[en] The purpose of this study was to introduce a new parameter which enhances breast cancer detection using X-ray mammography. We used the database of X-ray mammograms generated by the Japan Society of Radiological Technology. The new parameter called 'quasi-fractal dimension (Q-FD)' was calculated from the relationship between the cutoff values for the maximum image intensity in the lesion set at 21 levels from 20% to 100% at equal intervals and the number of pixels with an intensity exceeding the cutoff value. In addition to Q-FD, the image features such as curvature (C) and eccentricity (E) were extracted. The conventional fractal dimension (C-FD) was also calculated using the box-counting method. We used artificial neural networks (ANNs) as a classification method. When using C, E, C-FD and age as inputs in ANNs and taking the number of neurons in the hidden layer as 50, we found the area under the receiver operating characteristic curve (A_Z) was 0.87±0.07 in the task differentiating between benign and malignant masses. When Q-FD was added to inputs in addition to the above parameters, the A_Z value was significantly improved to become 0.93±0.09. These results suggested that Q-FD is effective for discriminating between benign and malignant masses. (author)

[en] We previously introduced a quasi-fractal dimension (Q-FD) to enhance breast cancer detection in X-ray mammography. In the present study, we evaluated the usefulness of this image feature for differentiating between benign and malignant masses using a support vector machine (SVM) with various kernels. The kernel computes the inner product of the functions that embed the data into a feature space where the nonlinear pattern appears linear. Q-FD was calculated using the method previously reported from the database of X-ray mammograms produced by the Japan Society of Radiological Technology. In addition to Q-FD, the image features such as curvature (C) and eccentricity (E) were extracted. The conventional fractal dimension (C-FD) was also calculated using the box-counting method. First, we investigated the SVM performance in terms of accuracy, sensitivity and specificity in the task of differentiating between benign and malignant masses by taking 5 parameters (C, E, C-FD, Q-FD and age) as input features in SVM. When using the linear kernel, the best accuracy was obtained at a regularization parameter of 50. For the polynomial and radial basis function (RBF) kernels, the best accuracy was obtained when the degree of polynomial and the width of RBF were 1 and 1, respectively. The accuracies were 0.746±0.089, 0.731±0.095 and 0.734±0.086 for the linear, polynomial and RBF kernels, respectively, when using C, E, C-FD and age as input features in the SVM. When Q-FD was added to the above input features, the accuracies were significantly improved to 0.957±0.045, 0.950±0.045 and 0.949±0.052 for the linear, polynomial and RBF kernels, respectively. These results suggest that Q-FD is effective for discriminating between benign and malignant masses and SVM is highly recommended as a classifier for its simple utilization and good performance, especially when the training set size is small. (author)

[en] We evaluated radiofrequency (RF) heating of various implants embedded in a gel phantom during magnetic resonance (MR) procedures. We examined the dependence of RF heating on variation in specific absorption rate (SAR) and angle between the implant and the static magnetic field (B₀) and on the displacement of the phantom in the irradiation coil using a 1.5-tesla MR system, and we compared the influence of RF heating on the same implant using a 3.0T MR system. Our results support the occurrence of RF heating of implants made of non-magnetizing metal. We observed greater RF heating when the implant was set parallel to B₀, embedded at a shallower depth, and placed at the center of the RF irradiation coil. We also confirmed that the rise in temperature was proportionate to the increase in SAR. We considered the difference in temperature elevation on depth of embedding to reflect the skin-depth effect of RF intensity for both the 1.5- and 3.0-T MR systems. (author)

[en] We investigated the feasibility of combining the active contour model with gradient vector flow (Snakes-GVF) to estimate left ventricular (LV) volumes from cardiac cine magnetic resonance imaging (MRI). MRI data were acquired from 27 patients, including 14 adults (9 men, 5 women, 55.0±23.3 years) and 13 children (10 boys, 3 girls, 2.7±2.1 years) using Gyroscan Intera (1.5 Tesla, Philips Medical Systems). LV volumes were calculated by adding the areas surrounded by the contour extracted by Snakes-GVF and compared with volumes estimated by manual tracing. Those estimated by Snakes-GVF [y (mL)] correlated well with those estimated by manual tracing [x (mL)]. In adult cases, the regression equation and correlation coefficient were y=1.008x-0.517 and 0.996, respectively. In pediatric cases, they were y=1.174x-2.542 and 0.992, respectively. In conclusion, Snakes-GVF is a powerful and useful tool for quantifying LV volumes using cardiac MRI. (author)

[en] Respiratory motion makes it difficult to quantify myocardial perfusion with dynamic magnetic resonance imaging (MRI). The purpose of this study was to evaluate an automatic registration method for motion correction for quantification of myocardial perfusion with dynamic MRI. The present method was based on the gradient-based method with robust estimation of displacement parameters. For comparison, we also corrected for motion with manual registration as the benchmark. The myocardial kinetic parameters, K₁ (rate constant for transfer of contrast agent from blood to myocardium) and k₂ (rate constant for transfer from myocardium to blood), were calculated from dynamic images with a two-compartment model. The images corrected by the present method were similar to those corrected by manual registration. The kinetic parameters obtained after motion correction with the present method were close to those obtained after motion correction with manual registration. These results suggest that the present method is useful for motion correction for quantification of myocardial perfusion with dynamic MRI. (author)

No abstract available

[en] We evaluate radiofrequency (RF) heating of two kinds of hip joint implants of different sizes, shapes and materials. Temperature rises at various positions of each implant are measured and compared with a computer simulation based on electromagnetic-field analysis. Two kinds of implants made of cobalt-chromium alloy and titanium alloy were embedded at a 2-cm depth of tissue-equivalent gel-phantom. The phantom was placed parallel to the static magnetic field of a 1.5 T MRI device. Scans were conducted at the specific absorption rate of 2.5 W/kg for 15 min, and temperatures were recorded with RF-transparent fiberoptic sensors. Temperatures of the implant surface were measured at 6 positions, from the tip to the head. Measured temperature rises were compared with the results of electromagnetic-field analysis. The maximum temperature rise was observed at the tip of each implant, and it was 9.0degC for the cobalt-chromium implant and 5.3degC for the titanium implant. The simulated heating positions with electromagnetic-field analysis accorded with experimental results. However, a difference in temperature rise was seen with the titanium implant. RF heating was confirmed to take place at both ends of the implants in spite of their different shapes. The maximum temperature rise was observed at the tip where there is large curvature. The value was found to depend on physical properties of the implant materials. The discrepancy between experimental and simulated temperature rises was presumed to be the result of an incomplete model for the titanium implant. (author)

[en] Increased x-ray exposure to physicians’ eye lenses during radiology procedures is a significant concern. In this study, x-ray exposure to the eye was measured using an anthropomorphic head phantom, with and without radiation-protective devices, to examine the dose of x-ray radiation that physicians are exposed to during endoscopic retrograde cholangiopancreatography (ERCP). X-ray exposure of the eye was measured using novel dedicated direct eye lens dosimeters that could specifically measure H _p(3) during the ERCP procedure. The spatial dose in the height direction of the physician was measured using an ionization chamber dosimeter. Eye dosimeters were attached inside and outside the lead (Pb) glasses attached to the head of the human phantom to demonstrate its protective effect. Irradiation from the system lasted for 30 min. When the overcouch x-ray tube system is used, the cumulative radiation dose over the 30 min x-ray fluoroscopy time, without the use of radiation-protective devices, to the left and right eyes was 3.7 and 1.5 mSv, respectively. This dose was estimated to be the dose to the lens per therapeutic ERCP examination. With radiation-protective glasses, the dose reduced to 1.8 and 1.0 mSv for the left and right eye, respectively. The results of our study indicated that radiation exposure to the eye was reduced by up to 80.0% using Pb glasses and by 96.8% using radiation-protective curtains. Our study indicates that a physician’s maximum radiation exposure to the eyes during an ERCP procedure may be above the level recommended by the International Commission on Radiological Protection when the physician does not use radiation-protective devices. The eyewear, which is larger and fitted more closely to the face, provided a better protection effect even with a low lead equivalence, demonstrating that the shape of eyewear is important for protective function. (paper)

[en] Gafchromic films have been applied to X-ray dosimetry in diagnostic radiology. To correct nonuniformity errors in Gafchromic films, X-rays in the double-exposure technique can be replaced with ultraviolet (UV)-A rays. Intensities of the incident and transmitted UV-A rays were measured. However, it is unclear whether the chemical color change of Gafchromic films affects the UV-A transmission intensity. Gafchromic EBT3 films were suitable to be used in this study because non-UV protection layers are present on both sides of the film. The film is placed between UV-A ray light-emitting diodes and a probe of a UV meter. Gafchromic EBT3 films were irradiated by UV-A rays for up to 60 min. Data for analysis were obtained in the subsequent 60 min. Images from before and after UV-A irradiation were subtracted. When using 375 nm UV-A, the mean ± standard deviation (SD) of the pixel values in the subtracted image was remarkably high (11,194.15 ± 586.63). However, the UV-A transmissivity remained constant throughout the 60 min irradiation period. The mean ± SD UV-A transmission intensity was 184.48 ± 0.50 μm/cm². Our findings demonstrate that color density changes in Gafchromic EBT3 films do not affect their UV-A transmission. Therefore, Gafchromic films were irradiated by UV-A rays as a preexposure. (author)

[en] The purpose of this study is to develop a method for use at extremely low-dose ranges and to decrease the uncertainty outside the recommended range of Gafchromic RTQA2 (RTQA2). By this method, the CT dose including the scattered radiation region can be grasped. The base density was increased by ultraviolet (UV)-ray preirradiation. RTQA2 was irradiated with UV-A rays for 26 and 40 h. Subsequently, RTQA2 was exposed to 2, 4, 6, 8, 10, 25, 50, 75, 100, 150, 200 and 250 mGy X-rays using a segmentation method. Calibration curves with and without UV-A irradiation were compared. The calibration curve with 40-h UV-A ray irradiation was the most linear, and a steeper slope area was not observed. The uncertainty in the calibration curve was reduced (p < 0.05). UV-A ray irradiation is an effective method for treating RTQA2; the accuracy in the extremely low-dose range of RTQA2 was improved. (authors)