x-ray equipment - ch 5
all exposure switches are of the
"dead-man" type - xray exposure may occur only while the switch is depressed
old phototimers had a minimum reaction time of
.05 second or less (photomultipliers)
specialized tomographic equipment generators may include
10, 15, 20, 25, 30 and 40 mA stations
diagnostic x-ray range for mA, seconds, and kVp
10-1200 mA, .001-10 seconds, kVp of 25-150
in the US, each hot wire carries
110-120 volts with 60 Hz alternating current. The 2 hot wires reverse their polarity 120 times per second.
a 2 stage exposure switch - description
1st stage - activates the rotating anode and heats the filament - creating the space charge cloud anode must turn at a high enough speed to avoide melting the target area with high kVp exposures 2nd half initiates the expsorue
nearly all x-ray equipment operates from an incoming line of
210-220 volts (V).
3-phase, 6 pulse current produces a voltage ripple of
3 pulses per half cycle, = 6 pulses per Hz (cycle) therefore 360 pulses per second.
full-wave rectified high-frequency generator ripple ranges from
3-4%
For the filament circuit, the incoming line must be modified to about
3-5 amperes and 6-12 volts.
filament circuits are usually adjustable to the equivalent of mA ratings of
50, 100, 200, 300, 400, 500, 600, 800, 1000 and 1200.
Most single phase units do not go beyond
500 mA.
on mobile equipment, the switch must be on a cord with a minimum of
6 feet - to permit the radiographer to move as far as possible from the x-ray tube during an exposure.
electricity in buildings in the US
60 Hz alternating current with nominal rms of 200-240 volts.
according to US Public Law 90-602, generators must terminate the exposure at
600 mAs for exposures above 50 kVp and 2000 mAs for exposures below 50 kVp - primarily during mammography.
most falling load generators will step down the mA at
70-80% of tube-loading capacity.
high-frequency generators use
AC and DC power converters to change the incoming-line voltage frequ from 60 Hz to the 6000 Hz range.
x-ray tube requires
DC for x-ray production - allows e- to flow only from the cathode to anode in the x-ray tube
mAs timers
Measure mAs located after the secondary coil of the high voltage transformer They monitor the product of mA and time on the 2ndary side of the high-voltage step-up transformer. Stop the exposure when desired mAs level is reached falling load generators as well as some capacitor discharge units. (mobile)
old-fashioned phototimers were calibrated by
setting the capacitor-discharge point (usually controlled by a set screw in the electronics cabinet) at a level that produced a satisfactory exposure to the IR. A trial-and-error procedure with a phantom.
advantages of falling load generators are
shorter times in heavy-load situations simpler operation
exposure switch
simply a remote control switch that permits current to flow thru the circuit. - resides on the primary side of the xray circuit around the same level as the autotransformer.
never use compression bands as the
sole restraint device
the tabletop must include
space for a tray to hold cassettes and a radiographic grid - bucky tray
the electrical device and location in circuit for rotor switch:
speed of rotating anode Stator separate circuit from stator of anode motor
backup times cannot exceed
the tube limit and should be set at 150% of the anticipated manual exposure mAs. Prevents tube overload and unnecessary radiation exposure to the pt.
the watt
the unit of power
unit of power
the watt (W)
the main circuit supplies
the x-ray tube with properly modified power - to produce x-rays has a primary (low voltage) side - location of all operator controls and a secondary (high voltage) side
tube suspension systems permit
the xray tube to manipulated Hold the tube immobile during the exposure
floating tabletops save radiographers
time and effort - especially when positioning large patients
with large patients, backup time may be reached and will
time out, underexposing the image If this happens, first look at the image Then, if underexposed, look at the S value See console for mAs used - may have to use manual technique in order to bump it up
purpose of the filament circuit
to create the thermionic cloud
purpose of the ground
to ground the equipment for safety
because of the higher-frequency current,
transformers for these units are significantly smaller
Upright units
upright cassette holder or Bucky unit - chest radiography, AC joints, abdominal obstructive procedures, C spine.) May include movable grid, cassette tray and automatic exposure control sensors). aka wall bucky movable grid - reciprocating or oscillating (film) cassette tray AEC
floor-to-ceiling suspension systems
use a pair of rails one on the ceiling and one of the floor for longitudinal positioning. Rooms w/high ceilings may use an overhead rail suspended from a wall.
falling load generators can shorten tube life because they
use higher mA settings, causing the filament of the tube to wear out more quickly.
floor suspension system
uses a tube-support column mounted on the floor. Must be carefully counterbalanced to avoid tipping, by adding counterweight to the back of the telescoping tube arm. It requires that both tube system and table be installed further back from the wall than other systems.
main switch and circuit breakers are
usually enclosed in an electrical power box
variable resistor allows
variances of mA selection on the control panel modifies the voltage and amperage to the correct level for the step-down transformer Determines the amperage needed to create the right space charge cloud
nominal
various resistance, voltages constantly fluctuate
radiation therapy simulator units
verify radiation therapy treatment set-ups prior to actual treatment
voltage ripple
when full-wave recitification is applied, producing this.
backup time - AECs
Systems to prevent overexposure are necessary if one forgets to activate the AEC in a wall unit, leaving the table unit waiting for exposure. The wall is not receiving radiation, so the exposure wouldn't cease until the tube overload protector is activated. = waste of tube life and excessive radiation dose to the pt.
when a high-frequency current is supplied with full-wave rectified power,
a 12-14 kHz waveform can be produced.
C-arm tube suspension system
a C-shaped arm supports the tube and IR. The tube and IR are fixed to opposite ends of the C-arm. Used in head units, mobile fluoro, and ceiling-suspended angiography and surgical units.
depression of the exposure switch on a capacitor discharge mobile unit triggers
a discharge to the xray tube.
a very slight shift in the quanitity of electrons in the thermionic cloud around the filament can have
a dramatic effect on the quantity of x-ray photons produced when the kilovoltage is applied
generator power varies as kilovoltage changes so
a high-power generator at 150 kVp may not be as powerful when the kVp is lowered into the middle diagnostic range
grid installation usually consists of
a mechanism that will automatically move the grid during exposure
the control console is located in
a radiation-shielded location outside the radiographic exposure room.
what indicates the readiness of a capacitor
a signal
3-phase, 12-pulse waveform is accomplished by a
combination of 12 diodes. Produces a voltage ripple of 4-10%, so tube voltage never drops below 90-96% of the peak kilovoltage setting. Produces 40% move average photon energy than full-wave rectified, single-phase.
AEC and AED
common acronyms for automatic exposure controls
computed tomography units
computerized sectional images
capacitors discharge more slowly as potential difference decreases, so
considerable residual kV may exist at exposure. May cause a leakage of radiation
the mAs value remains
constant, even when there is a slight fluctuation in the capacitor charging current
the exposure switch must be attached to the
control console so it is impossible for the operator to be exposed. The equipment must be designed to prohibit the x-ray tube from being manipulated into a position where it could expose the user.
the main breaker
controls the incoming-line current and protects the equipment from overload
purpose of rectifiers
converts AC from the high voltage transformer into pulsating DC for x-ray production
a two-step button
depress halfway to activate the anode rotation and depress completely to initiate the x-ray exposure. Most manufacturers recommend that these buttons be depressed completely in one motion to extend the life of the x-ray tube.
diagnostic x-ray control exposure switches are
designed to begin but not end exposures
medical x-ray units are classified as
diagnostic or therapeutic
tilting tables are used in
diagnostic radiographic and fluoroscopic rooms allow table to tilt 90° in one direction and 15° in the other direction Footboard attaches to table to allow for standing - assure it is secure shoulder supports keep pt from sliding off when head is tilted down (myelogram) Handgrips give pt an added feeling of security
2 disadvantages to curved tops
difficult for radiographer to maintain a patient accurately in oblique or lateral positions, and the top is entirely useless as a level support surface for a film cassette during tabletop radiography.
the filament circuit's supply is drawn
directly from the main circuit's supply
fixed tables
do not permit tilting - designed for diagnostic radiographic work only
a tabletop must be
easily cleaned hard to scratch and without crevices where CM can gather
wavetail cutoff
exposure begins at the peak voltage and then decreases.
dead-man type of exposure switch
exposures may only occur when the switch is depressed prevents the exposure from continuing when the tech is in the room on mobile equipment cord must allow tech to be at a minimum of 6' away
urologic units
facilitate urological and genital studies can be custom built to nearly any specifications
tables may be either
fixed or tilting
curved tops are used for
fluoroscopic exams - more comfy and permit the body part to be placed slightly closer to the IR for a more accurate image.
ancillary equipment for tilting tables
footboard - to stand on when table is upright - GI studeis when pt begins erect and then is brought hor'l. shoulder supports - keep pts from sliding off the table handgrips - give the pt an added feeling of security compression bands - to compress tissue
mammography units
for breast studies
head units
for cranial studies
panoramic dental and facial units
for facial structures
diagnostic units are designed for
general procedures, cardiac cath, head procedures, fluoro, etc. - in the diagnostic x-ray range of 10-1200 mA, .001-10 seconds, at a kVp of 25-150.
3-phase power is produced by the
generator and is the common form in which power is supplied to users by power companies
falling load generators permit
greater use of the acceptable tube limits without requiring a higher-power and more $$ generator that would permit higher mA loading. Begins exposures at the highest mA for the first .1 second and then reduces the mA to a lower range at .2 second, etc. until the appropriate lower mAs is reached.
generator power ratings are determined by the
greatest load the generator is capable of seinding to the xray tube. The unit of power is the watt
what helps with the leakage of radiation on capacitor discharge units
grid-biased xray tubes - cut photon emission at a set time by reversing the charge polarity of a wire grid in front of the filament. the tube collimator can be designed to automatically close its lead shutters after the exposure, stopping radiation leakage.
an electrogmagnetic brake on the table may be controlled by
hand, knee or foot
3-phase, 12 pulse generator - described
has 12 diodes (rectifiers) produces 12 pulses per cycle 720 pulses per second voltage ripple of 4-10% step-up transformer has a wye or delta configuration
3-phase, 6-pulse generator, fully rectified - described
has 6 diodes (rectifiers) produces 6 pulses per cycle produces a voltage ripple of 13-25%, so tube never falls below 75-87% of peak voltage 35% more efficient than full-wave rectified, single phase step-up transformer is a wye or delta configuration combines 3 phase current
to avoid the possibility of error, all x-ray units that use rotating anodes
have circuitry that prevents an exposure until the anode is turning at the correct speed.
voltage ripple varies as
higher voltage loads are applied
the mA meter is located
in the secondary side of the main circuit and connected near the electrical ground of the secondary winding of the high voltage transformer
"phototimer" today refers to
ionization chambers
timer circuit
is intended to end the exposure at an accurately measured, preset time. Wired in the circuit b/w the autotransformer and the high voltage transformer several types: electronic, milliamperage-second times, AEC timers
whenever the exposure switch is depressed,
it must be held until both the audible and visible indicators have ceased. (buzzer and light)
since xray generators operate in the kilovoltage and milliamperage range, the power ratings are stated in
kilowatts.
common diagnostic x-ray console controls
kvp selection, ma selection, time selection, rotor switch, exposure switch
the electrical device and location in circuit for timer selection:
length of exposure Timer circuit b/w exposure switch and step-up transformer
modern ionization chambers with SCRs may have a minimum reaction time of
less than .001 second.
a falling load generator utilizes the tube's
loading potential to a much greater extent than the constant-potential generator.
all radiographer-operated controls are located on the
low-voltage side of the circuit to protect operators from high-voltage shock hazards.
fixed tables have the ability to be
lowered and then raised to a working height
under automatic exposure controls, the radiographer still controls
mAs and kVp
the kVp selector and meter
makes contact with the taps on the autotransformer the voltage selected at this point by the tech is the input voltage to the high-voltage transformer kVp meter is a pre-reading voltmeter - it measures the voltage not the kVp to the x-ray tube positioned before the step-up transformer kVp readout on the console shows the kV that will be applied to the x-ray tube
With a 60 Hz line, an exposure in single-phase full-wave rectified unit results in
photons produced during the low-voltage periods - they are of such low energy that they may not exit the tube or do not contribute to the radiographic image b/c they are absorbed - not useful to x-ray production
2 types of AECs:
phototimers ionization chamber devices
3-phase power - described
power produced by the generator utilizes 3 armatures instead of just one (single-phase) as each wave peak begins to drop toward 0, the potential difference is boosted back to peak by the next phase voltage never drops to 0
release of the switch must terminate the exposure to
prevent the exposure from continuing when the operator enters the radiation area
automatic exposure controls are
programmed to terminate the radiographic exposure time - only control time
a primary concern for radiographers must be
proper assistance to pts when they are getting on or down from the table
the electrical device and location in circuit for mA selection:
provides filament current Variable resistor (rheostat) in filament circuit b/w incoming line and step-down transformer
Bucky
radiographic grid installed over the cassette tray Named for Dr. Gustav Bucky - inventor of the radiographic grid installed over the film cassette
x-ray tabletop
radiolucent - easily permit x-rays to pass thru floating stationary fixed - does not allow tilting up or down tilting - allows standing position and trendelenburg
the current control devices
regulate the amperages supplied to the filament in the xray tube - control on the control console by the radiographer - marked in increments of the mA available when the high -voltage supplly is released at exposure
functions of compression bands
restraint for pts who are unable to cooperate compress abdominal tissue for a more uniform sugject density
what is on the secondary side of the main circuit
secondary side of step-up transformer the ground mA meter rectification circuit
normal height of a table
30-40 inches or 75-100 cm from the floor
3-phase power is represented by symbol
3¢
compared to capacitor discharge machines, battery-operated mobile units have the advantage of
3¢ exposure consistency, higher rms voltage and no leakage possibility, mobility with recharging capabilities.
single-phase, fully rectified - described
4 diodes (rectifiers) produces 1-phase, 2-pulse DC utilizes both halves of the sine wave there are 2 pulses/cycle current still drops to 0 120 times/second has 100% voltage ripple
falling load generators
Modern generator that takes advantage of extremely short time capabilities and tube heat loading potential. Specially designed 3¢ of high-frequency generators. Take full advantage of the current loading capability of the xray tube by beginning the exposure with a high amperage and then allowing it to fall during the exposure. Accomplished with a constant potential generator. Requires that both the mAs and the kV be regulated independently by the radiographer. Calculates the most efficient method of obtaining the required mAs Always uses the shortest time possible to obtain a given mA Disadvantage: can't use a breathing technique
AEC timers
Used to provide consistency of radiographic quality (density/exposure) Relies on excellent positioning skills and knowledge of anatomy of interest - must be over correct cells - not much latitude for positioning! Consists of a flat ionization chamber b/w pt and IR When predetermined level of ionization is reached, an electronic switch terminates the exposure. Counts # of photons and shuts off
for a 3¢ generator, the power rating formula is
V x A = W. What is the power rating for a 3¢ generator capable of delivering 150 kVp at 1000 mA to the tube? 150 kV x 1000 mA = 150,000 W or 150 kW
for a 1¢ generator, the power rating formula is
V x A X .7 = W. What is the power rating for a 1¢ generator capable of delivering 120 kVp at 300 mA to the tube? 120 kV x 300 mA x .7 = 25,200 W or 25.2 kW
old-fashioned phototimer devices used
a thyratron to regulate exposure automatically - the beam would pass thru the pt, table, and cassette before striking a fluorescent screen that had the ability to absorb xrays and produce light photons. The light photons were directed toward a photomultiplier tube that had the ability to produce multiple electrons from each light photon. The electrons established a charge on the capacitor and when the capacitor reached its preset value, it discharged, triggering an electrical current that breaks the timer circuit - terminates the exposure
filament circuit consists of
a variable resistor - rheostat, potentiometer, or saturable reactor (like a dimmer switch) and step-down transformer (mutual induction)
single-phase generators with full-wave rectification produce
a voltage ripple of 2 pulses per Hertz or 120 pulses per second - 2 usable pulses per cycle. 1¢ 2P waveform with a ripple of 100%. The voltage in the tube drops to 0 twice per period or cycle.
3-phase, 6 pulse generators with full-wave rectification produce
a voltage ripple of 6 pulses per Hertz or 360 pulses per second. Produces 6 usable pulses per cycle = 3¢ 6P waveform. 3-phase, 6-pulse power produces a ripple of 13-25%, meaning the voltage in the x-ray tube never falls below 75-87% of peak kilovoltage setting on the console. Produces 35% more average photon energy than a single-phase, full-wave rectified generator.
exposure switch can be connected to the switch that
activates the rotating anode of the x-ray tube.
phototimer is often used to refer to
all AECs
purpose of the x-ray generator
allows the operator to select and control kVp and mA and time allows the operator to select focal spot size allows the operator to choose a manual or automatic exposure timing Increase the low voltage from the utility company to high voltage (kVp) necessary for x-ray production Converts AC from the utility co. to pulsating DC required by the x-ray tube to allow e- to flow from cathode to anode (rectification) Protects the operator and pt from electrical shock from high voltage involved when using the x-ray machine
in 3-phase power,
as each wave peak begins to drop to zero, the overall potential difference is boosted back to peak by the next phase wave. The result is that the sum of the phasing never drops to zero.
chest units
automate chest radiography by combining an upright Bucky unit with an automatic film handling mechanism
falling load generators must be used with
automatic exposure controls or rely on an mAs timer instead of independent mA and time controls.
the electrical device and location in circuit for kVp selection:
autotransformer b/w incoming line and exposure switch
exposure switch and timer are located
b/w the autotransformer and the step-up transformer
AEC sensors are located
b/w the table top and the bucky tray
capacitors discharge units' capacitor circuit is supplied from
batteries which are charged from line current.
angiography table surface may be
carbon graphite fiber - to reduce absorption of photons
overhead suspension system
ceiling suspension - allows longitudinal and transverse positioning as well as vertical distance. Each motion is locked in place by a solenoid (detent)
detents
centering locks - verify common tube positions - solenoids
purpose of mA meter
measures tube current (flow of e- across the xray tube during exposure) only gives readout during the exposure
All AECs have a
minimum reaction (response) time
capacitors discharge units are most commonly used for
mobile equipment.
The filament circuit
modifies the incoming-line power (210-220 V) to produce the thermionic emission from the filament wire of the xray tube by a sequence of devices. Regulates the number of e- available at the filament to produce x-rays Drawn directly from the main circuit's supply Different mA stations may be selected by the filament variable resistor All operator controls are located on low voltage side
the electrical device and location in circuit for exposure switch
moment of exposure Switch b/w autotransformer and timer circuit
electronic timers are the
most common and accurate timers in use today. Capable of accurate exposures as short as .001 second with only a 1 msec delay. Set by variation of the timer controls on the console
high-frequency generators
most efficient uses power converters to change incoming-line frequency from 60 Hz to 6000 Hz has 3-4% voltage ripple
mobile systems
mostly floor suspension system
the anode
must be turning at a sufficiently high speed to avoid melting the target area by the heat of the kilovoltage exposure.
incoming line current comes from the
neutral (or ground) wire and one of the hot wires produces a potential difference of 110-120 volts.
battery-operated mobile units operate on
nickel-cadmium (NiCd) battery-supplied AC current. these batteries are recharged by plugging the machine into an outlet the batteries provide power to propel the unit and operate the x-ray tube provides power similar to 3 phase, 12-pulse It will take more technique on a mobile unit than a high frequency radiography room unit
On battery-operated mobile units, the batteries supply
nonpulsating direct current to a rotary converter, which provides current like a 3¢ 12 P or greater frequency generator.
A single-phase power is
not as efficient as a three-phase power
a diagnostic x-ray generator is composed of
numerous basic electrical devices
tube support 5 versions
overhead floor-to-ceiling floor mobile c-arm
incoming-line current (mains)
supplied in the form of a 3-phase power cycle from our power company (KCPL). One of the hot wires is always half the incoming voltage above-ground potential, while the other is always the same voltage below-ground potential. incoming line current comes from the neutral (or ground) wire and one of the hot wires produces a potential difference of 110-120 volts. The two lines are not in phase with one another, using incoming current from both hot wires produces a potential difference that is less than the sum of the 2 single phases. Usually results in about 210 volts. We can't order anything else from KCPL. We simply get 3-phase
rectifiers are arranged
systematically in a circuit - called the rectifier circuit
Bakelite
table surface type
the autotransformer - described
takes the input line voltage consists of single electrical winding with a series of taps (self induction) used to select a PRE-DETERMINED kVp works on the principle of electromagnetic induction and is based on the transformer law Vs/Vp = Ns/Np # of coils determines what kVp selections can be used
Disadvantage - AECs may be incapable of
terminating exposures quickly enough, especially with extremely high-speed films and intensifying screens during high kVp chest radiography
the only items in a filament circuit are
the circuit variable resistor - rheostat filament step-down transformer
the main and filament circuits are combined to form
the complete basic x-ray circuit.
Because falling load generators start at the highest mA possible,
the control of mA is removed from the radiographer - which may be a disadvantage when needing to use long-time exposures like those of the T-spine and sternum breathing technique exams.
in the filament circuit, after regulation
the current is then sent to a step-down transformer that modifies it to the appropriate amperage that will reach the filament itself.
an x-ray circuit is established when
the devices are connected in a sequence capable of accelerating electrons to the speed necessary to cause the production of x-ray photons within an x-ray tube.
the filament circuit supplies
the filament of the x-ray tube with properly modified power - to create the appropriate thermionically emitted electron cloud at the filament.
the ground is located on
the high voltage side of the step-up transfomer
the main x-ray circuit modifies
the incoming-line power (210-220 V) to produce x-rays by a sequence of devices. The circuit must boost voltage to the range necessary to produce x-rays and to permit the radiographer to adjust the amperage, voltage and length of exposure as well as incorporate appropriate circuitry to increase the efficiency of x-ray production.
the minimum reaction (response) time is determined by
the length of time necessary for the AEC to respond to the radiation and for the generator to terminate the exposure.
basic x-ray circuits can be divided into
the main and filament circuits
what "lives" on the primary or low voltage side
the main breaker autotransformer kVp selector and pre-reading kV meter exposure switch exposure time primary side of step-up transformer
in 3-phase current, if any phase is at 0,
the other 2 phases are of equally opposite values so that the sum of the 3 currents is always zero - used in connecting the generator windings to combine the current.
the x-ray generator circuit can be divided into 3 sections
the primary side (low voltage) the secondary side (high voltage) the x-ray tube (filament circuit)
last component on the primary side
the primary side of the step-up transformer
who adjusts the various factors from a control console
the radiographer
capacitor discharge mobile unit's depression of a charge button causes
the rectification circuit to charge a capacitor instead of the xray tube.
the first component on the secondary side of the main circuit
the secondary side of the step-up transformer
The trigger value is set by the variation of
the timer controls on the console unit by the radiographer
when the desired mAs level is reached
the timers interrupt the circuit to stop the exposure
the effects of generator type on radiographic density and patient dose and contrast
with lower phase generators, fewer useful photons are being produced, and density gets greater with a higher frequency generator. Dose ends up higher with lower phase generators because lower phase generators provide fewer transmissions. You'd have to bump up technique on a lower phase generator to get the same result, resulting in higher pt dose. As kVp controls contrast, higher phase generators provide lower contrast (longer scale of contrast)
tomography units
with tubes that move in an arc during exposure
the filament transformer (step-down) - described
works with low voltage and high current transformer allows the tech to provide the appropriate power to the x-ray tube filament for e- emission the filament current (A) determines the temp of the filament - so the number of e- that will be available to bombard the target - size of space charge cloud when the temp is higher, the amount of e- available and quantity of x-rays emitted from the tube increases
ionization chamber description
xray beam passes thru the pt and tabletop before striking the ionization chamber. The IR (bucky/cassette) is positioned under the ion chamber. Uses a thin 5 mm thick parallel-plate chamber. The exact size, shape and position of the ionization chamber must be known to the radiographer. Ions are collected or counted in the chamber when preset limit is reached, the timer circuit is broken and the exposure is terminated If the area of interest is not centered or the wrong ionization chamber is selected, the radiograph will be either under or overexposed.
disadvantages of a capacitor discharge mobile unit
xray leakage and exposure often begins at the peak voltage and then decreases.
single-phase power
∩ - permits the potential difference to drop to zero with every change in the direction of current flow and is represented by the symbol 1¢. In a full-wave rectified circuit (direct pulsating current), this means that the xray tube is experiencing no potential difference and is producing no x-ray photons 120 times each second on a 60 Hz line. Also maximum current is experienced 120 times per second.