Evaluation and verification of a simplified lead equivalency measurement method

Abstract Purpose This technical note presents an inexpensive tool and method for determining lead equivalency using digital radiography x‐ray equipment. Methods A test tool was developed using commercially available lead tape (3M™ Lead Foil Tape 421). The test tool consisted of nine varying lead thick squares arranged in a larger square (0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, and 1.0 mm). It was imaged on a DR plate with a digital portable x‐ray unit across a range of energies (60–120 kVp) and two beam filtrations. Lead equivalency was determined by using the linear relationship between dose to the detector and pixel values in the raw images. The lead equivalency of the tape was validated using known lead thicknesses (physically measured with caliper). Additional lead equivalency measurements were made for protective eyewear, a thyroid shield, and a lead apron. Results The test tool and method measured the two known lead thicknesses to be –9.7% to 7.1% different from the actual values across the range of energies under normal x‐ray beam conditions and under a 1‐mm copper filtered x‐ray beam. The additional lead equivalency measurements of radiation protection apparel across energies ranged from –6% to 20% for both beam conditions when compared with the values provided by the manufacturer. Conclusion This work validates the test tool and methodology as an inexpensive alternative to checking the lead equivalency of radiation protection apparel in a clinical setting. The methodology is equipment independent with a few prerequisites.

standards available that address how to measure the attenuation properties of this radiation protection apparel: • ASTM F 2547-18 Standard test method for determining the attenuation properties in a primary x-ray beam of materials used to protect against radiation generated during the use of x-ray equipment 9 • ASTM F 3094-14 Standard test method for determining protection provided by x-ray shielding garments used in medical x-ray fluoroscopy from sources of scattered x-rays 10 • IEC 61331-1:2014 Protective devices against diagnostic medical x-radiation -Part 1: Determination of attenuation properties of materials 11 However, the ability for people, companies, and institutions to independently verify the lead equivalency of shielding materials using these standards can be cumbersome and requires specialized equipment for setup and measurement. This study investigates the development of an inexpensive test tool using lead tape and a process to determine the lead equivalency of materials using digital xray equipment.

| MATERIALS AND METHODS
To measure the lead equivalency of objects, there are some basic requirements. In the simplest iteration, one needs an x-ray source, test object, and a radiation detector. The first part of our setup was developing a test tool using commercially available lead tape - A portable digital x-ray system (Carestream DRX Revolution; Carestream DRX Plus 3543C) was used to make the measurements.
Before starting to use the test tool to evaluate radiation protection garments, there were some initial tests done to reduce potential errors and problems in the process. The tube voltage accuracy and exposure reproducibility were evaluated using a RTI Black Piranha Model 657 (RTI, Towaco, NJ, USA). The tube voltage accuracy was within 1.5% across the energy range used. The exposure reproducibility had a coefficient of variation of under 0.2%. Next, the linearity and uniformity of the digital detector were evaluated using a range of exposures (between 0 uGy and 50 uGy) at 70 kVp with 1 mm Cu in the beam and a source-to-detector distance of 150 cm.
The setup for validating the lead foil tape and measuring the lead equivalence of the different radiation protection apparel was at a source-to-image distance of 150 cm with the detector on the floor.
To validate the lead foil tape, known lead thicknesses were used.
Pieces of 1/32" and 1/64" lead were laid on the digital detector with the test tool placed in the center. There are typically tolerances of +/− 0.005" (0.127 mm) with regards to commercially available lead. 12,13 The thickness of the lead was determined using a caliper.
Similarly, the different radiation protection apparel (lead glasses (lead lenses), side shield of lead glasses (lead with vinyl sheet), thyroid (compositeantimony and lead), and apron (compositeantimony and lead)) were laid flat on the digital detector and exposed all at once. Exposures were made in increments of 10 from 60 kV to 120 kV under two beam conditions (no filter and 1 mm Cu filter). Table 1 shows the beam quality measured by the RTI Black Piranha for the portable digital x-ray system used.
The "FOR PROCESSING" images were exported Here B represents the transmission of x-rays for a given thickness, x, of shielding materials in mm. α, β, and γ are fitting parameters for Archer's eq. (1). Knowing B, α, β, and γ, eq. (2) can be used to calculate the lead thicknesses, which can then be compared with the known lead thicknesses. The calculated lead equivalencies of the radiation protection apparel were compared with the values supplied by the vendors and manufacturers. Table 2 shows the results of the caliper measurements of the lead sheets used for validation of the test tool. Table 3 shows the results of the detector uniformity at different detector air kerma levels. Table 4 has the fitting parameters results from fitting the transmission data to Archer's equation. Table 5 shows the calculated lead thicknesses against the caliper averaged lead thicknesses of the lead sheets.  Despite only using one type of digital x-ray system, this process is vendor independent. Some vendor's systems are even capable of making ROI measurements on the original data images on the system itself which streamlines the whole process.

| RESULTS
The geometry of this setup does not follow the geometrical design of any of the current standards mentioned above. However, it still can be used as a tool to effectively evaluate the attenuation properties of the radiation protection apparel.

| CONCLUSIONS
We have validated the use of an inexpensive test tool using commercially available lead foil tape in conjunction with a digital x-ray system for determining the lead equivalency of radiation protection apparel. It can serve as a useful tool to measure the attenuation properties in terms of lead equivalence for materials under different energy ranges and beam conditions. The methodology is equipment independent and has some prerequisites. It requires an x-ray system that can provide adequate accuracy and reproducibility results, known lead thicknesses for the test tool, detector uniformity without gross nonuniformities, known pixel values to detector entrance dose relationship, and the ability to draw ROIs on the acquired images.

CONF LICT OF I NTEREST
The authors have no conflict of interest to declare.