THE CONCEPT OF X-RAY CT DOSE EVALUATION METHOD USING RADIOCHROMIC FILM AND FILM-FOLDING PHANTOM

Backside Irradiation of Ultraviolet-A for Correcting Nonuniformity Error of Gafchromic XR-QA2 Films

Purpose

Radiochromic film is used for quality assurance and quality control of X-ray equipment in the diagnostic radiology. In addition, three-dimensional dose distribution of computed tomography (CT) is measured. To correct the nonuniformity and uncertainty of radiochromic films for dose measurement of CT, the films are preirradiated ultraviolet (UV)-A rays. There is a difference in the UV protection strength of radiochromic films. A concern exists about the effects of the UV-A irradiation intensity. We thus irradiated with UV-A rays from the backsides of the films to assess if backside irradiation was possible.

Materials and Methods

Gafchromic XR-QA2 and RTQA2 were used in this study. The UV-A rays were simultaneously irradiated on the front and backsides of each film for 12 h. The yellow layer of each film was scanned and imaged. The average pixel values ± standard deviations (SDs) were compared. In the statistical analysis, a paired t-test was performed. To compare, the active-layer densities engendered by the UV-A rays. Calibration curve was created with 48 h of preirradiation of UV-A.

Results

The mean pixel values ± SD for Gafchromic XR-QA2 on the front and backsides were 130.776 ± 0.812 and 81.015 ± 1.128, respectively. On the other hand, the mean pixel values ± SD for Gafchromic RTQA2 on the front and backsides were 62.299 ± 1.077 and 133.761 ± 1.365, respectively. The statistical results of the paired t-test were significantly different (P < 0.01) between both films. Fitting equation of the calibration curve is shown below. y = -390.47 ± 200 + (443.45 ± 10x80).5068 ± 0.0434.

Conclusion

Based on the relationship between the sensitivity of the active layer to UV-A rays and the strength of UV protection on the surface, we concluded that backside irradiation is recommended for Gafchromic XR-QA2, and frontside irradiation is recommended for Gafchromic RTQA2.

Identification of the computed tomography dose index for tube voltage variations in a polyester-resin phantom

The aim of this study is to measure the volumetric computed tomography dose index (CTDI_vol) for different tube voltages for a polyester-resin (PESR) phantom, and to compare it to values for a standard polymethyl methacrylate (PMMA) phantom. Both phantoms are head phantoms with a diameter of 16 cm. The phantoms were scanned by a CT scanner (GE Revolution EVO 64/128 slice) with tube voltages of 80, 100, 120, and 140 kV. The other scan parameters were constant (i.e. tube current of 100 mA, rotation time of 1 s, and collimation width of 10 mm). The CTDI_100,c and CTDI_100,p were obtained by measuring the dose with an ionization chamber inserted into five holes within the phantoms. The CTDI_vol was calculated based on the CTDI_100,c and CTDI_100,p values. The measurements were repeated three times for each hole. It was found that the CTDI_vol values for the PESR phantom were dependent on tube voltage value, and were similar to the dependency in a PMMA phantom. The maximum CTDI_vol difference between the PESR and PMMA phantoms was 7.5%. We conclude that the dose measured in the PESR phantom is similar to that in the PMMA phantom and that the PESR phantom can be used as an alternative if the PMMA phantom is not available.

Development of a three-dimensional dose evaluation method for computed tomography

During a single scan using computed tomography, an X-ray tube orbits along a 360°-circular path around the patient. A scan obtained using the half-cylindrical type phantoms with a radiochromic film sandwiched in between reveals a pixel value map illustrating the two-dimensional (2D) dose distribution. A three-dimensional (3D) dose distribution can be obtained with a 360° rotation of the 2D dose map. This study evaluates the concept and methodology of creating a 3D dose map to develop a phantom with a radiochromic film for obtaining the 3D dose distribution. The coronal and axial plane dose distributions were also evaluated. A single scan computed tomography image obtained using a half-cylindrical type of acrylic phantom with a sandwiched radiochromic film was studied. The diameters of the phantoms were 10 and 16 cm, and their lengths were 30 cm. A 2D image of the XR-QA2 film was obtained using an image scanner and image processing software. A red channel image was used to obtain the 3D dose distribution using a computing platform. A pseudo color was applied to the red channel image from which cross-sectional color images were obtained. Half of the cross-sectional pixel data were rotated by 360° to generate the data for each axial plane. The image created was saved, and a 3D pixel value map was constructed. The dose measurement procedure for the 3D dose distribution was developed using half-cylindrical acrylic phantoms with a radiochromic film.