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Physics: the unnecessary science > Medical Imaging > Flashcards

Medical Imaging Flashcards

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1
Q

Describe the structure of an X-ray tube.

A

A vacuum tube has a heated filament (cathode) and the tungsten anode with a high potential difference of 200kV.

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2
Q

Describe how X-ray photons are produced from an X-ray tube.

A
  • Electrons are emitted from heated filament via thermionic emission.
  • Electrons accelerate through high voltage towards anode.
  • Electrons hit the anode and decelerate.
  • KE is converted into high frequency X-rays.
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3
Q

What is the attenuation of X-rays?

A

The decrease in intensity of electromagnetic radiation.

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4
Q

X-rays attenuate as they travel through matter. What are the four mechanisms that explain why they attenuate?

A
  • Simple scatter
  • Compton effect
  • Pair production
  • Photoelectric effect
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5
Q

Name three factors that affect X-ray attenuation.

A
  • Intensities before and after material (I)
  • Amount of radiation absorbed (μ)
  • Thickness of material (x)

I = Io e^-μx

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6
Q

How is contrast media used in X-ray imaging?

A

Materials that have large atomic numbers (eg Iodine and Barium) absorb a lot of X-rays.
They are injected in the bloodstream to make internal structures more identifiable.

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7
Q

What is the relationship between the attenuation coefficient and the proton number?

A

μ ∝ z^3

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8
Q

Describe the simple scatter mechanism.

A
  • X-ray photons have energies 1-20keV
  • X-ray interacts with electron in atom but does not have enough energy to remove electron.
  • X-ray photons are scattered

Not used in hospitals as X-ray machines use p.d greater than 20kV.

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9
Q

Describe the photoelectric effect mechanism.

A
  • X-ray photons have energies < 100 keV
  • X-ray is absorbed by electron and electron escapes

Used in hospitals as X-ray machines use 30-100kV

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10
Q

Describe the Compton scattering mechanism

A
  • X-ray photons have energies of 0.5MeV to 5MeV
  • Electron absorbs some of the X-ray and is removed from atom.
  • Left over X-ray becomes scattered X-ray photons.
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11
Q

Describe the pair production mechanism.

A
  • X-ray photons have energies of 1.02MeV or greater
  • Energy is absorbed by nucleus of atom.
  • Positron and electron are ejected.
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12
Q

Describe the components of a CAT scan.

A

X-ray tube on one side and detectors opposite rotate around the patient.

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13
Q

How does a CAT scan work?

A
  • X-ray tube generates a fan shaped beam towards the patient.
  • Beam produces a thin cross section of the patient.
  • Tube and detectors rotate around the patient.
  • X-rays attenuated by tissues in the body.
  • Transmitted X-rays are absorbed by the detectors
  • Computer creates an image using the slices of images.
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14
Q

Name advantages and disadvantages of CAT scan over X-ray.

A

Advantages:
- 3D image can be used to assess shape, size and different tissues.

Disadvantages:

  • Exposed to ionising radiation for longer
  • Takes longer
  • Must stay still for ages
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15
Q

Why are gamma sources used as medical tracers?

A

They are least ionising and can penetrate through the patient (so can be detected externally).

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16
Q

What are the characteristics of radioisotopes and why are they important?

A
  • Short half-life
  • High activity

To limit exposure to harmful radiation, to retrieve a clear image quickly and to limit the amount of radioactive substance used.

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17
Q

Name two radioisotopes used as medical tracers and when they are used.

A

Fluorine-18 -> PET scans

Technetium-99m -> Major organs (heart, brain etc)

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18
Q

Why are radioisotopes made at hospitals?

A

They have a very short half-life so cannot be stored.

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19
Q

Describe the gamma camera and its components.

A

Collimator- lead tubes that allow photons to travel in one direction.
Scintillator- photons hit scintillator and photons of visible light are emitted (1/10 chance gamma hit scintillator)
Photomultiplier tube- VL photons enter photomultiplier tube and electrical pulse is sent.
Computer- produces high quality image of concentrations of medical tracers in patient.

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20
Q

State one advantage of the gamma camera over the X-ray.

A

Gamma camera produces an image showing the FUNCTIONS and PROCESSES of the body.

21
Q

Describe the structure of the PET scan

A

A ring of gamma detectors that contain a photomultiplier tube and sodium iodide scintillator to produce a voltage pulse when gamma photons are incident.

22
Q

Explain how a PET scan works

A
  • Decaying radioactive medical tracer emits positron
  • Positron annihilates with electron
  • Two gamma photons are emitted, travelling in opposite directions
  • Detected by gamma cameras
  • Arrival time of photons indicates location of tissue / tumour
  • Computer produces a 3D image.
23
Q

Name the medical tracer used in PET scans

A

FDG which has fluorine-18 nuclei

24
Q

How do you produce fluorine-18 nuclei?

A

Collide protons with oxygen-18 nuclei to form fluorine-18 and neutrons.

25
Q

State the advantages and disadvantages of using PET scans

A

Advantages: non-invasive, demonstrates organ function, observe the effects of medicine.

Disadvantages: expensive because there must be facilities that can make medical tracers onsite.

26
Q

Define ultrasound

A

Longitudinal sound waves with a frequency greater than 20kHz

27
Q

What is the frequency for medical ultrasounds?

A

1MHz to 15MHz

28
Q

What is the piezoelectric effect?

A

A piezoelectric material can:

  • Generate an e.m.f when the material experiences stress (eg stretched or compressed)
  • Change shape (stretch or compress) when a voltage is applied.
29
Q

How does an ultrasound transducer work?

A

To generate ultrasound, alternating p.d is applied to a crystal. Crystal contracts and expands at the natural frequency. Therefore, it resonates and produces an intense ultrasound signal.

Transducer emits pulses of ultrasound

Transducer detects ultrasound when the ultrasound is incident on the crystal. Crystal expands and contracts which generates an e.m.f that’s detected by a computer.

30
Q

What is an ultrasound A-scan?

A

Single transmitter is used to emit a short pulse of ultrasound and detect the reflected ultrasound.

Used to measure eyes and check on retina

31
Q

What is an ultrasound B-scan?

A

Transducer is moved across the skin. The transducer emits and detects the ultrasound. Time and speed of the ultrasound is used to create a 2D image.

32
Q

Why are ultrasounds pulsed?

A

To allow time for the reflected signal to be received by the transducer.

33
Q

State the major difference between A-scans and B-scans?

A

A-scan is one dimensional. B-scan is two dimensional and composed of many A-scans.

34
Q

What is acoustic impedance?

A

Z = ρc

Product of density and speed of ultrasound.

35
Q

What happens when an ultrasound hits a boundary of two different media?

A

Some of the wave energy is reflected and the rest is transmitted (into body)

36
Q

State the advantages and disadvantages of ultrasounds.

A

Advantages: no exposure to ionising radiation, cheap, portable, quick, good at imaging soft tissues.

Disadvantages: can’t penetrate through bone (can’t show fractures), cannot pass through air spaces (no image on behind the lungs)

37
Q

What is the Doppler effect?

A

When there is a change in frequency of a wave as the source of waves move

38
Q

What is Doppler imaging?

A

Non-invasive technique to measure blood flow

39
Q

Explain how Doppler imaging works to measure the speed of blood flow.

A
  • Ultrasounds are sent into the blood vessels.
  • Ultrasound is reflected back.
  • Frequency is shifted and can help work out the speed of the blood flow
40
Q

Why is the transducer not placed perpendicular to the patient’s skin?

A

There would be no change in frequency because cos90 = 0

41
Q

How are ultrasound waves produced using piezoelectric crystals?

A

Alternating p.d is applied to a crystal. Crystal contracts and expands at the natural frequency. It resonates and produces an intense ultrasound signal.

42
Q

Explain why contrast materials are used in the diagnosis of stomach problems.

A

Barium / iodine have a large attenuation coefficient, making it ideal for imaging the outline of soft tissue.

43
Q

Describe how the piezoelectric effect is use to emit and receive ultrasound.

A

Alternating p.d makes crystal vibrate and emit ultrasound.

Ultrasound makes crystal vibrate which sends alternating p.d.

44
Q

Explain why a gel is used when a patient goes for an ultrasound.

A

Gel fills in air gaps between transducer and skin.

Gel and skin have a similar acoustic impedance, therefore, maximum transmission of ultrasound can pass through the body.

45
Q

Describe how ultrasound scanning is used to obtain diagnostic information. Include A-scans and B-scans.

A

Ultrasound is pulsed into the body.
Ultrasound is reflected at the boundary of tissue.
Delay time is used to determine thickness of tissue.

A-scan is an ultrasound in one direction.
B-scan uses a sensor in different positions to produce an image.

46
Q

Describe the principles of ultrasound scanning.

A
  • Transducer sends out PULSES of ultrasound waves
  • Ultrasound is reflected at a boundary of tissue
  • Time delay indicates thickness
47
Q

Explain what is meant by non-invasive techniques and state an advantage.

A

No surgery / incisions are needed

Less trauma
Less prone to the risk of infection

48
Q

How can ultrasounds be used to determine the speed of blood?

A
  • Ultrasound is reflected by moving blood cells.
  • The frequency of ultrasound changes
  • Relationship between freq change and speed of blood

Physics: the unnecessary science (19 decks)

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