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PCP > Radiography > Flashcards

Radiography Flashcards

1
Q

Label the X-ray machine

A

A: + Spinning anode
B: Tungsten target
C: Glass envelope
D: Cathode
E: Focusing cup
F: - Hot cathode filament
G: Electrons
H: Anode heel effect
I: X-ray photon beam
J: Window
K: Anode
L: Bearings
M: Rotor
N: Stator

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2
Q

What is the main purpose of the cathode

A

Acceleration of the Electron

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3
Q

The cathode

a) How are electrons produced
b) How are electrons accelerated
c) How does filament size affect the X-ray

A

a) Current (mA) is applied through the wire, heating the wire. Electrons emmitted through thermionic emission, producing an electron cloud at the cathode
b) A large potential difference (kV) is applied across the X-ray tube to accelerate electrons produced at the cathode
c) Fine filament size: small electron beam, small focal spot, fine detail, dense heat concentration
Broad filament size: large electron beam, large focal spot, reduced detail, higher safe exposures

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4
Q

What is the main purpose of the anode

A

Deceleration of the electron

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5
Q

The anode

a) Properties od the anode (tungsten) target
b) How much kinetic energy of electrons is converted to x-rays. Where does this occur
c) Best angle for the focal spot
d) Large vs small focal spots

A

a) Efficiency due to high atomic number. High melting point and specific heat capacity. Thermoconductive
b) Less than 1%. Occurs at the focal spot
c) 6-20 degrees
d) Large actual focal spots: reduce overheating
Small effective focal spots: improve image detail, limit size of field of view

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6
Q

The anode

a) Why does overheating occur
b) Problems associated with overheating (3)
c) Tube ratings charts

A

a) Electon collisions at the anode causes the target to vaporise and condense on the glass envelope
b) 1. Hot anode may then emit electrons, reversing the tube current 2. Over time the anode will become pitted and cause reduced and uneven x-ray output 3. Filament may burn out from overheating, causing glass envelope cracks and implosion
c) Tube ratings are defined input parameters (kV, mA, exposure time) that can be safely used without causing damage (including overheating) to the x-ray tube itself

c) Tube ratings chart
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7
Q

The anode

Stationary anode vs rotating anode

A

Stationary: Low power portable machines. Copper stem dissipates heat. Oil provides heat sink and electrical insulation.
Rotating: Continually changing the actual focal spot. Heat loss by radiation into oil bath (NOT by conduction)

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8
Q

Types of radiation

Continuous radiation

A

Produced by the electrons colliding with the nucleus. Bremsstrahlung (= braking) effect. Energy released at X-ray. Continuous spectrom of radiation produced

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9
Q

Characteristic Radiation

A

Produced by electron colliding with a shell electron. Shell electron is ejected, outer shell electorn drops into inner shell, emitting energy as X-ray. X-rays produced at characteristic peaks, dependent on atomic number and shell energies

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10
Q

Soft radiation

A

Does not contribute to image
Low energy, long wavelength X-rays which are absorbed in patient - health hazard
Legal requirement to filter them out (inherent tube filtration - 1mm aluminium. Extra filtration 2-2.5mm aluminium)

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11
Q

Changing parameters of the X-ray Beam

a) Effect of increasing kV
b) Effect of increasing mA & time

A

a) ↑ number of x-rays, ↑ electron velocity, ↑ x-ray energy and penetration, ↓ x-ray wavelength
b)↑ number of x-rays, ↑ filament heating, ↑ x-ray exposure

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12
Q

a) Define intensity of the X-ray beam and what it’s affected by
b) Define quality of the X-ray beam and what it’s affected by

A

a) Amount of energy in the beam ∝ number of photons ∝ photon energy
Affected by: kV, mA, filtration, rectification, target atomic number
b) Penetration power of the x-ray beam ∝ x-ray energy
Affected by: kV, filtration, rectification

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13
Q

Transformers

What is the puropse of transformers in an X-ray machine

A

Alter voltage from mains supply into the x-ray machine

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14
Q

Transformers

Describe:
a) Autotransformers
b) Step-down transformers (aka low tension)
c) Step-up transformers (aka high tension)

A

a) Function: provide steady voltage from the mains (240V) to the machine. Ensures even x-ray production
b) Function: heat the filament. Input = 240V, output = 10V and 3-5A
c) Function: supply a large potential difference for acceleration of electrons. Input = 240V, output = 40 to 100 kVs

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15
Q

Rectifiers

What is the purpose of rectifiers in an X-ray machine

A

Mains AC current is required for transformers to function, but X-ray production requires a DC current → rectification changes current from AC to DC (a one-way current)

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16
Q

Rectifiers

a) Self-rectification
b) Half-wave rectification
c) Full-wave rectification

A

a) Simplest form for low power, cheaper machines. X-ray tube acts as a diode. Current (electrons) flow 50% of the time. Requires double exposure time. Reverse current may occur if anode overheats
b) Uses 2 diodes. Current (electrons) flow 50% of the time. More efficient and safer than self-rectification as no reverse current during overheating
c) Uses 4 diodes. Current (electrons) flow 100% of the time, but still a pulsatile current, so short exposure times are inaccurate

17
Q

Rectifiers

a) Three-phase rectification
b) Constant potential rectification

A

a) Uses 6 diodes. Requires a three phase supply, current 120 degrees out of phase. Current (electrons) flow 100% of the time, pulsatile flow but small ripples - almost constant x-ray production
b) Uses 4 diodes (rectify) and 2 condensers (charge and discharge). Constant tube current. Capacitor discharge units use this form of rectification

18
Q

Controlling mA and kV

a) What does mA control
b) What does kV control

A

a) Intensity of the x-ray beam. mA control alters current to the filament, which alters filament heating and tube current
b) Quality of x-ray beam. Higher kV = higher electron speed, higher electron energy, higher quality

19
Q

Types of x-ray machine

a)Portable machines
b) Mobile machines
c) Fixed machines
d) Fluoroscopy

A

a) Small transformer in tube head. Low maintenance. Widely used, esp equine and small animal. mA = 15-35, long exposure times, so potential for movement blur
b) Usually on wheels, but not portable. Large transformer in separate box. Not manoeuvrable enough for being near conscious horses
c) Dedicated room. Large transformer in separate area. mA up to 1000mA, 150kV. Used for all types of animal, but expensive and limited manoeuvrability
d) Moving, real-time image. X-ray beam directed at a fluorescent screen. Real time x-ray at high frame rate and low dose per image

20
Q

Fluoroscopy

Uses of image intensification

A

Contrast studies (swallowing)
Location of screws in orthopaedics
Interventional radiology: balloon valvuloplasty, intra hepatic shunt ligation, pacemaker implantation, ureteric stent implantation

21
Q

a) Inverse square law
b) Photoelectric effect

A

a) Intensity of the X-ray beam is inversely proportional to the square of the distance from the source
intensity ∝ 1/distance^2
b) The predominant interaction up to 60keV

b)
22
Q

a) What is the Compton effect
b) Consequences of scatter radiation
c) How to recude scatter production
d) How to reduce scatter reaching the film

A

a) The predominant interaction in the diagnostic energy range
b) 1. Decrease in contrast (film fogging) 2. Radiation hazard 3. Increased radiation dose
c) 1. Collimation of the primary beam (reduce size of primary beam to only cover area of interest) 2. Lower kV (allow photoelectric effect to predominant, not Compton, although not always feasable, depends on area of interest and power of machine) 3. Compress area under investigation (less scatter, but more image distortion)
d) 1. Air gap technique (move patient away from detector so less patient generated scatter) 2. Lead backed cassettes and intensifying screens (inbuilt scatter reduction) 3. Grids (scatter absorbed by lead as lower energy and different direction, primary beam passes through radiolucent gaps)

a)
23
Q

Types of grids (4) + pros and cons

A
24
Q

Grid parameters

a) Grid lines
b) Grid ratio
c) Grid factor
d) Grid efficiency

A

a) 24-60 lines per cm. More lines per cm = more expensive
b) Ratio of height of slats to width of interspace. Usually 6-8-10-12-14
c) Grids absorb x-ray photons, so need to compensate by increasing mA, usually by factor of 2-4 times (depends on other grid parameters). If too high, result can be counterproductive
d) Amount of primary beam allowed through, relative to scater absorbance ∝ grid ratio, grid lines, thickness of strip

25
Q

X-ray image formation

a) Describe a conventional film
b) Label diagram of film
c) How it works
d) Advantages (1)
e) Disadvantages (3)

b)
A

a) Image formed by exposing film inside a cassette to X-Rays. Film is developed using chemicals, revealing the image
b) Image
c) 1. Silver halide crystals are sensitive to x-rays and light 2. When exposed to x-rays and light from the phosphor layer of the intensifying screens, crystals are 3. Latent image is invisible to the eye until developed during processing 4. Developer turns exposed crystals into metallic silver, giving dark appearance
d) Spatial resolution is superior to digital imaging
e) 1. Low contrast resolution 2. Long processing time (sedation welfare) 3. Increased risk of error

b)
26
Q

X-ray image formation

Computed radiography
a) Features of this image acquisition
b) Determining spatial resolution
c) How thickness of phosphor layer affects resolution
d) Advantages (5)
e) Disadvantages (3)

A

a) Image is captured on a photostimulable phosphor
plate inside the cassette. Latent image stored in phosphor. A laser inside the cassette reader transforms the captured image into visible light
b) Determined by phosphor layer and pixel size
c) Thinner the phosphor layer, higher the resolution
d) 1. Low cost of installation 2. Multiple cassette sizes 3. More efficient than conventional films 4. High contrast resolution 5. Thin plate x-rays for intra-oral and dentals
e) 1. Expensive to maintain 2. Increased artefacts 3. Fragile

27
Q

X-ray image formation

Direct digital radiography
a) How it works
b) Most commonly used material for plates
c) Advantages (5)
d) Disadvantages (2)

A

a) Image captured on a detector and displayed on a computer workstation in seconds
b) Amorphous silicon
c) 1. Efficient workflow 2. Dose efficiency 3. Image visible in < 3 seconds 4. No need to remove detector for procesing 5. More efficient at conversion than computed/conventional
d) 1. Expensive 2. Fragile

28
Q

X-ray image formation

Direct digital radiography
a) Indirect flat panel detector (FPD)
b) Direct FPD

A

a) X-rays travel through outer scintillator layer (caesium iodide) to convert x-rays to visible light. Amorphous silicon photoiode layer channels light and converts to a digital output -> read out by thin film transistors (TFT)
b) X-ray photons absorbed by amorphous selenium photoconductor, creating positive and negative charge. Positive charges drawn to storage capacitors, forming latent image -> read out by TFT

29
Q

Radiation Safety

Summarise possibilities (3) of exposure to ionising radiation

A
  1. Cell correctly repaired (no increased cancer risk)
  2. Cell dies (DNA damaged beyond repair, deterministic effect)
  3. Cell repaired incorrectly (mutation occurs, stochastic effect)
30
Q

Radiation safety

a) What does the degree of damage caused by ionising radiation depend on (3)
b) What are deterministic effects

A

a) 1. Amount and type of radiation 2. Tissue irradiated 3. Effects are cumulative and irreversible
b) Related to a single high dose.
- Occur quickly after radiation
- Directly related to the absorbed dose
- Have a threshold below which effects do not occur
- Can cause significant cell damage/death

31
Q

Radiation safety

Effects of ionising radiation:
a) What are stochastic effects
b) What are somatic effects
c) What are genetic effects

A

a) Chance effects
- Dosage increases probability of occurrence
- Neoplasia more likely to develop with increased radiation
- Effects can be somatic or genetic
- No threshold dose
- Risk is small but never zero
b) A stochastic effect
- Irradiation of body tissues
- Restricted to the individual (not passed on via genetics)
- E.g. radiation induced cancer, cataracts
c) A stochastic effect
- Irradiation of the gonads
- Passed onto offspring
- Effects do not appear in the exposure recipient but in descendants
- E.g. Leukaemia

32
Q

Radiation safety

Radiation damage to tissues - what is observed
a) Bone marrow
b) Skin
c) GI tract
d) Gonads
e) What is the tissue weighting factor + examples

A

a) Bleeding, anaemia, infections
b) Alopecia, striation of nails, erythema, dryness, atrophy
c) Intestinal disruption
d) Temporary or permanent sterility
e) Reflects the sensitivity of tissues to radiation damage
- Gonads: 0.08
- Breast: 0.12
- Thyroid: 0.04
- Skin: 0.01

33
Q

Radiation safety

Complete the table
What is a mSv

A

mSv (millisievert) is a unit of measurement for whole body radiation dose

34
Q

Radiation Safety

Ways to protect from radiation (5)

A
  1. Lead shielding
  2. Increase distance
  3. Reduce time of exposure
  4. Accuate collimation
  5. Quality assurance testing to ensure accuracy of the light beam diaphragm

PCP (8 decks)

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