Ultrasound & Medical Imaging
A-Level Physics Topic 24 17:14 English narration · English + 中文 subtitles burned in
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The very first photograph of your life was probably taken with sound.
你这一生的第一张照片，很可能是用声音拍下的。
Before you were born, a doctor pressed a small probe to your mother's skin, and out of the darkness an image formed: a tiny face, a beating heart.
在你出生之前，医生把一个小探头贴在你母亲的皮肤上， 于是从一片黑暗中，一幅图像浮现出来：一张小小的脸，一颗跳动的心。
No cutting, no radiation, no harm.
没有切开，没有辐射， 没有伤害。
Just pulses of sound, far too high for any ear to hear, bouncing off you and coming back as echoes.
只是一串串声音的脉冲，高得任何耳朵都听不见，从你身上反弹回来，变成回声。
This is ultrasound — and it is only the beginning of how physics lets us see inside the body.
这就是超声波——而它，只是物理让我们看进身体内部的开始。
Physics lets us look inside the human body — with sound, with X-rays, and with antimatter.
物理让我们看进人体内部——用声音、用 X 射线，还用反物质。
Today: ultrasound and the piezo-electric effect, pulse-echo imaging, X-rays, and the PET scan.
今天：超声波与压电效应、 脉冲回波成像、X 射线，以及 PET 扫描。
Let's begin.
让我们开始吧。
It all starts with a special crystal — a piezo-electric crystal.
这一切始于一种特别的晶体——压电晶体。
Squeeze it, and an electromotive force appears across it.
挤压它，它两端就出现一个电压。
And the reverse: apply a p.d. and it changes shape.
反过来也一样： 加上一个电压，它就改变形状。
Feed it a rapidly alternating voltage, and it vibrates millions of times a second, pushing out ultrasound — sound far above human hearing.
给它一个快速交变的电压，它每秒就振动上百万次， 推出超声波——远高于人耳所能听到的声音。
The very same crystal then listens: returning echoes squeeze it, and it turns them back into a voltage.
同一块晶体接着又来倾听：返回的回声挤压它， 它就把回声变回一个电压。
One transducer, both a speaker and a microphone.
一个换能器，既是扬声器，又是麦克风。
Production, then. Here is what is actually inside the probe.
这就是探头内部真正的样子。
At its heart is the piezo-electric crystal with an electrode on each face.
它的核心是一块压电晶体，两个面上各有一个电极。
An alternating voltage of a few megahertz makes it vibrate at that same frequency, sending longitudinal waves into the body — above twenty kilohertz is ultrasound, and medical imaging uses one to ten megahertz.
几兆赫的交变电压让它以同样的频率振动，把纵波送进人体——高于二十千赫就是超声波， 而医学成像用的是一到十兆赫。
Behind the crystal sits backing material, which damps the ringing so each pulse stops quickly and the echoes stay separate.
晶体后面是背衬材料，用来抑制余振， 让每个脉冲迅速停下，回波之间才不会混在一起。
In front are a plastic cover and a lens, and the whole thing lives in an earthed metal case.
前面是塑料外罩和透镜， 整个结构装在接地的金属外壳里。
The clever part is that the same crystal does both jobs, switching between sending a pulse and listening for its echo.
巧妙之处在于，同一块晶体身兼两职， 在发射脉冲和聆听回波之间来回切换。
Here is how we build an image.
我们就是这样建立起一幅图像的。
The transducer sends a short pulse into the body.
换能器把一个短脉冲送进身体。
At each boundary between tissues, part of the pulse bounces straight back.
在每一个组织交界处， 一部分脉冲径直弹回来。
The transducer times how long the echo takes to return.
换能器测量回声返回所用的时间。
Since the pulse travels there and back, the depth of that boundary is the speed of sound, times the time, divided by two.
由于脉冲要走一个来回， 那个交界的深度，就等于声速乘以时间，再除以二。
Sweep the probe across the body, collect thousands of echoes, and a picture takes shape.
让探头扫过身体，收集成千上万个回声， 一幅画面就成形了。
Pulse-echo imaging is five steps.
脉冲回波成像分五步。
One: the transducer sends a short pulse into the body.
第一，换能器把一个短脉冲送入人体。
Two: at every boundary between tissues, part is reflected and part carries on deeper.
第二，在每一个组织交界面上， 一部分被反射，另一部分继续往深处走。
Three: the same transducer detects each returning echo.
第三，同一个换能器接收每一个返回的回波。
Four: the time delay gives the depth, and here is the step everyone drops a mark on — the pulse travels there and back, so the distance is d equals c t over two, with that factor of two.
第四，时间延迟给出深度，而这一步正是大家最常丢分的地方——脉冲是一去一回的， 所以距离是 d 等于 c t 除以二，别忘了那个二。
The echo's amplitude tells you how strong the reflection was.
回波的振幅则告诉你反射有多强。
Five: sweeping the probe across the body assembles all those depth readings into a two-dimensional image.
第五，把探头在身体表面扫过去，就把这些深度读数拼成一幅二维图像。
Let us do one.
我们做一道。
An ultrasound pulse returns sixty microseconds after it was sent, and the speed of sound in the tissue is fifteen hundred metres per second.
一个超声脉冲在发出后六十微秒返回，而组织中的声速是一千五百米每秒。
How deep is the boundary that reflected it?
反射它的交界面有多深？
Use d equals c t over two, and do not lose the factor of two — the sixty microseconds covers the trip out and the trip back.
用 d 等于 c t 除以二，别把那个二丢了—— 这六十微秒覆盖的是去程加回程。
Put the numbers in, remembering that a microsecond is ten to the minus six seconds.
把数字代进去，记住一微秒是十的负六次方秒。
That gives zero point zero four five metres, or four point five centimetres.
结果是零点零四五米，也就是四点五厘米。
If you had forgotten to halve it, you would have said nine centimetres and lost the mark.
如果忘了除以二，你会答成九厘米，那一分就没了。
This is what the raw signal looks like on an oscilloscope, and it is called an A-scan.
这是原始信号在示波器上的样子，称为 A 型扫描。
The tall spike on the left is the transmitted pulse itself.
左边那个高高的尖峰就是发射脉冲本身。
After it come smaller pulses: the echo from the fat–muscle boundary, then later the echo from the muscle–bone boundary.
在它之后是几个较小的脉冲：先是脂肪与肌肉交界面的回波，稍晚是肌肉与骨骼交界面的回波。
Two things are readable straight off the trace.
从这条曲线上可以直接读出两件事。
The horizontal position of each echo is its time delay, which you convert to depth; and the height of each echo is how much was reflected there.
每个回波的水平位置就是它的时间延迟，换算过去就是深度； 而每个回波的高度就是那里反射了多少。
A later echo is deeper, and a shorter echo is a weaker boundary.
回波越晚，位置越深；回波越矮，交界面越弱。
Sweep that measurement across the body and you get this.
把这套测量在身体上扫过去，就得到这样一幅图。
The fan shape comes from the transducer sweeping through a range of angles.
扇形是因为换能器在一系列角度上来回扫描。
Every bright speck in the picture is an echo from a boundary between two tissues, and its position on the screen was worked out from that echo's time delay — exactly the calculation we just did, repeated millions of times a second.
画面里每一个亮点，都是两种组织交界面上的一个回波，而它在屏幕上的位置， 正是由那个回波的时间延迟算出来的——就是我们刚才做的那道计算，每秒重复上百万次。
One practical detail makes it all possible: a coupling gel between the probe and the skin pushes the air out.
有一个实际细节让这一切成为可能：探头与皮肤之间要涂耦合剂，把空气挤出去。
Without it, almost all the ultrasound would reflect straight back at the skin–air boundary and never get inside at all.
没有它，几乎全部超声波都会在皮肤与空气的交界面上直接反射回来，根本进不了身体。
But what makes an echo?
可是，是什么产生了回声呢？
Every material has an acoustic impedance — its density, times its speed of sound.
每一种材料都有一个声阻抗——它的密度，乘以它的声速。
When a pulse meets a boundary where the impedance changes, part of it reflects.
当脉冲遇到一个阻抗发生变化的交界时，一部分就反射回来。
A big mismatch, like skin to air, reflects almost everything — which is why a gel is smeared on the skin, to push the air out and let the sound in.
若阻抗相差很大，比如皮肤到空气， 几乎全部都会反射——这正是为什么要在皮肤上抹一层凝胶，把空气挤出去，让声音进得来。
And as ultrasound travels deeper, it fades away, its intensity dropping exponentially.
而当超声波越走越深，它就渐渐衰减，强度按指数下降。
Why does a boundary reflect at all?
交界面为什么会反射？
Because the two materials differ in one property: their specific acoustic impedance, Z.
因为两种材料在一个性质上不同：它们的声阻抗 Z。
It is simply density times the speed of sound, Z equals rho c.
它就是密度乘以声速，Z 等于 rho c。
Its unit follows from that product: kilograms per metre squared per second.
单位也就从这个乘积得出：千克每平方米每秒。
The numbers are worth a feel.
这些数值值得建立一点直觉。
Bone has a large Z, because it is both dense and fast.
骨骼的 Z 很大，因为它既致密、声速又快。
Air has a tiny Z, because it is neither.
空气的 Z 极小， 因为两样都不占。
Soft tissue sits in between, and all soft tissues sit close to one another, which will turn out to matter enormously.
软组织介于两者之间，而且各种软组织彼此非常接近—— 这一点后面会变得极其重要。
A quick one.
来一道快的。
Find the specific acoustic impedance of soft tissue, given a density of one thousand and sixty kilograms per cubic metre and a speed of sound of one thousand five hundred and forty metres per second.
已知软组织的密度为一千零六十千克每立方米，声速为一千五百四十米每秒， 求它的声阻抗。
There is no trick here — multiply the two together.
这里没有什么花招——把两者相乘就行。
That gives one point six times ten to the sixth kilograms per metre squared per second.
得到一点六乘以十的六次方 千克每平方米每秒。
Keep that number in mind, because the whole art of ultrasound imaging is about how it compares with the impedance on the other side of a boundary.
把这个数记住，因为超声成像的全部技巧， 就在于它与交界面另一侧的声阻抗相比是大是小。
Now we can say exactly how much reflects.
现在我们可以准确说出反射了多少。
The intensity reflection coefficient is the fraction of the intensity that comes back, and it is Z one minus Z two, over Z one plus Z two, all squared.
强度反射系数就是返回的强度所占的比例， 它等于 Z 一减 Z 二，除以 Z 一加 Z 二，再取平方。
Look at what that expression does at the two extremes.
看看这个式子在两个极端会怎样。
With very different impedances, the top is nearly as big as the bottom, the fraction approaches one, and almost everything reflects — that is skin against air, and it is why we need the gel.
当两个声阻抗差别很大时，分子几乎和分母一样大，比值趋近于一，于是几乎全部被反射—— 这正是皮肤与空气的情形，也正是我们需要耦合剂的原因。
With very similar impedances the top is nearly zero, so almost nothing reflects and you cannot see the boundary at all.
当两个声阻抗非常接近时， 分子几乎为零，于是几乎不反射，你根本看不见这个交界面。
Good imaging needs the middle ground: different enough to give an echo, but alike enough that plenty of the pulse carries on to the structures deeper down.
好的成像需要中间地带： 差别足够大以产生回波，又足够接近，让相当一部分脉冲能继续深入到更里面的结构。
Even the part that carries on does not carry on unchanged.
即使是继续前进的那一部分，也不是原封不动地前进。
As ultrasound travels through tissue, its intensity falls with distance, and it falls exponentially.
超声波在组织中传播时， 强度会随距离下降，而且是指数下降。
The law is I equals I nought e to the minus mu x, where x is the distance travelled and mu is the attenuation coefficient, measured in inverse metres.
规律是 I 等于 I 零乘以 e 的负 mu x 次方， 其中 x 是传播的距离，mu 是衰减系数，单位是每米。
A large mu means the beam dies away quickly — bone attenuates far more strongly than soft tissue does.
mu 越大， 光束衰减得越快——骨骼的衰减比软组织强得多。
Remember this equation carefully, because the very same law describes X-rays passing through the body, and the exam will use it in both halves of this topic.
请把这个方程记牢， 因为完全相同的规律也描述 X 射线穿过人体的过程，考试会在本章的两半里都用到它。
For denser things, like bone, we reach for X-rays.
要看更致密的东西，比如骨头，我们就用上 X 射线。
In an X-ray tube, electrons are boiled off a hot wire, accelerated across a vacuum, and slammed into a metal target — and the sudden stop turns some of their energy into X-ray photons.
在 X 射线管里，电子从一根热丝上被"煮"出来， 在真空中被加速，狠狠地撞进一块金属靶——这猛然的急停，把它们的一部分能量变成 X 射线光子。
These pass through the body, but dense bone absorbs far more than soft tissue, casting a shadow.
这些光子穿过身体，但致密的骨头比软组织吸收得多得多，投下一道阴影。
Their intensity, too, falls exponentially with depth.
它们的强度， 同样随深度按指数下降。
Spin the tube around the patient, and a computer reconstructs a full three-dimensional scan.
让 X 射线管绕着病人旋转，计算机就能重建出一幅完整的三维扫描。
X-rays are made in an X-ray tube, in three steps.
X 射线是在 X 射线管里产生的，分三步。
First, a heated cathode gives off electrons by thermionic emission — heat alone frees them from the metal.
第一，被加热的阴极通过热电子发射放出电子—— 仅靠热量就把它们从金属里释放出来。
Second, a high potential difference, tens to hundreds of kilovolts, accelerates them across a vacuum onto a metal target, the anode, usually tungsten.
第二，几十到几百千伏的高电势差， 让它们在真空中加速，射向金属靶，也就是阳极，通常是钨。
Third, they slam into the target and stop very suddenly.
第三，它们撞上靶并骤然停下。
Their kinetic energy has to go somewhere, and most of it simply becomes heat, which is why the target has to be cooled.
它们的动能总得有个去处，其中大部分直接变成了热，这正是靶必须冷却的原因。
A small fraction leaves as X-ray photons — this is Bremsstrahlung, or braking radiation.
只有一小部分以 X 光子的形式射出——这就是轫致辐射，也叫制动辐射。
And some electrons knock out an inner electron of a target atom; when an outer electron drops in to refill that gap, it emits a photon of one exact energy, giving the characteristic lines.
还有一些电子会把靶原子的内层电子打出来；当外层电子落下来填补这个空位时， 就会发出一个能量确定的光子，形成特征谱线。
Here is the arrangement.
这就是它的结构。
On the left, a low voltage heats the filament, which is the cathode.
左边，一个低电压把灯丝加热，那就是阴极。
Across the evacuated tube, a high voltage pulls the electrons to the anode on the right.
在抽成真空的管子两端，一个高电压把电子拉向右边的阳极。
Notice that the tungsten target is set at an angle.
请注意钨靶是斜放的。
That is deliberate: it turns the beam sideways, out through the window, and it spreads the heat over a larger patch of metal than a face-on target would.
这是有意为之：它把射线束转向侧面，从窗口射出，而且比正对着放的靶 把热量摊在了更大的一片金属上。
There is a hard limit on how energetic these photons can be, and it makes a nice exam question.
这些光子的能量有一个硬性上限，而它是一道很好的考题。
The most energy a single photon can have is the entire kinetic energy of one accelerated electron, handed over in a single collision.
单个光子能获得的最大能量， 就是一个被加速电子的全部动能，在一次碰撞中一次性交出。
That kinetic energy is e V.
那份动能是 e V。
So set h f max equals e V, and since f equals c over lambda, rearranging gives lambda min equals h c over e V.
于是令 h f max 等于 e V，又因为 f 等于 c 除以 lambda，整理后得到 lambda min 等于 h c 除以 e V。
Everything on the right is known, so the accelerating voltage alone fixes the shortest wavelength the tube can produce.
右边全是已知量，所以仅凭加速电压 就决定了这支管子能产生的最短波长。
On the spectrum this shows up as a sharp cut-off: nothing at all below lambda min, and raising the voltage pushes that edge further to the left.
在谱图上，它表现为一个陡峭的截止： 在 lambda min 以下什么都没有，而提高电压会把这条边界继续往左推。
Put those two processes on one graph and this is the result.
把这两个过程画在同一张图上，就得到这个结果。
The broad continuous hump is the Bremsstrahlung: electrons brake by different amounts, so they give photons of every energy up to the maximum.
宽阔连续的隆起是轫致辐射： 电子减速的程度各不相同，所以给出的光子从零到最大值各种能量都有。
It begins abruptly at the minimum wavelength on the left, rises to a broad peak, then tails away.
它在左边的最短波长处陡然开始，升到一个宽阔的峰，然后逐渐拖尾。
Standing on top of it are the sharp characteristic peaks.
叠在它上面的是尖锐的特征峰。
Those are the inner-shell transitions, so their positions depend only on the target metal — change the accelerating voltage and the cut-off moves, but the peaks stay exactly where they were.
那些来自内层电子的跃迁，所以位置只取决于靶金属—— 改变加速电压，截止边界会移动，而这些峰纹丝不动。
An X-ray image is not a photograph, it is a shadow picture.
X 光片不是照片，而是一张影子图。
The beam passes through the patient onto a detector, and what you see is how much got through at each point.
射线束穿过病人打到探测器上， 你看到的是每一点透过了多少。
Bone attenuates strongly, so little reaches the detector behind it and it appears light.
骨骼衰减很强，后面到达探测器的就很少， 于是显得亮。
Lung is mostly air, attenuates hardly at all, and appears dark.
肺里几乎全是空气，几乎不衰减，于是显得暗。
The contrast is the difference in attenuation between two neighbouring tissues — and that is the weakness of the method, because two soft tissues attenuate almost identically and are hard to tell apart.
对比度就是相邻两种组织衰减程度之差——而这正是这种方法的弱点， 因为两种软组织的衰减几乎一模一样，很难区分。
The fix is a contrast medium, such as a barium meal or an iodine injection, which absorbs strongly and makes a soft structure stand out sharply.
解决办法是造影剂， 比如钡餐或碘注射，它吸收很强，能让软组织结构清晰地凸显出来。
Let us use the attenuation law.
我们来用一下衰减定律。
X-rays pass through three centimetres of tissue whose attenuation coefficient is forty per metre.
X 射线穿过三厘米厚、衰减系数为每米四十的组织。
What fraction of the intensity gets through?
有多少比例的强度能穿过去？
The ratio I over I nought is e to the minus mu x, so first work out the exponent: forty times zero point zero three zero metres — convert the centimetres — gives one point two.
I 比 I 零等于 e 的负 mu x 次方，所以先算出指数： 四十乘以零点零三零米——记得把厘米换算掉——得到一点二。
So the answer is e to the minus one point two, which is about thirty per cent.
于是答案是 e 的负一点二次方，大约是百分之三十。
A useful companion idea is the half-value thickness: the thickness that halves the intensity, x one-half equals natural log two over mu.
还有一个很有用的配套概念是半值厚度： 使强度减半的厚度，x 二分之一等于 ln 2 除以 mu。
It works exactly like half-life in radioactive decay, and it is often quicker than the full exponential.
它的用法和放射性衰变里的半衰期 完全一样，而且常常比算完整的指数更快。
A single X-ray flattens the whole body onto one plane, so organs overlap.
一张普通 X 光片把整个身体压扁到一个平面上，器官因此互相重叠。
Computed tomography solves that in four steps.
计算机断层扫描用四步解决了这个问题。
One: the tube and the detector array rotate around the patient, recording one thin slice from many different angles.
第一，球管和探测器阵列绕着病人旋转， 从许多不同角度记录同一个薄层。
Two: a computer combines those views into a two-dimensional cross-section of that slice.
第二，计算机把这些视图合成为该层的二维横截面。
Three: the patient moves along a little and the next slice is imaged.
第三，病人沿轴向移动一小段，再拍下一层。
Four: the slices are stacked into a three-dimensional image.
第四，把各层叠起来，构成三维图像。
Because each point has been viewed from many directions, overlapping soft tissues are separated out — which is exactly what a plain X-ray cannot do.
由于每一点都被从许多方向看过，互相重叠的软组织就被分离开来—— 而这恰恰是普通 X 光片做不到的。
The most remarkable of all uses antimatter.
最了不起的一种，用的是反物质。
A patient is given a tracer — a substance that emits positrons — the antiparticle of the electron. This is PET scanning.
给病人注入一种示踪剂——一种会放出正电子的物质， 正电子是电子的反物质孪生兄弟。
Each positron travels a few millimetres, meets an electron, and the two annihilate — their mass vanishing into two gamma-ray photons, flying off in exactly opposite directions because energy and momentum are both conserved.
每个正电子走上几毫米，遇到一个电子，两者就湮灭—— 它们的质量化作两个伽马射线光子，朝着恰好相反的方向飞出去。
A ring of detectors records the two simultaneous arrivals — a coincidence — and that pair pins down the line they came from.
一圈探测器同时接住这两个光子， 这一对光子就锁定了它们所来自的那条直线。
Millions of these lines build a glowing map of where the body is most active.
千百万条这样的直线， 拼出一幅发光的图，显示身体里哪里最活跃。
PET works differently from everything so far: instead of shining something through the patient, we put the source inside.
PET 的原理和前面所有方法都不同：我们不是让什么东西穿过病人， 而是把放射源放进体内。
A tracer is a substance containing radioactive nuclei that is injected into the body.
示踪剂是一种含有放射性核素的物质，注射进人体。
It is chosen so that it is taken up more by the tissue you want to study — a tumour, for instance, has a high metabolism, so it absorbs far more glucose than the tissue around it.
挑选它的标准是能被你想研究的组织更多地摄取——比如肿瘤代谢旺盛， 吸收的葡萄糖远多于周围组织。
In positron emission tomography the tracer is a positron emitter, a beta plus emitter.
在正电子发射断层扫描中，示踪剂是正电子发射体， 也就是 beta 加发射体。
The standard one is fluorine-eighteen attached to a glucose analogue, known as FDG, which follows glucose around the body and gathers wherever cells are working hardest.
最常用的是接在葡萄糖类似物上的氟十八，简称 FDG， 它跟着葡萄糖在体内运行，聚集在细胞工作最卖力的地方。
The energy of those two photons is not a free parameter — it is fixed, and you can derive it.
这两个光子的能量不是可以随意取值的——它是确定的，而且你可以推出来。
A positron and an electron have the same rest mass, and when they annihilate, energy is conserved: all of that rest energy becomes photon energy.
正电子和电子的静止质量相同，当它们湮灭时，能量守恒：全部静止能都变成光子能量。
So two h f equals two m e c squared.
于是二 h f 等于二 m e c 平方。
The twos cancel, which means each photon carries m e c squared exactly.
两边的二约掉，也就是说每个光子恰好带走 m e c 平方。
Put the electron mass and the speed of light in and you get about eight point two times ten to the minus fourteen joules — that is five hundred and eleven kilo-electronvolts per photon, with a wavelength of about two point four times ten to the minus twelve metres.
把电子质量和光速代进去，得到约八点二乘以十的负十四次方焦耳—— 也就是每个光子五百一十一千电子伏特，波长约为二点四乘以十的负十二次方米。
Every PET scanner in the world is built to look for photons of that one energy.
世界上每一台 PET 扫描仪，都是为了寻找这一个能量的光子而造的。
Because the two photons leave back to back, a ring of detectors around the patient can find where they came from.
由于两个光子是背对背飞出的，围绕病人的一圈探测器就能找出它们来自哪里。
When two detectors on opposite sides fire at the same instant — a coincidence — the annihilation must have happened somewhere on the straight line joining them.
当相对两侧的两个探测器在同一瞬间被触发——称为一次符合—— 湮灭就一定发生在连接它们的那条直线上的某处。
One coincidence gives a line; millions of them, crossing at every angle, let the computer reconstruct a three-dimensional map of where the tracer collected.
一次符合给出一条线； 上百万次符合从各个角度交叉，就让计算机重建出示踪剂聚集位置的三维图。
Comparing the two arrival times more precisely narrows down the position along that line, which is time-of-flight PET.
更精确地比较两个到达时间，还能把位置沿那条线进一步缩小，这就是飞行时间 PET。
The result is this: an image not of anatomy, but of activity, with the warm colours marking the most metabolically active tissue.
结果就是这样一幅图：它显示的不是解剖结构，而是活动强度， 暖色标出的是代谢最活跃的组织。
Three marks to secure.
三个要拿稳的分。
First, ultrasound reflects at boundaries where the acoustic impedance changes, and a coupling gel removes the skin-air reflection.
第一，超声波在声阻抗发生变化的交界处反射，而耦合凝胶能消除皮肤与空气之间的反射。
Second, X-rays are attenuated exponentially — intensity equals the start, times e to the minus mu x.
第二，X 射线按指数衰减——强度等于初始值乘以 e 的负 μx 次方。
Third, PET uses a positron emitter: annihilation gives two gamma-rays in opposite directions, located by the detector ring.
第三，PET 用一种正电子发射体： 湮灭给出两个方向相反的伽马射线，由探测器环定位。
Master these, and medical physics is yours.
掌握这些，医学物理就是你的了。