X-rays are high-frequency electromagnetic waves that pass through soft tissues but are absorbed by dense materials such as bone and metal. In medical imaging, X-rays cast shadows on a detector: bones appear white, soft tissues appear grey, and air-filled spaces appear black, allowing doctors to view internal structures without surgery.
What are X-rays and where do they fit in the electromagnetic spectrum?
X-rays are part of the electromagnetic spectrum — they are transverse waves that travel at the speed of light (3 × 10⁸ m/s in a vacuum). They sit between ultraviolet radiation and gamma rays on the spectrum, with:
- Wavelength: approximately 0.001–10 nm (shorter than UV but longer than gamma rays)
- Frequency: approximately 3 × 10¹⁶ Hz to 3 × 10¹⁹ Hz
- Energy: high enough to ionise atoms (remove electrons) — this is what makes them useful for imaging and potentially harmful with repeated exposure
| Part of EM spectrum | Wavelength | Key property relevant to medicine |
|---|---|---|
| Radio waves | cm–km | MRI (radio waves interact with hydrogen nuclei in a magnetic field) |
| Infrared | ~700 nm to 1 mm | Thermal imaging (heat emitted by infected/inflamed tissue) |
| Ultraviolet | 10–400 nm | Sterilisation; treatment of skin conditions |
| X-rays | 0.001–10 nm | Imaging bone and dense tissue; CT scans |
| Gamma rays | <0.001 nm | PET scans; cancer radiotherapy |
How are X-rays produced?
X-rays are produced in an X-ray tube:
- A high voltage (typically 30,000–150,000 V) accelerates electrons from a heated cathode (negative electrode) towards a metal anode (positive electrode).
- The electrons strike the anode — usually made of tungsten — at very high speed.
- The rapid deceleration of the electrons converts some of their kinetic energy into X-ray photons (and most of the rest into heat, which is why X-ray tubes must be actively cooled).
- The X-rays emerge from a window in the tube and are collimated (directed) towards the patient.
The higher the accelerating voltage, the higher the energy (and shorter the wavelength) of the X-rays produced. More energetic X-rays penetrate deeper and are used for imaging thick body parts or in cancer radiotherapy.
Why do X-rays produce images of bones and dense tissues?
X-ray absorption depends on the density and atomic number of the material:
- Bone contains calcium and phosphorus — both with relatively high atomic numbers (20 and 15 respectively). Dense structures with higher atomic number elements absorb X-rays strongly. Bone appears white on an X-ray image (radiograph).
- Soft tissues (muscle, fat, organs) have lower average atomic numbers and density. They absorb less, appearing grey.
- Air-filled spaces (lungs, bowel) absorb almost no X-rays. They appear black.
- Metal implants (pins, plates, joint replacements) absorb X-rays very strongly and appear bright white.
In a traditional radiograph, the transmitted X-rays darken photographic film (or activate a digital detector). Where more X-rays pass through, the image is darker; where more are absorbed, the image is lighter.
What is the difference between an X-ray and a CT scan?
A standard X-ray radiograph is a 2D shadow image produced by a single beam of X-rays passing through the patient onto a detector. It is fast, simple, and uses a relatively low radiation dose. It is excellent for bones, detecting pneumonia, and checking for foreign objects, but all structures are superimposed in 2D.
A CT scan (computed tomography, or computed axial tomography) takes many X-ray images from different angles as the X-ray tube rotates around the patient. A computer combines these into a detailed 3D reconstruction that can be viewed as slices through any plane. CT scans show soft tissue detail far better than plain X-rays, but use significantly higher radiation doses (a chest CT delivers about 100 times the dose of a chest X-ray) and take longer.
| Feature | Plain X-ray | CT scan |
|---|---|---|
| Image type | 2D shadow | 3D reconstruction (multiple slices) |
| Soft tissue detail | Poor | Good |
| Radiation dose | Low | Much higher |
| Speed | Seconds | Minutes |
| Best for | Bone fractures, chest, foreign objects | Brain, abdomen, trauma assessment |
What are the risks of X-ray imaging?
X-rays are ionising radiation — they carry enough energy to remove electrons from atoms and break chemical bonds. This can:
- Damage DNA directly (strand breaks)
- Create free radicals that damage DNA indirectly
- In rare cases, lead to cell mutations that cause cancer
The risk from a single diagnostic X-ray is very small but not zero. Medical imaging follows the ALARP principle (As Low As Reasonably Practicable) — the dose is kept as low as possible while still getting the diagnostic information needed.
Protective measures:
- Lead aprons shield body parts not being imaged (especially reproductive organs and the thyroid)
- Radiographers operate the equipment from behind a lead-lined screen or in a separate room
- Pregnancy: the foetus is particularly sensitive to ionising radiation; X-rays during pregnancy are avoided unless essential
- Children: because growing tissues are more sensitive to radiation, the risk is higher; CT scans in children are particularly carefully justified
What other imaging methods use electromagnetic radiation?
| Method | Type of radiation | How it works | Advantage over X-rays |
|---|---|---|---|
| MRI (Magnetic Resonance Imaging) | Radio waves + strong magnetic field | Detects how hydrogen nuclei in water molecules respond to radio pulses in a magnetic field | No ionising radiation; excellent soft tissue contrast |
| PET scan | Gamma rays | Patient given a radioactive tracer that emits positrons; annihilation with electrons produces gamma-ray pairs detected by the scanner | Shows metabolic activity; cancer cells show as "hot spots" |
| Infrared thermography | Infrared radiation | Maps heat distribution on the body's surface | Non-contact, non-invasive; identifies inflamed tissue |
Frequently asked questions
Why is bone white on an X-ray and air black?
An X-ray radiograph is a shadow image: wherever more X-rays pass through the body and reach the detector, the image is darker; wherever X-rays are absorbed, the image is lighter (because less radiation reaches the detector). Bone contains dense calcium and phosphorus which absorb X-rays strongly — very few X-rays pass through bone, so the detector records little exposure beneath it, and the image appears white (or pale). Air absorbs almost no X-rays, so all the radiation passes through air-filled spaces and the detector records maximum exposure, producing a very dark (black) area.
Why are X-rays harmful but radio waves are not?
Both X-rays and radio waves are electromagnetic radiation, but they differ fundamentally in photon energy. The energy of a photon is proportional to its frequency (E = hf). Radio waves have very low frequencies (kHz to GHz) and correspondingly low photon energies — far too low to ionise atoms. X-rays have frequencies millions of times higher and photon energies high enough to eject electrons from atoms and break chemical bonds. This ionisation is what makes X-rays capable of damaging DNA. The body can repair most single-strand DNA breaks, but double-strand breaks and accumulated damage over many exposures increase the risk of cancerous mutations.
How does contrast medium (contrast agent) improve X-ray imaging of soft tissue?
Dense contrast agents are introduced into the body to make soft tissue structures temporarily visible on X-rays, because they absorb X-rays far more strongly than the surrounding tissue. A barium swallow or enema introduces barium sulfate (very high atomic number, insoluble, non-toxic) into the gastrointestinal tract to outline the oesophagus, stomach, and intestines. Iodine-based contrast agents are injected intravenously to highlight blood vessels (angiography) and solid organs. Without contrast, the soft tissue boundaries in these structures have insufficient difference in X-ray absorption to be distinguished.
What is the difference between X-rays and gamma rays if both are ionising electromagnetic radiation?
X-rays and gamma rays occupy overlapping parts of the electromagnetic spectrum and cannot always be distinguished by wavelength alone — the shortest X-rays have shorter wavelengths than some gamma rays. The key difference is their origin: X-rays are produced by the deceleration of electrons or by electron transitions in the outer shells of atoms; gamma rays are produced by nuclear decay (changes inside atomic nuclei). In medical imaging, X-rays come from an electrical X-ray tube and can be switched on and off. Gamma rays in PET scans come from radioactive isotopes administered to the patient and cannot be turned off — they emit radiation until the isotope decays.
For predict-first GCSE physics with Professor Newton — connecting electromagnetic wave properties to diagnostic imaging before working through any calculation — visit aitutors.me.