Radiology Physics

K-Edge

  • Simple explanation:

    • All contrast agents have a certain number where they release energy which then allows them to absorb X-Rays and therefore produces the best images. The number that is needed to get the electron to release the energy which makes the best pictures is called the K edge. “K” because that is the outermost electron shell where the first round of electrons are released. Technically there are “numbers” for each electron shell but because K is outermost it will be the first and is most commonly used.

    • Essentially the value at which the most XR are absorbed and therefore the best images are produced.

  • Complex nerd answer

    • Energy level at which the contrast media undergoes a transformation which allows for more XR absorption

    • The contrast can absorb more XR at this level because the incoming XR provide enough energy to release the electrons from the K shell

    • Setting machine settings to the level of the K-edge allows for clearer images without needing higher radiation doses

  • Artifact

    • Data error in the K-space will result in striped (banding) artifact

      • Spacing and orientation of the bands will depend based on where the error is in the k-space

    • Error closer to center of k-space = banding lines will be closer

    • Error in k-space on X-axis = longitudinal banding lines

    • Error in k-spine on Y-axis = horizontal banding lines

    • Error in k-space on X & Y axis = diagonal banding lines

Beta decay

  • Neutron-poor nuclei

  • Proton converted to neutron with emission of a positron (beta plus particle) + neutrino. When the positron has lost all or most of its kinetic energy it will react with an orbital electron, leading to the annihilation of both particles and emission of 2 511 keV photons.

Foundations of Physics Part 1

  • Energy

    • Measured in Joules = heat —> typically used metric for body

    • Measured in Volts —> used when talking about bolts (how much energy an XR itself has)

  • Power

    • Energy/unit time

    • Measured in watts = (J/s)

    • 100 W = 100 J/s

    • 100 kW = CT scanner

    • 10 kW = portable XR

  • Electric Potential (EMF)

    • Voltage

    • Potential energy

      • If we lift a box above the ground it has high potential energy because we can drop it

      • In electricity we can basically put negatives next to each other then apply a field and let it flow the way it wants

      • The potential is how high the box is raised or how different the pos to neg shit is

      • 1 e-in 1V = 1 eV (electron volts)

  • Current

    • Flow of electrons

    • Measured pos to negative (even though we care about it flowing the opposite way)

    • Measured in amps (amps is too big so typically measured in mA)

    • Tube current measured this way

  • Atomic number = number of protons = Z

  • Atomic mass = number of protons + neutrons = A

  • Electrons shells

    • Innermost: K - L - M - N Outermost shell

    • 2nsquared = number of electrons in each shell where n is the shell number (K =1 , L = 2, etc)

    • Binding energy = highest at inner shell and lowest in outer shell

      • How much energy needs to be applies to knock the electron out of the shell

  • Electromagnetic Radiation

    • MR is only time we really think of this as a wave

      • Otherwise think of it as a packet of energy hitting people

    • Wavelength X frequency = constant = speed of light

      • So wavelength and frequency are inversely related because the number has to be the same constant

      • Wavelength

        • Distance between equivalent points (peak to peak for example)

        • Measured in distance (meters for example)

      • Frequency

        • Number of points flowing past a point

        • Measured in Hz

    • Higher frequency = higher energy

    • X-ray vs gamma ray

      • Not the energy they have (because you can have high energy XR)

      • Difference is that gamma ray come from nucleus vs XR comes from electrons

  • Ionization

    • Means you can knock an electron out of the atom

      • The electron then bounces off shit and hits DNA and causes damage and carcinogenesis

    • In order to have ionizing radiation ( in order to knock off that electron) the photon energy > binding energy

  • Power (W) = Voltage X Amp for how much power it can maintain for 0.1 s

  • XR tubes

    • Cathode side (filament) = where electrons come from

      • This heats up a wire which makes heat and causes electrons to come out

    • Anode (contains focal spot or target)

      • Positive side

        • Tube voltage (kV) is the potential we choose

          • This makes electric field and the electrons flow toward the anode and hit the target which makes the XRs

    • mA = how many electrons/sec are flowing from cathode to anode

      • We can choose this as well

    • 90% of the energy made becomes heat, only 1% is used for imaging

    • When electrons get set free they can do two major things

      • Bremsstrahlung

        • Electron his target and emits a photon

Foundations of Physics Part 2

  • When an XR or gamma ray gets thrown at a surface and hits it, it will undergo one of the following

    • Coherent reaction

      • Causes scatter

      • Does not deposit any energy (so no dose to patient from this)

      • Minimal role

    • Photoelectric effect

      • photon comes in, interacts with electron and gets completely absorbed

        • Causes binding energy to be reached and electron leaves energy shell and then gets transmitted to kinetic energy

      • Absorption of XR or gamma ray by an electron and then electron fucks off and does whatever it does

      • If <25 keV in soft tissue, or <40 in bone = this is major interaction

        • if > 25 or > 40 —> Compton

      • Can only occur if the energy of the photon is more than the binding energy of the electron it is hitting

      • PE = Z^3/E^3

        • Lead has high Z = so good at blocking shit

        • Calcium also has a decent Z in terms of material in the body

    • Compton scatter

      • XR/gamma ray hits out shell electron —> election ionizes and is sent off, the remaining ray continues on but has less energy since it have some to the electron

      • Compton = electron density/E

        • E = energy of the ray

      • Note scatter results in worsening contrast (higher energy = more scatter = worse contrast)

    • Backscatter

      • adds 30% to skin dose (for A&P CT)

      • Ray hits organ and reflects backwards and hits skin again causing increased dose to skin

    • Attenuation

      • Ability of material to block or scatter an incoming photon

      • high attenuation = less material needed to block radiation = better at blocking shit

      • Increased Z = increased attenuation = most important factor

      • Increased density = increased attenuation

      • Increased energy (more penetrating) = decreased attenuation

      • N = N0 (e^-ut)

        • t = material thickness

        • Basically the number of electrons coming out is equal to the number of photons coming in times the number of shit that gets block

          • e shit = percentage of shit that is not attenuated

      • What percent of photons are transmitted through 1 cm of tissue (u = 0.1)

        • 0.1 x 100 = 10%, so 10% blocked and 90% get through

  • Half Value layer

    • how much material is needed to block half the beam

    • at 80 kV you seen 3 cm (30 mm) of soft tissue

      • so at every 3 cm of soft tissue you will lose half the beam assuming a beam of 80 kV

        • As beam energy goes up you will need a thicker piece of tissue

  • K-edge

    • At low energy alot of shit gets blocked

      • The low energy is not enough to ionize an electron

    • then you get a jump

      • Because you just hit just enough energy (the bindng energy of the material in the k shell) which frees the electrons

      • Y axis = attenuationn

      • X axis = energy of photon

      • zig zag graph in photo

    • K edge is due to photoelectric effect

    • Where is this relevant

      • You want the goldilocks of just right energy

        • lower energy = dose and no image

        • high energy = bad too

      • When you use a filter with special traits of some material it filters out the bad shit tpget that sweet spot for imaging and dose

      • Also seen with iodine and barium

  • Kerma (quantity energy of beam)

    • kinetic energy released in matter

      • basically amount of energy your incoming ray gives to the electrons

      • Dose of material </= K material

      • typically proportion to the number of photons

    • Abdomen = 5 mGy

    • mGy when hit spatient, uGy that hits receptor becuase body eats most of it

  • Beam quality

    • penetrating ability of beam

    • Higher half value = high quality beam (harder beam)

    • High energy beam

    • Filtered beam = higher beam energy = higher quality

    • Higher energy (increased kV) =

  • Tube voltage

    • Energy and number of photons increases as voltage increases

    • As voltage increases

      • number of XR increases

      • average brehsstrahlung energy increases

      • photon beam penetration increases

    • Optimum tube voltage for angio of small vessels ~70 kV

      • Average effective energy f ~ 35keV

        • Iodine k-edge = 33 so you are just above it

  • Filtration

    • get rid of low energy photons

    • Filtration = reduced dose

    • Image stays the same because the lower energy photons weren’t going to hit the receptor anyway

  • Characteristic XR are the little peaks on the curve

    • Will change based on the target material

Foundations of Physics part 3

  • Primary beam

    • beam that gets shot out and hits detector and patient

  • Leakage

    • Small dose that hits the tech/doctor/whomever because it didnt come out of the tube perfect

  • Scatter

    • Dose that hits patient and bounces all over

    • This is the major source of dose to the doc/tech

  • Scatter in detail

    • Compton scatter - major effect

      • Gives dose to patient

    • Coherent - less effect

      • No dose to patient, all gets bounced off

    • The patient is the major source of scatter because that what the beam is hitting for shit to bounce off of

    • REDUCES CONTRAST OF IMAGE

      • Makes everything more gray vs black and white

        • Makes the whites darker and the black lighter

    • Factors involved

      • FOV

        • Increased FOV = more scatter

      • Thickness

        • Increased thickness = more scatter

        • For typical A&P = 5 bad photons (scattered photons) for every one good photon

      • Tube Voltage

        • Increased TV = more scatter

        • Note higher TV = more compton and less photoelectric effect

    • Note: if you move the object away from the detector then the scatter will bounce around but most will not hit the detector

      • Vs if the object is close to the detector there is no room for the scatter to go anywhere else so it will all hit the detector and get a shittier image

      • This distance is the air gap I believe

  • Grid

    • 70% of primary beam gets through

    • 90% of scatter absorbed

    • Grid ratio = thickness/density (normal is 10-14)

    • Bucky factor

      • Basically since the grid is filtering out so much shit, the BF is how much extra radiation needs to be given to keep the image quality the same

    • Use a grid for patient thickness of 10 (10-14 cm) cm and above (only exception is mammo)

  • Magnification = SID/SOD

    • SID = source to image distance

    • SOD = source to object distance

  • Characteristic curves

Foundations of Physics Part 4

  • Digital detectors

    • Integrating detectors

      • basically records how much energy hits them

    • Scintillator (indirect)

      • Crystal on top of detector element (dexel or dell aka)

      • XR hist crystal —> absorbs energy and emits light (photons) at a lower energy

        • crystal is high Z (cesium-iodide)

        • Thin crystal = less spread of photons when they come out = less blur = better spatial resolution

          • But less XR absorbed so need more dose

        • Thick crystal = more spread of photons = more blur

          • Absorbed more XR so less dose needed

      • Photons hits detector elements (photodiode) and turns it to electron

      • Electron stored

      • Cant have XR just hit detector because would go through it idk

    • Photoconductor (direct)

      • XR hits amorphous selenium (A-SE) and you get electrons (the A-Se is basically your crystal for this system)

        • Direct because XR to electron directly

      • Electrons then drawn to positive charged detector element

      • Higher spatial resolution because so light spread

    • Photostimulable phosphors

      • original digital systems

      • XR hits plastic that traps electrons and excites them —> so they are trapped in excited state

      • hit the plastic with a laser that frees the electron which is then picked up by the detector

      • Red light beam hits the plastic, blue or green light is color that is picked up by detector

  • Digital images

    • Pixel is basically one square that is given a number which corresponds to density or some shit

    • Bit depth

      • How many numbers can a pixel store

      • Cant store infinite numbers in a pixel because too much memory

      • 1 bit depth = 0 or 1 are only options (black and white basically)

      • 2 bits = 4 values =

      • Max value = 2^bitdepth

      • 6 = 64

      • Most medical images are 16 bit depth = 65,536 possible values

      • 1 byte (memory) = 8 bits

      • Each pixel needs 6 bits

      • 10 mega bite per image (radiograph)

  • Matrix size

    • Pixel size = how much size of the image does the pixel represent

    • Smaller pixel size = better picture = less blur

    • Larger pixels = more different colors and you have to take the average so makes the whole image blurry af

    • Typical radiograph = 2000 x 2500

    • CT = 512 x 512

  • Displays (monitors and shit)

    • Megapixels

      • Diagnostic monitors = 3MP

      • Mammo = 5 MP

      • Mp is for how muich of the image can you see at native resolution (1 pixel in matrix = 1 pixel in monitor) without having to move image around to see everything

      • Not about zooming

      • Zooming in is to help your brain focus it or some shite

    • Display contrast

      • Luminance = high levels is good

        • how much light thrown out

        • measured in candella/m2

        • Diagnostic = 350 cd/m2 (minimum)

        • Mammo = 420 cd/m2 (minimum)

      • Illuminance = high level is bad

        • Reduces contrast

        • Light rom outside shit hitting the monitors and bouncing into your eyes

        • If you bring monitor outside into sun and you cant see shit because its too bright

        • hence why we are in a dark room = less light = less

        • measured in lux

  • DICOM GSDF

    • bcasically standard so that images look same on all monitors

  • SMPTE

  • Low Pass filter

    • Get rid of noise but lose spatial resolution

    • low pass = blurry

    • Blurry because averages pixels together so less contrast next to each other and becomes this amorphous blur

  • Unsharp masking

    • Accentuates edges in image

    • Take original image and subtract a blurred version of the image so you are left with just the edges

    • Then take the edged image and add back the good version

    • Want this when looking for lines because can see line border better

    • Increases noise though

  • Dual energy

    • Low kV (low energy) some photoelectric and compton

    • High kV = minimal photoelectric and mostly compton

    • PE has most with high Z materials = bone, so do some subtraction and shit so you can get a pure bone and soft tissue image

  • v

Foundations of Physics Part 5

  • Contrast

    • Subject and image contrast

    • The difference in received signal

    • Higher contrast = better

    • What affects contrast

      • Z of the material —> high Z block more radiation

      • Density of material = high density blocks more

      • Scatter —> more scatter means worse contrast because basically levels the playing field of photons so everything because the same shade of gray

    • At higher energy

      • Material attenuation becomes more similar

        • If you were to have infinitely high energy then all the photons would penetrate and go through the material so doesn’t matter

      • Higher proportion of scatter

    • Contrast = (object - background) / background

    • Basically higher contrast means larger range

    • Contrast affected by tube voltage (kV)

  • Latitude and Dynamic Range

    • Wide latitude = less inherent contrast

    • For a display = range of how dark to how bright it can put out

    • For a detector = range of dose that can be picked up and reliably recorded

    • Wider range = better

  • Contrast agents

    • Positive agent = brighter = gadolinium, microbubbles on US, iodine

    • Negative = darker = air?

    • Neutral = water

  • Noise

    • how grainy is the image

    • less noise = better

    • Noise = 1/ square root of dose absorbed by image receptor

    • Low pass filter will reduce noise —> blurring filter that will reduce noise but also reduced spatial resolution

    • If source to image distance double the noise will double

      • Dose = 1/distance squared

        • Double the distance = quarter the dose

      • Noise = 1/sqrt Dreeptor

      • 1/sqrt1/4 = 2

  • XR Noise sources

    • Quantum mottle

      • Mottle is reduced by increasing tube current

        • Gives more photons, so more can get through pt and more hit detector

      • Controlling mottle

        • Increase dose = decreases noise = less mottle

        • Voxel size —> thinner slices and smaller pixel sizes = more noise

        • Image processing

      • High mottle will affect your ability to see low contrast items the most

    • Electronic

    • Structured

    • Anatomic

  • Quantum limited

    • Number of photons is what determines how much noise is in the image, basically means limited by photons and shit

    • More photons need to be captured to reduce the noise

  • Image quality is task specific, it depends on what you want me to do with the image

  • If you quadruple mAs = receptor dose will quadruple

Foundations of Physics Part 6

  • Spatial Resolution

    • Blur basically

      • WWhat affects blur

        • Motion artifact

        • Acquisitiont ime

        • Temporal resolution - faster image is obtained, less time for motion = better resolution and less blur

        • Focal spot blur

          • Larger focal spot = more blur

          • Smaller spot is better but then more heat

    • Bar phantom -

      • Looks at line pairs which is a piece of plastic with strips of lead on it. The lead strips are placed closer and closer together to see at what level we can no longer differentiate the lines from just a large block.

      • Eye cam see 5 line pais per mm at an arms distance

      • Limiting spatial resolution is the distance where you can no longer differentiate the lines

      • One line pair is one dark and one light image

  • Aliasing

    • Basically if you have too many of the dark and light lines per pixel it is too much and the machine or whatever merges it and you get a blur that looks like one line

    • Nyquist theory = need to sample at 2x the higher frequency

    • Want 1lp/mm then you need 2 pixels per mm

    • 2lp/mm = need

    • Have not taken enough samples

  • Line spread function

    • Go to tip of bell curve then go down half way (full width half maximum) and see the width = wider = more blur = worse spatial resolution

  • Modulation (contrast) transfer function

    • Do not evaluate contrast they evaluate spatial resolution

    • Idk man take the ratio of LP/mm (spatial resolution) and plot it, if you don’t do this the lines get blurred and you just see a blob

    • limiting spatial resolution is taken at 10% level - whatever the LP at this point is the spatial resolution, anything less you cant see

  • Contrast to noise ratio

    • Higher = better = easier to see shit

    • Relative measurement

  • Signal to noise ratio

    • Absolute measurement

    • SNR > 3-5 = can see shit, if less then cant see shit

Solid State Dosimeters

  • How do the badges work

  • Something like light or radiation hits the badge and causes a change in energy levels and then you’ll have to hit the badge with a laser beam and see some shit idk

  • Inside badge there is a packet of shit that absorbs the shit

  • Geiger counter

    • Very sensitive but cannot tell you how much dose

  • Shallow dose equivalent = basically dose to skin

  • Lens dose equivlanent = dose to eyes

  • Deep dose equivallent = dose to body = essentially effective dose

  • Total effective dose equivalent = deep dose + committed dose which is basically if you ingest something i guess and then this because the effective dose, if there is nothing eaten then TEDD will be same as DDE

  • Occupational limit (effective dose)

    • 50 mSV/year

  • Lens dose limit = 150 mSv/year equivalent dose

  • Extremity, any organ and skin= 500 mSv/year equivalent dose

  • Public ceffectiev dose = 1 mSv/year

  • Pregnant workers = if not declared then assumed not pregnant

    • If says pregnant then 0.5 mSv/month (5 mSv total) equivalent dose

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