An Invitro Study For The Influenceing Factors And Inhibiting Methods Of Magnetic Susceptibility Artifact And RF Shielding Artifact During MR Imaging

Objective this study was focused on the influencing factors such as material,size,MR sequence,perameter and inhibiting methods of Magnetic susceptibility artifact(MSA) during MR imaging.Methods 9 screws with different material(copper,titanium,stainless steel) and size fixed in water phantom were tested with different MR sequences(such as spin echo,turbo SE,gradient echo,short time inversion recovery,chemical shift selective fat suppression, SE-echo-planar imaging) and parameters(such as field of view,time of echo,Matrix, slice thickness,flap angle,number of acquisition,bandwidth,phase-frequency encoding direction and Sample orientation),the Diameter of artifact(DA) and value of Artifact-screw size ratio(ASR) were measured by SPSS 13.0 statistic software.Results①Artifacts were most marked in stainless steel,second in Cu and slightest in Ti, statistic difference was testified.Artifacts were characteristic of geometry deformation and signal distortion with SE,TSE,EPI-SE,STIR and CHESS-FS sequences,and signal loss with GRE.Artifacts with EPI-SE and GRE were much worse than these with SE,TSE and STIR,significant difference of artifacts between STIR and CHESS-FS was also testified (P＜0.001).②Second,artifacts were lightened more or less when increased Matrix,flip angle,number of acquisition(Naq) and bandwidth(BW),decreased field of view(FOV), slice thickness and TE were applied and when long axis of sample was parallel to main field orientation,however,some difference of diminishing amplitude of ASR with these parameters was demonstrated,meanwhile,artifacts were slighter on phase-encoding direction than that on frequency-encoding direction with SE group,it was reversed with EPI-SE and no difference with GRE(pairing t test,P＜0.05).Conclusion Susceptibility artifacts were clearly affected by implant material,pulse sequences and imaging parameters.MR artifacts can be minimized by choosing TSE,STIR instead of EPI,GRE and CHESS-FS sequences and by adopting small voxel size,short TE and widened BW,it is feasible to place the long axis of implants parallel to main field and to transform frequency-phase encoding direction with SE,TSE,IR and EPI sequences. Objective to investigate the mechanism,influencing factor and inhibiting methods of RF shielding artifacts of tubiform metal implants during MR imaging.Methods 3 samples with different materials(aluminium and plastic tubes),different diameters(1.5 mm and 2.8 mm) and different shape(tubiform and luminal) fixed in the center of water phantom were tested with turbo spin echo and gradient echo sequences by changing frequency-phase code direction and flip angle(θ) and sample position(L⊥BO and L∥B0),signal intensity(SI) and Artifact Signal shielding ratio(SSR) were measured in three regions of interest(ROI_A,B,C).Statistical analysis was carried out with different groups:①SI and SSR were compared between different ROI of alum tube held with the same material and size,the same sequence and parameters(one-way ANONA test).②Data of ROIa between alum tube,luminal alum tube and plastic tube with the same size were analyzed(one-way ANOVA test).③In the same condition,data of ROIa between sample with diameter of 1.5 cm and that of 2.8 cm were contrasted with independent -samples t test.④with different phase-frequency encoding direction,flap angle and sample position were analyzed respectively(one-way ANOVA test).Results①RF shielding artifacts were most noticeable in ROIa,second in ROIb and slightest in ROIc,Statistical differences of SI and SSR were demonstrated between different groups(P＜0.01).②Artifacts were most marked in ROIa of alum tube,second in that of luminal alum tube and no interference in that of plastic tube(ANOVA,P＜0.01).③Significant differences of SI and SSR in ROIa were shown between samples with diameter of 1.5 cm and 2.8 cm(P＜0.01).④Statistical differences of SI and SSR in ROIa were testified when frequency-phase code directions were changed(P＞0.05) with TSE sequence, however no difference of SI and SSR were displayed with GRE sequence(P＞0.05). Shielding effect in ROI_a when L∥BO was worse than that when L⊥BO(P＜0.01),SI and SSR increased when flap angle was added(P＜0.01).Conclusion①there are RF interference and shielding artifacts for tubiform metal implant during MR imaging.②RF shielding artifacts are related to changes of frequency-phase encoding direction with TSE,but no related to that with GRE.RF shielding artifacts were also related to sample position,shape,size and flip angle of RF.③Shielding effect is the worst in the center of lumens,choosing reticulodromous intravascular stent,increasing flip angle and major axis of stent vertical to main field are helpful to degrade the interference of RF shielding effect.