Abstract

This study investigates an X‐ray dose measurement method for computed tomography using Gafchromic films. Nonuniformity of the active layer is a major problem in Gafchromic films. In radiotherapy, nonuniformity error is reduced by applying the double‐exposure technique, but this is impractical in diagnostic radiology because of the heel effect. Therefore, we propose replacing the X‐rays in the double‐exposure technique with ultraviolet (UV)‐A irradiation of Gafchromic EBT2 and EBT3. To improve the reproducibility of the scan position, Gafchromic EBT2 and EBT3 films were attached to a 3‐mm‐thick acrylic plate. The samples were then irradiated with a 10 W UV‐A fluorescent lamp placed at a distance of 72 cm for 30, 60, and 90 minutes. The profile curves were evaluated along the long and short axes of the film center, and the standard deviations of the pixel values were calculated over large areas of the films. Paired t‐test was performed. UV‐A irradiation exerted a significant effect on Gafchromic EBT2 (paired t‐test; p=0.0275) but not on EBT3 (paired t‐test; p=0.2785). Similarly, the homogeneity was improved in Gafchromic EBT2 but not in EBT3. Therefore, the double‐exposure technique under UV‐A irradiation is suitable only for EBT2 films.PACS number(s): 87.53 Bn

Highlights

  • Gafchromic films are used for measuring X-ray doses of diagnostic examination, and the halfvalue layer and effective energy are indicators of quality assurance and control.[1,2,3,4,5] Gafchromic films are applicable to X-ray dose measurements in computed tomography (CT).(6-12)

  • We evaluated a method for correcting nonuniformity error in Gafchromic films

  • We removed the density irregularities caused by uneven thickness of the active layer by irradiating the films with UV-A rays

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Summary

Introduction

Gafchromic films are used for measuring X-ray doses of diagnostic examination, and the halfvalue layer and effective energy are indicators of quality assurance and control.[1,2,3,4,5] Gafchromic films are applicable to X-ray dose measurements in computed tomography (CT).(6-12)In the CT dose measurements, a Gafchromic film is rolled up with a certain thickness of flexible acrylic sheet,(7) enabling three-dimensional X-ray dose measurements in CT.[7,8] Several pieces of Gafchromic films are placed between the flexible acrylic sheets, whose thickness determines the spatial resolution of the dose measurement. Gafchromic films can be shaped into hemicylindrical acrylic phantoms. In this method, the Gafchromic film is sandwiched between the phantoms and is scanned during. CT, enabling radiation dose measurements of single or helical scans and conical-beam CT. This technique achieves a virtual three-dimensional rendering of the dose distribution.[12]. The information on radiation dose and active layer thickness is obtained as image information on the red and blue channels, respectively. Such information can be exploited to correct the nonuniformity error.[13,14]

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