Xray Machine

Question 1

Overall principle of an xray machine

Answer 1

Accelerate electrons towards a piece of metal in a vacuum

Electrons interact w metal = xrays

Question 2

Function of glass tube in xray

Answer 2

creates vacuum for xray production

Question 3

Cathode function in xray

Answer 3

generates electrons

Question 4

anode function in xray

Answer 4

receives electrons and produces xrays

Question 5

What is the power supply in xray machines

Answer 5

high/low voltage circuits

Question 6

What are the components of a cathode

Answer 6

tungsten filament

focusing cup

Question 7

Tungsten filament in cathode

Answer 7

connected to a low voltage circuit

Question 8

Focusing cup in cathode

Answer 8

Negatively charged concave reflector

Question 9

Functiono of tungsten filament in cathode

Answer 9

When heated - emits electron cloud

Source of electrons that will be sent to anode to produce xrays

Question 10

Focusing cup in cathode function

Answer 10

Focusing cup electrostatically focuses the electron beam to the focal spot on anode
Electrons are repelled by the negatively charged focusing cup

Question 11

Descibe anode in xray

Answer 11

Copper block
Tungsten focal spot = target
+ charged to attract electrons
Path from cathode to anode is the high voltage curcuit

Question 12

What is the path from cathode to anode

Answer 12

high voltage circuit

Question 13

Focal spot (anode)

Answer 13

Site of xray production
Small rectangular area in anode
+ charged
Attracts electron cloud from cathode

Question 14

What 4 things should an ideal target (tungsten focal spot) have?

Answer 14

1. High Z#
2. High melting point
3. High thermal conductivity
4. Low vapor pressure at high temperature

Question 15

Why is it advantageous to have a high Z# for an ideal target?

Answer 15

Promotes maximum conversion of electron energy to xray photon energy

Question 16

Tungsten satisfies all requirements of an ideal target except what? How is this addressed?

Answer 16

Tungsten is a poor heat conductor

Solved by copper block and cooling oil to assist in heat dissipation

Question 17

Why do we want a low vapor pressure at high temperature for an ideal xray target?

Answer 17

Can otherwise damage target

Question 18

Focal spot should be _____ to assist in heat dissipation

Answer 18

large

Question 19

Focal spot should be _____ to increase image sharpness

Answer 19

small

Question 20

What are the opposing issues regarding focal spot size?

Answer 20

Should be large to assist in heat dissipation
Should be small to increase image sharpness

SOLVED BY LINE FOCUS PRINCIPLE

Question 21

Line focus principle

Answer 21

Makes possible a large actual focal spot & “projected”/”effective” focal spot of small size
Placement of target at an angle
= better heat dissipation
= increased image sharpness

Question 22

Copper block of anode

Answer 22

Good thermal conductor
Removes heat from tungsten target
Reduces risk of melting of target

Question 23

Function of glass tube of xray machine

Answer 23

Vacuum tube

No gas to prevent collision of the electrons to the gas molecules which impair xray production

Question 24

Function of cooling oil in xray

Answer 24

dissipates heat

Question 25

Rotating anode

Answer 25

Tungsten target is a beveled disk that rotates when tube is operating Electrons strike successive small areas of target = acheives obj of having small focal spot Actual focal spot is the circumference of the disk = helps distribute heat over large areas DISADVANTAGE - $$$$$

Question 26

Most dental systems use what kind of anodes?

Answer 26

stationary | bc rotating is $$$$$

Question 27

Purpose of low voltage circuit

Answer 27

Heat filament to cause thermionic emission

Question 28

How does a low voltage circuit work

Answer 28

Variable resistor increases/decreases resistance in circuit and changes current in filament Controls # electrons produced at filament mA control of xray machine actually adjusts this current Increase mA = increase electrons sent to anode

Question 29

If you increase mA in a low voltage circuit, what is the result?

Answer 29

Increase electrons sent to anode

Question 30

Milliampere current (mA)

Answer 30

Electrons from heated filament to the target WITHIN the xray tube Controls # xray photons Increase mA for a given exposure time = increase receptor exposure

Question 31

Increase mA for a given exposure time, _____ receptor exposure

Answer 31

increase

Question 32

High voltage circuit

Answer 32

Creates high potential difference between cathode/filament and anode/target Gives high velocity to filament electrons Voltage varies between 50 - 100 kVp

Question 33

An increase in kVp results in a _____ in mean energy of polychomatic xray beam - ultimately influences ______

Answer 33

increase | image quality

Question 34

as kVP increases, velocity of electrons (energy) ____

Answer 34

increases

Question 35

As kVp increases, photon energy (quality, penetrability) ______

Answer 35

increases

Question 36

An increase in photon energy (quality, penetrabiity) results in...

Answer 36

Fewer photons interact in pt | Radiographic contrast decreases

Question 37

As kVp increases, # photons _____

Answer 37

increases

Question 38

An increase in the # of photons results in...

Answer 38

Increase in receptor exposure

Question 39

Autotransformer

Answer 39

Changes kVp Located in control panel Compensates for variations in incoming line voltage = VOLTAGE STABILIZER

Question 40

Receptor exposure

Answer 40

Measure of total xray beam exposure to the sensor Must be enough to properly penetrate/image pt with acceptable noise levels Must not be too much to over expose the pt (unnecessary dose) or receptor (poor image quality)

Question 41

The receptor exposure must be enough to...

Answer 41

properly penetrate/image pt with acceptable noise levels

Question 42

the receptor exposure must not be too much to...

Answer 42

over expose pt - unnecessary dose | over expose receptor - poor image quality

Question 43

How is receptor exposure controlled

Answer 43

kVp - quality, quantity mA - quantity Exposure time Collimation, filtration, SID

Question 44

kVP controls...

Answer 44

receptor exposure beam quality - image contrast beam quantity - exposure

Question 45

For intraoral dental imaging, kVp is often set at...

Answer 45

60-70 kVp

Question 46

mA controls...

Answer 46

receptor exposure beam quantity - exposure Reported on xray tube control head

Question 47

How is exposure time measured

Answer 47

Seconds Measured in pulses on older xray tubes Related to 60 cycle alternating current (AC) incoming power supply 1 pulse - 1/60 sec Newer machines report exposure time directly (ms)