Chapter 6 Xray Circuit And Tube Heat Management

Front 1

Control Console

Back 1

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.

Front 2

Autotransformer

Back 2

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.

Front 3

Exposure switch

Back 3

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

Front 4

Exposure timer

Back 4

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

Front 5

Filament Circuit

Back 5

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

Front 6

mA selector

Back 6

Controls the amperage in the filament circuit.

Front 7

High voltage circuit

Back 7

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.

Front 8

Rectification

Back 8

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).

Front 9

What are 3 ways in which current is rectified?

Back 9

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

Front 10

Diode

Back 10

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

Front 11

Half wave rectification

Back 11

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

Front 12

Full-wave rectification

Back 12

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.

Front 13

Single phase generators

Back 13

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

Front 14

Three phase generators

Back 14

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%!

Front 15

High frequency generators

Back 15

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.

Front 16

Synchronous timers

Back 16

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

Front 17

Electronic timers

Back 17

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

Front 18

Bucky control

Back 18

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

Front 19

Manual exposure control

Back 19

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

Front 20

Automatic exposure control (AEC)

Back 20

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).

Front 21

Phototimer and ionization chamber

Back 21

Types of AEC

Front 22

Back up time

Back 22

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

Front 23

Public Law 90-602

Back 23

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

Front 24

Anatomically programmed radiography control (APR)

Back 24

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.

Front 25

Exposure control

Back 25

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.

Front 26

Tube rating chart

Back 26

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

Front 27

Extending X-ray Tube Life

Back 27

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).

Front 28

Chapter 6 Summary

Back 28

Summary