The Study Of Multifunction QA Phantom For MRI

Magnetic Resonance Imaging (MRI) performance parameter test is an important content of MRI quality control and quality assurance. However, the performance test phantom is an important tool for performance test. Common MRI performance test phantoms include Magphan SMR 170&100, AAPM phantom, ACR phantom, phantom equipped with MRI system, et al. Many hospitals and research institutions bought import phantom, or used phantom equipped with MRI system to carry out performance test and quality estimation. The imported multifunction performance test phantoms were designed according to different criteria, such as AAPM reports, ACR reports, NEMA standards, et al. One phantom could not satisfy the requirements of domestic MRI equipment performance test and quality evaluation. The imported MRI phantoms were expensive and most of them only could test the axial plane, so some phantoms were needed to complete comprehensive performance test. The phantoms equipped with MRI system also could not satisfy the domestic requirements for the relatively simple internal structure, the simplex performance parameter test and difference caused by different manufacturers. Therefore, it was important to design MRI performance test phantom to satisfy the present related domestic and foreign criteria.The common phantoms and some related MRI performance parameters test reports (e.g., AAPM reports, ACR reports, NEMA standards) were introduced and analyzed in the thesis. Based on the current problems of MRI test phantoms, some research work had been done in the thesis as follow:(1) Based on the comprehensively analysis of MRI performance parameters test methods and the domestic and foreign multifunction phantoms, two sets of phantom scheme were designed to satisfy the current domestic and foreign criteria (e.g., Chinese Ministry of Health WS/T 263-2006 standard, AAPM reports, ACR reports, NEMA standards). The two sets of phantom were comprehensive MRI performance test phantom and multifunction, multi-aspect MRI performance test phantom respectively. The material of the shell and functional modules was Poly-methylmethacrylate, and the material of the nylon screws and sealing covers were Poly-formaldehyde.The major modules of the self-designed phantoms (comprehensive MRI performance test phantom and multifunction, multi-aspect MRI performance test phantom) were as follow:a. Slice thickness test moduleThe width of slopes were increased, so the distance between the slope image and the slope fixed structure was increased. Thus the designed slice thickness test module in the thesis could provide multi-profiles to reduce the random error and avoid artifacts. The "回"-shaped fixed structure avoided the fixed screws interference and a small cylinder on it could test the Ghost artifact which could reduce the interference from other test structures.b. Spatial resolution test moduleThere were three kinds of structures in the module, there were crossed line pairs card,0～180° periodic distribution circle holes and MTF test module respectively. Multi-directional spatial resolutions were achieved in a single scan without repeat scan.c. Low-contrast resolution test moduleMore abundant, delicate targets were added in this module, which had much more advantages in low-contrast object detectability aspect. Low signal targets were added in multifunction, multi-aspect MRI performance test phantom, so it could simulate clinical focuses higher or lower than the background.d. Grid test module:This module could test magnetic field homogeneity and geometric distortion.e. Geometric distortion ratio test moduleThe target objects were distributed in each functional module, which were through the center point or not. The design could test geometric distortion with a broad range of application, such as different slice positions and different positions in a plane.The two sets phantoms were assembled with tube and rod sliding assembly on the modules packaged aspect, which also could be disassembled and packaged freely and satisfied the MR1 performance test very well.(2) The experiments of 17 solution formulae were designed by referring to domestic and foreign literatures and the relevant standards. The experiment results showed that the 2,3,7,8,11,14 solution formulae were fit for the phantom of the MRI whose static magnetic field was equal to or greater than 1.5T.To compare with the phantom of medical equipment center, among all of solution formulae the 11 solution formula (that is 1g/L copper sulphate solution) was selected to fill the self-designed phantom. For the static magnetic field equal to or greater than 3.0T, solution formulae were needed for the MRI phantom.(3) The phantom comparative experiments of self-designed phantoms and Magphan SMR 170 phantom had been done by selecting common sequence of the MRI performance test on nine clinical MRI manufacturers. To validate and optimize the self-designed phantoms gradually by comparatively analyzing the MR images of self-designed phantoms and Magphan SMR 170 phantom.(4) The MRI performance parameters test software was programmed in the MATLAB environment in the thesis. The functions of the software were as follow:a. Slice thicknessThe boundary points of slope image were searched out by a derivative of the mean of multi-profiles on the slope image, so the slice thickness was obtained. This method reduced random error and improved resistance ability to interference (e.g., the inconformity background, the tilt case of the phantom).b. Spatial resolutionFirst, to get the ERF on the MTF test image and then LSF was obtained by the derivative of LSF. At last, the Fourier transform of ERF was MTF. This method could improve the objectivity and accuracy of spatial resolution test.c. Low-contrast resolutionFirst, the mean pixel value and standard deviation of region of interest (ROI) were obtained by searching the series of targets on the module image automatically. Second, the mean pixel values of the background ROI adjacent to the targets were obtained. Then to figure the contrast to noise ratio curve, which made the test of the low-contrast resolution much faster and more accurate.d. Geometric distortion ratioFirst, to detect the edge of the geometric distortion targets, then the length between two targets was automatically measured. Thus the geometric distortion ratios were calculated.e. Signal to noise ratio and image uniformityTo search the uniform image central region automatically (about 75% uniform image), then the mean, standard deviation, maximum and minimum of the central ROI were obtained.The innovations of the thesis lie in:1) The innovations of the comprehensive MRI performance test phantom were as follow:a. On the slice thickness test aspect, the slope width were increased to get much more profile without fixed structure interferences. Thus the random error was decreased.b. On the spatial resolution aspect, there were crossed line pairs card, the 0～180° periodic distribution circle holes and MTF test module. Multi-aspect spatial resolutions were achieved in a plane with a single scan.c. On the geometric distortion aspect, had made the geometric distortions of different slice positions and multi-aspect in a plane were tested through the design of more plane, more targets.d. On the low-contrast resolution aspect, there were more abundant, delicate targets.Based on the above comprehensive MRI performance test phantom, the innovations of the multifunction, multi-aspect MRI performance test phantom lie in:a. On the spatial resolution aspect, the crossed line pairs card had been improved.b. On the low-contrast resolution aspect, more abundant, delicate low signal targets were added opposite to the high signal targets.c. On the Ghost artifact aspect, a small cylinder was added to test Ghost artifact, which could reduce the interference from other test structures.d. The multi-aspect performance test function in the coil was realized with the multifunction, multi-aspect MRI performance test phantom had made.The comprehensive MRI performance test phantom and the multifunction, multi-aspect MRI performance test phantom were assembled with tubes and rods sliding assembly on the modules packaged aspect, which also could be disassembled and packaged freely.2) Based on the self-designed phantoms, the MRI performance parameter test software was programmed, which had made the relevant performance parameters automatic or semi-automatic test come true. The objectivity and accuracy of results were increased, and the test efficiency was also increased.Therefore, the self-designed phantoms not only satisfied the present domestic and foreign requirements, but also improved relevant performance parameters test methods. Meanwhile, the performance test software improved test efficiency and made the test results more objective and accurate. The MRI performance parameter test phantom designed in the thesis had already obtained patent of invention and patents for utility model.