E. Wong, BE208, UCSD Winter 2008 Bioengineering 208 Magnetic Resonance Imaging Winter 2008 Lecture 2 • The basic spinwarp pulse sequence – Slice Selection – Frequency Encoding – Phase Encoding – Other pulses • Basic image contrast – Proton Density –T 1 –T 2 E. Wong, BE208, UCSD Winter 2008 Spin-Warp Pulse Sequence G X (t) G Y (t) RF G Z (t) RF pulses •What for? •90°: Tip M Z -> M XY •180°: Refocus resonance offsets •How Big? • •What Shape? •~FT(! G z slice profile) " = # B 1 $ dt
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E. Wong, BE208, UCSD Winter 2008
Bioengineering 208
Magnetic Resonance Imaging
Winter 2008
Lecture 2
• The basic spinwarp pulse sequence
– Slice Selection
– Frequency Encoding
– Phase Encoding
– Other pulses
• Basic image contrast
– Proton Density
– T1
– T2
E. Wong, BE208, UCSD Winter 2008
Spin-Warp Pulse Sequence
GX(t)
GY(t)
RF
GZ(t)
RF pulses
•What for?
•90°: Tip MZ -> MXY
•180°: Refocus resonance offsets
•How Big?
•
•What Shape?
•~FT(! Gz slice profile)
!
" = # B1$ dt
E. Wong, BE208, UCSD Winter 2008
Spin-Warp Pulse Sequence
GX(t)
GY(t)
RF
GZ(t)
Slice Select Gradients
•What for?
•Map space into frequency
during RF pulses
•How Big?
•
•What Shape?
•Typically flat during RF pulse
!
GZ
=RF _bandwidth
"(slice_ thickness)
E. Wong, BE208, UCSD Winter 2008
Spin-Warp Pulse Sequence
GX(t)
GY(t)
RF
GZ(t)
Slice Refocussing Gradient
•What for?
•Rewind magnetization that
was dephased by the second
half of the slice select gradient
•How Big?
• Half the area of the slice
select gradient
•What Shape?
•Only the area matters
E. Wong, BE208, UCSD Winter 2008
Spin-Warp Pulse Sequence
GX(t)
GY(t)
RF
GZ(t)
Crusher Gradients
•What for?
•Right one destroys FID from
imperfect 180°
•Left one required to balance
right one
•How Big?
•
•Want several phase wraps
across slice
•What Shape?
•Only the area matters!
FID" MZ(z)e
i#z Gz dt$$ dz
E. Wong, BE208, UCSD Winter 2008
Spin-Warp Pulse Sequence
GX(t)
GY(t)
RF
GZ(t)
Killer Gradient
•What for?
•Destroy residual transverse
magnetization prior to next TR
•How Big?
•Same criteria as Crushers:
•Want several phase wraps
across voxel
•What Shape?
•Only the area matters
KZ(t)
FID
E. Wong, BE208, UCSD Winter 2008
Spin-Warp Pulse Sequence
GX(t)
GY(t)
RF
GZ(t)
ky
kx
1 2 3 4 5
1
2
3 4
5
XY Imaging Gradients
•What for?
•Sample space
•Frequency encode:
•
•Phase encode:
•Only the area matters
•Phase rewinder leaves phase
consistent across phase encodes!
GX =Acquisition _Bandwidth
"(FOV )!
K(" # Gdt)$
E. Wong, BE208, UCSD Winter 2008
Spin-Warp Pulse Sequence
E. Wong, BE208, UCSD Winter 2008
Spin-Warp Pulse Sequence
GX(t)
GY(t)
RF
GZ(t)
…
…
…
…
TE
TR
E. Wong, BE208, UCSD Winter 2008
Image Contrast - Proton Density
Bushberg - The Essential
Physics of Medical Imaging
E. Wong, BE208, UCSD Winter 2008
Image Contrast - T1
Bushberg - The Essential
Physics of Medical Imaging
E. Wong, BE208, UCSD Winter 2008
Image Contrast - T2
Bushberg - The Essential
Physics of Medical Imaging
E. Wong, BE208, UCSD Winter 2008
T1-weighted T2-weightedDensity-weighted
Slide Credit: T.T. Liu
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