MRI IQ and Artifacts PDF
Preview MRI IQ and Artifacts
The Plan ■ The Magnetic Resonance Phenomenon & Contrast (30) ■ Spatial Encoding (26) ■ The “Pulse Sequence” Rules Everything (3) Seventh Inning Stretch ■ Fast Imaging (14) ■ Functional MRI (18) ■ Diffusion and Summary (9) ■ Image Quality and Artifacts (48) ©2011 Mark Cohen, all rights reserved www.brainmapping.org 1 An “Equation” in Resolution Because MR is an emission modality the temporal resolution, spatial resolution and contrast are inter-dependent: Signal = kB (voxel size) imaging time 0 −contrast where B is the field strength. 0 ©2011 Mark Cohen, all rights reserved www.brainmapping.org Contrast to Noise Ratio ©2011 Mark Cohen, all rights reserved www.brainmapping.org CNR vs. Resolution Noise free Imaging time = 1X Imaging time = 2X Imaging time = 4X 64 X 64 128 X 128 Minimum Imaging Time 256 X 256 ©2011 Mark Cohen, all rights reserved www.brainmapping.org CNR vs. Resolution 256 X 256 Signal/Noise Ratio Held Constant 128 X 128 64 X 64 Imaging time = 16X Imaging time = 4X Imaging time = 1X ©2011 Mark Cohen, all rights reserved www.brainmapping.org CNR vs. Resolution Noise free 16 averages 4 averages 1 average 6644 XX 6644 112288 XX 112288 Imaging Time Held Constant 225566 XX 225566 ©2011 Mark Cohen, all rights reserved www.brainmapping.org Slice Thickness 5 mm 2 mm TR=500! TE=11! NEX=1 Matrix=256x256 BW=16kHz ©2008 Mark Cohen, all rights reserved www.brainmapping.org 7 Magnetostimulation He feels magnetostimulation Body Gradient dB/dt Head Gradient She feels Nothing ©2008 Mark Cohen, all rights reserved www.brainmapping.org 8 Interpolation This location was not acquired Native Resolution Bilinear Interpolation ©2011 Mark Cohen, all rights reserved www.brainmapping.org 9 Truncation in Fourier Domain Frequency- encoded Original Sample Signal … … -1 Apparent Signal F (s) Truncated Series ©2011 Mark Cohen, all rights reserved www.brainmapping.org 10
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