Computed Tomography (CT) Flashcards
computed tomography (CT) is also called _______ tomography
axial
CT images are of what plane?
-axial
-transaxial
-transverse
-acquired in the plane perpendicular to long axis of body
“tomos” is Greek for –
cutting
what axis represents LENGTH of patient?
Z axis!
what axis represents WIDTH of patient?
X axis!
what axis represents HEIGHT of patient?
Y axis!
Z axis is length of patient which splits them into –>
sup/inf
X axis is width of patient which splits them into –>
rt/lt lats
Y axis is height of patient which splits them into –>
ant/post
label the planes -
X, Y, Z
A = Z
B = X
C = Y
what is the gantry of CT?
the hole/donut
the CT gantry ranges from ____-____ cm
70-90 cm
CT scanner gantries __do/do not___ have the ability to tilt forward and backward
they do
-however, for CT sims we obvi don’t want this function
where are control panels located in CT room?
on CT gantry
what can you control with control panels in CT room on gantry?
-laser alignment
-gantry tilt
-table movement
-also houses mic and speaker
what are the major parts of CT? (7)
- slip rings
- generator
- cooling system
- xray source
- filtration system
- collimation
- detectors
describe CT slip rings -
slip rings are electromechanical devices with brush like apparatus
______ allow for continuous electrical power, electronic communication, and help the gantry rotate continuously
slip rings
_________ make helical CT scans possible
slip rings
_____ part of CT helps allow for higher energy and power of machine
generator
the CT generator produces what and how much energy? ___-___
120-140 kV
kilovolts
the ________ of CT keeps it’s temperature regulated
cooling system
the CT xray source has an anode focal spot of what size?
.5-1 mm
describe xray source of CT -
same rotating anode as LINAC with tungsten target as well
______ part of CT reduces range, and thus dose, of xrays reaching the patient
filtration system
how does the filtration system of CT reduce dose to patient?
by cleaning up lower dose xrays that wouldn’t help the image
the filtration system helps
1. create a more ______ beam
2. reduce _____
3. improve overall CT ____
- uniform
- artifact
- image
______ filter is typical filter used for head to reduce intensity to periphery of what we are imaging
bowtie filter
the following is what type of filter? -
bowtie filter in CT
-bowtie filter was on IX LINAC too
________ restricts xray beams to a specific area
collimation
_____ controls slice thickness and reduces ____
- collimation
- scatter
less scatter, better the image _____
contrast
how does collimation control slice thickness?
by narrowing or widening the xray beam
what is common CT slice thickness range?
.5 - 1 cm
________ are what collect information to create the image
detectors
multiple detectors collectively are called what?
detector array
how are detectors placed?
as close together as possible
what shape do detectors make when placed in CT gantry?
arc or full circle
arc = 3rd gen
full 360 = 4th gen
what are CT detectors made of?
solid-state crystal –> scintillation detectors
___larger/smaller___ detectors have better spatial resolution
smaller
describe differences in 3rd and 4th gen CTs -
3rd gen =
-detectors in arc shape
-the xray tube and detector array go around 360
-fan beam from xray tube
-single row detectors
4th gen =
-full circle of stationary detector arrays and one xray tube that goes around the 360
-fan beam from xray tube
-single row detectors
newer CTs (past 4th gen) have single row detectors or multidetector rows?
newer CT models have multidetector rows
describe differences between single row detectors and multidetector rows
single row =
-3rd & 4th gen
-700 - 4800 detectors in a single row arranged in arc or complete circle
-limits the upper limit of slice thickness (max 10 mm)
-one gantry rotation = 1 single slice
multidetector row =
-newer CTs
-4 - 64 parallel rows
- a single rotation can produce multiple slices
- increased scan speed
the introduction of __________ for newer CTs has increased scan speed
multidetector rows
single detector row CTs have about how many detectors? what about multidetector row CTs?
700 - 4800 detectors in a single row arranged in arc or complete circle
whereas multidetector rows have about 4-64 parallel rows
how many gantry rotations does it take to make 1 single slice for single row detectors CTs?
1 gantry rotation = 1 slice
whereas multidetector row CTs can make multiple slices in just one gantry rotation
single row detectors have an upper slice thickness limit of _____
max 10 mm
CT scanning is done in ___ scanning
helical
what are other names for helical scanning?
-spiral
-continuous acquistion
____ scanning increases scan speed and improves image resolution
helical
what was the OLD way of CT scanning?
-nowadays = helical scanning
used to be axial scanning –>
so similar process to step n’ shoot
-took too long
Pitch of CT scanning means/effects what?
pitch of CT is the relationship between couch movement and xray beam collimation
CT pitch is expressed how?
as a ratio
CT pitch ratio ranges from ___-____
.5:1 to 2:1
a CT pitch of .5:1 means what?
how about CT pitch of 2:1 -
.5:1 CT pitch = overlap and increased patient dose
2:1 CT pitch = extends the image & less patient dose
a __lower/higher___ pitch will increase scan speed
higher pitch will increase scan speed
–2:1 CT pitch = extends the image & less patient dose
–spread out spirals VS tight spirals that will take a longer time to accumulate
CT pitch and CT scan speed become very important for what type of patients?
breath hold patients
what are the 2 types of CT Scanning Data Types?
raw data and image data
describe the difference between raw data and image data -
raw data = what is acquired from scan
image data = what we can see and use for planning system
what type of data is acquired by system during scan?
raw data
– ie. scan data
raw data/scan data is sent where? to be turned into an image
sent to computer where it is turned into an image
____ data includes measurements obtained by detector array
raw data
____ data is used to create a variety of images
raw data
____ data = the processed raw data that is turned into an image
image data
for image data, the computer assigns ____ value to each pixel
HU (Hounsfield Unit)
for image data, the computer assigns an HU value to each ____
pixel
pixels represent what?
patient tissue
SFOV =
Scan Field Of View
______ determines the area within the CT gantry that raw data will be acquired
SFOV
Scan Field Of View
CT scan data is acquired in a ______ around the isocenter
circle
Scan Field Of View can also be called what?
calibration field of view
ex. if a 25 cm SFOV is selected, what is the diameter of the circle scan data/circle imaged going to be?
25 cm
if a patient is larger, or we want more anatomy what may be do to SFOV?
extend the SFOV
how many HU are there?
2000
what is HU of air, water, bone
-1000 = air
0 = water
1000 = bone
what would have an HU of 2000?
metal
what is considered baseline HU?
water at 0
monitors can only display how many shades of gray?
human eyes can only detect how many shades of gray?
monitor can display 256 shades of gray
and our eyes can only detect 40 shades of gray
describe difference between Window Width (WW) and Window Level (WL) -
WW = range of shades of gray; determines # of HU represented on an image
WL = median; window center; selects center CT value of WW
Window Width or Window Level affect brightness the most
WL (window level)
Window Width or Window Level affect contrast the most
Window Width (WW)
Window Width or Window Level increase/decrease the shades of gray of an image
WW (Window Width)
increasing or widening Window _____means more numbers/shades of gray are available
Window Width
__more/less__ shades of gray will decrease our image contrast
more gray !
– more shades of gray between stark black and stark white DECREASE contrast !
the following are other names for what term?
–> scout, surview, topogram, scanogram, preview, pilot
localizer scans
most CT scans begin w/ one or more ___ images
localizer
–scouts
scout images/localizer images are used for what?
to select the region of interest to be scanned (ie. SFOV)!
localizer scans/scouts are NOT axial scans!! they are what?
AP or lateral
region of interest to be scanned is also known as the _____
SFOV
localizer scans/scouts ensure anatomy is in ______ of imaging system
range
not having the patient centered on CT can lead to what?
artifacts and clipping
___________ estimates values between known values
interpolation algorithms
what makes RECONSTRUCTION possible?
interpolation algorithms
axial scans are reconstructed to _____ and ___ views
sagittal and coronal views!
without interpolation, what would happen to the reconstructed coronal and sagittal views?
they would be blurry!
label the planes -
-coronal
-sagittal
-transverse
blue = transverse
yellow = coronal
pink = sagittal
which corresponds to Window Width, which for Window Level
A = Window Width
B = Window Level
which corresponds to a .5:1 pitch?
which is 2:1 pitch example?
A = .5:1
B = 2:1
which corresponds to Gen 3 CT and which is a Gen 4 CT?
A = gen 3
B = gen 4
lower the HU value __darker/lighter___ the image
darker
higher the HU value __darker/lighter__ the image
lighter/whiter
T/F
localizer scans/scouts are NOT cross sectional scans
true
localizer scans/scouts are acquired when the tube is ___stationary/not stationary___
stationary !!!
the _____ image is used to select region of interest to be scanned/SFOV
scout
what estimates the value between known values and makes reconstruction possible?
Interpolation Algorithms
axial scans are reconstructed to ________ and __________ views?
coronal and sagittal views
CT images are a display of many small pixels with different shades of gray depending on the _______ rate
attenuation rate (ie. HU)
define “matrix”
the layout of cells (pixels) in rows and columns
define “pixel”
picture elements; pixels are each cell within a matrix; 2D representation
define “voxel”
volume element; 3D representation of tissue
a voxel is a _____ representation of tissue, whereas a pixel is a _____ representation
voxel = 3D
pixel = 2D
(pixel) x (__________) = VOXEL
pixel x slice thickness = voxel
bigger matrix sizes have more pixels, this leads to a ___better/worse___ image
better
what is the matrix size of CT scanner?
512 x 512 pixels
what measures the relative linear attenuation coefficient of tissue?
pixels
Hounsfield Units (HU) are also called what?
CT numbers
_____ represent tissue density
HU - Hounsfield Units (also can be called CT numbers)
what is the HU/CT # of water?
0
what is the HU/CT # of air?
-1000
what is the HU/CT # of bone?
1000
what is the HU/CT # of muscle?
50
what is the HU/CT # of blood?
ranges from 30-45
what is the HU/CT # of CSF?
15
what is the HU/CT # of fat?
ranges from -50 to -100
what is the HU/CT # of lung?
ranges from -400 to -600
if the HU/CT # is greater than 1000, what is the likely material/tissue?
metal
what are differences between CT simulation machines and regular CT machines? *key word = simulation
CT sim = flat table top (also usually carbon fiber), larger bore, external lasers, immobilization device friendly table, virtual sim software that can generate BEVs and DRRs
where are the external lasers in a CT sim room?
mounted on side walls/sides and ceiling/above
external lasers are how far apart from interal lasers?
500mm or 50 cm
are external lasers moveable? YES or NO
—if yes, then in what directions?
YES!
our external lasers move in the sag (x) (ceiling) (lt/rt) and vert (y) (lateral walls) (ant/post)
concept check: do the internal lasers go anywhere for CT sim?
no - they are just not used. they stay in CT bore since they are fixed position lasers!
is there sup/inf external lasers for CT sim? Why or Why not
no sup/inf laser usage for CT sim… we just move the table
what is the point of external lasers for CT sim?
the point of ext lasers for sim is to be able to mark the patient/isocenter outside of the CT bore
the higher contrast and higher resolution that we see on our DRRs is achieved how?
with smaller slice thicknesses and smaller spacing
…and adjusting mA - quantity of slices
slice thickness is adjusted where?
in the gantry of the CT using collimation
thinner slices have ___higher/lower___ image quality? and why
HIGHER quality due to reduced scatter
SBRT/SRS uses this technique
thinner CT slices are achieved how?
through more collimation that narrows the xray beam
one CT slice delivers about how much dose to patient?
1-6 Gy to skin surface
a tighter pitch leads to what?
increased patient dose!
recall: .5:1 ratio = overlap
what should CT spacing typically be?
< 5 mm
what happens when there is MORE space between each CT slice?
there is less info obtained, so, the scan then has to interpolate more to fill in the gaps
DRR is calculated from the _______
CT scan
what does DRR stand for?
Digitally Reconstructed Radiograph
DRRs are 2D or 3D representation of patient?
2D
on trt machine, we know we are looking at a DRR when we see what?
scan with inclusion of MLCs or blocks
DRRs are 2D images that show _______
treatment ports
we use DRRs to match ____ images
port images
kVp = _________
BEAM QUALITY
mA = _________
BEAM QUANTITY
as mA increases, patient dose ____decreases/increases_____
increases
NEMA stands for what?
*hint - it is an Association
National Electrical Manufacturers Association
ACR stands for what?
American College of Radiology
a virtual simulation is performed when?
after CT sim is complete
during a _____ simulation is when outlining of external contours, contours the target volumes, and OARs takes place
virtual
T/F: virtual simulation displays ports/beams
true
during a _____ simulation is when review of the plan and isodose distribution takes place
virtual
RV stood for what (this was on a class exam)
Record & Verify system
what does PACS stand for?
Picture Archiving and Communication System
what does DICOM stand for?
Digital Imaging and Communication in Medicine
the following is describing PACS or DICOM -
-allows for acquisition, interpolation, and storage of digital images
PACS
the following is describing PACS or DICOM -
-creates standards for transferring images and radiographic information
-standards are created for sharing images
DICOM
the standards for DICOM are produced by a joint committee of who?
NEMA and ACR
what are the 4 main principles of PACS?
- image acquisition
- display system
- network
- storage system
label the CT scan -
–green
–purple
–orange
–green = voxel
–purple = pixel
–orange = matrix
Slice thickness can affect _______
quality
smaller slices = _________ quality
higher
increase kvp by ___% = doubles the mas
15%
increasing mas, ___increases/decreases___ patient dose … and increasing kvp __does/does not____ increase patient dose
increase in mas - increases patient dose… and increasing kvp does not increases patient dose
