Noise 1Psy 8960, Fall ‘06

Noise, Part II: physiology and artifacts

• Types of noise– Thermal– Physiological– Artifacts

• Physiological noise– Respiration– Motion

• Segmentation and navigation

Noise 2Psy 8960, Fall ‘06

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

Noise 3Psy 8960, Fall ‘06

Last week: thermal SNRS

NR

Noise 4Psy 8960, Fall ‘06

This week: physiological noise

Noise 5Psy 8960, Fall ‘06

Noise 6Psy 8960, Fall ‘06

Phase errors accumulate through time FLASH: RO direction EPI: PE direction

Global B0 shift = spatial offset due to phase roll

Noise 7Psy 8960, Fall ‘06

Global B0 shift = spatial offset due to phase roll

+ =

Noise 8Psy 8960, Fall ‘06

Global B0 shift = spatial offset due to phase roll

+ =

Noise 9Psy 8960, Fall ‘06

Global B0 shift = spatial offset due to phase roll

+ =

Noise 10Psy 8960, Fall ‘06

Simulated respiration effects (on edge voxels)

thermal noise

thermal noise + B0 shift

Noise 11Psy 8960, Fall ‘06

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)

Noise 12Psy 8960, Fall ‘06

Segmented EPI

Noise 13Psy 8960, Fall ‘06

Segmented EPI

2nd shot

1st shot

Noise 14Psy 8960, Fall ‘06

Segmented EPI – imperfect timing

2nd shot

Phase shift added to 1st shot

Noise 15Psy 8960, Fall ‘06

Segmented EPI – respiration or motion

2nd shot

Phase roll added to 1st shot

Noise 16Psy 8960, Fall ‘06

Segmented EPI pulse sequence with navigators

RF

GSS

GPE

GRO

DAC

TENPE/NSEG

Noise 17Psy 8960, Fall ‘06

Segmented EPI and Navigation

2nd shot

1st shot

Phase information from navigators is compared to correct between-segment errors