Akihiko Tabuchi

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.

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.

Evaluation of two radiochromic films for HVL measurement

Although the half-value layer (HVL) is one of the important parameters for quality assurance (QA) and quality control (QC), constant monitoring has not been performed because measurements using an ionization chamber (IC) are time-consuming. To solve these problems, a method using radiochromic film and step-shaped aluminum (Al) filters has been developed. In this study, GAFCHROMIC EBT2 dosimetry film (GAF-EBT2) and GAFCHROMIC XR TYPE-R dosimetry film (GAF-R) has been used. The measurement X-ray tube voltage was 120 kV. Two radiochromic films were scanned using a flat-bed scanner. To remove the nonuniformity error of scanned images, image J version 1.40g image analysis software has been used. HVL was evaluated using the density attenuation ratio. The HVLs and second HVLs of GAF-EBT2, GAF-R, and an IC dosimeter were compared. The HVLs (second HVLs) at 120 kV using GAF-EBT2, GAF-R, and an IC dosimeter were 4.47 mm (10.97 mm), 4.17 mm (8.61 mm), and 4.03 mm (10.35 mm) respectively, and the effective energies were 40.6 keV, 39.3 keV, and 38.7 keV, respectively. The difference ratios of the second HVLs using GAF-EBT2 (GAF-R) and an IC dosimeter were 6.0% (– 16.8%), –10.6% (–11.6%), and 5.1% (–0.2%), respectively. In addition, the difference ratio of the second HVLs at 120 kV using GAF-EBT2 and GAF-R was 27.4%. GAF-EBT2 and GAF-R proved to be capable of measuring effective energy within an error range of less than 5%. However, in HVL measurements of devices operating in the high-energy range (X-ray CT, radiotherapy machines, and so on), GAF-EBT2 was found to offer higher measurement precision than GAF-R, because it shows only a slight energy dependency.

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°.

Evaluation of Scanner Image Properties of Radiochromic Film for Measuring Effective Energy

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 flatbed scanner and step-shaped aluminum filters has been developed. In this study, a mobile scanner was used for digital image acquisition of GAF-EBT2, because the mobile scanner has characteristics of low cost and increased portability compared with a flatbed scanner. GAF-EBT2 was scanned using the flatbed scanner in transmission mode and reflection mode, and the mobile scanner in reflection mode. The HVL (X-ray tube voltages of 120 kVp) was evaluated using the density attenuation ratio. The effective energies obtained using HVL of GAF-EBT2 (flatbed scanner, mobile scanner) and an IC dosimeter were compared. In reflection mode, both the flatbed scanner and the mobile scanner provided a flat density profile curve without the error of the light source. The difference ratios of the effective energies by each method using a flatbed scanner (reflection mode) and a mobile scanner (reflection mode) with GAF-EBT2 were less than 5 % compared with results of an IC dosimeter. 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.

Evaluation of Dose Distribution According to Patient Setup Errors in Pediatric Head CT Examination: A Phantom Study

Radiation doses during computed tomography (CT) examinations are generally evaluated by CT dose index (CTDI). The CTDI is measured by means of a CTDI phantom, which is placed in the center of the CT gantry aperture. However, patients are not always positioned in the centered in clinical settings. In this study, dose distributions associated with patient setup errors and head size were evaluated using phantoms developed for pediatric patients. The phantoms were made using flexible acrylic sheets in a cylindrical shape [diameters of 6 (premature baby), 8 (neonate), 10 (infant), and 12 cm (child)] and placed on the end of the CT bed. The bed position was lowered from the center by up to a radius length for each phantom size. Dose distributions in the phantoms were measured using radiochromic film. By lowering the bed position, the surface doses at 0° increased and those at 180° and the center doses decreased. However, the other measurement positions exhibited complex changes depending on the phantom size. In addition, the shapes of the dose profile curves varied according to the distance from the X-ray tube (i.e. location in the gantry aperture). In this study, the surface doses became smaller with decreasing distance from the X-ray tube. Therefore, decreasing the distance from the lens to the X-ray tube would be an effective way to reduce the absorbed dose of the lens. However, the dose distributions in the phantoms varied in a complex manner depending on patient setup errors and phantom size. In addition, beam slice width and pitch were important factors in determining the amount of the CT dose.

Radiographers as film reader in gastric cancer screening: A comparative study using the results of a film-reading workshop

The ability of radiographers to detect cancer on radiographs was compared to that of physicians to assess the competence of radiographers as film readers in gastric cancer screening. After obtaining permission for the screening and analysis of data, a test set of films from 100 patients who underwent gastric cancer screening using radiographic examination was evaluated. Films were selected from 137 744 patients and 197 cases of cancer. Screening was performed at the Osaka Cancer Prevention and Detection Center in Japan between April 2004 and March 2008. Eleven radiographers affiliated to the screening center and 37 physician readers scored the test set on a 4-point scale blinded to the other participants’ scores. Receiver operating characteristic (ROC) analysis was performed to evaluate the interpretation performance, and the area under the ROC curve (AUC) was defined as the ability of the radiographer or physician reader to detect cancer. The results showed that the average ability of radiographers to detect cancer (AUC, 0.82) was identical to that of physician readers (p = 0.96, Welch’s t test), suggesting that radiographers can assume the role of film readers in gastric cancer screening.

Diagnostic Validity of High-Density Barium Sulfate in Gastric Cancer Screening by Record Linkage with the Osaka Cancer Registry –AUC Analyses of Sensitivity and Specificity on Sex and Ages of Subjects–

High-density barium sulfate (HDB) has been recommended by the Japanese Society of Gastroenterological Cancer Screening (JSGCS) and used in many medical facilities. However, it is not confirmed that the new method using HDB is superior in diagnostic validity compared to the conventional method using moderate-density barium sulfate (MDB). Pre viously, the authors reported that both methods showed simi lar validity in terms of sensitivity and specificity in gastric cancer screening. In this study, sensitivity and specificity were re-examined in more detail by area under receiver-operating characteristic (ROC) curves (AUC) analysis, especially on the sex (male, female) and the ages (>50, 55, 60, <60) of subjects. The results showed that the diagnostic validity of the method using HDB is almost same to that of the conven tional method using MDB. Clinical improvement in gastric cancer screening is in need of further studies on the new me thod using HDB.