A novel radiation protection device based on tungsten functional paper for application in interventional radiology

Abstract Tungsten functional paper (TFP), which contains 80% tungsten by weight, has radiation‐shielding properties. We investigated the use of TFP for the protection of operators during interventional or therapeutic angiography. The air kerma rate of scattered radiation from a simulated patient was measured, with and without TFP, using a water‐equivalent phantom and fixed C‐arm fluoroscopy. Measurements were taken at the level of the operator's eye, chest, waist, and knee, with a variable number of TFP sheets used for shielding. A Monte Carlo simulation was also utilized to analyze the dose rate delivered with and without the TFP shielding. In cine mode, when the number of TFP sheets was varied through 1, 2, 3, 5, and 10, the respective reduction in the air kerma rate relative to no TFP shielding was as follows: at eye level, 24.9%, 29.9%, 41.6%, 50.4%, and 56.2%; at chest level, 25.3%, 33.1%, 34.9%, 46.1%, and 44.3%; at waist level, 45.1%, 57.0%, 64.4%, 70.7%, and 75.2%; and at knee level, 2.1%, 2.2%, 2.1%, 2.1%, and 2.1%. In fluoroscopy mode, the respective reduction in the air kerma rate relative to no TFP shielding was as follows: at eye level, 24.8%, 30.3%, 34.8%, 51.1%, and 58.5%; at chest level, 25.8%, 33.4%, 35.5%, 45.2%, and 44.4%; at waist level, 44.6%, 56.8%, 64.7%, 71.7%, and 77.2%; and at knee level, 2.2%, 0.0%, 2.2%, 2.8%, and 2.5%. The TFP paper exhibited good radiation‐shielding properties against the scattered radiation encountered in clinical settings, and was shown to have potential application in decreasing the radiation exposure to the operator during interventional radiology.


| INTRODUCTION
Lead has been employed in radiation protection in a clinical setting for many years; its uses include shielding for operator and patient protection in X-ray radiography and computed tomography, as well as during electron beam radiotherapy. In addition, radiation-shielding materials made of lead or lead equivalents are not flexible. 1,2 To overcome these problems, some researchers have explored ways of providing effective X-ray protection using alternative materials. [3][4][5][6] In an attempt to reduce the weight of protection materials, several vendors have developed composite lead-equivalent materials using mixtures of different elements such as lead, tin, copper, bismuthantimony, and yttrium. 7,8 Tungsten functional paper (TFP: Toppan Printing Co., Ltd., Tokyo, Japan), which is the paper with thickness of 0.3 mm and containing 80% tungsten powder by weight, has been developed as a lead free radiation shielding alternative, that has the advantages of paper in that it is easy to cut, fold, and stick onto other materials. These characteristics make it particularly flexible for many radiation-protection applications, such as in radiation-shielding surgical drapes, which have been reported based on bismuth and barium. [9][10][11][12][13][14][15] In a previous study, we have shown that TFP has radiationshielding properties for various peak X-ray kilovoltages in the range of 60-120 kVp. 16 We found that 10 TFP sheets (3.0 mm) could have 0.48 AE 0.02 mm lead equivalent for 100 kVp x-ray. 16 Furthermore, in electron-based radiotherapy, Fujimoto et al. reported an evaluation of the shielding characteristics of TFP against electron beams with clinically relevant energies and demonstrated that TFP is a useful radiation-shielding material with potential clinical applications. 17 Interventional radiology (IR) is a medical specialty that utilizes fluoroscopic guidance for the diagnosis and treatment of disease, and can involve the administration of relatively large doses of radiation. In IR, radiation exposure to the operator results mainly from scattered radiation from the patient. 18,19 Because the operator must stand close to patient during procedures and thus cannot utilize distance for radiation protection, occupational shielding is especially important. It is difficult for the operator to maintain a large distance of separation from the radiation source, because they must operate the equipment close to the patient. Moreover, during complex operations, a large number of x-ray fluoroscopy images may be required.
For these reasons, in IR, the operator is limited in terms of distance from the radiation source and the exposure time; therefore, shielding is especially important.
In IR, both the patient and operator face risks from radiation exposure. Patients are primarily at risk of potential skin effects from prolonged radiation exposure. Operators and staffs are at risk of radiation-induced cataracts and potentially stochastic long-term cancer risk. 20 These risks are minimized in appropriately controlled conditions; however, radiation hazards for workers conducting IR in inappropriate controlled conditions have been reported. 20 One principle of working with radiation is to ensure that the radiation exposure is "as low as reasonably achievable". 21 In order to achieve this, various practical protective measures have been developed to reduce the backscatter radiation from the patient, including ceiling mounted shields and table mounted leaded drapes. 22 Chida et al. measured operator air kerma rates using a luminescence dosimeter (Luxel Badge; Landauer Inc., Glenwood, IL, USA) during cardiac catheterization procedures, and found that the use of both the table and the ceiling lead drape reduced the occupational radiation exposure by approximately 42%. 23 The purpose of the present study was to investigate and measure the utility of a novel TFP as a new method to reduce the operator's radiation exposure from scattered X-ray radiation arising from the patient during IR procedures. We measured the reduction in the air kerma rate for the operator using various thicknesses of TFP, and compared measurements and computed simulations using the particle and heavy-ion transport code system (PHITS) 24 Monte Carlo (MC) simulation. The physical measurement of the air kerma rate from a phantom was difficult; this was because the air kerma rates for scattered radiation from the phantom were very low. 25

| MATERIALS AND METHODS
The TFP was constructed from tungsten powder on cellulose-based paper, with a density of 0.06-0.21 g/cm 2 and a thickness of 0.3 mm per sheet. This material has similar characteristics to paper, and can be cut, folded, and glued, so that it can take various forms. Figure 1 shows a schematic diagram of the measurement geometry. A C-arm digital angiography system (AXIOM-ArtisdFC: Medical Solutions, Erlangen, Germany) was used with the tube located beneath the table holding water equivalent slabs to simulate the patient. The X-ray equipment has two modes: fluoroscopy mode and image acquisition F I G . 1. A schematic diagram of the measurement geometry for scattered radiation from the water equivalent slab phantom. The TFP was taped to the operator side of the phantom and the number of TFP sheets was varied through 1, 2, 3, 5, and 10.
(or cine) mode. The X-ray tube has an added filtration of a 2.5 mm of Al and 0.6 mm of Cu and a 900-mm separation between the source and the detector. The patient table was in the beam path, and was modeled as 0.86 mm Aluminum equivalent for the Monte Carlo simulations. The field of view was 180 9 180 mm 2 , and the detector entrance dose rates were 32 nGy/pulse in fluoroscopy mode (with 15 frames s À1 and a pulse width of 3.6 ms) and 0.2 lGy/frame in cine mode (with 15 frames s À1 and a pulse width of 6.2 ms), which were measured using an area detector of an ionization chamber (Diamentor K2S; PTW, Freiburg, Germany) during IR automatically. Table 1 lists the imaging parameters used in this study. twice. Data regression was performed using the Igor Pro (Version 6.33J: WaveMetrics, Lake Oswego, OR, USA) software package, and measurements were compared with the MC simulations because it was difficult to measure the air kerma rate for the operator during angiography, the air kerma rate for scattered radiation from the phantom were very low. We used the particle and heavy-ion transport code system (PHITS; version 2.64: Japan Atomic Energy Agency, Japan), which has been used in many fields related to particle and heavy ion transport, including accelerator technology, radiotherapy, space radiation, and radiation protection. 19 The geometry as shown in Fig. 1 was modeled with PHITS, with the patient table modeled as aluminum of a 0.86 mm thickness. The X-ray spectrum T A B L E 1 Imaging parameters for the C-arm digital angiography system (AXIOM-ArtisdFC: Medical Solutions, Erlangen, Germany).

Mode Fluoroscopy Cine
Pulse/frame rate (s À1 ) 1 5 1 5 Pulse width (s) 3.6 9 10 -3 6.2 9 10 -3  The number of layers of TFP was varied through 0, 1, 2, 3, 5, and 10. The statistical uncertainties in the PHITS calculations were within 3% at waist level. Table 2 lists the results of measurements involving 0, 1, 2, 3, 5, and 10 sheets of TFP. The measured air kerma rates were decreased when an increasing number of TFP sheets were used for the eye, chest, and waist protection. However, at the knee level the measured dose varied little as a function of the number of TFP sheets.

| RESULTS
The trends were similar using fluoroscopy mode and cine mode. Figure 2 shows the air kerma rate as a function of the number of TFP sheets at eye, chest, waist, and knee level. Figure 3 shows the fitted curves for the percent reduction in rate at each level calculated from Fig. 2 data using Igor Pro software. With five sheets of TFP the rate reduction in the air kerma rates relative to those measured with no TFP shielding using cine mode were 50.4%, 46.1%, 70.7%, and 2.1% at eye, chest, waist, and knee levels, respectively.
Using fluoroscopy mode, the respective rate reduction in the air kerma rates as compared with no TFP shielding were 51.1%, 45.2%, 71.7%, and 2.8% at eye, chest, waist, and knee levels, respectively. Figure 4(a) shows a comparison between the calculations using PHITS and the measured data for the relative air kerma distributions at operator height without TFP shielding, and Fig. 4(b) shows the relative air kerma distributions using five sheets of TFP. The relative air kerma distributions at operator height were normalized to the value at waist level for each distribution. The MC simulations were in close agreement with our measurements, which the differences for each average were within 3% (Fig. 4).

| DISCUSSION
We evaluated the radiation-shielding ability of TFP regarding scattered radiation from a TM phantom. Although the statistical uncer-  Further development of TFP for applications such as surgical drapes will require additional modifications, including single-sided water or blood absorption layers, and a waterproofing layer on the rear. We have already attempted to create waterproof layers on TFP sheets. The simplest method is to laminate water-repellent material to one or both sides of the TFP sheet. TFP has the potential to be a practical substitute for existing protective devices such as lead drapes for the treatment table and ceiling, surgical drapes, and to serve as a general-purpose radiation-shielding material.

| CONCLUSION
We have described measurements and simulations of the radiationshielding ability of TFP for IR, especially concerning scattered radiation.
We expect that TFP will play an important role in reducing the level of scattered radiation more flexibly than lead, because TFP is easy to cut, paste, and roll as a result of its paper-like properties.