X-Rays Flashcards
Simple X-Ray Tube Labels (7)
- High Potential Difference
- Thick walled glass chamber to maintain a high vacuum
- Heated metal filament cathode
- Outer tube housing of steel, lined with lead
- Useful beam of X-rays
- Thin window to allow emission of X-rays
- Electrical connection
Method of Producing X-Rays Step 1 (3)
Producing Electrons
- Cathode, tungsten filament (similar to lightbulb)
- Heat filament by passing electric currentthrough it
- Electrons “boil” off filament and produce a cloud of electrons (known as thermionic emission)
Method of Producing X-Rays Step 2
Accelerating Electrons
- Apply a large P.D between cathode and anode, creating electric field
Method of Producing X-Rays Step 3 (3)
Decelerating Electrons
- Smash high energy electrons into anode target material (rapid deceleration)
- Electrons interact with nucleus of target atom to produce a continuous x-ray spectrum
- Known as Bremsstrahlung - breaking radiation
X-Ray Efficiency
1% of energy –> X-rays
99% of energy –> Heat
Energy of X-ray depends on the …
… proximity of electron to nucleus
Maximum energy X-rays created when …
… all electron’s energy lost in the collision with nucleus
What are Characteristic X-Rays? (6)
- Alternative to bremsstrahlung
- An inelastic interaction with orbiting electrons
- Electrons ionize an orbital electron of target atom
- Inner shell electron rearrange to fill vacancy
- Change in energy produces a line spectrum of photons
- (Energy depends on binding energy)
Charactersitic X-Ray Diagram (2)
- Collision of electrons, emission of delta-ray & creation of vacancy
- Fillin of vancancy by outer electron, emission of X-Ray
Shape of Diagnostic X-ray spectrum depend on what factors? (5)
- Potential applied across x-ray tube
- Current
- Filtration
- Z
- Waveform
What is a K-line?
The peak of an x-ray line
Spectrum Characterisitics of Max & Min Energy (2)
Max Energy - potential applied across X-ray tube
Min Energy - depends on absorption of low energy X-rays in:
- anode
- Exit port of tube
- Cooling Oil
- Any additional filters
What does Filtration do? (2)
- Unwanted soft radiation removed to save skin dose, therefore hardened beam
- Attenuator preferentially absorbs lower energy (soft) photons
Cathode Assembly
Part - Material - Properties (2)
Filament
Coiled Tungsten
- Good thermionic emitter
- Hard wearing
Focusing Cup
Nickel/stainles steel
- Poor thermionic emitter
- High melting point
What is the focussing cup? (2)
- Fits around filament to direct electrons to a small focal spot on the anode
- Otherwise large focus and poor resolution
- Focusing cup may be biased or unbiased
- Biased = tighter potential lines, tighter electron projectory.
Electrics of X-Ray Tube
Part - Description (3)
Filament Current
- Boils electrons off cathode filament
- ~5A @ 10V
- controls # X-rays produced
High Voltage
- attracts -‘ve e from cathode to anode
- 000’s of volts
- controls max x-ray energy
Tube Current
- current of e crossing X-ray tube to produce X-rays
- mA fluroscopy, 100mA for radiography
Stationary Anode characteristics (3)
- Small tungsten target
- Coppy block coducts heat way from target efficiently
- Anode face angled to spread out heat
Types of Digital Image Receptors (2)
- Radiography
- Fluroscopy
Photo-Stimulated Luminescence (3)
- X-rays absorbed
- elctrons:
- excited from valence band to cond band
- some fall back into electron traps
- stay in traps until stimulated by laser light
- leave traps & return to valence band
- light emitted
Scanning CR Plates method (3)
- laser scans across plate
- trapped electrons released, causing light to be emitted
- light is collected & converted from analog to digital signal
Scanning Plates Diagram (6)
- Laser
- Collimator
- Galvo
- Fiber Optic
- Photo Detector
- Phosphor Image Plate
How to clear CR plate once read?
Exposure with very bright white light (removes any electrons still in traps)
Imaging Chain method (5)
- image plate (IP) exposed to radiation
- absrobs X-rays & “stores” image
- IP put into reader
- remove phosphor IP from cassette
- scanned by laser beam
- emits visible light
- reader collects light
- sends dark/light signals to computer
- image displayed on monitor
- IP cleared for re-use
Why are CR Cassettes lead-backed?
For backscatter protection
What does DR stand for?
Digital Radiography
What is bit-depth?
Mapping images to different levels of gray (higher bit-depth = more number of levels)
Types of DR (2)
- Indirect (most common)
- Direct (used in mammography)
Benefits of DR Detectors (3)
Direct o/p of electronic signal for digitisation
- no manual cassette handling
- no separate exposure & readout stages
- image almost instantly produced on screen
Method of Indirect DR (5)
- X-rays converted into light at the scintillator
- Light converted to electric chrge in the photodiode
- Electric charge detected & stored in a 2D pixel array
- Stored charge forms a latent image
- Charge read out to create the image
What does the Scintillator layer do?
Converts x-rays into light
Reasons for pre-processing of DR Images (3)
- Raw image data is pre-processed to:
- minimise difference in response between pixel elements
- correct for broken pixels
- remove offset errors
- Pre-processing uses pixel correction maps & measurements taken during detector calibration
- Detector must be calibrated regularly
Direct DR method (4)
- X-rays converted directly into electric charge in the photoconductor
- Electric charge detected and stored in a 2D pixel array - a flat panel array
- Stored charge forms a latent image
- Charge read out to create the image
Advantages of DR over CR (3)
- Direct readout of latent image in electronic form, with no need to:
- handle cassettes
- separate reader
- faster image production
- higher patient throughput
- Higher Detective Quantum Efficiency than standard CR
- potential for patient dose savings
- Similar spatial reslution to standard CR
- inferior spatial resolution to High Res CR systems
Image Intensifiers (7)
- Video Camera
- Aperture
- Anti-scatter grid
- Table
- Collimation
- Filters
- X-ray Tube
Fluoroscopy Diagram (5)
- Path of electrons
- Electrodes of the electron lens
- Output fluorescent screen
- Input fluorescent screen
- Photo Cathode
What is S-Distortion? (3)
- S-shaped distortion across image
- Caused by external magnetic fields
- stray field from surroundings
- Earth’s magnetic field
- Usually negligible
What is Minification Gain?
- Large image on i/p focused to small image on o/p
- Intensity of light amplified
- Gain increases with increasing diameter of i/p phosphor
What is Maginification Mode? (5)
- Uses largest FoV limited by physcal size of ||
- Can use magnified FoVs
- X-ray field reduced
- Voltages applied to electrodes in || changed
- Brightness gain decreases –> i/p exposure rate increased to compensate
What does CCD camera stand for?
Charge Coupled Device Camera
- Digital spot images
- Can also use for cine
Advantages of Flat Panel Fluoroscopy (3)
- Smaller & Lighter
- Less Bulky detector
- better access to patient
- Less geometric distortion
- ability to reconstruct in 3D
