Ch. 5 The xray imaging system Flashcards
What are the 3 main components of an xray system?
- xray tube
- operating console
- high voltage generator
A high voltage generator contains…
the high voltage step up transformer and the rectification circuit
What are the voltages of an xray system?
25 - 150 kVp
What are the tube currents for an xray system?
100 - 1200 mA
Where is the fluoro tube usually located?
under the exam table.
To what degrees does a fluor table tilt? foot side and head side?
30 to the head and 90 degrees to the foot
Where would the generator be located within a xray room?
in an equipment cabinet against the wall
Radiation quantity is usually expressed in..
milligray (mGy) or mAs
How is radiation quality expressed?
kVp or HVL (half value layer)
All electric circuits that connect the meters and controls on the operating console are…
at low voltage to minimize the possibility of shock
Operating consoles are based on…
computer technology
Most xray systems are designed to operate on how much power?
220V..some are 110V or even 440V
Why would voltage provided to an xray machine vary by as much as 5%?
Because of variations in power distribution to the hospital and in power consumption by different sections of the hospital.
What does a line compensator do?
measure the voltage provided to the xray system and adjusts the voltage to 220V.
The power supplied to the xray machine is first delivered to the…
autotransformer
The autotransformer has a single winding and is designed to…
supply a precise voltage to the filament circuit and to the high voltage circuit of the xray imaging system.
What principle does the autotransformer work on?
the electromagnetic induction
How many windings and cores does the autotransformer have?
1 each
How much can the autotransformer step up?
it can step up voltage up to about twice the input voltage value
The voltage the autotransformer receives is called the … and the voltage it provides is called the…
Primary voltage
secondary voltage
The primary and secondary voltage are directly related to…
the number of turns of the transformer enclosed by the respective connections
Some older consoles have adjustments for kVp called…
major kVp and minor kVp
What do the major and minor kVp controls do?
it allows the technologist to provide precisely the required kVp.
Exactly what does the minor kVp control do?
it fine tunes the selected technique
The minor and major kVp controls represent…
2 separate series of connections on the autotransformer.
kVp determines the ___ of the xray beam.
quality
What is a prereading kVp meater?
A meter that registers but no exposure is made and no circuit has current. It allows the voltage to be monitored before an exposure.
What does mA measure?
The xray tube current that crosses from cathode to anode
The number of electrons emitted by the filament is determined by…
the temperature of the filament
What controls the filament temperature?
the filament current
What is the filament current measure in?
amperes
As the filament current increases…
the filament becomes hotter, and more electrons are released by thermionic emission
What do filaments normally operate at?
3-6 amperes
Why is a correction circuit incorporated into a tube?
to counteract the space charge effect
Thermionic emission is the…
release of electrons from a heated filament.
What is the space charge effect?
As the kVp is raised, the anode becomes more attractive to the electrons that would not have enough energy to leave the filament area. These electrons also join the electron stream, which effectively increases the mA with kVp.
Xray tube current is controlled through the…
filament circuit
Voltage from the mA selector switch is delivered to the …
filament transformer
What type of transformer is the filament transformer?
a step down.
What monitors the xray tube current?
with an mA meter that is in the tube circuit
Where is the mA meter connected?
at the center of the secondary winding of the high voltage step up transformer
What Hz does the secondary voltage alternate at?
60 Hz making the center of the winding always at 0 volts
What is the full title for the filament transformer?
filament heating isolation step down transformer
What does the filament transformer do?
It steps down the voltage to about 12 V and provides the current to heat the filament
Why are the secondary winding heavily insulated from the primary?
Because the secondary windings are connected to the high voltage supply for the xray tube
In a filament transformer, what are the primary windings made of?
thin copper
What current do the primary winding of the filament transformer carry?
a current of 0.5 to 1 A and about 150V
How are the secondary windings different from the primary windings?
They are thick and about 12V of electric potential and carry a current of 5-8 A (not mA)
The number of xrays that reach the IR is directly related to…
both the xray tube current and the time that the xray tube is energized.
What is a guard timer?
a timer that will terminate an exposure after a prescribed time
After how many minutes does the guard timer go off?
usually 6 seconds
What side is the guard timer usually located?
on the primary side of the high voltage transformer, where the voltage is lower
How many types of timer circuits are there?
4
Who controls the timer circuits?
3 are controlled by the radiologic technologist and the other is automatic.
In the US electric current is supplied at what frequency?
60 Hz
What is a synchronous motor?
an electric motor designed to drive a shaft at precisely 60 revolutions per second (rps)
Why do we care about synchronous motors in imaging systems?
because they are often used as timing mechanisms (60 rps)
How do we know if an imaging system is on a synchronous timer?
Because the minimum exposure time possible is 1/60 s (17ms) and intervals increase by multiples 1/30, 1/20.
Why can’t serial exposures use synchronous timers?
because they need to be reset after each exposure
Electronic timers are the most…
sophisticated, most complicated, and most accurate of all the timers
How are electronic timers set up?
they have rather complex circuitry based on the time required to charge a capacitor through a variable resistance.
How accurate are electronic timers?
They allow a wide range of time intervals to be selected and are accurate to intervals as small as 1ms.
What type of procedures and why are electronic timers preferred?
interventional radiology because they can be used for rapid serial exposures
Most exposure timers are electronic and are controlled by…
a microprocessor
mAs determines the…
number of xrays emitted and therefore the exposure of the image receptor
What monitors the product of mA and exposure time and terminates exposure when the desired mAs value is attained?
the mAs timer
The mAs timer is designed to provide the ___ ___ tube current for the ____ ____ for any mAs selected.
highest safe; shortest exposure
Where is the mAs timer located and why?
it is on the secondary side of the high voltage transformer because the timer must monitor the actual tube current
What type of imaging systems are mAs timers used?
falling-load and capacitor discharge imaging systems
AEC measures…
the quantity of radiation that reaches the IR.
AEC automatically..
terminates the exposure when the IR has gotten the required radiation intensity.
What AEC system is used by most manufacturers?
one with a flat, parallel plate ionization chamber positioned between the patient and the IR
What is the ionization chamber of an AEC made of?
radiolucent material
What happens in the ionization chamber?
ionization in the chamber makes a charge and when the appropriate charge has been reached, the exposure is terminated
During a QA check on system made by the tech, what is happening?
the tech selects they type of exam, which sets the appropriate mA and kVp. At the same time, the exposure timer is set to the backup time. When the electric charge from the ionization chamber reaches a preset level, a signal is returned to the operating console where the exposure is terminated.
Why should AEC be used carefully during a mammo study?
because of varying tissue thickness and composition, the AEC may not respond properly to a low kVp
for a diagnostic xray, the electric backup timer should be set to what?
1.5 times the expected exposure time if the AEC fails
What are solid state radiation detectors used for ?
exposure timer checks
Why do solid state detectors work well as exposure timer checkers?
because they operate with a very accurate internal clock based on a quartz crystal oscillator. They can measure time as short as 1 ms, and when used with a oscilloscope, can display a radiation waveform.
What is the high voltage generator responsible for?
increasing the output voltage from the autotransformer to the kVp necessary for xray production.
A high voltage generator contains 3 primary parts:
- the high voltage transformer
- the filament transformer
- rectifiers
What type of transformer is a high voltage transformer?
a step up transformer
What is meant when the term step up is used?
the secondary voltage is higher than the primary voltage because the number of secondary winding is greater than the number of primary windings.
Define turns ratio
the ratio of the number of secondary windings to the number of primary windings
voltage increase is proportional to…
the turns ratio, according to the transformer law…the current is also reduced proportionately.
What is the turns ratio for a high voltage transformer?
usually 500:1 and 1000:1
Why is the voltage waveform on both sides of a high voltage transformer sinusoidal?
because transformers operate only on AC
What is the difference between the primary and secondary waveforms?
their amplitude. the primary voltage is measured in V and the secondary voltage is measured in kV
The primary current is measured in ____ and the secondary current is measured in ___.
A; mA
Radiographers outside of the US and Japan may use what frequency?
50 Hz
What is the rectification process?
the process of converting AC to DC
What is the device that allows current flow in only one direction?
a rectifier
What type of current must a xray tube have?
DC
How are xrays produced?
by the acceleration of electrons from the cathode to the anode and cannot be produced by electrons flowing in the reverse direction.
What needs to be rectified?
the secondary voltage of the high voltage transformer (going from AC to DC)
Why must voltage be rectified?
to ensure that electrons flow from the tube cathode to the anode only
rectification is accomplished with…
diodes
What is a diode?
an electronic device that contains 2 electrodes.
Originally, all diode rectifiers were…
vacuum tubes called valve tubes
What have valve tubes been replaced with?
solid state rectifiers made of silicon
Why do semiconductors work?
tiny crystals in the semiconductors have some useful electrical properties and allow the semiconductors to serve as the basis for the solid sate microprocessors.
How many class types are there of the semiconductors? and what are they?
- They are n-type and p-type
Describe n-type semiconductors
they have loosely bound electrons that are mostly free to moe
Describe p-type semiconductors
they have spaces called holes where there are no electrons. These holes are like the space between cars in traffic. holes are as mobile as electons
What would happen if a p-type and n-type were put together and formed a p-n junction with the potential on the p side?
If a higher potential is on the p side, then the electrons and holes will both migrate toward the junction and wander across it. This flow of electrons and holes constitutes an electric current.
What would happen if a p-type and n-type were put together and formed a p-n junction with the potential on the n side?
If a positive potential is put on the n side, both the electrons and the holes will be swept away from the junction and no electrons will be available at the junction surface to form a current.
solid state p-n junctions only conducts electricity…
in one direction and is called a solid state diode
Why are solid state diodes rectifiers?
because they only conduct electric current in one direction
Where are rectifiers located?
in the high voltage section
During a negative half of a cycle, current can flow only…
from anode to cathode, but this does not occur because the anode is not constructed to emit electrons (during a unrectified voltage)
What is half wave rectification?
a condition in which the voltage is not allowed to swing negatively during the negative half of its cycle
During the positive portion of the AC waveform, what is allowed to happen?
the rectifier allows electric current to pass through the xray tube
Since the rectifier does not conduct during a negative portion of the AC waveform and no electric current is allowed, what is the result?
the electric current is a series of positive pulses separated by gaps when the negative current is not conducted. What results is a rectified current because electrons flow in only one direction.
Why does half wave rectification have its name?
because only one half of the AC waveform appears in the output
What machines may have a vacuum tube rectifier?
some portable and dental xray systems
What are the systems called if they have a vacuum tube rectifier?
self-rectified
half wave rectified circuits contain how many diodes?
0, 1, or 2
How many xray pulses are produced by a half wave voltage generator?
60 xray pulses each second
What is a shortcoming of the half wave rectifier?
it wastes half the supply of power and requires twice the exposure time.
What circuit rectifies the entire AC waveform?
the full wave rectification
How many diodes in a full wave rectified xray imaging system?
4 in the high voltage circuit
How does the full wave rectifier work?
the negative half cycle corresponding tot he inverse voltage is reversed so the anode is always positive.
What is the advantage of full wave rectifier?
the exposure time is cut in half
What is the pulsed xray output of a full wave rectifier?
120 times each second
Half wave and full wave rectifiers are produced by…
single phase electric power
single phase power results in a
pulsating xray beam
Why does the xray beam pulsate with the single phase power ?
because of the alternate swing in voltage from 0 to maximum potential 120 times each second under full wave rectification.
Why do single phase voltage waveforms have little to no diagnostic value?
because of their low energy they can’t penetrate well
How can single phase’s little diagnostic value be overcome?
with 3 phase power that generates 3 simultaneous voltage waveforms that are out of step with one another.
What uses multiple voltage waveforms that are superimposed on one another, resulting in a wave form that maintains a nearly constant high voltage?
a three phase power
How many pulses are in a 3 phase power?
6 pulses per 1/60s
What application is applied when the voltage applied across the xray tube is nearly constant, never dropping to 0 during exposure?
a 3 phase power
What are the limitations to the speed of starting an exposure and ending an exposure?
the initiation time and the extinction time
What is one advantage to a high frequency generator?
its size-they are a lot smaller
High frequency generators produce…
nearly constant potential voltage waveforms,
What do nearly constant potential voltage waveforms do?
improve the image quality at lower patient dose
What systems now use high frequency circuits?
mammo and CT
What kind of circuits do high frequency generators use?
inverter circuits
What are inverter circuits?
high speed switches, or choppers, that convert DC into a series of square pulses (A square wave (pulse) is a non-sinusoidal periodic waveform (which can be represented as an infinite summation of sinusoidal waves), in which the amplitude alternates at a steady frequency between fixed minimum and maximum values, with the same duration at minimum and maximum.)
What does SCR stand for?
Silicon-controlled rectifiers
What do portables high voltage generators use in order to generate square waves?
storage batteries and silicon controlled rectifiers (SCRs)
What is the input that is generated by the portable generators to the step up transformer?
500 Hz
A high voltage step up transformer at 500 Hz is about the size of a 60 Hz transformer..it is pretty…..
heavy
High frequency generators are grouped by…
frequency
What type of rectification is used in almost all stationary xray imaging systems?
full wave rectification or high frequency voltage generation
How does the capacitor discharge generator work?
the generator charges a series of SCRs from the DC voltage of a nickel-cadmium battery (NiCd). By stacking the SCRs, the charge is stored at very high voltage. During exposure, the charge is released (discharged) to form the xray tube current needed to produce xrays.
During capacitor discharge, the voltage falls about ____ kV/mAs.
1 kV/mAs
Why is there a need for precise radiographic technique charts when using a capacitor discharge generator?
because the falling voltage limits the available xray tube current and causes kVp to fall during exposure.
What could happen after a given exposure time when using a capacitor discharge generator?
the capacitor bank continues to discharge which can cause continued xray emission.
How is extra xray emission stopped during a capacitor discharge?
by a grid controlled xray tube, an automatic lead beam stopper, or both
how does the grid controlled xray tube work?
It has a specially designed cathode to control xray tube current.
Why are a lot of systems used today have a falling load technique?
To ensure the shortest possible exposure time.
What is the theory of the falling load voltage?
the initial tube loading is higher and drops during exposure.
What is the drop rate of a falling load?
it follows the cooling characteristics of the xray tube anode
Where does falling load voltage work best?
in high capacity xray imaging systems such as interventional radiology where shorter the exposure time the better
How much voltage ripple does a single phase power system have?
100% voltage ripple
How much voltage ripple for a 3 phase, 6 pulse power system?
14% voltage ripple and voltage never falls below 86% of the maximum value.
How much voltage ripple for a 3 phase, 12 pulse power system?
4% voltage ripple and voltage never falls below 96% of the maximum value
How much voltage ripple for a high frequency generator?
1% voltage ripple-making it the best for xray quantity and quality
Less voltage ripple results in greater….
radiation quantity and quality
Why is radiation quantity best with less voltage ripple?
a greater number of xrays are produces when the electron energy is high than when it is low when a projectile electron is emitted by the xray tube filament
Why is radiation quality best with less voltage ripple?
because fewer low energy projectile electrons pass from cathode to anode to produce low energy xrays.
Fluoroscopy generally stays in ____ phase.
single
Fluoroscopic mA is ___ than xray mA.
lower
Why is fluor mA lower than xray mA?
Because the xray cables are long and have considerable capacitance, which results in a smoother voltage waveform.
Transformers and high voltage generators usually are identified by..
their power rating in kW
Electric power for any device is specified in…
Watts
Power = …
power = current x potential
Watts = ….
watts = amperes x volts
How many kW is a high voltage generator for a basic unit?
30-50 kW
How many kW does an interventional radiology unit run?
up to 150 kW
High voltage generator power (kW) =
maximum xray tube current (mA) at 100 kVp and 100ms
Power is the product of…
amperes and volts
milliamperes x volts =
watts
