Psy 8960, Fall ‘06 Noise1 Noise, Part II: physiology and artifacts Types of noise –Thermal –Physiological –Artifacts Physiological noise –Respiration –Motion.

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Presentation transcript:

Psy 8960, Fall ‘06 Noise1 Noise, Part II: physiology and artifacts Types of noise –Thermal –Physiological –Artifacts Physiological noise –Respiration –Motion Segmentation and navigation

Psy 8960, Fall ‘06 Noise2 Types of noise Thermal –Random fluctuations in current in receive coil, amplifier … –Spatially independent Physiological noise –Respiration –Cardiac –Vasomotor –Uncontrolled cognitive factors Artifacts –Spikes –Fluctuations in Nyquist ghost –Segmentation ghosts –Motion

Psy 8960, Fall ‘06 Noise3 Last week: thermal SNR SNR

Psy 8960, Fall ‘06 Noise4 This week: physiological noise

Psy 8960, Fall ‘06 Noise5

Psy 8960, Fall ‘06 Noise6 Phase errors accumulate through time FLASH: RO direction EPI: PE direction Global B 0 shift = spatial offset due to phase roll

Psy 8960, Fall ‘06 Noise7 Global B 0 shift = spatial offset due to phase roll +=

Psy 8960, Fall ‘06 Noise8 Global B 0 shift = spatial offset due to phase roll +=

Psy 8960, Fall ‘06 Noise9 Global B 0 shift = spatial offset due to phase roll +=

Psy 8960, Fall ‘06 Noise10 Simulated respiration effects (on edge voxels) thermal noise thermal noise + B 0 shift

Psy 8960, Fall ‘06 Noise11 Motion (during k-space acquisition) Simulated early (t = 15ms) in EPI acquisition Middle (t = 30ms) Late (t = 45ms) (Much more relevant in 3D MP-RAGE and segmented EPI)

Psy 8960, Fall ‘06 Noise12 Segmented EPI

Psy 8960, Fall ‘06 Noise13 Segmented EPI 2 nd shot 1 st shot

Psy 8960, Fall ‘06 Noise14 Segmented EPI – imperfect timing 2 nd shot Phase shift added to 1 st shot

Psy 8960, Fall ‘06 Noise15 Segmented EPI – respiration or motion 2 nd shot Phase roll added to 1 st shot

Psy 8960, Fall ‘06 Noise16 Segmented EPI pulse sequence with navigators RF G SS G PE G RO DAC TE N PE /N SEG

Psy 8960, Fall ‘06 Noise17 Segmented EPI and Navigation 2 nd shot 1 st shot Phase information from navigators is compared to correct between-segment errors