Physics Ch 3 Gen XR Concepts Flashcards
photons –> inc vs dec –> noise inc/dec?
- inc photons –> dec noise
- dec photons –> inc noise
most important source of noise (quantum) noise?
quantum mottle –> XR beam is inhomogeneous –> not enough XR reach film/detector –> random noise
how to reduce quantum mottle? (2)
- inc XR (mAs)
- inc efficiency detection
what factors inc scatter? (3)
- inc kVp
- inc thickness (body habitus)
- inc field of view (lrger XR beam)
what is grid?
bw pt & image receptor –> device w mult thin lead stripes –> block scattered (angled) XRs
grid –> how affect contrast? scatter?
grid –> reduce scatter –> improve contrast
what is grid ratio? inc ratio –> how affect contrast?
ratio of lead ht to distance bw stripes
inc ratio –> dec scatter –> improve contrast
grid –> how affect dose?
inc grid ratio –> inc dose
tube-detector distance & noise –> relationship formula?
inc distance –> inc noise
inverse square law: inc distance 2x –> inc noise 1/4
noise & mottle –> relationship formula?
inc XR photons 4x –> dec mottle 1/2
to dec quantum mottle –> inc mA vs kVp?
correct answer: inc mA
inc kVp –> inc compton scatter –> inc noise
mAs -> most important factor for… sharpness vs contrast vs noise/mottle
noise/mottle
inc mA from 50 to 200 –> how change noise/mottle?
dec by 50%
which of following will reduce noise?
- use grid
- use air gap
- inc mA
ALL reduce noise –> but correct answer is “inc mA”
what is signal to noise ratio? main concept?
quantitative method for compare signal vs noise
strong signal –> can tolerate more noise
contrast to noise ratio –> main concept?
high contrast –> can tolerate more noise
what happen to contrast to noise ratio?
- inc mA?
- automatic exposure control (AEC) –> dec kV?
- inc mA –> dec noise –> improve contrast to noise ratio
- “fixed” noise –> dec kV –> inc contrast –> improve contrast to noise ratio
spatial resolution –> main concept? units?
how close 2 lines can be and still be seen as separate
line pairs per mm
what is spatial frequency?
spatial resolution
what is unsharpness?
loss of spatial resolution
what is motion unsharpness?
pt motion –> loss of spatial resolution
what is system unsharpness?
detector –> limiting factor –> max spatial resolution
geometric unsharpness:
- small focal spot
- dec source-object distance
- dec object-detector distance
- inc magnification
==> inc/dec blur?
- small focal spot –> dec blur
- dec source-object distance –> inc
- dec object-detector distance –> dec
- inc magnification –> inc
geometric unsharpness: small focal spot –> utilized in what exams? (2)
- mammo
- extremity
inc magnification –> affected by what 2 factors?
- source to obj distance –> dec
- object to detector distance –> inc
magnification –> formula?
source to detector (image) distance / source to obj (pt) distance
SID/SOD
what is modulation transfer fx? formula?
original perfect XR data that exited pt –> image that digital detector produced
MTF (image) = info recorded / info avail = <1.0
what is detective quantum efficiency (DQE)?
“prediction of dose” –> estimate required exposure level necess to create optimal image –> measure of efficiency of detector to convert XR energy to image signal –> better DQE –> less rad necess