Module 6.5 Flashcards
A-Scan
A method of scanning tissue that involves placing an ultrasound emitting
transducer on the surface of the body, and then measuring reflections of emitted
pulses. A-Scans are used to measure the fetal head size during pregnancy.
Acoustic Impedance
The product of the speed of sound through a given
medium, and the density of the medium.
Anode
A positively charged electrode
Attenuation of X-Rays
The reduction of X-ray intensity as they pass through
matter
B-Scan
A method of scanning tissue, used for more complex structures than
A-scans. Instead of the echo signals controlling the y-gain (as in A-scans), they
control the brightness of the oscilloscope spot. B-scans are used to determine the
placenta’s position during pregnancy.
Cathode
A negatively charged electrode
Compton Effect
The decrease in a photon’s energy when it is scattered by a
charged particle. This results in a decrease in the photon’s frequency and
therefore an increase in its wavelength.
Computerised Axial Tomography Scanning
A scanning method that produces
a cross section of the body by rotating a monochromatic x-ray beam around it, in
combination with a series of detectors. Whilst it produces higher resolution images
that ultrasound and is non-invasive, it is highly ionising and costly.
Contrast Media
A contrast medium is a substance that ensures that there is a
significant difference between the density of the area being scanned and the rest
of the body. Barium is often chosen due to its high proton number. It is consumed
by the patient.
Gamma Camera
A type of detector used in PET scanners, consisting of
photomultiplier tubes that convert gamma photons into electrical pulses.
Medical Tracers
Gamma emitters that have suitably short half-lives to be
ingested into the body, and be detected externally for the duration of a medical
process.
Pair Production
The production of a particle and antiparticle pair from a
sufficiently high energy photon
Photoelectric Effect
The emission of electrons from a metal surface when light
above a certain frequency is shone on it.
Piezoelectric Effect
An effect shown by crystals like quartz. When a potential
difference is applied, the crystal will mechanically deform. Likewise, when the
crystal is deformed, a potential difference is produced
Positron Emission Tomography Scans
A scanning technique that produces
cross-sectional and 3D images. It involves a radionuclide being injected into the
body, which then releases gamma photons that are detected by the scanning
machine.
Simple Scatter
The process of low energy photons scattering off a particle
without a change of momentum
Ultrasound
Sound waves with a frequency higher than the upper-frequency
audible to the human ear (20kHz).
X-Ray Tube
An evacuated tube which converts electrical signals into X-rays
How are X-rays produced
By rapidly accelerating or decelerating charged particles - their kinetic energy is transferred into high-energy photons
How can you differentiate between X-rays and Gamma rays
X-rays and gamma rays have frequencies that overlap, so you cannot distinguish them by their wavelengths. Instead you have to use their method of production - gamma rays come from radioactive decay or particle collisions with a mass defect, whereas X-rays are produced by accelerating charged particles
Why are X-rays used in medical imaging often referred to as soft X-rays
Because they have energies that are lower than gamma rays
Describe 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 X-ray tube work
Electrons are emitted from the heated filament via thermionic emission and drawn towards the anode. They collide with the anode and some of their kinetic energy is released as X-rays in all directions and the rest is transferred to heat energy within the anode
Why does the X-ray tube need a vacuum
To prevent electrons from colliding with molecules of air before they gain enough energy to release X-rays
How is the anode prevented from overheating
By either rotating it so that a new section of it is in contact with the X-rays all the time, or by using water as a coolant, circulating it through the anode
How are the X-rays focused into one beam
The vacuum tube is encased in a material that is thinner in one area, so only X-rays that pass through that section are released from the tube. They then pass through a collimator - a series of straight, parallel tubes that absorb any rays that are not travelling parallel to the axis of the tubes
Why its it better for X-rays to be a beam rather than emitted in all directions
Because it allows them to be directed at specific areas and minimises the patient’s exposure to them
Explain the process of taking an X-ray of a patient
X-rays are directed at an area of a patient’s body and pass through the bone and soft tissue. Since bone has a higher attenuation coefficient, it absorbs more X-rays than soft tissue does. If photographic film is placed behind the patient, the areas where the bone is will not blacken as much as the areas of soft tissue, creating an image of the inside of the patient’s body. However, nowadays, digital detectors are used in place of photographic film
The greater the attenuation coefficient
The more the material will absorb the incident X-ray
Explain the process of simple scattering
- X-rays of energy between 1 and 20keV are directed at a material
- The X-rays will reflect off layers of atoms or molecules in the material because they have insufficient energy to undergo more complex processes (like the photoelectric effect)
Explain the process of the photoelectric effect
- X-rays of energy less than 100keV are directed at a material
- The 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
Explain the process of the Compton effect
- X-ray of energy between 0.5 and 5MeV are directed at a material
- The X-rays will lose a small amount of their energy to electrons in the absorbing materials due to an inelastic collision between the photon and electron
- The scattered X-ray photon will have less energy than before (greater wavelength)
- The Compton electron will be scattered in a different direction as momentum must be conserved
Explain the process of pair production
- An X-ray of energy greater than 1.02MeV passes through the electric field of an atom
- An electron-positron pair is produced
- The positron will annihilate with another electron and produce two photons
- This process is not very important in medical X-rays as the photon energies are usually not high enough to cause pair production
Give 2 examples of contrast media
Barium (Z=56) or Iodine (Z=53) - compared to soft tissue (Z=7)
What does CAT stand for in CAT scan
Computerised Axial Tomography
What is a CAT scan and how do they work
A CAT scan is a 2D X-ray image of a patient’s body made up of lots of 2D images. An X-ray tube generates a fan-shaped beam that is directed onto a patient that is laying down. There is a ring of detectors behind the patient to detect the beam intensity. The tube and the detectors rotate around the patient and up and down their body to create a 3D image of their whole body
Compare CAT scans to conventional X-ray image
CAT scans give a better resolution image, and having a 3D image makes it easier to assess the injury. However, CAT scans take significantly longer than conventional X-rays so the patient is exposed for longer
How are tracers used in a non-invasive diagnosis and which type of radiation is best suited for it
The 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
what are the characteristics of radioisotopes used in medicine and why are they important
They usually have high activities and short half-lives. This is so the image can be obtained quickly, the patient is not exposed to harmful radiation for longer than necessary and only a small amount of the radioactive substance is required
Why are many of the radioactive sources needed for medical traces produced on-site
Because they have such short half-lives meaning they need to be used almost immediately
What is Technetium-99m
A gamma-only emitter. It is in a metastable state. Meaning it remains in an excited state for a prolonged period of time. In this state, it had a half life of 6 hours. After this, it will decay into Technetium-99 which is a stable isotope with a half life 210,000 years
What is a gamma camera used for
Detecting gamma photons emitted by medical tracers inside the body
Describe the general structure of a gamma camera
-Collimator which only allows photons travelling a certain direction through
- Scintillation crystal which emits many visible light photons for every incident high-energy photon
- Photocathode which produces an electron for every incident visible photon
- Photomultiplier tube which amplifies the signal
- Computer which detects the signal and displays the image on a screen
Explain briefly the process that occurs in a gamma camera when a gamma photons are incident on it
Photons are collimated and then incident on a scintillation crystal, which absorbs the gamma photons and releases thousands of visible light photons.
These are then directly to a photocathode where an electron is produced for every incident visible photon.
The electrons are passed into a photomultiplier tube which releases more electrons. The positions of impact on the scintillator is used to locate the emission site of the original gamma photon. The signal is detected by the computer and the image is displayed
Describe the structure of PET scanner
A ring of gamma cameras placed around a patient in order to create 3D image
Explain the process of a PET scan
A positron emitter is administered to the patient. The positrons travel only a few mm before annihilating with an electron and releasing two gamma photons which are detected at two diametrically opposed detectors in the camera ring. The arrival times are recorded and speed is known so the location of annihilation can be calculated. Since annihilation is very close to the initial positron emission, the location of the tracer can be estimated. This is repeated until a 3D image can be produced
Give an example of a radioisotope commonly used in a pet scan and its half life.
Flurorine-18 (made using Oxygen-18 with protons)
Has a half-life of approximately 110 mins
What is fluorodeoxyglucose and how is it used
Glucose substituted Flourine-18. It is used in PET scans to locate areas in the body with high respiration rates such as cancerous tumors or active areas of the brains
Evaluate the pros and cons of PET scans
Pros: non-invasive, accurately demonstrate organ function, can be used to observe the effects of various medications.
Cons: expensive, require tracers to be made on-site
What are the pros of using ultrasound
It is non-ionising, non-invasive , quick and affordable
What is ultrasound most commonly used for
Finding the boundary between two media
Explain the Piezoelectric effect
A piezoelectric material either:
1) Generates a pd when it is contracted or expanded
2) Will contract or expand when a pd is applied
How do piezoelectric crystals work
Applying a pd to a piezoelectric crystal will cause it to produce ultrasound waves, and a piezoelectric crystal absorbing ultrasound waves will produce an alternating pd. They tend to be made from quartz, polymeric or ceramic materials
How does an ultrasound transducer work
It has an alternating pd that causes a piezoelectric crystal to contract and expand at a resonant frequency of the crystal to max intensity. Once the ultrasound has been created, the pd is turned off and the reflected signal is detected by the trasducer
What is an ultrasound A-scan
It uses transducer to emit a signal and then detect the reflected signal. It is used to determine the distance from the device to the point of reflection (usually a boundary between media) by using the time and the speed of sound in the medium
What is an ultrasound B-scan
A series of A-scans that are stitched together to great a 2D image. The transducer is moved across the patient’s skin and uses the time and speed to calculate distance to a 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
Explain what happens when an ultrasound hits a 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
What is the reflection coefficient
The ratio of the reflected intensity to the original intensity
What happens when Z1 and Z2 are very close
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 minimized 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 that has a Z similar to skin, minimises reflection
What is Doppler imaging
A non-invasive technique to measure blood flow
Explain the process of Doppler imaging
1) Ultrasound waves are sent into a blood vessel
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
