computed tomography Flashcards
1
Q
what are the 2 major limitations of conventional radiography
A
- low in contrast
- lots of superimposition
2
Q
a large tomographic angle creates a ___ thickness of cut
A
small thickness
3
Q
a small tomographic angle creates a ___ thickness of cut
A
large thickness of cut
4
Q
what is the difference between conventional tomography, versus CT
A
conventional: axial tomography
CT: trans axial or transverse
5
Q
explain axial tomography
A
parallel to long axis of body
sagittal or coronal images
6
Q
explain trans axial or transverse CT
A
perpendicular to the long axis of the body
7
Q
CT was developed by who
A
Godfrey Hounsfield
8
Q
what determines slice thickness of first gen CT scanners
A
beam width
9
Q
first gen CT scanners aquired what type of images/slices
A
axial slices
10
Q
what was the scanning process of first gen CT scanners
A
the tube and detector scan across the subject
rotate 1 degree
scan again in opp direction
11
Q
how many views did first gen CT scanners collect over 180 degrees
A
180
12
Q
what type of motion did first gen CT scanners use?
A
translate-rotate motion **
13
Q
what was the imaging time for first gen CT scanners
A
5 minutes
14
Q
what was the beam difference in 2nd gen scanners
A
used a fan beam
15
Q
what kind of motion do 2nd gen use
in what increments
A
translate-rotate
5-10 degree increments
16
Q
how many cells make up the 2nd gen detector array
A
5-30 detector cells
17
Q
what are the pros of 2nd gens (2)
A
- faster, so less imaging time
- scan could be potentially performed in a breath hold (20-30 sec)
18
Q
about how fast could 2nd gens scan
A
20-30 seconds
19
Q
what are the cons of 2nd gen (3)
A
heavier tube with vibrating electronics
scan time still too long
unequal attenuation
20
Q
what type of filter must be used with fan beams
A
bow tie filter
21
Q
what type of movement do 3rd gen use
A
rotate-rotate
22
Q
how much does the 3rd gen fan beam cover
A
the entire patient
23
Q
what type of detector is used in 3rd gen
A
curvilinear detractor array
24
Q
what is the 3rd gen imaging time
A
sub-second image time
25
what are the 2 types of 3rd gen collimation
pre-pt collimator
pre-detector collimator
26
what is SSCT
single slice per revolution scanners
27
what is the major disadvantage to SSCT scanners
ring artifact
28
how do ring artifacts happen
just one faulty detector can cause it
29
what is CAT
computed axial tomography
30
how does interscan delay happen on CATs
table is moving to next location (when they had single rows)
31
what moves in CAT
CT table/couch moves and remains stationary while the tube rotates within the gantry
32
what gen is this
first
33
what gen is this
second
34
what gen is this
third
35
what gen is this
third
36
what generation are MDCT scanners
third
37
what is MDCT
multi detector computed tomography
38
what was improved with mdct
temporal resolution
39
what is temporal resolution
the ability to capture and resolve images of fast-moving structures
40
what are slip rings
set or parallel, stationary, electrically conductive rings
in contact with conductive brushes or blocks
41
what do slip rings do
supply voltage without the need to respool every 360 degrees
42
what is this
slip rings
43
the following are all functions of what?
* Provide the electrical power to operate tube and detector
* Provide scanning instructions to the gantry components
* Transfer detector signal into image reconstruction
computer
44
what are the advantages of slip ring technology (4)
facilitates continuous rotation (no interscan delay)
less motion artifacts
quicker scans
improved temporal reoslution
45
what is spiral/helical scanning
slices begin and end at different points on the same z-axis
46
what type of scanning is this
helical scanning
47
what removes the slant and blur resulting from helical scanning
interpolation
48
what do 4th gen scanners do
rotating tube within a circular detector array
49
what gen scanners is this
fourth
50
what are the 3 major steps of CT image formation
1. data acquisition
2. image reconstruction
3. image display
51
what happens during data acquisition
detectors measure attenuation values
data is converted into binary for input into the computer
52
what does DAS stand for
data acquisition system
53
what is the DAS
the electronics positioned between the detector array and computer
54
what does the DAS do
measures the transmitted beam, encodes into binary, and transmits to the computer
55
where is raw data stored
in the RAM of the computer attached to the CT system
56
what does the computer use for image reconstruction
the raw data stored in the RAM
57
the reconstructed image is in what form
numerical form
58
can the reconstructed image be viewed in numerical form?
no!
59
where are the images sent when complete?
into PACS
60
what is the analog signal
the attenuation measurements
61
where does the signal become digital
when translated to the numerical CT image
62
what is CT detector efficiency?
the ability to capture, absorb, and convert photons into electrical signals
63
what 3 types of efficiency must CT detectors possess
capture
absorption
conversion
64
what is capture efficiency
ability to capture photons transmitted from the patient
65
what determines capture efficiency
size of detector area
distance
66
what is absorption efficiency
number of photons absorbed by the detector
67
what does absorption efficiency depend on (4)
atomic number
physical density
size
thickness of detector face
68
what is conversion efficiency
how well the detector converts the incoming x-rays to a digital signal
69
what is the response time or resolving time
recovery time between detecting x-ray events
70
what is afterglow
the persistence of the image after the radiation is turned off
71
what shoulde the CT afterglow values be
<0.01%, 100 milliseconds after termination
72
what is the dynamic range
ratio of largest to smallest signal that can be measured
73
what is the average CT dynamic range
1 million : 1
74
what is total detector efficiency
product of capture efficiency, absorption efficiencym, and conversion efficiency
75
76
what are solid state scintillation detectors
detector elements are affixed to a circuit board
77
what materials are used for solid state scintillation detector photodiodes
cadmium tungstinate and ceramic
gadolinium oxysulsife with ultrafast ceramic
78
chem formula for cadmium tungstinate
CdWO4
79
chem for gadolinium oxysulfide
Gd2O2S
80
what are the 2 categories of multi-row CT detectors
matrix array detectors
adaptive array detectors
81
what are matrix array detectors
(fixed array detectors)
cells have equal dimensions
82
what are adaptive array detectors
cells have unequal dimensions
83
how big is the CT gantry
70-90 cm
84
R1
gantry
85
R2
gantry aperture
86
what needs to be changed regularly on the gantry
the filters
87
what is the tilting ability of gantry angulation for most CT machines
+/- 30 degrees
88
what is the rough weight limit of the CT table
200-300 kg
89
what is indexing
a way to ensure that the machine knows where it is on the table
90
what determines the image reconstruction time
microprocessors (RAM)
primary memory
91
the x plane goes where
left to right across pt
92
the y plane goes where
anterior to posterior on pt
93
the z plane goes where
head to toe on pt
94
what is the isocentre
the point in the gantry where all 3 axes intersect
95
what is the difference between a pixel and a voxel
voxel is basically a 3D pixel
96
what reconstruction matrix size do current systems have
512 x 512
97
Pixel size = ?
FOV / matrix size
98
what is a scout / scanogram / topogram
low dose x-ray taken prior to CT
used to choose FOV
99
Ni = ?
N0 e ^-mu x
100
what is the formula for slice width
Ni = No e ^-mu x
101
what was the first reconstruction technique
ART
algebraic reconstruction technique
102
what reconstruction technique do most systems use
analytical reconstruction algorithms
103
how do analytical reconstruction algorithms work
filtered back-projection
interpolation
104
how does iterative reconstruction work
start with an assumed value
compare to measured
make corrections until match or acceptable
105
what are the advantages of iterative reconstruction
reduce image noise and artifacts
lower pt dose
106
what is the disadvantage of iterative reconstruction
takes a long time, lots of effort from the computer
107
how does FBP work
filtered back projection
"smears back" the image to reconstruct
108
what is another term for FBP
convolution method
109
how is FBP blur supressed
using mathematical filters
110
what is the thing that reduces FBP blur
convolution filter
kernel
111
what does the sharpening filter do
reduces blur
accentuates noise
112
what does the smoothing filter do
decreases noise
increases blur
113
when do you use sharp kernels
when contrast is very high (to the point where the contrast overcomes noise)
114
when do you see soft kernels
when there is less contrast
115
what is interpolation
method of estimating the value of an unknown using known values on either side
116
what is pitch
ratio of pt table movement to width of beam
117
if distance traveled by table is equal to beam width, what is the pitch and pitch ratio
1, 1:1
118
what (geometrically) does a pitch of 2:1 indicate
table will move twice the distance of the slice thickness, each rotation of the gantry
119
in SDCT, beam width = ?
slice thickness
120
in MDCT, beam width = ?
slice thickness x n of slices
121
what is the pitch that creates contiguous slices
1
122
what happens to dose when pitch is >1
reduced
123
what happens to pt dose when pitch is <1
increased
124
CT number = ?
mu - muw / muw x1000
125
what is muw
attenuation coefficients of water
**changes based on keV
126
what is the hounsfield n of water
0
127
what is the hounsfield number of fat
-100
128
what is the hounsfield n of soft tissue
+50
129
what is the hounsfield n of air
-1000
130
what is the hounsfield n of bone/metal
+1000
131
why does beam hardening happen
polychromatic beams increase in quality as they penetrate
132
do deeper values have higher or lower mu n? why?
lower as deeper
due to beam hardening
133
beam hardening causes what
cupping artifact
134
how does CT simulation help in radiation therapy
localize and provide info
simulation plans treatment, dose distribution, and treatment isocentre and beams
135
what are the 3 steps of CT simulation
1. pt scanning
2. treatment planning and CT simulation
3, treatment setup
136
what is quantitative CT
most sensitive x-ray tehcnique
for measurement of BMC in osteoporosis
137
what is DECT
dual energy ct
138
what is dual energy ct
uses 2 energies to differentiate similar attenuating materials
139
what 4 methods can be used to perform DECT
- sequential acquisition of 2 separate scans
- rapid tube potential switching
- multilayer detectors
- dual x-ray sources
140
what are the pros and cons of temporally sequential scans
can be done using any CT scanner
risk of pt motion
141
what are the two ways that temporally sequential scanning (entire volume) can happen
through full volume
through each tube ritation
142
what are the pros of temporally sequential scans (tube rotation)
reduces interscan delay
can be done on any scanner
143
what are the cons of temporally sequential (tube rotation)
anatomy/organs prone to motion can still misregister
144
what is the pro of rapid kVp switching
near-simultaneous data acquisition of high and low
145
what are the cons of rapid kVp switching (4)
transitions must be abrupt
requires rapidly modulating mA
difficult to filter
requires very fast electronics and good generator
146
how does a multilayer detectors work
1 (high) tube potential
low E is collected by front detector
high E is collected by back detector
147
what is a pro of a multiplayer detector
low and high E sets are acquired simultaneously
148
what is a con of multilayer detector
need different detector thicknesses to get comparable noise
149
how does dual x-ray sources work
2 sources and 2 detectors, perpendicular to each other
150
what are the pros of the dual x-ray source
current and noise can be separately optimized
filtration can be optimized
151
what are the cons of dual x-ray source
scatter from one tue can pickup on the other
90 pahse shift between low and high energy data
