Module 6: C27 - Medical Imaging Flashcards
1
Q
What are some types of Non-Invasive Techniques
A
• X-ray
• MRI (not on spec)
• Ultrasound
• CT Scan/CAT Scan
• Fluoroscopes (like a video X-ray)
2
Q
What are the advantages of non-invasive techniques?
A
The advantages is that the person doesn’t need to be cut open or have open surgery
3
Q
What 2 things does Ionising Radiation do
A
Ionising Radiation does two things
• Kills living cells
• Mutates DNA (especially dangerous for rapidly growing organisms)
4
Q
What are X-rays
A
X-rays are a form of electromagnetic radiation
that are very penetrating.
X-rays have a very short wavelengths between 0.1
and 10 nm. (The size of a water molecule is about 0.3 nm.)
5
Q
What two things are X-rays 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 can X-rays ‘look inside’ objects?
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 absorption, appear white.
7
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.
8
Q
How are X-rays produced
A
X-rays are produced when fast moving electrons are rapidly decelerated.
High speed electrons are produced using a negatively charged cathode, that is heated (thermionic emission).
Conservation of energy means the loss of Kinetic Energy results in photons being emitted.
The deceleration of electrons is achieved by bombarding electrons onto a metal anode.
If the deceleration is great enough the photons will have energies in the x-ray range of the spectrum.
9
Q
What are CAT scans?
A
• In a CAT scan, an X-ray source is moved around the patient in in a circle.
• X-ray detectors are positioned opposite the X-ray source.
• The X-rays detected are used to build up many cross-sectional views of the body.
10
Q
How does Computerised axial tomography (CAT or CT) scans work?
A
A narrow, pencil-thin, X-ray beam is used to scan across and around the patient. At each position, a measurement of the amount of radiation transmitted through the patient is made. The process is repeated 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.
11
Q
What are Fluoroscopes and how do they work?
A
Fluroscopes are used to show a patient’s 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.
12
Q
What does the Maximum Energy of X-ray photon equal?
A
Maximum Energy of X-ray photon = Maximum Kinetic energy of a single electron
13
Q
How can we find the wavelength from an X-ray tube
A
The energy of a photon is equal to the Planck constant h x frequency f, and maximum frequency of the emitted X-rays, f, is the speed, c, divided by the minimum wavelength λ, so
hf = eV
hc/λ = eV
Therefore,
λ = hc/eV
The wavelength from an X-ray tube is inversely proportional to the accelerating potential difference. Increasing the tube current just increases the intensity of the X-rays.
14
Q
What is the relationship between Wavelength from an X-ray tube accelerating potential difference?
A
The wavelength from an X-ray tube is inversely proportional to the accelerating potential difference. Increasing the tube current just increases the intensity of the X-rays.
15
Q
What is the current in the X-ray machine?
A
The current in the X-ray machine is the rate of flow of electrons
I = N x q
where I = current in amps, N = number of particles flowing each second and q = charge on each particle in Coulombs
16
Q
What does Increasing the tube current do for the intensity if the x-rays (and for voltage)
A
Increasing the tube current increases the intensity of the X-rays.
Higher the voltage, the higher the frequencies of X-rays
17
Q
How does Voltages affects x-rays coming out of the x-ray tube
A
The higher the voltage the more ionising the x-rays are.
The wavelength from an X-ray tube is inversely proportional to the accelerating potential difference.
18
Q
The potential difference between the filament and metal target in an X-ray tube is 40kV.
The charge on an electron is 1.6x10^-19C and its mass is 9.1x10^-31 kg.
Calculate the speed of an electron as it reaches the target.
A
Ek = Ev
Ek = 1/2 mv^2
Ev = 1/2 mv^2
1.6x10^-19 x 40,000 = 1/2 x 9.11x10^-31 x v^2
6.4x10^-15 = 4.555x10^-31 x v^2
1.4x10^16 = v^2
1.19x10^8 = v
19
Q
What is intensity
A
Intensity is the power of an incoming beam of radiation divided by the area over which it is spread
20
Q
Intensity equation for radiation
A
Intensity = Power of Incident Radiation / Area
I (W/m^2) = P(W) / A(m^2)
21
Q
Attenuate Definition
A
To reduce in force, value, amount, or degree; weaken
22
Q
What happens when X-rays are attenuated
A
X-rays create image because they are attenuated. This means they are absorbed so the intensity (Wm-2) decreases.
23
Q
What are the 4 ways Attenuation occurs?
A
- Simple scatter
- Photoelectric effect
- Compton scattering
- Pair production
24
Q
What is attenuation
A
Attenuation is what happens to waves of energy as they travel through a medium because some of the energy is absorbed by it.
25
Simple explanations for attenuation
-Simple Scattering
-Photo-electric effect
-Compton scattering
-Pair Production
⚫ Simple scattering; X-rays can be scattered elastically by the atom
⚫ Photo-electric effect; The X-ray photon disappears and removes an electron from the atom
⚫ Compton scattering; the X-ray photon is scattered by an electron, it’s energy is reduced and the electron is ejected from the atom.
⚫ Pair production; the X-ray photon disappears to produced an electron-positron pair.
26
How does the Photoelectric Effect work in Attenuation
In the photoelectric effect, the incident photon
gives its energy to one of the orbital electrons in an atom of the absorbing material. This causes the electron to be ejected from the atom. This electron then travels through the absorber material ionising and exciting other atoms. An electron from a higher shell may then drop down to fill the 'hole' left by the ejected electron. When this electron drops down it gives out energy in the form of a characteristic X-ray photon.
27
How does Compton Scattering work in Attenuation
Compton scattering occurs at higher photon energies. The incident photon is scattered by an orbital electron of an atom in the absorbing material. Some of the photon's energy is given to the orbital electron. This "Compton electron" goes off in a direction different from that of the scattered photon and it may have any energy from zero up to about two-thirds the incident photon energy. The scattered photon can then undergo further scattering until it is completely absorbed in a photoelectric interaction.
28
How does Pair Production work in Attenuation
In pair production a photon, passing through the electric field of a nucleus, is 'catalysed' to become an electron-positron pair.
At the photon energies involved in medicine this process does not play an important part in absorption, so the key processes are photoelectric effect and Compton scattering.
29
How does Simple Scattering work in 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 scattered) without any change to its energy
30
How much energy is required for:
- Simple scatter
- Photoelectric effect
- Compton scattering
- Pair production
Simple Scatter: 1 —> 20keV
Photoelectric Effect: 20 —> 100keV
Compton Scattering: 0.5 —> 5MeV
Pair Production: > 1.02MeV
31
What are the 3 factors that affect x-ray attenuation
1. The amount of radiation the specific material
absorbs (the attenuation coefficient, μ)
2. The intensities before and after passing through the material (Io and I)
3. The thickness of the material (x)
32
What is attenuation dependant on?
⚫ Energy of the photons
⚫ Thickness of the substance
⚫ Type of substance
33
Equation for Intensity involving the attenuation coefficient
I = Io e ^-μx
Where Io is the initial intensity before any absorption, x is the thickness of the substance and μ is the attenuation coefficient or 5he absorption coefficient of the substance
34
Example Problem:
The attenuation coefficient of bone is 600 m-1 for x-rays of 20keV.
A beam of such rays has an intensity of 20Wm-2.
1. Calculate the intensity of the beam after passing through a 4.0 mm thickness of bone.
2. State the approximate what % of intensity remaining.
Io = 20Wm^-2
x = 0.004m
μ = 600m^-1
1.
I = Io e^-μx
I = 20e^-0.004 x 600
I = 1.81 Wm^-2
2.
(1.81/20) x 100 = 9.05%
35
Why do bones absorb more x-rays than flesh?
The dependence of photoelectric absorption on atomic number explains why bones absorb more X-rays than flesh. Flesh consists mainly of "light elements" such as: carbon, hydrogen and oxygen, so the effective atomic number of flesh is approximately 7. Bone contains high proportions of calcium and phosphorus, so it's effective atomic number is about 14. This means that the attenuation coefficient for bone is much greater than that for soft tissue, so bone casts an "X-ray shadow" on a film. To ensure that photoelectric absorption is the main process it is important to use low energy X-rays, since the probability of absorption depends on Z^3.
36
What is the difference between ionisation and
excitation?
In ionization, an electron is given enough energy to leave the pull of the atom altogether. This means that the electron leaves the atom, leaving a positive ion.
Excitation is when the electrons are given energy, but not enough to leave, only to jump up to a higher electron shell (energy level).
37
How are X-rays used for therapeutic use?
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. They do so by the mechanisms of Compton scattering and pair production.
38
Why is it important that the anode and cathode are charged in the way they are?
Electrons are negative so are repelled from the cathode and attracted to the anode.
39
What is Bremsstrahlung Radiation?
A continuous spectrum of different wavelengths being produced because there are different velocities of the electrons.
Ek = 1/2mv^2 = hc/λ
40
What creates the spikes on the Bremsstrahlung spectrum?
Inner movements of electrons within the atoms if the target give ought specific energy, relaxing to a lower energy level within the atom.
41
Energy spectrums depend on two things. What are they?
Material of the target (changes the k lines)
Voltage if the x-ray tube (changes the cut off wavelength).
Increase V = higher f, smaller λ
42
What is the relationship between the Attenuation Coefficient and the Proton (atomic) Number
Attenuation is directly proportional to the cube of the atomic number
μ ∝ Z^3
43
What are contrast media and when are they used?
Several types 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. In such cases, contrast media are used.
Contrast media are materials of high atomic number e.g. iodine and barium. Liquids containing iodine can be injected into blood vessels to study blood flow, and barium can be swallowed to outline the stomach and intestines.
44
Why are Contrast Mediums used?
Soft tissues have low absorption coefficients, so they don’t show up well in a regular X-ray. A contrast medium needs to be used to improve the visibility of their internal structures. The two most common are iodine and barium compounds – they are relatively harmless to humans as they don’t get absorbed into the blood stream.
45
What is a Contrast Medium
A contrast medium is a substance with a high attenuation coefficient.
When a contrast medium is in the body it absorbs x-rays producing more distinct images.
46
What is Barium used for as a contrast medium
Barium is used to investigate the gastro-intestinal system. Patients consume the element in the form of a ‘barium meal’, a white liquid mixture containing barium sulphate.
47
What is Iodine used for as a contrast medium
Iodine is administered intravenously to observe blood and fluid movement.
It is injected into blood vessels, so doctors can diagnose blockages in the blood vessels or structure of organs that have lots of blood vessels (heart, etc...)
48
What is Computerised axial tomography (CAT or CT) scans
A narrow, pencil-thin, X-ray beam is used to scan across and around the patient. At each position, a measurement of the amount of radiation transmitted through the patient is made. The process is repeated 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.
49
How does a CAT scan work (how are the images made)
• In a CAT scan, an X-ray source is moved around the patient in in a circle.
• X-ray detectors are positioned opposite the X-ray source.
• The X-rays detected are used to build up many
cross-sectional views (slices) of the body.
50
Advantages and Disadvantages of CAT scans
Advantages:
- You get a full 3D or sliced image – much more information displayed
- CAT scans have a better resolution – so can see more detail between different soft tissues
- Can distinguish between materials of similar densities (and therefore attenuation coefficients).
Disadvantages:
- Cheaper
- Quicker
- X-rays require only a few ‘photos’ taken while in CAT scans you’re exposed to much more ionising radiation
- You have to remain still throughout the process – this can be difficult for some
- Larger ionizing radiation exposure compared to standard x-rays.
51
Advantages of CAT scanners
CT scanners quickly produce threedimensional information, showing small differences in tissue density and the depth of particular structures. (Conventional X-rays can’t provide this kind of detail.)
It can be used to find the exact location of a head injury quickly. (This could be important to prevent brain damage.)
Knowledge of the precise location and size can also be important when treating a brain tumour.
52
Worked Example: Absorbed by bone
A collimated beam of X-rays from a 100kV supply is incident in bone. The initial intensity of the beam is 18Wm^-2. The attenuation coefficient of bone is 0.60cm^-1. Calculate the intensity of the beam after it has passed through 7.0mm of bone.
Step 1: Write down all the quantities given. It is important to have the value of μ and x in consistent units (here cm^-1 and cm, respectively).
Io = 18Wm^-2, μ = 0.60cm^-1, x = 0.70cm
Step 2: Substitute the values into the exponential decay equation and calculate the transmitted intensity I.
53
Why are gamma-emitting sources more suited to medical imaging
Gamma-emitting sources are most ideal because gamma photons are the least ionising and can penetrate through the patient to be detected externally. Alpha and beta sources cause more ionisation. They are dangerous and are not used for imaging techniques.
54
What are qualities needed for radioisotopes used in imaging
For a suitable radioisotope in medicine, it must:
- Emit gamma radiation and nothing else
- Half life ~ 4 hours - a few days
- Safe to drink/eat, inject, or breathe in
55
What things can a gamma camera detect?
Gamma cameras detect high energy photons, which includes both x-rays and gamma rays.
56
What makes Technetium-99m is a popular medical tracer
• Short half-life ensures high enough activity to build up an image.
• Short half-life ensures patient is not exposed to radiation for a long period after the procedure.
• Can be combined with other non-radioactive elements to make compounds that target different areas of the body, including the brain, heart, liver, lungs and kidneys (remember radioactivity of nuclei is unaffected by chemical bonds).
57
What happens to ensure that the radioisotope reaches the correct organ or tumour?
In order to use a radioactive source as a tracer it has to be combined with a chemical, so it will target the desired tissues/area of the body.
This is then called a radiopharmaceutical or medical tracer.
58
How can a medical tracer/radiopharmaceutical be followed in the body
Once the medical tracer is in the body, a gamma camera can follow the radiopharmaceutical’s course through the body as the tracer emits gamma photons. The concentrations of radioactivity can be used to find irregularities.
59
What are the 4 main parts of the gamma camera
- Collimator
- Scintillator / NAI Crystal
- Photomultiplier Tubes
- Computer
60
What is the Collimator in a gamma camera and what does it do
The collimator is a honeycomb of long, thin tubes made from lead. Any photons arriving at an angle to the axis of the tubes are absorbed by the tubes, so only those travelling along the axis of the tubes reach the scintillator.
61
What is the Scintillator/NAI Crystal in a gamma camera and what does it do
The scintillator material is often sodium iodide. A single gamma photo striking the scintillator produces thousands of photons of visible light. Not all the gamma photons produce these tiny flashes, because the chance of a gamma photon interacting with the scintillator is about 1 in 10.
62
What is the Photomultiplier Tube in a gamma camera and what does it do
The 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 (voltage). The outputs of all the photomultiplier tubes are connected to a computer.
63
What is the Computer in a gamma camera and what does it do
With these,p of sophisticated software, the electrical signals from the tubes can be processed 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 patients body. The final image is displayed on a screen.
64
What makes a gamma camera different from an X-ray
A gamma camera differs from an X-ray imaging technique in one very important way. It produces an image that shows the function and processes of the body rather than its anatomy.
65
Example Question:
Discuss the advantages of using a gamma-emitting tracer in the patient rather than a beta-emitting tracer.
Gamma photons will pass through the patient without doing much damage and will be detected by the gamma camera.
Beta particles would be absorbed by the body and damage it (as beta radiation is ionising). Additionally a beta-emitting tracer would not be detected by scanner.
Overall, gamma is more penetrating and less ionising than beta
66
There are two types of lead collimator tubes T and S available for a gamma camera.
Tube T is thin and long. Tube S is broad and short.
Discuss which type of tube would be more suitable in a gamma camera.
Tube T would be the better tube as it is long and thin. This means that gamma rays that are only travelling parallel to the tube would be able to fully pass through without hitting the sides.
67
Example Question:
The medical tracer technetium-99m is used in gamma scans. Technetium-99m has a half-life
of 6.0 hours and it emits gamma rays.
A fresh sample of a radiopharmaceutical containing technetium-99m is prepared in the radiography department of a hospital. The initial activity of the radiopharmaceutical is 820 MBq. The radiopharmaceutical is injected into the patient some time later when its activity has dropped to 630 MBq.
Calculate the time in hours between the radiopharmaceutical being produced and it being
injected into the patient.
λ = ln 2 / 6
λ = 0.1155 h^-1
A = Ao e^-λt
630x10^6 = 820x10^6 e^-0.1155t
630/820 = e^-0.1155t
ln 630/820 = 0.1155t
t = 2.3 hours
68
Why does it take some time for a gamma camera to build up a picture?
Radioactivity is a random and spontaneous process. The longer the time, the better the image, as more radiation takes place.
69
Advantages of PET scanners
- It’s a non-invasive technique (the patient doesn’t take the risks with surgery)
- They help diagnose different types of cancers and observe the function of the brain
- Can help doctors identify the onset of of disorders in the brain (like Alzheimer’s)
- They can assess the effect of new medicines/drugs on the body
70
Disadvantages of PET scanners
- PET is very expensive due to the facilities needed to provide medical tracers
- PET scanners are only found at larger hospitals
- Only patients with complex health problems are recommended for PET scans.
71
What medical tracer is used in positron emission tomography (PET)
Fluorine-18 is a versatile medical tracer used in PET. The isotope is a positron emitter with a half life of around 110 minutes. A nucleus of fluorine p-18 decays into a nucleus of oxygen-18, a positron, a neutrino, and a gamma photon.
72
Where and how can Fluorine-18 be made for PER scans
Fluorine-18 has to be made either on-site or in a specialist lab near the hospital using a particle accelerator (or a cyclotron).
One method is where high speed protons collide with oxygen-18 nuclei and produce fluorine-18 nuclei and neutrons. Non+radioactive oxygen-18 is easy to find, as it makes up about 20% of natural oxygen.
73
What happens in a PET scan/How does a PET scan work
⚫ Radio isotopes that emit positrons are injected into the blood in a tracer.
⚫ The isotope gathers where there is a lot of oxygen required (area of cancer)
⚫ The positrons are emitted and only travel a short distance before encountering an electron
⚫ The annihilation of an electron and positron is used to produce a gamma ray pair (because of conservation of momentum)
⚫ They are detected and an internal image of the body is produced
74
What happens in order to create an image for a PET scan
A radioisotope is injected into the patient intravenously. The radioisotope will travel go the affected areas, before it starts the emit positrons. These positrons collide with electrons to form two gamma rays by annihilating each other that travel in opposite directions (perpendicular to the collision of the positron and electron). This means the conservation of momentum is conserved. These gamma rays are both then picked up by the detectors that then work out where the collision took placed by using d = st (distance = speed x time).
75
What happens when a positron and an electron annihilate each other in a PET scan
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 arrival times of these photons at the two diametrically opposite detectors. The voltage signals from all the detects are fed into the computer which generates an image
76
What medical tracer is most often used for PET scans
Most PET scanners used a medical tracer caller fluorodeoxyglucose (FDG), which is similar to naturally occurring glucose, but is tagged with a radioactive fluorine-18 atom in place of one oxygen atom
The advantage of using FDG is that our bodies treat it like normal glucose. The activity from the FDG in the body is monitored using gamma detectors.
77
What are the two main medical tracers used in PET scans
- Fluorodeoxyglucose (FDG)
(Similar to glucose but is tagged with a radioactive fluorine-18 atom in place of one oxygen atom)
- Carbon monoxide
(Made using the carbon-11 isotope - very good at clinging onto haemoglobin molecules in red blood cells)
78
The mass of an electron or positron is 9.11x10^–31 kg. The speed of light is 3.00x10^8 m/s.
Show that the energy of an electron at rest is 8.2x10^–14 J.
E = mc^2
E = 9.11x10^-31 x (3x10^8)^2
E = 8.199x10^-14 J
E = 8.2x10^-14 J
79
What are some benefits of using Ultrasound
It is non-ionising and therefore harmless, it is non-invasive (no surgery is necessary, so no risk of infection), and it is quick.
80
What is an Ultrasound Transducer
An ultrasound transducer is a device used both to generate and to receive ultrasound. It changes electrical energy into sound and sound into electrical energy, by means of the piezoelectric effect.
81
What is Infrasound
Sounds with frequencies below 20Hz are
called infrasound.
82
What is the Crystal in an ultrasound scanner made from
The crystal in an ultrasound scanner is made of lead zirconate titanate (PZT). It has a thickness of half the machine’s working wavelength. This allows the crystal to resonate, producing a large signal.
83
What does a piezoelectric crystal do
At the right voltage and frequency, a piezoelectric crystal emits an ultrasound wave: it is an ultrasound transmitter. Correspondingly, it will turn an incoming sound wave into an alternating voltage, acting as a receiver.
84
What is the Piezoelectric Effect and how do piezoelectric crystal create and detect sound
Using an Alternating Current (where the voltage rapidly changes direction with a frequency f) very fast vibrations can be created.
This is called a piezoelectric transducer.
The piezoelectric effect also works the other way. By exposing the crystal to compressive or tensile forces an emf is produced.
This conversion of kinetic to electrical energy allows piezoelectric crystals to detect as well as produce sound.
The detections are the reflections of ultrasound from the boundary between different media in the body.
85
What are the 2 main types of ultrasound scans
A-Scans
B-Scans
86
What are A-Scans and what do they do?
A-scans are the simplest. They operate in one dimension, allowing distances between boundaries to be measured.
The most common use is in the eye, where boundary change echoes are detectable from the edges of the lens and the retina.
87
How are distances found using ultrasound
Distances are calculated using distance = (speed x time)/2, due to the echoes needing to travel to the boundary and back again
88
How do ultrasound transducers create a pattern of echoes from the boundaries
Ultrasound transducers create a pattern of echoes from the boundaries between different density media allowing an image of these boundaries to be created.
Like any other sound wave at each boundary some of the wave is reflected while the rest passes on through.
89
Where is the pulsed voltage of the ultrasound transducer shown
The pulsed go,step at the ultrasound transducer is displayed on an oscilloscope screen or computer screen as a voltage against time plot.
90
What are A-scans used for
An A-scan only gives information about tissue boundaries along a one-dimensional line. This is useful for very specific measurements, such as the depth of the eye or the diameter of the head of a growing fetus, and can provide enough information to indicate an abnormality.
91
What is the B-Scan used for
The B-scan is the standard technique used for producing a two-dimensional image of a growing fetus.
92
What is a B-scan
A brightness scan (B-scan) is a more complex imaging technique. It uses an array of transducers, each measuring amplitudes, and plots each returning signal as a shaded pixel on a screen, based on its amplitude.
93
What are B-scans and how do they work
B-scans are more complex. The ultrasound images you are familiar with are B-scans.
• ‘B’ stands for brightness. A computer creates dots of different brightness depending on the strength of the echo (and therefore the greater voltage produced by the piezoelectric crystal).
• This is done very rapidly, allowing the transducer to move and produce
images from different angles.
• B-scans is effectively multiple overlaid A-scans.
94
What is the equation for Acoustic Impedance (Z)
Z = ρv
SI unit: kgm^-2s^-1
95
What things can Ultrasound not be used to take images of
Ultrasound cannot be used to take images of the lungs or any other part of the body that contains an air cavity
Ultrasound is also absorbed by bone, which means it cannot be used to image the inside of the skull, but it has many other uses, especially in the diagnosis of heart problems.
96
Why is a coupling medium (gel) used
A coupling medium (gel) is needed to ensure most of the sound produced by the machine is transmitted into the body.
97
What does the amount of reflection of sound waves depend on in Ultrasound
The amount of reflection depends on the acoustic impedance of each material. Acoustic impedance
98
Example Question:
What is the acoustic impedance of wood, if its density is 650kgm^–3 and sound travels through it at 3400ms–1?
2.21x10^6 kgm^-2s^-1
99
What is the equation for the amount of a wave that is reflected (reflection coefficient α)
α = Ir/Ii = (Z2-Z1)^2 /(Z2+Z1)^2
Where Ir is the intensity of the reflected beam and Ii is the intensity of the incident (original) beam, and Z1 and Z2 are the acoustic impedances of the two materials.
100
What does a greater difference in Z (acoustic impedance) result in
The greater the difference in Z between the two
materials, more of the wave is reflected.
101
Calculation example:
Find the percentage of intensity reflected at the
muscle-bone boundary of a patient.
Zmuscle = 1.69x10^6 kgm^-2s^-1
Zbone = 7.6x10^6 kgm^-2s^-1
Ir/Io = (1.69x10^6 - 7.6x10^6)^2 / (1.69x10^6 + 7.6x10^6)^2
Ir/Io = 0.4047
0.4047 x 100 = 40.47%
102
What problem does using coupling gel solve
The air-skin boundary means that about 99.9% of the incident ultrasound will be reflected before it even enters the patient. To overcome this problem, coupling gel, with acoustic impedance similar to that of skin is smeared into 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 patient’s body.
103
What is Impedance Matching or Acoustic Matching
The terms impedance matching or acoustic matching are used when two substances (e.g coupling gel and skin) have similar values of acoustic impedance.
In this case, negligible reflection occurs at the boundary between the two substances.
104
Why must coupling gel be applied to the surface of the patient’s skin
This is to ensure that ultrasound waves are not
reflected back at the surface of the skin by ensuring there is no air between the transducer and the skin (the different in impedance between skin and air is very large due to the differing c and ρ values).
105
Example Question:
What is the wavelength of an ultrasound wave of frequency 4MHz and speed 1500m/s
v = fλ
v/f = λ
1500/4x10^6 = 3.75x10^-4 m
106
Example Question:
A material of density 1.5gcm^-3 transmits sound waves at a speed of 4000ms^-1. What is its acoustic impedance in kgm^-2s^-1?
Z = pc
1.5gcm^-3
Z = 1.5x10^3 x 4000
Z = 6x10^6 kgm^-2s^-1
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What is the Doppler Effect
The Doppler effect is the change of observed wavelength and frequency due to the relative motion between the emitter and the observer.
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How can the direction and speed of blood flow be determined in Doppler imaging
Blood flow causes compression (moving towards the transducer) or rarefaction (moving away from the transducer) of ultrasound waves.
A computer detects the change in frequency to determine the direction and speed of the blood flow.
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What is the formula you need to use to determine the speed of blood flow in Doppler ultrasound?
Δf/f = 2v cosΘ / c
You are given the formula but must know the
meaning of each quantity in it.
• f = original ultrasound frequency
• Δf = difference between original and received
ultrasound frequencies
• v = speed of moving cells in the blood
• c = speed of ultrasound wave
• θ = Angle between blood vessel and
transducer.
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Example Question:
Determine the speed of blood flow using the information below
θ = 60°
Frequency of original ultrasound = 1MHz
Frequency of received ultrasound = 998.5kHz
Speed of ultrasound in blood = 1.6km s-1
Δf/f = 2v cosΘ / c
(1x10^6 - 998.5x10^3) / 1x10^6
(2 x v x cos60) / 1.6x10^3
v = 2.4ms^-1
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What can Doppler Ultrasound be used to image
This technique can be used to reveal blood clots (thrombosis), identify the narrowing of the walls caused by accumulation of fatty deposits (atheroma), and evaluate the amount of blood flow to a transplanted kidney of liver
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How do we work out how fast blood cells move
If a source of sound moves towards the
listener, the waves begin to catch up with
each other. The wavelength gets shorter
and so the frequency gets higher – the
sound has a higher pitch.
We use this principle to work out how fast
blood cells move. Ultrasound reflects off
the blood cells and causes a Doppler shift
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What happens in Doppler imaging when a blood cell is stationary
• The ultrasound probe emits an ultrasound wave
• A stationary blood cell reflects the
incoming wave with the same wavelength: there is no Doppler shift
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What happens in Doppler imaging when a blood cell is moving away from the ultrasound probe
• The ultrasound probe emits an ultrasound wave
• A blood cell moving away from the probe reflects the incoming wave with a longer wavelength
• In reality, there is actually two Doppler shifts. The first one occurs between the probe and the moving blood cell (not shown here) and the second one occurs as the red blood cell reflects the ultrasound.
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What happens in Doppler imaging when a blood cell is moving towards from the ultrasound probe
Now, the blood cell moves towards the probe. It reflects the incoming wave with a shorter wavelength.
