Rumi Gotanda

Energy response of the GAFCHROMIC EBT3 in diagnosis range

Although GAFCHROMIC EBT3 (EBT3) as a radiochromic film shows only slight energy dependency errors in comparison with other radiochromic films, the influence of energy dependence in the diagnosis energy range (less than 100 keV) is larger in the high energy range (over 100 keV). Based on this characteristic, adaptation of the EBT3 dosimetry in the diagnosis range was investigated. The energy response of the EBT3 in the diagnosis range at 30, 40, 50, 60 keV was measured using the density-absorbed dose calibration curve of the absorbed dose versus film density for the EBT3. Various data (degree of leaning, coefficient of determination) of each effective energy were compared. The density - absorbed dose calibration curves were linearly correlated in each of the effective energies. There was an energy dependent error of approximately 0.2 % from 30 to 60 keV. As a result, it can be seen that the EBT3 is available in the diagnosis energy range. However, the influence of the non-uniformity error caused by the repeatability of the scan method must be considered because EBT3 distortion has a serious influence on measurement precision.

Comparing three UV wavelengths for pre‐exposing Gafchromic EBT2 and EBT3 films

Gafchromic films are used for X‐ray dose measurements during diagnostic examinations and have begun to be used for three‐dimensional X‐ray dose measurements using the high‐resolution characteristics of Gafchromic films for computed tomography. However, the problem of unevenness in Gafchromic film active layers needs to be resolved. Double exposures using X‐rays are performed during therapeutic radiology, although this is difficult for a diagnostic examination because of a heel effect. Thus, it has been suggested that ultraviolet (UV) radiation be used as a substitute for X‐rays. However, the appropriate UV wavelength has not been determined. Thus, we conducted this study to decide an appropriate UV wavelength. UV peak wavelengths of 245 nm (UV‐A), 310 nm (UV‐B), and 365 nm (UV‐C) were used to irradiate EBT2 and EBT3 films. Each UV wavelength was irradiated for 5, 15, 30, and 60 min, and irradiation was then repeated every 60 min up to 360 min. Gafchromic films were scanned after every irradiation using a flatbed scanner. Images were split into RGB images, and red images were analyzed using ImageJ, version 1.44, image analysis software. A region of interest (ROI) one‐half inch in diameter was placed in the center of subtracted Gafchromic film images, and UV irradiation times were plotted against mean pixel values. There were reactions in the front and back of Gafchromic EBT3 and the back of Gafchromic EBT2 with UV‐A and UV‐B. However, UV‐C resulted in some reactions in both sides of Gafchromic EBT2 and EBT3. The UV‐A and UV‐B wavelengths should be used. PACS number(s): 87.53 BnGafchromic films are used for X-ray dose measurements during diagnostic examinations and have begun to be used for three-dimensional X-ray dose measurements using the high-resolution characteristics of Gafchromic films for computed tomography. However, the problem of unevenness in Gafchromic film active layers needs to be resolved. Double exposures using X-rays are performed during therapeutic radiology, although this is difficult for a diagnostic examination because of a heel effect. Thus, it has been suggested that ultraviolet (UV) radiation be used as a substitute for X-rays. However, the appropriate UV wavelength has not been determined. Thus, we conducted this study to decide an appropriate UV wavelength. UV peak wavelengths of 245 nm (UV-A), 310 nm (UV-B), and 365 nm (UV-C) were used to irradiate EBT2 and EBT3 films. Each UV wavelength was irradiated for 5, 15, 30, and 60 min, and irradiation was then repeated every 60 min up to 360 min. Gafchromic films were scanned after every irradiation using a flatbed scanner. Images were split into RGB images, and red images were analyzed using ImageJ, version 1.44, image analysis software. A region of interest (ROI) one-half inch in diameter was placed in the center of subtracted Gafchromic film images, and UV irradiation times were plotted against mean pixel values. There were reactions in the front and back of Gafchromic EBT3 and the back of Gafchromic EBT2 with UV-A and UV-B. However, UV-C resulted in some reactions in both sides of Gafchromic EBT2 and EBT3. The UV-A and UV-B wavelengths should be used. PACS number(s): 87.53 Bn.

Correction of nonuniformity error of Gafchromic EBT2 and EBT3

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 BnThis 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.

Effective energy measurement using radiochromic film: Application of a mobile scanner

Abstract The effective energy calculated using the half-value layer (HVL) is an important parameter for quality assurance (QA) and quality control (QC). However constant monitoring has not been performed because measurements using an ionization chamber (IC) are time-consuming and complicated. To solve these problems, a method using radiochromic film (GAFCHROMIC EBT2 dosimetry film (GAF-EBT2) with slight energy dependency errors), a mobile scanner and step-shaped aluminum (SSAl) filter is developed. The results of the method using a mobile scanner were compared with those of the recommended method using an IC in order to evaluate its applicability. The difference ratios of the effective energies by each method using a mobile scanner with GAF-EBT2 were less than 5% compared with results of an IC. It is considered that this method offers a simple means of determining HVL for QA and QC consistently and quickly without the need for an IC dosimeter.

Noise reduction of radiochromic film: median filter processing of subtraction image

Pre-ultraviolet rays exposure is a useful method to reduce non-uniformity error of radiochromic films. However, dust and scratch noises such as spike noise disturb precise measurement. To reduce these noises, median filter processing is applied for pre-subtraction and subtraction images. To reduce non-uniformity error of the thickness unevenness of Gafchromic EBT film, ultraviolet rays were exposed to correct data. There were three kinds of images obtained: first ultraviolet exposure image, second ultraviolet exposure image and the subtraction image of both. Median filer processing was performed on all these images. Eleven kinds of median filter radius factors (0.0 to 5.0) were applied using image analysis software. Data and graphs were then estimated. The maximum pixels value of dust was 229 on the second ultraviolet exposure image of film 3. After median filter preprocessing, the pixel value of the noises were similar to the minimum value. A 2.0-radius median filer is a useful factor for processing. Noise reduction that affected data of estimated images may be applied to measure radiation doses on a variety of radiochromic films. Ultraviolet exposure and subtraction method with median filter processing enable precise measurement and high spatial resolution dose distribution.

Ultraviolet exposure of Gafchromic XR-RV3 and XR-SP2 films

Gafchromic film has been used for X‐ray dose measurement in diagnostic examinations. Their use has been initiated for three‐dimensional X‐ray dose measurement by using the high‐resolution characteristics of Gafchromic films in computed tomography. However, it is necessary to solve the problem of nonuniform thickness in the active layers of Gafchromic films. A double exposure technique using X‐rays is performed in therapeutic radiology; it is difficult to use in a diagnostic examination because of the heel effect. Therefore, it is suggested that ultraviolet (UV) rays be substituted for X‐rays. However, the appropriate UV wavelength is unknown. In this study, we aimed to determine which UV wavelengths are effective to expose Gafchromic XR‐RV3 and XR‐SP2. UV lamps with peak wavelengths of 245 nm, 310 nm, and 365 nm were used. The three UV wavelengths were used to irradiate Gafchromic XR‐RV3 and XR‐SP2 films for 60 min, and irradiation was repeated every 60 min for 600 min thereafter. Films were scanned after each irradiation period on a flatbed scanner. The images were split into their red‐green‐blue components, and red images were stored using ImageJ version 1.44o image analysis software. Regions of interest (ROI), 0.5 inches in diameter, were placed at the centers of the subtracted Gafchromic film images, and graphs of UV irradiation duration and mean pixel values were plotted. There were reactions to UV‐A on both Gafchromic XR‐RV3 and XR‐SP2; those to UV‐B were moderate. However, UV‐C demonstrated few reactions with Gafchromic XR‐RV3 and XR‐SP2. From these results, irradiation with UV‐A may be able to correct nonuniformity errors. Uniform UV‐A irradiation of Gafchromic films with large areas is possible, and UV rays can be used as a substitute for X‐rays in the double exposure technique. PACS number: 87.53 BnGafchromic film has been used for X-ray dose measurement in diagnostic examinations. Their use has been initiated for three-dimensional X-ray dose measurement by using the high-resolution characteristics of Gafchromic films in computed tomography. However, it is necessary to solve the problem of nonuniform thickness in the active layers of Gafchromic films. A double exposure technique using X-rays is performed in therapeutic radiology; it is difficult to use in a diagnostic examination because of the heel effect. Therefore, it is suggested that ultraviolet (UV) rays be substituted for X-rays. However, the appropriate UV wavelength is unknown. In this study, we aimed to determine which UV wavelengths are effective to expose Gafchromic XR-RV3 and XR-SP2. UV lamps with peak wavelengths of 245 nm, 310 nm, and 365 nm were used. The three UV wavelengths were used to irradiate Gafchromic XR-RV3 and XR-SP2 films for 60 min, and irradiation was repeated every 60 min for 600 min thereafter. Films were scanned after each irradiation period on a flatbed scanner. The images were split into their red-green-blue components, and red images were stored using ImageJ version 1.44o image analysis software. Regions of interest (ROI), 0.5 inches in diameter, were placed at the centers of the subtracted Gafchromic film images, and graphs of UV irradiation duration and mean pixel values were plotted. There were reactions to UV-A on both Gafchromic XR-RV3 and XR-SP2; those to UV-B were moderate. However, UV-C demonstrated few reactions with Gafchromic XR-RV3 and XR-SP2. From these results, irradiation with UV-A may be able to correct nonuniformity errors. Uniform UV-A irradiation of Gafchromic films with large areas is possible, and UV rays can be used as a substitute for X-rays in the double exposure technique. PACS number: 87.53 Bn.

Size and Shape of Spherical Objects on Full-Field Digital Mammography and Digital Breast Tomosynthesis Images

This study assessed the accuracy of shape and size representation of spherical objects on full-field digital mammography (FFDM) and digital breast tomosynthesis (DBT) images. Six 5-mm-thick polymethylmethacrylate slabs were positioned on the breast support table with 9 aluminum spherical objects of 30 (± 0.1) mm diameters between the first and second slabs. X-ray imaging was performed using FFDM and DBT (angular range 15°–40°, with correction of magnification), and repeated with the objects placed between the third and fourth slabs, and subsequently between the fifth and sixth slabs. The aspect ratio of the spherical objects and longer diameter were measured to evaluate the shape and size, respectively. A Steel-Dwass test was performed for comparative analysis. A P value <0.05 was considered significant. No significant differences in the aspect ratio of the spherical objects imaged using FFDM, DBT15°, or DBT40° images were observed (overall median: 1.02, overall range: 1.00–1.06). The longer diameter on the FFDM was increasingly magnified (median, range) with increasing distances of 20 mm (32.5, 31.8–33.5 mm) and 40 mm (33.6, 32.9–34.7 mm) between the breast support table and object center. However, in the case of DBT, the longer diameter was approximately the same as that of the actual object (overall, 30.4, 30.0–31.7 mm). At each height, the longer diameter was significantly different between the FFDM and DBT15° images and between the FFDM and DBT40° images (all P = 0.001), with no significant difference in that between the DBT15° and DBT40° images. The size on the FFDM images was magnified as compared to the size of the actual objects, and that on the DBT images was approximately the same as that of the actual objects. Thus, preoperative tumor size determination using FFDM images should be avoided.

Basic Study of the Imaging Conditions on Tumor Volume Measurement Using 3D-MR Imaging of the Liver While Patient Holds Breath

Tumor volumetric measurement using liver 3D-MR imaging tends to cause measurement errors because the liver moves with the diaphragm during imaging while the patient is holding the breath. The breath holding method of the patient when 3D-MR imaging for measurement of tumor volume was performed was discussed using a movable simulated tumor phantom. Furthermore, optimal imaging conditions (breath holding method, slice section, voxel setting and phase encoding direction) with the least influence by body motion were studied. Based on three breath holding methods, the simulated phantom was moved in the direction of the foot-head along the line of the MRI receiver coil. The image sequence was a 3D-T1 weighted fast field echo method. The imaging time was fixed at 20 s. As a result, the breath holding method with the smallest measured volume error is functional residual capacity (FRC) breath hold (0.1 mm/s). The optimum imaging condition is when the imaging section is set in the axial direction and the phase encoding direction is perpendicular to the moving direction with an iso voxel as small as possible. In the case where it is necessary to set the phase encoding in the direction parallel to the proceeding direction, imaging with the setting of rectangular voxel can suppress the measurement error of the volume. In order to measure accurate volume using liver 3D-MR imaging, it is necessary to know the direction of movement of the tumor during respiration and to set appropriate breath holding method, section, phase encoding direction and voxel size.

Optimal Variable Refocus Flip Angle Control Method and Echo Train Length for Suppressing Exposure to Radio Frequency

The popularization of 3-Tesla magnetic resonance imaging (MRI) has improved the quality of images and shortened typical examination times. However, a side effect of this is increased exposure to radio frequency (RF) radiation. The amount of RF exposure can be controlled using a technique called variable refocus flip angle (vRFA). Controlling vRFA is also an important for improving the signal to noise ratio (SNR) and for reducing blurring on MRI images. In this study, we examined the influence of controlling vRFA and echo train length (ETL) on SNR. To do this, we used a device that can arbitrarily control three angles—the fifth RFA, the RFA centered in k-space, and the final RFA. Using a phantom, T1 and T2 values were made equal to gray- and white-matter, respectively. The repetition time was 5000 ms and echo time 90 ms. By setting the fifth RFA to 40° and using an ETL of 11–15, the signal shifted smoothly to a pseudo steady-state (PSS), and a stable signal was obtained. Further, we were able to suppress blurring by gently changing the k-space-centered RFA. In the final echo, we were able to maintain PSS by increasing the final RFA up to 180°, resulting in a high SNR. Results of this study showed the changes reduced RF exposure. Using an ETL of 30, blurring was reduced, though RFA control was similar to that used with ETLs of 11–15; although slightly higher RF exposure was required to obtain a high SNR, the fifth RFA was required to be 60°–90°.

Influence of Image Resolution Property on Aliasing Error of Digital Wiener Spectrum

The noise properties of a radiography system are commonly described by its wiener spectrum (WS). The two-dimensional discrete Fourier transform (2D-DFT) methods is the most commonly used and accepted techniques for measuring the digital WS. The 2D-DFT method has been adopted by the International Electrotechnical Commission (IEC) as a noise-power spectrum. However, the digital WS contains the effects of aliasing error, and this error depends on the presampled modulation transfer function (MTF) of the digital radiography (DR) system. The aim of this work was to show the influence of the aliasing error of the digital WS when the image resolution property was changed. We examined the influence of the aliasing error using simulated noise images. Two types of noise images with same pixel size and different presampled MTFs were generated by using ImageJ (National Institutes of Health: NIH). These images were used to simulate the image resolutions of an indirect/direct flat panel detector (FPD). The theoretical WS of the simulation noise image can be derived from a standard deviation σ of the Gaussian filter and added noise. Simulated noise images were analysed using the 2D-DFT method. The WS values calculated from those simulation images were compared with the theoretical WS values. The WS values in the indirect and direct FPD increased, compared to the theoretical WS values. The average relative differences for frequencies up to the Nyquist frequency were 27.9% and 85.2%, respectively. The results showed that the degree of the influence of the aliasing error of the digital WS depends largely on the presampled MTF of the DR system. Therefore, we should take into account the impact of the aliasing error of the digital WS, in the comparison between DR systems with different presampled MTFs.