RADR 2209 - UNIT 2 EXAM
What are some things that the protective housing does? (3)
-absorbs most photons traveling in directions other than towards the patient -provides solid and stable mechanical support for the tube -electrical insulator for the tube and provides thermal cushion for the tube
What are good properties of rhenium
-added to tungsten because it increases thermal capacity and tensile strength
Define diode (1) and what is another name for it (3)?
-allows electrons to flow in only 1 direction -aka one way valve/solid state rectifier/rectifier
Why is a line compensator used with the main power switch?
-because of the incoming power supply -it is wired around the autotransformer and automatically adjusts the power supplied to the x ray machine
the anode radiates heat within the tube away from itself. List the 3 processes of heat transfer
-conduction of heat by heat tolerant materials -radiation of heat energy from the anode to the oil bath -convection of heat into the room via cooling fans
What are good properties of molybdenum
-has low thermal conductivity -light but strong (makes it easier for anode to turn)
What are good properties of tungsten
-has very high melting point -has excellent thermal conductivity
Name 2 notes of caution regarding the housing
-housing can become very hot -high voltage cables are not handles to maneuver the tube head assembly
Name the 3 technical factors of the tube rating charts
-kv peak -milliamperage -exposure time
What 3 things need to happen in order to produce an x ray
-large potential difference to give kinetic energy to the filament electrons (kVp setting) -a vehicle on which kinetic energy can ride (mAs setting) -place for interaction (target of anode)
What are the 2 major causes of tube failure
-maintain tube at high temps -prolonged excessive heating caused by prepping the rotor unnecessarily or arcing from filament
What are the 2 types of electromagnetic induction
-mutual -self induction
Name the 4 magnetic properties and examples for them
-nonmagnetic (glass, wood, plastic) -diamagnetic: weakly repelled by magnets (water, Au, mercury) -paramagnetic: weakly attracted by magnets (platinum, Al, Gd) -ferromagnetic: strongly attracted by magnets (Fe, Co, Ni)
Define a battery (3)
-produces electrons -keeps electrical charge for long term -provides electrical potential
Describe the rotating anode
-spreads heat over a larger surface area -has molybdenum rotating disc and coated with tungsten and mounted in a copper shaft
Define resistor (2)
-stops flow of electrons -regulates flow of electricity through part of circuit that it replaced
Define capacitor (2)
-stores electrical charge but only temporarily -cant provide new electrons
Describe the stationary anode
-target doesn't move -tungsten is embedded in a copper wire -damages easily because electrons hit the same target area over and over again -used in old x rays and some dental x ray machines
Describe how the 2 filament wires are done in the tube
-tiny coils of wire are housed in the cathode of the x ray tube -represented on operating console via a large focal spot and small focal spot
What is the MRI's range of magnets?
0.5 T - 5.0 T
Name the 3 laws of magnetism
1. every magnet has a north and a South Pole 2. like poles repel each other, opposite poles attract 3. the force of attraction or repulsion varies on the strength of the poles and inversely with the square of distance between them
Name the 5 principals of Electrostatics
1. like charges repel, unlike charges attract 2. Coulombs law 3. electric charges reside on external surface of conductors 4. the concentration of charges on a curved surface of a conductor is greatest where the curve is the greatest 5. only negative electrons are allowed to move in the solid conductors (NOT protons)
Name the 3 characteristics of flux
1. lines of flux travel from South Pole to North Pole inside the magnet and north to south outside of the magnet (this creates elliptical loops) 2. lines in the same direction repel each other, lines in opposite directions attract each other 3. magnetic fields are distorted by magnetic materials and unaffected by non-magnetic material
What is the correction factor for a single phase
1.0
What is the correction factor for a 3 phase 6 pulse
1.35
What is the correction factor for a 3 phase 12 pulse
1.41
What is the correction factor for a high frequency
1.45
What percent ripple is a single phase full wave
100%
What percent ripple can be used depending on the number of rectifiers
13% (3 phase 6 pulse) - 3.5% (3 phase 12 pulse)
How many filament wires are in a radiographic tube
2
Most tubes have how many filaments? What is another word for them?
2 filaments referred as dual focus tubes (represented by large and small focal spot options)
What degrees are used in the target angles
7-18 degrees in a tube (12 is the most common)
What is current measured in?
Amperes (A)
What is the AEC and where is it located
Automatic Exposure Control -in the bucky measures the amount of radiation leaving the patient and stops exposure once a quantity of radiation has been reached
Who identified electric induction first?
Faraday
What is the HU equation
HU = kVp x mA x s x c (exposure time)
Give an example of mutual induction
Imagine 2 coils. Coil A is the primary coil connected to an AC power source. Coil B is the secondary coil, and as the fluctuating magnetic field from A moves back and forth through the turns of B, a secondary current is induced
Using the crystals, how do electrons move
ONLY from N to P
Who identified electromagnetism first?
Oersted
What is resistance measured in?
Ohm (O)
What crystals are used to make rectifiers
P and N crystals
What is the strength of magnets measured in?
Tesla (T)
What is electric potential measured in?
Volts (V)
When the kV is applied to the x ray rube, what does it create?
a large positive charge on the anode and large negative charge on the cathode
Define electric potential
ability to do work because of a separation of charges
What is the p crystal made of
abundance of electron traps
What is the n crystal made of
abundance of freely moving electrons
How is the autotransformer controlled
adjustable via the kVp selector on the operating console and is controlled by the radiographer
Define rheostat (1)
adjustable/variable form of a resistor
What does the kVp do
adjusts the autotransformer and determines the number of turns on the secondary side necessary to produce voltage through self induction, that will be sent to the step up transformer
Define AC
alternating current flows in both directions
Define self induction
an induced current flows in the direction that opposes the action that induced it - the action being the change in magnetic field
Define anode heel effect
angle causes intensity of x ray beam to be less on anode side because of the heel of the target is in the path of the beam and some x rays are absorbed in the target heel
What does the anode consist of (2)
anode target induction motor (1/2 inside and 1/2 outside protective housing)
What is the anode target face used for
anode target face is angled to make use of the line focus principle
Which is the positive and negative side of the x ray tube
anode: positive cathode: negative
Define actual focal spot
area being bombarded by filament electrons -the size of the electrons depended on the size of the filaments (the smaller the electron stream, the greater the heat generated in a small area)
Describe the electron cloud
attracted to the anode target because of the huge potential difference and flows from the cathode to anode and is a continuation of the flow of electricity through the x ray circuit -as the flow of electrons penetrate the target surface, these filament electrons interact with the atoms of tungsten, generating heat and x rays
Define thermionic emission
boiling off electrons
Define suntanning
bronze discoloration on glass
Describe the line focus principle
by angling the face of the anode target, a large actual focal spot can be maintained and small effective focal spot size can be created (goal is balance between heat and focal spot)
What is the effect of lowering the intensity on the anode side of the beam
can cause the image to be lighter on that end
Define transformer (1)
can increase or decrease voltage by a predetermined amount
What would happen to the tube if electrons were in the air
cause arcing and damage
What kind of pathway is an electric current?
closed pathway
What is the focusing cup connected to
connected to secondary circuits
Define grounding
connecting an electrical device to the earth via a conductor
What is the rheostat controlled by
controlled by the mA selector on the operating console the higher the mA, the lower the resistance
What do motors do?
convert electrical energy to mechanical energy
What do generators do?
convert mechanical energy to electrical energy
Name 3 examples of good conductors/bad resistors
copper gold water
_______ flows with the arrows and _______ flows against the arrows
current electrons
In an x ray tube, where does the current flow
current MUST always flow from anode to cathode this is done by the rectifier
Following the sequence of events through the filament circuit, where does current flow to
current flows from the autotransformer to the rheostat, to the step down transformer, and to the selected filaments within the cathode of the x ray tube
What does a step down transformer do?
decreases voltage
Define DC
direct current flows in 1 direction
Describe what is inside the x ray tube
electric vacuum that consists of an anode, cathode, and induction motor all incased in a glass or metal envelope (enclosure)
Define Ohm
electrical resistance equal to the resistance between 2 points along a conductor
In an x ray tube, where does electrons flow
electrons MUST flow from cathode to anode this is done by the rectifier
Define electrodynamics
electrons in motion
Define ripple
energy fluctuates from the 0 (on the line) to the max voltage (peak of the wave)
What must the filament circuit do to produce the correct number of x ray photons for an exposure
filament circuit must work with the exposure timer in the primary circuit
Define current
flow of electrons in a conductor
What does the cathode consist of (2)
focusing cup filaments
What does the cathode consist of? (2)
focusing cup filaments
Name the 2 protective devices
fuse circuit breaker
Name 2 examples of bad conductors/good resistors
glass plastic
Define fuse (1)
glass encased wire that melts if current flow rises excessively, thus opening the circuit
Define half wave rectification
half the AC cycle flows through the x ray tube and the other half that would flow the wrong way in the x ray tube is blocked
Describe what the rotor is made of
has an iron core that is surrounded by coils
What do enclosure types have and why
have a target window for the desired exit point of the x rays produced made to minimally interfere with (absorb) the x rays
Define circuit breaker (1)
if the current flow rises, the internal switch of the breaker is tripped (opened) stopping the flow of electricity
How can you tell if a thing is safe or not
if the point is on or above the specified mA line, it is UNSAFE
What do transformers do?
increase or decrease voltage
What does a step up transformer do?
increases voltage
Describe what the stator does
induces an electric current with the rotor in a magnetized field -the rotor aligns with the stator as the stator magnetizes and a net electric current and magnetic field is induced, causing the rotor to turn
Define mutual induction
induction of electricity in a secondary coil by moving a magnetic field
Where is the stator and what is it made up of
is outside the tube enclosure made up of electromagnets
What is the tube assembly made of
lead
Describe the protective housing
lead lined metal structure with 2 large electrical cables that securely attach to the x ray tube through special high voltage receptacles
The anode side needs ______ exposure
less exposure
What is the goal of the filament circuit
literally boil off electrons out of the filament wire
Where is the timer switch/circuit located and why?
located in primary circuit because its easy to control a low voltage vs a higher one
What is the focusing cup made of and what does it do
made of nickel surrounds each filament on its back side leaving the front open and facing the anode target receives a negative charge from the secondary circuit that forces the electrons together on a cloud as they are boiled off of the filament via electrostatic repulsion
Define flux
magnetic field consisting of lines of force in space
What items are in the primary circuit? (5)
main power switch circuit breaker autotransformer timer circuit primary side of step up transformer
What is the main purpose of the cathode
maintain a vacuum within the tube
What does resistance depend on? (4)
material length temperature cross-sectional area
Electric charge is a property of ______
matter
What is the significance of electric timers
measures time as a device
What does the mA meter do
monitors the x ray tube current
The cathode side needs _____ exposure
more exposure
Why are rectifiers necessary in the x ray circuit
necessary to route the electricity through the x ray tube correcly
Define main power switch
on/off switch for the unit and is connected to the incoming power supply (around 220V)
When is the exposure terminated in the primary circuit
once the programmed magnitude of electric charge is reached
What is another name for a rectifier
one way valve/diode
Define switch (1)
opens circuit therefore breaking the pathway closes circuit therefore forming a pathway
Define effective focal spot
origin of the x ray beam and is seen from the patients perspective (the smaller the area is, the sharper the image will be)
Define leakage radiation
photons traveling in directions other than towards the patient
What is phase 2 of the 2 phase switch
prepping the rotor this phase initiates the x ray production process
Define circuit breaker
protects against short circuits and electrical shock
What does the metal envelope provide
provides a constant electric potential between the cathode and the disclosure
What does the cathode do
provides a source of electrons needed for x ray production
What 2 things are used to extend tube life
rating charts cooling charts
What items are in the filament circuit
rheostat step down transformer
How does an anode rotate
rotates by an induction motor that consists of a stator and rotor
What items are in the secondary circuit? (4)
secondary side of step up transformer/circuit mA meter rectifier bank x ray tube (except for the filaments)
What does the radiographer select the operating options for? (4)
selecting kVp mAs exposure time focal spot size
Describe properties of the anode
series of electrical thermal conductor and has a target window -all x rays that don't make it to the target window continue as a current flow through the x ray current
Define an autotransformer
set by a control panel and is self induction
What parts of the focusing cup affects how well it focuses the electrons on the target? (3)
size shape charge
Why are rectifiers used in pairs
so that the AC has an open path from each direction
What happens when the exposure switch is first pressed
some of the electricity is diverted to the induction motor of the tube to bring the rotor up to speed. with the rotor spinning, the selected filament is energized whirl the desired degree of thermionic emission is achieved
Define resistance
something in a current that resists/impedes the flow of electricity
What are the 2 types of anodes
stationary anode rotating anode
Define electrostatics
stationary electrons
Describe how the stator and rotor are situated
stator on the outside rotor on the inside
Who controls the filament circuit? (3)
technologist mA selector rheostat
What happens to the HU equation if multiple exposures are made
the answer from the equation is multiplied by the number of exposures
Define space charge. What does it do?
the charge is self limiting and once it reaches a size good for the current used, it becomes difficult for additional electrons emitted -the self limiting factor is the space charge effect -the focusing cup forms them to a cloud and becomes the space charge
What happens to the pathway if the circuit is broken?
the circuit becomes open (open circuit) EX: Turning on a switch, closes the pathway and turning off a switch opens the pathway
How is the anode target angle determined
the intended use of the tube and NOT by the radiographer
Define electromagnetic induction
the phenomenon of inducing an electric current in a conductor by moving a conductor through a magnetic field
What is the dividing line between the primary and secondary circuits
the step up transformer
What happens when the switch is pressed
the voltage from the autotransformer passes back to the step up transformer the voltage then passes through the rectifier bank before passing to the anode and cathode the x ray tube so that the anode is always positive and cathode always negative
What is part of the anodes job
to dissipate the heat that is massively produced here
Why is a step down transformer used
to increase current by reducing voltage that is applied to the filament
What surrounds the protective housing and why
tube is immersed in an oil bath and cooling fans that circulate air around the assembly and therefore protecting it from thermal damage
As the tubes age, what material can also be used to cause arcing and damage to the tube
tungsten
What are rectifiers used for
used to convert AC to DC
What is the step up transformer used for?
used to increase voltage from the autotransformer to the kV necessary for x ray production
What is the significance of the exposure timer
uses impulses of electricity to time the exposure
What is the significance of mAs timers
uses the mA on the control panel and automatically sets the mA to the largest setting and time to the shortest setting to obtain the desired quality of the x rays
What is Ohm's law and the equation for it?
voltage across the total circuit or any part of that circuit is equal to the current multiplied by the resistance V=IR
Following the sequence of events through the primary and secondary circuit, where does voltage flow to
voltage flows through the autotransformer to the step up transformer through the rectifiers to the x ray tube
When is full wave rectification used
when 4 rectifiers are used
Define magnetic materials
when orbital electrons of their atoms spin in 1 direction creating tiny magnets called magnetic dipoles
Define how a magnet works
when the dipoles form groups of similarly aligned atoms, they create domains. When magnetic materials are placed in a strong magnetic field, the domains align with the external field, which organizes them and magnetizes the material, thus creating a magnet
Using the crystals, when will the solid state diode conduct electricity
when there is a positive P and a negative N
Define three phase power
when we rectify each waveform as if they were a single waveform, the x ray tube sees a waveform
