Radiology Physics Flashcards
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
How do x-rays interact with tissue?
Transmission
- x-ray passes through patient unaltered or with a lower energy
- Strikes detector and contributes to image darkening
- Transmission and Adsorption good: Contribute to image quality
How do x-rays interact with tissue?
Scatter
- Common in soft tissue
- Compton scatter = most important type
- photon loses energy, changes direction, and ejects an electron
- Exposes personnel = safety issue
- Reduces image contrast
How to decrease scatter
Increase collimation
Decrease kVp
Decrease tissue thickness or use a grid
Remember, technique must be increased with a grid
Grids do not decrease personnel exposure to scatter, only imporve image quality
Imaging Artifacts
Errors in technique, processing, digital reconstruction
Things that make the image look odd that are not related to pathology
Imaging Artifacts:
Underexposure
- Film:
- poor contrast, lack of darkening
- Digital:
- quantum mottle (grainy texture)
- How to fix?
- add 15% kVp for soft tissue.
- Double mAs for bone
Imaging artifact:
Overexposure
- Film:
- overdarkening
- Digital:
- clipping, planking, paradoxical overexposure effect
- How to fix?
- Decrease kVp by 15% for bone
- ½ the mAs for soft tissue
Imaging Aftifacts:
Motion
Common problem
Results: blurring
Fix:
Lower mAs
sedate patient
retake
Imaging Artifacts:
Shape and Size Distrotion
Foreshortening
z-ray tube and detector in the same plane, radiographed part angled
Imaging Artifacts:
Shape and Size Distortion:
Elongation
Radiographed part and detector in the same plane, x-ray tube angled
Radiographed part and x-ray tube in the same plane, detector angled
Imaging Artifacts:
Shape and Size Distortion
Magnification
Radiographed object elevated away from detector
Object is enlarged and blurry
What about other modalities
- Computed tomography (CT)
- x-rays
- Magnetic resonance imaging (MRI)
- radio waves and magnetic fields
- Ultrasound
- sound waves
- Nuclear Scintigraphy
- gamma rays
