HomeStretch CRACK Physics X-Ray Interactions/General/Detectors

Question 1

What are the three ways x-rays interact with the patient

Answer 1

• Classical (aka elastic aka coherent aka thomson) = happens at low energies, no ionization, just scattered x-ray
• Compton scatter
• Photoelectric effect

Question 2

Probability of PE is inversely proportional to the =

Answer 2

IP Energy Cubed (1/E³)

Question 3

Probability relative to Z (atomic number)?

Answer 3

Prob of P.E. is directly proportional to the atomic number (Z³)

Question 4

High yield recap

Classic/elastic interaction

Answer 4

has 4 names. . . classic, coherent, elastc and thomson

low energy (<10keV)

No contribution to image

tiny contribution to dose

does not result in ionization (no e- lost)

Question 5

High yield recap

Compton

Answer 5

Bad boy

Major contributor to scatter / image fog

Involves the outer shell electron (Compton drive by)

part of the e- energy is transferred

variable energy

doesn’t care about Z (doesn’t give a Compton shit)!

depends on density

dominates at above 30 keV

Question 6

High yield recap

Photoelectric effect

Answer 6

Good boy

major contributor to image contrast

involves INNER shell e-

ALL of the photon energy is transferred

All or nothing!

depends on Z³

More important in higher Z elements

Dominates below 30 keV

Question 7

Compton and PE are about equal at ____ keV within tissue

Compton and PE are about equal at ____ keV within bone

Compton and PE are about equal at ____ keV within barium and iodine

Answer 7

30 keV

50 keV

300 keV

Question 8

Attenuation in tissue depends on what 3 things?

Answer 8

1. Effective atomic number in tissue
2. X-ray beam quality (energy average)
3. tissue density

Question 9

Linear attenuation coefficient

Answer 9

measure of the probability of the material to attenuate x-rays over a set distance

effected by the density of the material

Mass attenuation coefficient = LAC / Density

Question 10

Mass attenuation coefficient

Answer 10

Mass attenuation coefficient = LAC / Density

Measure of the RATE OF ENERGY LOSS BY AN XRAY as it traves through a material

independent of density

depends only on atomic number and photon energy

Question 11

half value layer

Answer 11

amount of material required to attenuate x-ray to half the original output

Question 12

increasing energy (kVp) does what to linear attenuation coefficient and half-value layer?

Answer 12

decreases the LAC

increases the HVL

Question 13

increasing density does what to linear attenuation coefficient and half-value layer?

Answer 13

increases LAC

decreases the HVL

Question 14

What is entrance skin dose and what are the factors that determine it?

Answer 14

radiation ABSORBED by the skin as the beam strikes the patient

tube current (mA) = proportional

Tume of exposure = proportional

kVp = square

Distance = inverse square law (double distance = dose reduced by 4)

Question 15

What is quantum mottle?

how do you reduce it?

Answer 15

it is most important source of random noise in imaging

noisy image because not enough x-rays are reaching the film/detector

reduce mottle by increasing more x-rays (mA) and more efficient detection

Question 16

Scatter is maximized with what three things?

Answer 16

1. high kVp
2. thick parts (or people)
3. large FoV

Question 17

What does collimation do to FoV and noise?

Answer 17

increase collimation = decreased filed size and decreased noise

Question 18

What does a grid do?

Answer 18

blocks off-angle xrays

decreases noise but increases dose

Question 19

what is grid ratio?

Answer 19

ratio of the height of the grid to the distance between them

Question 20

in regards to a grid, what does it do to dose ?

Answer 20

dose increases with use of a grid

higher grid ratio increases dose

Question 21

if a grid is placed upside down, what do you see?

Answer 21

edges of image will be “cut-off” and the central portion is preserved (flip it around)

Question 22

if given a choice to increase mA or kVp to decrease noise, you should increase which one?

Answer 22

increase mA

Question 23

noise will increase as the distance between the tube and detector increases by what increment?

Answer 23

inverse square law

Question 24

Quadrupaling the x-rays will decrease mottle by what factor?

Answer 24

mottle will be cut in half.

Question 25

What is spatial resolution

Answer 25

how close two lines can be to each other and still be visibly resolved as separate things

Question 26

what is unsharpness?

What are the three things that cause it?

Answer 26

• Motion
• System
  
  • film = size of grain of photographic chem
  • CR = size of laser use to read phosphor plate
  • DR = size of individual thermoluminescent transistor
• geometric (i.e mag)

Question 27

Maginifcation is calculated by

Answer 27

Source to image distance / source to object distance

Question 28

What is dose quantum efficiency?

relative to dose?

some things to know about?

Answer 28

DQE = estimate of the required exposure level that will be necessary to create optimal image (1.0 in ideal world)

High DQE = LOW Dose

DQE is inverserely proportional to spatial resolution

Question 29

increase pixel denisty and pixel pitch = ____ spatial resolution

Answer 29

increases

Question 30

What is the primary factor influencing image contrast in digital system?

in film systems?

Answer 30

LUT

kVp

Question 31

Contrast will be increased if?

It will not be changed by?

Answer 31

• increased with:
  
  • decreased kV
  • decreased filtration (which decreases quality/kVp)
  • something with higher atomic number is added (i.e. iodine)
  • something with higher density is added ( barium)
  • scatter is reduced
• No change with
  
  • symmetric overlying tissue
  • fucking around with mA

Question 32

4 cm of patient tissue will require how much increase in mA?

Answer 32

double

Question 33

What has better spatial resolution? Digital or plain film?

Answer 33

plain film (they don’t have the limitation of individual units ie pixels)

Question 34

Increase pixel density and decresed pixel pitch (spacing) = ____ spatial resoltion

Answer 34

better spatial resolution

Question 35

pixel bit depth

Answer 35

number of bits that determine the number of shades of grey that can be displayed on a computer monitor

Question 36

increased matrix is bigger or small pixel? what does it do to spatial resolution?

Answer 36

smaller pixels - increases spatial resolution.

Question 37

Classification of digital detectors?

Answer 37

Storage Phosphor (CR) = type of indirect

Flat panel (DR) =. can be direct or indirect

indirect (scintillators) = xrays -> light -> charge

Direct (photoconductor) = xrays -> charge

Question 38

how do you reset the CR plate? what happens if you don’t?

Answer 38

bright white light!

if you forget you’ll get ghosting artifact.

Question 39

Most flat panel detectors in IR are what?

Answer 39

scintillators (CsI)

Question 40

in Digital radiography, which subtype uses a scintillator and which uses a photoconductor?

which has a fill factor of nearly 100%

Answer 40

scintillator in indirect

photoconductor in direct

direct has a fill factor of nearly 100%

Question 41

The typical standard of care for a digital display is how many pixels?

Answer 41

3 Mega-Pixels!

Question 42

CR is cassette based and it is ____

DR is ______

centralized or decentralized?

Answer 42

CR is Centralized (same workflow as conventional film)

DR is Decentralized.

Question 43

Primary spatial resolution summary

Screen films = ?

CR = ?

DR = ?

Answer 43

screen films = screen thickness

CR = pixel size and light scattering/spreading

DR = spread of light photons in the x-ray to light conversion (inirect DR); size of the detector “del”