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Physics A Level > 6.5 Medical Imaging > Flashcards

6.5 Medical Imaging Flashcards

1
Q

Structure of X-ray tubes - heater

A

An external power supply connected to a cathode causes electrons to be produced by thermionic emission

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2
Q

Structure of X-ray tubes - anode (4)

A

The electrons are accelerated towards the positive anode
It is made from the target metal (e.g. tungsten) which has a high melting point
The electrons are decelerated by hitting the anode, producing X-ray photons which are directed towards a window
The anode is usually rotated to keep it cool as about 99% of the kinetic energy of the incident electrons is converted to thermal energy

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3
Q

How is the wavelength from an X-ray tube related to the accelerating PD?

A

Wavelength is inversely proportional to PD

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4
Q

What are K-lines?

A

Lines produced on a characteristic spectrum when ionisation occurs in target metal

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5
Q

How does changing current affect the characteristic spectrum of an X-ray?

A

At a higher current, the peak is higher but at the same photon energy

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6
Q

How does changing voltage affect the characteristic spectrum of an X-ray?

A

At a higher voltage, the peak moves up and to the right

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7
Q

Define attenuation

A

The decrease in the intensity of an electromagnetic radiation as it passes through matter

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8
Q

Attenuation mechanisms - simple scatter (2)

A

Energy range - 1-20 keV

A photon with insufficient energy interacts with an electron in the atom so the photon is scattered elastically

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9
Q

Attenuation mechanisms - photoelectric effect (2)

A

Energy range - <100 keV

The photon is absorbed by an electron in the atom, causing it to escape the atom

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10
Q

Attenuation mechanisms - Compton scattering (3)

A

Energy range - 0.5-5.0 MeV
The photon has more energy than it needs so after it interacts with an electron, causing it to leave the atom, the photon is scattered with reduced energy
Both energy and momentum are conserved

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11
Q

Attenuation mechanisms - pair production (2)

A

Energy range - >/ 1.02 MeV

The photon interacts with the nucleus of the atom causing it to disappear, creating an electron and a positron

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12
Q

Why is simple scatter insignificant for hospital radiography?

A

X-ray machines use PDs greater than 20keV

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13
Q

What does the transmitted intensity of X-rays depend on? (2)

A

The energy of the photons

Thickness and type of substance

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14
Q

What is a collimator used for?

A

It produces parallel X-rays by filtering out non-parallel beams, causing the image to be clearer

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15
Q

Why is a glass lens impractical for producing parallel X-rays?

A

The wavelength of X-rays is too short

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16
Q

Why are contrast media used?

A

To improve the visibility of internal structures because soft tissues have low absorption coefficients

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17
Q

Contrast medium - barium (2)

A

Barium sulphate used to image digestive systems

White liquid mixture (barium meal) that is swallowed before image is taken

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18
Q

Contrast medium - iodine (2)

A

Used to image the circulatory system

It is injected into blood vessels to diagnose blockages

19
Q

What is a CAT scan?

A

A series of cross-sectional X-ray images from different angles, used to produce a three-dimensional image

20
Q

CAT scanner components - X-ray tube (2)

A

The tube produces a fan-shaped beam and rotates opposite to an array of X-ray detectors
The thin beam irradiates a thin slice of the patient, producing a single 2D image

21
Q

CAT scanner components - computer and display

A

Used to combine the 2D slices into a three-dimensional image

22
Q

What are the advantages of a CAT scan over an X-ray? (2)

A

Used to create a 3D image

Can distinguish between soft tissues of similar attenuation coefficients

23
Q

What are the disadvantages of a CAT scan compared to an X-ray? (3)

A

X-rays are quicker and cheaper
CAT scans are longer so there is more exposure to ionising radiation
Patients have to remain very still in a CAT scan

24
Q

Why is technetium-99 used as a medical tracer? (4)

A

It decays by releasing gamma photons
It has a half life of around 6 hours
It can be injected into the bloodstream or ingested
it can be easily incorporated into different molecules to monitor a variety of organs

25
Q

What are medical tracers?

A

Molecules that are used so that the uptake of different substances by organs can be monitored

26
Q

Why is fluorine-18 used as a medical tracer? (2)

A

It emits positrons when it decays to oxygen-18

It has a half life of around 2 hours

27
Q

Gamma camera components - collimator

A

Long, thin tubes of lead that filter out non-parallel beams

28
Q

Gamma camera components - scintillator (2)

A

Typically a NaI crystal, the scintillator produces visible light photons
One gamma photon produces many visible light photons and around 1 in 10 photons interact with the scintillator

29
Q

Gamma camera components - photomultiplier tubes (2)

A

The photons collide with a metal, releasing electrons

The electrons are accelerated, producing multiple electrons so there is a measurable voltage

30
Q

Gamma camera components - computer and display

A

The impact of the photons and the size of the measured voltage is used to construct an image showing the concentration of the tracer

31
Q

How does the gamma camera differ to an X-ray?

A

The image shows the function and processes of the body rather than its anatomy

32
Q

How does a PET scan produce an image? (4)

A

Glucose is radio-labelled with fluorine-18 (FDG) and injected into the body
The FDG accumulates in tissues with a high rate of respiration
The positrons annihilate electrons, producing two gamma photons that travel in opposite directions
Gamma cameras surrounding the patient detect the two photons and the conservation of momentum is used to track the photons back to the position they were emitted from

33
Q

What are the advantages of PET scans? (3)

A

Non-invasive
Helps with identification of brain disorders
Used to assess the effect of new medicines on different organs

34
Q

What are the disadvantages of PET scans? (2)

A

Very expensive

Only found at larger hospitals due to cost

35
Q

What is the piezoelectric effect? (2)

A

When a stress is applied to a piezoelectric material, a potential difference is induced across the material
If a PD is applied to the same material, it undergoes mechanical stress, causing it to vibrate

36
Q

What sort of waves does ultrasound use? (2)

A

Longitudinal

Frequency above 20 kHz

37
Q

What is the procedure for an ultrasound? (4)

A

A high frequency alternating PD is applied to the crystal in the transducer, causing it to vibrate
At the resonant frequency, ultrasound waves are produced as the material resonates
The transducer emits pulses of ultrasound, causing pauses that allow the returning waves to be detected and acts as the receiver as the returning waves cause vibrations that induce a measurable EMF
The time and intensity of the returning waves is used to interpret the material and distance

38
Q

What is the pulse repetition frequency?

A

How often a pulse is emitted

39
Q

What results does an ultrasound scan produce?

A

At each boundary between materials, a reflection is produced which is dependent on the acoustic impedance of the two materials

40
Q

What is coupling gel used for? (2)

A

It provides a boundary between the skin and transducer
The air-skin boundary causes about 99.9% reflection so using gel ensures almost all of the ultrasound enters the body as negligible reflection takes place at the initial boundary

41
Q

Ultrasound - A-Scans (2)

A

A single transducer records along a straight line

Used to determine the thickness of bone or the distance between different structure

42
Q

Ultrasound - B-Scans (2)

A

Produces 2D image on screen
The transducer connects to a computer which uses the array to produce a row of dots whose brightness is proportional to the intensity of the reflected ultrasound at each boundary

43
Q

How is the Doppler effect produced in ultrasound? (2)

A

Ultrasound reflected off moving blood cells produces waves with a changed frequency
The frequency shift is directly proportional to the speed of the blood

44
Q

Why is an ultrasound receiver placed at an angle to the skin?

A

If it is perpendicular to the skin, it does not detect movement (cos(90) = 0)

Physics A Level (23 decks)

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