Midterm Review Apparatus And Image Flashcards
What is mA? mAs?
mA: the number of electrons available to move from cathode to anode
mAs: the # of electrons sent in 1 second
Why does the x-ray tube require DC current?
Electrons must only flow in one direction (cathode to anode)
What is the main cause/factor of tube failure?
Heat
When electrons stray and hit the anode in a different spot
Off-focus radiation
“Any combo of mA and time (as long as mAs is the same) will produce the same density”
Reciprocity law
The invisible image before its processed
Latent image
Purpose of glass/metal envelope
Maintain vacuum in the tube
Metal envelope extends tube life/avoids arcing
What is a beam restrictor? Types?
Device that attached to the opening in the x ray tube housing to regulate the size and shape of the beam. Control and minimize scatter
- Aperture diaphragm: flat piece of lead or lead lines material that fits onto x-ray tube head
- Cone and Cylinders: extension code or cylinder attached to an aperture diaphragm
- Variable Aperture Collimator: 2 sets of shutters. 2 set can be adjusted to restrict beam
Function of the step up transformer
Increases voltage from the autotransformer to the kV needed for x-ray production
Increases by a fixed amount
How does an electronic timer work?
- a capacitor takes a preprogrammed charge, once it is charged the exposure stops
- the resistance changes in order to charged the capacitor faster/slower for a longer/shorter exposure time
Types of body habitus
- Sthenic- average, muscular
- Aesthenic- small, frail
- Hypersthenic-large
- Hyposthenic- thin, slim, healthy
Types of pathologies
- Destructive: increases radiolucency
- Constructive: increases radiopacity
To produce x-rays you must have…
- source of electrons (cathode)
- means to accelerate electrons
- means to decelerate electrons (anode target)
Differential absorption
The difference in how different body tissues/parts absorb the beam
Advantages of less ripple
- greater radiation quantity and quality
- higher energy of x-rays
- high efficiency of x-rays
Factors affecting OD
- mAs
- SID
Components of the filament circuit
Rheostat, step down transformer
keV vs. kVp
keV: measurement used for binding energies and energy of the incident electron
kVp: the forces with which incident electrons interact with the tungsten target
What happens when the exposure switch is pressed halfway?
- electricity goes to induction motor and bring rotor up to speed
- selected filament in energized and electron cloud is built up
Focal Distance/ Grid Radius
Distance between grid and convergent line
Types of x ray tubes
Crookes tube: no vacuum, gas filled, on mA station
Coolidge tube: vacuum, glass/metal envelope, various mA stations
What is another name for Off center grid error
Lateral decentering
What is another name for the characteristic x-ray spectrum?
Discrete emission spectrum
Functions of the autotransformer
Provides voltage to the step up transformer depending on what kV is set. Different combos of the secondary connections allow for increasing sand decreasing voltage
Step up: more coils used
Step down: less coils used
Contrast improvement factor and formula
The ability of a grid to improve contrast
K= image contrast with grid/ image contrast without grid
Table characteristics
Shape: flat, curved Uniform Radiolucency Durable Easy to clean Hard to scratch High weight limit No crevices Can include cassette holder or detector
The absorption of the beam by the lead strips of a grid
Grid cut off
Functions of rectifier. Types of rectified voltage
Converts AC to DC to keep electrons flowing only from anode to cathode
Unrectified voltage: x-rays not made during second half of cycle
Half wave rectification: inverse voltage is removed, the rectifier does not conduct during the negative cycle, gaps when current is not being conducted
Full wave rectification: inverts the negative half so the anode is always positive 0-100% ripple
Three phase power: used 3 AC waveforms at the same time but out of sync 3 phase 6 pulse (4-14%ripple), 3 phase, 12 pulse (4% ripple)
The relationship between the actual and effective focal spots is direct or indirect?
Direct
APR and how it works
Anatomically programmed radiography
-Preprogrammed set of exposure factors for different body parts
Grid (Bucky) Factor
No grid- 1
5: 1- 2
8: 1- 4
12: 1- 5
16: 1- 6
Function of timer circuit
makes or breaks high voltage across tube
Located in primary circuit because it is easier to control low voltage
What does “quality” mean in regards to radiation?
Strength of radiation
Increasing mAs had what affect on scatter?
No affect
Principle Components of the equipment
- operating console
- x-ray tube
- high voltage generator
What will happen if the field size is too big when using AEC? Too small?
Too much scatter will reach the cell and cause the AEC to shut off prematurely
Too small: AEC will take longer to receive enough radiation to end exposure
Types of timing circuits
- Synchronous
- Electronic
- mAs
- AEC
Types of Anodes
Stationary: target doesn’t move, electrons always heating up same spot-heat builds up faster-limited to small exposures
Rotating: allows electron beam to interact with a greater target area (500x bigger)
What type of x-rays form a continuous emission spectrum?
bremsstrahlung
Probability of PE interactions increases when…
-Energy levels are closer (binding energy and photon energy)
Advantaged of high frequency generators
- less than 1% ripple
- smaller, lightweight
- less costly
- better exposure reproducibility
What does kVp control
Radiographic contrast
Factors that affect radiographic density
“Prime exposure factors”
-kVp, mA, exposure time, SID
What is the purpose of the back up timer?
To protect patient from overexposure
What happens to the emission spectrum with a high voltage waveform?
- Increased amplitude
- Increased average energy
Forces holding electrons in orbit?
Centrifugal: outwards force
Centripetal: center seeking
Function of step down transformer
Reduce voltage going and increase current going to filament so it doesn’t break
By what percent do we increase kVp to double the amount of photons hitting the IR?
15%-not directly proportional
What is the air gap technique?
A method to reduce scatter. Move IR 10-15cm away from body part.
How does a synchronous timer work?
Electric motor turns shaft at 60rpm. Exposure time selected determines the time it takes to go from the on switch to the off switch.
- shortest time is 1/60s and timing intervals increase by multiples thereof
- has to be reset in between
What if an image is to light?
- Low OD
- underexposed
- anatomy never there to begin with
When SID is increased, radiographic density is ________?
Decreased
Factors that affect scatter reaching IR
- kVp
- Field Size
- Patient Thickness
Function of tube housing
- Hold and protect tube
- Dissipate heat
- Reduce leakage radiation
Interactions that produce heat
- excitation
- ionization of an outer shell electron
Types of tube supports
- ceiling
- floor-to-ceiling
- floor
- c-arm
How does mA affect the spectrum?
Increasing mA increases the # of photons at all energy levels, increasing the amplitude of the spectrum
When do we use the reciprocity law?
- Motion (trauma/peds)
- Focal spot size
- Breathing technique (4-6 seconds long when you want to blur out something)
Anode heel effect is most apparent with…?
- larger IR size
- smaller SID
Parts of the induction motor
Stator: electromagnets arranged in pairs outside the envelope
Rotor: shaft made of iron surrounded by a copper cuff inside the envelope
Operated through mutual induction-stators create a magnetic field that the rotor follows/turns in sequence to
What affect does kVp have on the spectrum?
The peak moved up and to the right because the average energy is higher and more x rays are being produced (incident electrons hitting anode at greater speed)
Functions of the Anode
- Electrical Conductor
- Mechanical Support
- Thermal Dissipator
Types of tables
- fixed: height adjustable
- tilting: move to 90 degrees upright and 15-30 degrees head down, fixed height
What are grid frequency and ratio?
Grid frequency: # of lead lines per unit length
Grid Ratio: height of lead strips vs. distance between them
Factors affecting density
- Image quality factors (density, contrast)
- Exposure Technique factors (kVp, mAs, SID)
- Patient factors (cast?, size, pathology)
What is the most effective means to reduce scatter hitting the IR?
Grids
What is the k shell binding energy in tungsten (atomic #74)?
69.5 keV
What is the function of the cathode? Parts? What is it made out of?
To provide a source of electrons.
Filament and focusing cup
Made out of thoriated tungsten because high atomic # (74) and high melting point (3410C)
What does FFD stand for?
Focus to Film Distance
What is positive beam limitation?
“Automatic collimation”
Automatic collimators adjust to size of cassette put in tray to ensure field size is not larger than IR
Does not replace manual collimation
By how much do we need to change kVp to double density?
15%
Disadvantages of filtration
- more scatter (more matter more scatter)
- decreased image contrast
How does the air gap technique affect density and contrast?
Decreases density
Increases contrast
Grid Patterns
- Linear: Parallel, focused
- Crossed/Cross hatched
Types of radiation formed from interactions at target
Characteristic
Bremsstrahlung
How does a mAs timer work?
It monitors the current passing through the tube and will terminate the exposure when the set mAs has been achieved.
located in secondary circuit in order to measure actual tube current
Focal range
Recommended SIDs that can be used with a focused grid
Types of moving grids
- Reciprocating: motor driven, moved back and forth
- Oscillating: spring-like mechanism, moves in a circular motion
How does increased fog affect density and contrast?
- increase density
- decrease contrast
Types of Filtration
- Inherent: can’t be adjusted, built in
- Added: can be adjusted
- Compensating
Function of the rheostat
Controls filament temperature and rate at which electrons are boiled off, the higher the mA the lower the resistance
Define exponential absorption
As x-rays pass through tissue, for every increment of thickness, the x-rays are decreased in number by a certain percentage, never reaches zero
What factors can influence the effectiveness of the focusing cup?
- size
- shape
- charge
- size of filament
- shape of filament
- how filament is sitting in focusing cup
What is the density maintenance formula and when do we use it?
mAs1/mAs2 = SID1^2/SID2^2
- Portables
- To fit larger parts to a receptor
- Patient condition
What if an image is too dark?
- High OD
- Overexposed
- anatomy will still be there, just not visible
What causes the anode heel effect?
Line focus principle
What is the anode target shaft made out of? Why?
Molybdenum with a copper coating
Copper is a good thermal/electrical conductor. Molybdenum has a LOW THERMAL CONDUCTIVITY and it light weight
How is grid performance measured?
With the “K” contrast improvement factor
The fuzzy-edge of the image
Penumbra
What is an Atom?
Smallest particle that has all the properties of an element
Falling load generators
Initial tube load is higher and drops during exposure
Achieved mAs set in a shorter exposure
What is the limiter of the line focus principle?
Heat
When SID is increased, scatter is ________?
Not change
Moving grid errors
- incorrect grid installation
- grid movement is same direction as grid lines: grid lines will show
The area of the target where x-rays are emitted
Focal spot
Function of the line compensator
Automatically adjusts power supply to 220V
Why do we use filtration?
- Increase x -ray beam quality
- increase penetrability (higher strength photons)
How does AEC work? Types of AEC?
It measures the quantity of radiation reaching the IR and terminates the exposure when the required amount of radiation has been received
- Photodiode/Phototimer: behind IR (Exit), converts light to electrical energy, once certain charge is reached exposure is terminated
- Ionization Chamber: located between patient and IR (entrance), air inside cell becomes ionized as it is hit by radiation, ionization is directly proportional to amount of radiation. When predetermined charge has been reaching the exposure is terminated
What happens to the emission spectrum when filtration is added?
- Decreased amplitude
- Increased average energy (peak shifts right)
Types of electric current
Alternating Current: changes direction in cycles as electric potential changes
Direct Current: flows in only one direction
Ways to extend tube life
- Minimize exposure factors for each exam
- Utilize faster image receptors
- Warm up anode (small warm up exposures)
- Use shortest exposure times possible
What is tube current?
The movement of electrons from cathode to anode
What is the anode target made out of? Why?
Molybdenum core with a tungsten coating
Would be too heavy if all tungsten. Tungsten adds mechanical strength
Attenuation consists of
- Absorption
- Scatter
- Transmission
What is the line focus principle?
The relationship between the actual and effective focal spot.
Allows for a larger area for heating while maintaining a small focal spot-angling target
Components of main X-Ray circuit
Primary side: main power switch, line compensator, circuit breakers, autotransformer, step up transformer, timer circuit
Secondary side: mA meter, rectifier
Stationary grid errors
- off level: grid or tube is angled
- Off focus: SID out of focal range
- Upside down: grid is upside down
- Off center: grid/tube not centered
What is the autotransformer sometimes referred to as, and why?
-kVp selector
It directly impacts the voltage being supplied to the primary side of the step up transformer. The kVp selected on the console controls the autotransformer
What is a Bi-Angle target
2 target angles on anode = 2 focal tracks
Filaments stacked on top of one another instead of parallel
Imaginary lines drawn up from each of the lead lines on a focused grid, where they connect
Convergent point
If all the point were connected: convergent line
How is radiographic density measured?
By optical density: scale of 0-4 (human vision 0.25-2.5)
>3 will be seen as black
<0.2 will be seen as clear
What is the effective focal spot controlled by? What is the actual focal spot controlled by?
Effective focal spot controlled by actual focal spot. Actual focal spot controlled by:
- Filament length
- target angle
- charge on the focusing cup
- depth of filament in focusing cup
- shape and size of focusing cup
How is visibility achieved
The balance of contrast and density
How does increasing atomic # affect compton interactions?
There is no change
What is the main reason for using the line focus principle?
To decrease the size of the focal spot
