Define contrast in X-ray imaging.

Contrast is the difference in the degree of blackening between structures.

Define sharpness in X-ray imaging.

Sharpness is how well-defined the edges of structures are.

How are X-ray photons produced in an X-ray tube?

Electrons released by thermionic emission at the cathode are accelerated towards the anode; when they bombard the metal target, they rapidly decelerate and transfer their kinetic energy into X-ray photons .

Why does the X-ray spectrum have a sharp cut-off at short wavelengths?

At the cut-off, each electron gives up all its kinetic energy in a single collision to produce one photon, corresponding to the maximum possible photon energy and so the minimum wavelength.

Why are aluminium filters used in medical X-ray imaging?

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

Image sharpness can be improved by using a narrower X-ray beam and reducing scattering with a .......... or lead grid.

Image sharpness can be improved by using a narrower X-ray beam and reducing scattering with a collimator or lead grid.

True or False? Harder (higher energy) X-rays always give better image contrast, whatever tissue is being imaged.

False. Hard X-rays are used for imaging bone and soft X-rays for tissue; the correct level of hardness depends on the structure being imaged.

Define attenuation of an X-ray beam.

Attenuation is the reduction in the intensity of an X-ray beam as it passes through matter.

Why do bones appear white on an X-ray photograph?

Bones absorb X-ray radiation strongly, so little radiation passes through to reach the detector or film at that point.

Define half thickness .

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

As an X-ray beam passes through matter, its intensity decreases .......... with distance travelled.

As an X-ray beam passes through matter, its intensity decreases exponentially with distance travelled.

What condition on the intensity ratio I / I 0 indicates good contrast between two tissues?

There is a large difference between the intensities, so the ratio I / I 0 is much less than 1.0 .

True or False? The linear absorption coefficient, μ , is the same for X-ray photons of any energy.

False. The linear absorption coefficient depends on the energy of the X-ray photons , as well as the material.

Define a computed tomography (CT) scan .

A CT scan is an imaging technique in which an X-ray tube rotates around a stationary patient, taking X-ray images of the same slice at many different angles, which a computer combines to build a 3D image.

How is a 3D image built up during a CT scan?

X-ray images of the same slice are taken at many different angles This process is repeated for successive slices A computer combines the slice images together to build a 3D image , which can be rotated and viewed from different angles

State three advantages of a CT scan compared with a single X-ray image.

Produces images of much higher resolution Can distinguish between tissues with similar attenuation coefficients Produces a 3D image of the body

State two disadvantages of a CT scan compared with a single X-ray image.

The patient receives a much higher radiation dose There are possible side effects from the contrast media used

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

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

True or False? A CT scan produces only a single 2D image of the body.

False. A CT scan combines many 2D images, taken of successive slices from multiple angles, to build up a single 3D image .

A LevelPhysicsCambridge (CIE)Flashcards24. Medical PhysicsProduction & Use of X-rays

Production & Use of X-rays (Cambridge (CIE) A Level Physics): Flashcards

Exam code: 9702

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Define contrast in X-ray imaging.

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Define contrast in X-ray imaging.
Contrast is the difference in the degree of blackening between structures.
Define sharpness in X-ray imaging.
Sharpness is how well-defined the edges of structures are.
How are X-ray photons produced in an X-ray tube?
Electrons released by thermionic emission at the cathode are accelerated towards the anode; when they bombard the metal target, they rapidly decelerate and transfer their kinetic energy into X-ray photons.
State the equation for the minimum wavelength of X-rays produced by an accelerating voltage V.
$λ_{m i n} = \frac{h c}{e V}$
Why does the X-ray spectrum have a sharp cut-off at short wavelengths?
At the cut-off, each electron gives up all its kinetic energy in a single collision to produce one photon, corresponding to the maximum possible photon energy and so the minimum wavelength.
Why are aluminium filters used in medical X-ray imaging?
They absorb long-wavelength (soft) X-rays, which are less penetrating and more likely to be absorbed by the body without contributing to the image, reducing patient radiation dose.
Image sharpness can be improved by using a narrower X-ray beam and reducing scattering with a .......... or lead grid.
Image sharpness can be improved by using a narrower X-ray beam and reducing scattering with a collimator or lead grid.
True or False?
Harder (higher energy) X-rays always give better image contrast, whatever tissue is being imaged.
False.
Hard X-rays are used for imaging bone and soft X-rays for tissue; the correct level of hardness depends on the structure being imaged.
Define attenuation of an X-ray beam.
Attenuation is the reduction in the intensity of an X-ray beam as it passes through matter.
Why do bones appear white on an X-ray photograph?
Bones absorb X-ray radiation strongly, so little radiation passes through to reach the detector or film at that point.
State the equation for the attenuation of X-rays in matter, and give the meaning of each symbol.
$I = I_{0} e^{- μ x}$
- I₀ = intensity of the incident beam
- I = intensity of the emergent beam
- μ = linear absorption coefficient
- x = distance travelled through the material
Define half thickness.
The half thickness is the thickness of material that reduces the intensity of an X-ray beam of a particular frequency to half its original value.
As an X-ray beam passes through matter, its intensity decreases .......... with distance travelled.
As an X-ray beam passes through matter, its intensity decreases exponentially with distance travelled.
What two processes remove photons from an X-ray beam as it passes through the body?
- Absorption
- Scattering
What condition on the intensity ratio I/I₀ indicates good contrast between two tissues?
There is a large difference between the intensities, so the ratio I/I₀ is much less than 1.0.
True or False?
The linear absorption coefficient, μ, is the same for X-ray photons of any energy.
False.
The linear absorption coefficient depends on the energy of the X-ray photons, as well as the material.
Define a computed tomography (CT) scan.
A CT scan is an imaging technique in which an X-ray tube rotates around a stationary patient, taking X-ray images of the same slice at many different angles, which a computer combines to build a 3D image.
How is a 3D image built up during a CT scan?
- X-ray images of the same slice are taken at many different angles
- This process is repeated for successive slices
- A computer combines the slice images together to build a 3D image, which can be rotated and viewed from different angles
State three advantages of a CT scan compared with a single X-ray image.
- Produces images of much higher resolution
- Can distinguish between tissues with similar attenuation coefficients
- Produces a 3D image of the body
State two disadvantages of a CT scan compared with a single X-ray image.
- The patient receives a much higher radiation dose
- There are possible side effects from the contrast media used
A CT scanner takes X-ray images of the same slice, at many different ...........
A CT scanner takes X-ray images of the same slice, at many different angles.
True or False?
A CT scan produces only a single 2D image of the body.
False.
A CT scan combines many 2D images, taken of successive slices from multiple angles, to build up a single 3D image.

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