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28 Cards in this Set

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Control Console

The unit where the operator sets all of the exposure techniques: kVp, mA, exposure time. This includes the main switch, the autotransformer, kVp selector, exposure switch, exposure timer.

Autotransformer

Primary purpose: vary the voltage to the primary side of the step-up transformer. A single coil transformer that serves 3 functions: it provides the means for kVp selection, provides compensation for fluctuations in the line voltage, and it supplies power to other parts of the xray circuit.

Exposure switch

Closes the circuit, allowing electric current to flow through the primary side of the step-up transformer.

Exposure timer

A device that terminates the exposure and is set by the operator on the control console.

Filament Circuit

Primary purpose of the filament Circuit is to supply a low current to heat the x-ray tube filament for thermionic emission of electrons.

mA selector

Controls the amperage in the filament circuit.

High voltage circuit

Primary purpose is to supply the xray tube with voltage high enough to create x-rays. Includes the xray tube and the rectifier beginning and ending with the step-up transformer. Current flows in this circuit only during an exposure.

Rectification

Changes alternating Current into direct current. Rectification prepares the current for x-ray production by ensuring that it flows in the right direction (from the filament to the target).

What are 3 ways in which current is rectified?

Self-rectification. Half-wave rectification. Full-wave rectification. Self rectification is no longer used because it is inefficient.

Diode

An electronic device that permits current flow in one direction only.

Half wave rectification

Uses two diodes eliminating the negative half of the electric cycle and leaving a gap. Xrays pulse on off on off

Full-wave rectification

Uses 4 diodes to redirect the current during the negative half of the electric cycle so the current flows in the same direction during both the positive and negative halves of the cycle,utilizing the entire electrical cycle for x-ray production. The main advantage of Full-wave rectification is that exposure time can be cut in half due to doubling the x-ray output compared to Half-wave rectification.

Single phase generators

Powered by a single source of AC current, resulting in a pulsating xray beam. Lowest power, least expensive x-ray machines. Single phase full wave rectification 120 pulse of electricity per second creates 120 pulse of xray per second

Three phase generators

Powered by 3 separate sources of AC current at the same time. When full wave rectified, the wave form looks like a ripple. 360 pulses per second. A major advantage of three-phase current is that it is more efficient producing 40% more xrays than single phase and reducing exposure times by 40%!

High frequency generators

The most common generators used today. They use a single AC source but the 60 Hz Full-wave rectified circuit is converted to a significantly higher frequency of about 6,000 Hz for general use or up to 100,000 Hz. They are more efficient and smaller in size reducing the cost of the machines.

Synchronous timers

Controlled by a small electric rotating motor at 60 revolutions per second. Time settings stated in fractions from 1/60 of a second.

Electronic timers

Sophisticated timer designed for use with 3 phase and high frequency generators capable of ultra short exposure times. They are more accurate.

Bucky control

Computer needs to know if bucky/grid is being used because it affects the exposure technique.

Manual exposure control

The operator manually sets the mA, kVp, and exposure time.

Automatic exposure control (AEC)

Terminates the expos it e time when a certain quantity of radiation has been detected by the IR. (mA and kVp are set by operator, exposure time automatically determined).

Phototimer and ionization chamber

Types of AEC

Back up time

Set higher than the estimated exposure timer as a backup in case the AEC fails for any reason.

Public Law 90-602

Generators must terminate exposure at 600 mAs for exposures above 50 kVp

Anatomically programmed radiography control (APR)

One of the most widely used exposure control techniques. You select the body area and the projection, and the other controls are selected by the computer.

Exposure control

Exposure switch must be positioned so that exposures cannot be made from outside the control booth. Two switches the rotor switch preps the rotating anode to spin and begins heating the filament. A signal will sound when it is ready and The exposure switch is then pressed.

Tube rating chart

If the exposure is above the mA line, the exposure exceeds tube capacity. The maximum heat capacity (heat units HU) is found by multiplying the kVpxmAs

Extending X-ray Tube Life

1. Warm up the anode according to the manufacturer's instructions.


2. Do not hold down the rotor switch for long periods of time.


3. Use low mA setting whenever possible.


4. Use the low-speed rotor whenever possible.


5. Do not make repeated exposures near the tube limits.


6. Do not use the tube when you can hear the rotor bearings (call a service engineeer).

Chapter 6 Summary

Summary