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