5_MRI基本脉冲序列与MR图像对比度分辨率信噪比 87页PPT

如果M从和态完全恢复，每个FID都可 达到最大值
饱和条件?：Mzn00, n1
TR>>T2
3T2
Mxny 0 Mz0 1eTR/T1 ,
n1
The amplitude of the FID signal is:
A f M z 01 e T R /T 1 1 e T R /T 1
Approximate T2 value in ms
60-80
Whole blood (deoxygenated)
1350
50
Whole blood (oxygenated)
1350
200
Cerebrospinal fluid (similar to pure water)
4200 - 4500
2100-2300
IV. Basic Spin-Echo Imaging
SE ~ Spin echo


RF
slice
phase readout
signal
TE
பைடு நூலகம்
Gz
Gy Gx echo
1：90选择性RF脉冲与 Gz —层面选择 2：-Gz ：产生反方向相移，部分补偿+Gz
产生的正向相移—提高信号 3：Gy — 相位编码 4:180 RF 脉冲— 形成回波 5:-Gx 产生负向相移，部分补偿后面 + Gx
Ir r1 2 e T R T E / 2 / T 1 r e T R / T 1 r e T E / T 2
TETR
I r r 1eTR/T1r eTE/T2
Irr1 e T R /T 1 re T E /T 2
Mzn00, n1
饱和恢复序列 Saturation Recovery
M z ( t) M z ( 0 ) e tT 1 M z 0 ( 1 e tT 1 )
饱和恢复序列 Saturation Recovery
90 - (长间隔) - 90
每个90RF后，产生一个FID信号
Image Quality
– Contrast – Resolution – Noise, Signal-to-Noise Ratio – Artifact
Image Quality
– Contrast – Resolution – Noise, Signal-to-Noise Ratio – Artifact
Saturation? In MR, saturation is a nonequilibrium state with no net magnetization
The same amount of nuclear spins is aligned against and with the magnetic field
产生的正向相移—采集对称信号
6:+Gx —频率编码（采样、读数）
Irr1 2 e T I/T 1 r e T R /T 1 r
Ir r1 2 e T R T E / 2 / T 1 r e T R / T 1 r e T E / T 2
3D 采集
3D与2D采集的区别
RF脉冲是非选择性 的
选层梯度场 相 位编码
3D FT
扫描时间增加(因 为相位编码步数增 加)
为节省时间，可降 低Z方向的空间分 辨力
M S t S kx,ky I x, y
Image Characteristics
Gx
t
kx Gxt
0
kx
Skx xeikxxdx
0
kx
0
kx
k-Space
kt0tGd
Phase-Encoded Signals
Gy
St yeiG yyTpedy ei0t
ky GyTpe
MR成像方法和脉冲序列
M S t S kx,ky I x, y
Image Characteristics
上次课主要内容
层选 频率编码 相位编码 K空间 1D/2D/3D 采集
Slice Selection
Gz Resonant Frequency
Contrast
PDW
T1W
T2W
MR 基本脉冲序列
饱和恢复序列 ~ Saturation recovery 部分饱和恢复序列 ~ Partial saturation 反转恢复序列 ~ Inversion recovery Standard SE GRE
饱和恢复序列 Saturation Recovery
SE ~ Spin echo
Saturation-Recovery Spin-Echo
Inversion-Recovery Spin-Echo
Ir r1 2 e T I/T 1 r e T R /T 1 r e T E /T 2
Ix,yf ,T 1 ,T 2,T 2 * ,
T1-weighted, T2-weighted, Proton Density
At a main field of 1.5 T
Tissue Type
Approximate T1 value in ms
Adipose tissues
240-250

I
x,yeikyydy

Sk x,k y Ix ,ye ik yyd y e ik xx d x ??? Sk x ,k y Ix ,ye ik x x k y y d x d y
Gx
Phase encoding
8/31/2019
Phase Shift

B
Frequency X-Position Phase Y-Position
9
k-Space
kt0tGd
180
Frequency-Encoded Signals
St xeiGxxtdx
8/31/2019
RF
B
4
St xeiG xxtdxei0t

St
xeiGxxtdx

1D 采集 - Line by Line, Line Scan
Phase Encoding
2D Spatial Encoding – Frequency and Phase Encoding
（液体）流体抑制 FLAIR （ Fluid-Attenuated Inversion Recovery ）
选择TI使water信号过0点 – 其他组织：M可进入xy平面产生随T2衰减 的信号； – water ：没有M可以 进入oxy平面； – TI：2000ms
Diffusion MRI: A New Strategy for Assessment of Cancer Therapeutic Efficacy
PDW
TR Long
TE Short
T1W T2W
Short Long Short
Short Long Long
一般地:
短 TR：100—1000ms 长 TR：1600—3000ms
短 TE：10—30ms
长 TE：70—100ms
For one slice Imaging Time = TR*Npe*Nex
Gray matter of cerebrum
920
100
White matter of cerebrum
Liver Kidneys Muscles
780
490 650 860-900
90
40 60-75
50
In most situations (but not in principle) T1 is greater than T2.
短TR，最小的TE
T1W
Short TR T1W <500ms
III. 反转恢复序列 IR ~ Inversion Recovery
180°- 90 ° 在IR序列中，首先使用180°RF 脉冲，
然后等待一段时间再使用一个 90°RF 脉 冲。
III. 反转恢复序列 IR ~ Inversion Recovery
1-exp(-t/T1) 在IR序列中，M由–M0 恢复到M0，恢
复曲线：
1-2exp(-t/T1)
0点（Null point）
信号过0的点叫做null point 过0点的时间TI(null)
TI(null) = (ln2)T1 = 0.693T1
脂肪抑制~ Fat suppression STIR （Short TI Inversion Recovery）
Which is better？What is the difference？
Which is better？What is the difference？
Which is better？What is the difference？
Which is better？What is the difference？
选择TI使脂肪信号过0点 – 其他组织：M可进入xy平面产生随T2衰减 的信号；
– 脂肪：没有M可以 进入xy平面；
– 1.5T： TI ~ 140ms – 0.5T： TI ~ 100ms
Radiotherapy induced fatty marrow A 67 y o female , a known case of Ca cervix with radiotherapy. Now complains of severe backache, radiating to lower limbs. Referred to MRI lumbar spine to rule out metastasis and cause of pain. Diffuse homogeneously high marrow signals involving L5 and all sacral vertebral bodies on T1 and T2 w images with complete signal suppression on STIR implies to fatty marrow, it s a radiotherapy induced change, area of involvement corresponds to field of radiotherapy, should not be mistaken for neoplastic marrow infiltration.
Ir Cr1 e T R /T 1r
Long TR PDW >1500ms
II. 部分饱和恢复序列 Partial Saturation Recovery
Ir Cr1 e T R /T 1r
90-(短间隔)- 90
第2个90°RF脉冲后，M没有完全恢复， 后面的FID信号比第1个FID信号小。

Sky

yeikyydy
t
T pe
ky
kx
For 2D imaging
Sk x ,k y Ix ,ye ik x x k y y d x d y
ky kx
ky kx
FT-1
I x, y
Sx,ky
TI( Inversion Time) M z ( t) M z ( 0 ) e tT 1 M z 0 ( 1 e tT 1 )
Irr1 2 e T I/T 1 r e T R /T 1 r ?作业1
对于典型的SR序列T1 恢复曲线，纵向 弛豫恢复曲线：