Module 6: C27 - Medical Imaging Flashcards
What are some types of Non-Invasive Techniques
• X-ray
• MRI (not on spec)
• Ultrasound
• CT Scan/CAT Scan
• Fluoroscopes (like a video X-ray)
What are the advantages of non-invasive techniques?
The advantages is that the person doesn’t need to be cut open or have open surgery
What 2 things does Ionising Radiation do
Ionising Radiation does two things
• Kills living cells
• Mutates DNA (especially dangerous for rapidly growing organisms)
What are X-rays
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.)
What two things are X-rays used for
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.
How can X-rays ‘look inside’ objects?
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.
How are X-rays detected
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.
How are X-rays produced
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.
What are CAT scans?
• 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.
How does Computerised axial tomography (CAT or CT) scans work?
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.
What are Fluoroscopes and how do they work?
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.
What does the Maximum Energy of X-ray photon equal?
Maximum Energy of X-ray photon = Maximum Kinetic energy of a single electron
How can we find the wavelength from an X-ray tube
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.
What is the relationship between Wavelength from an X-ray tube accelerating potential difference?
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.
What is the current in the X-ray machine?
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
What does Increasing the tube current do for the intensity if the x-rays (and for voltage)
Increasing the tube current increases the intensity of the X-rays.
Higher the voltage, the higher the frequencies of X-rays
How does Voltages affects x-rays coming out of the x-ray tube
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.
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.
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
What is intensity
Intensity is the power of an incoming beam of radiation divided by the area over which it is spread
Intensity equation for radiation
Intensity = Power of Incident Radiation / Area
I (W/m^2) = P(W) / A(m^2)
Attenuate Definition
To reduce in force, value, amount, or degree; weaken
What happens when X-rays are attenuated
X-rays create image because they are attenuated. This means they are absorbed so the intensity (Wm-2) decreases.
What are the 4 ways Attenuation occurs?
- Simple scatter
- Photoelectric effect
- Compton scattering
- Pair production
What is attenuation
Attenuation is what happens to waves of energy as they travel through a medium because some of the energy is absorbed by it.
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.
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.
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.
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.
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
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
What are the 3 factors that affect x-ray attenuation
- The amount of radiation the specific material
absorbs (the attenuation coefficient, μ) - The intensities before and after passing through the material (Io and I)
- The thickness of the material (x)
What is attenuation dependant on?
⚫ Energy of the photons
⚫ Thickness of the substance
⚫ Type of substance
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
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.
- Calculate the intensity of the beam after passing through a 4.0 mm thickness of bone.
- State the approximate what % of intensity remaining.
Io = 20Wm^-2
x = 0.004m
μ = 600m^-1
- I = Io e^-μx
I = 20e^-0.004 x 600
I = 1.81 Wm^-2 - (1.81/20) x 100 = 9.05%
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.
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).
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.
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.
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
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.
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
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
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.
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.
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.
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.
