X-Ray Tubes (2)

Question 1

How are x-rays developed?

Answer 1

Generated when an electron is accelerated and decelerated rapidly - interacts with dense atomic particles (tungsten target)

Question 2

Prior to 1913 most x-ray tubes were referred to as___.

Answer 2

Gas tubes due to the fact that they generally contained a high amount of residual gases.

Question 3

prior 1913 x-ray tubes were

Answer 3

• unreliable (fluctuations in gas temp and pressure)
• max energy 130kV
• taget material = platinum

Question 4

What are the different types of x-ray tubes?

Answer 4

• bipolar
• unipolar
• rotating anode tube

Question 5

Bipolar x-ray tubes

Answer 5

• most common
• most portable
• built-in high voltage transformer
• 150-450kV max

different housing can emit 4000kV max

Question 6

Unipolar x-ray tubes

Answer 6

only cathode has a potential difference, anode is at ground potential

Question 7

Rotating Anode Tubes

Answer 7

Rotating target self-cools by constantly rotating

exposures can be performed in “fractions” of seconds

Question 8

list the X-Ray tube controls

Answer 8

• Milliamperage (mA)
• Kilovoltage (kV)
• timer

Question 9

What does the mA control do?

Answer 9

Governs the amount of electricity going to the negative cathode.

1amp = 6.3x10¹⁸ electrons through a conductor per second

most tubes are limited to 10mA

Question 10

What does the kV control do?

Answer 10

Governs the energy potential between the negative cathode and positive anode.

voltage that moves electrons (current) through the circuit, higher voltage = faster moving electrons.

amount of voltage required to move 1amp of current through the circuit per second.

Question 11

What does the Timer control do?

Answer 11

Governs the duration for the x-ray shot.

longer the tube is energized the more x-rays will penetrate the part and more photons will reach the film.

Question 12

Two types of X-ray Generation

Answer 12

AC (alternating current)

Constant Potential Systems

Question 13

What are AC x-rays?

Answer 13

Alternating current is the power supply for the x-ray tube, but the current will only produce x-rays during 1/2 of the 1/60th second AC cycle

• valiance in energy levels
• needs filters

Question 14

What are Constant Potential System x-rays?

Answer 14

tube rectifiers convert AC supply into a rectified AC that resemble a DC current
- uniform stream of mono-energetic radiation.

Question 15

What are fashion tubes?

Answer 15

X-Ray Tubes that make use of AC for the production of x-rays in short bursts or pulses dependant on the frequency of AC applied.

Question 16

What are Rectifiers?

Answer 16

Used to convert the AC power supply to emulate uniformity or DC.

Question 17

What are Transformers?

Answer 17

X-ray systems generally require 20-450kV +. most voltage supplies do not support this, transformers are used to “step up” voltage to something that the tube can use.

Amperage is normally to high so “step down” transformers are used

Question 18

Types of Transformers and what they do

Answer 18

Step up = convert to higher

Step down = convert to lower

auto = automatic adjustment for what is needed

Question 19

What are Rheostats?

Answer 19

Used to control the filament heating current for an x-ray tube

Question 20

Three different ways that Rectification can be done

Answer 20

• valves and diodes
• special rectification circuits
• semi-conductor rectifiers

Question 21

What is a Solid-state rectification system?

Answer 21

commonly used rectification system

• simplified circuits allow for a high voltage filtering and capacitor system
• use of thyristor-based circuits
• high-frequency square-waves (10-16kHz)

Question 22

Describe the Self-rectification x-ray circuit

Answer 22

The tube acts as a valve, only half the sine wave is used, unused portion dissipates as heat.
- anode may overheat forming electrons

Question 23

Describe the Half-wave rectification x-ray circuit

Answer 23

Adding a single valve (diode) into the circuit eliminates the reverse half of the sine wave, creating a pulsating x-ray.

Question 24

Describe a Full wave rectification x-ray circuit

Answer 24

Both positive and negative sine waves are rectified, leaving only the positive portion with a frequency that is 2x the half wave rectified current at the anode.

Question 25

three parts to producing an x-ray

Answer 25

• source of electrons (heated tungsten filament)
• way to accelerate electron movement
• high-density target to be impacted by electrons

Question 26

How much electrical energy actually produces the x-ray?

Answer 26

2% remaining 98% is the heat

Question 27

Name the two interactions that occur to produce an x-ray

Answer 27

• bremmstrauhlung

- characteristic

Question 28

Bremmstrauhlung

Answer 28

Braking

Electrons slow as they pass through the atoms composing the anode target.

x-ray energy released is dependent on original velocity - new slowed velocity.

Question 29

Characteristic

Answer 29

Electron knocks one of the anode target atoms electrons out, orbital electrons domino to fill inner rings.

x-ray energy released is = change in energy level of transferred shell electrons.

Question 30

What is an X-ray tube focal spot?

Answer 30

the area of the target that is impacted by the electrons (actual focal spot).

Question 31

What is so important about the x-ray tube focal spot?

Answer 31

the size of the area affects the amount of power that can be applied to the x-ray tube and the quality of the resultant image on the film.

must be large enough to transmit and dissipate heat, yet small enough to maintain image sharpness.

Question 32

actual focal spot vs. effective focal spot?

Answer 32

actual = area of anode target impacted with electrons

effective = portion of x-ray beam projected from the tube window (smaller than actual focal spot)

Question 33

The focal spot in an x-ray tube is supposed to be as ____ as possible to create a _____ image.

Answer 33

small as possible to create a sharper image.

Question 34

What is the anode hood (window)?

Answer 34

hood design over the anode that:

• filters radiation outside of the “cone” area
• provides an electrical shield with the glass envelope

Question 35

The anode hood and window act as a filter that creates a point that is referred to as_______

Answer 35

inherent filtration; the filtration of energy due to absorption by the tube envelope, insulators, and mechanical components

Question 36

Parts of the x-ray tube

Answer 36

• Glass envelope vacuum
• anode hood
  ]- anode +
• cathode -
• target
• window

Question 37

What is the x-ray window made of? and what is its purpose?

Answer 37

Beryllium

reduces the longer wavelength radiation while not affecting the short/hard radiation

Question 38

What role does the Glass envelope play on an x-ray tube?

Answer 38

traditionally pyrex glass, now metal-ceramic tubes made from a steel cylinder fused to alumina-ceramic insulating caps.

• hermetically sealed under an extreme internal vacuum.
• waves pass through the glass in areas that are thinner or have a beryllium window.

Question 39

How can x-ray tubes be cooled?

Answer 39

• suitable oil that is circulated through the anode on the back surface of the target
• coolant around target + anode
• constant water or air flow
• utilizing “duty cycle”

Question 40

What is an x-ray tube duty cycle?

Answer 40

tube is idle for a duration of time relative to the time it was activated for between exposures to avoid overheating.

50% duty cycle = 1min exposure, 1min cool down