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Radiology Review > Physics > Flashcards

Physics Flashcards

1
Q

cathode

A

negative charge (electrons flow from)

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2
Q

anode

A

positive charge (electrons move to)

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3
Q

multiphase generators

A

reduces ripple effect of AC current (overlaps several waves)

  • makes peaks = kVp (peaks)
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4
Q

Bremsstrahlung

A
  • increases with accelerating voltage (kV) and anode atomic number (Z)
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5
Q

characteristic radiation

A

incoming e- ejects inner shell e-

  • must exceed binding energy (only K shell is important)
  • 5-10% of total usually
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6
Q

all targets produce characteristic radiation just below 20 keV, why?

A

k-edge filtering

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7
Q

how can you get more characteristic radiation

A

lower Z anode (Mo in mammo)

  • get 25% characteristic (max contrast = mono-energetic spectrum)
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8
Q

x ray filter

A

stops low energy photons = reduces dose

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9
Q

inherent filtration

A

glass exit window on the tube

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10
Q

added filtration

A

preferential filtering at lower energies

  • mean energy shits UP (shifts to the right) = increasing QUALITY, decrease QUANTITY (decrease dose)
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11
Q

K-edge filtering

A

filter material just about the k-edge of the target to kill higher energy keV photons

  • makes spectrum narrower
  • emphasize characteristic peaks
  • both improve contrast
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12
Q

photoelectric absorption

A

totally absorbed

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13
Q

Compton scattering

A

lose part of energy and change direction

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14
Q

Photoelectric interaction

A
  • incoming photon hits tight inner shell
  • photon is totally absorbed (photoelectron ejected)
  • outer shell electron fills vacancy = emits characteristic “fluorescent” x ray
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15
Q

kedge depends on material, what is the k edge of Iodine

A

33 keV

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16
Q

when do you get the best image contrast

A

when the beam spectrum matches the energy-dependent interaction inside the person

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17
Q

probability of photoelectric interactions

A

increased with increased Z

decreased with increased energy

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18
Q

compton scatter

A

photon hits loosely bound outer shell e-

  • scatters in a new direction with less energy
  • proportional to electron density
  • falls off at higher energy
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19
Q

attenuation

A

Sum of all interactions in the patient

  • went in and didn’t come out = photoelectric effect
  • went in and didn’t get detected due to change in direction and energy = compton scatter
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20
Q

increase in energy does what to total attenuation

A

decrease in PE with about the same compton = decrease total attenuation

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21
Q

photoelectric effect dominates at what energies

A

lower energy

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22
Q

compton dominates at what energies

A

higher energy

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23
Q

linear attenuation coefficient (u)

A

fraction of incident photons lost from beam per distance unit

  • Beer-Lambert law
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24
Q

half value layer

A

thickness of material that cuts the beam intensity in half

  • increased energy -> increased HVL (need more stuff to stop beam)
  • increased density of material -> decreased HVL (for same energy, need less stuff to stop it)
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25
Q

what is the useful equation for HVL

A

HVL = 0.7 / u

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26
Q

what is the clinical significance of HVL

A

shows if there is enough filtering

lower HVL = decrease in quality/energy, increased attenuation, increased dose (bad stuff)

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27
Q

mass attenuation coefficient

A

linear attenuation coefficent divided by density (u / p)

  • independent of phase of matter of density
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28
Q

beam hardening

A

tissue attenuation acts like a filter (removes lower energy photons)

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29
Q

x ray exposure = mAs, what if you increase mAs

A

increased mAs = more photons, same energy distribution -> mean energy stays the same

  • direct proportion to increase dose = double mAs = 2 x photons and 2 x the dose
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30
Q

varying energy (kVp), what happens if you increase kVp

A

more photons and HIGHER engery

  • 50/15 rule = increase in kVp by 15% = 2x photons -> should cut mAs by 50% to get same exposure (in 60-90 kVp range)
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31
Q

what is meant by softer beam

A

decreased kVp

  • decreased penetration
  • increased attenuation in body tissues
  • INCREASED CONTRAST (small dynamic range used in mammo due to all soft tissue imaging)
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32
Q

what is meant by harder beam

A

increased kVp

  • increased penetration
  • decreased attenuation
  • DECREASED CONTRAST
  • larger dynamic range (CXR - different tissues)
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33
Q

Typical kVp of different exams from lowest to highest

A

Mammo - 20

Extremity - 40

head, spine, hip, angiography - 70

Abdomen - 80

Barium - 115

CXR - 120

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34
Q

if you decrease energy (kVP) what happens

A
  • decreased photons = AEC will increase mAs
  • increased contrast
  • increased dose (from increase in mAs)
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35
Q

lower kVp for peds reduces, dose, how?

A

decreased flux of x rays produced but partially limit the mA compensation (partial AEC)

  • decreased dose but increased image noise
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36
Q

does iterative recon affect dose

A

no

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37
Q

iterative recon does what to an image

A

decrease noise

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38
Q

what does scatter do to contrast

A

degrades contrast

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39
Q

how to reduce scatter

A
  • grid
  • collimation
  • air gap
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40
Q

Grid use

A

scatter is not aligned = removes a lot of scatter

  • some primary is filtered as well
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41
Q

grid ratio

A

higher the grid ratio the higher the scatter rejection

Grid ratio = height of strips / distance between strips (GR = h /D)

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42
Q

what is bucky factor

A

Bucky factor = ratio of input to output flux (usually 2-6)

  • need to increase exposure that much to get same exposure to the detector -> increases DOSE
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43
Q

Adding a grid does what if dose does not change

A
  • decreased scatter

- INCREASED noise due to decreased through transmission

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44
Q

Collimation to reduce scatter

A

decrease FOV = DECREASED scatter, INCREASED contrast

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45
Q

Air Gap to reduce scatter

A
  • scatter has a greater angle = doesn’t hit detector
  • scatter has a shorter distance to travel and falls off more quickly than primary (inverse square law)
  • Greater source to image distance and magnification
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46
Q

how much of the electron energy is converted to x rays vs heat

A

99% heat and 1% x rays

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47
Q

heat capacity of tube determined by what?

A
  • focal spot
  • anode
  • housing
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48
Q

what is the equation for heat units

A

HU = kVp x mA x secs

1 HU = 0.71 J

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49
Q

Line focus principle

A

bigger area for heat dissipation

determined by anode angle theta

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50
Q

if you increase anode angle theta what happens

A
  • increased heat dissipation
  • increased effective focal spot
  • increases FOV
  • decreased sharpness
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51
Q

Heel effect

A
  • decreased amount of higher energy photons on the anode side
  • worse for bigger FOV or shorter source to image distance
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52
Q

due to heel effect which side of the tube will you place the thickest (most dense) part of the body

A

cathode side

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53
Q

Grid artifacts

A
  • incorrect focal distance
  • off center grid
  • tilted grid
  • inverted grid
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54
Q

effect of increased kVp on contrast

A

increased kVp = increased penetration, decreased contrast, decreased dose

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55
Q

Resolution due to frequency

A

thick about sound (violin vs bass)

  • higher spatial frequency for smaller size
  • lower spatial frequency for larger, thicker structures
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56
Q

Resolution is characterized by what function

A

Modulation transfer function (MTF)
- range of 0-1 (1 is perfect)

  • in real life lower frequency is better preserved
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57
Q

resolution limits in line pairs

A

one line pair / mm is equal to two pixels (line pair = 1 bright and 1 dark = 2 pixels)

  • 5 lp / mm = 10 pixels / mm = pixel size of 0.100 mm
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58
Q

Limiting resolution

A

Limiting resolution = frequency at 10% MTF

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59
Q

what are the usual limiting resolutions for different exams from high to low resolution

A

Mammo - 5-10 lp/mm

Radiography - 3 lp/mm

CT - 1 lp/mm

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60
Q

Resolution is affected by what

A
  • motion
  • focal spot size and magnification
  • detector resolution and sampling
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61
Q

Motion unsharpness

A
  • patient motion or tube motion
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62
Q

how do you decrease motion unsharpness

A
  • shorter exposure times

- immobilize patients

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63
Q

how to decrease focal spot blur

A

use a small focal spot

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64
Q

what are the typical focal spots for typical radiography and for mammo

A
  • typical - 1.2 or 0.6 mm

- mammo - 0.3 or 0.1 mm

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65
Q

why can you not use the smallest focal spot all the time

A
  • reduced tube output -> longer exposure time -> more motion blur
  • more heat = shorter tube life
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66
Q

Focal spot blur

A

M = SID / SOD

  • Penumbra blur is directly proportional to the focal spot size and the mag
  • bigger the mage or focal spot = more blur
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67
Q

Geometric unsharpness

A

small FS and small mag = less blur

big FS and big Mag = greatly increased blur

small FS and big mag = standard blur

  • mag makes everything bigger including focal spot blur (use small FS to get back to standard)
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68
Q

Detector resolution

A
  • detector type, material, thickness
  • pixel pitch
  • binning
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69
Q

Overall resolution is determined by each individual MTF of the system, how do you improve resolution

A

must know each individual MTF and address the worst one

multiply each one together to get the overall MTF of the system

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70
Q

Noise power spectrum (NPS)

A

spatial frequency content of image noise

  • decrease dose = MORE noise
  • increase dose = LESS noise
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71
Q

what causes noise

A

mostly = finite number of photons used in the image (quantum mottle)

Limited absorption efficiency of the x ray by the detector

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72
Q

Quantum mottle

A

Poisson distribution (variance = mean)

  • relative noise = 1 / sq rt N
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73
Q

Signal to noise ratio

A

resolution in the numerator and noise is denominator (R/N)

SNR of > 5 is good

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74
Q

what happens to SNR if there more noise

A

both size and contrast of detectible lesions go down

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75
Q

what happens to SNR if there is more blur

A

size of detectible lesions goes down

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76
Q

Detective quantum efficiency (DQE)

A

dose is proportional to SNR^2

  • better DQE = better machine
  • DQE of 2x = same image quality for less dose (SNR same for 1/2 dose)
  • higher image quality for same dose (1.4 x SNR of same dose)
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77
Q

what is the range of DQE

A

0-1

  • higher = better to see small lower contrast at a lower dose
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78
Q

Digital imaging machines

A
  • improved dynamic range
  • post processing better quality
  • PACS
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79
Q

Indirect flat panel

A

phosphor -> converts x ray to light -> CCD or photodiode coverts light to signal

  • decreased signal
  • increased noise
  • increased blur from light spread

BUT, has higher DQE (increased absorption efficiency of materials)

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80
Q

Direct flat panel

A

photoconductor coverts x rays to signal directly

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81
Q

Computed radiography

A

storage phosphor hold x rays in latent storage -> laser stimulates light emission -> photomultiplier tube coverts light to signal

  • both indirect and direct are better than CR
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82
Q

MTF of direct, indirect, and CR

A

Direct > Indirect > CR

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83
Q

DQE of direct, indirect and CR

A

Indirect > Direct > CR

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84
Q

Pre processing

A
  • correct for detector defects
  • convert x ray to pixel values
  • display consistently with prior
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85
Q

Post processing

A
  • grayscale processing
  • edge enhancement
  • equalization
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86
Q

Luminance

A

brightness, cs/m^2

  • high >/= 350, 420 for mammo
  • low = 1 or 1.2 for mammo

great luinance = increased contrast

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87
Q

GSDF

A

grayscale display function - more consistent throughout the hospital

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88
Q

CR twin

A

double exposed plates (two images on top of each other)

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89
Q

Delayed readout of CR

A

decreased density in the periphery where the fading starts

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90
Q

Backscatter exposure

A

poor collimation or long exposure = backscatter from the cassette (CR) or detector electronics (DR)

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91
Q

CR roller artifact

A

dust on the roller = horizontal bright lines that can be traced outside the patient

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92
Q

Grid moire pattern

A

interference between the grid ratio and display matrix

  • can show up on monitor with to low resolution
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93
Q

DR lag (ghosting)

A

residual image of lead markers from previous image

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94
Q

DR incomplete grid suppression software

A

incomplete grid line suppression by post processing

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95
Q

DR detector saturation

A

overexposure in low/un-attenuated regions (like lung)

  • can’t recover the data
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96
Q

Fluoro tube changes

A

heat capacity

  • high speed anode rotation
  • water or oil heat sink

Focal spot

  • small focal spot for fluoro
  • large focal spot during spot or cine (greater tube current)
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97
Q

fluoro collimation

A

mulitple sets of shutters

beam shaping

  • decreased object glare
  • decreased scatter
  • decreased dose
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98
Q

Image intensifier

A

x ray - latight - electons - light - electronic signal

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99
Q

flat panel detector

A

x ray - light - electronic signal

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100
Q

II flux gain

A

e- gain kinetic energy when travelling across the high voltage

  • increases light by a factor of 50-100
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101
Q

Minification gain

A

BIG input, small output (affects only brightness)

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102
Q

Brightness gain of II

A

BG = flux gain x minification gain

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103
Q

Frame averaging

A

sacrifices temporal resolution to reduce noise

  • decrease dose with increased lag
104
Q

Automatic brightness control

A

controls mA and kV automatically

105
Q

magnification for II

A

projects only central part of the input layer onto output phosphor

  • decrease field of view = increase geometric mag (increased resolution)
  • less minification gain = increased exposure rate by ratio of FOV (reduce the noise)
  • mostly increases kVp to minimize increase in dose (keeps air kerma and tube heat lower)
  • less contrast and increase noise
106
Q

digital spot mag mode

A

mostly increase mA

107
Q

does flat panel mag increase dose?

A

yes, just not as much as II

108
Q

Sentinel event of fluoro

A

anything over 15 Gy

109
Q

Dose area product

A

DAP = emitted dose x field size (in Gy per cm^2)

  • NOT patient or skin dose
  • can estimate effective dose (whole body)
110
Q

air kerma

A

total accumulated dose during procedure (mGy)

111
Q

air kerma rate

A

air kerma rate per minute

112
Q

dose area product

A

air kerma x beam area

113
Q

peak skin dose

A

dose to highest area of skin (usually less than air kerma)

114
Q

how to minimize dose

A
  • keep detector close to the patient
115
Q

minimize dose in peds

A
  • remove grid

- use low dose (higher kV and lower mA)

116
Q

occupational exposure comes from what

A

scatter from the patient (compton)

  • stand on detector side
117
Q

reduce exposure

A
  • less time
  • distance
  • shielding
118
Q

Pincushion distortion (II)

A

curved input phosphor to flat output phosphor

  • reduced with mag use
119
Q

s-distortion

A

external magnetic field = spatial warping

120
Q

Vignetting (II)

A

darkening at the edges (flat panel does not get this)

121
Q

in mammo on which side do you place the cathode

A

chest wall side (thickest portion) - heel effect

122
Q

focal spot for mammo

A

0.3 mm for contact and 0.1 mm for mag

123
Q

normal compression range for mammo

A

110-180 newtons

124
Q

why compress breast

A
  • decrease scatter
  • decrease geometric blur
  • decrease tissue overlap
  • decrease dose
125
Q

Grid ratio for mammo

A

4-5 only

  • 6-16 to general radiography
126
Q

because of low energy in mammo what has to be true of filtration

A

filtration must be low

  • low Z material for tube port (beryllium, Z = 4)
127
Q

K-edge filtering in mammo

A

use same element for target and filter = narrow spectrum = more contrast and less dose

128
Q

what is the highest resolution of any imaging

A

mammo

  • smaller focal spot
  • smaller pixels
  • magnification
  • compression
129
Q

Mag view changes from contact view

A
  • small focus (0.1 mm) with breast closer to tube and further from detector via mag stand
  • 1.5-2x mag factor
  • no grid, decreased scatter due to air gap
  • similar resolution due to small focus
  • can use spot compression to reduce overlap
130
Q

electronic mag

A

takes same picture and just makes the pixel bigger

131
Q

geometric mag

A

real mag views

  • better visual of calcs etc.
132
Q

TOMO breast

A

limited angle cone beam CT

  • incomplete data -> non-isotropic resolution = never sees breast from side = worse z-direction resolution
133
Q

TOMO artifacts - emboss

A

embossing (intra-plane ringing of high contrast along the tube travel direction (think about biopsy clips)

134
Q

TOMO artifact - shadow

A

out of plane smearing of objects (think about calcs)

135
Q

what is the HVL of mammo

A

HVL usually 0.3 mm of Al

136
Q

Mean gladular dose (for average breast)

A

<300 mrad (3 mGy)

  • dense and thicker than 4.2 cm dose would be higher
137
Q

Scoring of ACR phantom

A

must have:

  • 4 / 6 fibers
  • 3 / 5 specks
  • 3 / 5 masses
138
Q

Mammo ghosting

A

shadow from previous image

  • worse on selenium
  • aggravated by high dose / contrast
  • aggravated by low temperature
139
Q

incorrect auto technique

A

impants (fails to pick right kVp/mAs = underexposure = increased noise

140
Q

Loss of skin line

A

breast periphery not fully compressed

  • loss of skin can be due to post processing
141
Q

Damage to filter

A

wrinkle or dent in filter

142
Q

Grid artifact in mammo

A

cross hatch

143
Q

detector calibration for mammo

A

detector line interface

144
Q

TOMO truncation artifact

A

some breast are outside the beam path

  • terrace or venetian blind artifact due to incomplete information
145
Q

Most common problems for mammo

A
  • bad compression
  • motion
  • bad positioning
146
Q

power rating for CT power supply is determined by what

A

focal spot size

147
Q

large focal spot size for CT is what

A

1-2 mm (high power rating ~65 kW)

148
Q

small focal spot size for CT is what

A

0.5-0.6 mm (low power rating ~25kW)

149
Q

what are the two collimators in CT

A

Pre-patient (source collimator)
- decreased dose

Post patient collimator
- decreases scatter from patient to detector

150
Q

CT filters

A

Al or Cu

151
Q

Bowtie filter for CT (usually made of Al, teflon, or graphite)

A

Contours beam

  • more uniform beam
  • decrease beam hardening artifacts
152
Q

Post patient collimator on single detector machine

A

determines slice thickness

153
Q

post patient collimator on MDCT

A

determines collimation width and minimum slice thickness

154
Q

types of CT detectors

A

Gas and solid state

Gas = xenon ionization chamber (60-80% efficency)

solid state = bismuth germinate or cadmium tungstate
(scintillator -> light -> photodetector -> electric signal) = ~ 100% efficent

155
Q

benefit of solid state over gas CT detector

A
  • higher absroption effiency
  • higher SNR
  • less beam hardening
156
Q

disadvantage of solid state over gas CT detector

A

less stable

157
Q

difference between single and multiple detector CT

A

single = single detector = single slice

multi = multiple detectors = multiple slices per rotation

158
Q

Axial CT

A

takes pictures then moves table then takes picture

159
Q

Helical CT

A

tube and table move at the same time

160
Q

what allows the tube to spin

A

slip ring between power and x ray tube

161
Q

when might you run in axial mode

A

if > 128 slice detector to reduce cone beam artifacts

  • look at dynamic structures that are moving (like heart)
162
Q

Minimum slice thickness

A

MST = collimation width / number of channels

  • can reconstruct images greater than, but not less than this number
163
Q

difference bewteen non-isotropic voxel vs isotropic voxel

A

non isotropic = rectangle

isotropic = cubic

164
Q

how is x and y axis determined

A

X and Y = DFOV (mm) / matrix size (always 512)

165
Q

how is the Z axis determined

A

z axis = slice thickness

166
Q

Line pairs

A

remember - take two voxels to define a line pair

same equation as the x and y axis multiplied by 2

Line pairs = (DFOV (mm) / matrix size) x 2

167
Q

types of reconstruction

A

filtered back projection
-assumes single beam CT = not good for low radiation dose

iterative reconstruction

168
Q

how do you get partial volume averaging

A

each voxel is assigned an attenuation coefficient (u)

  • if a beam straddles two different densities you get the mean (average) of the two
  • kVp dependant
169
Q

what is iterative reconstruction

A

may reduce noise by 30-70%

reconstruction longer

170
Q

types of reconstruction filters

A

Standard = balanced detail and noise

smooth = low detail and low noise

bone = high detail and high noise

171
Q

what is the memory needed for CT images

A

2 bytes / voxel

in 512 x 512 matrix = 262,144 voxels = 0.5 MB per slice

average for abdomen pelvis = 75 MB

172
Q

Hounsfield Unit

A

directly related to the linear attenuation coefficient

1 HU = 0.1% difference between the linear coefficient of tissue compared to water

173
Q

Window width

A

number of shades of grey

  • inversely proportional to image contrast
174
Q

window level

A

center shade of grey

  • adjusted to match tissue you want to look at
175
Q

formula for window/level

A

level +/- Window / 2

176
Q

typical window width and level for different tissues

A

ST = 400 / 40

Liver = 150 / 70

lung = 1000 / -600

Bone = 1000 / 600

177
Q

Pitch

A

pitch = Table travel per rotation / collimation width

  • pitch of 1 = no overlap (< 1 = overlap, > 1 = gaps)
178
Q

relationship between helical pitch and dose

A

higher the pitch = lower dose

179
Q

what is slice broadening

A

slice gets thicker with higher table pitch (ex. pitch of 1 = 20% broadening)

180
Q

what is the relation of gantry rotation and dose

A

time for gantry to move 360 degrees

  • linear to dose
  • shorter the gantry time = lower the dose
181
Q

radiation dose when changing kVp

A

exponential change

182
Q

radiation dose when changing mAs

A

linear change

183
Q

Noise is figured by…

A

sq rt of # of photons

184
Q

Motion CT artifact

A
  • parallel lines
185
Q

Undersampling on CT

A

aka aliasing

  • photon deficiency = streak artifacts
  • white rings on part of body outside the detector FOV
186
Q

Ring artifact for CT

A

detector malfunction or miscalibration

  • looks like drop of water in a pond
187
Q

Beam hardening artifacts CT

A

due to wide range of photon energies

  • low energy absorption
188
Q

Stair step artifact of CT

A

off axis reformations for thick slices (axial slices > 1.25 mm)

189
Q

Signal to noise ratio

A

more photons = less noise

  • SNR = # photons / voxel
190
Q

SNR is affected by what

A

kVp higher = less noise

mA higher = less noise

Pitch higher (faster) = more noise

gantry rotation time faster = more noise

DFOV smaller = less photons (increased spatial resolution but more noise)

191
Q

Radiation dose in gray vs sieverts

A

GrAy = Absrobed dose

SiEverts = Effective dose

192
Q

CTDIvol (CT dose index for one slice)

A
  • phantom based
  • includes both direct and scatter radiation
  • reflects technique, NOT total dose (mGy)
193
Q

Dose length product (DLP)

A

basically the CTDIvol x Anatomic length

  • this DOES reflect total dose in mGy cm
194
Q

Dose reduction for CT

A

Lower kVp = expoential

Lower mA = linear

higher pitch

shorter GRT

thicker slices

Avoid small DFOV

Iterative reconstruction

195
Q

Average speed of sound in the body

A

1540 m/sec (soft tissue)

  • fat and air are slower
  • muscle and bone are faster
196
Q

how many micro seconds does it take per 1 cm of tissue for a sound wave to go there and back

A

13 micro seconds

197
Q

what are the 4 ways that sound can interact with the body

A
  • reflection (straight back or same angle)
  • refraction (bent)
  • scattered
  • absorption (heat)
198
Q

Reflection

A

if acoustic impedance is very different = lots of reflection

if they are similar = near zero reflection (transmitted)

199
Q

Refraction

A

differences in speed of sound between two tissues = bent sound

200
Q

Scatter

A

is like reflection but due to very small reflectors

  • specular reflector = flat, perp to the beam -> large amount bounce back
  • nonspecular reflection = scatters at angles that may not be picked up
  • higher frequency = more scatter
201
Q

absorption

A

beam turned to heat

  • higher frequency = more absorption
202
Q

Attenuation

A

all of the reflection, refraction, scatter and absorption

  • higher freq = more attenuation = hard to get deep
203
Q

Signal loss in dB

A

Signal loss = 0.5 dB/MHz/cm x distance (cm) x Frequncy (MHz)

204
Q

what is the half value thickness of US

A

thickness of material that causes 3 dB decrease in signal

  • air, high (goes short)
  • water, low (goes further)
205
Q

attenuation leads to through transmission

A

no attenuation in the fluid (brighter posterior compared to surrounding ST)

206
Q

Curvilinear

A

good for abdomen

207
Q

Linear

A

high spatial resolution (small parts)

208
Q

Piezoelectric material

A

changes shape with voltage

  • PZT (lead-zirconate-titanate) most common crystal
209
Q

Best lateral resolution

A

at focal spot at end of near field

210
Q

Length of near field increases with what

A

diameter and higher frequency

211
Q

Damping degree

A

light damping = higher Q (narrow bandwidth) = good for doppler

heavy damping = low Q (shorter SPL) = better axial spatial resolution

212
Q

Phased array

A

smaller number of elements that sweep at different times to steer the beam (smaller footprint)

213
Q

how to make different focal zones

A

time difference between activating crystals on the inside vs periphery (shapes the beam)

214
Q

how large is an US image

A

0.25 MB per image

215
Q

Transmit power vs gain

A

transmit power = amount of signal (power of signal from radio station)

gain = how much amplification of the signal (changing the volume on the radio)

216
Q

pulse repetition frequency

A

how often 3-5 wavelets are sent out (~ 1 kHz)

217
Q

Spatial pulse length

A

relates to how small of physical distances can be distinguished

  • higher frequency transducer -> better spatial resolution
  • SPL = the number of little wavelets (usually 3)
218
Q

Axial resolution

A

dependent on SPL (0.5 x SPL)

  • NOT dependent on depth
219
Q

Lateral and elevation resolution

A

more dependent on depth and follow shape of the beam

  • lateral and elevational best in the focal zone

(elevational in plane of thickness)

220
Q

put the focal zone where on a lesion?

A

center it on the lesion

221
Q

time gain compensation

A

due to attenuation signal from deeper is lower -> time gain compensation turns up gain to try and correct this

222
Q

B mode

A

brightness mode (signal more = bright, low signal = darker)

  • gray scale
223
Q

M mode

A

movement of structures plotted on time

224
Q

A mode

A

amplitude mode (graph of signal)

225
Q

Spatial compounding

A

compound imagines from different angles (decreased artifacts) = see around things

226
Q

Doppler angle at 90 degrees

A

lowest signal and most error

227
Q

what is the good doppler angle

A

60 degrees

228
Q

Continuous doppler

A

no depth info

  • no aliasing
229
Q

Pulsed doppler

A

depth selection

aliasing possible

Used in association with 2D B mode (duplex doppler)

230
Q

Aliasing

A

wrap around on the spectral tracing due to undersampling of the frequency shift

  • increase scale (increases PRF as well)
231
Q

how to stop aliasing

A

increase PRF (PRF/2 is the smallest that can be measured)

  • lower freq transducer
  • use higher angle theta
  • use power doppler (no direction sense)
  • might have to decrease depth
232
Q

Mirror image artifact

A

due to a strongly reflective interface due to high difference in acoustic impedance values

233
Q

how to stop mirror image artifact

A

different acoustic window or angle

234
Q

Refraction artifact

A

misplaced anatomic position due to direction change in the beam at interfaces

235
Q

how to stop refraction artifact

A

change the angle or spatial compounding

236
Q

Speed displacement artifact

A

one path has material with different speed of sound than in adj tissues

  • apparent discontinuity of the diaphragm
237
Q

how to stop speed displacement artifact

A

change the angle

238
Q

Reverberation artifact

A

parallel lines = parallel reflectors

  • like white lines in the bladder
239
Q

how to stop reverberation artifact

A

change angle

change window

tissue pressure

harmonic imaging

240
Q

Comet tail artifact

A

reflectors close together and also lose signal (due to attenuation)

241
Q

Ring down artifact

A

create vibration, commonly seen with air

242
Q

Beam width artifact

A

looks like sludge in the GB,

extending adj tissue into a flluid structure

243
Q

how to stop beam width artifact

A

change focal zone

different angle or window

harmonics

244
Q

Shadowing

A

beam attenuation along one path

245
Q

how to stop shadowing

A

compound imaging

different window

246
Q

Larmor Frequency

A

42.6 MHz/Tesla = RF sent into the patient

247
Q

where are MRI voxels

A

in the Gradients

248
Q

Field strength of MRI per tesla

A

1 Tesla = 10^4 gauss

249
Q

Flip angle

A

how much the proton orients into the transverse plane

250
Q

Free induction decay

A

protons go out of sequence and lose signal due to field inhomogeneity

  • rate depends on T2*
251
Q

Acquisition MRI scan time

A

Time = TR x # of phase encoding steps x NEX x # of slices

  • NEX = # of k spaces averaged together
252
Q

what sequences are black blood

A

Spine echo sequences

  • blood getting the 180 degree pulse didn’t get the initial 90 degree pulse
253
Q

what gradient is changed with every repetition on spin echo imaging

A

phase-encoding gradient

254
Q

what is the primary determinant of noise in MRI

A

receiver bandwidth

255
Q

what sequences are bright blood on MRI

A

gradient echo sequences

Radiology Review (9 decks)

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