Mersey XR Flashcards
X RAY PRODUCTION
BREAKING RADIATION
MAX ENERGY = TUBE POTENTIAL
Characteristics XRAYS for tungsten
X-Ray Tube
A LOT OF HEAT is produced
Thermal conduction dissipates heat
- down anode shaft
- surrounding mineral oil for thermal INSULATION
-Surrounding lead shield absorbs x rays
Inherent filtration is 0.5-1mm Al
There will be SOME leakage radiation
RADIATION PROTECTION
Berilium - lower atomic number - used as a window for lower energy mamography
Normal XR - Aluminimum
CT - Copper and Tin
Mammography - Molybdenum or Rhodium Filters
RAYLEIGH SCATTERING
NO ENERGY TRANSFER
ONLY OCCURS AT LOW ERGY IN SOFT TISSUE
<10% OF XRAY INTERACTIONS
Photoelectric effect - provides contrast
1kEV soft tissue characteristic soft tissue x ray
Compton Scattering
Small scattering angle - x ray still has most of its energy
Larger scattering angle - x ray loses or transfers MORE energy
Graph showing P() of angles at different energy
At LOW energy = scattering through larger angles and smaller angles
You get LESS backscatter at HIGHER energies
P() of Compton scattering LOWERS with higher energy
INCREASES with MASS DENSITY (ρ) and number of electrons per gram
X-Ray interactions in tissues
In soft tissue >25KeV - Compton Scatter predominates
Attenuation
Linear Attenuation coefficient
- increases with density
- decreases with energy
LAC/DENSITY - MASS ATTENUATION COEFFICICIENT
Comparing air to soft tissue and bone
K edge filters - used to remove high energy x rays to IMPROE CONTRAST
K edge filters preferential attenation
K EDGE OF IODINE = 33.2kEV
So you want x-ray energies ABOVE this
So here’s a typical setup that you would use for plane or X ray imaging. You’ve got the X ray tube, obviously your patient supported on imaging table. Underneath the imaging table is your x ray image receptor, which we’ll look at. There’s different types an x ray image receptor. In between the table and the image receptor, there are two, two other objects that are often used. So there’s ionization chambers which are which are used to automatically control the exposure of the X ray. So the the AEC, as it’s often referred to, there’s that these ionization chambers are interspersed between the detector and the patient, and in some studies. There will also be an anti scatter grid placed between the patients and the detector
99% of the energy that they have gets converting converted into heat, and only about 1% gets converted into X rays. - thats why it rotates
the X rays are emitted from the target, from an area known as the focal spot. So the focal spot is where the electrons from the cathode interact with the target So the focal spo
SMALLER ANGLE = SMALLER EFFECTIVE FOCAL SPOT SIZE
Larger real spot = more heat dissipation
The target angle affects the effective focal spot size, leading to a smaller focal spot size with a larger heel effect.
More self attenuation in direct of ANODE
Tungsten has got a high atomic number and a high density, which makes it a good absorber of X rays, a good producer of X ray.
BUT MORE HEEL EFFECT at SMALLER TARGET angles
Heel effect MORE APPARENT at LOWER energies due to INCREASED PHOTOELECTRIC EFFECT.
X rays emitted towards cathode have less self attenuation, resulting in non-uniform intensity along axis.
Heel effect is more noticeable at small target angles, exaggerating non-uniformity at low tube voltages.
Spatial Resolution
Measurable ways to say it’s the ability to resolve objects as separate entities.
How far apart do you need to have two objects for them to be seen as separate in the image. So that’s that’s a distance that you can measure.
The size of the penumbra determines the space resolution in plain radiography, with smaller penumbras resulting in higher resolution.
This is what the test patterns typically look like, a series of attenuating and non attenuating rectangles of different thicknesses and separations. So with each of these, we have what’s known as a line pair. We have an non attenuating and attenuating rectangle of the same width and the same separation, and then you go through the next pair same with separation and so o
So in spatial resolution, in X ray imaging is typically referred to in this in this way. Line pairs per millimeter.
So typically you will get planar X ray imaging with resolutions of five line pairs per millimeter, that sort of magnitude.
BROAD = 1.2mm
FINE = 0.6mm
Focal spot size is actually selected by choosing a different coil. in the cathode.
Focusing also helps to direct electrons into a narrower beam as they have a tendancy to divge
The size of the penumbra determines the space resolution in plain radiography, with smaller penumbras resulting in higher resolution.
The focal spot size and source-to-image receptor distance are important geometric factors that affect the size of the penumbra.
X RAY IMAGE RECEPTORS
Instead, the three sets of detectors that we’re interested in for planar X ray imaging are
COMPUTED RADIOGRAPHY
Imaging plates with phosphor in a casette
Expose the imaging plates with X rays.
Electrons within the phosphor within this barrier of fluoro halide, electrons are excited to higher energy states and the trapped there. These electrons jump up in energy to a different energy states, and stay the, they don’t, they don’t drop back.
The READ OUT
Red light stimulation
Blue light released and detected by photomultiplier, amplifier and ADC
Plate erased by bright white causing all remaining -e’s to drop back
ADC conversion
Continuous analogue signal from photomultiplier tube. Each signal stored as a digital value in each pixel.
Sampling pitch = pixel size
Sampling frequency = pixels per unit length = 1 / pixel size
- CR Plates
Matrix size = no. pixels in both axes
Cesium iodide and gadolinium sulfide are common scintillators used in X-ray detectors.
Smaller imaging plate with same matrix = smaller pixel = better spatial resolution
Spatial resolution:
CR imaging resolution depends on sampling frequency, pixel size, and laser beam width.
so space resolution in CR imaging depends
- the sampling frequency, the pixel size during the read out process.
- by the size of the laser beam that’s used in the reader. if you have a very broad laser beam,
- the scattering of that laser light within the phosphor. So the thicker the phosphor is on the plate, more scattering of laser light will occur, and then the divergence of the light that’s emitted from the plate.
So digital radiography using flat panel detectors is another type of imaging detector that we use in planar X ray work.
Flat panel detector is a matrix of individual detectors. Cr play is just one continuous detector that’s digitized.
Digital radiography consists of a matrix of detectors, and you detect a signal that each individual detector to produce your image.
ith CR plates. You normally just stick they go under the imaging table in a holder, these different sized imaging plates. You take them away to read them out with flat panel detectors. They’re usually part of the X ray system. So it’s an example of one sort of fixed in place, or it can be fixed in place underneath. When you irradiate them, you don’t need to read out. You don’t put them in a reader. Afterwards, they produce a digital signal, almost immediately connected to a computer and produce a digital signal. There’s two types of the flat panel detector, indirect, direct. So look at each of those
INDIRECT FLAT PANEL DETECTORS
XRAYS to LIGHT to ELECTRICAL
Indirect flat panel detectors are better for general radiography and fluoroscopy due to lower patient doses and higher resolution images.
Different phosphors, different scintillators that you can use. Cesium iodide is, is a common one, gadolinium, oxysulfide is another one.
A continuous piece of scintillator, and it sits on top of matrix of photo diodes. So a photodiode is just a light detector
light that’s emitted from the scintillator will be detected by these filter diodes.
A matrix of light detectors detecting light from the scintillator.
The laser light scattered within phosphor spreads out, causing blurring of signal.
DIRECT FPDS
Direct conversion of XRs into electric current
The direct flat panel detector converts X rays into electric charge directly, without emitting light, and collects the electric charge through a high electric field on a transistor array.
Direct flat panel detectors have a lower divergence of signal compared to indirect flat panel detectors, resulting in higher resolution available.
AMORPHOUS SELENIUM - PHOTONCONDUCTOR
PIXEL SIZE = TRANSISTER SIZE
Comparing the two
Indirect = better with higher energies = lower patient dose
Direct = better with lower energies = better in mammography
Image intensifier
X-Ray Source
Detector = Image intensifier = detects WEAK XRs
Flux and minification gains determine brightness gain, with accelerated electrons producing more light at output phosphor.
So is side a schematic view side through of an image intensifier. X rays are entry through a very thin entrance window here, just on the inside of that there’s a phosphor or a scintillator, if you like, typically, caesium iodide that converts X rays into light. The light is then incident upon a photocathode, which converts the light into electrons. And the electrons that are liberated from here are accelerated by series of electrodes, they’re accelerated towards an output window. So these electrons, they’re generated accelerated by these electrodes.
And the electrons that are liberated from here are accelerated by series of electrodes. They’re accelerated towards an output window. So these electrons that are generated accelerated by these electrodes, you’ll notice that the output window, the output phosphor, is much smaller than the input phosphor, so we’ve got a sort of a concentration of energy going from a wide area to a small area, and that produces that helps to produce image brightness helps to amplify the brightness of the signal from here going down to here. So when these electrons reach this output phosphor, they convert their kinetic energy gets converted into light, so we get a visible image produced on a much smaller screen. that reflects what’s happening on the input screen.
Gives BRIGHTNESS GAIN - brightness between two screens
FLUX GAIN = acceleration by electrodes
MINIFICATION GAIN = Output image is much smaller = square of input screen diameter / square of output screen diameter
BG = FG X MG
INTENSIFIER ARTIFACTS
Pin cushion distorsion = Non-linear pathway of accelerated electrons through intensifier
Vignetting = image brighter at the centre
Veiling Glare = depicted here is a lead disc, which should be totally attenuating, so we shouldn’t get any signal. This is the profile of signal intensity through through the central line. We shouldn’t get any signal here, because this is totally attenuated led disc. But you’ll see that there’s a signal.
Magnification in Fluoroscopy
Magnification REDUCES MINIFICATION GAIN = reducing BRIGHTNESS GAIN = COMPENSATING BY INCREASING XRAYS
Digital subtraction angiography
MASK subtracted from pulse contrast or LIVE images
LOGARITHMIC SIBTRUCTION (due to exponential xray attenuation)
SUBTRACTION PROCESS = INCREASED NOISE
Uses >70kEVP as it uses iodine
Digital subtraction angiography
XRAY CONTRAST = VARIATION IN BEAM ATTENUATION
What we mean by contrast? First of all, contrast is the difference in sit and strength between two parts of an image. Obviously, if we’re going to detect features in an image, we need to have contrast, differences in signal between what we’re interested in and what we sort of our target features and background features.
Higher energies, there’s less attenuation,
More attenuation if high density or atomic number
Radiographic contrast
Amount of scattered radiation that reaches the image receptor will interfere with the amount of contrast that we can see, so we may need to remove scattered radiation. A
The actual display information on the computer screen, the way it’s displayed can affect our perceived contra
Subject contrast
Subject contrast
So we’ve got 6x rays in this schematic interacting with long 6x rays going through bone, six through soft tissue. You’ll notice that in each case, there’s a different number of X rays being transmitted through lung is a very weak attenuator. So most of the radiation pass through the soft tissue, there’s partial attenuation, and through bone, there’s much greater attenuation
Effect of Kevp
Effect of scatter
About two thirds will be absorbed in the patient as radiation dose, and only about 1% actually gets the image receptor.
Controlling Scatter
So if we can reduce the volume of tissue that’s irradiated, we will reduce the number of scattered photons being produced. So we do that by making the beam smaller. So we use beam coordination to make the beam area as small as possible. So that makes the area of the beam small, and we if we compress the tissue, we make the thickness of tissue that the radiation is passing through smaller as well. So these two techniques are simple ways of trying to reduce the amount of scatter that’s produced. So tissue compression is used principally in mammography. It’s used routine mammography, and that’s why to reduce the amount of scatter and enhance contrast.
Scatter Vs kVP
Higher energy XR = FEWER SCATTERED EVENTS = BUT MORE FORWARD SCATTERS = SO MORE GET TO THE DTECTOR
Increasing KVP leads to fewer scatter events, but those that do occur are more likely to reach image receptor.
Scatter Grid = series of attenuating strips
Lower grid ratio = less scatter
Grid ratio = H/d
Radiographic noise
Unwanted variation in signal intensity - INCREASE NUMBER POF XRAYS
Mammography
- Compression of breast to reduce thickness
- Xray tube angle cathode side along chest wall = where beam is stronger through thicker tissue
- Lower tube voltage 25-35KVP
- Mo Rh
looking at radiation output from X ray tubes, the focal spot is effectively a point source of X rays, which means we can use the inverse square law to determine the intensity of radiation with distance from the fogged spot. So the beam radiation is emitted in all directions. So the radiation field diverges with distance from a point source. The Inverse Square Law says that the intensity of the radiation falls off with the square of the distance that you are from the source. So this makes distance a very good way of protecting yourself against radiation. Coronavirus in lecture, looking under the radiation output from an x ray machine
Radiation output from XRAY tubes = Measured in AIR KERMA
Kerma = TOTAL ENERGY liberated by PE and CS events events per unit mass of air - mGy
Air Kerma Rate mGy/s = measured by IONISATION CHAMBER
Intensity Vs KvP - proportional to voltage SQUARED
Minimum dose to receptor to get useful image
Low KVp - higher ESD, Low penetration
Higher kVP - lower ESD, higher penetration - BUT LESS CONTRAST
Intensity Vs tube current - proportional to current
Entrance surface dose
DOSE AREA PRODUCT
Flat ionising chamber measures dose x beam area to measure how much going to patient
DOSE NOT VARY WITH DISTANCE
DAP is independent of distance from X-ray tube, making it useful for measuring patient dose
PATIENT DOSE PARAMETERS
DAP or kap
Reference Air Kerma
Entrance Surface Dose
Organ Dose
Receptor Dose
