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.