Miscellaneous imaging Flashcards
1
Q
useful range of film OD
A
0.3 (50% transmittance) to 2 (1% transmittance)
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
3
Q
film speed
A
~ 1/amt of light needed for development
4
Q
film latitude
A
exposure range of useful contrast
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
6
Q
common intensifying screen thickness
A
200 um
7
Q
intensification factor
A
ratio of exposures, without and with intensifying screen, to get a given film density
-30-50
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
9
Q
what is purpose of image intensifier
A
convert xrays exiting patient into a bright light image
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
11
Q
nuc med matrix size
A
128x128, 1 byte per pixel
1/64 MB
12
Q
MRI matrix size
A
256x256, 2 bytes per pixel
1/8 MB
13
Q
CT matrix size
A
512x512, 2 bytes per pixel
1/2 MB
14
Q
US matrix size
A
512x512, 1 byte per pixel
1/4 MB
15
Q
conversion efficiency of scintillator
A
% of absorbed E converted into light
2-20%
scintillators aka phosphors
16
Q
mammo matrix size
A
4096x6144, 2 bytes/pixel
50 MB
17
Q
CR, DR, and film matrix size
A
2560x2048, 2 bytes/pixel
10 MB
18
Q
digital photosopt/DSA matrix size
A
1024x1024, 2 bytes/pixel
2 MB
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
20
Q
CR intensities compared to screen-film
A
screen film= 5 uGy
CR plates can tolerate 100 X above and below
21
Q
direct flat panel detector
A
photoconductor
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)
23
Q
abdomen window level
A
window = 150
level = 60
24
Q
semi-annual mammo QA
A
-darkroom fog
-screen-film contact
-compression
25
what does raising film developer temperature do to fog?
increases it
26
how to increase film contrast and density
-increase developer temperature
-increase developer time
27
what is gamma in film
-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
28
what can cause film to fall outside of useful exposure range?
-incorrect exposure setting on film
-anatomy produces wider range of exposures
29
effect of overprocessing film
-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
30
effect of underprocessing film
-curve shifts to right- decrease in sensitivity
-shoulder drops, slope of curve decreases- less contrast and less density
31
what is detector geometric efficiency/sensitivity proportional to?
-proportional to radiation sensitive detector area and inversely proportional to square of source-to-detector distance for a pt source
32
what is detector intrinsic efficient related to?
-detector thickness
-Z
-mass density
-decreases with photon energy
33
define detector resolution
-FWHM of single energy peak at specific energy
34
gain of amplifier
log ratio of output power/voltage to input power/voltage
-measured in dB
35
intrinsic vs extrinsic performance in nuc med
-intrinsic looks at subpart of imager (ex detector without degrading effects of collimator)
-extrinsic look at total image; realistic conditions
36
how much power loading can a 1 mm focal spot tolerate?
100 kW
37
why are higher kV values (140 kV) used in head scanning?
help minimize beam hardening artifacts
38
typical kV values CT
regular= 120 kV
higher = 140 kV
lower (pediatric) = 80 kV
39
nyquist frequency
-highest frequency that can be faithfully reproduced
-half the sampling rate
-nyquist rate is 2X max frequency
40
what does aliasing cause in US?
-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
41
mottle in CT
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
42
quantum mottle in CT vs nuc med
nuc med is higher than CT because number of photons used to generate image is low
43
random bright and dark streaks in CT
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
44
what can beam hardening and scatter cause in CT?
-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
45
what do CT collimators do?
-define total beam width: 40 mm for 64-slice and 160 mm for 320-slice
-reduce scatter radiation
46
collimantor sensitivity
-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
47
what images do you use for QA of collimator?
flood images
48
do PET systems use collimators?
No
thus have increased sensitivity
49
scintillator in Anger camera
NaI
50
counts and PMTs in Anger camera
55 PMTs
500,000 counts
51
Anger camera max non-uniformity
5%
52
intrinsic resolution of Anger camera
3-5 mm
system resolution is R = square root (Ri^2+Rc^2)
i.e. intrinsinc and collimator
53
different types of SPECT collimators
-parrallel hole (constant FOV)
-converging- magnified, FOV~ 1/distance
-diverging- project smaller image size, FOV increases with distance
pin hole- magnify and invert
54
quantum efficiency of CT detectors
>90%
geometric efficiency also ~ 90% for detectors 1 mm wide and separate 0.1 mm
55
what determines slice thickness in CT
detector width
-0.5-0.6 mm
56
multi-detector CT beam width
number of slices times slice thickness
57
dual energy CT
-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
58
compare pros and cons of CT image quality vs radiography
-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)
59
CT FOV for head and body
25 cm
40 cm
60
matrix size CT
512x512
pixel size is FOV/number of pixels
61
compare CT generations
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
62
CTDI for head vs body phantom
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)
63
pitch
table movement in 1 rotation/beam width
< 1 = oversamples, more dose
> 1 = undersampled, less dose
64
cardiac imaging
-best in diastolic phase of cardiac cycle
-pitch 0.2-0.3
-high dose
-use dual energy scanner to improve temporal resolution
65
treshold of contrast for underexposed and overexposed film
<0.5 OD
> 2 OD
positive contrast= darker lesions (lesion absorbs fewer xrays)
66
relationship between film latitude and contrast
inverse
67
how to increase resolution in fluoro
-halving the image intensifier FOV electronically doubles resolution
-reducing the image intensifier by physical collimators has no effect on resolution
68
would larger matrix sizes improve CT resolution?
No
-because of focal spot blur and detector blur
69
what causes detector blur?
-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
70
does motion blur depend on image magnification
no
71
convert FWHM to limiting spatial resolution in lp/mm
FWHM = 1/(2LSF)
72
ROC curve for random guessing
(0,0) to (1,1)- straight line
73
contrast scale and acceptable limits
contrast scale = (ux-uwater)/(CTx-CTwater)
2*10^-4 /cmHU for 100-140 kV
74
formula for noise that uses contrast scale
noise = (contrast scale *SD*100%)/uwater
75
unit of US intensity
mW/cm2
76
does US velocity depend on frequency?
no
77
decibels for sound
+10 dB = 10X increases, -20 dB = 100X decrease, +30 dB = 1000 X increase
78
acoustic impedance
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
79
US intensity reflected vs transmitted at interface
reflected = [(Z2-Z1)/(Z2+Z1)]^2
-sum of reflected and transmitted equals 1
80
why do we use gel with US?
-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
81
can you US image through air or bone?
No, because it is all reflected
-lack of transmission results in areas void of echoes- shadowing
82
Z of air, tissue, bone, piezo
< 0.01
1
5
20
therefore if have tissue/air interface, only get reflection
83
specular vs non-specular reflection in US
-specular- from large, smooth surfaces- used to generate US images
-non-specular- diffuse scatter from rough surfaces
84
hyperechoic vs hypoechoic
hyper= higher scatter amplitude relative to background
and vice versa
85
only application of A mode US
ophthalmology
86
doppler US color convention
-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
axial and lateral resolution in US
axial: pulse length/Z
lateral: US beam width, improved by increasing number of lines/frame
88
when do you get scatter in US?
-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
US refraction
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
US attenuation
-increases with increasing frequency
-~0.5 dB/cm per MHz
91
thickness of transducer crystal
wavelength/2
92
US bandwidth for crustal
-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
what affects US resolution most?
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
does axial resolution vary with depth in US
no
95
does lateral resolution vary with depth in US
yes
best in focal zone
96
fundamental trade-off in US
spatial res and max imaging depth
97
what is piezoelectric
-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
what is fresnel zone
-near field
-used for imaging US
-length is r^2/wavelength; r is transducer radius
99
Fraunhofer zone
-far field
-starts where near field ends
100
US focal length
-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
what info does echo provide in US
-time span between return provided depth
-strength provides info about differences in Z
102
pulse repetition frequency in US
-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
thermal index in US
ratio of acoustic power produced by transducer to the power needed to raise tissue by 1 degree Celcius
104
harmonic frequencies in US
-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
what US artifact does refraction cause?
spatial distortions
106
US mirror image artifact
sounds is reflected off a large interface like the diaphghram, causing parts of image to be in wrong place
107
ghost image in US
-arise because of division of smooth transducer into a large number of small elements in multielement transducer array
108
reflection instensity between soft tissue and air, lung, bone, fat, muscle
air > 99%
lung 50%
bone 40%
fat 0.8%
muscle < 0.1%
109
what is spectral analysis in US
plot doppler frequency shift as function of time
-gives info on blood flow
110
isotope vs isotone
isotope = same number of protons
isotone = same number of neutrons
111
how long must the half life for isomeric state to be called metastable?
10^-9 second
112
how far do alpha particles travel
< 0.1 mm
113
how many half lives does it take to reach equilibrium?
-4 daughter half lives
-activities of parent and daughter are approximately equal
114
transient vs secular equilibrium
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
what does pulse height analyzer do
-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
spatial res of SPECT vs planar imaging
-SPECT spatial res is worse than planar imaging
-but SPECT has improved contrast due to elimination of overlapping structures
117
thickness of detectors in PET
thick: 20-30 mm
-to efficiently detect 511 keV photons
118
PET sensitivity of 3D scanner without septa vs 2D scanner
3D is 6X higher than 2D
119
main use of PET/CT
staging of malignant disease to monitor patient response to therapy
120
in PET, ratio of organ-specific uptake to unwanted uptake in other tissues
target-to-background ratio
121
edge packing in nuc med
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
cumulative activity
initial activity * 1.44 * half life
123
S factor in nuc med
-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
nuc med operator doses during injection
0.01-0.02 mSv/h
-annual effective doses range from 1-5 mSv
125
nuc med procedures with high effective dose
brain (10 mSv)
inflammation (20mSv)
cardiac (7 mSv)
others rango from 0.5 mSv
126
MIRD
medical internal radiation dose
-framework for assessing the absorbed dose to whole organs, tissue subregions, voxelized tissue structures, and individual compartments
127
99Mo breakthrough
-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
how many hydrogen protons per cm3 tissue
10^22
129
spin density of lung, bone, and fat
lung- 3%
bone- 5%
fat-98%
don't see bone or lung in MRI
130
where is T1 relaxation long?
-in liquid materials and in solids
131
what does T2 depend on?
-approximately independent of magnetic field strength
-T2 decreases with increasing viscosity and decreasing molecular mobility
132
MRI contrast agents
T1: gadolinium
T2*: paramgnetic and ferromagnetic materials
133
tissue differences in proton density (MRI)
10%
134
MRI resoluton
-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
MRI SNR
-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
ferromagnets
-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
is gadolinium paramagnetic?
yes
reduces T1
produces more intensity on T1 weighted images
138
BOLD
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
negative contrast agent
reduces T2*
-get less intensity on T2 weighted
140
formula for gyromagnetic ratio
eg/2m
g is g-factor
m is mass
141
gyromagnetic ratio of 1H
42.6 * 2 pi MHz/T
sign of ratio is the sense of precession
142
what causes SAR to increase?
-field strength
-RF power and duty cycle
-transmitter coil type
-body size
143
what does film sensitivity depend on?
-photon energy
-film with silver- 25 keV binding energy- absorbs 30 keV photons well
144
what is difference between ICRP, ICRU and NCRP
ICRP= radiation protection
ICRU= measurement- ec. replaced R with Gy
NCRP= advises regulators on radiation protection
145
objectives of radiation protection
-prevent significant deterministic effects
-minimize stochastic risks
146
lateral skull xray air kerma at 1 m
1.5 uGy
147
AP abdominal xray air kerma at 1 m
3 uGy
148
CT scan air kerma at 1 m
30 uGy
149
fluoro air kerma at 1 m
20 uGy for 1 min
150
what % more or less mAs do you use for infant vs large adult?
-infant- 45%
large adult- 160%
151
average glandular dose in mammo
3.5 mGy
4.2 cm breast with 50% glandularity
152
annual limit on intake
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
5 steps to take if dose limits are exceeded
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
how manyu cervical vertebrae do humans have
7
155
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
leukemia
156
A patient is suspected of having kidney stones. What is the most appropriate exam
CT
157
What is the most common location for cancer in the breast?
upper outer quadrant
158
For ionizing radiation, how does the OER vary as LET increases from 1 to 100 KeV/μm
decreases
159
According to TG-59, an ionization chamber reading is corrected to compensate for the temperature and pressure dependence of which of the following?
mass of air in ion chamber
160
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
mandible
161
equation for doppler frequency shift in US
delta f = 2 v cos(theta) *f/c
162
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?
20 mGy
110 * 0.9/0.5 and convert units...
163
What is the approximate exposure gamma ray constant for 131I?
R-cm2/mCi-hr?
3.3 R-cm2/mCi-h
164
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?
options were all > 10 mm except 1 option at 8 mm- chose that
Resolution degrades with depth in spect
165
3. 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.
C is false
166
integral uniformity in Nuc Med
100 % * (max counts - min counts)/(max counts + min counts)
167
differential uniformity in Nuc Med
100%*(high-low)/(high+low)
X and Y are calculated separately
168
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?
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
be able to calculate FWHM from data given as # of counts vs channel number and distance per channel
remember to subtract background which is square root of max number of counts
170
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?
4.5 cm
Radius plus 1 cm divided by radius, all squared, is IS factor for 710 to 340
171
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?
4.5 cm
340/710 = (r/r+1)^2
172
what is MTF
contrast from system as a function of frequency
173
sampling in space domain leads to what in frequency domain?
replication
174
nyquist ciriterion
must sample at twice the max frequency
175
number of interactions delta N in short distance delta x
delta N = delta x * N * u
us is linear attenuation coefficient
176
DQE equation
(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
quantum sink
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
quantization noise
error introduced into analog signal when it is digitized
179
probability of A occurring if A occurs m(A) times in M repetitions
P(A) = lim as M goes to infinity of (m(A)/M)
180
probability of A or B
P(A) + P(B)
181
probability of A and B
P(A)*P(B)
182
probability of A given B
(P(A)*P(B))/P(B)
183
probability density function
P(x= X) = integral from -infinity to X of p(x)dx
184
expectation value
= 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
equation for variance
sigma squared = <(x-xbar)^2>= -^2
186
what is characteristic function
FT of probability density function
187
what is wiener spectrum
FT of auto-correlation function
188
white noise
contains equal amounts of all frequencies of interest
189
rose model
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 <<<< 1
190
FT of rectangle
sinc function
191
FT of triangke
sinc^2
192
noise equivalent quanta
noise will make quanta appear to be reduced
DQE = NEQ/true input
\
NEQ = q is ideal system
193
does amplification improve DQE?
no because it also amplifies noise
194
slower film
needs more exposure for the same density
195
max optical density in film
# of grains/cm3 before exposure * cross section wrt visible light *thickness of film * 0.434
196
film-screen
screen efficiencies
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
how to increase efficiency of screen-film systems
-double emulsion or use thicker screens (hurts resolution)
-use optimized k edge energy
-match the screen emission wavelength to the film sensitivity
198
how is noise affected by absorption efficiency of screen?
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
pros and cons of high speed film system
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
quantum mottle vs structure mottle
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
resolution of film
FWHM of LSF
202
what is film speed proportional to?
# of light photons generated
thickness of screen (1/thickness ^2)
