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A Level Physics > Medical Imaging > Flashcards

Medical Imaging Flashcards

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1
Q

How are x-rays produced?

A

Fast moving electrons are decelerated by an interaction with a metal. The kinetic energy lost by the electrons is transferred into x-ray photons.

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2
Q

What is the basic structure of an x-ray tube?

A

An evacuated tube containing two electrodes. The cathode is a heater that produces electrons. These are accelerated towards the anode which is made from a target metal. X-rays are produced when electrons are decelerated by the target metal.

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3
Q

What does attenuation describe?

A

The decrease in intensity of x-rays as they pass through matter.

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4
Q

What is simple scatter attenuation?

A

The x-ray photon is scattered elastically by an electron.

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5
Q

What is photoelectric effect attenuation?

A

The x-ray photon disappears and removes an electron from an atom.

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6
Q

What is Compton scattering attenuation?

A

The x-ray photon is scattered by an electron, its energy is reduced and the electron is ejected from the atom.

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7
Q

What is pair production attenuation?

A

The x-ray photon disappears to create an electron-positron pair.

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8
Q

What is the attenuation coefficient of a substance?

A

How much it absorbs or attenuates x-rays.

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9
Q

What is a contrast medium used for?

A

Viewing the structure of soft tissues that would normally be impossible to see with x-rays.

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10
Q

How does a CAT scan work?

A

A fan shaped beam of x-rays is rotated around a patient, producing a slice-like image. This is repeated at several points through their body.

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11
Q

What are some advantages of CAT scans?

A

They can produce 3D images of the patient and they can distinguish between soft tissues.

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12
Q

What are some disadvantages of CAT scans?

A

They expose patients to very high levels of radiation and as they take so long, patients have to remain very still during the procedure.

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13
Q

What are medical tracers?

A

Substances such as fluorine-18 or technetium-99m that move to specific areas of the body and emit gamma photons.

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14
Q

What is the function of a collimator in a gamma camera?

A

A set of long thin tubes that only lets parallel photons pass through.

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15
Q

What is the function of a scintillator in a gamma camera?

A

A material that produces thousands of visible light photons when it is struck by a single gamma photon.

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16
Q

What is the function of photomultiplier tubes in a gamma camera?

A

Tubes that convert photons into an electrical pulse.

17
Q

What does a gamma camera image show?

A

It shows activity within the body rather than structure.

18
Q

How does a PET scan work?

A

The patient is injected with a medical tracer that produces two gamma photons travelling in opposite directions. The ring of detectors surrounding the patient detects these photons at different times and can use the time difference to calculate where the photons were created within the body.

19
Q

What is ultrasound?

A

A longitudinal wave with a frequency greater than 20kHz.

20
Q

What is the piezoelectric effect?

A

Special crystals that induce an emf when they are compressed or stretched. They also compress or stretch when a potential difference is applied across them.

21
Q

What is an ultrasound A-scan?

A

An ultrasound pulse is sent in a straight line through the body. At each boundary between two materials, the signal is partially reflected. The scanner can measure internal distances by timing how long it takes for the signal to return to the transducer.

22
Q

What is an ultrasound B-scan?

A

Multiple A-scans taken from several different angles to allow a full image to be created.

23
Q

What is the acoustic impedance of a substance?

A

The product of the density of the substance and the speed of sound in the substance.

24
Q

What is the intensity reflection coefficient of a substance?

A

The ratio of reflected intensity to incident intensity of ultrasound.

25
Q

What is the coupling gel used in an ultrasound scan?

A

A special gel with an acoustic impedance similar to that of skin so that very little ultrasound is reflected back at the boundary between the gel and the skin.

26
Q

How is the Doppler effect used in ultrasound images?

A

For determining the speed of blood flow in a patient.

A Level Physics (20 decks)

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