3.10.5 X-Ray Imaging Flashcards
When does TIR occur in a glass rod?
If the light (or any EM wave) enters at incident angle > critical angle
What is the core of an optical fiber surrounded by?
Cladding
Which has a higher optical density and why?
Core or Cladding
Core
So that TIR can occur
(TIR only occurs when travelling from dense to less dense medium)
Other than allowing TIR to occur, what does the cladding do?
Protects core from damage
Prevents ‘crosstalk’
What is ‘crosstalk’?
When EM waves cross into adjacent fibers in the bundle (creating blurred images.)
What are the 2 types of bundles?
Coherent
Incoherent
What are coherent bundles?
The relative positions of fibers remain constant.
Good for imaging
(smaller diameter fibers have better resolution)
What are incoherent bundles?
The relative positions of fibers do not remain constant.
Images would be ‘jumbled’
Good for transmitting light.
(cheaper and easier to lay)
What are the main components of an x-ray tube?
Glass envelope
Cathode
Filament
Vacuum
Anode (with focal spot)
Rotor
Fixed shaft
Bearings
How does an x-ray tube work?
Electrons are released from filament, through thermionic emissions; a filament (of tungsten) emits electrons when heated.
They are attracted towards positive anode.
When electrons are brought to a sudden stop, their energy is dissipated in the form of x-rays.
What is continuous spectra of x-rays?
Due to the deceleration of electrons, KE = transferred to x-ray photons, whose max. energy = limited by the KE.
The electrons are decelerated by diff. amounts, - continuous spectra emitted.
What is line spectra of x-rays?
Depending on material being struck by the electrons, certain energies (frequencies) of x-rays appear with greater intensity.
Photons of specific energies are able to remove inner electrons of atoms - creates vacancy that higher electrons drop in to fill = release specific photon of radiation.
What does p.d control (in regards to x-rays) ?
Max. photon energy
Peak value of radiation activity
Amount of energy of peak radiation (moves position of peak, with respect to x-axis)
What does p.d not effect?
Position of line spectra for given target material.
What is the anode?
The anode is a rotating tungsten disc; spinning at 3000rmp, with a beveled edge
Why does the anode rotate?
The rotating spreads the heat over a larger area to prevent overheating and reduces risk of melting.
Allows higher power use, allowing higher x-ray production.
Why does the anode have a beveled edge?
Means larger surface area for electron beam.
-reduces heat build up, while still giving small focal spot for a sharper image.
What are x-rays used for?
To view internal structures of objects and materials.
What happens when x-rays go through a body?
X-rays usually pass though soft tissue and muscles.
Absorbed by bones.
(how much energy absorbed by a material depends on atomic number - tissue and bones have diff. atomic number, so will contrast in x-ray image)
What is required for clear x-ray image?
A clear x-ray image requires a large diff. in proton number or density between target area and surrounding tissue.
How would soft tissue be imaged (e.g. to see something stuck in esophagus) ?
A contrast medium is used to improve image quality.
Contrast medium = high proton number compound (‘barium meal’).
- barium passes through gastrointestinal tract, making it clearly visible against surrounding tissues on the x-ray.
What happens to a beam of monoenergetic x-rays as they pass through matter?
It’s intensity is reduced/ attenuated
What does the amount of absorption of x-rays through a material depend on?
Thickness of medium
Atomic (proton) number of medium
Density of medium
What is a photon of an x-ray absorbed by an atom through?
Photoelectric absorption
What happens to the electron when a photon strikes an inner shell electron?
Electron escapes its shell - excess energy contributes to the electron’s KE.
What happens to the x-ray photon when it strikes the atom (photoelectric absorption)?
Atom completely absorbs the photon, so it does not make it to the detector.
How does proton number relate to absorption?
Higher proton number atoms contain more electrons, which increases chance of absorption, because there are more interactions.
What is linear attenuation coefficient? [μ]
The fractional change in number of photons [N], for each unit of distance through the material.
How does density relate to linear attenuation coefficient?
Higher density = higher μ
(more atoms per unit area = more interactions between photons and electrons)
What is mass attenuation coefficient?
μ/ρ = constant
What is the half value thickness?
The thickness of a material which will reduce the intensity of a beam of x-rays to half its original value.
(The thickness of a material defining the penetrating power of a monoenergetic beam of x-rays.)
What do x-ray filters do?
Remove the lowest energy photons
What are the 2 uses of x-ray beams?
Internal imaging
X-ray/Radio therapy
Why don’t we want low energy photons?
They are absorbed by ‘superficial tissue layers’ (skin) so do not contribute to the uses of x-rays.
They can also cause harm as they are ionising.
What is ‘beam hardening’/ what is meant when the beam has been ‘hardened’?
When the beam has had low energy photons filtered out.
What is the ideal scenario for beam hardening?
When the lower energy photons are removed and the rest are retained.
What is the realistic scenario for beam hardening?
The low energy photons are heavily suppressed
The overall intensity is lowered.
Why does the filter have to discriminate between photon energies?
We want as small an effect as possible on high energy photons.
What are the 2 causes of attenuation?
Photoelectric Absorption
[Compton] Scattering
What is photoelectric absorption?
Photon hits inner shell electron,
Passes on energy,
Electron escapes
What is the probability of absorption?
Inversely proportional to photon energy cubed.
What is [Compton] Scattering?
Outer shell electrons deflect the photons.
What is the probability of [Compton] Scattering?
Inversely proportional to photon energy.
What is the highest cause of attenuation for lower energy photons?
Photoelectric absorption.
(1/2 energy of photons = increase prob. by factor of 8)
([Compton] Scattering - 1/2 energy of photons = increase prob. by factor of 2)
What property materials are needed for filters to maximise photoelectric absorption?
High z number
What is the beam energy for radiotherapy?
> 250 keV
(lots of low energy photons to be removed)
What material is used to filter out photons for radiotherapy?
Copper
(z number is relatively high)
What is the beam energy for x-ray imaging?
> 150 keV
(less low energy photons need to be removed)
What material is used for x-ray imaging?
Aluminium
(lower z number so less PE absorption happens)
What is the ‘focal spot’?
The beveled edge of the anode where x-rays originate from.
What is the umbra?
The region under the object being imaged that is in full shadow.
(No photons reach the film.)
What is penumbra?
The regions under the object being imaged that are in partial shadow.
(Some photons reach the film, so film is partially blackened.)
What does an x-ray image look like, referring to the blackened film?
The image would be a transparent central region.
2 regions of semi-blackened film (‘fuzzy edges’) surround it.
Surrounding that is fully blackened film.
What is another phrase for the ‘fuzzy edges’?
Geometric unsharpness.
What must be done to minimise geometric unsharpness?
Make the focal spot size as small as possible.
What happens if the focal spot is too small?
Heat gets concentrated and can damage the anode, so damages the x-ray tube.
Why does the p.d between the filament and the target affect the focal spot size?
The p.d determines how much energy the beam carries and transfers to the anode, and therefore how much heating will occur.
What happens to x-ray penetration as p.d increases?
X-ray penetration increases, giving clearer images.
However, this means focal spot size would need to be increased to dissipate the heat, blurring the image.
What does exposure time affect?
Clarity of images
(longer exposure = darker image = clearer radiograph)
What are some limitations for exposure times?
- Part of body being examined
2.
Why does the body part being examined affect exposure time?
Some body parts can be held stationary (hand/arm) so longer exposure times are possible = clearer images.
Internal organs perform involuntary movements, increasing the movement unsharpness, blurring the images, so shorter exposure times used = less clear images.
What is used to minimise random scattering?
Grid of fine lead strips.
- absorbs any photons arriving at a non-zero angle.
What is one drawback of the grid of fine lead strips?
Some of the (un)scattered photons (primary radiation) can also be absorbed, so to compensate, exposure time is increased.
What are the for ‘layers’ of an intensifying screen?
Protective plastic coating
Calcium tungstate
Magnesium oxide
Stiff cardboard [1mm thick]
What does the calcium tungstate do in an intensifying screen?
As x-rays hit the calcium tungstate, atoms excite - when they de-excite, visible light is released (in same pattern as original x-rays).
What does the magnesium oxide do in an intensifying screen?
MgO = very white - reflects visible light towards film, which darkens it (as well as x-rays).
How does using an intensifying screen effect intensity of radiographic image?
The combination of x-ray photons and visible light photons striking film = more intensified/ heavily concentrated radiographic image.
x30 intensity
What is the benefit of the use of visible light photons in an intensifying screen?
Visible light photons = non-ionising
- x-ray exposure time can be reduced for same image quality (less x-rays used)
What is a cassette?
A pair of intensifier screens are used, surrounding a double sided film.
Surrounding this is a cassette front (plastic) and cassette back (metal).
What does the metal cassette back do?
Absorbs any photons that strike it, preventing photons reflecting back and interfering with the image.
What are the main components of an image intensifier?
Fluorescent screen
Photo cathode
(all parallel plates = ) electron lens
Fluorescent viewing screen
How does the first fluorescent screen work in an image intensifier?
X-rays sent towards screen in a specific pattern (after passing through patient).
Screen emits visible photons (through excitation and deexcitation of atoms), in the same pattern.
How does the photo cathode work in an image intensifier?
Absorbs visible photons and emits electrons, also in the same pattern.
What happens to the electrons in an image intensifier?
Accelerated across the gap by a p.d.
(p.d. gives them energy by doing work on them)
What does the electron lens do in an image intensifier?
Reduces the size of the pattern
What happens at the fluorescent viewing screen in an image intensifier?
When the electrons strike the viewing screen, higher energy photons of visible light are released, in the same original pattern, forming an image.
What factor can the intensity/ brightness of the image be increased by, in an image intensifier?
(by the electron lens and p.d)
x5000
Generally, how does a CT scan create an image?
Uses narrow, collimated, monochromatic x-ray beams and computers to create ‘slices’ of cross-sectional images.
These are ‘stacked’ to create a 3D image.
What are the benefits of CT scans?
Fast
360º views
Detailed/ greater clarity/ captures fine details, abnormalities and internal injuries.
Non-invasive (no surgery)
What does the collimator do in CT scans?
Reduces dose and restricts the scattered x-rays from outer parts of the related slice. (Makes beams parallel and go straight)
What happens within the gantry of a CT scan?
X-ray source rotates around object/patient.
X-rays passing thru are detected on opposite side (at multiple angles).
-transmitted intensity is measured.
What happens after the x-rays are detected in a CT scan?
X-rays are then processed on computer (using tomographic reconstruction algorithms) to produce tomographic/ cross-sectional images.
Why might a patient be asked to hold their breath during a CT scan?
Holding breath stops chest moving so helps focus the radiation beam, creating less blurry images.
This can be difficult for some patients.
What are some disadvantages of a CT scan?
X-rays = ionising = higher risk of cancer [CT scans use higher radiation exposure]
High costs due to high demand and need for specialized training. [tho less expensive than MRI]
Struggle to distinguish soft tissue of similar densities/ tissue boundaries [better than simple x-ray imaging]
What are the components of an indirect flat panel (FTP) detector?
Scintiller
Photodiode
Electric circuit
Image
What happens at the scintillator of a FTP detector?
Scintillator (e.g Césium Iodide) detects x-ray photons.
X-ray photons converted into light photons, which are directed towards photodiode array.
What happens at the photodiode of a FTP detector?
Photodiodes absorb light photons
Each photodiode corresponds to a pixel in final image.
What happens at the electric circuit in FTP detector?
Concerts absorbed light into stored electric charge.
The charge from each pixel is read out by electric scanner/scanning.
Scanned data converted to digital signals, which are processed by an image processor to create a digital image.
What are the benefits of FTP detector?
Faster
Produces clearer images (more sensitive) than film.
Images are more portable (saved to files that can be shared)
