X-RAY CIRCUIT (copy) (copy)

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Primary Circuit

Main power switch, circuit breakers, the autotransformer, the timer circuit, primary side of the
step-up transformer

Secondary Circuit

Secondary side of the step-up transformer, the milliampere meter, a rectifier bank, and the x-
ray tube (except for the filaments)

Filament Circuit

Rheostat, step-down transformer, the filaments

Line Compensator

incorporates a meter to measure the voltage provided to the x-ray machine and
a control to adjust that voltage to precisely 220 volts.

Circuit Breakers

Included in the primary circuit to protect against short circuits and electric shock

Autotransformer

\-Is an adjustable transformer controlled by the kVp selector on the operating console.

\-Operates in the principle of self-induction

Autotransformer

\-When a radiographer selects a kVp setting, he or she determines the number of turns on the secondary side to be included in the circuit element and with it the output voltage.

\-Sometimes called the kVp selector.

Autotransformer

Primary purpose is to provide a voltage that will be increased by the step-up transformer to produce
the kilovoltage selected at the operating console.

Autotransformer

The radiographer controls the autotransformer through the kVp selector on the operating console, and
through this directly determines the voltage applied to the x-ray tube to produce x-rays.

Autotransformer

When a radiographer selects a kVp setting, he or she determines the number of turns on the secondary
side to be included in the circuit element and with it the output voltage.

Step-up Transformer

\-Used to increase the voltage from the autotransformer to the kilovoltage necessary for x-ray production.

\-Not adjustable and increases the voltage from the autotransformer by a fixed amount.

Timer Circuit

Consists of mechanical and electronic devices whose action is to “make” and “break” the high voltage
across the x-ray tube.

Exposure Timers

\-Cause the x-ray tube to emit x-rays for a specific time as determined by the radiographer or by AEC

\-Controls the length of exposure

Mechanical Timers

\-Simple devices use in some portable and dental units

\-Operates by clockwork

\-Inexpensive but not very accurate

\-Exposure times greater than 250 ms

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Synchronous Timers

Minimum exposure time possible 1/60 second

Electronic Timers

Most sophisticated, most complicated, most accurate

Electronic Timers

\-Used for rapid serial exposures

\-Exposure time as small as 1 ms

\-Wide range of time intervals

\-Based on the time it takes to charge a capacitor through a variable resistor

mAs Timers

\-Designed for accurate control of tube current and exposure time

\- Terminates the exposure once desired mAs is attained

\-Located in the secondary circuit

Automatic Exposure control

Automatically terminates the exposure when sufficient radiation to provide the required optical
density has reached the image receptor.

Phototimer

Device that measures the quantity of radiation reaching the image receptor.

 Photomultiplier tube
 Ionization chamber

2 types of Phototimer

Spinning Top Test

Method used to check the accuracy of exposure timer

mA meter

Is a device placed in the secondary circuit that monitors x-ray tube current

Rectifiers

Solid-state rectifiers are commonly used

High-Voltage Generator

The high-voltage generator of an x-ray machine is responsible for converting the low voltage from the
electric power company into a kilovoltage of proper waveform.

 High-voltage transformer
 Filament transformer
 rectifiers

High-voltage generator contains the following three primary parts.:

High Voltage-transformer

Is a step-up transformer. The secondary voltage is greater than the primary voltage because the
number of secondary windings is greater than the number of primary windings.

Turns Ratio

The ratio of the number of secondary windings to the number of primary windings.

Rectification

Is the process of converting alternating voltage to direct voltage and therefor alternating current to
direct current.

Self-rectification

Property of an x-ray tube which means that it acts as a valve permitting the flow of current in only one
direction, from the cathode to the anode.

Half-wave Rectification

\- Represents condition in which the voltage is not allowed to swing negatively during the negative half of its cycle.

\-Contain zero, one, or two diode

\-X-ray output is pulsating, with 60 x-ray pulses per second

Full-wave Rectification

\- Contains at least 4 diodes

\- X-ray output – 120 x-ray pulses per second.

\-the negative half cycle corresponding to the inverse voltage is reversed so that a positive voltage is always directed across the x-ray tube.

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Single-Phase Power

 Results in pulsating x-ray beam
 Alternate swing from zero to maximum potential 120 times per second
 Low energy, low penetrability, low diagnostic value

Three-Phase Power

 Generates three simultaneous voltage waveforms out of step with one another.
 Voltage across x-ray tube is nearly constant and never drops to zero during exposure.
 Requires additional size and cost

High-Frequency Generator

 Can be place in the x-ray tube housing
 Produce nearly constant potential
 Improves image quality
 Lower patient dose

Inverter Circuits

 Are high-speed switches, or choppers, that convert DC into a series of square pulses.
 Used by a high-frequency voltage generation.

Capacitor Discharge Generator

Uses nickel-cadmium (NiCd) battery

Falling-load Generator Control Circuit

 Falling-load capability delivers the maximum possible mA for the selected kVp by considering the
instantaneous heat load characteristics of the x-ray tube.
 This delivers the desired amount of radiation to the image receptor in the shortest possible exposure
time.
 During capacitor discharge, the voltage falls approximately 1 kV/mAs

Capacitor Discharge on Mobile Units

works on the principle that a small current is drawn from the mains to charge a capacitor before the x-ray exposure, and this capacitor is allowed to discharge through the x-ray tube during exposure.

Voltage Ripple

Is the variation in peak voltage waveform.

Single-phase Generator

 100% voltage ripple
 The voltage varies from zero to its maximum value

13 % voltage ripple

 3-phased, 6 pulse generator =

4% voltage ripple

3-phased, 12-pulse generator =

High-Frequency Generator

Less than 3% voltage ripple

Power Rating

 Power = current x potential
 Watts = Amperes x volts

Maximum Available Power

When specifying high-voltage generators, the industry standard is to use the maximum tube
current(mA) possible at 100 kVp for an exposure of 100 ms.

Rheostat

\-A variable resistor controlled by the mA selector on the operating console

\- When the radiographer adjusts milliamperage on the operating console, he or she is adjusting this and ultimately the amount of current applied to the filament (filament current) in the x-ray tube.

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the lower

in rheostat, the higher milliamperage station number, _ resistance.

filament circuit

The goal of the this is to literally boil electrons out of the filament wire.

Rheostat

itRheostat controls filament temperature.

Step-down Transformer

Used in filament circuit to increase the current by reducing the voltage that is applied to the filament.

Filament Circuit

The purpose is to control the degree and duration that the filament is heated, which in turn controls
the number of electrons boiled off that will ultimately become the tube current.

Filaments

Represented on the operating console by the “large focal spot” and “small focal spot”.

 The kVp selected adjusts the autotransformer and determines the number of turns on the secondary
side necessary to produce a voltage, through self-induction, that will be sent to the step-up
transformer.
 The step-up transformer increases this voltage by a fixed amount and, through mutual induction,
produces the kilovoltage selected on the operating console.
 This kilovoltage must be rectified.
 The filament circuit draws electricity from the autotransformer, which then travels to the rheostat.
 The selected mA station sets the resistance in the filament circuit.
 From the rheostat, electricity travels to the step-down transformer.
 The kilovoltage applied to the x-ray tube creates a large positive charge on the anode and a large
negative charge on the cathode.

Principles of Circuit Operation

Operating Console/Control Panel

Allows radiographer to control the x-ray tube current and voltage so that the useful x-ray beam is of
proper quality and quantity.

Quantity

Refers to the number of x-rays or intensity of the beam usually expressed in mR or mR per
mAs.

Quality

Refers to the penetrating qualities of the x-ray beam an is expressed by kVp or HVL.

 mAs
 kVp
 Distance
 Filtration

Factors that Affect X-ray Quantity

 kVp
 Filtration

Factors that Affect X-ray Quality

 On/off switch
 mAs selection
 kVp selection
 Table or wall unit activation
 Exposure switch.
 mAs and kVp meters.

Some Basic Controls for Every Panel

 X-ray tube life may be extended by using minimum radiographic factors of ma, kVp, and exposure time
appropriate for each examination.
 Use of faster image receptors.
 Excessive heat results in reduced x-ray tube life.

Extending Tube Life

 Prepares the x-ray tube to receive high heat load.  An example of a warm-up technique is an initial exposure of 50 kVp, 100 mA, at 1/30 second followed by a second exposure in which the mA is raised to 200.

Warm Up Procedures

 Minimize filament boost (preparation) time.
 Limit rotor/start/stop operations.
 Use lower tube current (mA).
 Do not make a high mA exposure on a cold tube.
 Adhere to rating charts and anode heating and cooling curves.
 Limit operations to 80% of maximum single exposure ratings.
 Do not exceed the anode thermal capacity or dissipation rate of the target.
 Do not rotate the tube housing rapidly from one position to another.
 Avoid long intervals between spot-films

Other Ways of Extending Tube Life

1. DEPOSITION OF FILAMENT TUNGSTEN
2. PRODUCES GAS (GASSY TUBE)
3. PUNCTURED TUBE
4. CARELESSNESS
5. DAMAGES TO ANODE
6. DAMAGES TO ROTOR AND BEARINGS
7. DAMAGES TO FILAMENT
8. DAMAGES TO TUBE HOUSING
9. DAMAGES TO STATOR

X-ray Tube Aging