Dissertation > Medicine, health > Clinical > Diagnostics > Diagnostic Imaging > Magnetic resonance imaging

An Invitro Study for the Influenceing Factors and Inhibiting Methods of Magnetic Susceptibility Artifact and RF Shielding Artifact during MR Imaging

Author ZhangYuDong
Tutor HeNing
School Lanzhou University
Course Medical Imaging and Nuclear Medicine
Keywords Magnetic resonance imaging Magnetic susceptibility artifacts Spin-echo sequence Gradient echo sequence Echo-planar MR artifacts RF shielding effect RF interference
CLC R445.2
Type Master's thesis
Year 2009
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MR susceptibility artifacts influencing factors and suppression method purposes of MR susceptibility artifacts (Magnetic susceptibility artifact, MSA) with different metal materials, size, MR scan sequence and parameter settings, the relationship of the influencing factors, finding the best suppression method. Materials and Methods copper, stainless steel and titanium alloy screws each, the specification of the same, fixed in a water phantom center field strength 1.0T magnetic resonance instrument: ① select SE (Spin echo), TSE (turbo SE), GRE (Gradientecho), STIR (short time inversion recovery), CHESS-FS (chemical shiftselective fat suppression), SE-EPI (SE-echo planar imaging) in the equivalent parameters under the conditions of samples repeated scanning. ② SE, GRE, TSE IR and EP work sequence to change the field of view (field of view, FOV), matrix (Matrix), thickness (slice thickness), TE (time of echo), the receiver bandwidth (bandwidth, BW), the number of acquisitions (time of excitation) flip angle (flap angle), phase - frequency encoding direction and sample the long axis of the main magnetic field direction repeated scanning of the sample. Artifacts maximum diameter (diameter orlength of artifacts, DA), taking into account the sample specifications of the statistical results measured at all levels, while computing artifacts / sample specifications (artifact-screw size ratio ASR), the results of multiple comparisons . Results under the same conditions: ① titanium screw artifacts lightest, copper followed, stainless steel heaviest significant statistical difference between the results. The artifacts in the SE, TSE, STIR, CHESS-FS sequence performance for image geometric distortion and signal distortion, GRE performance for signal loss. EPI GRE sequence artifacts heaviest TSE and SE sequences followed by the the IR sequence lightest, STIR than CHESS light-FS sequence is the result of differences between the the compatibility group design ANOVA processing and the block was statistically significant (P <0.01 ). ② artifacts in short TE voxel, wide bandwidth, large flip angle, increase the number of acquisitions and the sample long axis parallel to lighten BO when each sequence, each parameter artifacts to reduce the degree of difference, paired t-test set of results the difference was statistically significant (P <0.01); artifacts in SE sequence along the phase-encoding direction light, light, EPI along the frequency-encoding direction switching frequency - phase-encoding direction did not change in the GRE sequence. Conclusion sample material, sequence and parameters of MSA of the same, with a reasonable sequence of parameter settings MSA interference reduction. , MR RF shielding artifacts influencing factors and inhibition of tubular metal objects in the the approach aims MR imaging RF shield the artifact formation mechanism, influencing factors and suppression method. Materials and methods of the cylindrical shape, with mesh the cylindrical tubes (diameter respectively of 1.5cm and 2.8cm) and the diameter of 1.0cm plastic tube (control group) stars long axis parallel, perpendicular to the main magnetic field ( BO) direction fixed in the center of the water model 1.0T magnetic field environment change the frequency on the TSE (turbo SE) and GRE (Gradient echo) sequence - phase-encoding direction and the excitation pulse flip angle repeated scanning of the sample area (in the pipe center A zone), marginal zone (B zone) and the outer edge of the zone (C zone) set the region of interest (Regions of interest, ROI), measuring the signal strength (signalintensity, SI) and signal shielding rate (Signal shielding ratio, SSR): analysis of variance the the SI, SSR differences in different regions of interest in comparison tubes under ① equivalent conditions. ② analysis of variance comparison of the same diameter cylindrical, with a mesh of a cylindrical aluminum tube and plastic tube the Region A SI and SSR difference. ③ equivalent conditions under independent sample t-test was used to compare the different diameter aluminum tube central area of ??SI SSR difference. ③ Under the same conditions, analysis of variance to compare the phase - frequency encoding direction, flip angle and sample location lumen central area SI, SSR difference. Results ① same conditions shield artifacts is heaviest in aluminum tubes Center, followed by marginal zone, the outer edge of the area lightest, SI, SSR group difference was statistically significant (P <0.01). ② under the same conditions, shielded unshielded artifacts artifacts in barrel-shaped aluminum tubes with mesh barrel tubes followed by plastic tubes, the tubes central area of ??SI, SSR difference was statistically significant (P <0.01) . ③ Under the same conditions, 1.5 cm diameter aluminum tube shield artifact heavy at 2.8 cm diameter aluminum tubes, t test two sets of SI SSR difference was statistically significant. (4) switching frequency - the possession of the central area of ??SI the TSE sequence before and after the phase-encoding direction SSR difference was statistically significant (P <0.05), the difference was not statistically significant (P <0.05) in the GRE sequence. The artifact sample long axis parallel to the BO when aluminum tube shield is more important than the long axis perpendicular to the BO. Changing the flip angle, SI, SSR statistically significant group differences, and tended to increase with the flip angle increases. Conclusion ① the presence of RF interference and radio frequency (RF) shielding effect of the the tubular metal objects in the MR examination. ② RF shielding artifacts and frequency - a phase-encoding direction, the metal placement related to the direction, shape, size, and the excitation pulse, the flip angle. ③ shielding artifact is the heaviest in the center of the tube, choose a mesh-like stent, the lumen long axis perpendicular to BO, appropriately increasing the flip angle can effectively reduce the shielding artifacts.

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