Rad Physics Ch. 5 (plus transformers)

T or F: electric current is passed to the rotor in an induction motor.

false; current is produced in the rotor windings by induction and the electromagnets surrounding the rotor are energized in sequence (produces a magnetic field)

What is the falling-load generator?

exposure begins at maximum mA, and the mA drops as the anode heats -result is a minimum exposure time

The different types of x-ray imaging systems are usually identified according to:

the energy of the x-rays they produce or the purpose for which the x-rays are intended

The high-voltage generator may be housed in:

the equipment cabinet positioned against a wall -always close to the x-ray tube, usually in the exam room -a few installations place these generators out of sight above the exam room -new generators with high frequency circuits require less space

X-ray imaging systems with synchronous timers are recognizable because:

the minimum exposure time possible is 1/60 seconds -timing intervals increase by multiples ( 1/30, 1/20, etc)

The voltage the autotransformer receives (primary) and the voltage it provides (secondary) are related directly to:

the number of turns of the transformer enclosed by the respective connections

The 3 main components of an x-ray imaging system are:

the operating console the x-ray tube the high-voltage generator

What does the prereading kVp meter allow?

the precise adjustment of the supply voltage and monitoring the kVp before the x-ray exposure

What is the purpose of the minor kVp control?

"fine tunes" the selected technique -minor and major kVp represent 2 separate series of connections on the autotransformer

A timer circuit consists of an electronic device whose action is to:

"make" or "break" the high voltage across the x-ray tube -separate from other circuits in the x-ray unit -nearly always done on the primary side of the high voltage transformer (where the voltage is lower)

What is the autotransformer law?

*SAME AS THE TRANSFORMER LAW*

How does an electric generator work?

-a coil of wire is placed in a strong magnetic field between 2 magnetic poles -coil rotated by mechanical energy -current is induced in the coil of wire

Name an example of a transformer.

-an electromagnet with a ferromagnetic core bent around into a continuous loop -if a secondary coil is wound around one side of the primary coil, almost all of the magnetic field produced by the primary passes through the center of the secondary -change of current in the primary coil induces a changing current in the secondary coil

Describe a closed-core transformer.

-built around a square of ferromagnetic material -laminated layers of iron -helps reduce energy losses (greater efficiency)

Describe a shell-type transformer.

-confines magnetic field lines of the primary winding because the secondary is wrapped around it -essentially 2 closed cores -more efficient than closed-core -most currently used

Describe the mA meter located within the tube circuit.

-connected at the center of the secondary winding of the high-voltage step-up transformer -secondary voltage is alternating at 60 Hz so the center of the winding is always at zero volts -no part of the meter is in contact with the high voltage -meter may be safely put on operating console -sometimes allows mAs to be monitored

How does an electric motor work?

-electric energy is supplied to the current loop to produce a mechanical motion (rotation of the loop in a magnetic field) -same components as an electric generator

Describe an autotransformer.

-iron core with only one winding of wire around it -acts as primary and secondary windings -generally smaller -only a small step-up or step down in voltage is required

Describe an electronic timer.

-most sophisticated, most complicated and most accurate of the x-ray exposure timers -rather complex circuitry -allow a wide range of time intervals to be selected and are accurate to intervals as small as 1 second -can be used for rapid serial exposures -suitable for interventional radiology procedures -controlled by a microprocessor

List the 5 major controls on the operator's console.

-on/off control -kVp selection -mA selection -time (mAs selection) -automatic exposure controls

How does an induction motor work?

-powers up the rotating anode -rotating rotor is a shaft made of bars of copper and soft iron fabricated into one mass -external magnetic field is supplied by several fixed electromagnets called stators -current produced in rotor windings by induction -induced current generates a magnetic field

Why are some of the secondary connections on the winding of an autotransformer located closer to one end of the winding than the primary connections?

-this allows the autotransformer to increase voltage -other connections located closer to the middle of the winding allow a decrease in voltage

Most x-ray imaging systems are designed to operate on:

220 V power, although some can operate on 110 V or 440 V. -electric power companies are not capable of providing 220 V accurately and continuously

Diagnostic x-ray imaging systems usually operate at voltages of:

25-150 kVp and at tube currents of 100-1200 mA

Filaments normally operate at currents of:

3-6 amperes

Because of variations in power distribution to the hospital and in power consumption by various sections of the hospital, the voltage of the x-ray unit may vary by as much as:

5% -results in a larger variation in the x-ray beam, which is inconsistent with production of high quality images

In the U.S., electric current is supplied at a frequency of:

60 Hz

Describe the movements of a patient couch.

90 degrees to vertical so that the patient is standing 30 degrees head down

Define automatic exposure control (AEC).

electronic device designed to terminate x-ray exposure after properly exposing image receptor

What example that we have previously learned is the basis of the electric motor?

Oersted's experiment -an electric current produces a mechanical motion (motion of the compass needle)

With most systems, the x-ray tube is located in the _____, and the operating console is located in _____.

examination room; adjoining room with a protective barrier separating the 2

What is the Transformer Law?

Vs/Vp=Ns/Np

Define a semiconductor.

a material that can sometimes conduct an electric current and sometimes not, depending on the voltage applied across that material

Define line compensation.

a method of adjusting the voltage to the x-ray imaging system to a constant value, to compensate for fluctuations in voltage supplied by the power company

The usual x-ray room contains:

a radiographic imaging system and a fluoroscopic imaging system -fluoroscopic x-ray tube is usually located under the exam table -radiographic x-ray tube is attached to an overhead moveable crane assembly that permits easy positioning of the tube

Define radiolucent.

allow relatively undiminished intensity of x-rays through the tabletop

Define a capacitor.

an electric circuit device that stores electric charge

Just under the patient couch is:

an opening to hold a thin tray for a cassette and Potter-Bucky grid -for fluoro, tray must be moved to the foot of the couched and shielded for radiation protection with a Bucky slot cover

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 increases the mA with the kVp

Where is the power supplied to the x-ray unit first delivered to?

autotransformer -voltage from the autotransformer to high-voltage transformer is variable but controlled.

What is the most preferred type of examination couch and why?

carbon fiber couches -strong and absorb little x-radiation -contribute to reduced patient radiation dose

The x-ray tube current crosses from:

cathode to anode -measured in milliamperes (mA)

A transformer will operate ONLy with:

changing electric current (AC) -a direct current applied to the primary coil will induce no current in the secondary coil

What are the 3 types of transformers?

closed-core autotransformer shell-type

Operating consoles are based on:

computer technology -controls and meters are digital -techniques are selected with touch screen -numeric technique selection replaced by icons indicating body part, size and shape -features are automatic, but tech must know their purpose and proper use

What is a transformer?

device that transforms electric potential and current into higher or lower intensity

Describe a diode.

electric device that contains 2 electrodes

The autotransformer works on the principle of:

electromagnetic induction

T or F: The conversion process of a generator to change mechanical energy into electrical energy is 100% efficient.

false; there are frictional losses in the mechanical moving parts and heat losses caused by resistance in the electrical components

X-ray tube current is controlled through a separate circuit called the:

filament circuit -connections on the transformer provide voltage for the filament circuit

The filament temperature is controlled by:

filament current (amperes)

What is the full name of the filament transformer and what is its purpose?

filament heating isolation step-down transformer -steps down the voltage to approximately 12V and provides the current to heat the filament

The voltage from the mA selector switch is delivered to the:

filament transformer -which is a step-down transformer, so the voltage to the filament is lower than the voltage supplied to the filament transformer -current is increased across the transformer in proportion to the turns ratio

Most patient couches are:

floating (easily unlocked and moved by the tech) or motor driven

Describe a voltage ripple.

fluctuation in the voltage applied to the x-ray tube expressed as a percentage of kVp.

What is the only type of high-voltage generator that can be positioned in or on the x-ray tube housing?

high-frequency generator

What is the difference between a high-voltage generator and a high-voltage transformer?

high-voltage transformer is just one component of a high-voltage generator

Why does the x-ray circuit require rectification?

in order that a reverse voltage is not applied across the x-ray tube -x-ray tubes operate most efficiently on a DC current

What type of motor is used with x-ray tubes?

induction motor

The low voltage from the autotransformer becomes the:

input to the high-voltage step up transformer that increases the voltage to the chosen kVp

Ideally, the exam room should be designed so that:

it is possible to reach the operating console without having to enter the "radiation area" of the exam room

What is a pre-reading kVp meter?

kVp meter that registers even though no exposure is being made and the circuit has no current -allows voltage to be monitored before an exposure

The operating console usually provides control of:

line compensation, mA, kVp, and exposure time -meters are provided -imaging systems with AEC may have separate controls for mAs

What determines the total number of x-rays emitted and therefore the exposure of the IR?

mAs

Some older x-ray operating consoles have adjustment controls labeled:

makor kVp and minor kVp -by selecting one of these controls, tech scan provide precisely the required kVp

What example that we have previously learned is the principle on which the electric generator operates?

mechanical motion (the motion of a magnet near a coil of wire) induces electricity in a coil of wire

As filament current increases, the filament becomes hotter, and then:

more electrons are released by *thermionic emission*

Radiation quantity refers to:

number of x-rays in an x-ray beam -usually measured in mGy or mGy/mAs -*x-ray intensity*

The part of the x-ray imaging system most familiar with radiologic techs is the:

operating console

What is required to be apart of every x-ray imaging system?

patient-supporting examination couch -may be flat or curved but MUST be uniform in thickness and as transparent to x-rays as possible

Describe the mA meter location.

placed in the tube circuit, connected at the center of the secondary winding of the high-voltage step-up transformer in series with the x-ray tube. -reduces the possibility of shock

What is a synchronous motor?

precision device designed to drive a shaft at precisely at 60 revolutions per second -used as timing mechanisms in some x-ray systems

How does primary voltage relate to secondary voltage in an autotransformer?

primary and secondary voltage are in direct relation to the number of turns in the transformer

What is rectification?

process of changing alternating current and voltage to direct current and voltage

Define power.

product if voltage ad current -Watt

What are precision resistors used for?

reduce the voltage to the filament circuit to a value that corresponds to the selected mA -result in fixed stations so that x-ray current is not continuously variable

In a step-up transformer, the current on the secondary side is ______ than the current on the primary side.

smaller

A transformer with a turns ratio less than 1 is a:

step down transformer

A transformer with a turns ratio greater than 1 is a :

step up transformer -voltage is increased or stepped up from the primary side to the secondary side

What is a guard timer?

terminates an exposure after a prescribed time, usually at about 6 seconds -this makes it impossible for any timing circuit to continuously irradiate a patient for an extensive period

What is the effect of the transformer law on current?

the change in current across a transformer is in the opposite direction from the voltage change but in the same proportion; an inverse relationship

Radiation quality refers to:

the energy of the x-ray beam -expressed in kVp or more precisely, HVL -*x-ray penetrability*

Paramount in he design of all timing circuits is that:

the radiographer starts the exposure and the timer stops it -during fluoro, if the radiographer releases the exposure switch, the exposure is terminated immediately

The number of electrons emitted by the filament is determined by:

the temperature of the filament

What does the transformer law state?

the voltage change is proportional to the turns ratio (number of turns of the secondary coil to the number of turns in the primary coil)

The number of x-rays that reach the IR is directly related to:

the x-ray tube current and the time that the tube is energized

The operating console allows radiologic techs to control:

the x-ray tube current and voltage so that the useful x-ray beam is of proper quantity and quality

Fluoroscopic couches tilt and are identified by:

their degree of tilt -ex: a table would tilt 90 degrees to the foot side and 30 degrees to the head side

Why can synchronous timers not be used for serial exposures?

they must be reset after each exposure

In a filament transformer, the secondary windings are:

thick and at approximately 12 V electric potential and carry a current of 5-8 A

In a filament transformer, the primary windings are made of:

thin copper -carry a current of 0.5-1 A and approximately 150 V

What is the purpose of an electric generator?

to convert mechanical energy into electric energy

Why are all of the circuits, that connect the meters and the controls on the operating console, at a low voltage?

to minimize the possibility of hazardous shock

What is the purpose of the autotransformer?

to vary and control the amplitude of the voltage supplied to the high-voltage step-up transformer and the filament transformer

T or F: It is safer and easier to control a low voltage and then increase it.

true -it is much harder to increase a low voltage to the kilovolt level and then control its magnitude

An autotransformer can be designed to step up voltage to approximately:

twice the input voltage value

What is an autotransformer?

type of transformer having a single winding

What does the line compensator measure?

voltage provided to the x-ray imaging system and adjusts that voltage to precisely 220 V. -today's x-ray units have automatic line compensation and have no meter

The kVp meter is placed across the output terminals of the autotransformer and therefore actually reads:

voltage, not kVp -scale of the kVp meter still registers kilovolts because of the known multiplication factor of the turns ratio

The protective barrier must have a:

window for viewing the patient during the exam