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Huda > Miscellaneous imaging > Flashcards

Miscellaneous imaging Flashcards

1
Q

useful range of film OD

A

0.3 (50% transmittance) to 2 (1% transmittance)

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2
Q

base + fog OD

A

0.2
base= density of film base alone
fog= level of blackening due to few grains being developed in absence of radiation
-fog is mainly related to silver halide grain size

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3
Q

film speed

A

~ 1/amt of light needed for development

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4
Q

film latitude

A

exposure range of useful contrast

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5
Q

why use intensifying screens?

A

-absorb 50X more of incident xrays than a radiographic film
-decreases exposure time= lower patient dose, lower x-ray tube loading, less blurr

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6
Q

common intensifying screen thickness

A

200 um

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7
Q

intensification factor

A

ratio of exposures, without and with intensifying screen, to get a given film density
-30-50

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8
Q

what is the light output of an image intensifier proportional to?

A

-the input area of the image intensifer and the radiation exposure
-reducing the imagine intensifier by a factor of 2 reduces the exposed region by a factor of 4
-a 4fold increase in radiation exposure would be required to maintain a constant brightness at the output of the image intensifier

-electronic magnification by decreasing the exposed area of the image intensifier results in increased skin doses

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9
Q

what is purpose of image intensifier

A

convert xrays exiting patient into a bright light image

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10
Q

how many MB for chest x-ray digitized to 2k x 2.5 k matrix using 2-byte coding of each pixel?

A

210242.510242 = 1.05*10^7 kB

A MB is 1024^2 kB
thus 10 MB required

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11
Q

nuc med matrix size

A

128x128, 1 byte per pixel
1/64 MB

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12
Q

MRI matrix size

A

256x256, 2 bytes per pixel
1/8 MB

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13
Q

CT matrix size

A

512x512, 2 bytes per pixel
1/2 MB

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14
Q

US matrix size

A

512x512, 1 byte per pixel
1/4 MB

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15
Q

conversion efficiency of scintillator

A

% of absorbed E converted into light
2-20%
scintillators aka phosphors

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16
Q

mammo matrix size

A

4096x6144, 2 bytes/pixel
50 MB

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17
Q

CR, DR, and film matrix size

A

2560x2048, 2 bytes/pixel
10 MB

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18
Q

digital photosopt/DSA matrix size

A

1024x1024, 2 bytes/pixel
2 MB

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19
Q

reading out CR plates- colors of the lights

A

re light- stimulate and empty electron traps
blue light: emitted and measured
white light: use to erase

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20
Q

CR intensities compared to screen-film

A

screen film= 5 uGy
CR plates can tolerate 100 X above and below

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21
Q

direct flat panel detector

A

photoconductor

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22
Q

window level for lung

A

window= 1500
level = -500
level is overall brightness
narrower window= more contrast for tissues within the widow range (but lungs invisible)

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23
Q

abdomen window level

A

window = 150
level = 60

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24
Q

semi-annual mammo QA

A

-darkroom fog
-screen-film contact
-compression

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25
Q

what does raising film developer temperature do to fog?

A

increases it

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26
Q

how to increase film contrast and density

A

-increase developer temperature
-increase developer time

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27
Q

what is gamma in film

A

-max slope of characteristic curve
-expressed in terms of density difference associated with exposure of 10:1
-film gamma is 3.32 max contrast factor

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28
Q

what can cause film to fall outside of useful exposure range?

A

-incorrect exposure setting on film
-anatomy produces wider range of exposures

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29
Q

effect of overprocessing film

A

-curve shifts to left with rise in tow
-as toe rises, slope decreases- less contrast
-indicates increase in sensitivity because a given density value is produced with a lower exposure
-increased density and fog

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30
Q

effect of underprocessing film

A

-curve shifts to right- decrease in sensitivity
-shoulder drops, slope of curve decreases- less contrast and less density

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31
Q

what is detector geometric efficiency/sensitivity proportional to?

A

-proportional to radiation sensitive detector area and inversely proportional to square of source-to-detector distance for a pt source

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32
Q

what is detector intrinsic efficient related to?

A

-detector thickness
-Z
-mass density
-decreases with photon energy

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33
Q

define detector resolution

A

-FWHM of single energy peak at specific energy

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34
Q

gain of amplifier

A

log ratio of output power/voltage to input power/voltage
-measured in dB

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35
Q

intrinsic vs extrinsic performance in nuc med

A

-intrinsic looks at subpart of imager (ex detector without degrading effects of collimator)
-extrinsic look at total image; realistic conditions

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36
Q

how much power loading can a 1 mm focal spot tolerate?

A

100 kW

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37
Q

why are higher kV values (140 kV) used in head scanning?

A

help minimize beam hardening artifacts

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38
Q

typical kV values CT

A

regular= 120 kV
higher = 140 kV
lower (pediatric) = 80 kV

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39
Q

nyquist frequency

A

-highest frequency that can be faithfully reproduced
-half the sampling rate

-nyquist rate is 2X max frequency

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40
Q

what does aliasing cause in US?

A

-can show highest velocities in center of a vessel as having a reverse flow
-pulse repetition frequency must be at least twice the doppler frequency shift to avoid artifacts

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41
Q

mottle in CT

A

3 HU
-random variations of photons incident on detector
-random fluctuations in attenuation coefficients of 0.3%
-quadrupling the mAs would halve the mottle
-doubling slice thickness will double number of x-ray photons and reduce mottle

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42
Q

quantum mottle in CT vs nuc med

A

nuc med is higher than CT because number of photons used to generate image is low

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43
Q

random bright and dark streaks in CT

A

noise
-statistical error of low photon counts
-appear preferentially along direction of greatest attenuation
-fix by combining data from multiple scans, increase mAs, bowtie filters, iterative reconstruction

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44
Q

what can beam hardening and scatter cause in CT?

A

-dark streaks between 2 high attenuation objects (like metal or bone) with surrounding bright streaks
-fix with increasing kVp, dual energy CT can reduce beam hardening but not scatter

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45
Q

what do CT collimators do?

A

-define total beam width: 40 mm for 64-slice and 160 mm for 320-slice
-reduce scatter radiation

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46
Q

collimantor sensitivity

A

-fraction of gamma rays reaching it that pass through the holes
-high sensitivity: larger holes and lower resolution
-high resolution: smaller holes and lower sensitivity
resolution is degraded with increasing distance from collimator

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47
Q

what images do you use for QA of collimator?

A

flood images

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48
Q

do PET systems use collimators?

A

No
thus have increased sensitivity

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49
Q

scintillator in Anger camera

A

NaI

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50
Q

counts and PMTs in Anger camera

A

55 PMTs
500,000 counts

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51
Q

Anger camera max non-uniformity

A

5%

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52
Q

intrinsic resolution of Anger camera

A

3-5 mm
system resolution is R = square root (Ri^2+Rc^2)
i.e. intrinsinc and collimator

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53
Q

different types of SPECT collimators

A

-parrallel hole (constant FOV)
-converging- magnified, FOV~ 1/distance
-diverging- project smaller image size, FOV increases with distance
pin hole- magnify and invert

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54
Q

quantum efficiency of CT detectors

A

> 90%

geometric efficiency also ~ 90% for detectors 1 mm wide and separate 0.1 mm

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55
Q

what determines slice thickness in CT

A

detector width
-0.5-0.6 mm

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56
Q

multi-detector CT beam width

A

number of slices times slice thickness

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57
Q

dual energy CT

A

-80 and 140 kV
-better temporal resolution for cardiac imaging
-delineates materials with similar attenuation
-temporal resolution is half of gantry rotation time for single source vs 1/4 for dual source

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58
Q

compare pros and cons of CT image quality vs radiography

A

-CT has better contrast- can detect lesions that differ by 0.3 % from surroundings- radiography requires 3 % difference
-resolution better in radiography
-pixels in CT 0.6 mm vs 0.2 mm in radiography
-CT is higher dose (chest CT is 100X dose of radiograph)

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59
Q

CT FOV for head and body

A

25 cm
40 cm

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60
Q

matrix size CT

A

512x512
pixel size is FOV/number of pixels

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61
Q

compare CT generations

A

1st: translate/rotate + pencil beam, 30 min scan, 1 detector
2nd: translate/rotate + multiple detectors, 90s scan, 30 detectors
3rd: rotate/rotate+ large array of detectors+ fan beam, 5 s scan, 300-700 detectors
4th: rotate + fixed ring of detectors, 2 s scan, 2000 detectors

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62
Q

CTDI for head vs body phantom

A

0.2mGy/mAs for head and 0.1 mGy/mAs for body
-at 120 kV

-in head scans, central and surface doses are similar wheres in body scans they differ more (attenuation)

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63
Q

pitch

A

table movement in 1 rotation/beam width

< 1 = oversamples, more dose
> 1 = undersampled, less dose

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64
Q

cardiac imaging

A

-best in diastolic phase of cardiac cycle
-pitch 0.2-0.3
-high dose
-use dual energy scanner to improve temporal resolution

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65
Q

treshold of contrast for underexposed and overexposed film

A

<0.5 OD
> 2 OD

positive contrast= darker lesions (lesion absorbs fewer xrays)

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66
Q

relationship between film latitude and contrast

A

inverse

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67
Q

how to increase resolution in fluoro

A

-halving the image intensifier FOV electronically doubles resolution
-reducing the image intensifier by physical collimators has no effect on resolution

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68
Q

would larger matrix sizes improve CT resolution?

A

No
-because of focal spot blur and detector blur

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69
Q

what causes detector blur?

A

-physical size of detector
-screen thickness
-light diffuses before being absorbed
-image thickness or detector area should be smaller than smallest objects you want resolved

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70
Q

does motion blur depend on image magnification

A

no

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71
Q

convert FWHM to limiting spatial resolution in lp/mm

A

FWHM = 1/(2LSF)

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72
Q

ROC curve for random guessing

A

(0,0) to (1,1)- straight line

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73
Q

contrast scale and acceptable limits

A

contrast scale = (ux-uwater)/(CTx-CTwater)
2*10^-4 /cmHU for 100-140 kV

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74
Q

formula for noise that uses contrast scale

A

noise = (contrast scale SD100%)/uwater

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75
Q

unit of US intensity

A

mW/cm2

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76
Q

does US velocity depend on frequency?

A

no

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77
Q

decibels for sound

A

+10 dB = 10X increases, -20 dB = 100X decrease, +30 dB = 1000 X increase

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78
Q

acoustic impedance

A

Z= density* sound velocity, units of Rayl
-independent of frequency
most tissues have Z = 1.6*10^6 Rayl
-air and lung have low Z
-bone and piezoelectric crystal have high
-differences between acoustic impedence at interfaces determine amt of energy reflected at interface

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79
Q

US intensity reflected vs transmitted at interface

A

reflected = [(Z2-Z1)/(Z2+Z1)]^2
-sum of reflected and transmitted equals 1

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80
Q

why do we use gel with US?

A

-tissue/air interfaces reflect 100% of incident US beam
-gel displaces the air and minimizes reflections that would prevent US transmission into patient
-strongest echoes from abdomen imaging are from gas bubbles

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81
Q

can you US image through air or bone?

A

No, because it is all reflected
-lack of transmission results in areas void of echoes- shadowing

82
Q

Z of air, tissue, bone, piezo

A

< 0.01
1
5
20

therefore if have tissue/air interface, only get reflection

83
Q

specular vs non-specular reflection in US

A

-specular- from large, smooth surfaces- used to generate US images
-non-specular- diffuse scatter from rough surfaces

84
Q

hyperechoic vs hypoechoic

A

hyper= higher scatter amplitude relative to background
and vice versa

85
Q

only application of A mode US

A

ophthalmology

86
Q

doppler US color convention

A

-blood flowing away from probe is in blue
-blood flowing towards heart is in red
-opposite of heart… BART

-moving toward detector = higher frequency

87
Q

axial and lateral resolution in US

A

axial: pulse length/Z
lateral: US beam width, improved by increasing number of lines/frame

88
Q

when do you get scatter in US?

A

-when object is smaller than wavelength
-kidney, pancreas, spleen, and liver have scattering sites

-organs that have liquids like bladder and cysts don’t scatter and have almost no echoes (show black)

89
Q

US refraction

A

sin(theta1)/sin(theta2)=v1/v2
-When US passes from one tissue to another having a different speed of sound, the frequency remains the same, but the wavelength changes

90
Q

US attenuation

A

-increases with increasing frequency
-~0.5 dB/cm per MHz

91
Q

thickness of transducer crystal

A

wavelength/2

92
Q

US bandwidth for crustal

A

-related to range of frequencies generated by the crystal
-narrow BW = pure frequency- persists for a long time (ring down time)
-damping material like tungsten or rubber are placed behind transducers to reduce the ring down time. Damping broadens BW and shortens pulses

93
Q

what affects US resolution most?

A

pulse length-axial- axial res is 1/2 of pulse length
lateral is US beam width

lateral res is 4X worse than axial
-transducer frequency determines axial res?

94
Q

does axial resolution vary with depth in US

A

no

95
Q

does lateral resolution vary with depth in US

A

yes
best in focal zone

96
Q

fundamental trade-off in US

A

spatial res and max imaging depth

97
Q

what is piezoelectric

A

-pressure electricity
-converts electrical energy into ultrasonic energy
-electrical energy causes crystal to change shape
-change in shape increases and decreases pressure in front of transducer, thus producing US waves
-when the US echoes return, they create pressure changes that are converted back to electrical energy signals

98
Q

what is fresnel zone

A

-near field
-used for imaging US
-length is r^2/wavelength; r is transducer radius

99
Q

Fraunhofer zone

A

-far field
-starts where near field ends

100
Q

US focal length

A

-distance from transducer to center of focal zone
-focal zone is region over which beam is focused
-focusing increases echo intensities can be done using curved crystal or acoustic lens

101
Q

what info does echo provide in US

A

-time span between return provided depth
-strength provides info about differences in Z

102
Q

pulse repetition frequency in US

A

-number of separate pulses of sound sent out every second
-aka pulse rate
-duration of pulse is 1 us
-between pulses, transducer acts as a reveriver
-common pulse rate is 4 kHz i.e. 4000 pulses/s
-high pulse rate means there is a short listening time when echoes can be detected
-choice of pulse rate controls penetration depth that can be detected

103
Q

thermal index in US

A

ratio of acoustic power produced by transducer to the power needed to raise tissue by 1 degree Celcius

104
Q

harmonic frequencies in US

A

-integral multiple of US pulse frequencies
-arise from non-linear interactions in tissues
-usually use the first harmonic (i.e. 2X the fundamental frequency); higher frequencies have too much attenuation

105
Q

what US artifact does refraction cause?

A

spatial distortions

106
Q

US mirror image artifact

A

sounds is reflected off a large interface like the diaphghram, causing parts of image to be in wrong place

107
Q

ghost image in US

A

-arise because of division of smooth transducer into a large number of small elements in multielement transducer array

108
Q

reflection instensity between soft tissue and air, lung, bone, fat, muscle

A

air > 99%
lung 50%
bone 40%
fat 0.8%
muscle < 0.1%

109
Q

what is spectral analysis in US

A

plot doppler frequency shift as function of time
-gives info on blood flow

110
Q

isotope vs isotone

A

isotope = same number of protons
isotone = same number of neutrons

111
Q

how long must the half life for isomeric state to be called metastable?

A

10^-9 second

112
Q

how far do alpha particles travel

A

< 0.1 mm

113
Q

how many half lives does it take to reach equilibrium?

A

-4 daughter half lives
-activities of parent and daughter are approximately equal

114
Q

transient vs secular equilibrium

A

transient- parent is short lived
secular- parent is long lived

-both secular and transient equilibrium occur after 4 daughter half lives with both parent and daughter activities being approx equal

115
Q

what does pulse height analyzer do

A

-reduce scatter
-determines which portion of the detected spectrum is used to create images
-maximizes number of photopeak events while minimizing the detected photons that would degrade image quality

116
Q

spatial res of SPECT vs planar imaging

A

-SPECT spatial res is worse than planar imaging
-but SPECT has improved contrast due to elimination of overlapping structures

117
Q

thickness of detectors in PET

A

thick: 20-30 mm
-to efficiently detect 511 keV photons

118
Q

PET sensitivity of 3D scanner without septa vs 2D scanner

A

3D is 6X higher than 2D

119
Q

main use of PET/CT

A

staging of malignant disease to monitor patient response to therapy

120
Q

in PET, ratio of organ-specific uptake to unwanted uptake in other tissues

A

target-to-background ratio

121
Q

edge packing in nuc med

A

artifact
increased brightness at edge of crystal
due to internal reflection, absence of PMT beyond edge

-crystals are made larger than FOV to minimize edge packing

122
Q

cumulative activity

A

initial activity * 1.44 * half life

123
Q

S factor in nuc med

A

-absorbed dose in a target organ per unit cumulative activity in a source organ
-S factors increase as size of organ decreases

depends on:
1.number of emissions per transformation
2.energy associated with each emission
3.fraction of emitted energy deposited in the target organ

124
Q

nuc med operator doses during injection

A

0.01-0.02 mSv/h
-annual effective doses range from 1-5 mSv

125
Q

nuc med procedures with high effective dose

A

brain (10 mSv)
inflammation (20mSv)
cardiac (7 mSv)

others rango from 0.5 mSv

126
Q

MIRD

A

medical internal radiation dose
-framework for assessing the absorbed dose to whole organs, tissue subregions, voxelized tissue structures, and individual compartments

127
Q

99Mo breakthrough

A

-results in unnecessary dose
-degrades image wuality b/c of septal penetration
legal limit is 5.5 kBq of 99Mo/ 37 MBq of 99mTc

128
Q

how many hydrogen protons per cm3 tissue

A

10^22

129
Q

spin density of lung, bone, and fat

A

lung- 3%
bone- 5%
fat-98%

don’t see bone or lung in MRI

130
Q

where is T1 relaxation long?

A

-in liquid materials and in solids

131
Q

what does T2 depend on?

A

-approximately independent of magnetic field strength
-T2 decreases with increasing viscosity and decreasing molecular mobility

132
Q

MRI contrast agents

A

T1: gadolinium
T2*: paramgnetic and ferromagnetic materials

133
Q

tissue differences in proton density (MRI)

A

10%

134
Q

MRI resoluton

A

-depends on data acquisition matrix, not display matrix
-stronger gradients = higher res
-high res requires high SNR as well as large data acquisition matrix
-higher res may involve loss of signal intensity and/or increases in image acquisition time

135
Q

MRI SNR

A

-increases with: increased slice thickness
-decreasing matrix size
-reducing RF BW during detection
-magnetic field strenght
-square root of number of image acquisitions
-using smaller surface coils improves SNR

136
Q

ferromagnets

A

-have residual magnetization even after external field is removed
-property of large group of atoms whereas diamagnetism and paramagnetism are properties of individual atoms or molecules
-iron, nickel, cobalt
-have unpaired electrons that are strongly coupled, resulting in large local fields and high susceptibilities

137
Q

is gadolinium paramagnetic?

A

yes
reduces T1
produces more intensity on T1 weighted images

138
Q

BOLD

A

The BOLD technique takes advantage of the fact that the change from diamagnetic oxyhemoglobin to paramagnetic deoxyhemoglobin that takes place with brain activation results in decreased signal intensity on MRI

139
Q

negative contrast agent

A

reduces T2*
-get less intensity on T2 weighted

140
Q

formula for gyromagnetic ratio

A

eg/2m
g is g-factor
m is mass

141
Q

gyromagnetic ratio of 1H

A

42.6 * 2 pi MHz/T
sign of ratio is the sense of precession

142
Q

what causes SAR to increase?

A

-field strength
-RF power and duty cycle
-transmitter coil type
-body size

143
Q

what does film sensitivity depend on?

A

-photon energy
-film with silver- 25 keV binding energy- absorbs 30 keV photons well

144
Q

what is difference between ICRP, ICRU and NCRP

A

ICRP= radiation protection
ICRU= measurement- ec. replaced R with Gy
NCRP= advises regulators on radiation protection

145
Q

objectives of radiation protection

A

-prevent significant deterministic effects
-minimize stochastic risks

146
Q

lateral skull xray air kerma at 1 m

A

1.5 uGy

147
Q

AP abdominal xray air kerma at 1 m

A

3 uGy

148
Q

CT scan air kerma at 1 m

A

30 uGy

149
Q

fluoro air kerma at 1 m

A

20 uGy for 1 min

150
Q

what % more or less mAs do you use for infant vs large adult?

A

-infant- 45%
large adult- 160%

151
Q

average glandular dose in mammo

A

3.5 mGy
4.2 cm breast with 50% glandularity

152
Q

annual limit on intake

A

activity of a radionuclide that will deliver effective dose of 20 mSv during 50 years after radionuclide taken in by someone >18 or for period starting at intake and ending at 70 for someone < 18

153
Q

5 steps to take if dose limits are exceeded

A

Notify person and CNSC
Stop them from working
Determine dose and cause
Take action to prevent similar incident
Report to CNSC in 21 days

154
Q

how manyu cervical vertebrae do humans have

A

7

155
Q

A fetus receives a dose of 2 Gy during weeks 20 to 39 of pregnancy. After birth, the child has an increased risk for what condition

A

leukemia

156
Q

A patient is suspected of having kidney stones. What is the most appropriate exam

A

CT

157
Q

What is the most common location for cancer in the breast?

A

upper outer quadrant

158
Q

For ionizing radiation, how does the OER vary as LET increases from 1 to 100 KeV/μm

A

decreases

159
Q

According to TG-59, an ionization chamber reading is corrected to compensate for the temperature and pressure dependence of which of the following?

A

mass of air in ion chamber

160
Q

In using kV cone-beam CT for image-guided radiation therapy of head and neck cancer, which of the following structures receives the largest imaging dose?
A. Brain stem
B. Mandible
C. Parotid
D. Lens

A

mandible

161
Q

equation for doppler frequency shift in US

A

delta f = 2 v cos(theta) *f/c

162
Q

pencil ionization chamber has an active length of 10 cm. A CT dose measurement is made
with the body CTDI phantom and the chamber is positioned appropriately. A single axial CT scan
is performed at 130 kV, 120 mAs, and 5-mm slice thickness. The chamber reading is 110 mR.
(Assume an f-factor of 0.9 cGy/R.) What is the CTDI for this measurement?

A

20 mGy
110 * 0.9/0.5 and convert units…

163
Q

What is the approximate exposure gamma ray constant for 131I?
R-cm2/mCi-hr?

A

3.3 R-cm2/mCi-h

164
Q

A spatial resolution measurement of a SPECT system is performed using line sources of 99mTc
according to the NEMA protocol. If the spatial resolution (FWHM) is 10.5 mm in the center of the
phantom, what is the peripheral tangential spatial resolution (FWHM) at 7.5 cm from the center of
the phantom?

A

options were all > 10 mm except 1 option at 8 mm- chose that

Resolution degrades with depth in spect

165
Q
  1. Which of the following statements about imaging techniques is FALSE?
    A. Image quality is reduced when111In is used for imaging with a collimator designed for
    99mTc.
    B. 99mTc with a pinhole collimator can be used to image a 3-mm diameter lesion.
    C. High-energy collimators will provide good spatial resolution for 18F.
    D. Tungsten is a better collimator material than lead for imaging 201Tl.
    E. Because the energy of 57Co is so close to that of 99mTc, it can be used for high-count flood
    correction in SPECT imaging.
A

C is false

166
Q

integral uniformity in Nuc Med

A

100 % * (max counts - min counts)/(max counts + min counts)

167
Q

differential uniformity in Nuc Med

A

100%*(high-low)/(high+low)
X and Y are calculated separately

168
Q

A generator is eluted and yields 300 mCi of 99mTc. For clinical studies to be done up to 6 hours
after elution, what is the maximum acceptable number of microcuries of 99Mo allowable at the time
of elution?

A

24 uCi

decay to 6 h and then multiply by 0.5 % allowed Mo 99… or 5.5 kBq per 37 MBq?

Half life of Tc is 6 h
Decay it to then because Mo decays super slowly so at 6 h would have higher percent of Mo to Tc

169
Q

be able to calculate FWHM from data given as # of counts vs channel number and distance per channel

A

remember to subtract background which is square root of max number of counts

170
Q

Assuming the balloon can be considered as a perfect sphere in a MammoSite application, using
an 192Ir high-dose-rate source with a single dwell position at the center of the balloon surface, the
prescribing dose is 340 cGy to a planning target volume (PTV) 1.0 cm from the balloon surface in
three dimensions. The dose-volume histogram is calculated with a constant anisotropy factor for
the source in a large homogeneous tissue-equivalent medium, and the maximum dose within the
PTV is 710 cGy. What is the diameter of the balloon?

A

4.5 cm

Radius plus 1 cm divided by radius, all squared, is IS factor for 710 to 340

171
Q

Assuming the balloon can be considered as a perfect sphere in a MammoSite application, using
an 192Ir high-dose-rate source with a single dwell position at the center of the balloon surface, the
prescribing dose is 340 cGy to a planning target volume (PTV) 1.0 cm from the balloon surface in
three dimensions. The dose-volume histogram is calculated with a constant anisotropy factor for
the source in a large homogeneous tissue-equivalent medium, and the maximum dose within the
PTV is 710 cGy. What is the diameter of the balloon?

A

4.5 cm

340/710 = (r/r+1)^2

172
Q

what is MTF

A

contrast from system as a function of frequency

173
Q

sampling in space domain leads to what in frequency domain?

A

replication

174
Q

nyquist ciriterion

A

must sample at twice the max frequency

175
Q

number of interactions delta N in short distance delta x

A

delta N = delta x * N * u
us is linear attenuation coefficient

176
Q

DQE equation

A

(SNRout/SNRin)^2

SNR in depends on photon stats (ie. N/root(N))
SNR out depends on photobs absorbed by the detector (N(1-exp(-ux))/(root(N(1-exp(-ux)))

177
Q

quantum sink

A

limiting stage with the worst SNR since it has the fewest particles
-without increasing the number of quanta at the quantum sink, the noise of the system cannot be improved

178
Q

quantization noise

A

error introduced into analog signal when it is digitized

179
Q

probability of A occurring if A occurs m(A) times in M repetitions

A

P(A) = lim as M goes to infinity of (m(A)/M)

180
Q

probability of A or B

A

P(A) + P(B)

181
Q

probability of A and B

A

P(A)*P(B)

182
Q

probability of A given B

A

(P(A)*P(B))/P(B)

183
Q

probability density function

A

P(x= X) = integral from -infinity to X of p(x)dx

184
Q

expectation value

A

= integral from minus inifnity to infinity of x^n(p(x))dx
<(x-xbar)^n>= integral from minus infinity to infinity of (x-xbar)^n * p(x)dx

185
Q

equation for variance

A

sigma squared = <(x-xbar)^2>= -^2

186
Q

what is characteristic function

A

FT of probability density function

187
Q

what is wiener spectrum

A

FT of auto-correlation function

188
Q

white noise

A

contains equal amounts of all frequencies of interest

189
Q

rose model

A

SNR > 5 for detection
SNR = contrast * root(N)
N is number of photons per unit area

contrast is (photons per unit area - photons per unit area in ROI)/(photons in unit area)
could be (phi - phi exp(-ux))/phi

exp(-ux) is about 1 -ux for u «&laquo_space;1

190
Q

FT of rectangle

A

sinc function

191
Q

FT of triangke

A

sinc^2

192
Q

noise equivalent quanta

A

noise will make quanta appear to be reduced
DQE = NEQ/true input
\
NEQ = q is ideal system

193
Q

does amplification improve DQE?

A

no because it also amplifies noise

194
Q

slower film

A

needs more exposure for the same density

195
Q

max optical density in film

A

of grains/cm3 before exposure * cross section wrt visible light *thickness of film * 0.434

196
Q

film-screen
screen efficiencies

A

1- # of xrays absorbed by screen/number of xrays incident on screen - 30-80%
2- # of optical photons * their energy / (number of xrays photons absorbed * their energy) - 5 %
3- screen efficiency - accounts for optical energy that escapes the screen - 50%

197
Q

how to increase efficiency of screen-film systems

A

-double emulsion or use thicker screens (hurts resolution)
-use optimized k edge energy
-match the screen emission wavelength to the film sensitivity

198
Q

how is noise affected by absorption efficiency of screen?

A

when absorption efficiency increases, less incident xrays are required, but the fraction of xrays that are detected increases. Total number of detected xrays stays the same… therefore noise is not affected by absorption efficiency

199
Q

pros and cons of high speed film system

A

high speed= lower patient dose, higher quantum mottle. Mottle is higher because less quanta are required- therefore get more noise
-higher contrast
-con is reduced latitude (dynamic range)

200
Q

quantum mottle vs structure mottle

A

quantum: fewer photons reaching the image receptor = fluctuation in image densities
structure: uneven distribution of phosphors in screen leads to irregular structure of screen

201
Q

resolution of film

A

FWHM of LSF

202
Q

what is film speed proportional to?

A

of light photons generated

thickness of screen (1/thickness ^2)

Huda (20 decks)

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