A Study On Optimization Of X-ray Exposure Parameters (kVp And MAs) And Relationship Between Exposure Parameters, Image Quality And Radiation Dose

PurposeTo optimize X-ray exposure parameters and estimate the relationship between exposure parameters, image quality, and radiation dose, so as to minimize the radiation dose and sustain the image quality simultaneously.Materials and methodsPhantoms: 25 pieces of acrylic phantom with dimensions of 30cm×30 cm×1.0 cm each were employed to simulate various thickness values of human body. The phantom placed in the middle contained 25 pairs of cylindrical vessels 1.2mm in diameter and 40mm in length drilled along its midplane at intervals of 11mm apart. Iodine contrast derived from Ultravist (300mgI/m1, Shering, China) diluted in heparin solution with concentrations ranging from 50mg/ml to 4mg/ml was filled to each pair of vessels, with the ratio of concentration in any two adjacent vessels being 0.9. Different concentrations of iodine in those vessels can thereby represent different blood vessels containing iodine in angiography or various intensities of signal.Image acquisition systemⅠ: 2000 GTR H.F. 80kw remote-controlled X-ray Unit (Wangdong,China), with an image intensifier 12 inches in diameter, focal spot size 0.6mm and an equivalent filtration of aluminum 3mm in thickness.Image acquisition systemⅡ: DR2000 system (E-COM, Zhuhai, China) equipped with DirectRay DR 1000 amorphous selenium flat-panel detector (Hologic, U.S.). This detector is equipped with a 35×43cm X-ray sensing surface with a 2560×3072 matrix and a 139-μm pixel size. Focal spot size was set to be 0.6mm, and equivalent filtration of aluminum 3mm in thickness.Dosimeter: PMX—Ⅲdosimeter (RTI Electronic, Sweden).PartⅠ: A stacked phantom 10cm in thickness was employed to simulate the circumstance of digital substraction angiography (DSA) under the image intensifier (image acquisition systemⅠ). Radiographs were acquired with different combinations of exposure parameters (kVp and mAs) while keeping the detector entrance dose at a certain level. With a subjective evaluation by a 5-point scoring scheme to assess images obtained combined with radiation dose measured and the corresponding exposure parameters, curve fitting was used to evaluate the relationship between the three, and discuss whether could be obtained the optimum exposure parameter that can meet the requirements of both minimum radiation dose and concentration of iodine.PartⅡ: A stacked acrylic phantom with a thickness of 8 centimeters was used to represent lung of average-sized adult patients under the amorphous selenium flat-panel detector (a-Se FPD), to acquire images with different combinations of exposure parameters (kVp and mAs). The image quality of the digital radiographs assessed by a criterion of 5-point image scoring scheme was graded according into different levels, in each of which the minimum radiation dose and the corresponding exposure parameter were concluded. Results1. In PartⅠ: A quadratic regression model was obtained to describe the relationship between tube voltage and iodine detection threshold, and a cubic model to describe the relationship between tube voltage and phantom incident dose. The optimum kVp setting for the phantom with a thickness of 10cm was found to be 97.2. In PartⅡ: The values of minimum phantom incident dose were mainly located among tube voltages of 60-70kVp, and 70kVp was found to have the maximum number of values of radiation dose. A linear model fitted to the scatter points of logarithm values of both detection threshold concentration and radiation dose with high coefficient of determination (the square of the correlation coefficient) effectively explained the relationship between them.Conclusions1. While sustaining the detector entrance dose at a certain level, the concentration of iodine required for angiography increased with tube voltage, and the quantitative relationship between the two can be described by a quadratic regression equation. The quantitative relationship between tube voltage and phantom incident dose can be described by a cubic regression equation.2. On the basis of some specific thickness of phantom, through the grading of image quality of radiographs acquired into different levels, the minimum radiation dose in each level can be concluded to obtain the corresponding optimum exposure parameter.3. With the acrylic phantom 8cm in thickness to simulate lung of average-sized adult patients and kVp set at 70, the radiographs obtained by the a-Se system can satisfy the condition of minimum radiation dose while sustaining image quality simultaneously.