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Transcript
Susceptibility artifacts at MRI Thomas R McCauley Clinical Associate Professor Yale School of Medicine Artifacts in MRI • Strength of MRI is weakness of MRI • Appearance depends on many factors: – T1, T2, PD, Flow, motion, susceptability, chemical shift, diffusion, mag transfer, saturation effects, magic angle… • These many factors we exploit to detect pathology, but result in multiple artifacts Artifacts in MRI • Every study in MRI contains imaging artifacts • Discuss a small subset: artifacts due to field inhomogeneity (susceptibility) 40 yo old R hand numb Gado fat sat GRE 40 yo old R hand numb Gado FS GRE 40 yo old R hand numb Gado fat sat GRE Gado MRA next day. No fat sat T2 FSE fat sat Susceptibility • Def: Distortion of the magnetic field due to placement of materials in the magnet • Create uniform magnetic field within 1ppm. 1.5 T: 64,000,000 Hz +/- 32Hz • Patient distorts field especially at metal, edge of magnet bore, air soft tissue interfaces (worse: complex anatomy) Frequency Metal increases field. Soft tissues decrease field. Distorion of field is often complex 64MHz Position in x direction Inadvertent water sat Signal Frequency Inhomogeous fat sat is bad Inadveratant water sat is disaster • If fat sat fails or is inhomogeneous: MAKE SURE THERE IS NO WATER SAT • If water sat occurs the images are useless • If autoshim or manual tune does not correct – Remove fat sat – Use STIR instead of T2 fat sat – Use subtraction for pre post contrast SE T1 GRE Metal artifact • Magnetic field increased at/adjacent to metal • Artifact due to three effects – Inhomogeneous fat saturation – Dephasing: Inhomogeneous magnetic field in voxel causes dephasing signal loss – Misregistration: Signal is displaced in the frequency direction due to distortion of frequency encoding gradient Frequency Metal increaseBlow field up of voxel and thus frequency 64MHz Frequency varies voxel across voxel resulting in dephasing signal loss during TE. Dephasing decreased: SE > FSE >> GRE Short echo time. Small voxel Position in magnet Frequency 64MHz Double bandwidth Error in x direction is smaller With frequency encode gradient Signal in x locations in grey bar is incorrectly assigned to location corresponding to high field edge of the bar Position in x (freq) direction GRE localizer GRE SE PD TE 20 SE T1 TE 10 Swap freq SE TE 20 FSE FLAIR TE same swap freq eff 20 etl 6 62 yo right hip pain TEeff 129 TR 4000 ETL 14 BW 42 TEeff 8.7 TR 600 ETL 3 BW 62 BW 62 TEeff 8.7 TR 600 ETL 3 BW 125 TEeff 7.8 How to scan with metal • No fat sat: – STIR for edema detection – Subtraction of pre from post contrast images • Fast spin echo>spin echo>>gradient echo • Minimize TE. Fast spin echo for long TE • Maximize bandwidth! Decreases misregistration. Decrease TE and echo space which decreases dephasing signal loss How to scan with metal • Swap phase and frequency if needed • 1.5T better than 3T • Smaller voxels decrease dephasing: thinner slices >> increase matrix, but decrease SNR • If available, specialty metal sequences Susceptability artifacts • Understanding the mechanism – Recognize – Decrease