LECTURE 4: X-RAY TUBES 2

Question 1

Stationary Anode: What is the anode angle?

Answer 1

15-20deg

Question 2

Stationary Anode: What materials are used?

Answer 2

2mm thick tungsten, embedded in a mass of copper

Question 3

Stationary Anode: Why is tungsten embedded in copper?

Answer 3

Copper is a good thermal conductor - meaning it is good at distributing heat

Question 4

What is a thermal conductor

Answer 4

Distributes/dissipates heat well

Question 5

Anode Angle: If it is smaller, then is the focal spot smaller or larger?

Answer 5

Smaller

Question 6

Rotating Anode: rpm? Limitation? Actual rpm?

Answer 6

3000 rpm. Never really reach this speed because of technical factors; friction etc… 2500 rpm

Question 7

What are the material components of a rotating anode?

Answer 7

Tungsten anode, molybdenum stick connecting to copper cylinder

Question 8

What is the function of the molybdenum stick?

Answer 8

Transfers heat from anode to copper tube

Question 9

What is the difference in focal spot size in a stationary vs rotating anode?

Answer 9

No difference - still the same, only total SA is changed

Question 10

How does the anode rotate in an enclosure with no mechanical connection to the outside?

Answer 10

Electromagnetic induction motor is used to turn the anode: Stator and Rotor

Question 11

Describe how electromagnetic induction motor works in terms of x-ray tube

Answer 11

Stator surrounds a rotor. Stator has coil windings that produce a magnetic field, which induces a current that provides power for the anode to rotate.

Question 12

Why is tungsten chosen as a target material for x-ray tubes?

Answer 12

• High melting point
• High atomic number (means more e- e- interaction probable)
• Good at heat absorption and dissipation

Question 13

Why is the tungsten target embedded in copper?

Answer 13

Copper is a better heat conductor than tungsten so it can increase the thermal capacity of anode and dissipates heat fast

Question 14

Does the whole tungsten target get bombarded?

Answer 14

No- only a small area but we need to have a larger area because temperature of the area will rise and copper has a lower melting point so we don’t want direct interaction with copper

Question 15

Is a graphite or molybdenum anode disc better? Why would you use graphite?

Answer 15

Molybdenum as graphite will become hotter (does not conduct heat as well we moly); but graphite may be used to reduce disc inertia

Question 16

What is a “focal spot”?

Answer 16

The area of the TARGET from which the x-rays are emitted (angle from the anode)

Question 17

If the focal spot size is smaller, how does this affect image quality?

Answer 17

Spatial resolution is better!

Question 18

How is the size of the focal spot size controlled?

Answer 18

Depends on the angle of the anode

Question 19

What is “Line-focus Principle”?

Answer 19

Focal spot - angle with anode

Question 20

What is “apparent (effective) focal spot”?

Answer 20

The actual area that is projected onto the patient/image receptor

Question 21

What is the difference between actual and apparent focal spot size?

Answer 21

Actual is what actually hits the anode, and apparent is what apparently hits the patient

Question 22

How do you calculate apparent focal spot?

Answer 22

Actual focal spot x sine (angle of inclination)

Question 23

As the angle of inclination decreases (focal spot size), does the effective focal spot size increase or decrease?

Answer 23

decrease!

Question 24

Can an anode have 2 angles?

Answer 24

yes of course! this means there can be 2 focal spot sizes

Question 25

What is NEMA and what does it stand for?

Answer 25

National Electrical Manufacturers Association- regulates the variances in focal spot size

Question 26

What are the measurements of a slit camera?

Answer 26

Slit is 0.01mm wide. Placed in middle of a metal 5cm long and 1.5mm thick

Question 27

Where is the slit camera positioned to take an image?

Answer 27

About 10cm from the focal spot; and is between x-ray tube and film (like an IR)

Question 28

How is the slit camera used to determine focal spot size?

Answer 28

1. Slit along axis of x-ray tube for length, then

2. Rotate 90deg for width

Question 29

What are the standard measurements of focal spot size? and what does it increase in (increments)?

Answer 29

Starts at 0.3mm, up to 2.0mm, in 0.1mm increments

Question 30

How does a pinhole camera work?

Answer 30

Same setup as slit - except the size of focal spot will show up on the image; must apply come correction factors if there is geometric magnification present

Question 31

What is the Resolution Star TEst object?

Answer 31

Measures the effective blur size of the focal spot - tests spatial resolution

Question 32

What is the Anode Heel Effect?

Answer 32

Reduced x-ray intensity on the ANODE SIDE of the useful beam because of absorption in the ‘heel’ of the target (anode)

Question 33

Anode Heel Effect: on which side of the x-ray tube is there a reduced x-ray intensity?

Answer 33

ANODE SIDE

Question 34

What is the special term we call the x-ray beam due to the anode heel effect?

Answer 34

Non-homogenous beam

Question 35

How do we counteract the Anode Heel Effect? Explain

Answer 35

Increase FFD - so that the less intense x-ray beams do not hit the IR - leaving the ones that have same intensity to hit it

Question 36

What is the typical focal spot size for radiography vs mammography?

Answer 36

radio is 0.6-1.2mm

mammo is 0.1-0.3mm

Question 37

Know how to calculate heat unit (HU)

Answer 37

!