What is Lead and Lead equivalence?

Lead is a metal which has the property of blocking/ absorbing x-ray radiation due to its heavy density. Hence lead sheets or shielding are used in the world of radiology for x-ray protection. Various thickness of pure lead provides different levels of protection.

Following list gives a comparison of protection provided by various lead thicknesses when x-ray machine is working at 80kV

0.01 mm Pb	- 25% protection from x-ray radiation
0.04 mm Pb	- 55% protection
0.10 mm Pb	- 70% protection
0.125 mm Pb	- 80% protection
0.25 mm Pb	- 96% protection
0.35 mm Pb	- 97% protection
0.50 mm Pb	- 99% protection
1.00 mm Pb	- More than 99% protection

Since pure lead is in metal form, it is brittle and cannot be worn as apparel. Hence lead in its pure form or as lead oxide is mixed with some percentage of plasticizers or binders to prepare lead sheeting or protective sheeting from which protection apparel are prepared.

The protection provided by this type of sheeting is compared with that of pure lead and referred to as “Lead Equivalence”. If apparel prepared using any of these lead sheeting is giving 80% protection from x ray radiation, then it is said to be having a lead equivalence of 0.125mm Pb.

There are other metals such as Antimony, Tin, Tungsten and Bismuth which falls into same category of materials as Lead also blocks or absorbs radiation. The protective sheeting prepared using these metals or combination of these metals also provides protection from x-ray and is referred in “Lead Equivalence” or mm Pb.

How to measure Lead equivalence:

There are various standards which explain how to measure Lead Equivalence. But the simple and common method is to compare the protection provided by pure lead sheet against that provided by the protective sheeting. This is an indigenous method developed at Kiran which provides very accurate results if not 100%. This test method is in use since few decades at Kiran and is accepted by certification bodies such as ISO.

Lead equivalence is measured at various kilo voltages starting from 60kV to 120kV. Current and time is set according to different voltages.

Following equipments are needed to measure the lead equivalence.

X-ray machine
Calibrated pure Lead reference with known optical density
Density measurement tool
X-ray film processor
Test sample of size minimum 1 square inch.
X-ray cassette

Test procedure

Cut the protective sheeting required to be tested which is at least 1 square inch in size
Prepare the x-ray cassette by inserting a film.
Place the cassette below the x-ray tube with tube side up.
Place the calibrated pure lead reference strip on the one side of the cassette. Please refer the
Place the samples to be tested on the cassette as shown in picture.
Set the x-ray machine to the voltage at which we want to test the lead equivalence of the sample.
Take the x-ray and process the film.
Now we have the developed test film from which we measure the optical density of the reference pure lead and test sample.

How to get the test result?

The developed film is placed on an illuminator and measure the density with densitometer.
Note down the density readings for the calibrated lead step wedge
Plot the lead equivalent values in X-axis (scale e.g. 10 mm = 0.01 mm Pb ) and Density on the Y-axis (scale e.g. 10 mm = 0.1 D) on the graph paper as shown in the Figure 2.
Join all the points and draw a straight line.
Take the density of the product at least of three points starting at the centre of the test sample.
Plot the lead equivalence of the test sample as explained in the figure-2, against the known values of calibrated pure Lead reference.
Now plot the density values for the test samples on the Y-axis.
Draw a line parallel to X-axis and intercept the line drawn for reference.
From the point of interception, draw a line parallel to Y-axis and connect X-axis.
The point at which the line joins X-axis is the Lead equivalence for the test sample

Figure 1: Cassette under X-ray tube with Reference Lead & Test lead sample

Figure 2: Cassette under X-ray tube with Reference Lead & Test lead sample

Figure 3: Cassette under X-ray tube with Reference Lead & Test lead sample