Magnetic Resonance Imaging
磁共振成像
Basic Physics & Brief History
物理基础 & 简史
Magnetic resonance imaging is based on
磁共振成像基于
the physics principle of
核磁共振的物理学原理
Nuclear Magnetic Resonance or NMR
简称NMR
first described in the 1930s and 40s
最初提出于20世纪30年代和40年代
The basic principle of NMR deals with
NMR的基本原理涉及
the interaction of certain atomic nuclei
特定的原子核的相互作用
radio frequency energy
射频能量
and a strong magnetic field
在强磁场中的相互作用
This is a tough concept to grasp
这是个很难理解的概念
so for comprehension purposes
所以为了理解
We'll start with the basics
我们从基础开始
First a few definitions...
首先看一些定义
Radio frequency energy is part
射频能量
of the electromagnetic spectrum
是电磁波谱的一部分
that includes visible light and X-rays
包括可见光到X-射线
As you can see
正如你所见
the radio frequencies are at the far left of the spectrum
射频在波谱左边很远处
visible light in the middle
可见光在中间
and X-rays to the right
X射线在右边
All of these waves are defined by
所有的这些波由
a  wavelength and frequency
频率的一个波的长度决定
The wavelength is the distance
波长是
between peaks of the wave
波峰之间的距离
and the frequency is how many cycles
频率是每秒
are completed every second
所能完成的周期数
The Amplitude is the height or power of the wave
振幅是波的高度或力度
When talking about two or more waves
当说到两个或更多的波时
we can also describe a Phase
我们还会用到相位来描述
The blue and orange waves on the screen
片中蓝色和橙色的波
have the same frequency
有着同样的频率
and line up completely so they are also in phase
是完全一致的一条线，所以也是"同相"的
If we shift the orange wave to the right
如果我们把橙色波往右移
the frequency remains the same
频率依然一样
but the waves are now out of phase
但波是"反相"的
or have been phase shifted
或者说“相移”
On the other hand these two waves have
另一方面，这两条波有着
different frequencies but start
不同的频率，但起始时间相同
at the same time so they are in phase
因此他们是“同相”的
For the purpose of NMR and MRI
出于NMR和MRI的目的
radio frequency signals will be
射频信号会被描述为
described by their unique Phase and Frequency
其特定的"相位"和"频率"
A point on this computer screen is
显示屏的一个点
called a pixel
叫做注视点（pixel）
a concatenation of the words picture & element
连接着图片和元素
MR images are made up of a series a Voxels
MR图像由一系列的体素（Voxels）构成
or volume elements
或者说体积元素（体元）
Each square on the picture corresponds to a
图片上的每个方块都与
volume of tissue on the body
身体上的一个体积化的组织有关
The MR machine is designed to measure the NMR signal
MR机器被设计用于测量NMR信号
from each of the small volumes, localize them in 3D space
从每一个小块，并将其定位于3D空间里
and plot them on a 256 by 256
并把它们绘制于256*256
or 512 by 512 matrix to make a visible picture
或者512*512的矩阵里，以获得可视的图像
With this in mind we'll first describe the principles
记住这一点，我们首先描述出NMR的基本原理
of NMR in a single small square or box of tissue
即以单个小方块或者立方体的组织为基础
I suspect most of us have some recollection
我猜想我们大多数在童年时期
of constructing an electromagnet as a kid
都收集过自制的电磁体
utilizing a common battery a switch
用一个普通电池作为开关
an iron nail and a coil of wire
一个铁钉置于清晰的导线中
Throw the switch, pick up some small steel balls
通过开关，可以吸起一些小钢球
and consider yourself a mad scientist. Great stuff!
然后感觉自己就是一个疯狂科学家之类的伟大之物
What we unknowingly demonstrated with that simple project
我们通过这个小工程无意识中所展示出的
was the basic principle of Electromagnetism
正是电磁学的基本原理
where a flowing current electrons
流动电子流
has an associated magnetic field
都有着相连的磁场
oriented 90 degrees from the direction of current flow
或者和电流方向呈90度
The battery pushes the electrons around the wire
电池把电子流推向导线周围
We coil the wire to add up all those little magnetic moments
我们将导线做成线圈，加在所有小的磁性活动上
and add the steel nail to conduct the magnetic field to the
在钢钉上产生出
object we want to pick up
指向要吸起的物体的磁场
The direction of the magnetic field associated with
磁场的方向
the coill is defined by the right hand rule
由右手定定则来确定
Curl the fingers of your right hand in the direction
弯曲你的右手手指
of the current flow through the coil
四指指向的方向就是线圈上的电流方向
and your outstretched thumb will  point at the direction of
你翘起的拇指将指向
the associated magnetic field
其相关的磁场方向
With that in mind
记住这一点
let's look at the simplest of all elements—Hydrogen
让我们看一看最简单的元素——氢
The Hydrogen atom consists of a single proton
氢原子由单个质子构成
in the nucleus with a single circulating electron
质子位于原子核内，有一个电子
orbiting around that proton
围绕其进行轨道旋转
for the principal of NMR
对于NMR的基本原理来说
we don't care much about the electron
我们不太关心电子
We're  interested only in that single central proton
我们只关心一个原子核里
or nucleus  hence the descriptor nuclear
的单个中心质子，所以
in the acronym NMR
用NMR的首字母N来描述“核”
That unopposed proton in the nucleus just doesn't sit
这种原子核里的没有对手的质子
statically in the centre of the atom
不会乖乖呆在原子的中央
It actually rotates on its axis similar to the planets and stars
事实上它和星球、星星类似，是围绕着轨道运行的
Because the proton is positively charged
因为质子带正电荷
If we curl our fingers in the direction of rotation
如果我们弯曲四指，向着旋转的方向
our thumb'll point in the direction of the small magnetic field
拇指会指向小磁场的方向
associated with the each and everyone at the hydrogen protons
是我们体内的每一个氢质子
or spins in our body
都是相联系的自旋体
and fortunately for imaging purposes we have
幸运的是，我们体内有许多氢原子
a lot of hydrogen atoms in our bodies
可以达到成像的目的
An adult is about 60 percent water which contains
一个成年人含有60%的水
two hydrogen atoms for every oxygen
每个氧原子有两个氢原子
The energy storing molecules of fat and carbohydrates
是储藏着一个脂肪和碳水化合物的能量
also contain an abundance of hydrogen with glucose sporting
并且包含大量的带有葡萄糖的氢
12 hydrogens on a carbon oxygen hexagonal backbone
在一个碳氧六角结构上运送12个氢
and the free fatty acid containing 2-3 hydrogens
和包含了223个氢的自由脂肪酸
attached to every atom of the linear
附带着线形或者树形
or branched chain carbon backbone
碳链结构的每一个碳原子
Now, while we're walking around the earth,
现在，当我们行走在地球上时
all of these little proton magnets or spins are randomly oriented
所有的这些小质子磁体都是随机指向的自旋体
cancelling each other out and therefore
互相都抵消了,于是
we're usually not magnetic. However
通常是没有磁性的  然而
when we put any of us in a strong magnetic field
当我们把自身某部分置于强磁场中
such as the one found in a standard MRI machine
比如标准MRI机器所制造的磁场
all of these little spins or proton magnets lineup
所有这些小自旋体或者质子磁场排成队
Most of them line up with the main magnetic field
大部分都和主磁场一致的方向
a few line up directly opposite
少数的直接和主磁场相反方向
the main magnetic field and nothing in between
并且不存在两种状态的中间态
What determines the orientation is the amount of energy
决定方向的是与每一个
associated with each of the individual atoms or protons
独立的原子或质子的能量的总量
The ones with a little extra energy possibly
没有多余能量的,可能
from some local increased heat
可能来自一些上升的热量
will line up against the main magnetic field
将会呈和主磁场相反方向
and therefore are considered to be in a high-energy state
于是被认为是高能状态
the ones lining up with the main magnetic field
那些和主磁场方向一致的
are in a low energy state
是低能态
These protons don't simply point with
这些质子不仅仅是指向
or against the main magnetic field
或者反向于主磁场的
They actually precess much like a spinning top
其实原先很像一个旋转陀螺
as it falls to the solid surface
掉在一个固体平面上
The rate of precession can be determined exactly by
进动频率会完全由
the Larmor frequency equation
拉莫尔频率方程决定
which states that the rate of rotation is directly
这个方程表示旋转的频率直接
proportional to the strength of the local magnetic field
和当前磁场的力度成比例
At 1 Tesla, the Larmor frequency of a hydrogen proton
在1T时，一个氢质子或者一个自旋体的
or spin is 42.58 megahertz
拉莫尔频率是42.58兆赫
At 2 Tesla, 85.16 megahertz
在2T时,是85.16兆赫
and at 3 Tesla 127.74 megahertz
3T时是127.74兆赫
or basically 42.58 megahertz per Tesla
或者基本上可以说42.85兆赫每T
This will be important when we're talking about
这一点会很重要,当我们讨论
making an image from NMR data later on
用之后的NMR数据来成像的时候
For demonstration purposes
出于演示的目的
let's move all of our precessing protons
让我们先不管所有的进动质子
to a common origin on a 3D graph
先来看看3D图像的来源
As before, most of the protons are in a low energy state
如前文所说,大部分的质子都处于低能态
pointing in the direction of the main magnetic field
指向主磁场的方向
a few energetic protons are oriented
而很少的一些能量质子是
against the main magnetic field in the high-energy state
起初是反向于主磁场的高能态
Even though each proton is precessing in space
尽管每一个质子在空间上进动着
when you cancel all the opposing vectors
如果你去掉所有对立的矢量
you end up with a net magnetization
你会得到一个净磁场
pointing with the large main external magnetic field
指向最大的外部主磁场
as demonstrated in the simplified diagram on the right
正如右边的简化图所示
This is called the longitudinal magnetization
这叫做"纵向磁化"
Because it is in the same direction
因为是在与最大
as the large main external magnetic field
外部主磁场同一个方向
it cannot be measured or detected directly
它不能被直接测量或者检测到
and is therefore inferred.  But we can change all that.
所以是推测值 但我们可以改变它
Let's see what happens when we put energy
来看看当我们把能量加到自旋体
into the spins or protons
或者质子中时会发生什么
assuming our protons are sitting in a homogeneous
假设所有质子是同质的
1 Testla field
在1T的磁场中
the Larmor equation states that the precession rate
拉莫尔方程呈现的这些自旋体的
of these spins is 42.58 megahertz
进动率是42.58兆赫
if we transmit a radio frequency pulse of exactly
如果我们以42.58兆赫
42.58 megahertz in the vicinity of the protons
传输射频脉冲在质子附近
two things happen
会发生两件事
To make this a little easier to see
为了便于看明白
we're going to stop the precession
我们要把质子的
of the protons for a moment
进动暂停一会儿
First, the protons will absorb that energy and flip
首先,质子会在高能态化的自旋下
the spins into the higher energy state
吸收能量后进行翻转
If we put in enough energy to push 50 percent
如果我们投入足够的能量来
of the proton population into the high state,
把质子总和的50%推向高能态
in our case 4 up and 4 down
就我们的例子而言,就是上推和下推
you can see our longitudinal magnetization reduces to 0
你会看到它的纵磁化量减小到了0
as the opposing magnetic forces cancel each other out
随着相对的磁力互相抵消掉
In addition, the sinusoidal radio frequency pushes
此外,正弦无线电频率自一致起
the protons to synchronize and spin together
把质子推向了同步状态
This is the Resonance portion of an NMR
这是NMR的“共振”部分
if we add up all the magnetic moment
如果我们加上所有的磁距
you can see that we now have a net magnetic force
你会看到我们能得到一个净磁力
oriented horizontally or 90 degrees
以水平或者90度角
to the longitudinal magnetization
相对于纵磁化
this is called the Transverse Magnetization
这叫“横磁化”
and this magnetization can be detected
这种磁化能够被
with a coil or antenna.
一种天线检测到
Just as a current can create a magnet
正如电流可以产生磁场
a magnet can create a current
磁场也可以产生电流
If we have a coil of wire
如果我们有一个线圈
connected to an ammeter or current meter
和一个安培计或者电流表相连
and we place a magnet through the coil
然后我们把一个磁体穿过线圈
we will generate an electrical current through the wire
我就能通过导线产生一个电流
when we pull the magnet back
当我们把磁体拉回来
the current flows in the opposite direction
电流就会反方向流动
if we spin the magnet we generate a sinusoidal or
如果我们旋转磁体,就会产生来回摆动
alternating electrical current- the basis of a generator
是发电机的交流电基础
Similarly, the transverse magnetization rotates around
类似地,横磁化不停转动
as the protons precess and generates a small
随着质子的进动,并且在区域
but measurable current in a regional coil of wire
线圈中产生出很小但可测到的电流
This is the result we're looking for in nuclear
这就是我们在磁共振中
magnetic resonance but it's still not the whole story
得到的结果   但还不是全部
After we remove the radio frequency signal
在我们去掉频射信号之后
the protons will relax back into their baseline position
质子会归于基线位置
Again for demonstration purposes
同样为了说明的目的
we'll temporarily stop the precession
我们会暂停活动
the first thing that happens is the protons or spins,
首先会发生的是，质子是自旋体
being all positively charged,
带正电荷
will repel each other and move apart
会互相排斥并分离
as they spread apart we lose that transverse magnetization
随着他们的四散分离，就会失去横磁化
This process is called the T2 or Spin-Spin Relaxation
这个过程叫做T2，或者“自旋驰豫”
because it has to do with the interaction
因为这和相互作用有关
of the protons or spins themselves
所以质子是自旋的
No net energy transfer occurs with this relaxation
这种驰豫并没有发生净能量转移
The other relaxation occurs as the high-energy protons
另一种驰豫发生于高能质子
fall back into the low energy state
回落到低能态时
As this happens, the energy that was previously absorbed
这时候，之前被质子吸收的能量
by the protons is dissipated into the surrounding tissues
消散到周围的组织里
in the form of heat and thus
以热量的形式
involves an actual transfer of energy
实现了真正的能量转移
As these protons fall back down to baseline,
随着这些质子回到基线
we regrow the longitudinal magnetization
我们再次形成纵磁化
This is referred to as the T1 or Spin-Lattice Relaxation
这被称为T1,或者"自旋-点阵弛豫"
because it involves the transfer energy from the spins
因为包含了自旋体向周围组织
to the surrounding tissues or lattice
或者点阵的能量转移
Putting this all together
把这些总和起来
a sufficient radio frequency pulse
充足的频射脉冲
tuned to the natural precession frequency
调整为进动质子的
of the precessing protons
自然进动频率
is put into the tissues to flip 50 percent of the spins
就是把50%自旋体的翻转到组织里
into the high-energy state and cause the protons
成为高能态,并且使得质子
to synchronize in phase or spin together
在相位上同步,或者同步旋转
thus moving the longitudinal magnetization
因此移动纵磁化
90 derees into the transverse plane
90度到横磁化
The transverse magnetization, precessing
横磁化在本地质子的
at the resonant frequency of the local protons
共振频率下进动
produces a radio signal of the same frequency
产生了一种同样频率的无线电信号
that can be detected by a coil of wire
能够被线圈检测到
As the energy is removed,  the protons first move apart
随着能量的移去，质子第一次
in a T2 or Spin-Spin Relaxation
在一个T2驰豫或者自旋驰豫中相互分开
destroying the transverse magnetization
破坏了横磁化
And then, through T1 or spin lattice relaxation,
然后通过T1驰豫或者“自旋-点阵弛豫”
fall back into the lower energy state
落回到低能态
dissipating the previously absorbed energy
把之前吸收的能量
into the surrounding tissues in the form of heat
以热量的形式扩散到周围的组织
while regrowing or restoring 
在再次形成或者恢复
the original longitudinal magnetization
最初的纵磁化的时候
Because these protons in our bodies
因为这些我们身体里的质子
have different local environment,
有着不同的所属环境
some associate with the free-flowing water molecules
有些与与自由流动水分子有关
while others are fixed in position associated with the structural
而其它的与结构有关保持在原地
or energy storing molecules of protein and fat,
或者能量强分子一个蛋白质和脂肪
they have characteristic differences in their T1 and T2 relaxations
他们的T1和T2驰豫有着不同的特点
We can accentuate and measure these differences
我们可以通过强调和测量这些不同
by changing how quickly we put
通过改变加入不同的
in the radio frequency energy
频射能量
or the Repetition Time designated TR
或者重复时间（表示为TR）
and how quickly we choose to listen to the signal
以及我们选择接收
coming back from the transverse magnetization
来自横磁化信号有多快
or Echo Time designated TE of the precessing protons
或者用进动质子的"回声时间"(TE)来表示
This process is referred to as the Pulse Sequence
这个过程被称为脉冲序列
and we will use a sample of fat and water
我们用水中的脂肪作为例子
to demonstrate these differences
来展示这些不同
As before to simplify and maximize comprehension
和之前一样, 为了简化和便于解释
we're going to stop the precession of the protons
我们会暂停质子的进动
On the left is a group of spins or protons
左边是一群质子自旋体
associated with fat and on the right, water
和脂肪相似, 而右边
When we put in our resonant radio frequency Pulse,
当我们加入磁共振射频脉冲
all the protons absorbed that energy,
所有质子吸收了能量
flip into the high-energy state
翻转成高能态
and spin together to produce a 90 degree Pulse
并且一起自旋,形成一个90度脉冲
or transverse magnetization
或者横磁化
If we wait a sufficient amount of time,
如果我们等待足够的时间
the protons will move apart in a T2 or Spin-Spin Relaxation
质子将在T2驰豫或者自旋驰豫中分开
and the transverse magnetization will decay
于是纵磁化就衰退了
The protons associate with the free fatty acids,
质子就像自由脂肪酸
being relatively fixed in position, decay rapidly
在原地相对快速的衰退
as the spins push away from one another
随着自旋体互相排斥
They also give up their absorbed energy
它们还更快地放掉了吸收的能量
more rapidly as they fall back to base line
随着退回到了基线
in a T1 or spin-lattice relaxation,
在T1驰豫或者自旋-点阵驰豫中
depositing heat energy into the surrounding tissues
存储的热能散入周围组织
and regrowing the longitudinal magnetization
重新形成纵磁化
on the other hand
另一方面
the protons in the freely flowing water
在自由流动水中的质子
can hold on to their energy and
会保持其能量
continue to spin together in phase
继续同相地一起自旋
maintaining the transverse magnetization
就是保持了横磁化
At this point, when we turn on our receiving coil
此时当我们打开接受器
and measure the signal coming back from the protons
测量来自质子的信号反馈
the relatively large transverse magnetization in water
相对很大的横磁化和水
will give a strong signal   while the smaller or
会显示很强的信号  而更小的
absent transverse magnetization in fat
或者抽象的类似脂肪的横磁化
will give a weak signal
就会显示很弱的信号
by convention, the strong water signal
按照惯例，强烈的水信号
will be assigned a grayscale color of white
会指定为白色的灰度颜色
and the weak signal of fat will be dark gray or black
脂肪的弱信号会表示为深灰或者黑色
Hence, to accentuate the different T2 relaxations
因此为了强调我们体内的
of the protons in our bodies
质子的T2驰豫
we would wait a long period between radio pulses
我们会在频射脉冲之间等很长一段时间
referred to as a long repetition time TR
叫做长重复时间TR
and wait a long time to listen for the return signal or
并在接受返回信号或者回声时等很长一段时间
echo referred to as a long echo time TE
叫做长回声时间TE
And these differences can be measured and recorded
这些区别都可以被测量和记录
Using the same tissues in their baseline state
用基线状态下的相同组织
to accentuate the differences in T1 relaxations
来强调T1驰豫之间的区别
we again put in a 90-degree resonance radio frequency pulse
我们又一次加入了90度磁共振频射脉冲
which flips the protons into the high-energy state
这种脉冲把质子翻转到高能态
and pushes them in phase
并把它们推向同相
to produce our transverse magnetization
从而产生横磁化
T2 relaxation occurs as the protons move apart-
T2驰豫发生于质子分开的时候
faster in fat then water
比水中的脂肪要快
and then the protons fall back into the low energy state
然后质子落回低能态
dissipating the absorbed energy as heat
以热能的形式消散吸收的能量
into the surrounding tissues
到周围的组织中
and regrowing the longitudinal magnetization
重新形成了纵磁化
Again, because the protons in water
同样因为水里质子
are fluid and move freely
是液体能自由移动
they tend to hold on to that energy longer
它们倾向于更长时间持有能量
and the protons stay in the higher energy state
而质子保持在高能态
with little regrowth of the longitudinal magnetization
很少重构纵磁化
whereas the tightly bound fat protons
然而紧紧合并的脂肪质子
more rapidly give up that energy
更快地释放能量
and return to the lower energy state
作为对低能态的反馈
rapidly regrowing the longitudinal magnetization
做出纵磁化的快速重构
If we then quickly put in another resonant
如果我们快速加入另一个
90-degree radio frequency pulse,
90度角的共振频射脉冲
the fully recovered fat protons will produce a large
完全恢复了的脂肪质子会产生一个大的
transverse magnetization and a strong measured signal
纵磁化和一个强可测信号
that we can record if we listen to the return signal
如果我们接收返回信号，我们就能记录下来
or echo shortly after the second radio frequency pulse
或者在第二次频射脉冲之后很快出现的回声
However
然而
the water protons are still in a high-energy state
水质子依然是高能态
with little regrowth of the longitudinal magnetization
很少重构纵磁化
Therefore the new radio frequency pulse pushes more of
因此新的频射脉冲翻转出更多的
the low energy protons into the high-energy state
低能质子为高能态
and can only produce a small transverse magnetization
只能产生出很小的横磁化
as well as a net longitudinal magnetization
加上指向180度的
oriented 180 degrees
净纵磁化
or directly opposite the main magnetic field
或者直接相反于主磁场
This configuration in the water protons
这种在水质子中的情形
will give a low amplitude or low energy way
会得到一种低振幅或者低能量的方式
when we listen for the echo or return signal
当我们接收回声或者返回信号时
in other words
换句话说
the protons in the water are saturated with energy and
水中的质子被能量充满
can no longer produce a strong transverse magnetization
再也无法产生一个强横磁化
therefore to accentuate the differences
因此为了强调T1驰豫中
in the T1 relaxation between the protons in fat and water
脂肪和水的质子之间的区别
we want to rapidly put in our
我们想快速加入
resonance radio frequency pulses
磁共振频射脉冲
or a short repetition time TR
或者短重复时间TR
and quickly listen for the return signal
然后快速接收返回信号
or short echo time TE
或者短回声时间TE
Now this is a difficult concept and you
这是一个难懂的概念
may have to listen to it a few times to get it to sink in
你可能不得不听多次才能理解到位
but to summarize, the T1 relaxation effects
但是总而言之，T1驰豫效应
are accentuated by rapidly exposing the protons
就是由快速暴露的质子
to radiofrequency energy
强化为高频能量
and keeping the spins in the high energy state
并在高能态持续自旋
and thus reducing the effective longitudinal magnetization
从而减少了实际纵磁化
tissues that recover quickly will have a large signal
快速恢复的组织会有一个强信号
although slow to recover will have a low signal
而缓慢恢复的组织会有一个弱信号
T2 effects on the other hand are accentuated
另一方面,T2效应是由
by a prolonged echo time to allow the spins
延长的回声时间从而允许自旋体
to move away from each other
互相分离
and accentuate the differences in T2 relaxation
并且强调了T2驰豫
between regional tissues or chemicals
在不同区域的组织或化学物质的区别
so a T2-weighted image
所以T2图像
is obtained with a pulse sequence of a long TR
是由一个长TR和一个长TE的脉冲序列得到的
and a long TE decreasing the T1 affect
随着TR而减少了T1效应
along TR and enhancing the T2 affect with along TE
并随着TE而升高了T2效应
a T1-weighted image on the other hand
另一方面，一个T1图像
is obtained with a pulse sequence of a short TR
由一个短TR的脉冲序列得到
in a short TE enhancing the T1 relaxation
在短TE中升高T1驰豫
and minimizing the T2 affects
并最小化了T2效应
in between these two is the proton density Pulse sequence
在这两者之间是质子密度脉冲序列
which is obtained with a long TR and a short TE
由长TR和短TE得到
effectively minimizing the T1 and T2 affect
有效地最小化了T1和T2效应
and basically giving us an idea of the absolute number
基本上给我们一个绝对值
or density of protons in the region
或者区域内一个质子的密度
the first to propose the use of NMR
首先提出使用NMR
to diagnose disease in humans was Dr. Raymond Damadian
来诊断人类疾病的是Raymond Damadian医生
who in the March 1971 addition of the journal Science
他在1971年3月的科学杂志里
published a short article entitled
发表了一篇简要文章
Tumor Detection by Nuclear Magnetic Resonance
使用磁共振来检测肿瘤
in what he suggested that the NMR signal
他提出来自体内肿瘤的
from tumors in the body
NMR信号
could be differentiated from normal tissues
能够和正常组织区分开
and therefore could be detected
因此能够被检测
his original concept was not to make an anatomic picture
他本来的想法不是构造一个解剖学图像
but to build a detector large enough to accommodate a human
而是建造一个足够大的适用于人类的探测器
perform a whole body NMR
做全身的NMR
and look for a characteristic T1 and T2 tumor signal
寻求特定的T1和T2肿瘤信号
that would suggest you have a malignancy somewhere
它会告诉你你有一个恶心肿瘤
in your body sort of a quick screening tool for malignancy
在身体的某个部位,有点像恶性肿瘤的快速筛选工具
while Dr.Damadian's tumor detector never came to fruition
尽管Damadian医生的肿瘤始终没能实现
we do use MR Spectroscopy in conjunction with a standard
我们确实使用MR光谱学来连接标准
MR image to help distinguish benign
MR图像，有助于从体内的恶性肿瘤
from malignant processes in the body
中甄别出良性肿瘤
to make an NMR picture here
为了构建一个NMR图像
MRI as we know it today we have to be able to localize
正如我们如今所知的MRI，我们必须能够
the signals coming from the sample or tissues in 3D space
通过3D空间的样本或者组织，定位信号
the first to propose a technique to do just that
最先提出这种技术的
was Dr. Paul Lauterbur
是Paul Lauterbur医生
in the March 19, 1973 addition of nature
在1973年3月19日的自然杂志
Lauterbur published a brief article entitled
史无前例地发表了一个简要文章
Image formation by induced local interactions
通过诱发本地相互作用来进行成像
examples employing nuclear magnetic resonance
示例使用核磁共振
In the article he describes a technique
文章中他描述了一种技术
using magnetic radiance to identify the location of
使用磁辐射来区分充满水的
two 1-millimeter capillary tubes filled with water
两个1毫米的毛细管
both of which were submerged in a larger
两者毛细管浸在一个
4.2 millimeter tube filled with heavy water
4.2毫米的充满重水的大管内
Heavy water has the symbol D2O
重水表示为D2O
because the hydrogen atoms in heavy water
因为氢原子和重水
are isotopes of the standard Protium configuration
是标准“氕”结构的同位素
with one proton in the nucleus
在原子核里有一个质子
the deuterium atom on the other hand contains a neutron
另一方面“氘”原子包含一个中子
as well as the proton which limits the spin of the nucleus
并且质子限制了原子核的自旋
and therefore does not give a good NMR signal
因此得不到好的NMR信号
Lauterbur placed the apparatus in high field NMR Magnet shown
强磁场NMR磁体中的许多设备
and developed a technique to identify the actual orgin
并发展成为真正的把NMR信号
of the NMR signals from the standard water
从标准水中区别开的一种检测技术
this is the image produced
这是生成的图像
very primitive by today's standards
以现在的标准来看非常粗糙
but clearly shows the location of the two capillary
但是清晰地显示出两个毛细管
tubes of  substandard water
在标准水中位置
lot ever called his technique Zeugmatography
把他的这种技术称为Zeugma成像
from the Greek Zeugma which means
来自希腊Zeuma，意思是
"that which is used for joining"
“用来连接的”
to show you how we make an actual MR picture
为了给你演示我们是如何做出真正的MR图像的
we're going to move our perspective of our spins
我们把自旋体的透视图
from the side to the top
从侧面移到顶部
and represent each grouping simply by
呈现出每个组 仅仅通过
the net magnetic field rotating around the axis
绕着轴旋转的净磁场
remember that the Larmor or resonant frequency of our spins
记住自旋体的拉莫尔或者共振频率
is determined by the strength of the local magnetic field
是由所属磁场的强度决定的
when we first get into the MRI machine
当我们第一次进入MRI机器时
a superconducting magnet creates
一个超导磁体产生出
a near homogeneous magnetic field
一个邻近的同质磁场
from one side to the other
从一头到另一头
that determines the strength of MRI machine
决定着MRI机器的强度
common systems are 1
常规系统是1、
1.5 and 3 Tesla in strength
1.5及3 T的强度
there are 3 sets of gradient magnets in the MRI
MRI中有3套梯度磁体
used to localize signals in 3D space
用于3D空间的定位
the Z-axis
Z轴
X-axis, and Y-axis
X轴,以及Y轴
to select a particular slice of tissue in the body
为了选出体内特定的一层组织
we can turn on the set of electromagnets along the z-axis
我们可以沿着Z轴打开电磁梯度
that create a magnetic gradient from head to toe
从而从头部到脚部形成一个磁梯度
we now put in a radio pulse
现在我们放入一定频率的频射脉冲
with the frequency that will cause
这种频率能使待观察部位
the desired area to resonate as described earlier
发生如前文所述的共振
we have now selected our slice to the body
现在我们选择了身体的一个截面
because the local magnetic gradient is homogeneous
因为所在磁场的梯度是同质的
all of these net magnetic moment in a slice
一个截面里的所有净磁距
are in phase spinning together in sync
都是同相的，在一起同步自旋
and can't be distinguished from one another
无法互相区分出来
to further localised these magnetic moments
为了通过其特有的强度
in their signal strength we have two more gradients
进一步定位这些磁距，我们有另外两个梯度
that we can use to isolate the source of the signals
可以用来区分信号来源
the first gradient is called the phase encoding gradient
第一个梯度叫做相位编码梯度
to demonstrate the effect
为了展示出效应
we're going to slow down our net magnetic moments
我们会减慢净磁距
the phase encoding gradient is briefly turned on
把相位编码梯度打开
creating a gradient along the Y-axis in this particular case
在此特定情形下沿着Y轴建立一个梯度
resulting in the magnetic moments
使得梯度底部的
at the bottom of the gradient to slow down
磁距减慢
and the ones at the top
而磁场顶部的
where the local magnetic field is stronger to speed up
用来使磁距加快
the gradiient is quickly turned off
磁距很快关闭
and the spinning magnets return to the base frequency
自旋磁体回到基线频率
spinning at the same rate
以相同速率自旋
but they have now experienced a phase shift in the Y-axis
但它们现在Y轴上进行着相移
which we can use to localize the spins in the Y-direction
这就使我们可以在Y方向上定位自旋体
we then tune our system to focus on a particular phase in the matrix
然后我们调制系统，聚焦在矩阵的特定相位上
and use our third gradient to the X-direction
然后使用在X方向上的第三个梯度
to definitively localized each of the signals
从而准确地把每一个信号定位
in the selected row
在选定行里
this gradient again causes the spins
这种梯度同样使
to the right to slow down
向右的自旋体减速
and the ones on the left to speed up
使向左的自旋体加速
this frequency encoding gradient remains on
这种频率编码梯度
while the signals are recorded
在记录信号的时候一直开着
now each of the signals has a unique phase  and frequency
现在每一个信号都有一个特定的相位和频率
which can be localized in 3D space
能够在3D空间里进行定位
the whole process is repeated for each row
整个过程在每一行上重复着
localizing, in this example
在本例中的定位
in the Y-direction with the phase encoding gradient
用相位编码梯度定位于Y方向
and in the X-direction with the frequency encoding gradient
用频率编码梯度定位于X方向
until the entire matrix is complete
直到整个矩阵完成
each of the squares or voxels
每个正方形或体素
are assigned a grayscale value
都指定为一个灰度值
corresponding to the strength of the local signal
对应于其信号强度
by convention, white being a strong signal
根据惯例，白色是强信号
and black being no signal at all
黑色是完全无信号
in this simplified example
在这个简化示例中
our 4 by 4 matrix doesn't look like much
是不太好看的4*4矩阵
but a standard MRI with a 256 by 256 or 512 by 512 matrix
而256*256或者512*512的矩阵的标准MRI
will provide exquisite in anatomic detail of the body
能提供原子细节的精确信息
for Giuliana
送给朱莉安娜
Good Luck with Science Fair
祝你科研展览会好运
字幕\时间轴\翻译：核儿
