R. Gotanda

Computed tomography phantom for radiochromic film dosimetry

To evaluate in detail the dose distribution during computed tomography (CT), a sheet roll CT dosimetry phantom (SRCT-P) with a radiochromic film (RF) was experimentally developed. The SRCT-P was made by rolling up a vinyl chloride sheet in a cylindrical shape to arbitrarily select the SRCT-P diameter, dose measurement position, and depth. The SRCT-P centre core consisted of a plastic hose in which a 10 mm acrylic bar with a RF was inserted. To determine the availability of the SRCT-P, the surface and centre doses (at a 5 mm radius) at each SRCT-P diameter (6–16 cm; every 2 cm) were measured. The ratios of the centre-to-surface doses (Dcentre/Dsurface) systematically increased, from 80 to 111%, for decreasing SRCT-P diameters, between 16 and 6 cm, respectively. The centre dose approached the surface dose as the SRCT-P diameter decreased. To use a RF for a CT dose measurement, further detailed research and analysis is necessary. However, this study has shown that a SRCT-P is useful and beneficial for the measurement of the dose distribution during a CT examination.

Dose Distribution in Pediatric CT Head Examination Using a New Phantom with Radiochromic Film

Performing a detailed dose measurement is important to keep radiation doses during computed tomography (CT) examinations as low as reasonably achievable. To estimate in detail the dose distribution during pediatric CT examination of the head, a sheet roll CT dosimetry phantom (SRCT-P) with radiochromic film (RF) was developed. The dose distributions in the SRCT-P (diameters of 6 [premature baby], 10 [neonate], and 14 [infant] cm) were evaluated. The SRCT-Ps were made by rolling up flexible acrylic sheets (1.1 g/cm3). RFs were positioned every 5 mm along the radius at each SRCT-P, starting at 10 mm (center) and ending on the surface. The dose distribution along the z-axis at the center or on the surface showed a flat or wave pattern, respectively. When the mean surface dose at 10 cm diameter was taken as 100%, the mean surface doses at 6 or 14 cm diameters were 105 or 96%, respectively, and the mean center doses at 6, 10, and 14 cm were 109, 99, and 74%, respectively. The maximum-minimum doses and dose distribution of a CT examination can be measured separately by using the SRCT-P with RF.

Evaluation of Effective Energy for QA and QC: Measurement of Half-value Layer Using Radiochromic Film Density

The effective energy of diagnostic X-rays is important for quality assurance (QA) and quality control (QC). However, the half-value layer (HVL), which is necessary to evaluate the effective energy, is not ubiquitously monitored because ionization-chamber dosimetry is time-consuming and complicated. To verify the applicability of GAFCHROMIC XR type R (GAF-R) film for HVL measurement as an alternative to monitoring with an ionization chamber, a single-strip method for measuring the HVL has been evaluated. Calibration curves of absorbed dose versus film density were generated using this single-strip method with GAF-R film, and the coefficient of determination (r2) of the straight-line approximation was evaluated. The HVLs (effective energies) estimated using the GAF-R film and an ionization chamber were compared. The coefficient of determination (r2) of the straight-line approximation obtained with the GAF-R film was more than 0.99. The effective energies (HVLs) evaluated using the GAF-R film and the ionization chamber were 43.25 keV (5.10 mm) and 39.86 keV (4.45 mm), respectively. The difference in the effective energies determined by the two methods was thus 8.5%. These results suggest that GAF-R might be used to evaluate the effective energy from the film-density growth without the need for ionization-chamber measurements.

Evaluation of effective energy using radiochromic film and a step-shaped aluminum filter

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 and complicated. To solve these problems, a method using radiochromic film and step-shaped aluminum (Al) filters has been developed. To this end, GAFCHROMIC EBT2 dosimetry film (GAF-EBT2), which shows only slight energy dependency errors in comparison with GAFCHROMIC XR TYPE-R (GAF-R) and other radiochromic films, has been used. The measurement X-ray tube voltages were 120, 100, and 80xa0kV. GAF-EBT2 was scanned using a flat-bed scanner before and after exposure. To remove the non-uniformity error caused by image acquisition of the flat-bed scanner, the scanning image of the GAF-EBT2 before exposure was subtracted after exposure. HVL was evaluated using the density attenuation ratio. The effective energies obtained using HVLs of GAF-EBT2, GAF-R, and an IC dosimeter were compared. Effective energies with X-ray tube voltages of 120, 100, and 80xa0kV using GAF-EBT2 were 40.6, 36.0, and 32.9xa0keV, respectively. The difference ratios of the effective energies using GAF-EBT2 and the IC were 5.0%, 0.9%, and 2.7%, respectively. GAF-EBT2 and GAF-R proved to be capable of measuring effective energy with comparable precision. 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.

Half-value Layer Measurement: Simple Process Method Using Radiochromic Film

Although the half-value layer (HVL) is one of the important parameters for QA and QC, constant monitoring has not been performed because the measurements using an ionization chamber (IC) are time-consuming and complicated. To solve these problems, the use of radiochromic film (GAFCHROMIC XR TYPE R: GAF-R) with step-shaped aluminum (Al) filters, referred to herein as the simple process method, has been developed. The measurement X-ray tube voltages were 120 kV, 100 kV, and 80 kV. The Al filter area, the full exposure area, and the unexposed area were set on the GAF-R so as to obtain correct data. The HVL was evaluated using the density attenuation ratio. The HVLs obtained using the GAFR and an IC dosimeter were compared. HVLs with X-ray tube voltages of 120 kV, 100 kV, and 80 kV using the GAF-R were 4.10 mm, 3.55 mm and 2.97 mm, respectively. The difference ratios of the HVLs using the GAF-R and the IC were 1.2%, 7.6%, and 10.0%, respectively. The HVL at 120 kV can be routinely and quickly measured using the simple process method. Therefore, an IC dosimeter is not needed for HVL measurements for QA and QC. However, the HVL measurements of low energy (100 kV and 80 kV) need attention.

Evaluation of GAFCHROMIC EBT2 dosimetry for the low dose range using a flat-bed scanner with the reflection mode

Recently developed radiochromic films can easily be used to measure absorbed doses because they do not need development processing and indicate a density change that depends on the absorbed dose. However, in GAFCHROMIC EBT2 dosimetry (GAF-EBT2) as a radiochromic film, the precision of the measurement was compromised, because of non-uniformity problems caused by image acquisition using a flat-bed scanner with a transmission mode. The purpose of this study was to improve the precision of the measurement using a flat-bed scanner with a reflection mode at the low absorbed dose dynamic range of GAF-EBT2. The calibration curves of the absorbed dose versus the film density for GAF-EBT2 were provided. X-rays were exposed in the range between ~0 and 120xa0mGy in increments of about 12xa0mGy. The results of the method using a flat-bed scanner with the transmission mode were compared with those of the method using the same scanner with the reflection mode. The results should that the determination coefficients (r2) for the straight-line approximation of the calibration curve using the reflection mode were higher than 0.99, and the gradient using the reflection mode was about twice that of the one using the transmission mode. The non-uniformity error that is produced by a flat-bed scanner with the transmission mode setting could be almost eliminated by converting from the transmission mode to the reflection mode. In light of these findings, the method using a flat-bed scanner with the reflection mode (only using uniform white paper) improved the precision of the measurement for the low absorbed dose range.

Reducing Non-uniformity Error of Radiochromic Film in the Diagnostic Range by Ultraviolet Exposure: Preliminary Study

Thickness irregularity of active layer is made to express density irregularity. True data by the X-rays are extracted by exposing Ultraviolet (UV) rays that prohibited exposure are exposed for radiochromic film (RF). When UV is exposed, the density irregularity is corrected. In addition, RF is initialized, thereby improving of data acquisition.

Functional residual capacity breath hold for subtraction image of dynamic liver MRI

PURPOSEnIn dynamic liver magnetic resonance imaging (MRI) studies, there are problems with misregistration when subtraction images are processed. For reduction of the misregistration, a functional residual capacity (FRC) phase breath-hold (FRC B-H) method was used.nnnMATERIAL AND METHODSnSixty patients (32 males and 28 females, aged 33-85 years, median age 69 years) were examined. The subjects were chronologically categorized into two groups: a voluntary expiratory (VE) B-H group and a FRC B-H group. The blood-flow phase images were classified as plain, arterial, portal and parenchymal phases. To evaluate the reproducibility of liver positions between VE B-H and FRC B-H in each phase (between Plain and Arterial, Arterial and Portal, Plain and Parenchymal), the misregistration areas were compared on the top of the liver.nnnRESULTSnThe misregistration area between Plain-Arterial, Arterial-Portal and Plain-Parenchymal in VE B-H was 731.0+/-1153.6, 1134.9+/-1357.2 and 628.4+/-844.5 (cm(2)), respectively. The misregistration area between each phase in FRC B-H was 386.4+/-874.9, 574.5+/-1086.1 and 279.8+/-551.2 (cm(2)), respectively. Using the Mann-Whitney U-test as quantitative analysis, the difference in misregistration areas between two groups was statistically significant (p<0.05). Differences in the qualitative analysis were also significant according to the chi(2) test (p<0.05).nnnCONCLUSIONnThe liver positions with FRC B-H were markedly more reproducible than those with VE B-H. To improve the registration accuracy of subtraction dynamic liver MRI, the FRC B-H should be used.

Improved detection of gastric cancer during screening by additional radiographs as judged necessary by the radiographer

PurposeThe aim of this study was to determine whether additional radiographs, as judged necessary by the radiographer, improves cancer detection during gastric cancer screening.Materials and methodsWe analyzed 144 gastric cancer cases among 137 744 individuals who underwent X-ray screening for gastric cancer. Radiographs were obtained by 17 radiographers at a screening center in Japan from April 2004 to March 2008. Additional radiographs were taken based on the radiographer’s judgment in cases of suspected cancer. During double-blind reinterpretation of the cancer case radiographs by two radiologists, we determined the number of cancer cases that were detected by standard radiographs alone. We next determined the number of cancer cases detected using both standard radiographs and additional radiographs.ResultsCompared to the number of cancer cases detected with standard radiographs alone (120 cases detected, 24 cases undetected), the number of cancer cases detected with both standard and additional radiographs (137 cases detected, 7 cases undetected) significantly increased (17 cases; P < 0.001, McNemar test).ConclusionWe found that taking additional radiographs, when judged necessary by the radiographer during radiographic gastric cancer screening, improves cancer detection.

Film-reading ability of radiographers in detecting gastric cancer during screening using X-ray examination

PurposeThe aim of this study was to evaluate the film-reading ability of radiographers in detecting gastric cancer during screening X-ray examinations.Materials and methodsA test set of 100 patients (50 negative and 50 positive; mean age 62 years, range 33–78 years) given a stomach X-ray examination were selected from those who underwent gastric cancer screening in Osaka, Japan, between 2000 and 2003. Eleven radiographers and four radiologists scored the test set on a five-point scale. A receiver operating characteristic (ROC) analysis was performed, and the area under the ROC curve (AUC) was defined as a measure of film-reading ability to detect cancer.ResultsNo significant difference (two-tailed P = 0.962, Welch’s t-test) was observed between averaged AUC values from radiographers (0.76, range 0.85–0.62) and radiologists (0.75, range 0.86–0.62).ConclusionFilm-reading ability of radiographers in detecting gastric cancer during screening X-ray examinations was not significantly different from that of radiologists. Our results suggest that radiographers can assist radiologists to detect gastric cancer during screening.