# A Medical Student’s Guide to: X-Ray Imaging – What is an X-Ray?

Two-dimensional X-Ray images are created by firing a beam of X-Rays through a patient and measuring the number of photons reaching a detector placed behind the patient. X-Rays are attenuated (absorbed) as they pass through more dense tissues, and able to travel more freely through soft tissues, allowing doctors to see patient anatomy.

## What is an X-Ray?

An X-Ray is a type of electromagnetic wave, meaning it belongs on the electromagnetic spectrum. A wave is the movement of energy through a medium. Electromagnetic rays can also be modelled as a particle of energy called a photon.

When describing electromagnetic waves, we talk about their properties. The key properties of a wave are Wavelength (λ) and Frequency (f). Wavelength is the distance from one peak to the next, or from one trough to the next, and is measured in metres. Frequency is the number of waves that pass a fixed point each second, and is measured in Hertz.

For X-Rays, the wavelength is approximately between 10-9 and 10-11 metres. That’s between 1 and 0.01 nanometres! For comparison, this is roughly the width of an individual atom.

The Frequency of X-ray radiation is between 1018 and 1019 Hz.  That is between 1 and 10 quintillion waves per second.

Another important property of electromagnetic waves is the energy. The energy transferred by a photon (the discrete packets of energy) is proportional to the frequency of the wave. Energy is calculated by multiplying frequency by the Planck Constant (h). The Planck constant is equal to 4.1 × 10-15 and is measured in electronvolts. One electronvolt is equivalent to the kinetic energy gained by accelerating an electron from rest through a voltage of 1V.

E = h f

Energy is calculated by multiplying frequency (f) by the Planck Constant (h).

### Summary

So, that was a lot of information. I hate to break it to you, but if you’re studying for an Imaging Sciences degree, you’ll need to know it. However, most medical students will only need to know that X-rays are a high energy form of electromagnetic radiation.

This is a continuation of my A Medical Student’s Guide To: series, and the first post about x-ray technologies. Checkout the others below, and you might also be interested in reading my guides to Ultrasound: The Basics and Ultrasound: Imaging Modes. 