Module 6: Chapter 27 - Medical Imaging Flashcards
What are examples of non-invasive medical diagnosis techniques?
- X-ray
- MRI
- Ultrasound
- CT scan / CAT scan
- Fluoroscope
What can ionising radiation cause?
Ionising radiation can:
* Kill living cells
* Mutate DNA
How were x-rays discovered?
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.
Why were x-rays named x-rays?
“x” signifies an unknown quantity as it was unknown what x-rays where when they were first discovered
What can x-rays be 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 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?
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.
Describe the structure of an x-ray tube
Tube is also evacuated
Describe how and x-ray tube produces x-rays
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.
What precautions are taken when using x-rays and why?
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
How can X-rays be used for medical imaging?
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.
What is an X-ray tube?
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
Why is an x-ray tube evacuated?
So that electrons can pass through the tube without interacting with gas atoms
What is the target metal?
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
Why is tungsten often used as the target metal in an X-ray tube?
Tungsten has the highest known melting point of all elements
Why does the target metal in an X-ray tube have to have a high melting point?
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
What is the equation for the minimum wavelength of X-ray produced?
V = potential difference between anode and cathode in X-ray tube
Explain how the equation “λ = hc/eV” is derived
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
What happens if you increase the potential difference in an X-ray tube?
- The frequency of x-ray produced increases
- The energy of the X-ray produced increases
What happens if you increase the current in an X-ray tube?
The intensity of X-rays increases
What is a CAT/CT scan?
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
What does “CAT” scan stand for?
Computerised Axial Tomography Scan
What is a Fluroroscope?
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
What are contrast media?
Contrast media are materials of high atomic number and thus have high absorption coefficients
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
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
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
What does a typical X-ray spectrum look like?
Faster electrons form shorter wavelength x-rays and slower electrons form longer wavelength x-rays
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
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
Describe the intensity of a collimated X-ray beam
The intensity remains constant
What is attenuation?
The decrease in the intensity of electromagnetic radiation as it passes through matter and/or space
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
What are the 4 X-ray attenuation mechanisms?
- Simple Scatter
- Photoelectric Effect
- Compton Scattering
- Pair Production
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
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.
This type of mechanism is cominnant when an X-ray image is taken as the machines use 30-100kV supplies
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
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
What is the mechanism for Simple Scattering X-ray attenuation?
What is the mechanism for Photoelectric effect X-ray attenuation?
What is the mechanism for Compton Scattering X-ray attenuation?
What is the mechanism for Pair Production X-ray attenuation?
What range of energy for an incoming x-ray will result in x-ray attenuation occuring via simple scattering?
1 - 20 keV
What range of energy for an incoming x-ray will result in x-ray attenuation occuring via the photoelectric effect?
20 - 100 keV
What range of energy for an incoming x-ray will result in x-ray attenuation occuring via compton scattering?
0.5 - 5 MeV
What range of energy for an incoming x-ray will result in x-ray attenuation occuring via pair production?
. >1.02 MeV
What 3 factors affect x-ray attenuation?
- The attenuation coefficient
- The incident intensity of the X-ray
- The thickness of the material
What is the equation for X-ray attenuation?
FOR A COLLIMATED BEAM OF X-RAYS
μ = Attenuation coefficient, x = thickness
What are 2 common contrast media?
Barium and Iodine compounds
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.
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
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
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
In the Photoelectric Effect, μ ∝ Z³
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
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
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
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.
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
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
What is the decay equation of Technetium-99m?
Draw the structure of a gamma camera:
Components: Collimator, Scintillator, Photomultiplier tubes, Computer, Display
What is the equation for the decay of Fluorine-18?
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
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
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.
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.
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.
What is the structure of an ultrasound transducer?
Key components: Cable, Damping Material, Piezoelectric Crystal, Electrodes, Acoustic window
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
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.
What is the equation for the intensity reflection coefficient, α?
α = intensity reflection coefficient, Iᵣ = Reflected Intensity, I₀ = Incident Intensity, Z₁ = Acoustic Impedance of Incident Medium, Z₂ = Acoustic impedance of second Medium
What is the equation for doppler imaging?
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
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.
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!
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.
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
