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7th Form > Medical Physics > Flashcards

Medical Physics Flashcards

1
Q

What is ultrasound?

A

A longitudinal wave with frequency greater than 20 kHz.

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2
Q

What is the piezoelectric effect?

A

The material expands or contracts when a p.d. is applied across its opposite faces.

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3
Q

Describe the X-ray attenuation mechanism of simple scatter

A

X-ray (photon) is scattered by an atomic electron.

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4
Q

Describe the X-ray attenuation mechanisms of pair production.

A

An X-ray (photon) transforms into an electron and positron pair.

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5
Q

What is impedance matching?

A

Using a coupling medium between the transducer and body with an impedance closer to that of the body tissue to reduce reflections.

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6
Q

Define acoustic impedence.

A

The density of the material multiplied by the speed of sound in the material. (Can use equation if you define all symbols)

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7
Q

What is a piezoelectric transducer?

A

A device that can convert sound waves into alternating voltages and can also convert alternating voltages into sound waves. We use them to produce ultrasound.

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8
Q

What are the advantages of ultrasound imaging?

A

There are no known hazards (especially, no exposure to ionising radiation). Good for imaging soft tissues as you can produce real time images. Relatively cheap and portable. Quick procedure during which the patient can move.

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9
Q

What are the disadvantages of ultrasound imaging?

A

Ultrasound doesn’t penetrate bone. Ultrasound cannot pass through air spaces, so can’t produce images from behind the lungs. Can’t give detail about solid masses. Can’t give information about any solid masses found.

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10
Q

What are the two types of ultrasound scan.

A

Amplitude-scans (A-scans) and Brightness-scans (B-scans).

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11
Q

What are ultrasound A-scans used for?

A

Working out distances between structures in the body.

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12
Q

What are ultrasound B-scans used for?

A

Producing a two-dimensional image of a section of the body.

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13
Q

What is the Doppler effect?

A

An effect which produces a change of observed frequency of a wave for an observer that is moving with a speed relative to the source.

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14
Q

How can ultrasound imaging be used to detect the speed of blood in the artery?

A

The transducer is placed at an angle to the arm or artery and ultrasound is reflected by the moving blood cells. The wavelength or the frequency of the reflected ultrasound is altered. Using the Doppler effect equation, the change in frequency is proportional speed of the blood.

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15
Q

What are the components of a gamma camera?

A

Collimator, scintillator, photomultiplier tubes, computer and display.

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16
Q

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

A

Stops radiation from other sources entering the camera.

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17
Q

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

A

A piece of lead with thousands of vertical holes in it such that only gamma rays parallel to the holes can pass through it.

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18
Q

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

A

Emits a flash of light whenever a gamma ray hits it

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19
Q

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

A

Detecting the flashes of light from the scintillator and converting them into electricity.

20
Q

What is the function of the computer and electronic circuit in a gamma camera?

A

The circuit collects signals from the photomultiplier tubes and sends them to the computer, which displays them as an image.

21
Q

What is the shield in a gamma camera made of?

A

Lead

22
Q

What is the collimator in a gamma camera made of?

A

Lead

23
Q

What is the scintillator in a gamma camera made of?

A

Sodium iodide

24
Q

What isotope is generally used in PET scans?

A

Fluorine-18

25
Q

What isotope is generally used as a medical tracer?

A

Technetium-99m

26
Q

Why is Technetium-99m suitable as a medical tracer?

A

Produces gamma rays and has a half life of 6 hours (long enough for data to be recorded but short enough to limit exposure).

27
Q

Why is Fluorine-18 suitable for use in PET scanning?

A

Undergoes beta-plus decay to produce positrons and has a short half-life (110 minutes) to limit exposure.

28
Q

How does a PET scanner work?

A

A radioactive substance that beta-plus decays is injected into the body. The substance is given time to move through the body. Positrons emitted by the isotope collide with electrons in the organs, causing two gamma photons to be produced by annihilation. Detectors are used to record the gamma rays. This is used to generate a radioactivity map of the body, which matches the metabolic activity of the body.

29
Q

What are the microscopic dangers of using radiation in medicine?

A

Cell mutations and cancer. Cell sterility Cell death

30
Q

What are the macroscopic dangers of using radiation in medicine?

A

Tumours, skin burns, sterility, hair loss, radiation sickness and death.

31
Q

What is an advantage of using medical tracers?

A

A diagnosis can be made without the patient having to undergo surgery.

32
Q

What are the advantages of CAT scans over regular X-ray imaging?

A

CAT scans produce more detailed images, especially for soft tissue. The data from CAT scans can be used to produce a 3D image of the body.

33
Q

What does CAT stand for?

A

Computerised axial tomography

34
Q

What radiation does a CAT scan use?

A

X rays

35
Q

How does a CAT scan work?

A

Patient lies on a table with a ring of detectors around him and a rotating X-ray source. An X-ray beam fans out from the source and rotates around the body. A computer calculates how much attenuation has been caused by each part of the body and produces an image.

36
Q

How are X-rays produced?

A

A beam of electrons, produced by thermionic emission from a cathode, is aimed at a piece of tungsten.

37
Q

What is the purpose of a contrast material in an X-ray scan?

A

To distinguish different tissues when they have similar absorption coefficients.

38
Q

What is the beam intensity?

A

The power per unit area transferred by the beam.

39
Q

How can you improve the beam intensity of an X-ray?

A

Increase the tube voltage. Increase the current provided to the filament of the thermionic emission gun.

40
Q

State the mechanisms that cause X-ray attenuation.

A

The Photoelectric effect, the Compton effect and pair production.

41
Q

Give two examples of contrast medium.

A

Barium and iodine.

42
Q

What is Compton scattering?

A

A high energy photon knocks an electron out of an atom, causing the photon to lose energy and scatter.

43
Q

What is the Photoelectric effect?

A

A photon with around 30 keV is absorbed by an electron in an atom and the electron is ejected. Another electron falls into the hole left by the electron and this emits a second photon with lower energy.

44
Q

What is the difference between Compton scattering and the photoelectric effect.

A

In the PE effect, the photon is fully absorbed and a new photon is produced by another electron. In Compton scattering, the energy of the photon is only partly absorbed by the electron. Compton scattering uses much higher energies.

45
Q

What is pair production?

A

A very high energy photon spontaneously creates an electron and a photon.

46
Q

Why is an X-ray source rotated rapidly?

A

Only 1% of the energy is used to produce X-rays, the rest goes into heat. It is rotated to cool it.

7th Form (20 decks)

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