Imaging with X-Rays Flashcards
(157 cards)
1
Q
What are X-rays?
A
High frequency photons arising from the orbital electron shell of an atom.
2
Q
What is Bremsstrahlung (breaking radiation)?
A
The slowing down of electrons, causing them to release X-rays to conserve energy.
3
Q
What is the main way of generating X-rays in medical imaging?
A
Bremsstrahlung
4
Q
What are characteristic X-rays?
A
X-rays thar are created when electrons ‘move’ between energy states within an atomic structure. The energy difference between the states defines the energy of the X-ray emission.
5
Q
Describe the overall planar X-ray imaging process
A
6
Q
What equipment is used to produce X-rays?
A
An X-ray tube
7
Q
What is the role of the cathode in an X-ray tube?
A
The cathode is the source of electrons. It is a heated filament that emits electrons via thermionic emission.
8
Q
What is a cathode normally made of? Why?
A
Tungsten (W) becasue it has a high melting point
9
Q
What is the role of the focussing cup in an X-ray tube?
A
The focussing cup surrounds the cathode filament. It uses a small electric field to focus the electrons and create a small ‘focal spot’ on the anode.
10
Q
What is the role of the anode in an X-ray tube?
A
The positively charged ‘target’ that pulls electrons towards it. As they collide with the anode they decelerate, releasing electrons (mainly through Bremsstrahling) and heat.
11
Q
What is the efficiency of an X-ray tube?
A
< 1% X-ray
> 99% heat
12
Q
What mechanisms are in place to prevent X-ray tubes from melting?
A
- High melting point of anode
- High thermal conduction to move heat away from the focal spot
- Larger focal spots to spread the heat out
- Larger anode to conduct the heat
13
Q
Why is the anode of an X-ray tube angled?
A
It results in:
- A smaller effective focal spot but larger actual focal spot
- Higher heat dissipation due to larger surface area
14
Q
Why does the focal spot of an X-ray tube have to remain small?
A
Focal spot size is a large contributor to the absolute limit on spatial resolution.
15
Q
What is the limitation of an X-ray tube having a small focal spot?
A
A smaller focal spot results in a slower exposure time because more time is needed to allow the heat to dissipate.
16
Q
Why does the anode of an X-ray tube rotate?
A
Spinning the anode around spreads out the electron impact. This:
- Reducded the heat to an individual area
- Increases the heat dissipation
17
Q
An X-ray tube is held in a _______.
A
Vacuum
18
Q
What are the benefits of keeping an X-ray tube in a vacuum?
A
It facilitates free movement of electrons and prevents arcing from the cathode to the anode.
19
Q
What is the role of the X-ray tube housing?
A
It maintains the vacuum and provides substantial shielding from ionising radiation.
20
Q
What is the role of the X-ray tube window?
A
The tube window allows the useful X-rays to exit the tube whilst maintaining the vacuum.
21
Q
What is an X-ray tube window normally made of?
A
Beryllium
22
Q
Describe the characteristics of the X-ray spectrum
A
Low energies: filtered out
Majority of spectrum: bremsstrahlung
Peaks: characteristic radiation
23
Q
What is the maximum energy (keV) of an X-ray produced in an X-ray tube?
A
The potential difference of the tube
24
Q
What are the units for X-ray intensity?
A
mAs
25
What are the 2 types of filtration of an X-ray beam?
- Inherent filtration (from the beryllium window and self-absorption)
- Deliberate added filtration (to reduce low energy X-rays)
26
Why are low energy X-rays filtered?
Because they would be absorbed by the patients skin, increasing their overall dose without providing any clinical benefit.
27
What is the Heel effect?
The variation in X-ray intensity across the beam as a result of X-rays being immediately re-absorbed within the target after being generated.
28
In what circumstances can the Heel effect be beneficial?
In mammography the Heel effect can be used to reduce patient dose by placing the soft tissue on the anode side where X-ray intensity is lowest, and the thicker (chest wall) side over the cathode.
29
What factors impact the dose of X-rays delivered?
- Tube current (mA)
- Exposure time (s)
- Mean voltage across the tube (kV)
- Peak voltage across the tube (kVp)
- Current x time (mAs)
30
State 3 things that increase if the kVp of an X-ray tube increases
1) Number of photons produced (greater efficiency)
2) Mean X-ray energy
3) Maximum X-ray energy
31
What 3 things can happen when a photon passes through a medium?
1) No interaction
2) Complete absorption
3) Scatter/deflection
32
What are the 4 methods of interation between an X-ray and a medium?
1) Total absorption (Photoelectric)
2) Inelastic scatter (Compton)
3) Pair production
4) Elastic scatter (Rayleigh)
33
Give an example of a total absorption X-ray interaction
The Photoelectric effect
34
What is the photoelectric effect?
A photon interaction in which the photon is completely absorbed by an atom, releasing an electron from the orbital shell (ionisation). This can only happen is the energy of the incident X-ray is greater than the electron binding energy.
35
What factors influence the likelihood of the photoelectric effect occurring?
- Photon energy (higher energy = lower occurence)
- Atomic number (higher Z = higher occurence)
- Mass attenuation coefficient
36
What is the mass attenuation coefficient (µ/ρ)?
A measure of how easily a material can attenuate an X-ray beam as it passes through the material, per unit mass of the material.
37
Give an example of an inelastic scattering X-ray interaction
Compton scatter
38
What is Compton scattering?
A type of inelastic scattering where a photon collides with an electron, transferring some of its energy to the electron. This results in a change in direction and reduction in energy of the photon and the ejection of a electron (ionisation).
39
Give the equation for the change in photon energy due to Compton scattering
ΔE = change in energy
E = initial energy
m_e = electron mass
c = speed of light
θ = deflection angle
40
The change in energy due to Compton scattering is related to ______ of scatter.
Angle
41
How does the direction of Compton scatter relate to the energy of the photon?
As energy increases, the Compton scattering direction shifts forwards.
42
What scatter considerations need to be made at diagnostic energies?
There is appreciable 'backscatter' at diagnostic energies so radiation protection considerations need to be made.
43
What scatter considerations need to be made at high energies?
Any forward scatter undergoes further interactions.
44
What factor influences the likelihood of Compton scatter occurring?
Atomic number (Z)
45
What is the secondary beam in a planar X-ray?
The radiation emerging from the patient. This beam is significantly reduced in intensity as it has been attenuated.
46
A large component of a secondary X-ray beam is ________ radiation.
Scattered
47
Why does the secondary X-ray beam have a higher average energy than the primary X-ray beam?
Beam hardening
48
What is beam hardening?
The phenomenon where the average energy of an X-ray beam increases as it passes through a body/other material. This happens because lower-energy (soft) X-rays are absorbed more easily than higher-energy (hard) X-rays, leaving the remaining beam with higher average energy.
49
How can scatter be reduced?
- By decreasing the kV of the beam
- By collimating the beam
- By compressing the patient to reduce thickness
- By introducing an air gap so scattered X-rays don't reach the detector
- By introducing an anti-scatter grid (physical collimation)
50
What are the pros and cons of decreasing the kV of a beam?
+ Scattering is reduced
+ Increase in contrast
- Absorption in the patient is increased so a higher mAs is required
- There is a decrease in forward scatter
51
What are the pros and cons of introducing an air gap between the patient and the detector?
+ Scattered X-rays don't reach the detector
- Increase in patient dose
- Need a larger imaging device/detector
- Will get magnification
52
What are the pros and cons of introducing an anti-scatter grid between the patient and the detector?
+ Scattered X-rays don't reach the detector
- Increase in patient dose
53
What does X-ray image quality depend on?
- Scatter
- Detector efficiency
- X-ray spectrum/beam hardening
- Field non-uniformities (Heel effect)
- Patient motion
- Imaging technology
54
What is X-ray film made of?
Silver bromine ions within an emulsion
55
How does X-ray film work?
X-rays liberate electrons from the bromine ion, which attracts the silver particles. This can then be processed into an image.
56
State the positives and negatives of X-ray film
+ Very high resolution
- Processing requires specialist systems
- Not reusable
- Large skill requirement
57
Where are digital images stored?
Electronically on PACS
58
How is the profile of an analogue image different to a digital image? Why are they different?
The resolution of a digital image is lower because it is based on the value of each individual pixel compared to continuous variation in dose to the film.
59
What is the spatial resolution of a digital X-ray image limited by?
- Pixel size
- Matrix size
60
What is the intensity resolution of a digital X-ray image limited by?
- Bit depth
- Greyscales
61
What is computed radiography?
A digital imaging technique that uses photostimulable phosphor (PSP) plates to capture and store X-ray images, which are then scanned and converted into a digital format.
62
How does digital imaging using computed radiography work?
1) Incoming X-ray excited electron
2) Electron moves to the conduction band
3) Electron drops back to the valence band, releasing light
4) Some electrons drop to trapping sites where they will stay until they are read out
63
What are the two ways to 'read out' trapped electrons in computed radiography?
1) Heat: thermoluminescence (TLD)
2) Light: photoluminescence (CR)
64
How does photoluminescent read out work for compted radiography PSP plates?
CR plates require laser read out using a fine focused red light. This laser stimulates the plate which then emits blue light which is collected and viewed.
65
How can the efficiency of digital X-ray imaging be improved?
By increasing the thickness of the phosphor plate to trap more photons (however this decreases the resolution)
66
What are the pros and cons of computed radiography?
+ Similar workstyle to film (the image is 'developed')
+ Digital imaging provides improved contrast and image processing
- Poorer resolution than film (due to pixel/matrix size, laser scattering, laser beam diameter, size of phosphor grains)
- Readout times are around 20-30 seconds
67
What is digital radiography?
A form of radiography that captures images using digital sensors instead of traditional film. These sensors convert x-ray energy into digital signals, which are then displayed on a computer screen for immediate viewing and analysis.
68
What are the 2 forms of digital radiography?
- Indirect
- Direct
69
How does indirect digital radiography work?
Phosphor based scintillators are used to convert X-rays into visible light photons. These photons are then used to generate electron-hole pairs in amorphous silicon, converting the energy to electric charge. This charge is stored and read out using a TFT.
70
How does direct digital radiography work?
X-rays directly generate electron-hole pairs in amorphous silicon. This converts energy to change which is stored then read out using a TFT.
71
What is a TFT?
A thin film transistor; it is a device which amplifies an electric signal.
72
How can digital X-ray images be dangerous from a radiation protection standpoint?
It is difficult to determine whether there has been an under/over exposure of a patient as they have a wide dynamic range and 'correction' functions.
73
Why is it a problem to 'correct' underexposed digital X-ray images?
Windowing is required, which may exaggerate the noise of the image as fewer photons = lower signal to noise.
74
Why is it a problem to 'correct' overexposed digital X-ray images?
The image quality is too good for the task, meaning that the patient has had a higher exposure for little benefit. Imaing should be optimised instead.
75
What is kerma dose-area product (DAP)?
DAP is a common way to measure exposure in X-ray. It is the product of the absorbed dose and the area of tissue irradiated. It gives an idea of the total energy delivered to the patient.
76
Define mass attenuation coefficient (µ)
The attenuation of X-rays per unit mass as they pass through a medium, measured in cm²/g.
77
What does the mass attenuation coefficient depend on?
- Photon energy (keV) ~ 1/E³
- Atomic number of the material (Z) ~ Z³
- The number of Photoelectric and Compton interactions
78
Give the equation for the intensity of an X-ray passing through a homogenous medium
I = intensity
I_0 = initial intensity
µ = mass attenuation coefficient
t = thickness
79
Give the equation for the intensity of an X-ray passing through an inhomogenous medium
I = intensity
I_0 = initial intensity
µ = mass attenuation coefficient
t = overall thickness
z = thickness of each individual medium
80
Why is contrast needed?
It shows any physical and/or chemical differences throughout a medium
81
How is contrast displayed on a medical image?
Contrast manifests itself as a variation in intensity
82
What are the two types of contrast?
- Subject contrast
- Detector contrast
83
Define subject contrast
Contrast due to different amounts of radiation exiting the patient due to variations in tissue density, atomic number, X-ray energy, scatter, etc.
84
Define detector contrast
Contrast due to the properties of the detector and image processing.
85
Contrast is a measure of signal above __________.
Background
86
Give the equation for Weber contrast (local contrast)
C = contrast
I = intensity
87
Give the equation for Michelson contrast (modulation contrast)
C = contrast
I = intensity
88
What is the difference between Weber contrast and Michelson contrast?
Weber contrast is for features on a uniform background.
Michelson contrast is for patterns where both bright and dark features take up similar fractions of the image.
89
How is the contrast of a system tested?
Using a threshold contrast
90
What is threshold contrast?
The limit at which a detail cannot be distinguished from background (the minimum perceptible contrast).
91
_____ us a limiting factor in contrast.
Noise
92
Why is artificial contrast useful in patient imaging?
Tissues can be very similar in attenuation so introducing an artificial medium can accentuate the image.
93
Define spatial resolution
The ability to separate different object details within an image.
94
What are the 3 ways to define spatial resolution?
- Number of details per unit distance (Lp/mm or Lp/cm)
- Width of the Point Spread Function (PSF)
- Modulation transfer function (MTF)
95
What is a point spread function (PSF)?
The inherent blur from imaging caused by the scatter of photons that 'spread out' as they pass through a medium.
96
How can a PSF be used to resolve two objects?
The overlap of two PSFs can be used to measure whether two objects can be resolved.
97
Geometric blur is a limit on _______ _________.
Spatial resolution
98
How does the focal spot size influence image blur?
The smaller the focal spot, the sharper the image.
99
How can patient blur impact spatial resolution?
Patient motion (voluntary/involuntary) and variation in edge boundaries can cause a decrease in spatial resolution.
100
What is the square wave transfer function (SWTF)?
The SWTF quantifies how well an imaging system reproduces the contrast of square wave inputs at different spatial frequencies, helping assess the spatial resolution and contrast performance of the system (i.e. how sharp or blurry an image will appear).
They are demonstrated in a plot of amplitude (modulation) against frequency (Lp/mm).
101
What is the modulation transfer function (MTF)?
The MTF quantifies how effectively an imaging system can reproduce detail by measuring the decrease in contrast as spatial frequency increases. It is a fundamental measure of spatial resolution and image quality.
102
What is noise in an image?
The variation in signal for a constant input.
103
Noise is governed by _______ statistics.
Poisson
104
Give the equation that relates noise to signal
Noise = k √(mean signal)
k = constant
105
Signal to noise ratio increases with ______.
Signal
106
What is the disadvantage of increasing signal to get a better image?
It gives the patient more dose
107
High noise can mask _______.
Contrast
108
Distortion and unsharpness increase with ____________.
Magnification
109
Give the equation for magnification
110
What is mammography?
The imaging of breast tissue with X-rays to look for microscopic classification/small lesions.
111
What are the imaging requirements for mammography?
- Excelent spatial resolution (objects as small at 50µm)
- Good contrast (to differentiate between different soft tissues)
- Good latitude/dynamic range (due to variations in breast thickness from chest to nipple)
- Minimal patient dose (breast tissue is radiosensitive)
112
Name the 4 key components of a mammography units
- Gantry mount X-ray/detector
- Appropriate X-ray tube for low kV imaging
- Compression device to minimise scatter and motion
- Automatic Exposure Control (AEC) to terminate exposure at the correct detector dose
113
Why is a low kV imaging tube used for mammography?
To exaggerate the photoelectric effect, which emphasises soft tissue differences
114
How do radiographers choose mammography target/filter combinations?
Based on breast thickness
115
State 2 typical mammography target/filter combinations
- Molybdenum
- Rhodium
116
What imaging technique is used for mammography?
The half-field imaging technique
117
How does mammography utilise the Heel effect?
The breast is positioned so that the maximum dose is at the chest wall and dose decreases towards the nipple.
118
Why is compression necessary for mammography?
- Reduced dose
- Spread tissue
- More uniform thickness (narrow dynamic range for a high contrast image)
- Reduced motion
- Reduced scatter and beam hardening
119
What is fluoroscopy?
Real time X-ray imaging that follows similar principles to planar X-rays.
120
Give 2 examples of dynamic processes that are imaged with fluoroscopy?
- Aiding the positioning of intra-operative devices
- Diagnosis or assessment of patient anatomy
121
What are image intensifiers?
Devices that convert low-intensity X-ray images into bright, visible light images in real time, making it possible to view internal structures and motion.
122
How do image intensifiers work?
1) X-rays reach an input phosphor, which generates light photons
2) Light photons go to the photocathode, which generates electrons
3) The electrons are accelerated and focused to the output phosphor
4) Light photons are generated when the electrons reach the output phosphor
5) This creates an electro-optically intensified light image
123
Why does fluoroscopy have such high dose rates?
Low dose rates are used but there are very long screening times.
124
State 3 fluoroscopy techniques that reduce dose
- Pulsed fluoroscopy
- Frame grab/last image hold
- Virtual collimation
125
What is pulsed fluoroscopy?
A fluoroscopy exposure pulse that reduces patient dose whilst still allowing dynamic imaging.
126
How can pulse settings in pulsed fluoroscopy be customised?
The following settings can be adjusted (either manually or automatically):
- Pulse rate
- mAs per pulse
- Pulse width
127
What is frame grabbing in fluoroscopy?
The process of selecting and saving a single image (or frame) from the live fluoroscopic feed for documentation, analysis, or reporting.
128
What is virtual collimation in fluoroscopy?
A digital technique that allows the operator to simulate and adjust the collimation field on the screen without activating the X-ray beam, helping reduce radiation exposure to the patient and staff.
129
What is automatic brightness control (ABC)?
A setting on fluoroscopy units where the system will modulate different parameters to maintain image brightness according to patient/body part size.
130
What are fluoroscopy kV/mA curves?
Fluoro kV/mA curves describe how a fluoroscopy system automatically adjusts kilovoltage (kV) and tube current (mA) in response to changes in patient thickness or imaging requirements. They’re part of the Automatic Brightness Control (ABC) system that maintains image quality while minimizing radiation dose.
131
What is the consequence of magnifying a fluoroscopy image?
Magnifying the image decreases the field size, resulting in a reduced output. Due to the ABC, other factors are increased to maintain output so paitent dose increases.
132
What is computed tomography?
A method of X-ray imaging that produces a 3D reconstruction of a 3D object, displayed in scrollable 2D slices.
133
CT scanners have a ___ beam that contains multiple rotating ________.
Fan
Detectors
134
True or false: CT scanners utilise both rotational and translational motion.
FALSE: they only rotate
135
CBCT scanners have a ____ beam rather than a fan beam.
Cone
136
Describe the pros and cons of cone beam CT scanners
+ Large volume imaging
+ Faster scanning
- Loss of scatter rejection
- Tricky reconstruction (non-ideal geometry)
137
What are the two ways that a CT scanner can scan?
- Axial scanning
- Helical scanning
138
Describe the process of axial scanning
A stop and shoot method that involves one rotation (to image one slice), moving the patient by one acquired slice thickness, then repeating.
139
Describe the process of helical scanning
An imaging method that involves moving the patient as the CT assembly rotates. This increases the imaging speed, however, each rotation can be incomplete so slices require interpolation.
140
Define pitch
The distance that a CT couch travels per rotation divided by collimation.
141
What are multi-detector CT scanners?
CT scanners with several rings of detectors, allowing a large volume to be imaged in fewer rotations as multiple 'slices' can be imaged simultaneously.
142
What are the pros and cons of multi-detector CT scanners?
+ Faster imaging
- Increased patient dose
143
What are adaptive array CT scanners?
CT scanners with adaptive detector arrays, allowing the desired slice thickness to be chosen.
144
What is the difference between a CT pixel and a CT voxel?
Pixel = picture element = 2D
Voxel = volume element = 3D (pixel x slice thickness)
145
What are the 2 most common approaches to CT image reconstruction?
- Filtered back-projection (FBP)
- Iterative reconstruction (IR)
146
What are Houndsfield Units (HU)?
Hounsfield Units (HU) are a scale used in CT imaging to quantify how much a tissue attenuates X-rays compared to water (which has a HU of 0), they provide a standardized way to describe tissue density on CT images.
147
Give the equation for the CT number of a CT voxel (in Houndsfield Units)
µ = mass attenuation coefficient
148
What is the window level of a CT image?
The CT number of the "mid-grey" level betwen black and white.
149
What is the window width of a CT image?
The range of CT numbers from black to white.
150
How is a CT window selected?
Based on clinical need, using anatomical pre-sets.
151
What are the 4 main types of artefact in CT?
1) Partial volume effect
2) Streak artefacts
3) Beam hardening
4) Ring artefacts
152
What is the partial volume effect?
CT scans can't contain images smaller than a voxel, meaning that the average value of a voxel containing multiple densities is provided regardless of the ratio of tissues within the voxel. This effect increases with pixel size and/or slice thickness.
153
What are streak artefacts?
Artefacts caused by:
- Motion (the streaks are seen in the direction of motion)
- Components of the image outside the FOV
- Edges of high attentuation medium (like implants)
154
What are beam hardening artefacts?
Artefacts resulting from high attenuation materials 'starving' the intensity of the tissues behind.
155
What are ring artefacts?
Artefacts due to non-uniformities amongst detectors, meaning that each projection has a change in the measured attenuation coefficient at one point. This discrepancy appears as a ring.
156
What is dose modulation?
Customisation of a scan due to the differences between patients (e.g. patient size/scan location) in order for similar numbers of photons to reach the detector each time. Modulation parameters are selected based on a pre-scan radiograph.
157
State the 3 types of dose modulation
1) mAs modulation
2) kV modulation
3) Breast/hand saving
