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Physics Revision > Module 6.5 - Medical Imaging > Flashcards

Module 6.5 - Medical Imaging Flashcards

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1
Q

How does an x-ray tube generate and accelerate electrons into the target material?

A
  • An x-ray tube contains two different circuits
  • A high current is passed through a thermionic emission circuit containing a filament. The filament heats up, giving electrons in it enough energy to escape and be emitted from the filament
  • Another circuit contains a ‘focusing’ cathode behind the filament and tungsten plate acting as an anode in front with a high p.d. between the two
  • The emitted electrons are accelerated by this p.d. across the gap between the filament and anode and strike the tungsten anode
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2
Q

Why do electrons in an x-ray tube striking a tungsten target emit x-rays?

A

Two ways:

1) Bremsstrahlung Radiation
- When electrons strike the tungsten target most of their kinetic energy is converted to heat
- However some of their kinetic energy (~1%) is converted to electromagnetic radiation in the form of x-ray photos
2) Energy level jumps
- An incoming electron can knock out other electrons from the inner shells of tungsten atoms
- Electrons in the outer shell of the atom will move into the vacancy in the inner shell by releasing energy in the form of an x-ray photon

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3
Q

What does the spectrum of x-ray emissions look like for an x-ray tube?

A

The tungsten anode will emit a continuous spectrum of x-ray radiation due to Bremsstrahlung Radiation. In this spectrum there will be peaks in intensity at line spectra characteristic to Tungsten due to energy level jumps.

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4
Q

How do you calculate the maximum energy of X-ray photons produced by an X-ray tube?

A

Emax = eV = 1/2mv^2

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5
Q

How do we prevent the Tungsten anode in an x-ray tube form overheating?

A
  • The tungsten plate is rotated around so the heat gets spread out across it’s surface.
  • The plate is mounted on copper which conducts heat effectively
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6
Q

How/why can you increase the Intensity of an x-ray beam?

A

Increasing the tube voltage - electrons gain more energy to knock out electrons in lower shells, or emit higher energy x-rays when decelerating
Increasing the filament current - More electrons emitted per second, more x-rays per second, more energy per second per unit area

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7
Q

Describe simple scatter

A

Low energy x-rays are absorbed by electrons in atoms, but don’t have enough energy to move the electron so the x-ray is just re-emitted without any change to its energy.

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8
Q

Describe the photoelectric effect for x-ray attenuation

A

An x-ray photon is absorbed by an electron in an atom and the electron uses this energy to escape from the atom. If this was a lower energy level electron a higher energy level one will emit a visible light photon and drop down to fill its space.

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9
Q

Describe Compton scatter

A

A high energy x-ray photon interacts with an electron inside the atom causing it to be ejected from the atom at high velocity. The x-ray photon gets scattered with reduced energy.

If this was a lower energy level electron a higher energy level one will emit a visible light photon and drop down to fill its space.

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10
Q

Describe how pair production affects x-ray attenuation?

A

A high energy x-ray photon decays into an electron positron pair

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11
Q

What is the equation for the thickness of material at which x-ray intensity halves?

A

x½ = ln(2)/μ

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12
Q

What is the purpose of a contrast medium?

A

If tissues in an area of interest have similar attenuation coefficients a contrast media can be used to improve the visibility of their internal structures.

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13
Q

How do contrast media produce a better image?

A

Contrast media have high atomic numbers, so absorb more x-rays. This lets certain internal structures show up more clearly. They can also be tracked as they move around the body.

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14
Q

What are the two contrast media used for x-rays?

A

Barium, Iodine

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15
Q

What is Iodine used for in x-rays?

A

Iodine is used as a contrast medium in liquids, for example to view blood flow.
e.g. an organic compound of iodine can be injected into blood vessels so doctors can diagnose blockages

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16
Q

What is barium used for in x-rays?

A

Barium is used to image the digestive system. It is swallowed by a patient before an x-ray and can show the structure of the digestive system.

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17
Q

How does a CAT scan work?

A

1) The patient lies of a table that can be slid in and out of a ring
2) The ring is made up of detectors and a rotating x-ray tube
3) A fan-shaped x-ray beam is emitted and rotated around the patient’s body. It is picked up by the detectors.
4) A computer works out how much attenuation has been caused by each part of the body to produce a high quality 2D image of a ‘slice’ through the patients body.
5) By sliding the patient through the ring, taking multiple 2D ‘slice’ scans, a 3D image can be made

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18
Q

What advantages do CAT scans have over conventional x-rays?

A
  • The can distinguish between soft tissue with similar attenuation coefficients easier
  • Can be used to create a 3D image making it easier to identify positions, size etc of organs
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19
Q

What disadvantages do CAT scans have compared to conventional x-rays?

A
  • Much higher radiation dose
  • Patients must stay very still
  • much more expensive
  • Takes much longer
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20
Q

At what energies does simple scatter occur?

A

1-20KeV

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21
Q

At what energies does photoelectric effect occur? (X-rays)

A

20-100KeV

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22
Q

At what energies does Compton scattering occur?

A

0.5-5.0MeV

23
Q

At what energies does pair production occur? (x-rays)

A

> = 1.1 MeV

24
Q

What effect is the dominant cause of x-ray attenuation in medical imaging?

A

Photoelectric effect as medical imaging typically uses p.d.s of 30-100KeV

25
Q

What is a medical tracer?

A

Medical tracers - A radioactive substance bound to a substance used by the body, used to show tissue or organ function

26
Q

What is the advantage of using a medical tracer over an x-ray?

A

Unlike x-rays which just show structure, medical tracers can show organ structure and function

27
Q

What are the two medical tracers we need to know about?

A

technetium-99

fluorine-18

28
Q

What is the structure of a medical tracer?

A

A medical tracer consists of a radioactive isotope (radioisotope) bound to a substance used by the body (e.g. glucose)

29
Q

How is a medical tracer used to diagnose patients?

A

1) The medical tracer is injected into/swallowed by the patient
2) The radioisotope travels with the substance to wherever it is normally used by the body
3) The radiation emitted by the radioisotope can be recorded by a gamma camera/pet scan to create a map of radiation.

30
Q

Give an example of a condition that could be diagnosed with a medical tracer and how?

A
  • Areas of damaged tissue in the heart by detecting areas of decreased blood flow
  • Identify active cancerous tumors by showing metabolic activity in tissue
  • Show blood flow and activity in the brain to help treat neurological conditions
31
Q

Why is technetium-99m used for medical tracers?

A

It is widely used for medical tracers because:

  • it emits gamma radiation
  • Has a half life of only 6 hours
  • Decays into a much more stable isotope
32
Q

Why is fluorine-18 used for medical tracers?

A
  • It is used in PET scans as it undergoes β+ decay (releasing positrons)
  • It has a short half life of only 110 minutes
33
Q

What are the 5 parts of a gamma camera and what do they do?

A

1) Lead enclosure - stops radiation from other sources form entering the camera
2) Lead Collimator - only lets gamma rays parallel to the camera pass through so that their origin can be determined
3) Scintillator - Crystal that emits a flash of light whenever a gamma ray hits it
4) Photomultiplier tube - Converts the flashes of light into an electric signals and amplifies the signals
5) Electronic circuit - Collects signals from photomultiplier tube
6) Computer - processes signals to form a 2D image

34
Q

What are some pros and cons of gamma cameras?

A 
Pros:
- Cheaper than PET scan
- No need for surgery
Cons:
- Fairly expensive
- Uses ionising radiation
35
Q

What are some pros and cons of PET scans?

A

Pros:
- No need for surgery
- Short half life isotopes lower exposure
- Only uses weakly ionizing gamma radiation
Cons:
- Limited time in which patient can be scanned due to short half life
- Expensive

36
Q

How does a PET scan work?`

A

1) The patient is injected with a positron emitting radioisotope
2) It is given time to move through the body
3) Positrons emitted by the radioisotope annihilate with electrons in organs to produce gamma ray pairs moving in OPPOSITE directions
4) Detectors in a ring around the body record the gamma ray pairs and a computer uses these to build up a map of radioactivity in the body (a ‘slice’ through the patient) BY LOOKING AT THE DIFFERENCE IN THEIR ARRIVAL TIMES
5) The distribution of radiation matches up with metabolic activity because cells doing more work use more of the substance found in the tracer

37
Q

What is the scintillator used by a gamma camera?

A

A sodium iodide crystal

38
Q

What is ultrasound?

A

Ultrasound waves are longitudinal waves with frequencies higher than the range of human hearing (> 20 kHz)

39
Q

What are the typical ultrasound frequencies used for medical purposes?

A

1 to 15 MhZ

40
Q

Briefly, how does ultrasound imaging work?

A

When ultrasound meets a boundary between materials some is reflected and some passes through undergoing refraction. By sending ultrasound pulses and timing how long for the reflected waves to return you can determine the distance of these boundaries

41
Q

What are some advantages of ultrasound imaging?

A
  • No known hazards
  • Good for imaging soft tissue, as you can obtain real time images
  • Relatively cheap and portable
  • Scan is quick and patient can move during scan
42
Q

What are some disadvantages of ultrasound imaging?

A
  • Ultrasound doesn’t penetrate bone so can’t be used to detect fractures or look at brain
  • Ultrasound can’t pass through air spaces in the body
43
Q

Why is a coupling medium necessary for ultrasound imaging?

A

Soft tissue has a very different acoustic impedance than air so almost all ultrasound energy is reflected at the surface of the body

44
Q

What does a coupling medium do?

A

A coupling medium displaces the air between the transducer and body. It has a very similar acoustic impedance to that of body tissue so little energy is lost at the surface of the body

45
Q

How is a piezoelectric crystal used to receive ultrasound?

A

Receiving ultrasound:

1) Piezoelectric crystals produce a potential difference across them when they are deformed
2) This is because the centers of symmetry of their electric charges is displaced
3) Incoming sound waves stretch and squash the crystal making it produce an alternating p.d.

46
Q

How is a piezoelectric crystal used to produce ultrasound?

A

1) When a potential difference is applied across the opposite faces of a piezoelectric crystal it deforms
2) If an alternating p.d. is applied across the crystal will vibrate at the same frequency as the alternating p.d.
3) This lets you convert analog voltages to sound waves

Not sure if we need to know why this works. I think it’s because the electric charges in the crystal are attracted to their opposing charges on opposite sides of the crystal created by the applied p.d. This makes the crystal stretch/squash due to its internal structure.

47
Q

Why is the thickness of the PZT crystal in an ultrasonic transducer half the wavelength of ultrasound it produces?

A

Ultrasound with wavelength double the thickness of the PZT crystal will cause it to resonate producing a larger signal when receiving. Applying an alternating p.d. of this frequency across the crystal will also cause it to resonate producing ultrasound waves with greater intensity.

48
Q

Why is the PZT crystal in an ultrasonic transducer heavily damped?

A

Heavily damping the PZT crystal produces short pulses of ultrasound and increases the resolution of the device for receiving ultrasound.

49
Q

What is the structure of an ultrasonic transducer?

A

1) Acoustic window (Protects PZT crystal)
2) PZT crystal
3) Heavy Damping

50
Q

What is the unit for acoustic impedance?

A

kg m^-2 s^-1

51
Q

What is the difference between an A-scan and a B-Scan

A
  • An A-Scan shows intensity of ultrasound using a display of voltage against time. Whereas a B-scan displays intensity using the brightness of a dot on a screen
  • A-scan: electron beam sweeps across screen, B-scan: down screen
52
Q

Why would you use a B-scan over an A-scan?

A

Using a linear array of ultrasonic transducers a two-dimensional image can be produced by moving it across the patient’s skin using a B-scan.

53
Q

Why can you determine the speed of blood flow in a patient using ultrasound?

A
  • Ultrasound waves reflected at an angle to moving cells undergo a change in frequency due to the Doppler effect.
  • By measuring this change of frequency you can determine the speed at which the blood cells are moving
54
Q

Why would you use an A-scan over a B-scan?

A
  • The horizontal distance between reflected pulses (appearing as vertical voltage deflections on an A-scan) is the time the echo took to return
  • If you know the speed of sound in the material you can calculate the distance the ultrasound wave traveled to get between the two boundaries (twice)
  • An a scan can be used to determine e.g. the thickness of bone or distance between lens and retina in the eye

Physics Revision (36 decks)

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