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MIT 524 > Xray tube > Flashcards

Xray tube Flashcards

1
Q

How does characteristic X-rays occur?

A

Projectile electron interacting with an inner shell electron of an atom. when the outer shell electron fills this vacancy left behind by the ejected electron, this releases X-Rays as the excess energy in the process. Needs 69.5keV/kVp to occur in General Imaging.

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2
Q

How does bremsstrahlung xrays occur?

A

Projectile electron passes through the atom avoiding hitting any electrons, the electron is influenced by the pull of the protons inside the nucleus, causing the electron to slow down (Braking Radiation), when the electron slows down, it loses kinetic energy which is given off in the form of X-Rays. Bremsstrahlung outnumbers characteristic x-rays (anything below 69kVp in general imaging is bremsstrahlung radiation. The shortest wavelengths are made when the electron is slowed or ‘braking’ around the nucleus.

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3
Q

What does an auto timer do?

A
  • Allows for consistent quantity of X-Rays to occur when used correctly
  • It controls the exposures length and will switch off when patient has had the correct amount of exposure
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4
Q

what are 2 factors that would cause the AEC to not work correctly?

A

Prosthetics and not being positioned over the correct chambers

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5
Q

5 types of interaction with matter?

A 
Coherent/classical scatter
Compton scatter 
Photoelectric absorption 
Pair production
Photodisintegration
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6
Q

What 2 interactions with matter are present in diagnostic imaging?

A
  • Photoelectric effect- Absorption of an xray by the patient or lead due to ionization. (characteristic xray)
  • Compton scatter – Interaction between an incident x-ray and a loosely bound outer shell electron that results in ionisation and x-ray scattering. Contributes to degrading the image and making it appear foggy. (referred to as noise) Does effect general imaging. Can go back towards the tube (backscatter) or forward which effects the image (forward scatter) when an xray has had interaction with skin.
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7
Q

what 2 interactions with matter only happen at very hight energy levels?

A

Pair production and Photodisintegration

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8
Q

What interaction with matter uses the lowest energy?

A

Classical/coherent scatter

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9
Q

Inherent filtration

A

Inbuilt into the machine, cannot change. Eg. Tube insert glass, Oil around the insert.

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10
Q

Additional filtration

A

Sliding filter in or below the tube port, can be removeable. 2.5mm Aluminium or equivalent

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11
Q

Compensatory filter

A

Slots into the base of the collimator, removeable. Eg. Wedge Filters

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12
Q

what is another name for scatter?

A

noise and grey fog/veil

-When we refer to DR, scatter is more often referred to as noise

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13
Q

What does scatter do to the image?

A

lowers radiographic contrast by by leaving a grey fog / noise over the image

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14
Q

what contributes to more scatter?

A

Patient thickness and large field sizes

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15
Q

What is the purpose of a grid?

A

To reduce scatter from reaching the IR

which increases radiographic contrast

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16
Q

What are the 2 types of grids and how is the lead strips positioned?

A

Parallel - strips run parallel to each other

Focus - strips are angled towards the centre to align with the diverging beam

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17
Q

when do you use a grid?

A

When the body part is thicker than 10cm

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18
Q

when using a grid, what factor needs to be altered and by how much?

A

mAs - needs to be increased with accordance. ie - grid factor 4 = times mAs by 4.

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19
Q

Why is it important for the ‘tube side’ of the focus grid to be facing the tube?

A

If tube side is facing down then the angled lead strips won’t let the diverging bean through to the IR.

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20
Q

Grid ratio is measured by the?

A

Height of the lead strips and the width between each strip.

Grid ratio = Height divided by width

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21
Q

What does grid factor mean?

A

its how much we need to multiply the mAs by compared to the mAs we had originally

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22
Q

What is gridless digital technology?

A

A software that allows us to not have to use a conventional grid due to the software cleaning up/filtering the image for us

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23
Q

What is a benefit of gridless technology?

A

We can image body parts to uptown 13cm without needing to use a conventional grid. As a result, the patient receives a lower radiation dose

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24
Q

What is the misalignment maximum for the collimators?

A

1% is okay but anything over 1.5% (15mm) is when the engineer needs to correct it.

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25
why do we want to use the smallest IR possible?
IRs have the same number of phosphors so the smaller the IR the smaller the phosphors there for gives a more detailed image.
26
Why does the extremity IR provide a better quality image?
because it has more crystals in them
27
What is the process once the IR has been inserted into the CR
- IR is inserted into the reader. - reader takes the IP (image plate) out - the reader reads the IP with a red laser in a zig-zag pattern - IP emits a cyan light while it is being read. - This cyan light is then read by the PMT (photomultiplier tube) in CR or the charged couple device (DR) and that along with the co-ordinates, gives us our intensity/radiographic image. - After scanning the IP is blasted with a bright white light to erase the IP and it can now be re-used.
28
FPD - Flat Panel Detectors TFT - Thin Film Thyristors AMA - Active Matrix Array are also called?
Digital detectors (in digital radiography)
29
The smallest picture element that you can see is?
a pixel
30
What makes up the digital plate?
a del (digital element)
31
What are the components of a Del?
``` TFT = acts as a switching gate Storage capacitor (stores electrical charge) DDS = del detector surface (where the image is produced) ```
32
How big is a Del and what unit are they measured in?
100 - 200µm (microns) in size (0.1 – 0.2 mm) -very very small
33
What does the PMT do? (photomultiplier tube)
The Photomultiplier tube (PMT) or charged couple device (CCD) measures the cyan light along with the coordinates to put together the image
34
Direct image capture?
Uses one layer of material that converts the x-ray photon energy directly into an electrical charge.
35
Indirect image capture?
Has two layers in the detector -Top layer is a scintillation layer (produces light). This converts invisible X-Ray light photons into a visible light. The lower layer then converts the visible light into an electrical signal.
36
Refers to any emission of light?
Luminescence
37
What is Fluorescence?
immediate emission of light under stimulation. i.e. the light emission ceases the instant the stimulation source ceases.
38
Delayed emission of light some time after the original stimulus (exposure) has occurred. Think glow in the dark.
Phosphorescence
39
F centres are also described as?
Defects
40
Where are the F centres located?
between the valance band and conduction band.
41
What happens to the electrons after an exposure?
- In some cases, the energy from the red laser may be enough for the electrons trapped in the F centres to reset back to the Filled Band. - After the laser scanning, the IP is blasted with bright white light (Erase). - This resets any trapped residual electrons in the F Centres and returns them to the Filled Band. - Imaging Plate is now ready for re-use
42
What are the 3 steps involved in turning analog to digital images?
Scanning, sampling and quantisation
43
What does 'scanning' refer to when digitising an image?
In which a matrix of tiny cells is formatted. Each cell (pixel or dexel/del) will have an X,Y location.
44
What does 'sampling' refer to when digitising an image?
In which brightness measurements are taken for every pixel or dexel/del.
45
What does 'quantisation' refer to when digitising an image?
In which a discrete value is assigned for each brightness measurement for every pixel or dexel/del.
46
How space and value change from continuous to discrete when going from analogue to digital
An analogue radiographic image is said to be o Continuous in space o Continuous in value After the SCANNING stage, the radiographic image is o Discrete in space o Continuous in value After the SAMPLING stage, the radiographic image is still o Discrete in space o Continuous in value After the QUANTISATION stage, the radiographic image is o Discrete in space o Discrete in value
47
What element is used in TLD badges?
Lithium fluoride
48
Why is lithium fluoride used in the TLD?
Because it is a similar atomic number to skin so it can give a representation of the skin dose in the badges. -they can be re used
49
What are the filters in the TLD made of?
Plastic and aluminium as they will both give different readings depending on weather you have been exposed to short or long wavelengths - aluminium absorbs longer wave lengths - plastic absorbs shorter wavelengths
50
What does TLD stand for?
Thermoluminescence dosimeter
51
What is Thermoluminescence ?
when energy absorbed is stored and when the material is heated, this energy is released in the form of light.
52
5 advantages of TLD badges
Cheap Lightweight Robust Mailable to centres Re-useable
53
3 disadvantages of TLD badges
Longs delays are possible between incident and results Requires specialist equipment to process the chip The ‘evidence’ is destroyed when the chip is heated
54
6 factors that contribute to image quality
- Brightness (Density) - Contrast (Grey Scale) - Noise - Sharpness (Spatial Resolution) - Magnification (Size Distortion) - Shape Distortion
55
movement unsharpness can be caused by?
- voluntary or involuntary movement by the patient | - heart or breathing movement
56
screen unsharpness can be caused by?
CR -screen unsharpness is likely to be seen in radiographic images produced on general vs extremity IRs DR - Scintillator material, dispersal of the scintillator layer and the size of each dexel may mean that lines or structures may be unsharp
57
Geometric unsharpness is penumbra and is caused by?
- The larger to focal spot, the larger the penumbra - Increase in OID will increase the size of the penumbra - Increase in SID without an increase is OID will decrease the penumbra
58
what war the EI values for chest?
1500
59
what are the EI values for extremities?
1800-2100
60
what are the EI values or thicker body parts?
1500-1800
61
What do we look at to know if we have used too much or too little mAs?
Exposure Index (EI)
62
what is the formula for inverse square law?
New mAs = New SID2 | Old mAs = Old SID2
63
why does quantum mottle appear on the image?
too little mAs
64
What exposure factor controls the quality of an image?
kVp
65
What exposure factor controls the quantity of photons on an image?
mAs
66
if too much_____, cortex will be burnt out, we will lose the finer details and over penetrated
kVp
67
if not enough ____, thicker tissues will appear white and under penetrated
kVp

MIT 524 (1 decks)

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