Diagnostic Imaging - Generation of X-rays

Question 1

What is radiation?

Answer 1

Electromagnetic energy travels through space in electrical & magnetic waves

Question 2

What are properties of X-rays?

Answer 2

• have very short wavelength & are high energy
• cause ionization
• travel in straight lines
• can cause biological damage to tissue

Question 3

How does an x-ray tube work?

Answer 3

photons are produced by applying a charge across the x-ray tube head which contains a negative electrode, the cathode & a positive electrode (the anode)

Question 4

X-ray tubes are lead-lined on the…

Answer 4

outside

Question 5

Tungsten in the x-ray tube is used for…

Answer 5

both the filament of the cathode and the focal spot of the anode because it has a high melting point

Question 6

Applying a small current to the x-ray tube gives enough energy to…

Answer 6

free some elctrons from the cathode-thermionic emission

Question 7

the focusing cup of an x-ray tube is kept more…

Answer 7

negatively charged to keep the freed electrons together in an electron cloud

Question 8

Applying a potential difference between the cathode and the anode occurs when

Answer 8

kilovoltage attracts the free electrons to the anode where they interact with the focal spot to produce xrays

Question 9

increase in KV causes

Answer 9

an increase in the number & penetrating power of x-rays

Question 10

increase mA causes

Answer 10

increase in the number of x-ray photons produced but not the penetrating power

Question 11

overheating in the x-ray tube is a major problem of

Answer 11

the filament and the focal spot

Question 12

How can over-heating be prevented in the x-ray tube?

Answer 12

• use a broad focus for the cathode
• use effective focal spot as larger than actual focal spot to spread out the heat generated
• use tungsten target in the anode
• use copper and colling fins with oil
• use rotating anode
• minimise exposure settings and repeat rads

Question 13

Broad focus should be used for

Answer 13

higher exposure

Question 14

fine focus should be used for

Answer 14

smaller exposures for better detail

Question 15

Rotating anodes rotates…

Answer 15

so that electrons hit a different part of the focal spot

Question 16

Aside from the x-ray tube, an x-ray machine is composed of…

Answer 16

light beam diaphragm
control panel

Question 17

The control panel controls…

Answer 17

KV
mAs-mA and time settings
Exposure button

Question 18

exposure factors are expressed in terms of

Answer 18

KV & mAs

Question 19

With the exposure switch, what does the first position on the 2-step switch do?

Answer 19

• prepares the x-ray tube
• heats the filament
• rotates the anode

Question 20

With the exposure switch, what does the second position on the 2-step switch do?

Answer 20

• produces the x-rays (exposure)

Question 21

What are the types of x-ray machines available?

Answer 21

portable, high-frequency portable, mobile, fixed

Question 22

x-ray photons can penetrate tissue and…

Answer 22

some photons will pass through unchanged
some will be attenuated by the tissue

Question 23

What is attenuation?

Answer 23

the reduction of the intensity of the primary x-ray beam as it passes through matter

Question 24

Attenuation =

Answer 24

photoelectric absorption + Compton scatter

Question 25

What is secondary radiation?

Answer 25

radiation produced by the matter that is irradiated

Question 26

The photoelectric effect means absorption is greater at

Answer 26

exposures <70 KV

Question 27

The Compton effect means scatter increases as KV

Answer 27

increases especially >70 KV

Question 28

Absorption is dependent on…

Answer 28

the atomic number
physical density of the tissue
the thickness of tissue

Question 29

What are the 5 opacities from blackest to whitest?

Answer 29

Air
Fat
Soft tissue/fluid
Bone
Metal/contrast agents

Question 30

What are factors that you control that affect image sharpness?

Answer 30

source image receptor distance (SID)
object image receptor distance (OID)
motion

Question 31

What factors are outside your control that affect image sharpness?

Answer 31

• focal spot size
• digital machine software

Question 32

What is the inverse square law?

Answer 32

affects the intensity of the beam & exposure factors required

Question 33

How does the focal spot size differ?

Answer 33

• small focal spot should be used to produce a fine beam of x-rays
• large focal spot increases the penumbra effect

Question 34

Movement during exposure leads to

Answer 34

blurring of the image

Question 35

It is important to prevent movement due to…

Answer 35

the patient’s breathing

Question 36

to prevent the patient’s breathing from causing movement, exposure should be taken…

Answer 36

after expiration, except for thoracic radiographs, and should be short in duration

Question 37

What are ways to prevent motion?

Answer 37

• sedation
• shortest time for exposure settings
  -shorter SID (will lose sharpness)
• stabilisation of equipment

Question 38

What is SID?

Answer 38

source to image detector distance

Question 39

What is OID?

Answer 39

object to image detector distance

Question 40

to minimize distortion, one should center the beam over…

Answer 40

the area of interest

Question 41

Distortion increases towards…

Answer 41

the periphery of the IR due to obliquity of the x-ray beam