To evaluate the ability of periodically rotated overlapping parallel lines with enhanced reconstruction diffusion-weighted imaging (PROPELLER DWI) to distinguish between vessel occlusion and slow flow. Materials and Methods: Using a flow phantom with various velocities (1.37 to 11.1 cm/s), the signal-intensity ratios of the phantom, with the intensity of no flow as baseline, were measured using the following imaging sequences: PROPELLER DWI, spin-echo TI-weighted imaging (SE TIWI), fast-spin-echo T2-weighted imaging (FSE T2WI), two-dimensional phase-contrast imaging (2D PC), and two-dimensional time-of-flight imaging (2D TOP). The b-factor of PROPELLER DWI was varied from 0 to 1000 s/mm2. The velocity encoding of 2D PC was varied from 2 to 30 cm/s. Results: At the lowest flow velocity (1.37 cm/s), the signal-intensity ratio was 0.0075 for PROPELLER Dwell (b-factor=1000 s/mm^2), 1.8 for SE T1WI, 0.67 for FSE T2WI, 11 for 2D PC (velocity encoding=2 cm/s), and 1.4 for 2D TOP. The signal-intensity ratio was smallest for PROPELLER DWI, even when the reciprocals of the signal-intensity ratio of 2D PC or 2D TOP were considered. Conclusion: The results indicate that PROPELLER DWI provides the best signal intensity-ratio between vessel occlusion and slow flow. Although DWI with single-shot echo-planar imaging (EpI) or multi-shot EPI may have similarly high sensitivity for slow flow, these sequences do not have high spatial resolution or robustness to susceptibility artifacts. PROPELLER DWI would be a better choice for distinguishing between occluded and low-velocity arteries in the skull base or parasellar regions.
Although the principal dosimetric quantity in computed tomography (CT) can be assessed using a pencil ionization chamber with an active length of 100 mm, standard CT dosimetry phantoms of polymethylmethacrylate (PMMA), and plates of aluminum, most facilities do not possess the requisites. We present a practical method of estimating CTDI_<100,c>, CTDI_<100,p> and the half-value layer (HVL) from CTDI_<100,air>, which is measured parallel with the axis of rotation of the scanner to free-in-air. The three data chosen for this method of estimation were as follows: l) the relation of HVL to CTDI_<100,air> per radiographic exposure (mAs); 2) the relation of HVL to CTDI_<100,c> per CTDI<100,air>; 3) the relation of HVL to CTDI_<100,p> per CTDI_<100,air>. The data were based on the measured values of six CT scanners, so as to avoid dependence on the technical characteristics of a specific manufacturer. The estimated value has a possible maximum uncertainty of 20 %, although this method of estimation is practical for dose assessment.
Radiochromic film (RC-film) is of great interest as a film-type dosimeter for radiation oncology applications. We present a two-dimensional image-based evaluation of the measurement accuracy of a commercial RC-film product (Gafchromic MD-55-2 film, ISP TECHNOLOGIES, Inc.) by using a commercial Laser Densitometer (Model 1710, Computerized Medical Systems, Inc.) as an optical density imaging system. The coefficient of variation of the density (pixel-value) in one sample was approximately 3% to 11 % at 3 Gy or less, and 3% or less at 4 to 60 Gy. Although the coefficient of variation between three samples at the same dose was about 14% at 1 Gy, it decreased as the dose increased, reaching several percent. In 1 to 6 Gy samples, geometric imaging artifacts [interference (moire) patterns] were observed, and it was found that scan-sampling pitch influenced the accuracy of measurement of the density of the sample. To improve the accuracy of density measurement, sufficient knowledge about characteristic features of the density measuring system is essential.
The contrast-to-noise ratio (CNR.) is often used to evaluate magnetic resonance images, because it has two components, contrast and SNR, and indicates the detectability of clinical lesions. Two methods (using a phantom and using clinical images) are employed to measure CNR. In addition, there are some methods of measurement that use clinical images. In this report, the accuracy of measurement and correlation for signal detectability were evaluated in four methods of measuring CNR using clinical images. The results indicated that the inter-tissue method using an air signal provided good accuracy and was consistent with signal detectability using observer performance. In addition, a small region of interest (ROI) was better suited as the target for CNR measurement using clinical images.
The recent introduction of multi-detector row computed tomography (MDCT) has made it possible to scan the entire abdomen within approximately 10 sec in procedures such as interventional radiology computed tomography (IVRCT), which are associated with operator exposure. Therefore, anxious patients and patients who are not able to remain still can be examined with an assistant. In the present study, radiation exposure to the assistant was estimated, and the distribution of scattered radiation near the gantry was measured using an optically stimulated luminescence (OSL) dosimeter. Simultaneous measurements were obtained using a direction storage (DIS) dosimeter for reference. The maximum value of 1.47 mSv per examination was obtained at the point closest to the gantry's center (50 cm from the center at a height of 150 cm above the floor). In addition, scattered radiation decreased as the measurement point was moved further from the gantry's center, falling below the limit of detection (0.1 mSv or less) at 200 cm and at the sides of the gantry. OSL dosimeters are also employed as personal dosimeters, permitting reliable values to be obtained easily. They were found to be an effective tool for the measurement of scattered radiation, as in the present study, helping to provide better understanding of the distribution of scattered radiation within the CT room.
There is a strong tendency in Japan to perceive even the smallest amount of radiation or radioactivity as dangerous or detrimental. An investigation was done to identify the age at which this perception is formed. It was found that the perception was initially formed during elementary school, especially when the description of an atomic bomb was given during the history course, which is part of the sociology curriculum. The description in the sociology textbook emphasizes the damage produced by radiation. In contrast, the textbook has few descriptions of the positive uses of radiation. The perception, formed in elementary school, of radiation as something dangerous and detrimental remains vivid until adolescence. The only textbook to correctly describe the benefits of radiation was a high school physics textbook. However, only 30% of high school students take physics. The system of selecting science subjects in high school is preventing students from obtaining a correct understanding of radiation. Little improvement can be found in the latest textbook published following the 2002 governmental guidelines for education.
The standardized uptake value (SUV) is a relative measure of tracer uptake in tissue used in <I8>^F-FDG PET. However, the quality of ordered subset expectation maximization (OS-EM) images is sensitive to the number of iterations, because a large number of iterations leads to images with checkerboard noise. The main advantage of data acquisition in the three-dimensional (3D) mode is the high sensitivity to better exploit the intrinsic spatial resolution and the lower injection dose given to patients. In the 3D mode, the scatter fraction is higher, and, for a given administered dose, the random fraction is higher than that in the two-dimensional mode, which implies that correction methods need to be more accurate. Moreover, in clinical oncology 18F-FDG PET studies, patients have a wide variety of body shapes and sizes, which may impact image statistics. Consequently, it is necessary to make constant the acquisition (true) counts. The purpose of this study was to optimize injection dose and acquisition time in consideration of body mass index (BMI) for 3D whole-body <18>^F-FDG PET. Methods: A dedicated PET scanner, SIEMENS ECAT EXACT HR^+, was used to scan images of clinical data. The injection dose for BMI of < 14-19, 19-22, 22-25, and 25<(kg/m^2) were, 92.5 MBq, 111.0 MBq, 129.5 MBq, and 148.0 MBq, respectively. The emission scan time per bed position for BMI of < 14-19, 19-22, 22-25, and >25 (kg/m^2) were, 120, 120, 180, and 240 sec, respectively. A total of 20 patient subjects were evaluated as to true counts per bin (T/bin) of sinogram data and measured activity concentrations for the region of interest in the liver section. Results: T/bin was stable using an optimized protocol that took into consideration the BMI for any type of body morphology. The overall coefficient of variation was 7.27% for radioactivity concentration. Additionally, Gaussian filtering (8 mm FWHM) after reconstruction by the OS-EM method provided stable SUV values even when the iteration number was increased 30 times over. Conclusion: Optimization of injection dose and acquisition time indicated that BMI was a clinically useful acquisition protocol for 3D whole-body <18>^F-FDG PET.
This report deals with the image quality of film used in mammography in terms of changing the processing time of the film and reducing the radiation dose. We initially performed phantom experiments to investigate the extended process that could provide high image quality of the film. We then measured silver grain sizes using an optical microscope to investigate processing time, so that silver grains would not be coarse during the extended process. It was found that micro-calcifications could be clearly detected by the extended process by using a lower radiation dose and that the extended process did not affect coarsening of the silver grains.
As the PROPELLER sequence is a combination of the radial scan and fast-spin-echo (FSE) sequence, it can be considered an FSE sequence with a motion correlation. However, there are some differences between PROPELLER and FSE owing to differences in k-space trajectory. We clarified the imaging characteristics of PROPELLER T2-weighted imaging (T2WI) for different parameters in comparison with usual FSE T2WI. When the same parameters were used, PROPELLER T2WI showed a higher signal-to-noise ratio (SNR) and lower spatial resolution than usual FSE. Effective echo time (TE) changed with different echo train lengths (ETL) or different bandwidths on PROPELLER, and imaging contrast changed accordingly to be more effective.
Calculation of in-air or in-water dose for 4 MV X-ray irregular fields could be accurately performed using the collimator scatter factor (S_c) and phantom scatter factor (S_p) concepts. It has been revealed that the equivalent square field for a multi-leaf collimator (MLC) irregular field can be evaluated accurately by using the S_p-Clarkson or S_c-Clarkson integration method; however, the S_c-Clarkson integration method is more straightforward because the S_c factor expresses the in-air X-ray output factor. It has been found that when the MLC field is relatively much smaller than the main collimator field, the S_c factor can be accurately evaluated by introducing the small segment correction (SSC) factor (except for the case in which the MLC field is less than 1×1 cm^2). It has also been found that both the 5P factor and the tissue-phantom ratio (TPR) can be precisely evaluated by introducing the FMLC factor in cases in which the ratio of the MLC equivalent square field side to the main collimator equivalent square field side is less than about 0.7.
The output factor of high-energy X-ray machines varies with collimation. According to Khan's theory, collimator and phantom scatter factors contribute to total scatter factor. For precise X-ray irradiation, the two factors need to be taken into consideration. To obtain proper factors, we made two original polystyrene cylindrical mini-phantoms. These phantoms are both 4 cm in diameter and have a pinpoint ion chamber placed at a depth of 5 cm and 10 cm, respectively. Using a 6 MV X-ray machine, collimator scatter factors were calculated for various field arrangements (I.e., field sizes ranging from 4 cmx4 cm to 40 cm×40 cm at isocenter). To determine if calculated values were appropriate, we measured point doses of 20 X-ray irradiation patterns using a Farmer-type ion chamber with a water equivalent phantom at depths of 5 cm and 10 cm, respectively. Two hundred Mus were irradiated to the above-mentioned depths for each field. Based on the measured doses, variations were obtained for four calculation methods. Accounting for l) secondary collimator (jaw) setting, 2) blocked field (multi-leaf collimator) setting, 3) Khan's theory using a 5 cm mini-phantom, and 4) Khan's theory using a 10 cm mini-phantom. Dose variations in each method of calculation were as follows: l)+0.3 to +10.2% (mean, +2.0 to +3.2%), 2}-2.3 to 0.0% (mean, -0.8 to -0.6%), 3) 0.0 to + 1.5% (mean, +0.1 to +0.3%), 4) 0.0 to +1.4% (mean, -0.1 to +0.1%).