303 Week 2 - Pulse Sequences

What is each axis?

X-Axis = Frequency Encoding Grading

Y-Axis = Phase Encoding Gradient

Z-Axis = Slice Select Gradient

Spin Echo

• 90 deg. → 180 deg. → Rest → Repeat

• Takes too Long

• Great SNR in T1 images

Fast Spin Echo

• 90 deg. → 180 deg. → Repeat

• Signal does slowly and slowly get smaller so eventually a 90 degree pulse would be needed

FatSat

• 180 deg. (fat + water then recover at diff. times) → 90 deg. pulse (to saturate out the signal)

• T2 makes fat bright so we use a T2 fat sat to keep fluid bright but Fat Dark

FLAIR

Fluid Attenuated Inversion Recovery

• Good for : M.S. + Plaque around Lateral Ventricles

• 180 deg. → 90 deg.

• Suppresses Fluid

• Used for Brain Path to supress Lat Ventricles

STIR

Short Tau Inversion Recovery

• Used for MSK to supress fat over Large FOV

Why is Fat still bright in T2?

• J Coupling: Fat appears bright in T2 when it should be Dark

• When are RF is sent, it is constantly excited and never relaxes and gives artefact of a signal

Gradient Echo

• Alpha Pulse (<90 deg.) → We then Change Frequency Encoding Gradient (X-axis) [changing the Magnet, north to south rapidly]

• INCLUDES: SWI (Susceptiblity Weighted Image)

• USED FOR: Head Scans

• Downside: Susceptible to artefacts (unlike spin echo)

• SAFETY CONSIDERATION: Due to rapid changing of Magnetic Field --> Twitching -> Nerve Stimulation -> Faradays Law of Induction

• FLOWING BLOOD: Bright

• Stationary Blood: Dark (for SWI)

What is bottom red line pointing to

FID: Fourier Induction Decay

Signal to Noise Ratio:

• Larger Voxel = Better SNR = Faster Scans (BUT WE LOSE Resolution)