Week 1 Flashcards
(133 cards)
1
Q
what % of radiation comes from CT
A
60%
2
Q
difference between gen rad & CT
A
CT reconstructs cross sectional images of internal structures according to x-ray attenuation coefficients for almost every part of body
gen rad only makes 2d image where superimposition poses a potential issue in hiding key info
3
Q
CT use examples
A
- diagnosis, monitoring, screening diseases
- image guided therapy such as target localization & treatment planning
- attenuation correction of radionuclide imaging & localization
4
Q
similarity between CT & linear tomography
A
produces detailed images of specific layers of body
5
Q
what does CT allow for
A
- new clinical applications
- decreased dose
- iodinated contrast media
6
Q
CT was previously known as
A
computerized axial tomography (CAT) scans
7
Q
how does CT work
A
x-ray tube & detector rotate around patient to acquire data > processed by computer > reconstructs detailed slices of cross sectional anatomy of various body regions
8
Q
what imaging technique was used to visualize brain before CT
A
radiography / pneumoencephalography
9
Q
which mathematician introduced transformation operator that laid foundation for tomographic reconstruction
A
Johann Radon; radon & inverse radon transform
10
Q
who validated Allan Cormack’s work by building 1st CT prototype
A
Sir Godfrey Hounsfield
11
Q
which gen of CT scanner uses wide fan beam
A
3rd, 4th, 6th
12
Q
which gen of CT scanner uses narrow fan beam
A
2nd
13
Q
which gen of CT scanner uses cone beam
A
7th
14
Q
which gen of CT scanner uses pencil beam
A
1st
15
Q
which gen of CT scanner uses electron gun firing beam
A
5th
16
Q
which gen of CT scanner uses translate-rotate
A
1st, 2nd
17
Q
which gen of CT scanner uses rotate-rotate
A
3rd, 6th, 7th
18
Q
which gen of CT scanner uses rotate-fixed
A
4th
19
Q
which gen of CT scanner uses stationary-stationary
A
5th
20
Q
which gen of CT scanner starts the use of helical/spiral
A
6th
21
Q
what is slip ring indicative of which gen
A
3rd
22
Q
purpose of pre-patient collimators
A
- lower radiation dose
- prevent over-beaming
23
Q
purpose of post-patient collimators
A
- reduce scatter & penumbra
- defines slice thickness
SPS
24
Q
purpose of slip-rings
A
- allows scan frame to rotate continuously without stopping to rewind cables
- power & control signals comms to rotating frame through slip ring
25
image formation process
1. tube generates radiation
2. radiation penetrates body & attenuated by tissue
3. attenuated xray processed by detector
4. detector converts radiation into electrical signals
5. computer converts data into high res images
26
GOS
gadolinium oxysulfide
27
Pr
Praesodynium-doped
28
Lu
Lutetium
29
UFC
ultrafast ceramic
30
Gemstone Clarity Detector uses
Lu-based garnet
31
NanoPanel Prism Detector uses
Top layer: yttrium based garnet
bottom layer: GOS
32
Stellar Detector uses
UFC
33
Quantum Vi Detector uses
Pr GOS
34
PURE vision CT detector uses
Pr as an active additive of Toshiba GOS
35
commonly used solid state detectors
GOS, CsI, UFC
36
properties of solid state detectors
high detection, high geometric, small DELs
37
purpose of solid scintillator layer is to
convert x-ray photons to light photons
38
purpose of photodiode layer is to
convert light photons to electrical signal
39
purpose of beam shaping filter is to
remove low energy / soft x-rays to narrow x-ray energy spectrum and create monochromatic beam (beam hardening)
40
what happens if the patient is off iso-center
filtering mismatch > x-ray output needs to be compensated = increased dose
41
sequential scan
patient moves gradually layer-by-layer through gantry during scan
42
spiral scan
patient continuously moves through gantry during scan
43
advantages of using multi-row detectors
- reduced scan time
- less motion artifacts
- shorter breath holds needed
- multiphase contrast enhanced scanning possible
44
multi-slice CT benefits over single-slice CT
- efficient use of x-ray beam with increasing slices
- reduced dose
45
what configs do array detectors offer
fixed & adaptive array configs
46
pitch definition
table distance traveled in 1 360 degrees gantry rotation divided by beam collimation / x-ray beam width
47
typical detector pitch ranges from
0.75 - 1.5
48
choice of pitch influences __
1. image quality
2. patient dose
3. scan time
49
helical scan is only used for
larger structures
50
what factors involved in 'scan & data acquisition'
kVp, mA, scan time, Pitch, Beam Width, scan FOV (body coverage)
51
what factors involved in 'image reconstruction'
slice thickness, FOV, matrix, filter
52
what factors involved in 'display control'
zoom, window width, window level
53
Process for scan & data acquisition
1. scout / topogram / localizer AP &/or lateral
2. Plan CT scan region
3. CT acquisition
54
SFOV
area within gantry that raw data is collected
55
DFOV
determines how much of the SFOV is reconstructed into an image
56
pitch less than 1 is used for
imaging small structures & highly detailed ones
57
purpose of adjusting DFOV
1. exclude unnecessary area from image
2. focus on specific anatomical regions
3. include more anatomical info in image
58
pitch higher than 1 is used for
temporal structures & screening
59
constant kVp used for CT is
120 / 130 kV
60
when should you consider lowering kVp
to give more contrast esp with iodinated contrast & exploit k-edge
61
what is the downside of lowering kVp
increased noise which can be compensated with increased mAs but leads to increased dose
62
CT dose is approximately ___ of applied kVp
proportional to the square
63
diff between kVp & keV
kVp = peak voltage applied to xray influencing max energy of xray photons produced
keV = energy of individual xray photons
64
diff between average & max keV
avg = mean energy of xray photons in beam which influences image contrast & quality
max = highest energy of xray photons which indicates most penetrating photons
65
why does iodine attenuation increase at lower tube potential
decreased compton scatter & increased photoelectric effect as photon energy approaches k-edge of iodine (33.2keV)
66
if u want reconstructed images at a certain slice thickness, will you use a smaller or larger detector width to acquire raw data
wider anatomy = larger detector width = lower quality & dose
highly detailed anatomy = smaller detector width = higher quality & dose
67
what happens before reconstruction
interpolation
68
how does CT compensate this assumption of a circular path instead of a helical one
helical raw data set is interpolated into series of 2D image data sets before filtered back projection
69
how is interpolation achieved
weighing helical data average from either side of reconstructed plane
70
simple back projection is used to
emulate acquisition process in reverse
71
what does each ray in each view represent
individual measurement of attenuation factor
72
what is needed for simple back projection algorithm to regain image
take into account the position & angles of each detector
73
assumptions for simple back projection
1. infinitely small DELs
2. monochromatic spectrum for determination of micro
3. small FSS
74
how does the computer store simple back projection
in sinograms
75
what does the horizontal & vertical axis of simple back projection refer to
horizontal = different rays in each projection
vertical = projection angle
76
vertical line on sinogram means
bad detector
77
what is the issue of simple back projection
produces lots of blurring artifacts since assumptions are not met
78
filtered back projections are used to
filter blurring artifacts, noise, low contrast using different filters that offer tradeoffs between spatial resolution & noise
79
bone filter provides
fine details but noisy images
80
soft tissue filter provides
smoothing but less noise & spatial resolution
81
___ used to reconstruct CT images
iterative algorithms
82
how does iterative algorithms work
1. pulls out image assumption from vendor
2. compares it to real time data while making constant adjustments
3. repeats till both assumption & real data are in agreement
83
iterative reconstruction advantages
1. reduced noise
2. increased CNR & spatial resolution
84
iterative reconstruction disadvantages
1. slow
2. less predictive behavior
85
process of mapping x-ray attenuation back to its original voxel is ___
back projection
86
CTDIvol
Volume CT Dose Index
87
CTDIvol unit
mGy
88
DLP unit
mGy.cm
89
absorbed dose formula
CTDIvol x DLP
90
DLP
dose length product
91
effective dose formula
DLP x conversion factor
92
effective dose unit
Sv
93
DLP formula
CTDIvol x CT length of scan range
94
anything with Gy is __
absorbed dose
95
anything with Sv is ___
effective dose
96
deterministic risk primarily described by
CTDIvol
97
stochastic risk described by
DLP & effective dose
98
CTDI weighted formula
1/3 CTDI center + 2/3 CTDI peripheral
99
____ values are used to describe radiation to patients
CTDI; not actual dose values
100
CTDIvol formula
CTDIvol = CTDI weighted / pitch
101
what does CTDIvol account for
effect of pitch on dose
102
what does CTDI weighted account for
approximate average dose per slice
103
DLP represents
total radiation energy deposited in patient's body
104
effective dose is used for
estimation of risks
105
CT dose reduction strategies
DIMAR
1. automatic tube current modulation (mA)
2. reduce tube voltage in appropriate patients (kV)
3. DRLs
4. minimize scan ranging
5. iterative reconstruction
106
what are DRLs
surveys of dose estimates from diff modalities to highlight substantial variations in dose between diff facilities for same exam & similar patient group
107
purpose of DRLs
monitoring practices to promote improvements in patient protection
108
national & local DRLs set for
1. each exam
2. each clinical indication
3. each patient group
109
what CT DRLs are recommended quantities
CTDIvol & DLP
110
DRLS are determined as
median values observed for representative samples of particular group of patients
111
good CT images characteristics
1. high spatial resolution
2. high low contrast detail
3. acceptable temporal resolution
4. minimal noise
5. no artifacts
112
pixel size formula
DFOV / matrix size
113
voxel size is dependent on
1. FOV
2. Matrix
3. Slice thickness
114
how is spatial resolution expressed
lp/mm , lp/cm
115
how is spatial resolution measured
using resolution bar patterns
116
what is important in 3D/MPR reconstruction
detector array thickness
117
thinner slice thickness for spatial resolution leads to
1. higher spatial resolution
2. less partial volume effect
3. lower contrast
4. more noise
118
thicker slice thickness for spatial resolution leads to
1. more partial volume effect
119
when dose partial volume effect occur
when object doesnt fill entire depth of scan plane which is dependent on slice thickness = CT number underestimated
120
what does contrast refer to
sensitivity of the system; ability to differentiate between intensity differences in image
121
how to improve contrast resolution
1. reduce noise by increasing mAs & slice thickness (increased dose)
2. lower tube voltage
3. reconstruction algorithms
122
what are the risks of too long scan time
breathing artifacts & motion artifacts which can lead to double contour of organs
123
what is noise
quantum noise determined by x-ray flux & number of detected photons which is influenced by
1. detector efficiency
2. scanning technique
3. patient habitus
124
the greater the noise, the ___ the SD
greater
125
how to increase details in CT
1. decrease slice thickness
2. increase matrix
3. decrease FOV
126
why is patient preparation before Ct important for
- optimal image quality
- efficient workflow
- safety
- patient cooperation
127
importance of checking clinical history & previous images
determines appropriateness of imaging choice & best protocol to do so
128
helical CT allows for ___
rapid volumetric scanning; common among modern CTs
129
Cine / Cardiac CT allows for ___
multiple rotations with/without cardiac gating
130
CT perfusion allows for ___
multiple time points post processing to compute tissue dynamics
131
4D lung planning allows for ___
multiple time phase reconstruction during respiratory cycles
132
TZM means ___
titanium / zirconium / molybdenum
133
what is afterglow
persistence of image after radiation has been turned off
