Chapter 27 - Medical Physics Flashcards
What are the components of an X-ray tube
- Cathode
- Anode
What is the role of an anode in an X-ray tube
A target metal, often tungsten, where the electron beam is aimed. It is propelled by a high voltage across the tube, which accelerates the electrons towards it.
What is the role of a cathode in an X-ray tube
Emits a focused beam of electrons when heated by an electric current passing through the filament, shapes like a cup to direct electrons efficiently
How do you increase the X-ray beam intensity
1) Increase the accelerating potential difference, heightening the energy of the electrons.
2) Boost the heating current of the filament to produce more electrons per second
Why do X-rays diminish in intensity?
- Absorption
- Scattering
Why do we X-ray bones?
Bones absorb X-rays more efficiently than soft tissue, making them more prominent on X-rays images.
Contrast can be further improved by administering contrast media, which are visible on X-rays and help in visualising internal structures
How does CAT scans work?
- Rotating X-ray beam and detected to capture images of body slices
- These images are processed to produce detailed cross-sectional views of soft tissue
- Combining multiple slices generate 3D models of internal body structures
What are the key features of medical tracers?
- Tracers provide insights into both the structures and function of organs, unlike traditional X-rays which only show structure
- They concentrate in areas with high metabolic rates, indicating active processes or abnormalities.
- Specific tracers are designed to target distinct tissues and biological functions
Application of Medical tracers
- Areas with reduced blood flow can indicate heart tissue damage or blockages in the coronary artery.
Tracers help identify heart attacks by highlighting areas of dead muscle tissue. - Due to their high metabolic rate, cancer cells consume more glucose, making glucose-based tracers ideal for identifying tumours.
- Tracers are used to map cerebral blood flow and metabolic activity, valuable for diagnosing disorders like Alzheimer’s and Parkinson’s disease.
Key properties of medical ultrasound
- Frequencies span from 1 to 15 MHz
- Upon encountering a boundary between different materials, ultrasound wave is subject to refraction and reflection
- It’s the reflected waves that are used to create images
How are X-rays produced?
Rapidly accelerating or decelerating charged particles - their kinetic energy is transferred into high energy photons
How can differentiate between X-rays and gamma rays
Cannot distinguish them by wavelength
Have to use their methods of production - gamma come from radioactive decay or particle collisions with a mass defect
X -rays produced by accelerating charged particles
Why are X-rays used in medical imaging often referred to as ‘soft X-rays’
Energies lower than gamma rays
What is the general structure of an X-ray tube
Heated filament cathode and tungsten anode with a potential difference between them of up to 200kV and sealed in a vacuum tube
How does an X-ray tube work?
Electrons emitted from the heated filament via thermionic emissions and drawn towards the anode.
They collide with the anode and some of the kinetic energy is released as X-rays in all directions
Rest of energy transferred to heat energy within the anode
Why does the X-ray tube need a vacuum?
To prevent electrons from colliding with molecules in the air before they gain enough energy to release X-rays
How is the anode prevented from overheating
Rotating it so a new section is in contact with the X-ray at all times
OR
Using water as a coolant, circulating it through the anode
How are the X-rays focused into one beam?
encased in a material that is thinner in one area so only X-rays that pass through that section are released from the tube.
Pass through collimator - a series of straight, parallel tubes that absorb any rays that are not travelling parallel to the axis of the tubes
Why is better for X-rays to be in a beam rather than emitted in all directions?
Because it allows them to be directed at specific areas and minimise patient’s exposure to them
X-ray attenuation
When a material absorbs X-rays, decreasing intensity exponentially
Explain the process of taking an X-ray of a patient
- Directed at an area of a patient’s body and pass through the bone and soft tissue
- Bone has higher attenuation coefficient, absorbs more X-rays than soft tissue does
What does a greater attenuation coefficient mean?
The more the material will absorb incident X-rays
Simple scattering process
- X-rays of energy between 1 and 20keV are directed at the material
- X-rays will reflect off layers of atoms or molecules in the material because they have insufficient energy to undergo more complex processes
Photoelectric effect
- X-rays of energy less than 100keV are directed at a material
- X-rays can be absorbed by electrons in the material if they have the same energy as the ionisation energy of the atoms
- When an X-ray is absorbed, a photoelectron is released and another electron may de-excite, releasing another photon in the process
Compton Effect
- X rays of energy between 0.5 and 5MeV are directed at the material
- X-rays will lose a small amount of energy to electrons in the absorbing material due to an inelastic collision between the photon and electron
- Scattered X-ray photon will have less energy than before due to a greater wavelength
- Compton electron will be scattered in a different direction as momentum must be conserved.
Explain the process of pair production
- X-ray of energy greater than 1.02Mev passes through the electric field of an atom
- An electron-positron pair is produced
- Positron will annihilate with another electron and produce two photons
Contrast media and examples of it
High attenuation coefficient materials that have heavy atoms with large proton numbers. They are easily identified in X-ray images as they absorb a lot of X-rays
Examples: Barium and Iodine compared to soft tissue
what does CAT stand for in CAT scan?
Computer Axial Tomography
What is a CAT scan and how do they work?
- Produces a 3D inage of a patients body using alot of 2D images
- X-ray tube generates a fan-shaped beam that is directed onto a patient that is laying down
- Ring of detectors behind the patient to detect the beam intensity
- Tube and detector rotate around patient to create 3D image
Compare CAT scans to conventional X-rays
Pros: better resolution, 3D
Cons: longer, more exposure
Medical tracer
A compound made from radioactive isotope and specific elements that collects in a particular location in the body, used to locate cancerous tumours
How are tracers used in non-invasive diagnosis and which type of radiation is best suited for it?
Tracer is administered to the patient and its radioactive emissions are detected from the outside.
Gamma emitters are the best for this since they are the least ionising and most penetrative
Characteristics and importance of radioactive isotopes in medicine
Higher activities and short half lives
Shorter exposure and image obtained quickly
Why are many of the radioactive sources needed for medical tracers produced on site?
Because they have such short half-lives, meaning they need to be used almost immediately
Technetium-99m
Gamma only emitter, metastable state, remains in an excited state for a prolonged period of time
What are gamma cameras used for?
Detecting gamma photons emitted by medical tracers inside the body
General structure of gamma camera
- Collimator only allows photon travelling a certain direction through
- Scintillation crystal which emits many visible light photons for every incident high-energy photon
-Photomultiplier tube amplifies the signal - Computer detects signal and displays image on the screen
Explain briefly the process that occurs in a gamma camera when gamma photons are incident on it
- Photons collimated and incident on scintillation crystal, absorb gamma photons and release thousands of visible light photons
- Directed to the photocathode where an electron is produced for every incident visible photon
- Electrons are passed into the photomultiplier tube which releases more electrons
- Position of impact on the scintillator is used to locate the emission site of the original gamma photon
Describe the structure of a PET scanner
Ring of gamma cameras placed around a patient in order to create a 3D image
Explain the process of a PET scan
- Positron emitter is given to patient
- Positron travels only a few millimeters before annihilating with an electron and releasing two gamma photons which are detected at two diametrically opposed detectors in a camera ring
- Arrival times are recorded and speed is known so the location of annihilation can be calculated
- Since annihilation is very close to initial positron emission, locate estimated
Examples of radioisotopes used in PET scans
Fluorine 18
Half life of 110 minutes
What is fluorodeoxyglucose and how is it used?
Glucose substitued with Fluorine 18
Used in PET scans to locate areas in the body with high respiration rates such an cancerous tumours
Pros and Cons of PET scans
Pros: non invasive, accurately demonstrate organ function
Cons: expensive, requires tracers to be made on-site
Ultrasound
Longitudinal sound waves with a frequency greater than the range of human hearing
Pros of ultrasound
Non-ionising
Non-invasive
Quick and affordable
What is ultrasound most commonly used for
Find the boundary between two media
Piezoelectric effect
A material that can either:
1) Generates PD when it contracts and expands
2) Will contract and expand when PD is applied
How do piezoelectric crystals work?
Applying a PD will cause it to produce ultrasound waves, piezoelectric crystals absorbing ultrasound will produce an alternating PD
How does an ultraound transducer work?
Alternating PD causes piezoelectric crystals to contract and expand at resonant frequency of the crystals to maximum intensity
Once the ultrasound is created, PD is turned off and reflected signal is detected by transducer
What is an ultrasound A-scan?
Uses a single transducer to emit a signal and detect reflected signal
Determine distance from the device to the point of reflection by using time and speed of sound in the medium
What is an ultrasound B-scan?
A series of B scans that are stitched together to create a 2D image
Transducer moved across the patient’s skin and uses the time and speed to calculate the distance to the boundary at each point
Why are ultrasounds pulsed?
To allow time for the reflected signal to be received by the transducer
Why do smaller wavelengths give more detailed images
They allow the sound waves to diffract around smaller points of detail on the object that is being scanned
What is acoustic impedence
The product of a sound wave’s density and speed of sound in the medium
Explain what happens when an ultrasound hits the boundary between two media
A fraction of the wave’s energy/intensity is reflected and the rest is transmitted.
The amount is dependent on the acoustic impedance of each medium
Reflection coefficient
The ration of the reflected intensity to the original intensity
How can you calculate the reflection coefficient using the impedances of two media
Z1 is the impedance of the first medium and Z2 is the impedance of the incident medium
Formula only applies when the ultrasound is incident on the boundary at 90º
When Z1 and Z2 are very close what happens?
Most of the energy/intensity is transmitted
When Z1 and Z2 are very different, what happens?
Most of the energy/intensity is reflected
How can reflection be minimised when using a transducer against a patient’s skin
Since the impedances of air and skin are very different, most of the intensity would be reflected
However, using an impedance matching gel between the transducer and the skin minimise reflection
Doppler Effect
There is a change in frequency of a wave when it is reflected or emitted by a moving source
What is Doppler imaging
A non-invasive technique used to measure blood flow
Explain the process of Doppler Imaging
1) Ultrasound waves are send to the blood vessels
2) The iron in the blood cells reflects the waves back to the transducer and the frequency is shifted depending on the direction and how fast the cells are moving
