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OCR A level Physics > 6.5: Medical Imaging, F > Flashcards

6.5: Medical Imaging, F Flashcards

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1
Q

Explain one in which x-rays are produced?

A

When fast moving electrons with high levels of energy decelerate rapidly by smashing into a metal
or
When a fast moving electron ejects an inner shells electron of the metal atoms, outer shell electrons then move down levels and release energy(x-rays)

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2
Q

Explain another way in which x-rays are produced?

A

When a fast moving electron ejects an inner shells electron of the metal atoms, outer shell electrons then move down levels and release energy(x-rays)
or
When fast moving electrons with high levels of energy decelerate rapidly by smashing into a metal

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3
Q

Roughly what percentage of electrons’ kinetic energy is converted into x-rays? and what is the rest converted into

A

1% and the rest is converted into heat

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4
Q

Describe and briefly explain 1 way to increase the intensity of an x-ray beam

A

-Increase the tube voltage ∴ electrons gain more kinetic energy and can knock out electrons from deeper shells
or
-Increase the current supplied to the filament ∴ temp rises, liberating more electrons per second, producing more x-ray photons per second

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5
Q

Describe and briefly explain an alternative way to increase the intensity of an x-ray beam

A

-Increase the current supplied to the filament ∴ temp rises, liberating more electrons per second, producing more x-ray photons per second
or
-Increase the tube voltage ∴ electrons gain more kinetic energy and can knock out electrons from deeper shells

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6
Q

Define: Attenuation

A

Gradual decrease in intensity

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7
Q

List the 4 types of interactions x-rays can have with matter

A
  • Simple scattering
  • Photoelectric effect
  • Compton scattering
  • Pair production
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8
Q

Explain what is simple scattering (in terms of an x-rays)

A

Low-energy x-ray encounters the electrons in an atom and is does not contain sufficient energy to cause ionisation and ∴ is deflected

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9
Q

Explain what the photoelectric effect is

A

A photon is absorbed by an electron which is then ejected from out of the atom. Another electron then fill the gap, which emits a photon

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10
Q

Explain what Compton scattering is

A

A high energy photon knocks an electron out of an atom, which causes the photon to lose energy and be scattered.

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11
Q

Explain what Pair production is

A

Extremely high energy photon decays into an electron and positron from interaction with the nucleus of an atom

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12
Q

Explain the purpose of using a contrast media

A

If tissues in a region of interest have a similar attenuation coefficients, a contrast media allows you to see details clearly in x-ray images as they have high density which allows for a bigger contrast (e.g.barium)

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13
Q

What CAT(Computerised axial tomography) scan

A

Process of using multiple x-ray scans to produce images of ‘slices’ through the body, creating a 3D image.

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14
Q

Briefly describe the process of a CAT scan

A

1) Patient lies on table which slides in and out of a ring made up of detectors and rotating x-ray beams
2) X-ray beam fans out and rotates around the body, being picked up by the detectors
3) Computer then calculates total attenuation caught by part of the body ∴ creating a high quality image

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15
Q

List the advantages of using a CAT/CT scan over regular x-rays

A
  • Accurate and doesn’t get obstructed by other structures of the body
  • Sensitive to changes in density
  • 3D images allows you to rotate image and edit what you want to see
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16
Q

What is the purpose of medical tracers

A

A radioactive substance that is used to diagnose functions of organs. Unlike x-rays which only display the structure, they display the structure and function

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17
Q

What do medical tracers usually consist of

A

Radioactive isotopes (such as technetium-99 or fluorine-18) bound to a substance that is used by the body (such as water or glucose)

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18
Q

How is a tracer inserted and guided into a region of interest

A

Injected or swallowed and will go where ever the bounded substance would normally go.

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19
Q

How are gamma photons detected

A

A tracer emits gamma photons which are picked up by a gamma camera

20
Q

Define: Collimator

A

Device for producing a parallel-sided beam of EM radiation (A bit like a polariser)

21
Q

State and explain 3 uses of Tracers

A
  • Show areas of damaged tissue by detecting decreased blood flow
  • Identify active cancer tumours by detecting increased metabolic activity
  • Show blood flow and activity in brain which helps research and treat neurological conditions.
22
Q

List the 5 main parts of a gamma camera

A
  • Lead shield
  • Lead collimator
  • Sodium iodide crystal/Scintillator
  • Photomultiplier tubes
  • Electric circuit
23
Q

Define: Scintillator

A

A material that produces (many photons of) visible light when struck by a high-energy photon

24
Q

Define: Photomultiplier tube

A

A device used to detect flashes of light from the crystals and turns them into pulses of electricity

25
Q

What is the purpose of a Lead shield in a gamma camera

A

To stop radiation from other sources entering the camera

26
Q

Define: Positron emission tomography(PET)

A

The use gamma photons, produced by positrons annihilating with electrons inside the body mapping out biologically active areas within the body

27
Q

Describe the a skeleton process of a PET scan

A

1) Patient injected with radioactive tracer
2) Positrons emitted by radioactive isotope annihilate with electrons in organs creating pairs of gamma rays at π radians to each other
3) Detectors around the body record these and build up a map of radioactivity

28
Q

How can a doctor use the information given by a PET

A

Distribution of radioactivity matches up with metabolic activity (tracer used by harder working cells) ∴ you can diagnose illnesses(e.g. higher activity of cancer cells)

29
Q

State some positives of using a PET scan

A
  • Diagnostics without the need for surgery
  • Radioactive tracers have a short half-life
  • Equivalent to a standard x-ray exposure unlike(CAT, 5 years worth of background radiation)
30
Q

State some negatives of using a PET scan

A
  • Limited window for scanning
  • Expensive
  • Long(2-4 hours)
  • Not as detailed as CAT
31
Q

Describe 3 possible side effects from ionising radiation

A
  • Cell mutation/cancerous tumours by altering/damaging cell DNA
  • Cell sterility by halting cell reproduction
  • Cell death. Destruction of cell
32
Q

What are some of the macroscopic(visible to the naked eye) effects of ionising radiation

A
  • Tumours
  • Skin burns
  • Sterility
  • Radiation sickness
  • Hair loss
  • Death
33
Q

When is ionising radiation used

A

When the benefits outweigh the risks

34
Q

Describe 2 properties of Ultrasound

A
  • Longitudinal wave

- frequencies of >20,000 (higher f than humans can hear)

35
Q

What range of frequencies in ultrasounds are used for medical purposes

A

1 - 15 MHz

36
Q

Define: Transducer

A

E.g. Microphone, converts non-electrical signals into an electrical signal

37
Q

Define: Piezoelectric effect

A

Change in volume of a material when a voltage is passed through it opposite faces
or
Production of an induced e.m.f when certain crystals are placed under stress

38
Q

Define: Acoustic impedance

A

Defined by equation Z=pc, where p = density and c is the speed of sound. Units are kg/m^2/s

39
Q

Define: Impedance matching

A

When 2 substances have similar or identical acoustic impedances ∴ reduces the intensity of reflected ultrasound at the boundaries between them

40
Q

List the main advantages of using Ultrasound imaging

A
  • No known hazard
  • Real-time images
  • Cheap and portables
  • Quick
41
Q

List the main disadvantages of using Ultrasound imaging

A
  • Cant penetrate bone
  • Cannot pass through air
  • No detail on solid masses
42
Q

What is the purpose of a coupling medium

A

Displaces air and has an impedance much closer to that of body tissue(usually a gel)

43
Q

How are piezoelectric crystals used

A

When an alternating p.d. is applied across a crystal, the crystal vibrates at the same frequency. It can act as the transmitter and receiver of ultrasound

44
Q

What is an A-scan

A
  • Amplitude scan a measuring system, which sends short pulses of ultrasound into the body simultaneously with an electron beam sweeping across a screen.
  • Position of reflected pulse reflects time the ‘echo’ took to return allowing you to find the distance between structures
45
Q

What is a B-scan

A
  • Brightness scan, electrons sweep down the screen instead of across
  • Amplitude of reflected pulses is displayed as the brightness of the spot
46
Q

How are ultrasounds affected by the doppler effect

A

Ultrasounds reflected at an angle moving cells undergo a change in frequency. The change in frequency allows doctors to find the speed at which the cells are moving

OCR A level Physics (22 decks)

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