Physics Ch 3 Gen XR Concepts

Question 1

photons –> inc vs dec –> noise inc/dec?

Answer 1

• inc photons –> dec noise

- dec photons –> inc noise

Question 2

most important source of noise (quantum) noise?

Answer 2

quantum mottle –> XR beam is inhomogeneous –> not enough XR reach film/detector –> random noise

Question 3

how to reduce quantum mottle? (2)

Answer 3

• inc XR (mAs)

- inc efficiency detection

Question 4

what factors inc scatter? (3)

Answer 4

• inc kVp
• inc thickness (body habitus)
• inc field of view (lrger XR beam)

Question 5

what is grid?

Answer 5

bw pt & image receptor –> device w mult thin lead stripes –> block scattered (angled) XRs

Question 6

grid –> how affect contrast? scatter?

Answer 6

grid –> reduce scatter –> improve contrast

Question 7

what is grid ratio? inc ratio –> how affect contrast?

Answer 7

ratio of lead ht to distance bw stripes

inc ratio –> dec scatter –> improve contrast

Question 8

grid –> how affect dose?

Answer 8

inc grid ratio –> inc dose

Question 9

tube-detector distance & noise –> relationship formula?

Answer 9

inc distance –> inc noise

inverse square law: inc distance 2x –> inc noise 1/4

Question 10

noise & mottle –> relationship formula?

Answer 10

inc XR photons 4x –> dec mottle 1/2

Question 11

to dec quantum mottle –> inc mA vs kVp?

Answer 11

correct answer: inc mA

inc kVp –> inc compton scatter –> inc noise

Question 12

mAs -> most important factor for… sharpness vs contrast vs noise/mottle

Answer 12

noise/mottle

Question 13

inc mA from 50 to 200 –> how change noise/mottle?

Answer 13

dec by 50%

Question 14

which of following will reduce noise?

• use grid
• use air gap
• inc mA

Answer 14

ALL reduce noise –> but correct answer is “inc mA”

Question 15

what is signal to noise ratio? main concept?

Answer 15

quantitative method for compare signal vs noise

strong signal –> can tolerate more noise

Question 16

contrast to noise ratio –> main concept?

Answer 16

high contrast –> can tolerate more noise

Question 17

what happen to contrast to noise ratio?

• inc mA?
• automatic exposure control (AEC) –> dec kV?

Answer 17

• inc mA –> dec noise –> improve contrast to noise ratio

- “fixed” noise –> dec kV –> inc contrast –> improve contrast to noise ratio

Question 18

spatial resolution –> main concept? units?

Answer 18

how close 2 lines can be and still be seen as separate

line pairs per mm

Question 19

what is spatial frequency?

Answer 19

spatial resolution

Question 20

what is unsharpness?

Answer 20

loss of spatial resolution

Question 21

what is motion unsharpness?

Answer 21

pt motion –> loss of spatial resolution

Question 22

what is system unsharpness?

Answer 22

detector –> limiting factor –> max spatial resolution

Question 23

geometric unsharpness:

• small focal spot
• dec source-object distance
• dec object-detector distance
• inc magnification

==> inc/dec blur?

Answer 23

• small focal spot –> dec blur
• dec source-object distance –> inc
• dec object-detector distance –> dec
• inc magnification –> inc

Question 24

geometric unsharpness: small focal spot –> utilized in what exams? (2)

Answer 24

• mammo

- extremity

Question 25

inc magnification --> affected by what 2 factors?

Answer 25

- source to obj distance --> dec | - object to detector distance --> inc

Question 26

magnification --> formula?

Answer 26

source to detector (image) distance / source to obj (pt) distance SID/SOD

Question 27

what is modulation transfer fx? formula?

Answer 27

original perfect XR data that exited pt --> image that digital detector produced MTF (image) = info recorded / info avail = <1.0

Question 28

what is detective quantum efficiency (DQE)?

Answer 28

"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

Question 29

detective quantum efficiency (DQE) --> relationship to... - MTF? - signal to noise ratio? - spatial resolution?

Answer 29

- directly proportional to MTF - inversely proportional to signal to noise ratio - inversely proportional to spatial resolution

Question 30

digital imaging --> matrix --> contain pixels --> what is pixel density? pixel pitch?

Answer 30

- pixel density: #pixels per unit area | - pixel pitch: "pixel spacing" --> distance from center or 1 pixel to next

Question 31

inc/dec pixel density --> better spatial resolution?

Answer 31

inc

Question 32

inc/dec pixel pitch --> better spatial resolution?

Answer 32

dec

Question 33

what kind of detector has best spatial resolution?

Answer 33

photoconductor

Question 34

lrger/smaller matrix --> smaller pixels?

Answer 34

larger matrix --> more pixels --> smaller pixels

Question 35

lrger/smaller matrix --> better spatial resolution?

Answer 35

lrger

Question 36

dec source to object distance (SOD) --> change what factor to compensate for blur inc? - inc kVp - dec focal spot size - inc focal spot size

Answer 36

dec focal spot size

Question 37

inc focal spot size --> what factor is improved? why? - noise (mottle) - contrast - blur (motion)

Answer 37

- blur (motion) inc focal spot size --> inc mA --> dec exposure time --> less time for pt to mv --> dec blur from motion

Question 38

dec/inc kVp --> dec/inc contrast?

Answer 38

- dec kVp --> inc contrast | - inc kVp --> dec contrast

Question 39

inc scatter --> dec/inc contrast?

Answer 39

inc scatter --> dec contrast

Question 40

what factors improve contrast? - inc/dec kVp - inc/dec atomic number - inc/dec density

Answer 40

- dec kVp - inc atomic number - inc density

Question 41

inc filtration --> what happen to avg energy beam (penetration power)? what happen to contrast?

Answer 41

inc filtration --> dec low energy XRs --> inc avg energy of XR beam --> dec contrast

Question 42

most important factor affecting contrast in digital system? in film system?

Answer 42

- digital --> look up table (LUT) | - film --> kVp

Question 43

narrow vs wide window width --> inc contrast?

Answer 43

narrow

Question 44

window level --> determines what?

Answer 44

brightness

Question 45

which has wider dynamic range? --> digital imaging or film screen?

Answer 45

digital imaging

Question 46

window level --> inc vs dec --> good for looking at what?

Answer 46

- inc --> inc pixel --> better for looking at dark --> ie lungs - dec --> dec pixel --> better for looking at light --> ie bones

Question 47

XR --> underexposed --> cause?

Answer 47

too low mA

Question 48

XR --> overexposed --> cause?

Answer 48

#1 too high mA

Question 49

what kind of XRs usu DON'T have automatic exposure control (AEC)? (3)

Answer 49

- portable XR - extremity XR - baby XR

Question 50

inc exposure time 4x --> what happen to mottle?

Answer 50

dec by 1/2

Question 51

inc exposure time --> what happen to patient motion/blur?

Answer 51

inc

Question 52

tabletop extremity exam --> no automatic exposure control (AEC) --> inc exp time --> what happen... - contrast improve - motion artifact dec - image mottle/noise improve

Answer 52

image mottle/noise improve

Question 53

tabletop extremity exam --> automatic exposure control (AEC) --> inc exp time --> what happen... - pt dose inc - image sharpness worsen - image mottle/noise improve

Answer 53

image sharpness worsen

Question 54

filter --> thicker --> what happen to image contrast?

Answer 54

inc filtration --> higher energy XRs remain --> dec contrast

Question 55

tell XR tech to improve image contrast --> f/u XR shows no change --> what factor was changed? - kV - mA - grid ratio

Answer 55

mA --> NOT change contrast

Question 56

which of following uses highest kV? - abd - PA chest - extremity

Answer 56

PA chest

Question 57

to inc penetration (exposure, "black"ness) --> should inc what factor? by how much?

Answer 57

mA 30% rule --> inc mA by at least 30% to make visible change in density

Question 58

obesity --> __cm pt tissue --> require __ inc in mA?

Answer 58

every 4cm of pt tissue --> 2x mA

Question 59

obesity --> lrg inc kV, small inc mA --> what happen to dose? contrast?

Answer 59

- lower dose | - lower contrast

Question 60

obesity --> small inc kV, lrg inc mA --> what happen to dose? contrast?

Answer 60

- higher dose | - higher contrast

Question 61

what is S/P ratio?

Answer 61

ratio of secondary XRs (scatter) to primary XRs (useful XRs)

Question 62

S/P ratio --> depend on what 2 factors?

Answer 62

- thickness of body part: thicker --> more scatter --> inc S/P ratio - beam area: bigger --> more scatter

Question 63

reduce scatter & improve contrast --> 4 methods?

Answer 63

- collimate: dec beam area - compress body part: dec thickness of body part - dec kVp: dec scatter, inc PE - use grid/airgap: dec scatter than strike detector

Question 64

infant XR --> how different from adult XR? (3)

Answer 64

- no grid - lower kVp - lower mAs

Question 65

pediatric: how much mAs compared to normal adult mA? - 0-5yo - 6-12yo

Answer 65

- 0-5yo: 25% mA of an adult | - 6-12yo: 50%

Question 66

fracture --> cast --> how change mA? - fiberglass cast - plaster cast

Answer 66

- fiberglass: no change | - plaster: dry --> inc mA 2x, wet --> inc 3x

Question 67

XR: - h/o splenomeg --> should change what factor? - h/o ascites? - h/o sclerotic phase Paget's? - h/o lytic phase Pagets? - h/o emphysema? - h/o osteoporosis? - soft tissue neck XR?

Answer 67

- h/o splenomeg --> inc kVp - h/o ascites --> inc kVp - h/o sclerotic phase Paget's --> inc kVp - h/o lytic phase Pagets --> dec kVp - h/o emphysema --> dec kVp - h/o osteoporosis --> dec kVp - soft tissue neck XR --> inc kVp

Question 68

radiographic density (blackness) --> controlled by kVp vs mAs?

Answer 68

mAs

Question 69

radiographic contrast --> controlled by kVp vs mAs?

Answer 69

kVp

Question 70

need to inc mAs by what% for noticeable difference in "density"?

Answer 70

30%

Question 71

dec mA by 50% --> need to dec/inc kVp by how much% to maintain same density?

Answer 71

inc 15%

Question 72

4cm of tissue --> require how much more mA?

Answer 72

2x