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A Level Physics OCR > 6.5 Medical Imaging > Flashcards

6.5 Medical Imaging Flashcards

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1
Q

What is the basic structure of an X-ray tube;

A

components – heater (cathode) where electrons are emitted by hot filament, cup shaped to focus beam on target metal

Anode (target metal)

High voltage supply

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2
Q

How are x-ray photons produced from an x-ray tube?

A

X-ray tubes produce X-rays by accelerating electrons in a high-voltage electric field then rapidly decelerating them via collisions with a hard metal anode (positive electrode) e.g. tungsten

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3
Q

What happens when the electrons smash into an anode like tungsten?

A

They decelerate and some of the KE converts into EM energy, in the form of x-ray photons. The tungsten anode emits a continuos spectrum of x-ray radiation.

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4
Q

Why is the tungsten anode rotated in a x-ray tube?

A

most of the KE of the electrons are converted to thermal energy, so rotating avoids overheating.

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5
Q

Describe what happens in terms of electrons when an electron hits tungsten atom

A
  • Incoming electron
  • inner “tungsten” electron ejected
  • outer electron drops down to fill gap
  • x-ray photon emmited
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6
Q

What are 2 ways of increasing the intensity of X-ray beams?

A
  1. Increase tube voltage - Gives electrons more KE so knocks out deeper shell electrons in tungsten atom
  2. Increase current supplied to filament - Temp rises, frees more electron per second producing more x-ray photons per second
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7
Q

The intensity of an X-ray beam decreases (attenuates) exponentially with what?

A

The intensity of an X-ray beam decreases (attenuates) exponentially with distance from the surface(x)

Intensity=Io e ^ -(μx)

x is in cm
μ is in cm^-1

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8
Q

How are X-rays attenuated?

A

By absorption and scattering

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9
Q

What are the 3 causes of X-ray attenuation?

A
  1. The photoelectric effect
  2. Compton scattering
  3. Pair production
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10
Q

What is the photoelectric effect?

A

Photon absorbed by electron, which is ejected from its atom. The gap in the shell is filled by another electron, which emits a photon.

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11
Q

What is the Compton effect?

A

(X-ray) photon interacts with an (orbital) electron

The (scattered) photon has a longer wavelength
AND
The electron is ejected (from the atom at high speed)

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12
Q

What is pair production?

A

high energy Incoming photon decays and produces electron-positron pair

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13
Q

How much energy is absorbed by a material is dependent upon what?

A

Atomic number

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14
Q

What is simple scattering (another X-ray attenuation mechanism)

A

X-rays of energy 1-20 keV will reflect off layers of atoms or molecules in the material as they do not have enough energy to undergo more complex processes

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15
Q

Describe the use of contrast media when X-rays are used to produce images of internal body structures (4 marks)

A
  1. Iodine / barium (used as contrast material)
  2. *High Z number / large attenuation coefficient / large absorption coefficient (used to improve image contrast)
  3. Contrast media are ingested / injected into the body
  4. *Scan shows outline / shape of soft tissue
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16
Q

Describe the use of contrast media when X-rays are used to produce images of internal body structures (4 marks)

A
  1. Iodine / barium (used as contrast material)
  2. High Z number / large attenuation coefficient / large absorption coefficient (used to improve image contrast)
  3. Contrast media are ingested / injected into the body
  4. Scan shows outline / shape of soft tissue
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17
Q

What components are needed for CAT scan

A

Computerised axial tomography (CAT) scanning; components – rotating Xtube producing a thin fan-shaped X-ray beam, ring of detectors, computer
software and display

18
Q

What are 3 advantage of CAT scan over X-ray image?

A
  1. X-ray image is 2D / CT scan produces 3D image
  2. Greater detail / definition / contrast with CT scan / ‘soft tissues can be seen’
  3. Image can be rotated
19
Q

Describe a CAT scan

A

X-ray tube rotates around (the patient) / X-ray beam passes through the patient at different angles

A thin X-ray beam is used

Image(s) of slice(s) / (cross) section(s) through the patient are taken

X-ray tube moves / spirals along (the patient)

The signals / information(from the detectors) are used by the computer (and its software) to produce a 3D image

20
Q

Explain how the production of a CAT scan image differs from that of a simple X-ray image

A
  1. Simple X-ray is one directional / produces single image
  2. CT image(s) taken at different angles / X-ray tube is rotated
  3. Computer processes data / image constructed from
    many slices
21
Q

Name 2 medical tracers and what they do

A

Technetium-99m emits γ radiation, halflife of 6 hours, decays to stable isotope

flourine-18 used in pet scans, half life 110 mins, undergoes beta plus decay

22
Q

Describe the use of medical tracers to diagnose the condition of organs

A
  1. Tracer is injected into the body / placed inside the
    body / circulates the body
  2. Tracer is absorbed by organ / shows blockage
  3. Beta detector / gamma camera (is used to detect radiation from the body)
23
Q

What are the components in a gamma camera? (5)

A
  1. Lead shield-stops radiation from other sources entering the camera
  2. Lead collimator - A piece of lead with thousands of vertical holes in it - only γ rays parallel to the holes pass thorugh
  3. Sodium iodide crystal - emits a flash of light (scintillates) whenever a γ-ray hits it
  4. Photomultiplier tubes - Detect the flashes of light from the crystal and turn them into pulses of electricity
  5. Electronic circuit - collects the signals from the photomultiplier tubes and sends them to a computer for processing unto an image to help diagnose patients
24
Q

Evauluate how useful gamma cameras are

A
  • Diagnose patience without surgery
  • Cheaper then pet scans but still expensive
  • uses ionising radiation which is bad for you
25
Q

Explain the basic principles of PET scanning, including how the image is formed (5 marks)

A
  1. The brain / body is surrounded by a ring of (gamma)
    detectors /gamma camera(s).
  2. The positrons (from the injected F-18 nuclei) annihilate electrons.
  3. The annihilation of a positron and an electron produces two (identical gamma) photons travelling in opposite directions.
  4. The delay time between these two photons / gamma
    rays is used to determine the location of the annihilation / F-18 / tracer.
  5. Computer connected to detectors / gamma camera
    and an image is formed by the computer (using the
    electrical signals from the detectors).
26
Q

Explain Diagnosis using PET scanning

A

Distribution of radioactivity matches with metabolic activity because more of the radioactive glucose (or whatever injected) is taken up and used by cells doing more work.

Cells doing more work help diagnose higher activity of cancer cells

27
Q

Evaluate PET scans

A
  • Diagnose patience without surgery
  • radio tracers have short half life so exposed for short time
  • limited time when patient can be scanned
  • Very expensive
28
Q

Cell damage due to radiation is bad for what 3 reasons?

A

Cell mutation and cancerous tumours by altering DNA

Cell sterility (stop cells reproducing)

cell death

29
Q

What are ultrasound waves?

A

longitudinal wave with frequency greater than 20 kHz.

30
Q

How are ultrasound images produced?

A

using the piezoelectric effect

31
Q

Describe the piezoelectric effect?

A

The application of a p.d. across a material / crystal causes an expansion / contraction / vibration

32
Q

Describe the principles of ultrasound scanning

A
  1. A (piezoelectric) crystal / transducer is used to send
    pulse(s) of ultrasound (into the patient)
  2. ultrasound wave is reflected (at the boundary of tissue)
3. The (intensity of the) reflected signal depends on the
acoustic impedances (at the boundary)
  1. The (time of) delay is used to determine the depth /
    thickness
33
Q

Describe how the piezoelectric effect is used in an ultrasound transducer both to emit and
receive ultrasound

A

Emission: (Piezoelectric film / crystal connected to an)
alternating p.d making it vibrate (and hence emits ultrasound)

Reception: (Ultrasound makes the piezoelectric film /
crystal) vibrate and this produces (alternating) p.d

34
Q

Explain why a gel is used between the ultrasound transducer and the patient’s skin during a
scan.

A

Without the gel, the ultrasound would be reflected at the skin

Gel and skin has similar acoustic impedance / Z (values)
or There is a large difference between the Z (values) of
air and skin

35
Q

The amount of reflection of ultrasound is dependent on what?

A

The change in acoustic impedance

36
Q

Describe and explain the principles of a B-scan

A

Produces a 2D image by moving the transducer over the patient’s skin.

At each position, the scan produces a measure of the time interval and so the distance to the reflection point between signal production and reception.

The B scan is a series of A scans that are stitched
together to form an image

37
Q

Describe and explain the principles of a A-scan

A

Uses a single transducer to emit a signal and
then later receive the reflected signal back.

It is used to determine distances from the ultrasound device to the point of reflection (usually the boundary between two media).

This is achieved by measuring the time delay between generating and receiving the signal and using the speed of sound in the media to approximate the distance.

38
Q

Explain what is meant by Doppler effect.

A

Change in the frequency / wavelength because of source / ‘observer’ moving

39
Q

Whats the difference between A scan and B scan

A

A-scan in one direction only

B-scan uses a number of sensors or a sensor in different positions / angles (to build up a 2D image)

40
Q

How do doctors determine the speed v of blood in a

patient?

A
  1. Ultrasound transducer emits and detects ultrasound
  2. The transducer is placed at an angle (to the artery)
  3. Ultrasound is reflected by the blood / cells
  4. change in frequency / wavelength (of the reflected ultrasound) is related to speed of blood
41
Q

What are the advantages of ultrasounds?

A

no known hazards

good for imaging soft tissue, obtains real time image

relatively cheap and portable

Quick procedure and patient can move

42
Q

What are the disadvantages of ultrasounds?

A

Don’t penetrate bone, so cant check brain or fractures

can’t pass thorough air in body due to mismatch of impedance

can’t give detail on solid masses

A Level Physics OCR (22 decks)

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