Diagnostic Imaging - Generation of X-rays Flashcards
What is radiation?
Electromagnetic energy travels through space in electrical & magnetic waves
What are properties of X-rays?
- have very short wavelength & are high energy
- cause ionization
- travel in straight lines
- can cause biological damage to tissue
How does an x-ray tube work?
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)
X-ray tubes are lead-lined on the…
outside
Tungsten in the x-ray tube is used for…
both the filament of the cathode and the focal spot of the anode because it has a high melting point
Applying a small current to the x-ray tube gives enough energy to…
free some elctrons from the cathode-thermionic emission
the focusing cup of an x-ray tube is kept more…
negatively charged to keep the freed electrons together in an electron cloud
Applying a potential difference between the cathode and the anode occurs when
kilovoltage attracts the free electrons to the anode where they interact with the focal spot to produce xrays
increase in KV causes
an increase in the number & penetrating power of x-rays
increase mA causes
increase in the number of x-ray photons produced but not the penetrating power
overheating in the x-ray tube is a major problem of
the filament and the focal spot
How can over-heating be prevented in the x-ray tube?
- 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
Broad focus should be used for
higher exposure
fine focus should be used for
smaller exposures for better detail
Rotating anodes rotates…
so that electrons hit a different part of the focal spot
Aside from the x-ray tube, an x-ray machine is composed of…
light beam diaphragm
control panel
The control panel controls…
KV
mAs-mA and time settings
Exposure button
exposure factors are expressed in terms of
KV & mAs
With the exposure switch, what does the first position on the 2-step switch do?
- prepares the x-ray tube
- heats the filament
- rotates the anode
With the exposure switch, what does the second position on the 2-step switch do?
- produces the x-rays (exposure)
What are the types of x-ray machines available?
portable, high-frequency portable, mobile, fixed
x-ray photons can penetrate tissue and…
some photons will pass through unchanged
some will be attenuated by the tissue
What is attenuation?
the reduction of the intensity of the primary x-ray beam as it passes through matter
Attenuation =
photoelectric absorption + Compton scatter
What is secondary radiation?
radiation produced by the matter that is irradiated
The photoelectric effect means absorption is greater at
exposures <70 KV
The Compton effect means scatter increases as KV
increases especially >70 KV
Absorption is dependent on…
the atomic number
physical density of the tissue
the thickness of tissue
What are the 5 opacities from blackest to whitest?
Air
Fat
Soft tissue/fluid
Bone
Metal/contrast agents
What are factors that you control that affect image sharpness?
source image receptor distance (SID)
object image receptor distance (OID)
motion
What factors are outside your control that affect image sharpness?
- focal spot size
- digital machine software
What is the inverse square law?
affects the intensity of the beam & exposure factors required
How does the focal spot size differ?
- small focal spot should be used to produce a fine beam of x-rays
- large focal spot increases the penumbra effect
Movement during exposure leads to
blurring of the image
It is important to prevent movement due to…
the patient’s breathing
to prevent the patient’s breathing from causing movement, exposure should be taken…
after expiration, except for thoracic radiographs, and should be short in duration
What are ways to prevent motion?
- sedation
- shortest time for exposure settings
-shorter SID (will lose sharpness) - stabilisation of equipment
What is SID?
source to image detector distance
What is OID?
object to image detector distance
to minimize distortion, one should center the beam over…
the area of interest
Distortion increases towards…
the periphery of the IR due to obliquity of the x-ray beam
