M2 Topic 2: Sources of Medical Radiation Flashcards
(54 cards)
1
Q
Concept of energy spectrum
A
Radiation sources can produce a collection of waves/particles
Can be…
- Monoenergetic
- Polyenergetic
- Mixed emissions
2
Q
Monoenergetic
A
All radiation waves/particles in radiation field have same energy
3
Q
Polyenergetic
A
Waves/particles in field have different range of energies
4
Q
Mixed emissions
A
Emission of mixture of different types of radiation, e.g. gamma photons and beta particles
5
Q
Collimation
A
Control shape and edges of radiation beam
6
Q
What do watts (W) measure?
A
How much electrical energy is being converted into radiation energy each second
- As rad sources don’t produce all radiation waves/particles at once, rate of production varies over time
Watts = Energy/Time
= Joules (J) / Seconds (s)
7
Q
What do becquerel’s (Bq) measure?
A
Measures number of disintegrations each second, represents 1 nuclear decay each second (radioactivity)
8
Q
Source configurations
A
- Reflection (ultrasound)
- Transmission (radiography, radiation therapy)
- Emission (nuclear medicine)
9
Q
What is divergence?
A
Phenomenon whereby radiation spreads out as it travels away from source
- Occurs in all directions
- Continues until all radiation energy has been transformed into another form of energy
10
Q
Effect on divergence when object-to-image distance decreases?
A
- Less divergence
- Less magnification
- More image sharpness
11
Q
Effect on divergence when object-to-image distance increases?
A
- More divergence
- More magnification
- Less image sharpness
12
Q
What is intensity?
A
Amount of radiation energy (J) per unit area at a given distance from source at a particular time (s)
- Also influences image quality
13
Q
Effect on intensity as source-to-image distance decreases
A
Increased intensity
14
Q
Effect on intensity as source-to-image distance increases
A
Less intensity
- As radiation is diverging, energy becomes more spread out, further reducing intensity as area increases
15
Q
Similar triangles formula
A
d1/s1 = d2/s2
d1 = height of triangle 1
d2 = height of triangle 2
s1 = base of triangle 1
s2 = base of triangle 2
ratio between the 2 triangles
16
Q
Change in intensity formula
A
l1/l2 = (d2)^2/(d1)^2
l1 = intensity of source 1
l2 = intensity of source 2
d1 = distance from source 1
d2 = distance from source 2
17
Q
What does amperes (A) measure?
A
Electrical current, amount of charge passing through a point in the conductor per unit time.
1A = 1C/s
- Movement of particles that carry electric charge through closed looped conductor
18
Q
How do electrically generated radiation sources work
A
- Power source with cathode and anode, separates subatomic particles into +ve and -ve components, representing form of potential energy
- +ve and -ve charges in heightened energy state, waiting to reunite with each other and return to stability
- As soon as pathway exists between cathode and anode, potential energy stored in charged particles is transformed into kinetic energy
- This pushes electrons along pathway/circuit from anode towards cathode
- Electrons join +vely charged ion to create stable atom
19
Q
What do volts (V) measure?
A
Represents potential energy per unit charge available to move electrons through circuit
1V = 1J/C
20
Q
What is direct current (DC)?
A
Current flowing consistently in one direction
21
Q
What is alternating current (AC)?
A
Flow changes direction periodically as power source (usually coil of wire) rotates in magnetic field, changing the +ve and -ve terminals
22
Q
Radioisotope selection in nuclear medicine
A
- Need to have half-life suitable for procedure so optimal amount of radiation being emitted has enough time allowed for radiopharmaceutical to be metabolised by cells of interest
- Also informed by type of emission and energy of emissions
- Nearly all naturally occurring radioactive elements unsuitable, thus all radionuclides in nucmed are artificially generated
23
Q
What is a radionuclide
A
Any atom with unstable nucleus that emits radiation
24
Q
What is a radioisotope?
A
Specific version of radionuclide
- All diff radioisotopes have same number of protons (atomic identity)
- But have diff numbers of neutrons, making them a unique radioisotope
25
What are the two main mechanisms used for radionuclide generation?
- Simple bombardment
- Nuclear fission
26
What is simple bombardment
- Hitting nucleus with subatomic particles to upset balance of protons to neutrons
- Electrically generated in another type of particle accelerator, called a cyclotron
27
What is nuclear fission
Hitting the nucleus of heavier element with subatomic particles can cause original nucleus to split into two daughter elements, releasing EMR or particulate radiation in process
- Occurs in nuclear reactor
- Once reaction starts, unable to stop until all parent atoms have undergone fission and become stable
28
Radioactive decay formula
A(t) = A0*e^(-Dt)
A0 = initial radioactivity at t=0
D = decay constant
A(t) = amount of radioactivity at time (t)
29
Decay constant formula
In(2)/Half-life
30
Overview of ultrasound unit
- Monitor
- Transducers (probes)
- System controls
- Power supply
- Computer
31
Different types of transducers
Number of different transducers available in range of shapes and sizes to suit procedures and body sites.
- Linear probe = wide footprint, high frequency range, best for imaging shallow body regions such as neck
- Curved (convex) probe, curvilinear array with moderate frequencies = best for deeper structures such as abdomen
- Phased array probe = narrow, triangular shape, lower frequency range = suitable for cardiac and brain imaging
32
How do ultrasound probes work?
Generate reverberation by reflecting sound waves from various body tissues
- Material called piezoelectrical crystal transforms electrical energy into mechanical pressure in the form of sound waves, and vice-versa
- Examples of these crystals include quartz, tourmaline, rochelle salts, synthetic ceramics, polymers
- When electric current is applied to these crystals, they vibrate to produce ultrasound waves, exact frequencies produced can be altered by changing nature of electric current applied
- Object of interest reflects transmitted wave back to transducer, causes compression of crystal that is converted back into electrical current
- Processed by computer to produce image in real time
Probe must be placed in direct contact with imaged object to ensure signal transmission
- To assist, coupling gel often used to eliminate air gaps and reduce friction on patients skin as probe is moved
33
Overview of x-ray unit
- High voltage circuit and filament circuit with anode and cathode
- Filament
- Tungsten or Rhenium target
- Glass envelope
- Port/window
- Vacuum
34
How does an x-ray tube work
- Heating causes filament to emit electrons though thermionic (production of charged particles through heat) emission
- Electrons emitted from filament material replaced by those flowing in electric current so filament stays atomically stable
- Emitted electrons are accelerated towards anode via application of high voltage between anode and cathode
- Voltage can fluctuate during x-ray exposure due to conversion from AC to DC current, concept of peak voltage used to describe max different being applied between cathode and anode
- Voltage range from 25000 to 120000
- The high-speed electrons, accelerated by voltage potential, collide with tungsten target, produces continuous spectrum of polyenergetic x-ray photons
35
What are the two main mechanisms of interaction resulting in production of photons?
- Bremsstrahlung interactions
- Characteristic interactions
36
Bremsstrahlung interactions
Occur when high-speed electrons are decelerated or deflected by nucleus of atom
37
Characteristic interactions
Occur when high-speed electrons eject inner shell electrons from atom, causing outer-shell electrons to fill vacancies and emit x-ray photons with specific energies
38
How does the collimator work
Consists of two sets of adjustable lead shutters mounted at different levels
- Each set made of two pairs of lead leaves/blades that can be moved to shape x-ray beam
- Positioning on top of the other, allowing full opening and closing
- Entrance shutters (focal spot) located immediately below window of x-ray tube, limits initial size of beam as it exits tube
- Second set of shutters refines and shapes beam once its passed aluminum filter (absorbs low energy x-rays that don't have enough energy to travel to detector)
- This helps reduce absorbed dose, improves image quality by removing non-diagnostic radiation, + beam hardening
- Light source and mirror inside collimated used to simulate radiation beam path
39
What is beam hardening
Filtering out soft photons on lower end of spectrum, leaving behind radiation beam that has higher overall energy
40
Types of x-ray tube technology in diagnostic radiography
- General x-ray
- Computed tomography (CT)
- Dual energy x-ray absorptiometry (DEXA)
- Orthopantomograms (OPG)
- Angiography
- Mammography
- Fluoroscopy
41
Overview of linear accelerator (linac)
- Power supply
- Gantry
- On-board imager
- Treatment couch
Most RT treatments delivered in AUS use MV (megavoltage) x-rays
- 1000 times more energetic than kV x-rays, achieved by accelerating electrons inside x-ray tube to near speed of light using linac
42
Use of linac during treatment
- Patient lies on treatment couch whilst linac rotates around them
- Delivers high energy x-rays once RT have confirmed position of patient and tumour target
- Also include on-board imager, used to take x-ray images of patient before treatment starts to confirm above point before treatment
43
Components of the linac's power supply
Two components
- First provides AC power to electron gun (similar to anode of x-ray tube)
- Second provides high-frequency radio waves to wave guide
As electron gun produces electrons, they are accelerated through wave guide where they 'surf' the radio waves, gaining speed as they move through the wave guide.
Once at machine head, high energy electrons pass through series of components that helps convert and shape them into high energy x-ray beam.
44
Components of the linac's machine head
1. Bending magnet redirects electron beam (often 90 or 270 degrees) so radiation beam exits head in correct direction
2. Electrons converted into photons when they hit tungsten target (mostly occurs through Bremsstrahlung interactions)
3. After x-rays generated, set of primary collimators shapes beam into roughly circular or rectangular field, also help absorb scattered radiation
4. Flattening filter (typically made of metal), placed in beam's path to even out intensity, creating uniform radiation beam across entire field
5. Monitor unit chamber used independently to validate and monitor radiation dose being delivered as electrical currents and voltages of radiation hardware can fluctuate
6. Radiation beam shaped by series of collimators to shape radiation field to patient's tumour
7. Multi-leaf collimators (MLC's) series of individual leaves that can also move independently to create freeform shapes, more precisely aligning radiation field to tumour
45
Overview of scanners used in nuclear medicine
- Don't produce radiation
- Instead, radiopharmaceuticals that have been metabolised by patient, emit radiation (patient is source of radiation)
- Scanners detect this emitted radiation
46
Scintigraphy
Detectors used in nucmed scanners operate on scintigraphy, where certain materials emit light (scintillate) when they absorb high-energy photons or particles
- Type of luminescence
- Use scintillation crystals, with sodium iodide doped with thallium (most common)
47
What are the two imaging technologies used in nuclear medicine?
- Single Photon Emission Computed Tomography (SPECT)
- Positron Emission Tomography (PET)
48
Basis of SPECT
Uses two gamma cameras which continuously rotate around patient
49
Basis of PET
Uses ring of static scintillation detector elements
50
SPECT components
- Power supply
- 2 Gamma cameras
- Patient positioning monitor
- Gantry
- Patient couch
- Control console
51
Gamma camera structure (from radiation source)
- Collimators
- Scintillation crystals
- Light guide
- Photomultiplier tubes
- Position decoding circuits
- Y and X signals
52
How does a gamma camera (anger camera) work?
- Consists of series of collimators (diff function to others), help collect only desired emission inputs from radiation source
- Collects and analyses photons that have been emitting along paths perpendicular to camera
- Radiation captured by collimators interact with scintillating crystal, emitting light photons proportional to amount of radiation energy deposited by emitted photons
- Light is then channeled by light guide to series of photomultipier tubes
- Photomultiplier tubes amplify signal and convert it into electrical signal used to create final image
53
PET components
- Power supply
- Gantry
- Bore
- Patient couch
- Control console
54
How does a PET scan work
Relies on specific type of radioactive decay involving an annihilation reaction
1. Radiopharmaceutical used in procedure must be a positron emitter
2. Positron will interact with electron, causing annihilation reaction
3. Outcome of reaction is two gamma photons emitted exactly 180 degrees to each other
4. Photons then detected by ring of scintillation detectors surrounding patient
5. Detection of simultaneous scintillation events in 2 opposing directions allows scanner to determine exact location of radiopharmaceutical that caused reaction
