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Introduction to X-Rays

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Learning Objectives

How Do X-rays Work?

What Can We Actually See?

Test Your Knowledge

Risks

Introduction to X-rays

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Designed/illustrated/created by Charlie James

This resource hes been designed to give you a quick introduction/reminder of some foundational concepts of x-rays. The content covered is by no means exhaustive, but will hopefully help you prepare for your radiography sessions. On having completed this resource, you'll hopefully be able to:

Learning Objectives

What's the point?

  • Describe, in simple terms, howx-rays are generated in a medical setting
  • Describe, in simple terms, how an image is generated using x-rays
  • Explain how the density of a tissue will affect its tone on an x-ray
  • Explain how the orientation of the body can result in magnification of structures
  • Describe the difference between determinstic and stochastic effects of
ionising radiation
  • Use the correct SI unit for describing exposure to ionising radiation
  • Describe common natural sources of ionising radiation and compare
their exposure risk to medical sources of ionising radiation

How Do X-rays Work?

How Are X-Rays Generated?

What Happens to the Beam?

X-rays are a special type of electromagnetic radiation that, unlike some other sources of EM radiation (such as radio waves or visible light), contain sufficient energy to pass through solid objects. Whilst these rays can occur naturally, they can also be generated artificially to help visualise the body. In medical imaging, this process starts with a small, glass tube known (with characteristic imagination) as an x-ray tube. Inside this tube are two major components – a negatively-charged tungsten filament/cathode and a positively-charged copper anode. Both elements are contained within a vacuum.

How Are X-rays Generated?

Totally tubular

X-ray Tube

Copper anode

Tungsten filament

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+

First, electrical energy heats up the filament until electrons are released - this is known as thermionic emission (1). These electrons pass from the negatively-charged cathode towards the positively charged anode (2). Since they’re passing through a vacuum (and therefore encounter no air resistance), the electrons travelling towards the anode are able to accelerate to almost half of the speed of light.As they reach the anode, they strike a tungsten ‘target’ at its centre (3). Energy from this interaction causes x-ray photons (small packets of EM radiation) to be released (4). A series of metal sheets, known as collimators, are used to direct these photons into an x-ray beam that’s aimed towards the patient (5).

How Are X-rays Generated?

Totally tubular

5

X-ray Tube

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3

Copper anode

Tungsten filament

2

Collimators

1

X-rays fired at a patient typically do one of two things. They might penetrate the body, where they’ll reach a detector plate (1). This detects the intensity of the electromagnetic signal in order to build an image. The more x-rays detected in a single spot, the more intense the signal and the darker this portion of the radiograph appears. However, a proportion of x-rays will be absorbed by tissues in the body (2). Generally speaking, the denser a tissue, the higher proportion of x-rays it absorbs. This reduces the intensity of the signal that reaches the detector plate, creating a lighter region on the radiograph. This process of absorption means that areas of the body composed of denser tissues - for example, the abdomen - require a higher dose of radiation to image than areas - such as the thorax - that are less dense.

What Happens to the Beam?

2

1

Detector plate

What Can We Actually See?

Density

Magnification

The density of a tissue will determine how it appears on a radiograph. Typically speaking, denser tissues absorb a higher proportion of x-rays, reducing the intensity of the signal on the detector and creating a lighter tone on a radiograph. Bone is the densest tissue and appears almost white. Conversely, as the least dense substance, gas appears black. Muscle is light grey and fat tends to be dark grey. Finally, metal or contrast media absorb/reflect nearly all x-rays and appear bright white. Click on the boxes (right) to explore these different tones on an x-ray.

What Are We Actually Seeing?

Dense and densibility

Bone

Fat

Muscle

Air

High density

Low density

However, it's not just the type of tissue that will affect how it appears on a radiograph (1). The thickness of a tissue also determines its appearance – thicker tissues absorb more x-rays and appear lighter (2). Finally, x-rays will pass through every tissue between the x-ray tube and the detection plate. So, the final image doesn’t just reflect the density of a single tissue, but the combined density of structures behind and in front of it. For example, a muscular area will appear white if the beam also passes through bony tissue (3).

What Are We Actually Seeing?

Dense and densibility

3

1

2

Although collimators try to focus the x-rays into a narrow beam, they’ll still diverge slightly as they head towards the detector. This can modify the apparent size of a structure, depending on its distance from the beam. You can visualise this as like shining a torch on the floor. The torch sends out a diverging beam of light, just like the x-ray tube. The beam hits the floor, like x-rays hitting a detector plate. If you place your hand close to the floor (left), the shadow appears roughly the same size as your hand. However, as the hand moves closer to the torch (right), its shadow becomes larger and less well-defined.

What Are We Actually Seeing?

Magnifying the issue

Exactly the same thing happens with x-rays. The closer an object is to the x-ray source, the larger it appears. For example, in a PA chest x-ray (the beam passes from posterior to anterior) the heart is close to the detector plate, and appears roughly its actual size. However, in an AP chest x-ray (the beam passing from anterior to posterior), the heart sits closer to the source of the beam and appears magnified. Click the buttons below to compare AP and PA views of the chest.

What Are We Actually Seeing?

Magnifying the issue

PA View

AP View

Exactly the same thing happens with x-rays. The closer an object is to the x-ray source, the larger it appears. For example, in a PA chest x-ray (the beam passes from posterior to anterior) the heart is close to the detector plate, and appears roughly its actual size. However, in an AP chest x-ray (the beam passing from anterior to posterior), the heart sits closer to the source of the beam and appears magnified. Click the buttons below to compare AP and PA views of the chest.

What Are We Actually Seeing?

Magnifying the issue

PA View

AP View

Risks

Risks of X-rays

How Do We Measure Exposure?

Natural vs Medical Radiation

When x-rays encounter living tissue, they can have detrimental effects. The high energy photons can displace electrons from any atoms they hit, producing an ion (a negatively or positively charged atom). X-rays are therefore described as a form of ionising radiation. The ionisation process can result in the formation of free radicals – unstable and highly reactive atoms that can alter molecular structures such as DNA, RNA or enzymes.

Risks of X-rays

Not so radical

1. An x-ray photon displaces an electron from the atom, forming an ion.

2. The electron-deficient atom turns into a free radical (considerably less cool than the sunglasses imply).

Changes following exposure to ionising radiation can be divided into two categories: Deterministic effects occur when a threshold level of radiation exposure has been exceeded. At this point, cells death occurs, and symptoms such as hair loss or skin erythema may be observed. The severity of symptoms will increase with higher doses.Stochastic effects are the result of cell changes/mutations and can result in cancer or genetic abnormalities. These effects occur by chance and can happen at any radiation level – although become increasingly likely with higher levels of exposure. Usually there’ll be a lag period of several years between radiation exposure and the occurrence of stochastic effects.

Risks of X-rays

Not so radical

Stochastic effects become more likely with higher exposure levels

Deterministic effects occur past threshold levels of exposure

Effective dose of ionising radiation

Likelihood of changes occuring

Exposure to radiation is measured in sieverts (Sv). This unit represents the stochastic risk of ionising radiation – essentially, how likely it is to cause radiation-induced cancer or mutations. The more sieverts of radiation a person is exposed to, the greater their risk of stochastic effects. In the context of healthcare, one sievert represents a large dose of ionising radiation, and would be liable to cause nausea, vomiting and haemorrhage. Typically, medical radiation is significantly lower and is measured in millisieverts (mSv), each representing one thousandth of a sievert. For example, a normal chest x-ray exposes a patient to around 0.05 mSv of radiation. However, an abdominal x-ray (which requires more radiation to penetrate the denser tissue) delivers an effective dose of 0.6 mSv.

How Do We Measure Exposure?

1 Sv

10 mSv

Level at which immediate radiation sickness would occur

Average CT scan(6.6 mSv)

Chest x-ray(0.05 mSv)

However, it’s worth highlighting that we’re routinely exposed to natural sources of radiation, from buildings, cosmic rays, even our food and drink. But how do these exposure levels compare to man-made radiation? Well, click the logo below to play a millisievert-modelled game of Higher or Lower...

Natural vs. Medical Radiation

HIGHER

LOWER

HIGHER

LOWER

HIGHER

LOWER

HIGHER

LOWER

Test Your Knowledge!

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1-2: Stochastic effect 3-4: Knowledge absorbed 5: Effective dose

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