How much radiation does the human body receive when eating a banana, taking a flight, or having a chest X-ray?

丨Author: Hao Zhixin, Peking Union Medical College Hospital 丨Reviewer: Luo Yaping, Peking Union Medical College Hospital

When talking about the word "nuclear", many people think of the mushroom cloud over Hiroshima, Japan, or the Chernobyl nuclear power plant accident, or the nuclear leak at the Fukushima Daiichi Nuclear Power Plant in Japan 10 years ago...

Nuclear science and technology have played an increasingly important role in the fields of energy, medicine, science and technology, industry and agriculture, and have brought great social and economic benefits to human society. However, people still change color when talking about "nuclear medicine". In other words, the public is still unfamiliar with nuclear medicine. Even patients who undergo examinations or treatments in nuclear medicine departments often worry about the adverse effects of radiation on their health, and even have unnecessary panic. Let's talk about the relationship between "nuclear medicine" and health.

Radiation is everywhere. In physics, radiation refers to a form of energy transfer. Depending on the level of its energy, it is divided into ionizing radiation and non-ionizing radiation.

The signs for electromagnetic radiation (left) and ionizing radiation (right). The one on the left is for "good people" and looks like a signal tower; the one on the right is a warning sign commonly seen in radiology or nuclear medicine departments in hospitals.

Non-ionizing radiation refers to low-energy electromagnetic waves (also called electromagnetic radiation). It has low energy and is harmless to the human body, so no protection is required.

Ionizing radiation refers to high-energy rays that can ionize atoms or molecules and cause damage to matter, so protection is necessary.

Ionizing radiation includes high-energy particle flows such as alpha rays, beta rays, and neutrons, and high-energy electromagnetic waves such as gamma rays and X-rays. The effect of radiation on the human body is measured in equivalent doses, with the commonly used units being millisieverts (mSv) and microsieverts (µSv).

Radiation is everywhere, invisible and intangible, but that doesn't mean we are living in danger. Ionizing radiation can be divided into two categories: natural radiation and artificial radiation.

Radiation in life

Natural radiation refers to the radiation that occurs naturally in nature. People living on Earth are exposed to natural background radiation from food, houses, the sky, the ground, mountains, rivers, and vegetation at all times. The natural radiation dose varies from region to region. The global average annual dose is 2.4 mSv, and the average annual dose in my country is 3.1 mSv. For normal natural background radiation, no special protective measures are required.

Artificial radiation mainly comes from medical exposure, of which 90%-95% comes from X-ray diagnosis and radiotherapy, and 5%-10% comes from nuclear medicine diagnosis and treatment.

Impact of radiation on health Since ionizing radiation can change the properties of biological macromolecules in the human body, does that mean that the human body will be damaged or even develop cancer as long as it is exposed to ionizing radiation?

The answer is no

We must remember: any discussion of toxicity without considering dosage is irrational!

The extent of damage to human health depends on the intensity of the radiation and the duration of exposure. Short-term exposure to more than 4000 mSv can cause neurological damage and rapid death, and exposure to 2000-4000 mSv can cause radiation sickness.

But my dear friends, such terrible high-dose radiation is beyond the reach of ordinary people like us.

As mentioned earlier, we do come into close contact with ionizing radiation in our daily lives.

To give a few examples: eating a banana will give you a radiation dose of 0.1 µSv, flying for 10 hours will give you a radiation dose of 20 µSv, and having a chest X-ray will give you a radiation dose of 0.1 mSv.

You see, although we have had more than one close contact with these "terrible" radiations, our bodies are still very strong!

In fact, radiation doses below 100 mSv/year will not affect the incidence of cancer in the population. How much is 100 mSv? It is roughly the radiation dose that would be achieved by taking 1,000 chest X-rays a year.

Water can drown people, but drinking a glass of water will not; electricity can kill people, but using a flashlight will not. The same is true for the impact of radiation on health. We don't have to be afraid of the little bit of radiation and be scared of the necessary examinations. The potential danger of ionizing radiation to the human body is an indisputable fact. While we have a scientific understanding of radiation, we should stay away from unnecessary radiation to reduce the chance of it harming the body.

Radiation dose and health

The “mysterious” nuclear medicine examination Nuclear medicine examination is a medical imaging method that introduces drugs labeled with radionuclides into the patient's body and uses the rays emitted by the nuclides for imaging.

Nuclear medicine examinations can not only display the anatomical information of organs and lesions, but more importantly, they can reflect changes in blood flow, function, metabolism and even molecular levels of organs and lesions, thereby showing lesions at the stage when there are only functional changes but no morphological structural abnormalities, and achieving early diagnosis of diseases. This is exactly the specialty of nuclear medicine!

The core machines of nuclear medicine: PET/CT (left) and SPECT/CT (right). Although they look very similar to CT machines, they can examine very different things.

PET/CT and SPECT/CT are two core examinations of nuclear medicine. PET/CT is the most advanced molecular imaging examination and is called "the magic tool for screening tumors" by many clinicians.

So how does this magical function work? Because tumor cells grow fast and divide vigorously, their metabolism is significantly higher than that of normal cells. The most commonly used tracer for PET examination is 18F-FDG, which is injected into the human body to track high-metabolism cells and accumulate in these cells, so that the diseased tissue can be displayed on the PET image.

Let's play a little game to understand the principles of PET/CT.

In the picture below, the little animals are having a great time in the forest, but one of them is secretly crying. Can you find who it is? Is it hard to find?

▼ CT images

Who is shedding tears secretly?

This picture is like a CT scan. The anatomical structure is very clear, but it is difficult to find the lesion. What if all other animals are gray, but only the one that secretly sheds tears is in color? Wouldn't it be easy to see at a glance?

▼ PET images

This picture is similar to a PET image, in which only lesions with high metabolism are clearly visible, while normal tissue is only a light gray.

▼ PET/CT fusion images

The picture above has both clear lines and specific colors. You can recognize at a glance that it is a little cat secretly shedding tears!

This is the magic of PET/CT images. By combining the advantages of PET and CT, the metabolic activity and anatomical location of the lesion can be displayed simultaneously. Therefore, even early tumors can be detected in time by PET/CT. In addition, epilepsy, myocardial infarction, infectious diseases, rheumatic and immune diseases, etc. also require the help of PET/CT.

Compared with PET/CT, SPECT has been used in clinical practice earlier and in a wider range. SPECT has a wide range of examination items, including whole-body bone imaging, renal blood flow function imaging, myocardial blood perfusion imaging, thyroid imaging, parathyroid imaging, pulmonary ventilation/perfusion imaging, gastrointestinal bleeding localization imaging, etc. It has more advantages in providing blood flow, function and metabolism of organs or diseased tissues.

The combination of SPECT and CT forms SPECT/CT, which can provide diagnostic-grade CT anatomical images when necessary for more comprehensive early diagnosis of diseases.

Currently, the radioactive nuclides commonly used in clinical practice include 99mTc, 18F, 68Ga, etc., all of which are short half-life nuclides with half-lives of 6 hours, 109 minutes, and 68 minutes, respectively.

You can think of the half-life as the "shelf life". After each half-life, half of the radioactive drug will become ineffective, and this process is ongoing all the time. In addition, the radioactive nuclides injected into the human body are mainly excreted through the urinary system. Drinking more water and urinating more after the examination can appropriately reduce radiation.

The radiation dose received from a chest CT scan is about 6.0 mSv, and the radiation dose received from a nuclear medicine 18F-FDG PET scan is 3.5-8.0 mSv.

The radiation dose produced by radioactive nuclides in a PET examination is roughly equivalent to that of a chest CT examination. The radiation dose produced by most nuclear medicine examinations is even lower than that of a chest CT examination. Therefore, there is no need to look at nuclear medicine examinations with "tinted glasses". The radiation dose produced is very safe for the human body!

It should be noted that since children are more sensitive to radiation than adults, a low-dose scanning scheme will be used during the examination to minimize radiation exposure. In addition, like most other radiological examinations, pregnant women should try to avoid nuclear medicine examinations unless under special circumstances.

Radiation is everywhere, but it is difficult for us to be exposed to a single large dose of radiation that can damage the human body in our daily life. The low-level ionizing radiation that we are often exposed to in our daily life is not enough to affect our health. The various nuclear medicine examinations that patients undergo for the purpose of diagnosing and treating diseases are safe for patients, and there is no need to panic excessively. We should look at "nuclear" with a scientific attitude, so that nuclear science and technology can better serve us.

Image sources ▶ howstuffworks, wikipedia, HKNIC, pinterest, siemens-healthineers, itnonline

丨Editor: Liu Yang, Zhao Na丨Reviewer: Li Na, Li Yule, Dong Zhe丨Supervisor: Wu Wenming