
2. Measure the protection factor of each sample with each spectrum.
3. Determine lead-equivalent thickness of each sample with each spectrum based on the measured protection factor.
4. Confirm that scatter from the samples is comparable with scatter from pure lead of the same thickness.
5. Determine the lead-equivalent thickness of each sample plus a piece of pure lead of known thickness. The lead result is used to confirm accuracy of the analysis by demonstrating that the lead-equivalent thickness is equal to the actual thickness.

Samples were examined as described in the following table.

Stated Lead Equivalence
Sample #1 Two layers 0.5 mm
Sample #2 one layer  0.25 mm

Pure Lead Pure lead, 1/64th inch thick 0.4 mm

1. Characterize Spectra
Ten spectra were used, each using 10 mm of added Al filtration to harden the beam and approximate the spectrum of scattered radiation that would escape from a patient. The generator was calibrated to ensure accurate kV, and the half-value layer (HVL) of each spectrum was measured experimentally and compared with the theoretical value to confirm the spectral shape was well characterized. The following table describes the spectra used.

kV Added Al (mm) Theoretical HVL
(mm Al)
Measured HVL
(mm Al)
60 10 4.7 4.8
70 10 5.5 5.6
80 10 6.2 6.2
90 10 6.9 6.9
100 10 7.5 7.4
110 10 8.0 7.9
120 10 8.5 8.3
130 10 8.9 9.0
140 10 9.3 9.5
150 10 9.7 9.9

2. Measure Protection Factor
The protection factor of each sample for each spectrum was measured using an air ionization chamber and Keithley electrometer (Model 35617). Narrow-beam geometry was used to prevent contamination from scatter. Results are summarized in the following
table.

kV mA
nCoul Protection Factor
No Sample Sample Pure Sample Sample Pure

Sample #1 #2 Lead #1 #2 Lead
60 250 3.808 0.033 0.162 0.050 115 23.5 76.2
70 50 1.427 0.026 0.099 0.045 55.0 14.4 31.7
80 50 2.834 0.030 0.271 0.145 25.6 8.8 16.4
90 50 3.588 0.230 0.573 0.334 15.6 6.3 10.7
100 50 5.01 0.389 0.922 0.550 12.9 5.4 9.1
120 50 8.48 0.743 1.743 1.019 11.4 4.9 8.3
130 80 0.332 0.0279 0.0681 0.0402 11.9 4.9 8.2
140 80 0.385 0.0340 0.0720 0.0470 11.3 5.3 8.2
150 80 0.437 0.0428 0.0938 0.0543 10.2 4.7 8.0

3. Determine Lead-Equivalent Thickness
The protection factor for a range of lead thicknesses between 0 and 1 mm was calculated theoretically for each spectrum. Results are shown in Fig 2.

1 10 100 1000
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Equiv
Pb
Thickness
(
mm)
50 kV
60
70
80
90
100
120
140
150

Protection Factor

Figure 2. Calculated protection factor associated with different lead thicknesses for each spectrum. This data is used to determine the lead equivalent thickness of measured protection factors.

Figure 2 is used to determine the lead-equivalent thickness corresponding to measured protection factors for each sample and spectrum. Results in Figure 2 apply to any material as long as the protection factor is measured using the spectra as stated in this report.

4. Scatter
All measurements described in this report were performed using narrow-beam geometry to avoid contamination from scatter radiation. As a result, negligible scatter is generated within the protective devices. In practice, a small amount of scatter can be generated within both lead and non-lead protective devices. Radiation “build-up” was measured in all three samples. A build-up of up to 15% was observed in both the lead and non-lead materials, confirming that the non-lead material is equivalent to the pure lead sample.

RESULTS

The calculated lead equivalence for each sample is summarized below and in Fig. 3.

kV Measured Lead-Equivalent Thickness (mm)
Sample #1 Sample #2 Pure Lead (0.40 mm)
60 0.47 0.27 0.40
70 0.50 0.30 0.42
80 0.50 0.29 0.40
90 0.49 0.29 0.40
100 0.49 0.29 0.40
120 0.49 0.28 0.39
140 0.49 0.31 0.41
150 0.48 0.29 0.42

The accuracy of the calculated lead-equivalent thickness is estimated to be 0.02 mm of lead. The equivalent thickness of the pure lead sample agreed with the known true thickness within this uncertainty. This agreement confirms that the analysis was
conducted accurately.

Sample #1 was found to have a 0.5 mm lead-equivalent thickness within the accuracy of the measurement over the energy range tested.

Sample #2 was found to have a 0.25 mm lead-equivalent thickness within the accuracy of the measurement over the energy range tested.

60 70 80 90 100 110 120 130 140 150
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Pb-Equivalent
Thickness
(mm)
a) 0.40 mm Pure Pb
b) Sample #1
c) Sample #2
60 70 80 90 100 110 120 130 140 150
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Pb-Equivalent
Thickness
(mm)
a) 0.40 mm Pure Pb
b) Sample #1
c) Sample #2
kV

Figure 3. Summary of measured lead-equivalent thicknesses for: a) 0.40 mm pure Pb; b) Sample #1
(nominal 0.5 mm); and c) Sample #2 (nominal 0.3 mm). Results are accurate to approximately 0.02 mm of lead.