Study And Practice Of Measurement Method Of CT Temporal Resolution Based On Dynamic Phantom

Computed tomography(CT) has become one of the most important support method of medical diagnosis, since the advent of CT in1972, people are dedicated to improving the quality of imaging and gantry rotation speed of CT scanner. With the upgrading of CT, the quality of imaging continuously improved, achieving the dynamic scanning and volumetric imaging. But the improvement of CT temporal resolution is always restricted by the gantry rotation speed. In2005, Siemens launched the dual source computed tomography which greatly improved the CT temporal resolution and the time of gantry rotates one circle is330ms, expanding the range of clinical application of CT.CT temporal resolution mainly refers to the sum time of image acquisition and reconstruction in the field of scanning, in other words, CT temporal resolution is the time of gantry rotates one rotation. CT temporal resolution is also an important factor to affect the quality of cardiac imaging, CT that temporal resolution is very high can "freeze" the movement cardiac in a particular time phase and this overcomes the impact of motion artifacts on medical diagnosis. At present, there is not an unified method and corresponding tool to measure CT temporal resolution no matter at home and abroad. In order to evaluate the performance of CT scanner and imaging quality preferably, In this study, we proposed a measurement method of CT temporal resolution which based on the principle of the third generation CT ring artifact, designing and producing the corresponding phantom and achieving the measurement of CT temporal resolution.Firstly, the paper described the design and production of CT temporal resolution dynamic phantom, the design of the shape of temporal resolution dynamic phantom was based on the Catphan500phantoms(which was produced by United States of American phantom laboratory that can be used to evaluate the performance of CT) and the measurement principle of CT temporal resolution. The shape of temporal resolution phantom is cylinder, the diameter is15cm, the thickness is5cm and the material is polymethylmethacrylate(PMMA). The two high density attenuation objects are stainless steel wire and aluminum wire which are at the distance of the center of cylinder3cm and5cm respectively, and the diameters of the two are3mm. The temporal resolution phantom can be rotated when it was driven by the two phrases four wires hybrid stepper motor and the drive of stepper motor adopted by the method of the combination of software and hardwire, the control system of stepper motor is made of MCU STC12C2052AD which produced by Hong Jing technology and drive chip THB6064H which produced by Toshiba. That is, this phantom was designed to implement movement like circular by PC control and the speed of temporal resolution phantom was controlled by pulse frequency and the direction rotation was as the same of CT gantry.Secondly, we analyzed the relationship between temporal resolution and cardiac imaging, and pointed out that high or low of temporal resolution affected directly the quality of cardiac imaging and the medical diagnosis level, therefore, the research on temporal resolution is very important to improve medical diagnosis level; at the same time, the paper exploited the principle of the third generation CT ring artifact creation, in the third generation CT, if one(or more) detector(s) is(are)abnormal, it will produce ring artifact(s) on the reconstruction images. According to the principle, the temporal resolution phantom containing high density attenuation objects was rotated at the same speed and direction as the gantry, creating partial ring artifact, where the angular extent of ring artifact provides a direct indication of the temporal resolution; then the paper introduced data acquisition equipment and phantom speed verification tool and also designed the brief measurement method of CT temporal resolution, completing the data acquisition.There are two stainless steel beams in the temporal resolution dynamic phantom complete device, so when the image is reconstructed they would generate severe radial metal artifacts. Metal material has a higher attenuation coefficient than the other organizations of the human body and polymethylmethacrylate and so on, thus the x-ray passing it will be attenuated rapidly, leading to the corresponding projection data distortion. Artifacts which really don’t exist will appear on the reconstruction image. Metal artifact distorts the quality of image, therefore, the first step is to preprocess the image when measured CT temporal resolution. In this paper, the step of preprocessing the original image was mostly manipulated as the followings:first of all, in order to reduce the noise and smooth original image, the metal artifacts damaged original image was filtered by traditional adaptive filter; then dividing the image into metal parts and artifact parts by segmentation algorithm (because of reconstruction image includes two ring artifacts, the artifact parts included the smaller diameter ring artifact); then putting the two parts projection separately transformed into sinogram space; following filling the deviation of projection data by the application of hybrid interpolation, that is, filling the deviation of metal projection by spline interpolation and artifact parts by linear interpolation, in this paper, in order to process the later date easily, the smaller diameter ring artifact was eliminated; finally, putting the sum of the two parts and making filtered back projection(FBP) reconstruction, thereby getting the image which removed the radial metal artifact and the smaller arc, completing the image preprocessing.Then, in order to determine the circle of arc and the center of circle, it needed to detect the center of circle after the image preprocessing. Generally, we usually adopted Hough Transform algorithm to detect the center of circle. Hough Transform is a method that connected the edge pixels of image into closed region boundary by using the global feature of image, and Hough Transform realized the mapping relationship from the image space to the parameter space. This paper described in details the line detection and circle detection based on traditional Hough Transform. The basic idea of traditional Hough Transform detection algorithm is to transform the image space domain into the parameter space, and described the curve boundary of image with the boundary points which met some form of parameters. Because of the parameter space of circle is three dimensional, circle detection needed to consume a amount of computation time and occupy a lot of storage space. In this case, this paper proposed a method of circle detection which based on the image gray information and the circle gradient information, the algorithm reduced the ineffective cumulative detection, the amount of computation time and the need of resource greatly, hence improved the detection speed. This paper described in details the gradient calculation and the steps of circle detection algorithm which based on gradient information, then achieved the circle location detection of the arc and drew the circle.Finally, this paper introduced traditional Prewitt operator, the Prewitt operator has the character of strong anti-noise performance, low detection failure rate and good detection effect. Therefore, this paper realized the edge detection of arc adopted by the Prewitt operator, and determined accurately the arc starting point and destination point coordinates, at the same time combined with the circle center coordinates which was detected by Hough Transform circle detection algorithm based on gradient information, then calculated the angular extent under the condition of the speed of CT gantry is0.5s/240°, the size of angular extent is(240.0791°±0.6637°), and then according to the CT temporal resolution calculation formula, calculated the corresponding temporal resolution that is(0.5002±0.0014)s. Therefore, the CT temporal resolution of calculating out fitted in with the0.5s/240°scan mode of CT. The measurement method is reproducible, this suggests that the measurement of CT temporal resolution and the corresponding uniform phantom could be used to measure the CT temporal resolution. So this paper provides a new study method and evaluation tool for us to evaluate the performance and quality assurance of CT.