Apparatus and Image Management Flashcards
When the field size is decreased, what happens to the density?
Density is decreased because there are fewer photons (less scatter) reaching the IR. Scatter still contributes to density although not useful
Latent Image
the image before it is processed-invisible
Interspace material is made out of what?
Usually aluminum or plastic fibre
Types of Exposure timers
- synchronous timer
- electronic timer
- mAs timer
- AEC: photodiode/phototimer, ionization chamber
Types of Filtration
Inherent: can’t be adjusted
Added: can be adjusted
Compensating: for different body part thickness
How does scatter affect contrast?
More scatter=less contrast
Density
The amount of blackness produce on an image after processing
As we increase the atomic number of the target material, the efficiency of the characteristic and bremsstrahlung radiation____________
Increases: because the binding energy for each shell increases
Positive Beam Limitation
When an exam is selected, the field size will automatically change or when an IR is place in the Bucky tray the collimators automatically adjust to the size
How do grids affect radiographic density?
Decrease because less photons reaching the IR
Grid (Bucky) Factors
No grid-1
5: 1- 2
8: 1- 3
12: 1- 4
16: 1- 5
What is the Line Focus Principle?
The relationship between the actual and effective focal spot
Types of stationary grids
- Linear: parallel, focused
- Crossed/ cross hatched
Limitations of the Air Gap technique
- increased technique
- magnification on part of the image
- increased image blur
- not as effective with high kVp setting
Primary factors that affect scatter reaching the IR
kVp
Field size
Patient thickness
Grid ratio equation
Grid ratio = h/D
Height of grid divided by interspace width
3 disadvantages of crossed grids
- positioning of grid is critical
- tilt-table techniques are only possible if tube and table are properly aligned
- higher exposure technique required
What happens when the exposure switch is pressed half way?
The rotor is brought up to speed
The filament is energized and begins to boil off electrons
Attenuation consists of:
Absorption
Scattering
Transmission
Less scatter reaching the IR=________ in radiographic density
Decreased
Components of the Cathode
Filament
Focusing cup
Factors that affect optimal radiographic technique
- Image Quality Factors (density, contrast)
- Exposure Technique Factors (kVp, mAs, SID)
- Patient Factors
Types of moving grids
- Reciprocating: motor driven, moves back and forth
- Oscillating: spring like mechanism, circular motion
The contrast improvement factor is high for _____ ratio grids
Higher ratio grids
Grid frequency
The number of strips per centimeter
The fuzzy edge of an image
Penumbra
Grid Conversion Factor
Used to determine the new technique when changing grid ratio (new grid)
mAs1/mAs2 = GCF1/GCF2
Exponential Absorption
For every increment of thickness, the x-rays decrease in number, by a certain percentage
Filament electrons that stray and hit the anode at different spots
Off-focus radiation
What must you have to produce X rays?
- Source of electrons
- Means to accelerate electrons
- Means to decelerate electrons
- Vacuum
Air Gap Technique
Method used to reduce scatter without using grids
IR is moved 10-15cm away from part-increased OID
Functions of the Anode
Electrical Conductor
Thermal Dissipator
Mechanical support for target
Grid errors
- Off Level: x ray beam is angled across grid, grid is angled
- Off Center: CR is not aligned side to side with the center of focused grid
- Off Focus: SID out of focal range for grid
- Upside Down: grid is used upside down
Increase OID allows _____ scatter to reach the IR resulting in _______ density
Less scatter reaches the IR with increased OID resulting in decreased density
Ways to extend tube life
- Minimum exposure factors
- Faster image receptors
- Warming up anode
- Use shortest exposure time
Types of Interactions (with target)
Bremsstrahlung radiation
Characteristic Radiation
mA
Milliamperes: the number of electrons available to move from cathode to anode
How does increased scatter affect image density and image contrast?
Increase overall density
Decrease contrast
What does the parallel and crossed grids do that is undesirable?
Grid cutoff
Components of the Anode
Anode target
Stator
Rotor
Anode stem
Tungsten
Atomic #74
Boiling point: 3410C
K shell binding energy: 69.5 kEv
Types of Beam Restrictors
Aperture Diaphragms: flat piece of lead or lead lined metal that fits onto the x ray tube head
Cones/Cylinders:
Variable Aperture Collimators: two sets of shudders
What is the anode target made out of?
Tungsten with a Molybdenum core to reduce weight and withstand heat expansion/contraction
When kVp is increased there are: (with regards to interactions and scattering)
Fewer reactions overall, but of the reactions that occur, more are scatter
The density produced on a radiograph remains the same for any combo of mA and time
Reciprocity Law
Normal range of optical density that humans see
0.25-2.5 OD
>3=black
<0.2=white
Contrast
The difference in OD between adjacent structures
High contrast: short scale
Low contrast: long scale
Contrast improvement factor equation
k = image contrast with grid/image contrast without grid
The probability of PE interactions increased if
Energy levels are closer together (binding and photon)
How does kVp effect x rays?
It affects the quality and the quantity
What is the filament made out of?
Thoriated Tungsten:
High boiling point (3410C)
Thorium extends filament life and increases thermionic emission
Doesn’t vaporize easily
When kVp is increased, fog is _______
Increased
Body Habitus
Sthenic: average, muscular
Hyposthenic: thin, slim, healthy
Hypersthenic: large
Aesthenic: small, frail
Another name for the direct square law
Density Maintenance Formula
What happens to the density as the SID is increased?
Same number of photons covering a bigger area=less density
