Module 6: Chapter 27 - Medical Imaging Flashcards
1
Q
What are examples of non-invasive medical diagnosis techniques?
A
- X-ray
- MRI
- Ultrasound
- CT scan / CAT scan
- Fluoroscope
2
Q
What can ionising radiation cause?
A
Ionising radiation can:
* Kill living cells
* Mutate DNA
3
Q
How were x-rays discovered?
A
Wilhelm Röntgen discovered x-rays when investigating light emitted by different gases when a pd is applied between the 2 electrodes. The light radiation emitted was found to pass through skin but was mostly absorbed by bone. This light radiation also developed photographic film in the same way as visible light.
4
Q
Why were x-rays named x-rays?
A
“x” signifies an unknown quantity as it was unknown what x-rays where when they were first discovered
5
Q
What can x-rays be used for?
A
- Imaging - X-rays are very penetrating and can pass through many forms of matter. They are used in medicine, industry and security to take pictures of the inside of objects
- Crystallography - X-rays are used to work out the arrangement of atoms in various substances, including crystals
6
Q
How are x-rays detected?
A
An x-ray film is made up of a plastic sheet coated with silver halide crystals. When the film is exposed to X-rays the silver halide molecules become ionised. the image is then produced by “developing and fixing” the film. Hence a black and white image is produced. The degree of blackening depends on the amount of exposure to x-rays
7
Q
How are x-rays produced?
A
High speed electrons are produced using a negatively charged cathode that is heated, resulting in thermionic emission. The electrons are accelerated through a large pd from the cathods to the anode. X-rays are produced when the fast moving electrons are rapidly decelerated. The electrons are decelerated by bombarding electrons onto a metal anode. Conservation of energy means that the loss of kinetic energy results in photons being emitted. If the deceleration is great enough the photons will have energies in the x-ray range of the spectrum.
8
Q
Describe the structure of an x-ray tube
A
Tube is also evacuated
9
Q
Describe how and x-ray tube produces x-rays
A
High speed electrons are produced using a negatively charged cathode that is heated, resulting in thermionic emission. The electrons are accelerated through a large pd from the cathods to the anode. X-rays are produced when the fast moving electrons are rapidly decelerated. The electrons are decelerated by bombarding electrons onto a metal anode, in this case a tungsten target. Conservation of energy means that the loss of kinetic energy results in photons being emitted. If the deceleration is great enough the photons will have energies in the x-ray range of the spectrum.
10
Q
What precautions are taken when using x-rays and why?
A
X-rays are a form of ionising radiation and precautions must be taken due to the harmful effects of ionising radiation. X-rays are only taken when necessary and the radiographer stands behind a lead barrier whike the x-rays are being taken
11
Q
How can X-rays be used for medical imaging?
A
X-rays pass through soft tissue, such as skin and muscle without being absorbed. Denser tissue, such as bone, can absorb X-rays. Film that is exposed appears black and areas that are not exposed, because of X-ray absorbtion, appear white.
12
Q
What is an X-ray tube?
A
A piece of equipment that produces X-ray photons by firing electrons from a heated cathode across a large p.d in an evacuated tube - X-ray photons are produced when the electrons are decelerated by hitting the target metal of the anode
13
Q
Why is an x-ray tube evacuated?
A
So that electrons can pass through the tube without interacting with gas atoms
14
Q
What is the target metal?
A
A metal with a high melting point used for the anode in an X-ray tube, for example tungsten. It is angled so that the X-rays are emitted in the desired direction through a window
15
Q
Why is tungsten often used as the target metal in an X-ray tube?
A
Tungsten has the highest known melting point of all elements
16
Q
Why does the target metal in an X-ray tube have to have a high melting point?
A
The energy output of X-rays is less than 1% of the kinetic energy of the incident electrons. The remainder of the energy is transformed into thermal energy of the anode. Therefore a target metal with a high melting point is required
17
Q
What is the equation for the minimum wavelength of X-ray produced?
A
V = potential difference between anode and cathode in X-ray tube
18
Q
Explain how the equation “λ = hc/eV” is derived
A
Each electron produces one X-ray photon, therefore the maximum energy of a photon produced is equal to the maximum kinetic energy of a single electron. Therefore, hf = eV. This will find the minimum wavelength of the X-ray as you are using the maximum energy and wavelength is inversily proportional to energy
19
Q
What happens if you increase the potential difference in an X-ray tube?
A
- The frequency of x-ray produced increases
- The energy of the X-ray produced increases
20
Q
What happens if you increase the current in an X-ray tube?
A
The intensity of X-rays increases
21
Q
What is a CAT/CT scan?
A
In a CAT scan a narrow, pencil thin, fan shaped X-ray beam is used to scan across and around the patient. X-ray detectors are positioned opposite the X-ray source. The X-rays are attenuated by different amouns by different tissues so at each position, a measurement of the amount of radiation transmitted through the patient is made. Each time the X-ray tube and detectors make a 360 degree rotation, a 2D slice is aquired. The process is repeated along the patient until the machine has made a complete scan of the patient. A computer program is then used to reconstruct the data and produce a 3 dimensional image
22
Q
What does “CAT” scan stand for?
A
Computerised Axial Tomography Scan
23
Q
What is a Fluroroscope?
A
Fluoroscopes are used to show a patients organs working. For example, they can be used to detect blocked blood vessels. They consist of an X-ray source and an X-ray detector attached to a digital video camera. The patient is placed between the X-ray source and the detector
24
Q
What are contrast media?
A
Contrast media are materials of high atomic number and thus have high absorption coefficients
25
When are contrast media used?
They are used when you would like to use an X-ray to view soft tissues in the body. Several tpes of soft body tissue have almost the same average atomic number, so they produce very little difference in attenuation. This means that they are not easily visible. Therefore a contrast medium is used
26
Why are the electrons focused onto a single point on the target metal?
In order to create a point x-ray source rather than an extended x-ray source. This will produce a much sharper image of what you are attempting to image
27
What is Bremsstahlung radiation?
"Braking radiation". It is a continuous spectrum of different wavelengths being produced because the electrons have got a range of velocities
28
What does a typical X-ray spectrum look like?
Faster electrons form shorter wavelength x-rays and slower electrons form longer wavelength x-rays
29
Describe how the characteristics of the X-ray spectrum are formed
**Bremsstrahlung radiation** - The range of decelerating electrons inside the X-ray tube produces the broad background of bremsstrahlung
**K lines** - The narrow, intense K lines are characteristic of the target metal. The bombarding electrons can remove electrons in the metal atoms close to the nuclei. These gaps created in the lower energy levels are quickly filled by electrons dropping from higher energy levels. These transitions release photons of specific energies and therefore wavelengths
30
Describe the intensity of an X-ray beam from a point source
An X-ray beam from a point source obeys the inverse square law for intensity
31
Describe the intensity of a collimated X-ray beam
The intensity remains constant
32
What is attenuation?
The decrease in the intensity of electromagnetic radiation as it passes through matter and/or space
33
What is the attenuation coefficient?
A measure of the absorption of X-ray photons by a substance. Also known as absorption coefficient. Unit = m⁻¹
34
What are the 4 X-ray attenuation mechanisms?
* Simple Scatter
* Photoelectric Effect
* Compton Scattering
* Pair Production
35
Explain Simple Scattering X-ray attenuation?
The X-ray photon interacts with an electron in the atom, but has less energy than the energy required to remove the electron, so the X-ray photon simply bounces off (is scatted) without any change to its energy. The X-ray is scattered elastically by the atom
36
Explain the Photoelectric effect X-ray attenuation?
The incident X-ray photons is absorbed by one of the electrons in the atom. The electron uses this energy to escape from the atom.
## Footnote
This type of mechanism is cominnant when an X-ray image is taken as the machines use 30-100kV supplies
37
What is the Compton Scattering X-ray attenuation?
The incoming X-ray photon interacts with an electron within the atom. The electron is ejected from the atom, but the X-ray photon does not disappear completely - instead it is scattered with reduced energy. In this interaction, both energy and momentum are conserved
38
What is the pair production X-ray attenuation?
An X-ray photon interacts with the electric field of the nucleus of the atom. It disappears and the electromagnetic energy of the photon is used to create an electron and its antiparticle, a positron
39
What is the mechanism for Simple Scattering X-ray attenuation?
40
What is the mechanism for Photoelectric effect X-ray attenuation?
41
What is the mechanism for Compton Scattering X-ray attenuation?
42
What is the mechanism for Pair Production X-ray attenuation?
43
What range of energy for an incoming x-ray will result in x-ray attenuation occuring via simple scattering?
1 - 20 keV
44
What range of energy for an incoming x-ray will result in x-ray attenuation occuring via the photoelectric effect?
20 - 100 keV
45
What range of energy for an incoming x-ray will result in x-ray attenuation occuring via compton scattering?
0.5 - 5 MeV
46
What range of energy for an incoming x-ray will result in x-ray attenuation occuring via pair production?
. >1.02 MeV
47
What 3 factors affect x-ray attenuation?
* The attenuation coefficient
* The incident intensity of the X-ray
* The thickness of the material
48
What is the equation for X-ray attenuation?
| FOR A COLLIMATED BEAM OF X-RAYS
## Footnote
μ = Attenuation coefficient, x = thickness
49
What are 2 common contrast media?
Barium and Iodine compounds
50
How is iodine used as a contrast medium?
Iodine is used as a contrast medium in liquids, such as to view blood flow. An organic compound of Iodine is injected into blood vessels so that doctros can dioagnose blockages in the blood vessels and the structure of organs such as the heart.
51
How is barium sulfate used as a contrast medium?
Barium sulfate is often used to image digestive systems. It is given to the patient in the form of a white liquid mixture (a barium meal), which the patient swallows before an X-ray image is taken
52
How can X-rays be used for therapeutic use?
Specialised X-ray machines called linacs (linear accelerators), are used to creatae high-energy X-ray photons. These photons canm be used to kill of cancerous cells. They do so through the mechanisms of Compton scattering and pair production
53
Why are barium and iodine commonly used as contrast media?
Barium and iodine are elements with large atomic numbers, Z. For X-ray imaging the predominant interaction mechanism is the photoelectric effect, for which **the attenuation coefficient is proportional to the cube of the atomic number (μ ∝ Z³)**.
Barium and Iodine are also relatively harmless to humans as they don't get absorbed into the blood stream
## Footnote
**In the Photoelectric Effect, μ ∝ Z³**
54
What are the advantages of a CAT scan?
* 3D X-rays - allowing you to view the depth of particular structures and find the exact location of an injury quickly (important for head injuries/brain tumours)
* Allows "slices" to be viewed
* Can distinguish between materials of similar densities (and therefore attenuation coefficients) as the resolution is much higher
55
What are the disadvantages of a CAT scan?
* Larger ionizing radiation exposure compared to standard x-rays
* Patient has to remain very still for prolonged periods or time, or images will blur
56
What are the advantages of X-rays over CAT scans?
* Cheaper
* Quicker
* X-rays require only a few "photos" taken while in CAT scane you're exposed to much more ionising radiation
57
58
Describe the structure of a modern CAT scanner
The gantry (the vertical ring) houses an X-ray tube on one side and an array of electronic X-ray detectors on the opposite side. The X-ray tube and the detectors opposite it rotate around within the gantry.
59
How does the temperature of the cathode affect the minimum wavelength of an X-ray?
The temperatue of the cathode does not affect the minimum wavelength
60
How does the material of the cathode affect the minimum wavelength of an X-ray?
The minimum wavelength of the X-ray is independent of the cathode material
61
How do you choose a radioisotope for use in medical imaging?
For a suitable radioisotope in medical imaging:
* It must emit gamma radiation and nothing else
* It must have a half life long enough for use in imaging but not too long as to remain in the body (i.e 4 hours to 5 days)
* It must be safe to eat/drink, inject, or breath in
62
Why must a suitable radioisotope in medical imaging emit gamma radiation and nothing else?
Radioisotopes for use in medical imaging must be placed inside the patients body and their radiation detected from the outside, therefore gamma-emitting sources are most suitable as:
* Gamma photons are the least ionising and therefore do not damage the patient. Alpha and beta sources cause more ionisation, they are dangerous and are not used for imaging techniques
* Gamma photons are the most penetrating so can easily penetrate through the patient to be detected externally
63
Why must radioisotopes chosen for medical imaging have a short half life?
* To ensure high activity from the source, therefore only a small amount is required to form the image
* It also ensures that the patient is not subjected to a high dosage of radiation that continues long after the proceadure
## Footnote
However, the half life should be sufficiently long enough as to be viable to use in medical imaging, i.e not seconds or minutes
64
What is a radiopharmaceutical?
A radioisotope chemically combined with elements that will target particular tissues in order to ensure that the radioisotope reaches the correct organ or tumour for diagnosis or treatment
65
What is a medical tracer?
A radioisotope chemically combined with elements that will target particular tissues in order to ensure that the radioisotope reaches the correct organ or tumour for diagnosis or treatment
## Footnote
it is a **radiopharmaceutical**
66
What are 2 examples of radioisotopes used in radiopharmaceuticals/medical tracers?
* Technetium-99m
* Fluorine-18
67
What is the decay equation of Technetium-99m?
68
What is the half-life of Technetium-99m?
6 hours
69
What does "m" mean in Technetium-99m?
"m" means "metastable" and refers to a nucleus that stays in a high-energy state, with more energy than the stable nucleus, for a longer period than expected
70
What is the energy of the gamma photon released in the decay of Technetium-99m?
140 keV
71
Why are medical tracers required?
Medical tracers are required so that the radioisotope reaches the correct organ or tumour rather than just spreading out through the entire body
72
Draw the structure of a gamma camera:
**Components: Collimator, Scintillator, Photomultiplier tubes, Computer, Display**
73
What does a gamma camera do?
It detects the 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
74
What is a collimator?
A collimator is part of a gamma camera, a honeycomb of long, thin tubes made from lead that absorbs any photons arriving at an angle to the axis of the tubes so that a clear picture is obtained
75
What is a scintillator?
Part of a gamma camera, often made of sodium iodide, which produces thousands of photons of visible light when struck by a single gamma photon. However, not all the gamma photons produce these flashes as the chance of a gamma photon interacting with the scintillator is only 1 in 10.
76
What are photomultiplier tubes?
An apparatus that converts a photon of visible light into an electrical pulse (voltage), for example as part of a gamma camera
77
How does a computer and display form part of a gamma camera?
The computer receives the electrical pulses from the photomultiplier tubes and through the use of a sophisticated software, locates the impacts of the gamma photons on the scintillator. These impact positions can be used to construct a high-quality image that shows the concentrations of the medical tracer within the patient's body. The final image is displayed on a screen.
78
How does the role of a gamma camera differ from that of an X-ray?
* A **gamma camera** produces an image that shows the **function** and **processes** of the body
* An **X-ray** produces an image that shows the **anatomy** of the body
79
Why must some medical tracers be produced on-site in a hospital?
The half life may be too short to be produced elseswhere and transported to the hospital
80
Why is Tc-99 a suitable product from the decay of Tc-99m?
Technetium-99 has a very long half-life of 210,000 years. Its activity would be extremely small compared with the time taken for the scan so it would not interfere with the scan. It is also safe as the activity would be incredibly low.
81
How is the activity of a radioisotope affected by the radioisotope bonding to other elements/compounds?
It is completely unaffected as it is only dependent on the nuclei, not the electrons
82
What compound gives off visible light when hit by an x-ray or gamma photon?
Sodium Iodide
83
What is produced in the annihilation of a positron and an electron?
A pair of gamma photons emitted in opposite directions to each other
84
Why are the 2 gamma photons emitted in the annihilation of an electron and a positron emitted in opposite directions?
Due to the law of conservation of momentum, if the initial momentum was zero the sum final momentum must be zero
85
What is the equation for the decay of Fluorine-18?
86
What is the half life of Fluorine-18?
110 minutes
87
Why must Fluorine-18 be produced on-site or near the hospital it is going to be used at?
It has a short half life of 110 minutes
88
What is a method of producing Fluorine-18?
High-speed protons collide with an Oxygen-18 nuclei and produce Fluorine-18 nuclei and neutrons. Oxygen-18 is commonly found in the atmosphere (making up about 20% of natural oxygen) and is non-radioactive
89
What are 2 medical tracers PET scanners usually use?
* Fluorodeoxyglucose (FDG)
* Carbon monoxide (made with carbon-11)
90
What is Fluorodeoxyglucose (FDG)?
It is a medical tracer commonly used in PET scanners, it is similar to naturally occuring glucose but one of the oxygen atoms has been replaced by a radioactive Fluorine-18 atom
91
What are the advantages of Fluorodeoxyglucose (FDG)?
Human bodies treat it like normal glucose, therefore when it is injected it accumulates in tissues with a high rate of respiration.
92
What is Carbon monoxide (made with carbon-11)?
A medical tracer commonly used in PET scanners. Carbon-11 is an isotope that emits a positron and has a half-life of around 20 minutes. Carbon monoxide will cling 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 using the PET scan.
93
What is the structure of a PET scanner?
* The patient lies on a horizontal table surrounded by a ring of gamma detectors
* Each detector consists of a photomultipler tube and a sodium iodide scintillator, and produces a voltage pulse or signal for every gamma photon incident at its scintillator. The detectors are all connected to a high-speed computer
94
How does a PET scanner work?
* The patient is injected with a radioisotope that emits positrons, such as FDG. The isotope gathers in the body where a lot of oxygen is required, such as a cancerous tumour.
* When the Fluorine-18 in the FDG decays, the positrons emitted only travel a short distance (about 1mm) before encountering an electron. The annihilation is used to produce a pair of gamma photons moving in opposite directions (due to the law of conservation of momentum).
* These gamma photons are detected by the ring of gamma detectors. The computer can determine the point of annihiliation using the difference in the arrival times of these two photons at the diametricaly opposite detectors and the speed of light.
* The voltage signals from all the detectors are fed into the computer, which analyses and manipulates these signals to generate an image (scan) on a dislpay screen in which different concentrations of the tracer show up as areas of different colours and brightness.
95
What does "PET scan" stand for?
Positron Emission Tomography scan
96
What is tomography?
Tomography is a technique used in medical imaging to create cross-sectional images or slices of an object
97
What are the advantages of PET scans?
* PET is a non-invasive technique (the patient is not subjected to the risks of surgery).
* PET scans can be used to diagose different types of cancers, help plan complex heart surgery, and to observe the function of the brain which can lead to diangoisis of certain disorders of the brain - such as alzheimers
* PET scans can also be used to assess the effect of new medicines and drugs on organs
98
What are the disadvantages of PET scans?
PET scans are very expensive due to the facilities required to produce the medical tracers. Therefore, PET scanners are only found at larger hospitals, and only patients with complex health problems are recommended for PET scans
99
What is SONAR?
SONAR is an acronym for "**SO**und **N**avigation **A**nd **R**anging"
100
What can SONAR be used for?
It can be used by ships to detect:
* The seabed
* Other ships
* Other dangers such as icebergs
101
What is the human range of hearing?
20Hz - 20,000Hz
102
What is ultrasound?
A longitudinal sound wave above the human range of hearing - above 20,000Hz
103
What is infrasound?
A longitudinal sound wave below the human range of hearing - below 20Hz
104
How can animals use ultrasound?
Echolocation - Animals such as bats and dolphins echolocate by emitting ultrasound and listening for the reflection off nearby objects. This allows them to locate and navigate their surroundings
105
What are the benefits of ultrasound imaging?
* It is completely non-ionising and harmless
* It very quick, producing real-time images of the soft tissues of the body
* It is non-invasive
106
What are the disadvantages to ultrasound imaging?
* It produces fairly low resolution images
* They cannot be used to image the lungs or anything else which contains an air cavity
* Ultrasound is absorbed by bone so cannot be used to image the inside of the skull
107
What is the pitch of a sound wave?
The frequency
108
Why does sound require a medium to travel in?
It is a longitudinal mechanical wave
109
What is the speed of sound in air?
330ms⁻¹
110
What is an ultrasound transducer?
A device used both to generate and to receive ultrasound, which changes electrical energy into sound and sound into electrical energy, by means of the piezoelectric effect
111
How are ultrasounds produced?
Sounds are produced by a source vibrating back and forth. The faster the vibrations, the greater the frequency. Therefore to produce ultrasounds a source must vibrate at an extremely high frequency in to move air back and forth at a high frequency, producing a sound wave.
111
What is the main use of ultrasound in medicine and why?
Medical imaging - especially antenatal screening. This is because there are no known risks to the patient or pratitioner why is extremely important when imaging an unborn child.
112
What is the piezoelectric effect?
The production of an electromotive force (e.m.f) by some crystals, such as quartz, when they are compressed, stretched, twisted, or distorted, This is also a reversible process meaning when an external pd is applied across opposite faces of the crystal, the electric field can either compress of stretch the crystal.
113
What is the frequency range of ultrasound used for medical imaging?
1-15MHz
114
What is the resolution of images produced by ultrasound waves in the body? (and why)
The wavelength of ultrasound in the human body is less than 1mm, so ultrasound can be used to identify features as small as a few millimetres. Features any smaller would not be able to be distinguished due to diffraction of ultrasound waves.
115
How does an ultrasound transducer work?
* To generate ultrasound, a high frequency **alternating pd** is applied across opposite faces of the crystal. This repeatedly compresses and expands the crystal. The frequency is chosen to be the same as the natural frequency of oscillation of the crystal, and as a result the crystal resonantes, producing an intense ultrasound signal. An ultrasound transducer emits pulses of ultrasound, typically 5000 every second.
* The same transducer can detect ultrasound. Any ultrasound incident on the crystal will make it vibrate, so the crystal is compressed and expanded by tiny amounts. This vibration generated an alternating emf across the ends of the crystal which can then be detected by electronic circuits.
116
What is the structure of an ultrasound transducer?
**Key components:** Cable, Damping Material, Piezoelectric Crystal, Electrodes, Acoustic window
117
What piezoelectric crystal is used in an ultrasound transducer?
* **Quartz**
* **Lead zirconate titanate**
* Polyvinylidene fluoride
118
What are the 2 types of ultrasound scans?
* A-scans
* B-scans
119
What happens when an ultrasound wave meets a boundary with a different medium?
There is a reflection of the original pulse when there is a change in the density. The bigger the change in density, the bigger percentage of energy is reflected.
120
What is an A-scan ultrasound scan?
An A-scan is the simplest type of ultrasound scan. A single transducer is used to record in one dimension and allow distances between boundaries to be measured.
121
How does an A-scan ultrasound scan work?
* A transducer sends ultrasound pulses into the body of a patient. Each pulse of ultrasound will be partially reflected and partly transmitted at the boundary between two different tissues. The reflected or "echo" pulse will be received at the transducer and it will have less energy than the original pulse due to energy losses within the body and due to some of the energy being transmitted through the boundary.
* The voltage of the reflected ultrasound pulse is displayed on an oscilloscope screen as a voltage against time plot. The distance can then be calculated by using the equation: distance = (speed x time)/2
122
What can A-scan ultrasound scans be used for?
As they are used to record the distances between boundaries, they are useful in:
* Measuring the thickness of bone
* Measuring the distance between the lens and retina in the eye
123
What is a B-scan ultrasound scan?
A B-scan ultrasound scan is a "Brightness-scan" and unlike an A-scan, it uses an array of transducers which allows you to produce a two-dimensional image rather than just calculate distances along a single line.
124
How does a B-scan work?
A transducer is moved over the patients skin. The output from each transducer is connected to a high-speed computer. For each position of the transducer, the computer produces a row of dots on the digital screen - each dot corresponds to the boundary between two tissues and the brightness of the dot is proportional to the intensity of the reflected ultrasound pulse. The collection of dots produced correspond to different positions of the transducer over the patient, producing a two-dimensional image of a section through the patient. It is effectively many overlaid A-scans used to produce an image.
125
What happens to a wave at a boundary?
Some of the energy of the wave is:
* Transmitted (includes refraction)
* Reflected
* Absorbed
126
What is acoustic impedance, Z?
The product of the density, ρ, of a substance and the speed, c, of ultrasound in that substance
127
What is the unit for acoustic impedance?
kg m⁻² s⁻¹
128
What is acoustic matching (impedance matching)?
The use of two substances with similar acoustic impedance to minimise reflection of ultrasound at the boundary between them.
129
What is the equation for acoustic impedance?
Z = ρc
Acoustic Impedance = Density x Speed of sound (in substance)
## Footnote
c = speed of **sound wave** in the substance, NOT the speed of light
130
What is a coupling medium/gel?
A gel with an acoustic impedance similar to that of skin which is smeared onto the transducer and patient's skin before an ultrasound scan in order to fill air gaps and ensure that almost all the ultrasound enters the patient's body.
131
What happens when a sound wave meets a boundary between air and another medium?
It is almost entirely reflected
132
Why is a coupling medium/gel required in ultrasound imaging?
When an ultrasound transducer is placed on the skin of a patient, air pockets will always be trapped between the transducer and the skin. The air-skin boundary means that about 99.9% of the incident ultrasound will be reflected before it even enters the patient. To voercome this problem, a coupling medium/gel is used to fill any air gas between the skin and the ultrasound transducer. This ensures that almost all the ultrasound enters the patients body.
133
What does the fraction of ultrasound intensity reflected at a boundary between 2 media depend on?
The intensity reflection coefficient, α
134
What is the equation for the intensity reflection coefficient, α?
## Footnote
α = intensity reflection coefficient, Iᵣ = Reflected Intensity, I₀ = Incident Intensity, Z₁ = Acoustic Impedance of Incident Medium, Z₂ = Acoustic impedance of second Medium
135
What happens if the difference in acoustic impedance between 2 media is increased?
The greater the proportion of the intensity of the wave is reflected
136
When can the intensity reflection coefficient equation be used?
For a **collimated beam** of ultrasound travelling **perpendicular** to the boundary
137
What is the intensity reflection coefficient?
The ratio of reflected intensity over incident intensity for ultrasound incident at a boundary
138
What must you be careful doing when calculating using acoustic impedance?
Be careful converting density units!
139
What is 1g cm⁻³ in kg m⁻³?
1000
140
What is 1kg m⁻³ in g cm⁻³?
0.001
141
What is the equation for doppler imaging?
## Footnote
f = original ultrasound frequency, Δf = difference in original and received ultrasound frequencies, **v = speed of object being monitored**, **c = speed of ultrasound wave**,**θ = angle between transducer and velocity of moving object**
142
What is medical doppler imaging most commonly used for?
**Evaluating blood flow through major arteries and veins**. It can be used to reveal blood clots, narrowing of the walls of the blood vessel, and evaluate the amount of blood flow to a transplanted organ.
143
Determine the speed of blood flow when:
θ = 30°, f₀ = 1MHz, fᵣ = 998.5kHz, Speed of ultrasound in blood = 1.6 km s⁻¹
2.4 ms⁻¹
| remember where the angle is measured from!
144
What can you do with the data from doppler imaging?
It can be combined with the data from ultrasound imaging to produce an image of a blood vessel and the surrounding tissues, with the velocity of blood flow overlaid ontop in colour to indicate the direction and speed of blood flow. This can be particularly useful in the heart to see which direction the blood is flowing.
145
Explain how the use of doppler imaging can differentiate between soft tissue and blood vessels.
Ultrasound reflected of tissues will return with the same frequency and wavelength, but ultrasound reflected off moving blood cells will have a changed frequency and wavelength.
146
Why must the ultrasound transducer be held at an angle to the skin in doppler imaging?
Holding it at a right angle to the skin would give no observed change in frequency because cos(90°) = 0.
147
What is the energy of a positron at rest?
8.2x10⁻¹⁴ J
148
Why does anti-matter not exist naturally within the Universe?
It would annihilate immediately when it came into contact with normal matter
149
Calculate the energy of a gamma ray produced in the annihilation of an electron and a positron?
8.2x10⁻¹⁴ J
150
Where is θ measured from for doppler imaging?
between the line extending from the transducer and velocity of moving object
**NOT** to the normal of the velocity
