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Medical Physics Flashcards

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1
Q

How are x-ray photons produced

A

When fast-moving electrons are decelerated by interaction with atoms of a metal

The kinetic energy of the electrons is transformed into X ray photons

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2
Q

What does an X-Ray consist of

A

An evacuated tube containing two electrodes - the tube is evacuated so that electrons pass through the tube without interacting with gas atoms

An external power supply is used to create a large p.d between these electrodes

The cathode is a heater - produces electrons by thermionic emissions

These electrons are accelerated towards the anode

The anode is made from a metal - known as the target metal - that has a high melting point

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3
Q

When are x-ray photons produced

A

When the electrons are decelerated by hitting the anode

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4
Q

What is the energy output of X-Rays

A

Less than 1% of the Kinetic Energy of the incident electrons

The remainder of the energy is transferred into thermal energy of the anode - oil is circulated to cool the anode or the anode is rotated to spread the heat over a large surface area

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5
Q

For what reason is the anode shaped

A

Anode is shaped so that the X-rays are emitted in the desired direction through a window

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6
Q

Why is the X-Ray tube lined with lead

A

To shield the radiographer from any X-rays emitted in other directions

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7
Q

What happens when one electron releases one X-ray photon

A

The maximum energy of a photon from an x-ray tube must equal the maximum kinetic energy of a single electron due to the conservation of energy

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8
Q

X ray photon energy and shortest wavelength calculations

A

hf = eV
hc/lambda = eV
lambda = hc/eV

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9
Q

What is the wavelength from an X-ray tube inversely proportional to

A

The accelerating potential difference

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10
Q

What does increasing the tube current do

A

Increase the intensity of the X-rays

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11
Q

What is the term attenuation used to describe

A

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

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12
Q

What happens to the intensity of parallel beams of X-rays as they pass through matter

A

They will decrease

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13
Q

How many attenuation mechanisms are there by which X-ray photons interact with atoms and what do they do

A

4

Reduces the intensity of the collimated beam in the original direction of travel

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14
Q

What is Simple Scatter

A

Mechanism important for X-Ray photons with energy in the range 1-20 keV

X-ray photon interacts with an electron in the atom - but has less energy than the energy required to remove the electron - X-ray photon simply bounces off without any change to its energy

Insignificant for hospital radiography

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15
Q

Photoelectric Effect as an Attenuation Mechanism

A

Significant for X-ray photons with energy less than 100 keV

X-ray photon is absorbed by one of the electrons in the atom

Electron uses this energy to escape from the atom

Attenuation of X-rays by this type of mechanism is dominant when an X-ray image is taken

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16
Q

What supplies do X-ray machines typically use

A

30-100 kV

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17
Q

When is the Compton Scattering mechanism significant

A

Significant for X-ray photons with energy in the range 0.5-5.0 MeV

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18
Q

What is the Compton Scattering mechanism

A

Significant for X-ray photons with energy in the range of 0.5-5.0 MeV

Incoming X-ray photon interacts with an electron within the atom

Electron is ejected from the atom - but the X-ray photon does not disappear completely - instead is scattered with reduced energy

Both energy and momentum are conserved

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19
Q

What is the Pair production mechanism

A

Only occurs when X-ray photons have energy equal to or greater than 1.02 MeV

An X-ray photon interacts with the nucleus of an atom

It disappears and the electromagnetic energy of the photon is used to create an electron and its antiparticle - a positron

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20
Q

What does the transmitted intensity of X-rays depend on

A

The energy of the photons and the thickness and type of the substance

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21
Q

What happens to the intensity of a given substance and the energy of photons

A

The intensity falls exponentially with the thickness of the substance

The transmitted intensity is given by the equation I = (I0) x e^-(mu)x

I0 = Initial intensity before any absorption
x = Thickness of the substance
mu = attenuation coefficient / absorption coefficient (m^-1)

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22
Q

When are contrast mediums used

A

To improve the visibility of materials with low absorption coefficienst

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23
Q

What are the two most commonly used contrast mediums

A

Iodine and Barium

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24
Q

What is the predominant interaction mechanism for Barium and Iodine and why

A

Photoelectric Effect - they have a large atomic number and the attenuation coefficient is proportional to the cube of the atomic number

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25
Q

When is Iodine used as a contrast medium

A

In liquids

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26
Q

When is Barium Sulfate used as a contrast medium

A

To image digestive systems

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27
Q

How can X-rays be used for therapy instead of imaging

A

Specialised X-ray machines called linacs (linear accelerators) are used to create high energy X-ray photons

These photons are used to kill of cancerous cells and do this through Compton Scattering and Pair Production

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28
Q

What does CAT stand for

A

Computerised Axial Tomography

Axial refers to the images taken in the axial plane

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29
Q

What does a CAT Scanner record

A

A large number of X-ray images from different angles and assembles them into a 3D image with the help of sophisticated software

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30
Q

How do the scanning process and analysis for a CAT scan work

A

Controlled by a graph

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31
Q

How does a CAT scan work

A

The patient lies on their back on a horizontal examination table that can slide in and out of a large vertical ring or gantry

Gantry houses an X-ray tube on one side and an array of electronic X-ray detectors on the opposite side

X-ray tube and the detectors opposite it rotate around within the gantry

X-ray tube produces a fan-shaped beam of X-rays that is typically only 1-10mm thick. Thin beam irradiates a thin slice of the patient, and the X-rays are attenuated by different amounts by different tissues. Intensity of the transmitted X-rays is recorded by the detectors - which send electrical signals to a computer

Each time the X-ray tube and detectors make a 360 degrees rotation - a 2D image or slice is acquired - by the time the X-ray tube has made one complete revolution the table has moved about 1cm through the ring

In the next revolution - the X-ray beam irradiates the next slice through the patients body - so the X-ray beam follows a spiral path during the 10-30 minute scan

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32
Q

Advantages and Disadvanatges of CAT scans

A

Advantages:
CAT scans can be used to create a 3D image which helps doctors when analysing
CAT scans can distinguish between soft tissues of similar attenuation coefficients

Disadvantages:
High exposure to radiation
Have to remain very still as any movement blurs the slice

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33
Q

What are radioactive isotopes used for in medicine

A

Diagnosis and therapy

With diagnosis doctors try to find out what is wrong with the patient

With therapy they attempt to cure the patient using ionising radiation

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34
Q

How an tumours be targeted with ionising radiation

A

When tumours can be targeted by gamma radiation or high energy X rays from outside the patient or through brachytherapy

Brachytherapy is when a radioactive source is implanted in or next to the tumour inside the patient

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35
Q

What is the gamma camera

A

A diagnostic tool where a detector of gamma photons emitted from radioactive nuclei injected into the patient

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36
Q

Why are gamma emitting sources ideal for medical imahing

A

Radioisotopes have to be placed inside the patient and their radiation detected from the outside

Gamma emitting sources are the least ionising and can be penetrate through the patient

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37
Q

Why must radioisotopes chosen for medical images have a short half lfie

A

Ensure high activity from the source so that only a small amount is required to form the image

Patient is not subjected to a high dosage of radiation that continues long after the procedure

38
Q

What is Fluorine-18

A

Radioisotope used in PET scans that has a short half life and has to be produced on site at the hospital using a particle accelerator

39
Q

What is Technetium-99m

A

An extremely versatile radioisotope that can be used to monitor the function of major organs

40
Q

What is Technetium-99m

A

An extremely versatile radioisotope that can be used to monitor the function of major organs

m stands for metastable - the nucleus stays in a high energy state, with more energy that the stable nucleus for a longer period than expected

41
Q

How is Technetirum-99m produced

A

Natural radioactive decay of molybdenum-99

42
Q

How does molybdenum-99 decay

A

Beta minus emission with a half life of 67 hours

43
Q

How does the Tc-99m isotope lose energy

A

Emits a gamma photon with energy 140 keV with a half life of about 6 hours

Stable Tc-00 remains - has a half life of 210,000 years

44
Q

What is done to ensure the radioisotope reaches the correct organ or tumour

A

Radioisotope has to be chemically combined with elements that will target the desired tissues to make a radiopharmaceutical - also known as a medical tracer

45
Q

How are irregularities identified in the function of the body

A

Through the concentration of the radiopharmaceuticals

46
Q

What does a gamma camera detect

A

Gamma photons emitted from the medical tracer injected into the patient and an image is constructed indicating the concentration of the tracer within the patient’s body

47
Q

How does a gamma camera detect

A

Gamma photons travel towards the collimator

Any photons arriving at an angle to the axis of the tubes are absorbed by the tubes - only those travelling along the axis of tubes reach the scintillator

A single gamma photon striking the scintillator produces thousands of photons of visible light

Photons of visible light travel through the light guide into the photomultiplier tubes

These tubes are arranged in a hexagonal pattern

A single photon of light entering a photomultiplier tube is converted into an electrical pulse

Outputs of all the photomultiplier tubes are connected to a computer

Software can process the electrical signals of the tubes very quickly to locate the impacts of the gamma photons on the scintillator

These impact positions are used to construct a high-quality image that shows the concentrations of the medical tracer within the patient’s body

48
Q

What is a collimator

A

A honeycomb of long, thin tubes made from lead

49
Q

What is often the scintillator material

A

Sodium iodide

50
Q

What is the chance of a gamma photon interacting with the scinitillator

A

About 10%

51
Q

How does a gamma camera differ from an X-ray imaging technique

A

It produces an image that shows the function and processes of the body rather than its anatomy

52
Q

What does PET stand for

A

Positron emission tomography

53
Q

What does PET stand for

A

Positron emission tomography

54
Q

What does PET stand for

A

Positron emission tomography

55
Q

What is used in PET scans and why

A

Fluorine-18

It is a positron emitter with a half-life of about 110 minutes

56
Q

What does Fluorine-18 decay into

A

A nucleus of oxygen-18, a positron, a neutrino and a gamma photon

57
Q

How can Fluorine-18 be made through collision

A

High speed protons collide with oxygen-18 nuclei and produce fluorine-18 nuclei and neutrons

58
Q

What medical tracer do most PET scanners use

A

FDG (Fluorodeoxyglucose)

59
Q

Advantages of using FDG

A

Similar to naturally occurring glucose but is tagged with a radioactive fluorine-18 atom in place of one oxygen atom - our bodies treat it like normal glucose and it accumulates in tissues with a high rate of respiration

60
Q

What is used to detect activity from FDG

A

Using gamma detectors

61
Q

What is another medical tracer used for PET scanning

A

Carbon monoxide using the carbon-11 isotope

62
Q

How does carbon monoxide act as a medical tracer

A

Emits as a positron and has a half-life of about 20 minutes

Very good at clinging onto haemoglobin molecules in the red blood cells, so it can be transported through the body and the concentrations of carbon monoxide can be monitored in a PET scan

63
Q

PET scanner process

A

The patient lies on a horizontal table and is surrounded by a ring of gamma detectors

Each detector consists of a photomultiplier tube and a sodium iodide scintillator and produces a voltage pulse or signal for every gamma photon incident at its scintillator

The patient is injected with FDG - the PET scanner detects the gamma photons emitted when the positrons from decaying fluorine-18 nuclei annihilate with electrons inside the patient

Gamma photons detected for the PET scan come from the annihilation of the positrons, not the gamma photons emitted by the decaying fluorine-18 nuclei

The annihilation of a positron and an electron produces two gamma photons travelling in opposite directions so momentum is conserved

The computer can determine the point of annihilation from the difference in the arrival times of these photons at the two diametrically opposite detectors and the speed of photons

The voltage signals from all the detectors are fed into the computer - which analyses and manipulates these signals to generate an image on a display screen in which different concentrations of the tracer show up as areas of different colours and brightness

64
Q

Advantages of PET

A

Non invasive technique

Help diagnose different types of cancers, plan complex heart surgery, observe the function of the brain

Help doctors identify the ones of certain disorders of the brain

Help assess the effect of new medicines and drugs on organs

65
Q

What is Ultrasound

A

Longitudinal sound waves with frequency greater than 20 kHz

66
Q

Benefits of ultrasound

A

Form images of the internal structure ps of the body

Non ionising

Non invasive

Quick

67
Q

Range of frequencies for Ultrasound used for medical imaging

A

1-15 MHz

68
Q

What is the wavelength of ultrasound in the human body and what are the advantages

A

Less than 1mm - can be used to identify features as small as a few millimetres

69
Q

What is an ultrasound transducer

A

Device used to generate and to receive ultrasound

70
Q

How does an ultrasound transducer work

A

It changes electrical energy into sound and sound into electrical energy - by means of the piezoelectric effect

71
Q

What is the piezoelectric effect

A

When crystals produce an EMF when they are deformed

72
Q

Is the piezoelectric effect a reversible process

A

Yes as when an external p.d is applied across the opposite faces if the crystal - the electric field can either compress or stretch the crystal

73
Q

How is ultrasound generated

A

A high frequency alternating p.d is applied across opposite faces of a crystal - this repeatedly compresses and expands the crystal

The frequency is chosen to be the same as natural frequency of oscillation of the crystal

The crystal resonates and produces an intense ultrasound signal

74
Q

What does an ultrasound transducer emit

A

Pulses of ultrasound - typically 5000 pulses every second

75
Q

How can a transducer be used to detect ultrasound

A

Any ultrasound incident on the crystal will make it vibrate - so the crystal is compressed and expanded by tiny amounts

This vibration generates an alternating e.m.f across the ends of the crystal - this can be detected by electronic circuits

76
Q

What do modern ultrasound transducers use

A

Lead zirconate titanate or polyvinylidene fluoride

77
Q

What is the simplest type of ultrasound scan called

A

An A-scan

78
Q

How does an A-scan work

A

A single transducer is used to record along a straight line through the patient

The transducer sends ultrasound pulses into the body of a patient

Each pulse of ultrasound will be partly reflected and partly transmitted at the boundary between any two different tissues

Reflected or echo pulse will be received at the transducer

It wiki gave less energy due to energy loss

The pulsed voltage at the ultrasound transducer is displayed on an oscilloscope screen or computer screen as a voltage against time plot

79
Q

What is the time interval in an A-scan

A

Ultrasound pulse to travel from the front of the transducer to the retina and then back to the transducer

80
Q

What is the total distance travelled by the ultrasound pulse

A

2L

Where L is the distance between the transducer and the retina

81
Q

How do B-scans work

A

The transducer is moved over the patients skin

Output of the transducer is connected to a high speed computer

For each position of the transducer- the computer produces a row of dots on the signal screen - each dot corresponds to the boundary between two tissues

The brightness of the dot is proportional to the intensity of the reflected ultrasound pulse

Collection of dots produces correspond to the different positions of the transducer over the patient - making a 2D image of a section through the patient

82
Q

What happens at boundary

A

When a uniform beam of ultrasound is incident at a boundary between two substances - a proportion of its intensity will be reflected and the remainder will be refracted

The fraction of the ultrasound intensity reflected at the boundary depends on the acoustic impedance of both media

83
Q

What is acoustic impedance of a substance defined as

A

The product of the density and the speed of ultrasound in that substance

84
Q

Units for acoustic impedance

A

kg m^-2 s^-1

85
Q

What does reflected intensity depend on

A

The acoustic impedances

86
Q

What is the formula for the ratio of reflected intensity given that the angle of incidence is 0 degrees

A

(Z2 - Z1)^2 / (Z2 + Z1)^2 = Ir / Io

Ir/ Io is also known as the intensity reflection coefficient

87
Q

What is a coupling gel

A

A substance with acoustic impedance similar to that of the skin is smeared onto the skin and the transducer

The gel fills air gaps between the transducer and the skin and ensures that almost all the ultrasound enters the patients body

88
Q

When is the term impedance matching / acoustic matching used

A

When two substances have similar causes of acoustic impedance

89
Q

What is Doppler ultrasound

A

The reflection of ultrasound from iron-rich blood cells to help doctors to evaluate blood flow through major arteries and veins

90
Q

What can Doppler ultrasound reveal

A

Blood clots, the narrowing of walls due to the accumulation of fatty deposits, the amount of blood flow to a transplanted kidney or liver

91
Q

How does Doppler ultrasound work

A

The ultrasound transducer is pressed lightly over the skin above the blood vessel

The transducer sends pulses of ultrasound and receives the reflected pulses from inside the patient

Ultrasound reflected off tissues will return with the same frequency and wavelength - but that reflected off the many moving blood cells will have a changed frequency

Frequency is increased when the blood is moving towards the transducer and decreased when the blood is receding from the transducer

The frequency shift is directly proportional to the speed of approach or recession of the blood

92
Q

Formula for change in observed ultrasound frequency

A

Change in frequency = (2 x observed frequency x speed of moving blood cells x cos(theta)) / (speed of the ultrasound in blood)

Physics (19 decks)

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