Define thermionic emission .

Thermionic emission is the liberation of electrons from a heated cathode, which then gather in a cloud near its surface.

Name the four main components of an X-ray tube .

A heated cathode , an anode , a metal target and a high voltage power supply .

Why is the metal target in an X-ray tube usually tungsten , and why is the anode spun and sometimes water-cooled?

Tungsten is heat-resistant. Only about 1% of the electrons' kinetic energy is converted to X-rays, with the rest becoming heat, so the anode is spun at 3000 rpm and sometimes water-cooled to avoid overheating.

The anode and cathode of an X-ray tube are housed inside a .......... to stop the electrons colliding with particles on their way to the target.

The anode and cathode of an X-ray tube are housed inside a vacuum chamber to stop the electrons colliding with particles on their way to the target.

What are the two methods by which X-ray photons are produced when fast electrons collide with the target?

Bremsstrahlung radiation and characteristic radiation .

Define Bremsstrahlung radiation .

X-rays produced when fast-moving electrons undergo a steep deceleration on colliding with the target, with photon energy up to a maximum equal to the electron's kinetic energy, giving a continuous spectrum of wavelengths.

What causes characteristic radiation in an X-ray tube?

An incoming electron knocks an inner-shell electron out of a tungsten atom. An outer electron then falls to fill the vacancy, emitting an X-ray photon with energy equal to the difference between the two energy levels, producing sharp, discrete spikes.

Define X-ray attenuation .

The reduction in energy, or intensity, of a beam of X-rays due to their interaction with matter.

Name the four mechanisms by which X-rays can be attenuated.

Simple scattering , the photoelectric effect , Compton scattering and pair production .

Why does simple scattering cause blurring, or 'noise', in X-ray imaging?

Photons are deflected from their path with no change in energy and no absorption, so scattered X-rays arrive at the detector from several angles as well as from the main beam.

Define the photoelectric effect in X-ray attenuation.

An X-ray photon is absorbed by an inner-shell electron, causing it to be ejected as a photoelectron. All of the photon's energy is imparted to the photoelectron.

In Compton scattering, the X-ray photon's .......... increases as it transfers some of its energy to an orbital electron.

In Compton scattering, the X-ray photon's wavelength increases as it transfers some of its energy to an orbital electron.

True or False? In both simple scattering and Compton scattering, the X-ray photon's energy is unchanged.

False. In simple scattering the photon's energy is unchanged. In Compton scattering the photon transfers energy to an ejected electron, so its energy decreases and wavelength increases.

What distinguishes Compton scattering from simple scattering ?

In Compton scattering, the photon transfers some energy to an orbital electron, which is ejected, and the photon's wavelength increases. In simple scattering, there is no energy transfer and no electron is ejected.

Define half thickness .

The thickness of a material that reduces the intensity of an X-ray beam of a particular frequency to half its original value.

The intensity of an X-ray beam decreases .......... with the distance travelled through a material.

The intensity of an X-ray beam decreases exponentially with the distance travelled through a material.

What does a large linear absorption coefficient tell you about a material?

It absorbs X-rays strongly, so the beam is attenuated more over a given distance, for example bone has a much higher linear absorption coefficient than muscle.

What value of the ratio I / I 0 indicates good contrast between two tissues?

A ratio much less than 1.0 , showing a large difference between the transmitted intensities.

True or False? Bone has a lower linear absorption coefficient than muscle, which is why bone appears white on an X-ray image.

False. Bone has a much higher linear absorption coefficient than muscle, so it absorbs more X-rays and appears white.

Define contrast in X-ray imaging.

The difference in degree of blackening between structures.

Define sharpness in X-ray imaging.

How well defined the edges of structures are.

Name three ways the sharpness of an X-ray image can be improved.

Using a narrower X-ray beam , reducing scattering with a collimator or lead grid , and using a smaller pixel size .

Why are aluminium filters used in X-ray tubes?

They absorb the longer-wavelength X-rays, which are less penetrating and more likely to be absorbed by the body without contributing to the image, reducing patient dose.

Define contrast media .

A substance, such as barium or iodine , that is a good absorber of X-rays, given to a patient to improve the contrast on an X-ray image.

Why do contrast media have a large attenuation coefficient ?

Because they are made of elements with a large atomic number , for example barium ( Z = 56) and iodine ( Z = 53).

.......... X-rays are used to image bone, while .......... X-rays are used to image soft tissue.

Hard X-rays are used to image bone, while soft X-rays are used to image soft tissue.

True or False? Iodine is used as a contrast medium to image the digestive system.

False. Barium sulphate , taken as a barium meal, is used for the digestive system. Iodine is used in liquids, such as to observe blood flow, and is usually injected.

Define a CAT (computerised axial tomography) scan .

An imaging technique in which an X-ray tube rotates around a stationary patient, taking images of the same slice at many angles. This is repeated for successive slices, and a computer combines the images into a rotatable 3D image .

Name two advantages of a CAT scan over a simple X-ray image.

Produces much more detailed images, can distinguish tissues with similar attenuation coefficients, images soft tissue and bone in a single process, produces a 3D image, and has no overlapping structures.

Name two disadvantages of a CAT scan compared with a simple X-ray.

The patient receives a much higher radiation dose , and there are possible side effects from contrast media .

A CAT scanner takes X-ray images of the same .......... , at many different angles.

A CAT scanner takes X-ray images of the same slice , at many different angles.

Why does a CT scan expose a patient to a much higher radiation dose than a single X-ray image?

A CT scan requires several exposures of a slice from many different angles, repeated for multiple slices, before being combined into a 3D image, whereas a simple X-ray image is a single exposure.

True or False? CAT scans and MRI scans both use X-rays to produce images.

False. CAT scans use X-rays. MRI scans use magnetic fields, not X-rays.

A LevelPhysicsOCRFlashcards6. Particles & Medical PhysicsX-rays

X-rays (OCR A Level Physics): Flashcards

Exam code: H556

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Define thermionic emission.

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Cards in this collection (37)

Define thermionic emission.
Thermionic emission is the liberation of electrons from a heated cathode, which then gather in a cloud near its surface.
Name the four main components of an X-ray tube.
A heated cathode, an anode, a metal target and a high voltage power supply.
Why is the metal target in an X-ray tube usually tungsten, and why is the anode spun and sometimes water-cooled?
Tungsten is heat-resistant. Only about 1% of the electrons' kinetic energy is converted to X-rays, with the rest becoming heat, so the anode is spun at 3000 rpm and sometimes water-cooled to avoid overheating.
The anode and cathode of an X-ray tube are housed inside a .......... to stop the electrons colliding with particles on their way to the target.
The anode and cathode of an X-ray tube are housed inside a vacuum chamber to stop the electrons colliding with particles on their way to the target.
What are the two methods by which X-ray photons are produced when fast electrons collide with the target?
Bremsstrahlung radiation and characteristic radiation.
Define Bremsstrahlung radiation.
X-rays produced when fast-moving electrons undergo a steep deceleration on colliding with the target, with photon energy up to a maximum equal to the electron's kinetic energy, giving a continuous spectrum of wavelengths.
What causes characteristic radiation in an X-ray tube?
An incoming electron knocks an inner-shell electron out of a tungsten atom. An outer electron then falls to fill the vacancy, emitting an X-ray photon with energy equal to the difference between the two energy levels, producing sharp, discrete spikes.
True or False?
Increasing the accelerating voltage in an X-ray tube increases the minimum wavelength of X-ray produced.
False.
Minimum wavelength and accelerating voltage are inversely proportional, $λ_{m i n} = \frac{h c}{e V}$ , so a higher voltage gives a shorter minimum wavelength.
Define X-ray attenuation.
The reduction in energy, or intensity, of a beam of X-rays due to their interaction with matter.
Name the four mechanisms by which X-rays can be attenuated.
Simple scattering, the photoelectric effect, Compton scattering and pair production.
Why does simple scattering cause blurring, or 'noise', in X-ray imaging?
Photons are deflected from their path with no change in energy and no absorption, so scattered X-rays arrive at the detector from several angles as well as from the main beam.
Define the photoelectric effect in X-ray attenuation.
An X-ray photon is absorbed by an inner-shell electron, causing it to be ejected as a photoelectron. All of the photon's energy is imparted to the photoelectron.
In Compton scattering, the X-ray photon's .......... increases as it transfers some of its energy to an orbital electron.
In Compton scattering, the X-ray photon's wavelength increases as it transfers some of its energy to an orbital electron.
State the minimum photon energy condition for pair production to occur.
$E_{m i n} = h f_{m i n} = 2 m_{e} c^{2}$ , the total rest mass energy of the electron-positron pair produced.
True or False?
In both simple scattering and Compton scattering, the X-ray photon's energy is unchanged.
False.
In simple scattering the photon's energy is unchanged. In Compton scattering the photon transfers energy to an ejected electron, so its energy decreases and wavelength increases.
What distinguishes Compton scattering from simple scattering?
In Compton scattering, the photon transfers some energy to an orbital electron, which is ejected, and the photon's wavelength increases. In simple scattering, there is no energy transfer and no electron is ejected.
State the equation for X-ray attenuation, defining each symbol.
$I = I_{0} e^{- μ x}$
I₀ = incident intensity (W m^-2), I = transmitted intensity (W m^-2), μ = linear absorption coefficient (m^-1), x = distance travelled through the material (m).
Define half thickness.
The thickness of a material that reduces the intensity of an X-ray beam of a particular frequency to half its original value.
The intensity of an X-ray beam decreases .......... with the distance travelled through a material.
The intensity of an X-ray beam decreases exponentially with the distance travelled through a material.
What does a large linear absorption coefficient tell you about a material?
It absorbs X-rays strongly, so the beam is attenuated more over a given distance, for example bone has a much higher linear absorption coefficient than muscle.
What value of the ratio I/I₀ indicates good contrast between two tissues?
A ratio much less than 1.0, showing a large difference between the transmitted intensities.
True or False?
Bone has a lower linear absorption coefficient than muscle, which is why bone appears white on an X-ray image.
False.
Bone has a much higher linear absorption coefficient than muscle, so it absorbs more X-rays and appears white.
In the equation $I = I_{0} e^{- μ x}$ , what are the SI units of μ and x?
μ is measured in m^-1 and x is measured in m.
Define contrast in X-ray imaging.
The difference in degree of blackening between structures.
Define sharpness in X-ray imaging.
How well defined the edges of structures are.
Name three ways the sharpness of an X-ray image can be improved.
Using a narrower X-ray beam, reducing scattering with a collimator or lead grid, and using a smaller pixel size.
Why are aluminium filters used in X-ray tubes?
They absorb the longer-wavelength X-rays, which are less penetrating and more likely to be absorbed by the body without contributing to the image, reducing patient dose.
Define contrast media.
A substance, such as barium or iodine, that is a good absorber of X-rays, given to a patient to improve the contrast on an X-ray image.
Why do contrast media have a large attenuation coefficient?
Because they are made of elements with a large atomic number, for example barium (Z = 56) and iodine (Z = 53).
.......... X-rays are used to image bone, while .......... X-rays are used to image soft tissue.
Hard X-rays are used to image bone, while soft X-rays are used to image soft tissue.
True or False?
Iodine is used as a contrast medium to image the digestive system.
False.
Barium sulphate, taken as a barium meal, is used for the digestive system. Iodine is used in liquids, such as to observe blood flow, and is usually injected.
Define a CAT (computerised axial tomography) scan.
An imaging technique in which an X-ray tube rotates around a stationary patient, taking images of the same slice at many angles. This is repeated for successive slices, and a computer combines the images into a rotatable 3D image.
Name two advantages of a CAT scan over a simple X-ray image.
Produces much more detailed images, can distinguish tissues with similar attenuation coefficients, images soft tissue and bone in a single process, produces a 3D image, and has no overlapping structures.
Name two disadvantages of a CAT scan compared with a simple X-ray.
The patient receives a much higher radiation dose, and there are possible side effects from contrast media.
A CAT scanner takes X-ray images of the same .........., at many different angles.
A CAT scanner takes X-ray images of the same slice, at many different angles.
Why does a CT scan expose a patient to a much higher radiation dose than a single X-ray image?
A CT scan requires several exposures of a slice from many different angles, repeated for multiple slices, before being combined into a 3D image, whereas a simple X-ray image is a single exposure.
True or False?
CAT scans and MRI scans both use X-rays to produce images.
False.
CAT scans use X-rays. MRI scans use magnetic fields, not X-rays.

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