Chapter 27 Medical imaging Flashcards

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1
Q

Who discovered X-rays?

A

Wilhelm Rontgen

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2
Q

How does an x-ray tube work?

A

A tube is evacuated and a cathode electrode is placed at one end which is heated with electricity. An anode is placed at the other end of the tube and is made from tungsten. A high voltage supply gives each electrode a charge and the heat from the cathode causes electrons to be released (called thermionic emission) and these are accelerated towards the tungsten target. When they RAPIDLY DECELERATE on impact, x-rays are produced. These are filtered out of a window so the x-rays only cover a certain area

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3
Q

What happens to the tungsten in the x-ray tube and why does this happen?

A

It becomes very hot and needs to be cooled with oil or by rotation the tungsten target so that the heat is spread out.
This happens because only 1% of the energy of the electrons is turned into x-rays and most is turned into heat

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4
Q

How many electrons does it take to release 1 x-ray photon?

A

One electron produces one x-ray photon

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5
Q

What is the equation that is used for x-rays?

A
λ = hc / eV
λ = wavelength of x-rays produced
h = planks constant
c = speed of light
e = elementary charge
V = potential difference
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6
Q

Maximum energy of x-ray photon = …

A

= maximum kinetic energy of electron

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7
Q

Increasing the current…

A

… Increases the intensity of the x-rays

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8
Q

What does attenuation mean?

A

This is the change in the intensity of the electromagnetic waves as it passes through matter

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9
Q

What are the 4 ways that x-ray photons interact with matter?

A

Simple Scatter
Photoelectric effect
Compton Scattering
Pair production

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10
Q

Describe x-ray Simple scattering

A

An x-ray photon collides with an electron of an atom and the x-ray gets scattered because the x-ray has not enough energy to remove the electron. The energy of the photon needs to be in the range 1 - 20 keV

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11
Q

Descrive x-ray Photoelectric effect

A

An x-ray photon collides with an electron and this causes the electron to be removed from the atom because it has enough energy to escape the attractive force of the nucleus. The energy of the photon needs to be less than 100keV

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12
Q

Describe x-ray Compton scattering

A

An x-ray photon collides with an electron and this causes a low energy photon to be scattered and the electron to be removed from the atom. The energy of the x-ray needs to be between 0.5 - 5 MeV. Some of the energy goes into removing the electron and some of the energy goes into producing the low energy photon

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13
Q

Describe x-ray Pair production

A

This is when an x-ray photon interacts with the nucleus of the atom and disappears and produces an electron and a positron. The energy of the x-ray needs to be equal to or greater than 1.02MeV

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14
Q

How does that thickness of a substance change a photons intensity?

A

It falls exponentially

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15
Q

What is the equation for the change in intensity of a photon passing through a substance?

A
I = I0e^-µx
I = intensity
I0 = Starting intensity (before it entered the substance)
µ = The attenuation coefficient
x = thickness of the substance
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16
Q

What is the attenuation coefficient?

A

This is coefficient for a substance which is a measure of how easily it absorbs photons

17
Q

How does the atomic mass affect the attenuation coefficient?

A

µ ∝ Z^3
µ = attenuation coefficient
Z = the atomic mass

18
Q

With atomic mass affecting the amount of absorption of x-rays, what can this be used for?

A

This can be used to image the body and body functions. Safe elements injected or taken into the body and this can then be seen on an x-ray image

19
Q

How can x-rays be used for therapy?

A

They can be used to kill cancer by firing high energy x-rays which can kill cancer. They use Compton scattering and pair production to kill the cancer