Introduction

X-rays are a form of electromagnetic radiation widely used to diagnose various medical conditions and even treat cancers. The idea of being exposed to radiation may be alarming to some, especially those new to X-ray imaging or radiotherapy. However, the procedure is quick and painless. The low doses from diagnostic X-rays generally have no ill effects on patients, with the amount of radiation involved often lower than the dose received each year from natural radiation sources.¹

Over a century of research in the medical use of X-rays means this technology is now routine in many clinical procedures. X-ray imaging is often done by trained specialist doctors known as radiographers – though the ease of this procedure means it can be done by other healthcare professionals, such as dentists. As new X-ray techniques have developed, patient safety has also advanced. In this article, we will explain exactly what X-rays are, how they are used, and what to expect when undergoing X-ray imaging or radiotherapy using X-rays. 

How do x-rays work?

X-rays were discovered over a hundred years ago by a physicist named William Roentgen. He observed these rays whilst experimenting and named them ‘X’ rays as he didn’t know what they were at the time, and the name stuck.

The X-rays, Roentgen observed, were caused by the collision of electrons with a chemically-treated plate, causing it to glow. Electrons are subatomic particles that carry a negative charge – you can think of this charge in the same way as the negative end of a magnet. Electrons can shuffle between substances, transferring energy with them. X-rays are generated when electrons are accelerated into another substance at high speed. The energy is transferred to other particles, knocking other electrons loose that accelerate away as a beam of X-ray electromagnetic radiation.^2,3 

Put simply, electromagnetic radiation describes whenever a charged particle speeds up or slows down. We give different names to different speeds of this movement as they form a spectrum. At the ‘slow’ end of the spectrum are radio waves and infrared. Visible light is another type of electromagnetic radiation, and even faster speeds generate ultraviolet light. X-rays are generated by particles accelerated even faster than this, but not as fast as gamma rays. Unlike visible light, X-rays are powerful and fast enough to fly through different types of matter, even solids.

X-ray equipment

Radiography equipment consists of an X-ray tube (a negatively charged ‘cathode’ and positively charged ‘anode’ in a vacuum tube), high-voltage cables, and an X-ray-sensitive sensor. These components, often ceiling-mounted for flexibility, can be positioned at various angles around the patient.

Electrons are generated at the start of the X-ray process by heating a coil of wire (the ‘cathode’), becoming energised and colliding with the positively charged anode (usually made of tungsten). As mentioned before, electrons are negatively charged and, like magnets, are attracted to a positive charge. Electrons are drawn to the positively charged anode, and as they collide with it at high speed, they generate a beam of X-rays on the other side.^3,4

Tungsten is used due to its high ‘atomic number’ (the number of electrons it has). By having more electrons, more electron collisions will happen, and therefore more X-rays will be produced. The wider the ‘potential difference’ in charge between the negative cathode and positive anode – also known as the ‘voltage’ – the faster the electrons will accelerate and the higher the intensity of the X-ray beam.

The radiographer adjusts the voltage to get an X-ray beam suitable for what they are trying to examine. By adjusting the voltage, they can control the strength and quantity of X-rays the patient is exposed to.^4,5

X-ray images

When X-rays pass through the patient, they collide with electrons. Sometimes all the energy of the X-ray is absorbed, other times only some of it gets absorbed and the rest is scattered. Denser materials with a higher atomic number are more likely to absorb X-rays because there are more electrons to hit. This is why dense bone, high in calcium, absorbs X-rays well. Softer tissues are less dense and made mostly of substances with lower atomic numbers.

Unabsorbed X-rays pass through the body and hit the sensor. This creates images where dense tissues appear white, whilst softer parts appear darker. This used to be done with X-ray-sensitive films, but newer digital sensors work with lower X-ray doses and provide better images.^,4,5

Types of medical X-ray

X-rays in diagnostics

Since their discovery, various techniques have been developed for using X-rays to diagnose disease. The main categories include radiography, fluoroscopy and computed tomography (CT scanning).⁶

Radiography

Conventional radiography is the ‘standard’ X-ray technique, working as described with a simple arrangement of an X-ray tube and a sensor positioned on either side of the patient. Newer techniques like CT scanning are becoming more widespread, but due to its ease and speed, standard radiographs continue to be widely used for imaging the chest, abdomen, and skeleton, and also for dentistry.⁶ It is commonly used to diagnose fractures, dental health issues, spinal issues, tumours, and lung or heart problems. It can also help diagnose infectious diseases such as pneumonia.²

Fluoroscopy

Fluoroscopy is similar to a standard X-ray radiograph but captures the image in real time. This means movement through the body can be seen – particularly useful for examining the passage of substances through the body (e.g. the movement of blood through the heart).

To do this, a substance with a high atomic number (known as a ‘radiographic contrast agent’) is administered. This may be through swallowing or injection. This allows the X-ray to see the movement of the contrast agents through the body.¹

Computed tomography (CT)

The main limit of traditional radiography is it only gives a single angle. For example, a dense tumour might be detected, but it may not be possible to tell how deep it is. By taking multiple images from different angles and combining these using computers, we get a much better idea of what is happening in the body.

To do a CT scan, the patient lies inside a doughnut-shaped apparatus (similar to an MRI), and an X-ray beam and sensor are rotated around the patient. The CT scan tends to move along the body, and the images are digitally reconstructed to give a 3-D model.

Whilst this method gives a much clearer image of the body, it requires expensive equipment and isn’t required for routine X-ray imaging. The X-ray radiation doses involved in CT scans are also up to 1000 times higher than conventional X-ray radiographs, meaning they shouldn’t be used unless necessary.^1,6

Radiotherapy: x-rays in treatment

X-rays are also used in the treatment of cancer – a process known as radiotherapy. High-energy X-rays can be focused in a tight beam with enough energy to kill cancer cells whilst minimising damage to the healthy cells around them. Whilst the dosages involved are typically higher than those used for imaging, advances in radiotherapy mean cancer cells or tumours can be targeted precisely. CT scans are often used beforehand to determine where the beam should be focused, and recent advances in the technology allow radiographers to even control the shape of the beam as it passes through the body.⁷

Safety concerns

Risks and side-effects

Whilst it can be used to treat cancers, X-rays are a form of ionising radiation. Ionising radiation has sufficient energy to potentially cause DNA damage, elevating the risk of cancer over a person's lifetime. If not managed correctly, repeated and excessive exposure can pose a risk to human health, including a small increase in cancer likelihood.

However, X-rays are less damaging to human cells than higher energy types of radiation (e.g. gamma radiation). The risk of cancer is also considered very small, and the benefits of X-rays generally outweigh the risks. Other side effects are rare, but some can occur at higher radiation levels (e.g., cataracts, skin reddening, hair loss, reduced fertility).

Standard X-rays have no after-effects and patients can resume normal activities straight away. In fluoroscopy, there is a small risk of potential reaction to contrast agents that may be injected, swallowed or delivered via enema. Some temporary side effects may occur with contrast agents, such as changes in stool colour or temporary blurriness of vision.^1,2,5

Minimising x-ray exposure

Radiologists take steps to minimise any unnecessary exposure. For example, different X-ray techniques use different amounts of radiation and the method involving the lowest dose that gives the desired image will be chosen.

To give some context to how much radiation the patient will be exposed to, it’s common for a comparison to be made of the dose from one X-ray to the equivalent dose from natural sources of radiation such as naturally occurring radioactive substances or radiation from the sun. For example, a standard spinal radiograph is a third of the dose of radiation you are naturally exposed to in a year.^1,5

The general principle for minimising the dose of radiation from X-ray diagnostics and therapy is ‘ALARA’ or ‘As Low as Reasonably Achievable’. Even high X-ray doses can be justified if the benefits outweigh the risks and there are no set limits to X-ray dosages, though any dose must be justified based on benefits versus risks.⁵

Patient considerations

Pregnancy

Pregnant individuals can safely undergo X-ray examinations, though some additional safety measures should be taken. Unborn children are more sensitive to radiation, so low X-ray doses are used. Non-urgent procedures that expose the pelvic region may be delayed until after pregnancy.

Repeat investigations

Doctors will avoid repeating X-rays as much as possible, though sometimes repeat investigations are necessary for monitoring treatment progress. If you are receiving treatment from multiple centres, it may be helpful to keep a record of X-rays taken and inform doctors about these.

Summary

X-rays are essential in medical diagnostics and radiotherapy. Despite concerns about radiation exposure, the low doses used in routine X-ray imaging pose minimal risk, often lower than natural background radiation. Radiographers are trained to obtain the best image or treatment possible whilst reducing unnecessary exposure.

X-rays, ranging from conventional radiography to techniques like fluoroscopy and CT scans, provide crucial insights into many medical conditions. In radiotherapy, high-energy X-rays can precisely target and kill cancer cells. While X-rays are a form of radiation and carry potential risks, strict safety measures, dosing, and newer X-ray technologies should relieve these concerns. For almost all patients, X-ray examinations are safe and the benefits in diagnosis and treatment far outweigh the risks.