MRI MRI

Magnetic Resonance Imaging 磁共振成像

Tris Budiyono (ATRO CB) Tris Budiyono (ATRO CB)

a general overview of MRI. 核磁共振成像的概况。

1.the patient is placed in a magnet. 1.病人被放在磁铁里。

2.a radio wave is sent in. 2.无线电波发出。

3.the radio wave is turned off. 3.无线电波已关闭。

4.the patient emits a signal,which is received 4.病人发出信号，收到信号
and used for . 和用于。
5.reconstruction of the picture. 5、图片重建。
Some very basic physics 一些非常基础的物理学

A proton has a spin,and thus the electrical charge of 质子有自旋，因此质子的电荷也会移动。

the proton also moves.

A moving electrical charge is an electrical current, 移动的电荷是电流，这伴随着磁场。

and this is accompanied by a magnetic field.

Thus,the proton has its own magnetic field and 因此，质子有它自己的磁场，它可以被看作

it can be seen as a little bar magnet. 是一个小棒磁铁。
 magnetic force 磁力
nucleus (magnetic field) nucleus (magnetic field)

shell shell

proton little bar magnet proton 小棒磁铁

 Can we use every other nucleus for imaging? 我们能用每一个其他的核来成像吗?

We can only use nuclei that have 我们只能使用有

1H 1H
19F 19F

A spin, and 31P Aspin,and 31P

23Na 23Na

An odd number of protons 17O 质子的奇数 17O

39K 39K

The hydrogen nucleus is best for MR imaging 氢核最适合MR成像

as hydrogen occurs in large abundance 因为氢气大量出现
throughout the body. 整个身体。
What happens,when we put a patient into the magnet of an MR 当我们把病人放进MR机器的磁铁中时，会发生什么
machine

N N

Normally protons are aligned 通 外部磁场 。

external magnetic field in a
random fashion.

S S
external magnetic field 外部磁场

N N
The protons may align with their 质子可能与它们的
South and North poles in the 向外场方向的南极和北极，与之平
direction of the external field,parallel 行。
to it.
Or they may point exactly in the 或者它们可能正好指向完全相
complete opposite direction,anti- 反的方向，反平行。
parallel.

S S
These types of alignment are on different energy 这些类型的对齐方式具有不同的能级
levels

The difference number is very small and depends on 差数很小，取决于施加磁场的强度。

the strength of the applied magnetic field.

要得到一个粗略的想法
To get a rough idea
for about 10million protons ”Opposite direction, anti - 约1000万质子"相反方向，反平行"，约有10000007"
parallel”,there are about 10000007”direction of the external 外场方向"。
field”.
 10million 10million

10million 10million
10million 10million
Opposite direction, anti -parallel 相反方向，反平行
20million ΔE 20million ΔE

direction of the external field”. 外部字段的方向。"

10million+3 10million+3

10million+6 10million+6
10million+9 10million+9
0T 0.5T 1.0 T 1.5 T 0T 0.5T 1.0 T 1.5 T
Bφ Bφ
N N
The protons do not just lay there, 质子不只是躺在那里，对齐平行或
aligned parallel or anti-pallarel to the 反帕拉雷尔的磁场线。
magnetic field lines.

S S
 It is important to know how fast the protons process. 了解质子过程的速度是很重要的。

This speed can be measured as precession frequency, 这个速度可以测量为进动频率，

that is, how many times the protons process per second. 即质子每秒过程的次数。

This precession frequency is not constant. 这个进动频率不是常数。

It depends upon the strength of the magnetic field, in 这取决于磁场的强度，在

which the protons are placed. 这些质子被放置。

The stronger the magnetic field, the faster the procession 磁场越强游行越快
rate and higher the procession frequency. 速度和更高的游行频率。
 Larmor equation Larmor equation
ω0＝γB0 ω0＝γB0
ω0 is the precession frequency (in Hz or MHz). ω0是进动频率（以Hz或MHz为单位）。
It is possible and necessary to precisely 精确计算这个频率是可能和必要的。
calculate this frequency.
This is done by using an equation called 这是通过使用一个叫做
Larmor equation. Larmor equation.
B0 is the strength of the external magnetic field, B0是外部磁场的强度，在Tesla（T）中给出
which is given in Tesla (T)
γ is the so-called gyro-magnetic ratio γ是所谓的陀螺磁比
 ω0＝γB0 ω0＝γB0
The equation states that the precession frequency becomes 等式说明进动频率变为
higher when the magnetic field strength increases. 当磁场强度增加时。
The exact relationship is determined by the gyro-magnetic 确切的关系由陀螺磁比γ决定。
ratio γ.

This gyro-magnetic ratio is different 这个陀螺磁比不同

for different materials 用于不同的材料

The value for protons is 质子的值为

42.5MHz/T
Magnets used for imaging mostly have field 用于成像的磁体大多具有介于0.2至1.5T
strengths somewhere between 0.2 to 1.5 T. 之间的磁场强度。

The earth’s magnetic field is 0.3 and 0.7 G, 地球磁场为0.3和0.7G，

the magnet of a refrigerator door has about 冰箱门的磁铁大约有
100G=0.01T 100G=0.01T
precession frequency(ω0 ) for protons 质子的进动频率（ω0）

0.5T 1.0T 1.5T 0.5T 1.0T 1.5T

21.7MHz 42.5MHz 64.2MHz 21.7MHz 42.5MHz 64.2MHz
Introducing the coordinate system 介绍坐标系
Z Z
N To make communication easier,let N 为了便于沟通,让我们开始使用
us start using a coordinate system. 坐标系。

Y Y

X X
S S
longitudinal magnetization longitudinal magnetization
Z The z-axis runs in the direction of Z Z轴沿磁场线方向运行。
the magnetic field lines.

We will also illustrate the protons 我们还将把质子说成是小箭头

as vectors as little arrows. 的矢量。

A vectors represents a certain force 向量表示作用于某个方向（箭头

Y
(by its size) that acts in a certain 方向）的某个力（按其大小）。
X direction (direction of the arrow). X
 longitudinal magnetization longitudinal magnetization

WeAs cannot
this magnetization
measure thisismagnetic
in direction along
force, as it/ 我们不能测量这个磁力，因为它是同一方向
islongitudinal to the external
the same direction, parallelmagnetic fields, it
to the external ，平行于外部磁场。
is also called
magnetic field.longitudinal magnetization.
Z Z
It is actually this new magnetic vector 它实际上是这个新的磁向量，可能被
that may be used to get a signal. 用来得到一个信号。

Y Y
We cannot measure this magnetic force, as it 我们不能测量这个磁力，因为它是同一方向
is the same direction, parallel to the external ，平行于外部磁场。
magnetic field.

For this a magnetization transverse to the 为此，必须有一个横向于外部磁场的磁化

external magnetic field is necessary. 强度。
Primary Magnetic PrimaryMagnetic
Resonance Imaging ResonanceImaging
Magnets
Main magnet 主磁铁
Permanent magnets 永久磁铁

This magnet always magnetic and does not use 这种磁铁总是磁性的，不使用任何能量来工

any energy for work,which are its advantage. 作，这是它的优势。

Possible disadvantages are thermal instability,its 可能的缺点是热不稳定，

limited field strength,and its weight. 有限的场强及其重量。

A magnet of 0.3 T may weight about 100 tons! 0.3T的磁铁可能重约100吨！

 Main magnet 主磁铁
Resistive magnets 电阻磁体
In a resistive magnet, an electrical current is passed 在电阻磁体中，电流通过导线环产生磁场。
through a loop of wire and generates a magnetic field.

They are only magnetic as long as there is an electrical 只要有电流流过它们，它们就是磁性的。

current flowing through them.

As there is a resistance to the flow of the electricity though 由于电线对电流有阻力，这些磁铁在运行时会变暖，

the wire, these magnets get warm when in operation, and 必须冷却。
have to be cooled.
Compared with permanent magnets 与永磁体相比

They achieve a higher filed strength. 他们取得了更高的实力.

Resistive magnets are not very practical with 电阻磁铁不是很实用，具有很高的磁场强度

very high field strengths because they create lots ，因为它们会产生大量的热量，必须被消散
of heat that must be dissipated. 。
 Main magnet 主磁铁
Superconducting magnets 超导磁体
Superconducting magnets are the ones most 超导磁体是目前MR机中应用最广泛的磁体。
widely used in MR machines at the present time.

They also make use of electricity, but they have 他们还利用电力，但他们有一个特殊的电流

a special current carrying conductor. 携带导体。

This is a cooled down to superconducting 这是一个冷却到超导的温度.

temperature.
About 4°K or -269℃ 约4°K或-269℃
superconducting temperature. 超导温度。
At this temperature, the current conducting 在这个温度下，电流导电材料失去了电
material loses its resistance for electricity. 阻。

So if you send in an electrical current once, 所以如果你发送一次电流，它就会在那

it flows in there permanently, creating a 里永久流动，产生恒定的磁场。
constant magnetic field.

So called cryogens(helium, nitrogen) are 所谓的冷冻剂（氦气、氮气）是用来冷

used for cooling of these magnets, and have 却这些磁体的，不得不偶尔重新填充一
to be refilled once in a while. 次。
Cross-sectional of superconducting magnet 超导磁体的横截面
Liquid nitrogen 液态氮

RF Coil RF Coil

Gradient Coil 梯度线圈

新线圈（ShimC
Shim Coil oil）

superconducting Coil 超导线圈

Vacuum Liquid helium Vacuum 液态氦

 Quench Quench
When for some reason the temperature rises above 当由于某种原因，温度上升到这些磁体的超导
the superconducting temperature in these magnets, 温度之上，这些磁体就会出现超导性的损失，
these magnets, there will be a loss of 对电流的突然阻力。
superconductivity, and sudden resistance to the
flow of electricity.

This results in rapid heat production, which causes 这导致快速产生热量，导致冷冻剂迅速沸腾。

cryogens to boil off rapidly.
Advantage of superconducting magnets 超导磁体的优势

High magnetic field 高磁场

Excellent magnetic field homogeneity 优异的磁场均匀性

This is in the order of 10-50 ppm over a 这在直径45厘米的区域上大约为10-

region 45cm in diameter. 50ppm。
Disadvantage of the superconducting 超导磁体的缺点
magnets

High costs 成本高昂

Use of rather expensive cryogens 使用相当昂贵的冷冻剂

 Characteristics of a magnet (MRI) 磁铁的特性（MRI）

Permanent Resistive Superconducting Permanent Resistive Superconducting

Magnetic filed (B0) Up to 0.3T Up to 0.3T 0.3T to 4T 磁性归档（B0） 直至0.3T 直至0.3T 0.3T to 4T
磁场（Magneticfield）
Magnetic field 50-100ppm 10-50ppm 1-10ppm 50-100ppm 10-50ppm 1-10ppm
homogeneity eity
Weight 90.000kg 4.000kg 10.000kg Weight 90.000kg 4.000kg 10.000kg
Cooling None Water,heart exchanger Criogenic Cooling None Water,heart exchanger Criogenic
Power consumption 20kw 80kw 20kw 耗电量 20kw 80kw 20kw
Distance to 0.5 mT <1m 2m 10m 到0.5mT的距离 <1m 2m 10m
fringe field 边缘场
a general overview of MRI. 核磁共振成像的概况。

1.the patient is placed in a magnet. 1.病人被放在磁铁里。

2.一个无线电波被发送进来。
2.a radio wave is sent in.
3.the radio wave is turned off. 3.无线电波已关闭。
4.the patient emits a signal,which is received 4.病人发出信号，收到信号
and used for . 和用于。
5.reconstruction of the picture. 5、图片重建。
Radio frequency pulse (RF pulse) 射频脉冲（RF脉冲）

We sent in a radio wave. 我们用无线电波发送。

The term radio wave is used to describe an 术语"无线电波"用于描述电磁波,如果您在无
electromagnetic wave, that is in the frequency 线电中接收到的波,则在频率范围内。
range if the waves which you receive in your
radio.

Energy exchange is possible when protons 当质子和射频脉冲具有相同频率时，能量交

and the radio frequency pulse have the same 换是可能的。
frequency.
What happens with the protons, when they 当质子暴露于这个射频脉冲时，质子会发
are exposed to this RF-pulse. 生什么。

The radiowave has two effects on the protons 辐射波对质子有两种影响

lifts some protons to a higher level of energy. 将一些质子提升到更高的能量水平。

also causes the protons to precess in step, in phase. 也导致质子在步进，在相中进动。

lifts some protons to a higher level of energy. 将一些质子提升到更高的能量水平。

Z Z Z Z

RF pulse RF pulse

The RF exchanges energy 射频与质子交换能量

with the proton
Y Y Y Y

X X X 其中一些被提升到更高的能量
Some of them are lifted to a
higher level of energy, pointing 水平，在图中指向下方。
downward in the illustration.
 Z Z
In effect the magnetization along RF pulse 实际上，沿着z轴的磁化强度会 RF pulse
the z-axis decrease, as the 降低，因为指向下方的质子会"
protons which point down 中和"相同数量的指向上方的质
“neutralize” the same number of 子。
protons pointing up.
Y Y

X
X X
X
 also causes the protons to precess in step, in phase. 也导致质子在步进，在相中进动。

Z Z Z Z

RF pulse RF pulse

The RF gets the precessing RF同步获取进动质子

protons in synch
Y Y Y Y

X X X X
a new transversal magnetization,which 一种新的横向磁化强度，它随进动质
moves around with the precessing 子移动。
protons.
Secondary Magnetic 二次磁性
Resonance Imaging ResonanceImaging
Magnets
Volume coils 体积块线圈
Surface coils 曲面线圈
 Volume coils 体积线圈
Volume coils are used in all MR units. 体积线圈用于所有MR单元。

These completely surround the part of the body that is 这些完全围绕着要成像的身体部分。

to be imaged.

These volume coils should be closed to the size of the subject. 这些卷线圈应关闭到对象的大小。

The body coil is a permanent part of the scanner, and 身体线圈是扫描仪的永久部分，围绕病人。

surrounds the patient.

It is important, as it is the transmitter for all types of 这一点很重要，因为它是所有类型考试的发射器。

examinations.

It also receives the signal when larger parts of the body are 当身体较大的部分成像时，它也接收信号。
imaged.
 Surface coils 曲面线圈
Surface coils are placed directly on the area of interest, and have 曲面线圈直接放置在感兴趣的区域上，并具有与要检查的零件
different shapes corresponding to the part to be examined. 对应的不同形状。

They are receiver coils only, most of the received signal coming 它们只是接收器线圈，大部分接收到的信号来自附近的组织
from tissues near by; deeper structure cannot be examined ；这些线圈不能检查更深的结构。
with these coils.

As with the head coils, the RF pulse is transmitted by the body 与头部线圈一样，在这些情况下，射频脉冲由身体线圈传递
coil in these cases. 。
 a general overview of MRI. 核磁共振成像的概况。

1.the patient is placed in a magnet. 1.病人被放在磁铁里。

2.a radio wave is sent in. 2.无线电波发出。

3.the radio wave is turned off. 3.电波被关闭。

4.the patient emits a signal,which is received 4.病人发出信号，收到信号
and used for . 和用于。
5.reconstruction of the picture. 5、图片重建。
Further details about MR signal 有关MR信号的详细信息

If our protons rotated around in synch, in phase, 如果我们的质子同步旋转，同相，什么都不会

and nothing would change, then we would get a 改变，那么我们就会得到一个信号。
signal .
This, however, is not what happens . 然而，事实并非如此。

As soon as the RF pulse is switched off, the whole RF脉冲一关掉，受到RF脉冲干扰的整个系统就回

system, which was disturbed by the RF pulse, goes 到原来安静、平静的状态，放松下来。
back to its original quiet, peaceful state, it relaxes.
The newly established transverse 新建立的横向磁化强度开始消失
magnetization starts to disappear, ，

a process called transversal relaxation . 一个叫做横向松弛的过程。

The longitudinal magnetization grows back 纵向磁化强度恢复到原来的大小。

to its original size.

a process called longitudinal relaxation . 一个叫做纵向松弛的过程。

longitudinal relaxation 纵向松弛

These protons no longer 这些质子不再像以前那样抵

ZZ
cancel out the magnetic ZZ 消相同数量质子指向上方的
RFvectors
pulse of the same number R 磁向量。
of protons pointing up, as
Some
they didprotons
After the pick up
RF pulse
before. is
energy
switched, start
off, to walk
protons on
go
Y
Y So, the magnetization in this Y
Y 所以，这个方向的磁化强度
their
back hands, and higher
from their thus to
direction, the longitudinal ，纵向磁化强度增加，最后
decrease
the lowerthe amount
state of i. e.
of energy,
X
X
magnetization increases, and X
X 回到原来的数值。
longitudinal
point up again.magnetization
finally goes back to its
original value.
longitudinal relaxation (spin-lattice-relaxation) 纵向松弛（自旋晶格松弛）

What happens to the energy which they had 他们所拥有的能量会发生什么

picked up from the RF pulse? 从射频脉冲中拾取？

This energy is just handed over to their 这种能量只是交给他们的周围，所谓

surroundings, the so called lattice. 的晶格。

And this is why this process is not only 而这就是为什么这一过程不仅被称为纵向

called longitudinal relaxation, but also spin- 松弛，还被称为自旋晶格松弛。
lattice-relaxation.
 Z Z
If one plots the longitudinal magnetization 如果在RF脉冲关闭后绘制纵向磁化强度与
vs. time after
Y the RF pulse was switched 时间的关系图，则得到所谓的T1曲线。
off,Xone gets a so-called T1-curve.

Longit. Longit.
T1-curve T1-curve
magn. magn.
MZ=M0･(1-e-t/T1) MZ=M0･(1-e-t/T1)

time time
The time that it takes for the longitudinal 纵向磁化强度恢复所需的时间，要回到其原
magnetization to recover, to go back to its 值，用纵向松弛时间来描述，也称为T1。
original value, is described by the longitudinal
relaxation time, also called T1.
This actually is not the exact time it takes , 这实际上不是它所花费的确切时间，而是一
but a time constant, describing how fast this 个时间常数，描述这个过程有多快。
process goes.

T1 is a time constant comparable to the time T1是与例如描述放射性衰变的时间常数相

constants that for example describe 当的时间常数。
radioactive decay.
Funning out, they point less and less in the same 它们在同一方向上指向越来越少，因此横向磁化强度
direction, and thus transversal magnetization decreases. 降低。
Z Z Z

Y Y Y Y Y Y

X X X X X X
 Transversal relaxation time (T2)
If oneZplots transversal magnetization vs. 如果在RF脉冲关闭后绘制横向磁化强度与
time after the RF pulse is switched off, one 时间的关系图，则得到一条曲线，如所示
gets a curve as illustrated, which called a ，称为T2曲线。
T2-curve. Y
X

Transv. Transv.
magn magn
T2-curve T2-curve
MXY=M0･e-t/T2 MXY=M0･e-t/T2

Time Time
 How long is a relaxation time? 放松时间有多长？

T1 is longer than T2, and just to give you idea: T1比T2长，只是为了给你想法：

T1 is about 2-5-10 times as long as T2 . T1大约是T2的2-5-10倍。

Or in absolute terms in biological tissues: 或者在生物组织中的绝对值：

T1 is about 300 to 2000 msec, T1约300至2000msec，

T2 is about 30 to 150 msec, T2约30至150msec，

T1value T1value
T1 was defined as the time when about 63% of the T1定义为达到原始纵向磁化强度约63%的时间。
original longitudinal magnetization is reached.

T2 value T2 value
T2 is the time when transversal magnetization T2是横向磁化强度降至原值37%的时间。
decreased to 37% of the original value.
 Approximate
ApproximateTT12relaxation
relaxationtimes
times at
ataafield
field 一个场的近似T2松弛时间
strength
strengthof
of1.0T
1.0Tfor
forvarious
varioustissues
tissues 各种组织的1.0T强度
Tissue T1 (ms) T2 (ms) Tissue T1 (ms) T2 (ms)
Fat 180 90 Fat 180 90
Liver 270 50 270 50
Renal cortex 360 70 肾皮质 360 70
White matter 390 90 白质属 390 90
Spleen 480 80 480 80
灰色物质（graymat
Gray matter 520 100 ter） 520 100
Muscle 600 40 600 40
Renal medulla 680 140 肾髓质 680 140
Blood 800 180 血脑嵴液 800 180
Cerebrospinal fluid 2000 300 2000 300
Water 2500 2500 2500 2500
a general overview of MRI. 核磁共振成像的概况。

1.the patient is placed in a magnet. 1.病人被放在磁铁里。

2.a radio wave is sent in. 2.无线电波发出。

3.the radio wave is turned off. 3.无线电波已关闭。

4.the patient emits a signal,which is received 4.病人发出信号，接收到信号
and used for . 和用于。
5.reconstruction of the picture. 5、图片重建。
 90°pulse 90°pulse
If after the RF pulse, the number of protons on the 如果在射频脉冲之后，较高能级上的质子数等
higher energy level equals the number of protons 于较低能级上的质子数，纵向磁化强度就消失
on the lower energy level, longitudinal 了，由于相位相干而只存在横向磁化强度。
magnetization has disappeared, and there is only
transversal magnetization due to phase coherence.

The magnetic vector seems to have been “tilted” 磁向量似乎已经向侧面"倾斜"了90°。

90°to the side.
The corresponding RF pulse is thus also called a 相应的射频脉冲因此也称为90°脉冲。
90°pulse.
 Z Z Z Z
RF pulse RF pulse
The The
longitudinal 90° longitudinal 90°
magnetization magnetization
Y Y

X X X X

Two protons precessing One of the two protons 两个质子进动 两个质子之一吸收能量

- inaround
effectthe
- that a longitudinal
z-axis. magnetic
pick up energy, to go into 实际上是一个纵向磁场 态
the higher
vector is tilted 90°to state of energy.
the side. 向量向侧面倾斜90°。
Both processes are due to entirely different 这两个过程都是由于完全不同的机制，并
mechanisms, and occur independently even 且即使同时发生也是独立发生的。
though at the same time .
These magnetic vectors add up to a sum 这些磁向量加起来等于一个和向量。
vector.
The sum vector will actually perform a 求和向量实际上会进行螺旋运动。
spiraling motion . Z

Y Y

X X
 FID signal (free induction decay) FID信号（freeinductiondecay）

The sum vector constantly changes its 求和向量在执行螺旋运动时不断改变其方向

direction and magnitude while it performs its 和大小。
spiraling motion.
The sum vector induces an electrical current in 总和矢量在天线中感应出电流,即MR信号.
antenna, the MR signal.
This is greatest magnitude immediately 这是在RF脉冲关闭后立即减小的最大幅度
after the RF pulse is switched off and then 。
decreases. Z

Y Y

X X
 FID (free induction decay) 自由感应衰变（FID）
Signal 信号强
intensity 度

The signal disappears 但是,信号随着时间的推

with time, however, has a 移而消失,频率是恒定的.
constant frequency. time time
Thus, by changing the time between successive 因此，通过改变连续的时间
RF pulses, we can influence and modify 射频脉冲可以影响和改变组织的磁化强度和信
magnetization and the signal intensity of tissues. 号强度.

When you use more than one RF pulse - a 当你使用一个以上的射频脉冲一个连续的射

succession of RF pulses - you use a so - called 频脉冲你使用一个所谓的脉冲序列。
pulse sequence.

As you can use different pulses, e. g. 90°or 由于可以使用不同的脉冲，例如90°或180°

180°pulses, and the time intervals between 脉冲，而连续脉冲之间的时间间隔可以是不
successive pulses can be different pulse 同的脉冲序列。
sequences.
 TR = time to repeat TR=重复时间

The pulse sequence that we used was made up of 我们使用的脉冲序列仅由一种脉冲组成，即90°

one type of pulse only, the 90° pulse. 脉冲。

This was repeated after a certain time, which is 这在一定时间后被重复，称为TR（重复时间）

called TR (time to repeat)
 How did TR influence the signal TR如何影响信号

With a long TR we got similar signals from both tissues, 有了很长的TR，我们从两个组织得到了相似的信号，

both would appear the same on a MR picture. 两者在MR图片上都会出现相同的。

Using a shouter TR, there was a difference in signal 使用呼喊TR，组织之间的信号强度有差异，这取

intensity between the tissues, determined by their 决于它们在T1中的差异。
difference in T1.

The resulting picture is called a T1-weighted picture. 得到的图片称为T1加权图片。

This means, that the difference of signal intensity 这意味着，该图中组织之间信号强度的差异，即组

between tissues in that picture, the tissue contrast, is 织对比，主要是由于它们在T1中的差异。
mainly due to their difference in T1.
How do we obtain a T2-weighted image? 如何获得T2加权图像?

First, we use a 90°pulse. 首先，我们使用90°脉冲。

The longitudinal magnetization is tilted, we get a transversal 纵向磁化倾斜，得到横向磁化强度。

magnetization.
After the pulse is switched off, longitudinal magnetization 脉冲关闭后，纵向磁化强度开始重新出现，横向磁化强度
starts to reappear, the transversal magnetization, however, 却开始消失。
starts to disappear.

Now we do something new 现在我们做一些新的

After a certain time we send in a 180°pulse. 过了一段时间，我们发出180°脉冲。

which we call TE/2, half of TE 我们称之为TE2,即TE的一半
 Spine
The echoserves
180°pulse sequence
After the 90°pulse protons
to “neutralize” are
the external 180°脉冲作用于"中和"外部
dephaseing
Themagnetic
spine echo due to external
sequence
field and
consists of
inhomogeneities a internal 磁场不均匀性
TR
magnetic field inhomogeneities
90°and 180°pulse

90° 180° 90° 180° 90° 180° 90° 180°

RF pulse RF pulse
by choosing
With longer different
TEs, the TEs the
signal signals can
intensity be T2-
difference 使用更长的TE时,信号强度差异
weighted in varying
between tissues degrees
will be with very short
depending much TEs, T 2-
on their 组织之间的关系将在很大程度上取决于组织之间的关系
effects
T2s,have
theirnot yet had time
transversal to reallytimes
relaxation showup T2s，它们的横向松弛时间
spin echo 自旋回声

TE/2 TE TE/2 TE

The 180°pulse rephases the dephasing protons, and 180°脉冲会重新设计去相位质子,

a stronger signal,MR image
the spin echo results 一个更强的信号，自旋回波结果
It isThe
possible
signaltointensity
Thelongitudinal determine
of thesignal
magnetization intensity
after
tissue after thea timefor
TEa
TR 通 此时TE可以从T2曲线推断出TE后组织的信号强
This transversal magnetization immediately
tissue
is using
equal
can
starts toto
then abespin
the echo
amount
inferred
disappear sequence
aof
by fromtransversal
ratethe by
T2-curve
which combining
at this
is determined 回 度。
magnetization
bytime
the T1- and
the TE. we start
the t2-curve
transversal out
forwith,
relaxation that astissue.
time, it is thus
and “tilted”
by
90
thedegrees.
T2-curve.

signal signal
Longit. relax. Longit. relax.

Transv. relax. Transv. relax.

TR TE time TR TE time
 When
With awe only wait a very short
in TTE, inthen 当我们只等待一个很短的TE时，由于T2的差异而
A long TR long
and TR,
adifferences
short TE 1 longitudinal 长T 导致的信号强度的差异还没有时间发音。
differences
magnetizationin signal intensity
time are dueimportant
not very to differences
any in
T 2 have
more, asnot
all yet hadhave
tissues timeregained
to become pronounced.
their full
Thelongitudinal magnetization.
T1- and T2-curves for two different tissues are depicted 描

signal signal

TR time TR time
TE TE
The resulting picture is thus neither T1-nor T2- 由此得到的图片既不是T1-norT2加权，但大
weighted, but mostly determined by the 多由组织的质子密度决定。
proton density of the tissues.

Proton density weighted image 质子密度加权图像（自旋密度

(Spin density) ）
 When TR
A long we wait
and a long TR and
a long TElong TE, differences in T2 长当我们等待很长的TR和很长的TE时，T2中的差异已
have had time enough to become pronounced, the 经有足够的时间被发音，结果是T2加权。
Withresulting
a long TR there isare
picture T2no prevailing differences in T1.
-weighted. 长
With the long TE however, differences in T2 become pronounced. 然而随着TE的延长，T2的差异变得明显。

signal signal

TR TE time TR TE time
Thus the resulting picture is T2-weighted. 因此，得到的图片是T2-weighted的。

T2-weighted image T2-weighted image

 With a short TR, tissues have not recovered their T
A longitudinal
shorter
When TETR and
is short, a short
differences TE
in T2 cannot really 当TE短时，T2中的差异不能真正表现出来，因此得
magnetization, thus differences in T1 will 到的画面仍然是T1加权。
manifest themselves, so the resulting picture is still T1-
Whenshow
we up inaform
wait of signal
shorter time intensity
TR, differences.
differences in T1 influence tissue 当 织
weighted.
contrast to a larger extent, the picture is T1-weighted, especially 对比 TE时
when we also wait a short TE. 。
signal signal

TR time TR time
TE TE
When TE is short, differences in T2 cannot really 当TE短时，T2中的差异不能真正表现出来，因此得
manifest themselves, so the resulting picture is still T1- 到的画面仍然是T1加权。
weighted.

T1-weighted image T1-weighted image

Weighting of spin echo imaging as a function of 自旋回波成像加权作为重复时间（TR）和回波
repetition times (TR) and echo time (TE) 时间（TE）的函数

Repetition times (TR) 重复时间（TR）

Short (<300ms) Intermediate Long 短（<300ms）中级 Long

(300ms-1000ms) (>2000ms) ms) (>2000ms)
Short TE T1 PD & T1 PD 短TE（< T1 PD & T1 PD
(<40ms) -weighted -weighted -weighted 40ms） -weighted -weighted -weighted

long TE T1 &T2 PD,T1 &T2 T2 LongTE T1 &T2 PD,T1 &T2 T2

(>80ms) -weighted -weighted -weighted (>80ms) -weighted -weighted -weighted
 TE (short) (ms) TE (short) (ms)

10 10

Proton density image 20 质子密度图 20

40 40

100 100
T1-weighted image T1-weighted image
T2-weighted image 250 T2-weighted image 250

500 500
TR (short) TR (short)

25 50 100 200 400 1000 2000 3000 (ms) 25 50 100 200 400 1000 2000 3000 (ms)
 a general overview of MRI. 核磁共振成像的概况。

1.the patient is placed in a magnet. 1.病人被放在磁铁里。

2.a radio wave is sent in. 2.无线电波发出。

3.the radio wave is turned off. 3.无线电波已关闭。

4.the patient emits a signal,which is received 4.病人发出信号，收到信号
and used for . 和用于。
5.reconstruction of the picture. 5、图片重建。5.重建图片。