XR Physics

Lecture 2

  • Source to image distance

    • Large SID used in fixed CXR because it reduces magnification

    • Closer source is to the target (patient) = the more the structures will be magnified

    • Same thing as PA vs AP

    • So if you have two masses, one in anterior chest and one in posterior chest they will look different sizes

  • Contrast

    • Contrast is basically how different are the gray levels

    • Better contrast is achieved by

      • Lowering kV

      • Lowering FOV

      • Post image processing

  • Noise = Quantum Mottle

    • Basically differences in gray color for same material

    • Decrease noise = better image = achieved by

      • Increase mAs = more radiation to detector

      • Increase kV = more radiation to detector

      • Post image processing

  • Spatial resolution

    • How much blur is there, ability to see small objects

    • Improve (increase) spatial resolution via

      • Decreasing focal spot size

      • Decrease pixel size (decreased detector element size)

      • Magnification has varying effect

  • Detector Pitch

    • Detector element size

    • This is a small detector that says how much radiation is hitting it and gives you an average

    • If you have one then will average all the radiation hitting it = no bueno

    • More detectors = can see smaller shit = better spatial resolution

  • Magnification

    • Increasing focal spot = more blur

    • Decreases detector blur but increases focal spot blur

    • Seems important - maybe look into this more

  • Contrast to noise ration

    • If you decrease FOV = less scatter = better contrast (noise unaffected) = better C/N ration

    • Focal spot only affects blur

    • Decrease tube current = more noise = worse C/N

  • dN = 1/sqrt of dose = Need to know this

    • As dose to receptor goes up = noise goes down

    • dN = change in noise

    • If you have the mAs the noise willi ncrease 40%

  • Leakage - small amount of radiation that escapes machine and doses the non-patient

  • Primary beam - what gives the dose to the patient

  • Scatter - any time the beam hits anything it will get redirected and go other places

    • Includes hitting the patient which is usually the largest source of scatter

  • Exposure index

    • Amount of radiation hitting the detector

    • Typical doses

      • 150-300 for body XR (KUB)

      • 700-1000 = Extremity = want less noise and also more radiation hitting detector because there is less soft tissue and shit

  • CXR

    • 125 Kv (high)

    • 1 mAs (low)

    • 5 ms of time

    • mostly air so

    • AEC used

    • Grid used

    • Exposure index = 3 uGy

    • entrance air kerma (Kair) = 0.1 mGy

  • Portable CXR

    • 80 kV

      • Lower Kv (Kv is the same as tube voltage) because has less scatter, do this because cannot use a grid, cannot use grid because it is bedside and pt cannot be exactly positioned correctly perpendicular and would result in artifact

    • 1 mAs

    • No grid

    • No AEC (it is manual)

  • Adult KUB

    • 80 kV

      • Low Kv = gives better contrast and the abdominal shit is all gray anyway so need better contrast (if fat need more)

      • If go lower than this the beam will not penetrate the abdomen

    • 20 mAs

    • AEC used

    • Grid used

  • Extremity XR

    • Small focal spot = increased spatial resolution = Kv & ma are lower so can get away with it and not have too much heat

    • 55 kV

    • <1 mAs

    • AEC optional -

    • Grid optional

  • Peds CXR

    • lower kV = 60 kV

      • Better contrast and lower radiation

    • lower mAs = 1

    • Basically same as an extremity

    • Exposure index is the same = still need same amount of radiation hitting detector - the dose to patient is different _because they are different size and therefore less absorbed in smaller people (i think?) but dose to the receptor is the same because need to keep same noise and shit the same

  • Artifacts

    • Lag/Ghosting

      • After your take a pic, the electrons stay in detector and are not cleared out, then stuff from the old image persists on the next picture taken

      • Usually with high Z materials (prosthesis)

      • Fix via put acrylic and take blank pic if needed, this usually resolves on its own

    • Dust on laser (CR)/Dead pixel row (DR)

      • Row of detector elements that stop working

      • Looks like a straight line for no reason

    • Poor calibration

      • AEC is seen basically

      • Normally the AEC is not seen because there is calibration or some shit to make them invisible

      • Needs to be recalibrated to fix - called flat field correction

    • Grid Cutoff

      • Need detector to be perpendicular to the grid

      • If it is not then a lot of the good radiation that we need to make the image is blocked by the grid

      • Correct by making beam perpendicular to grid

      • Can lines and if radiation does not hit detector or is blocked by detector by the grid will be white (as if it were bone)

    • Gridlines

      • Multiple horizontal lines - looks like beat bord

      • Should not see normally because one of the follow

        • Grid is shaken so cannot see

        • Some other shit

      • If you see gridlines —> it stopped shaking or post image processing failed

      • Correct via removing grid (or fix the reason it failed, motor to shake not working)

    • Electromagnetic interference

      • Only on digital detectors

      • looks like bunch of lines similar to gridlines

      • Usually because there is a device that interferes with the XR machine by giving out electromagnetic radiation and interefers with sending shit to the computer (such as pain pump or some shit)

  • Effective dose

    • CXR = 0.02 mSv = should know this number

    • KUB = 0.7 mSv

  • d

Lecture 3 - Mammo/Tomo

  • Filter used to get rid of high and low energy photons

    • Higher energy photons will decrease your contrast

  • Grid is used

  • At high kV it is hard to differentiate between different substances because attenuation doesnt change much

    • At lower energy it is easier to differentiate different tissue densities (better contrast) because their attenuation differences are exaggerated (more different, more contrast)

    • 20-30 kV (tube voltage) is about where you want to be

      • Gives us a beam energy after filtration of about 20 keV

  • Tube current 100 mA - typically fixed

    • because we use small focal spot (to keep blur low), it is easy to overhead system with the small focal spot, so we use exposure time to control amount of radiation and keep the mA fixed so as not to overheat

  • Typical exposure time = 500 ms

  • 1 cm HVL (?)

    • less tissue needed to block half the beam because we are using lower energy beams

  • Focal spot size of 0.3 (screeners) to 0.1 mm (magnification views) (smaller than radiography)

  • Need high spatial resolution (means low blur)

    • Need this to see the micro-calcs

    • The spatial resolution in mammo is 2x that as regular radiography (radiography has more blur)

  • AEC

    • Does more than in radiography

    • Will choose target material, filter material, kV and filter time

    • Takes in a bunch of factors like distance and shit to determine other stuff

  • Heel effect

    • Cathode side (side where more radiation comes out) is closer to body because that part of breast is thicker than where the nipple is

    • Cathode/anode will also be tilted toward nipple slightly

      • Prevents all this radiation hitting the chest wall which does nothing and allows you to see breast better

  • Target material - curved line shit

    • Material that radiation hits to create XR

    • Remember 20 keV is target for what we want

    • One way = Mo or Rh

      • Mo = thinner breasts

        • Blocks low energy photons and high energy photons

        • Lets middle energy photons pass through it

        • It does this because of its k-edge

        • Mo target Mo filter for thin breasts

        • Mo target + Rh filter for medium breasts

        • Never use Rh target with Mo filter (i think this is what he said)

      • Rh = thicker breasts

        • Need little bit more higher energy photons to get through the breast

        • blocks low energy photons and very high energy photons but allows some higher energy photons through that Mo would have blocked

        • Will have slightly lower contrast because of this

        • Rh target, rh filter for very thick breasts

    • Other way = W (tungsten)

      • Doesn’t matter breast size

      • Uses silver or Rh for

        • Silver is for thicker breasts (>6 cm)

  • Port window is made of beryllium (low Z)

    • if you used glass too much of the radiation would get blocker

  • Compression

    • Decreases focal spot blur

      • Tissue is closer to detector

    • Decreases scatter

      • Less room for scatter to occur

    • Decreases dose

    • Less overlapping of breast tissue on itself

  • MQSA maximum initially = 25-45 lbs

  • Detectors

    • Csl (indirect)

    • a-Sel (direct)

      • Offers best characterization of microcalcifications

      • Best spatial resolution

        • Does not create intermate light which causes shit to get fucked up

    • MQSA requires a line to be present on the detector

      • Allows you to know where the detector is and therefore where to put the breast over it

  • Image processing

  • DICOM Gray scale display function

    • Basically the image should look the same from one monitor to another, should not be a change

    • Includes luminance, how much light needs to be put out so that you can see a difference in the shade of gray

  • Display

    • Needs to be certified for mammo

    • 5 mp (mega pixel) (5 million pixels)

    • Brightest white that the monitor can give off cannot be below 420 cd/m2 = 420 is lit = need to know this number

    • Typical pixel size = 70 um

      • Benign calc is about 150 um so you ideally want more than one pixel per calc

  • d

Lecture 4 Tomo/Mammo pt 2

  • MQSA mag must be 1.4-2x (same as regular mammo)

  • Mag views in mammo

    • A smaller focal spot must be used

      • less photons can be used otherwise will get too hot

        • so need to use lower mA

          • we counteract this by having 3x exposure time to get enough radiation for the image

          • Exposure time is longer in mag vs regular contact mammo

  • No grid in mag mammo

    • Would increase scatter but we have air gap effect which counteracts this

    • Air gap effect - by leaving a space between the breast and detector there is a normal loss of scatter because the particles that move super lateral hit nothing and dose is also lower because of this

  • Tomosynthesis

    • kV 25-40

    • ~1mm slices

    • Takes 4-25 s to acquire

  • Phantom

    • 4.2 cm compressed

    • Represent 50/50 glanularity

    • 3 objects inside

      • Fibers = architectural distortion = 1.5 mm (0.75 mm is needed to pass test)

      • Calcifications (speck groups) = o.3 mm (0.2-0.3 mm is needed to pass test)

      • Masses = 0.75 mm

    • Phantom is tested weekly

    • To pass MQSA you need to be able to see 4 fibers, 3 speck groups, 3 masses, for acr it is 2-3-2

  • Thinner the breast = better contrast (assuming same glandularity)

  • MQSA stuff

    • Need to keep last study for 10 years

    • Need to keep other studies that are not the last one for 5 years

    • Idea is that you have at least 5 prior studies to compare to

  • Patient dose

    • Mean glandular dose

      • Absorbed dose to the glandular tissue

      • Estimated, not exact

      • Must be <3 mgy

      • This is not the dose to the pt

      • Typical dose is 1.5 (half the max)

    • Effective dose

      • 0.4 mSv is typical

  • Screening tomo

    • Between 500 mb and 3gb of data

  • Artifacts

    • Slinky artifact

    • Staircase/terrace artifact

    • Halo artifact

Lecture 6 Knobology Dose Artifacts

  • KAP = average absorbed dose X area

    • Basically same as DAP

  • Increased DAP —> Increased effective dose

  • Pulse width

    • So you are hitting patient with beam

    • Pulse width is basically how long you leave the beam on

    • Left on longer = more time for motion to occur = worse temporal resolution

      • Called continuous mode

    • Shorter pulse width = less time = sharper image

      • Needs more radiation = more heating vs

      • Called pulsed mode

  • Increasing mA or pulse width may compensate for shitty images at lower pps (7.5 pps vs 30 for example)

  • Electronic magnification (mag on the fluro machine)

    • Increases spatial resolution

    • Increases Kair and decreases area so the KAP stays about the same

    • The skin is getting a higher dose though

  • AEC (automatic brightness control)

    • Keeps image bright

    • Changes kV and mA

    • Can increase mA = higher dose and better contrast OR

    • Can increase kV = lower dose and worse contrast

  • Collimation

    • No change in spatial resolution

    • Decreases pt dose, beam strength the same but less hitting pt, skin dose stays about the same

    • Note in mag the dose in beam goes up but target area decreases so stays about the same

  • Maximum entrance air kerma 88mGy/min or 10 roentingen/min

  • High level control (Boost)

    • Idk something for fat pts to get around the max kerma shit

  • Effective dose - basically how bad to cause cancer

  • Pin cushion distortion - only on II systems

    • Basically image looks like its pulled in centrally

  • Vignetting - II only

    • Reason why central area is white and periphery is little darker

    • Something to do with curve in the machine

  • Signal saturation - shit is so bright like seen on the fluoro spine nonsense dictation ones

  • S wave artifact

    • makes center of image look very slightly wavy

    • something with magnetic field messing it up (ie if MRI machine is close to scanner)

  • Parallax artifact

    • what happens when doing LP when machine is not directly over the needle and looks like its angled basically

    • beam diverges

    • Worse in fluoro because there is a shorter source to image ratio

  • most fluor is a few mSV (modifieds are less tho)

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