6.5: Medical Imaging, F Flashcards
Explain one in which x-rays are produced?
When fast moving electrons with high levels of energy decelerate rapidly by smashing into a metal
or
When a fast moving electron ejects an inner shells electron of the metal atoms, outer shell electrons then move down levels and release energy(x-rays)
Explain another way in which x-rays are produced?
When a fast moving electron ejects an inner shells electron of the metal atoms, outer shell electrons then move down levels and release energy(x-rays)
or
When fast moving electrons with high levels of energy decelerate rapidly by smashing into a metal
Roughly what percentage of electrons’ kinetic energy is converted into x-rays? and what is the rest converted into
1% and the rest is converted into heat
Describe and briefly explain 1 way to increase the intensity of an x-ray beam
-Increase the tube voltage ∴ electrons gain more kinetic energy and can knock out electrons from deeper shells
or
-Increase the current supplied to the filament ∴ temp rises, liberating more electrons per second, producing more x-ray photons per second
Describe and briefly explain an alternative way to increase the intensity of an x-ray beam
-Increase the current supplied to the filament ∴ temp rises, liberating more electrons per second, producing more x-ray photons per second
or
-Increase the tube voltage ∴ electrons gain more kinetic energy and can knock out electrons from deeper shells
Define: Attenuation
Gradual decrease in intensity
List the 4 types of interactions x-rays can have with matter
- Simple scattering
- Photoelectric effect
- Compton scattering
- Pair production
Explain what is simple scattering (in terms of an x-rays)
Low-energy x-ray encounters the electrons in an atom and is does not contain sufficient energy to cause ionisation and ∴ is deflected
Explain what the photoelectric effect is
A photon is absorbed by an electron which is then ejected from out of the atom. Another electron then fill the gap, which emits a photon
Explain what Compton scattering is
A high energy photon knocks an electron out of an atom, which causes the photon to lose energy and be scattered.
Explain what Pair production is
Extremely high energy photon decays into an electron and positron from interaction with the nucleus of an atom
Explain the purpose of using a contrast media
If tissues in a region of interest have a similar attenuation coefficients, a contrast media allows you to see details clearly in x-ray images as they have high density which allows for a bigger contrast (e.g.barium)
What CAT(Computerised axial tomography) scan
Process of using multiple x-ray scans to produce images of ‘slices’ through the body, creating a 3D image.
Briefly describe the process of a CAT scan
1) Patient lies on table which slides in and out of a ring made up of detectors and rotating x-ray beams
2) X-ray beam fans out and rotates around the body, being picked up by the detectors
3) Computer then calculates total attenuation caught by part of the body ∴ creating a high quality image
List the advantages of using a CAT/CT scan over regular x-rays
- Accurate and doesn’t get obstructed by other structures of the body
- Sensitive to changes in density
- 3D images allows you to rotate image and edit what you want to see
What is the purpose of medical tracers
A radioactive substance that is used to diagnose functions of organs. Unlike x-rays which only display the structure, they display the structure and function
What do medical tracers usually consist of
Radioactive isotopes (such as technetium-99 or fluorine-18) bound to a substance that is used by the body (such as water or glucose)
How is a tracer inserted and guided into a region of interest
Injected or swallowed and will go where ever the bounded substance would normally go.
How are gamma photons detected
A tracer emits gamma photons which are picked up by a gamma camera
Define: Collimator
Device for producing a parallel-sided beam of EM radiation (A bit like a polariser)
State and explain 3 uses of Tracers
- Show areas of damaged tissue by detecting decreased blood flow
- Identify active cancer tumours by detecting increased metabolic activity
- Show blood flow and activity in brain which helps research and treat neurological conditions.
List the 5 main parts of a gamma camera
- Lead shield
- Lead collimator
- Sodium iodide crystal/Scintillator
- Photomultiplier tubes
- Electric circuit
Define: Scintillator
A material that produces (many photons of) visible light when struck by a high-energy photon
Define: Photomultiplier tube
A device used to detect flashes of light from the crystals and turns them into pulses of electricity
What is the purpose of a Lead shield in a gamma camera
To stop radiation from other sources entering the camera
Define: Positron emission tomography(PET)
The use gamma photons, produced by positrons annihilating with electrons inside the body mapping out biologically active areas within the body
Describe the a skeleton process of a PET scan
1) Patient injected with radioactive tracer
2) Positrons emitted by radioactive isotope annihilate with electrons in organs creating pairs of gamma rays at π radians to each other
3) Detectors around the body record these and build up a map of radioactivity
How can a doctor use the information given by a PET
Distribution of radioactivity matches up with metabolic activity (tracer used by harder working cells) ∴ you can diagnose illnesses(e.g. higher activity of cancer cells)
State some positives of using a PET scan
- Diagnostics without the need for surgery
- Radioactive tracers have a short half-life
- Equivalent to a standard x-ray exposure unlike(CAT, 5 years worth of background radiation)
State some negatives of using a PET scan
- Limited window for scanning
- Expensive
- Long(2-4 hours)
- Not as detailed as CAT
Describe 3 possible side effects from ionising radiation
- Cell mutation/cancerous tumours by altering/damaging cell DNA
- Cell sterility by halting cell reproduction
- Cell death. Destruction of cell
What are some of the macroscopic(visible to the naked eye) effects of ionising radiation
- Tumours
- Skin burns
- Sterility
- Radiation sickness
- Hair loss
- Death
When is ionising radiation used
When the benefits outweigh the risks
Describe 2 properties of Ultrasound
- Longitudinal wave
- frequencies of >20,000 (higher f than humans can hear)
What range of frequencies in ultrasounds are used for medical purposes
1 - 15 MHz
Define: Transducer
E.g. Microphone, converts non-electrical signals into an electrical signal
Define: Piezoelectric effect
Change in volume of a material when a voltage is passed through it opposite faces
or
Production of an induced e.m.f when certain crystals are placed under stress
Define: Acoustic impedance
Defined by equation Z=pc, where p = density and c is the speed of sound. Units are kg/m^2/s
Define: Impedance matching
When 2 substances have similar or identical acoustic impedances ∴ reduces the intensity of reflected ultrasound at the boundaries between them
List the main advantages of using Ultrasound imaging
- No known hazard
- Real-time images
- Cheap and portables
- Quick
List the main disadvantages of using Ultrasound imaging
- Cant penetrate bone
- Cannot pass through air
- No detail on solid masses
What is the purpose of a coupling medium
Displaces air and has an impedance much closer to that of body tissue(usually a gel)
How are piezoelectric crystals used
When an alternating p.d. is applied across a crystal, the crystal vibrates at the same frequency. It can act as the transmitter and receiver of ultrasound
What is an A-scan
- Amplitude scan a measuring system, which sends short pulses of ultrasound into the body simultaneously with an electron beam sweeping across a screen.
- Position of reflected pulse reflects time the ‘echo’ took to return allowing you to find the distance between structures
What is a B-scan
- Brightness scan, electrons sweep down the screen instead of across
- Amplitude of reflected pulses is displayed as the brightness of the spot
How are ultrasounds affected by the doppler effect
Ultrasounds reflected at an angle moving cells undergo a change in frequency. The change in frequency allows doctors to find the speed at which the cells are moving
