Radiology Physics Flashcards
(35 cards)
What are X-rays
- Ionizing, electromagnetic radiation
- have enough energy to create ions by ejecting electrons
- Act as a wave and a particle
- Creates free radicals and can damage cells/DNA
How are x-rays produced?
High energy electrons strike a high anatomy weight metal, and energy is released as X-rays
How x-rays are produced?
- electrical current is applied to the cathode filament → thermionic emission of electrons
- A voltage (potential difference) is applied to make the cathode negatively charged and the anode positively charged
- Negatively charged electrons fly across the vacuum and strike the positive anode
- Energy is released form the anode in the form of x-rays
How electrons make x-rays
- 2 types of radiation produced in the x-ray tube
- bremsstrahlung (breaking) and characteristics
- 99% of electron energy lost as heat, 1% lost as x-rays
- Wide range of different energy level x-rays
- many are too low energy to be useful and increase patient dose and scatter
- Filters used to remove low energy x-rays and increase the average (no max) energy of the beam.
- usually copper or aluminum
Collimator
Limits the beams area
Reduces scatter
reduces radiation dose
imporves image quality
Grid
- Plate with thin lead strips between patient and detector
- Absorbs scattered x-rays to improve image quality
- Use when imaging patients/structures greater than 10cm
Detector
- Film (analog)
- Single-use film in cassettes
- Computed Radiography (CR)
- multiple-use cassettes
- Plate reader
- Digital Radiography (DR)
- wired/wireless plate
Detectors: Films
Pros
highest spatial resolution
inexpensive upfront cost
Detectors: films
Cons
More technique sensitive
Higher dose
Slow
Caustic development chemicals
Large space needed for storage
Detectors: Computed Radiography
Older form of digital radiography
Detector in cassette
Plate reader scans imaging plate inside cassette
Digital processor converts into digital image
Detectors: Digital Radiography
- Direct:
- x-rays → digital data
- Indirect:
- x-rays → visible light → Digital data
- Digital systems are able to “correct” to some extent technique related errors
- makes previously non-diagnostic imagaes diagnostic
- Decreases dose due to decreased re-takes
- Conversely, overexposure is less easily identified
Detectors:
Direct / Indirect Digital Radiography
Pros
excellent contrast
less sensitive to technique
Smaller space required
Pre/post-processing
Image transfer
Lower dose
Detectors:
Direct / Indirect Digital Radiography
cons
expensive
lower spatial resolution
can inadvertently cause overexposure
requires modern technology infrastructure and know-how
How to take a radiograph
- Postition and Collimate
- Set technique
- Expose
What is technique
Adjustments made to determine number and energy of x-rays produced
kVP
mA
mAs
Technique: Elecrrical Basis
mA
mA = milliampere
- Current applied to cathode
- Affects how many electrons in the “cloud”
- Keep high as possible
- Not frequently adjusted
Technique: Electrical Basis
mAs
mAs = milliampere seconds
- mA * seconds = mAs
- current over as period of time
- How long the current is applied to the cathode
- Also affects number of electrons in the “cloud”
Technique: Electrical Basis
kVp
kVp = Kilovoltage Peak
- Magnitude of voltage (potentail difference) between the anode and cathode
- Affects primary speed and energy of electrons
- Determines maximum energy and electron or photon can possess
Technique: Clinical Basis
mAs
mAs = milliampere seconds
Number and density of photons in the beam
Primarily controls image exposure/blackening
Minimize to reduce sensitivity to motion
Technique: Clinical basis
kVp
kVp = Kilovoltage peak
Peak energy of photons in the beam
“How hard” the beam “pushes” to get through the patient
Primarily controls image contrast and scatter
Technique: Contrast
15% CHANGE IN KvP = ½ change in mAs
Contrast: difference between parts of an image
High contrast: black and white
Low contrast: lots of grey
Technique: Contrast
Scale
- Short Scale:
- high contrast
- black and white
- musculoskeletal
- High mAs, low kVp
- Moderate Scale:
- Moderate contrast
- Many shades fo grey
- abdomen
- Moderate mAs, moderate kVp
- Long Scale:
- Low contrast
- more shades of grey
- Thorax
- Low mAs, High kVp
How do x-rays interact with tissue?
- Absorption
- Transmission
- Scatter
Changes to the x-ray beam via any of the above is termed attenuation
How do z-rays interact with tissue?
Absorption
- Results form photoelectric effect
- x-ray ejects an electron form a tissue atom
- Creates low-energy radiation and an ion pair
- Increases with moderate to lower kVp
- x-rays do not strike the detector
