Lecture03_PreprocessI&II_artifacts_motion

Lecture03_PreprocessI&II_artifacts_motion - Ghost...

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1 Ghost Correction Geometric Distortion Timing Correction Motion Correction Prevention Prospective Postprocessing Ghost Artifacts Fat chemical shift EPI misalignment of odd/even k-space lines… N/2 Artifact
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3 Slice Timing Slice Timing fMRI slices are most often acquired using a 2D pulse sequence. Thus different slices are acquired at different times. Slice 1 started at 0.0 s Slice 2 started at 2.0 s Slice 2 started at 2.0 s Slice 3 started at 1.0 s Slice 3 started at 1.0 s Slice 4 started at 3.0 s Slice 4 started at 3.0 s TR=4s, interleaved acquisition
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4 Slice acquisition Interleaved Slice acquisition Sequential Slice acquisition Sequential needs small ‘gap’ Slice acquisition ‘effective slice’ thickness always same Slice acquisition Other issues with acquisition Slice acquisition Other issues with acquisition
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5 Slice acquisition Spatio-temporal blurring problematic with sequential, compounded by interleaved Time between slices: Sequential: 70ms Interleaved: 1-2 secs Slice Timing This introduces a bias into the statistics performed on a given slice (important for event-related studies and crucial for latency estimation). For example, consider a 5s boxcar signal in slice 1 and slice 4. Data from slice 1 Data from slice 4 Data from slice 4 Function used for Function used for statistics statistics ->Good results >Good results ->Bad results >Bad results 3s delay 3s delay Timing correction via sinc interpolation This timing difference is often corrected by using sinc interpolation. This is applied by invoking the Fourier Shift Theorem. We thus calculate the value of the data at slice 4 at timepoint m, and t=-3.0s for example, and compare this data with the reference function         0 2 0 js t ft Fs ftt e   Results
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This note was uploaded on 02/10/2010 for the course TBE 2300 taught by Professor Cudeback during the Spring '10 term at Webber.

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Lecture03_PreprocessI&II_artifacts_motion - Ghost...

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