Japanese Journal of Radiological Technology

The rotation speed of X-ray CT is known to influence the quality of images. We examined changes in imaging characteristics according to various physical characteristics. The physical characteristic of rotation speed (1.0, 0.8, 0.7, 0.6, 0.5 sec/rot) was evaluated at the FOV center (isocenter) and 90 mm outside the center (off-center). The items of evaluation included low contrast resolution, high contrast resolution, noise, definition, and artifacts. High contrast resolution and definition declined at a rotation speed of O.6 sec/rot or less at off-center. An increase in shower artifact and change in the shape of the wire section were found at a rotation speed of 0.6 sec/rot or less at off-center. It was determined that image quality was especially influenced in terms of definition characteristics when the speed of rotation was high. A decrease in the number of views can be considered the main cause ; that is, the use of high-speed rotation degrades image quality. Therefore, it is important to consider proper usage as it corresponds with purpose.

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It has been found that in general 10 MV X-ray dose calculation can be made accurately for multi-leaf collimator irregular fields by using the total scatter factor (S_<cp>), collimator scatter factor (S_c), and phantom scatter factor (S_p) proposed by Khan et al. With respect to the collimator scatter factor (Sc), we used the field-mapping method of Kim et al. to obtain equivalent square fields of irregular fields (the collimator reverse effeet can be accurately dissolved using the field-mapping method). Even for extremely small multi-leaf fields compared with the main collimator opening, X-ray output calculations could be made accurately by introducing the small segment correction (SSC) factor. With respect to the phantom scatter factor (S_p), highly accurate calculations could be made for irregular field irradiation by applying an F_<MLC> (MLC radiation leakage) factor to the equivalent square field (in cases in which the ratio of the multi-leaf equivalent square field side to the main collimator equivalent square field side is less than 0.6). However, it has been found that highly accurate dose calculations can, in general, be performed when the main collimator is limited just at the opening determined by the multi-leaf collimator field.

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The current method of verification for external beam radiation therapy visually compares a simulation image with a portal image. However, because this method depends largely upon the observer's experience, it sometimes results in inter-observer differences. In this study, we developed software to measure atomatically the quantitative difference between the simulation image and portal image using an image-analysis method. The feasibility of this software was evaluated on a rectangular field in the pelvic region. We took 12 Simulation images of a pelvic phantom, setting 4 different field shapes on each of 3 isocenters. We then obtained 84 portal images setting 7 known distances from each of the 12 simulation images. Using this software, the direction of shift was detected correctly, and the distance of shift was detected correctly to within less than 3 mm. We consider that this software could be a useful method of verification.

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A study was performed to evaluate the total entrance skin dose (ESD) of patients during diagnostic and interventional radiology procedures (IVR) and to estimate ESD with body mass index (BMl) and fluoroscopy time. The study included 26 cases of transcatheter arterial embolization therapy (TAB) for hepatocellular catcinoma (HCC) and 19 cases of diagnostic digital subtraction angiography (DSA) for HCC. The ESD of patients was evaluated with a zinc-cadmium sensor linked to a digital counter (SDM : skin dose monitor). Exposure doses were measured with SDM attached to the front of the X-ray beam-limiting device like a dose area product monitor. ESD was calculated from the measured exposure dose. In 26 TAE for HCC, ESD was 1793.7±739.1 mGy, with the mean fluoroscopic time of 23.5 minutes and 4.4 DSA acauisitions. The fluoroscopic dose rate was 52.4±11.5 mGy/min. In 19 diagnostic DSA for HCC, ESD was 962.9±375.2 mGy, with the mean fluoroscopic time of 11.1 minutes and 4.0 DSA acquisitions. The fluoroscopic dose rate was 32.7±12.7 mGy/min. Although 33.2% of ESD was from fluoroscopy in diagnostic procedures, the figure was 68.8% in TAE procedures. lt was demonstrated that the increase in ESD during IVR was caused by the rise of fluoroscopy dose rate caused by high-magnification fluoroscopy and the extension of fluoroscopy time. ln order to reduce BSD, it is necessary to use a low fluoroscopy dose rate with low-rate pulse fluoroscopy, in addition to shortening fluoroscopy time. Fluoroscopy time was a poor predictor of risk because it did not correlate well with ESD during IVR (diagnostic procedures r^2=0.897, IVR r^2=0.594). However,ESD correlated well with the product of BMI and fluoroscopy time (diagnostic procedures r^2=0.910, IVR r^2=0.783). The linear relationship between ESD and the product of BMI and fluoroscopy time provides a simple monitoring mechanism of the ESD delivered to the patient during interventional radiology procedures. This linear relationship needs to be established for other types of interventional procedures.

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