X Ray Physics Flashcards
- CATHODE produces -e’s via thermionic emission
- high melting point
- high atomic number
Tungsten = high electric resistance = heats with current = excitation of valence -e’s
-e’s attracted towards the POSITIVE anode and strike it
Accelerated through
Two tungsten filaments and focussing cup
- ANODE is also made of tungsten - collision site is where x-rays are generated
But only using 1% - rest wasted as heat
Anode angle beam can be change to alter xray geometry
Anode generates a lot of heat - it spins to distribute the heat
Vacuum in the x-ray machine to avoid electron flow disruption
Lead housing to absorb anyone who should not be irradiated
- FOCAL SPOT
You have the bombarding electron beam,
ACTUAL focal spot (red square)and EFFECTIVE focal spot (by patient)
Line focus principle, ANODE ANGLE will change the EFFECTIVE focal spot
Smaller angles or smaller filament = narrower effective focal spot
Larger anode angles = larger FIELD SIZE
Effective focal spot changes due to geometry:
Closer to anode side = gets narrower
Closer to cathode side = gets larger
- ANODE HEEL EFFECT
- The variation in x-ray beam intensity between the anode and cathode side
- When x-rays are produced 360 degrees / isotropic / all directions
- x rays cathode side travel a shorter distance
- those travelling through the HEEL of the anode (bottom part of blue triangle)
Larger anode angles = heel effect / distal through anode (blue triangle) is LESS
Source to image distance
if you move detector away = increasing distance = less variation
Changing field size = collimation
The effect can be exploited so that you have better penetration of denser structures
e.g. pelvic images or mammography
Xrays of varying energy are produced
- most photo electrons are LOW energy - contributing to patient dose and NOT image as absorbed
- Filtration process via PHOTOELECTRIC effect
tau = likelihood of photoelectric effect
p = density
Z = atomic number
E = energy of x-ray photo
- INHERENT FILTRATION
- cannot control - due to components glass, conducting oil and x-ray window within x-ray tube - all will attenuate x-ray tube
Beryllium Z = 4
Aluminium Z = 13
Or mirror during collimation
Equivalent to 0.5 - 1.5mm of Aluminium
- ADDED FILTRATION
You could add more sheets to filter
- Notice you never reduce the maximum energy
- Reduces x-ray beam quantity
Compensation filters give more even exposure
You want inherent + added filtration to be around the equivalent of 2.5mm Aluminium
- Wedge
- Bowtie filter
- Trough filter
- COLLIMATION
Lead sheets which will attenuate x-rays
Bulb and mirror allows you to see effective collimation
xrays
1. Transmitted - go straight through
2. Attenuated = absorption via photoelectric effect
3. Scatter = adds noise and REDUCES CONTRAST
Collimation reduces the amount of scatter
8 XRAY CIRCUIT - OVERVIEW
You need to
1. Change potential to 100,000 Volts
2. Convert it to direct current to provide a constant stream
3. Adjust voltage depending on the density being imaged
8A. PRIMRARY CIRCUIT
- Line monitor
- Autotransformer - allows you to select kVp
- Exposure timer
- Red square - circuit breaker if current too high
Primary circuit - forms first part of step up transformer
8B SECONDARY CIRCUIT
Changes current to flow in one direction
Use diodes = semiconductors only allowing current in one direction
Tube current = no. electrons flowing from anode to cathode manipulate by
- Increases kVp Increases tube current
- If you increase kVp by 15% you need to reduce tube current by 50%
- FILAMENT CIRCUIT
Controls thermionic emission from cathode
V = IR
Voltage is the same so -> Varying resistance to vary current
Step down transformer then increases current
Filament current directly influences tube current
- BREMSSTRAHLUNG Radiation
ATTRACTIVE FORCE BETWEEN ELECTRON AND NUCLEUS
(German for BREAKING)
- BREMSSTRAHLUNG Radiation
- Cathode produced -e’s accelerated to anode focal spot
(<1% energy converted into x-rays) - Energy of electron = tube potential accelerating it
- Energy released as it breaks and changes direction due to nucleus
Further away - loses less!
This produces a BREMSSTRAHLUNG Spectrum
The pic is UNFILTRERED
FILTERED:
Preferentially remove lower energy photos via PHOTOELECTRIC effect
Inherent will remove anything below 12
Filtered BREMSSTRAHLUNG Spectrum
- Max is determined by kVp
- No. XRs = area under curve - determined by kVp, target material AND filtration
- CHARACTERISTIC RADIATION
- Accelerated electron interacts with the inner shell electron
- K shell electron will be ejected IF it exceeds binding energy
- Released K shell -e = photoelectron
-e from a higher shell will drop down releasing characteristic radiation with the energy = difference between binding energies
Difference between K and L = Kα Peak
Difference between K and M = Kβ Peak
Specific for the element
- X-RAY SPECTRUM
Combining characteristic radiation peak with BREMSSTRAHLUNG curve = GIVES X RAY SPECTRUM
X-Ray beam quality = AVERAGE Energy = Green Line
Quantity = Area under the curve
6A. X-RAY SPECTRUM + Filament Current
Current through tungsten filament on CATHODE
Higher current = hotter = more -e’s released
So increases QUANTITY not quality
6B. Affect of tube potential
- Pulls more -e’s away from cathode = so increases TUBE CURRENT - increases area under curve
Tube potential and tube current has an exponential relationship
6B. Affect of tube potential
Average ENERGY also increased - shifts curve to the right (as well as up due to current)
6C. FILTRATION
- REDUCES QUANITITY AND DOSE
- INCREASES AEVERAGE ENERGY (by filtering out the lower ones)
6D. Changing target material
Bremstraughlung radiation = EXPONENTIALLY related to atomic number
More attractive force from larger nuceli = more breaking radiation
Characteristic xrays will also change
6E. Generator waveform
Ripples = reduction in photons and average energy
So reduces quantity and quality
3 ways x-rays can interaction with matter
- PHOTOELECTRIC EFFECT
All incident x-ray energy is deposited into the -e
Releasing a PHOTOELECTRON
with energy, E = E of the original x-ray - binding energy
CREATING AN ION
Outer shell -e’s within the patient will drop down releasing characteristic radiation, but energy is very low 1-2 kEV and will be attenuated.
Probability of photoelectric interaction τ
p = Density
Z = Atomic number ^3
E = 1/ Energy of Xray ^3
K Edge = above an energy threshold to overcome binding energy = new pool of available -e’s
Iodine atomic number Z = 53
Tissue Z = 7
- COMPTON SCATTER
Contributes to patient dose
Not congruent with incident XR
= Increases noise within the image
NOT ALL XR energy is conferred to electron, but enough to free it = photoelectron
Energy of photoelectron proportional to SCATTER ANGLE
Larger angle = LARGER energy conferred
Compton scatter - largely depending on electron density of the tissue
Atomic number going up - number of electrons go up - so electron density similar
Not really affected by photoenergy
INCREASING photon energy
Compton Scatter predominates as it levels as photoelectric interactions drop.
- Rayleigh Scatter = Elastic Scatter
Does not infer dose to patient
Generally occurs are lower energies / longer wavelengths
Generally if atom is SMALLER than wavelength
ALL -e’s in the atom are excited, oscillate and energy released equal to energy of incident x-ray.
BUT energy is INDEPENDANT OF angle
NO DOSE TO PATIENT
Rayleigh scatter drops with increasing energies
- LINEAR ENERGY TRANSFER
When CHARGED particles travel through TISSUE
Smaller = electron = lower LET (lower)
LET is proportional to
Charge, Q ^2
Energy, 1/E
As the electron loses energy MORE energy is transferred.
- LINEAR ATTENUATION COEFFICIENT
Determines no. XRs either ATTENUATED or SCATTERED
μ
Combo = photoelectric + compton + rayleigh
Dependant on 3 Main things:
Photon Energy
Material Density
Atomic Number
LINEAR ATTENUATION COEFFICIENT
Same number proportion of xrays reduced with a set distance.
As you increased distance, its not linear it is actually exponential
LINEAR ATTENUATION COEFFICIENT
determines HALF VALUE LAYER
Tissue thickness to HALVE no. XRs
HIGHER LINEAR ATTENUATION COEFFICIENT = SMALLER HALF VALUE LAYER
INCREASING photon ENERGY
REDUCES the Linear attenuation coefficient (less interactions at higher energies)
Linear attenuation coefficient increases with DENSITY.
So higher energies gives you LESS CONTRAST as degree of transmissions through different densities become similar
- MASS ATTENUTATION COEFFICIENT
- Takes into consideration the MASS and AREA being exposed to XRAYS
it ACCOUNTS for density
MAC = linear attenuation coefficient / density (ρ)
- Back to Half Value Layer (cm)
INVERSELY PROPORTIONAL to linear attenuation coefficient
HVL = 0.693 / μ
Photon reduction factor = 2 ^ no. Half Value Layers
- DIGITAL RADIOGRAPHY
Indirect digital radiography requires:
Scintillation = converting XR energy into light
Computed Radiography and Direct Digital Radiography does not require Scintallation
- COMPUTED RADIOGRAPHY uses a cassette and an analogue to digital converter
Detector = Barium Flurobromide crystals doped with Europium -> disrupts crystal structural integrity -> creating F centres = Positively charged Flouride ions
Disrupted structure generates
- Valence Band = low energy state with lots of electrons available
- Conduction Band = Theoretical energy level in order to reach it
XRs oxidise Europium, released electron REDUCES F+ to F
Latent image is the F and Eu 3+
The last bit is the READOUT
Using 700nm red light = oxidises F to F +
Release of energy is in BLUE light spectrum 400-450nm
Blue light detected with a PHOTOMULTIPLIER and RED FILTER
This therefore uses PHOSPHORESCENCE
Delay in XRAY to light conversion
IMMEDIATE XRAY to light conversion would be FLUORSCENCE
Back 15. DIGITAL RADIOGRAPHY
- Indirect - XRs need light conversion - SCINTILLATION LAYER
- Direct - XRs directly into digital signal
Uses Cesium Iodide - photon converted into light funnelled down a colour to the film = better spatial resolution
15A. INDIRECT Radiography = Charged Coupled Device = CCD chip
3 Layers
- Scintillation layer made of CSI
- COUPLING LAYER - using fibre optics down to CCD chip or lens that can focus
- Crystalline Silicon with dexals = same as pixels with voltage gates
Signal drop off = secondary quantum sink, loss of energy as it goes down, also when it is focused onto a smaller area.
The more electrons that accumulate in each dexel = the DARKER it will be.
15B. INDRECT Radiography = THIN FILM TRANSISTOR ARRAYS
3 Layers again:
- Scintillation layer made of CSI
- PHOTODIODE LAYER (hydrogenated amorphous silicon)
- Thin film transistor (TFT) array
Making up a “DEL” = Detector Element
Bottom right = capacitor - STORES CHARGE
Top left = switch = CAN RELEASE CHARGE
Capacitor charged stored proportional to electrons released from scintillation.
Wasted electrons - Fill factor about 80% better resolution and less signal loss
15C. DIRECT DIGITAL RADIOGRAPHY
Also uses a DEL layer
But has an AMORPHOUS SELENIUM SEMICONDUCTOR LAYER on top
- Striking -e’s generate ION PAIRS
- charge differential is created with a positive anode end at the TOP
-negative CATHODE adjacent to TFT array
Moving -e’s generate an ELECTRON HOLE where other -e’s move in
THIS CURRENT FROM THE HOLE is measured by the TFT array
EXCELLENT SPATIAL resolution
But cannot deal with HIGH ENERGIES
So good for mammography - compressed breast tissue with low energies = you can use lower energy x-rays with good contrast
- SCATTER
- Decreases contrast in image
- Increases background noise
- Increases occupational exposure
Scatter to primary ration
Or Fraction of scatter
Measures of scatter = doesn’t take into account photelectric effect so NOT a direct measure of image quality
Increasing tissue thickness increases scatter
Collimating to reduces the scatter and patient dose
Tissue thickness and field size both increase scatter
INCREASING ENERGY Increases scatter = less photoelectric effect and Compton scatter predominates
- SCATTER REDUCTION
- Primary XR Beam enters
- EXIT XR Beam leaving patient
Collimation using lead shields
Reduced field Size
Reduce Patient Dose
Eliminates Scatter
High proportion of scatter = blurs the imae
Tissue compression gives better contrast and reduces scatter
AIR GAP =
Increasing object distance from detector means some of the scattered photons would not reach detector
BUT
1. You magnify
2. Get geometric blurring
Anti-Scatter Grids
- Multiple septa - usually lead
- Scattered photos NOT PARALLEL to the grid will be attenuated
Non-linear grids in line with divergent beam
Issues with grids!
h = Height = Grid Thickness
W = Between septa = interspace material
t = SEPTAL THICKNESS
GRID RATIO = H / W - iNCREASES IT reduces photons getting through
BUT with these grids - in order to keep the exposure the same YOU INCREASE dose to patient
- XRays released in isotropic manner
- Focal spot is not a single spot but has some size
Reducing source to object distance REDUCES geometric blurring
(Remember geomtric blurring is because the focal spot is FAT)
Magnification = SID / SOD
Source to image distance / Source to object distance
SID / SOD
How to decrease magnification
- Increase source to object distance
- OR DECREASE SOURCE TO IMAGE DISTANCE
Decrease SID
iNCREASE SOD
Reducing anode angle reduces focal spot
SO REDUCES GEMETRIC blurring and the smaller the penumbra
Geometric Blurring = f x (OID/SOD)
f = Focal spot
OID = Object to image distance
SOD Source to image distance
