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This syringe contains a radioactive form of glucose known as FDG.
这管注射器里含有放射性的葡萄糖，也就是氟代脱氧葡萄糖FDG。
The doctor will soon inject its contents into her patient's arm, whom she's testing for cancer using a PET scanner.
医生将马上会将其注入患者的胳膊中，再通过PET扫描仪来检测癌症。
The FDG will quickly circulate through his body.
FDG会迅速循环至全身血管中。
If he has a tumor, cancer cells within it will take up a significant portion of the FDG,
如果这位患者有肿瘤，那么其中的癌细胞将会摄取很多FDG，
which will act as a beacon for the scanner.
并在扫描仪上显示信号。
PET tracers such as FDG are among the most remarkable tools in medical diagnostics,
诸如FDG这样的显像剂是医学检测中最有效的工具之一，
and their life begins in a particle accelerator, just hours earlier.
FDG来源于粒子加速器，是在检查开始几小时前产生的。
The particle accelerator in question is called a cyclotron, and it's often housed in a bunker within hospitals.
这种粒子加速器，也被称为回旋加速器，往往被放置在医院的仓库里。
It uses electromagnetic fields to propel charged particles like protons faster and faster along a spiraling path.
它通过电磁场来加速带电粒子，例如质子，使其在螺旋轨道中运动得越来越快。
When the protons reach their maximum speed,
当质子达到最高速度时，
they shoot out onto a target that contains a few milliliters of a type of water with a heavy form of oxygen called oxygen-18.
就被喷射到含有几毫升特殊的水的目标物中，这种水中含有更重的氧原子，也被称为做氧-18原子。
When a proton slams into one of these heavier oxygen atoms, it kicks out another subatomic particle called a neutron.
当质子和这些氧原子猛烈撞击时，会激发出一种亚原子微粒，那就是中子。
This impact turns oxygen-18 into fluorine-18, a radioactive isotope that can be detected on a PET scan.
碰撞过程也让氧-18原子转变为氟-18原子，氟-18原子是一种能被PET扫描仪检测到的放射性同位素。
In a little under two hours, about half the fluorine will be gone due to radioactive decay,
在不到两个小时的时间里，放射性氟原子衰变到仅剩半数，
so the clock is ticking to get the scan done.
因此，整个扫描过程是有时限性的。
So how can fluorine-18 be used to detect diseases?
那么，如何用氟-18原子来检测疾病呢？
Radiochemists at the hospital can use a series of chemical reactions
医院里的放射化学家会利用一系列化学反应
to attach the radioactive fluorine to different molecules, creating radiotracers.
使放射性氟原子与不同分子相结合，形成放射性示踪剂。
The identity of the tracer depends on what doctors want to observe.
示踪剂的性质取决于医生想观察到什么。
FDG is a common one because the rate at which cells consume glucose can signal the presence of cancer;
FDG是常用的示踪剂之一，因为细胞消耗葡萄糖的速率可表明癌细胞的存在；
the location of an infection; or the slowing brain function of dementia.
感染处的位置；或者由痴呆导致的脑功能减退。
The FDG is now ready for the patient's scan.
现在FDG已经备好，患者检测准备就绪。
When a radiolabeled tracer enters the body,
当放射性示踪剂进入人体内后，
it travels through the circulatory system and gets taken up by its target
它会随着循环系统到达靶点，
whether that's a protein in the brain, cancer cells, or otherwise.
无论这靶点是脑部的蛋白，还是癌细胞或其他物质。
Within a few minutes, a significant amount of the tracer has found its way to the target area and the rest has cleared from circulation.
在数分钟之内，大量示踪剂就会到达靶点，而其余的示踪剂在循环过程中已被人体清除。
Now the doctors can see their target using a PET, or positron emission tomography, scanner.
现在医生可以通过PET扫描仪观察靶点，PET全称为正电子发射断层扫描仪。
The radiation that the tracer emits is what makes this possible.
这个过程依赖于示踪剂的放射性。
The isotopes used in PET decay by positron emission.
PET中的放射性同位素通过释放正电子衰变。
Positrons are essentially electrons with positive charge.
正电子本质上是带有正电的电子。
When emitted, a positron collides with an electron from another molecule in its surroundings.
当被发射时，正电子会与另一个来自周围其他分子的电子发生碰撞。
This causes a tiny nuclear reaction in which the mass of the two particles is converted into two high-energy photons,
碰撞会形成一个微小的核反应，导致两种粒子转换为两个高能量光子，
similar to X-rays, that shoot out in opposite directions.
和X光相似，这两个光子以相反的方向发射出去。
These photons will then impact an array of paired radiation detectors in the scanner walls.
这些光子随后到达扫描装置的放射物探头上。
The software in the scanner uses those detectors to estimate where inside the body the collision occurred
扫描装置中的软件借助于探测器来观测这种碰撞发生的位置，
and create a 3D map of the tracer's distribution.
并绘制出示踪剂的3D分布图。
PET scans can detect the spread of cancer before it can be spotted with other types of imaging.
PET扫描能够先于其他影像检查检查出癌症的存在。
They're also revolutionizing the diagnosis of Alzheimer's disease by allowing doctors to see amyloid,
此外，PET也革新着阿尔兹海默症的诊断，它帮助医生观察到淀粉样蛋白的存在，
the telltale protein buildup that otherwise couldn't be confirmed without an autopsy.
以前这种有警示作用的蛋白形成后，只有通过尸检才能检测到。
Meanwhile, researchers are actively working to develop new tracers and expand the possibilities of what PET scans can be used for.
同时，研究人员正积极研发新的示踪剂，并探索PET扫描的更多可能性。
But with all this talk of radiation and nuclear reactions inside the body, are these scans safe?
然而，关于进入人体的放射物和核反应仍然处于争论中，这些检查足够安全吗？
Even though no amount of ionizing radiation is completely safe,
尽管任何剂量的电离辐射都不是绝对安全的，
the amount of radiation the body receives during a PET scan is actually quite low.
但是人体在PET检查中接受的放射线剂量相对较低。
One scan is comparable to what you're exposed to over two or three years from natural radioactive sources, like radon gas;
每次检查所受的射线量相当于在2到3年时间里暴露在自然放射源，比如氡气中所接受的射线量；
or the amount a pilot would rack up from cosmic radiation after 20 to 30 transatlantic flights.
也相当于飞行员在20至30次横渡大西洋的飞行中所接受的累计宇宙射线量。
Most patients feel that those risks are acceptable for the chance to diagnose and treat their illnesses.
绝大多数患者会认为，为了获得诊治疾病的机会，这样的风险是可以接受的。
