Week 3 Lecture 1 Flashcards
scanning
defined by the beam geometry used
- size, shape, motion of the beam, path
what are the 2 methods of CT data acquisition?
- axial
- helical
when are scouts acquired?
PRIOR to both axial or helical scans
AP (in terms of scouts)
tube above patient
Lateral (in terms of scouts)
tube is beside patient
what part of the equipment is stationary?
the tube is
the table will be moving the patient into the gantry
scouts can be used to set what?
- DFOV
- Image center
- gantry tilt (tube angle)
in ap scouts, where should your image center be to indicate that you are isocentered?
image center over the spine
what can result due to miscentering?
- not scanning all required anatomy
- out-of-field artifacts
- higher patient doses
if the indicated patient position is wrong, what can happen?
misdiagnosis
during image display, what can scouts do for us?
it can be used to reference the location of a cross-sectional slice
- can show us the selected SFOV, DFOV and location of that slice
what are the steps to axial acquisition?
- 360 degree tube rotation around patient
- tube stops + table moves in
movement of the table correlates to?
slice thickness
what are the advantages to axial acquisition?
+ image quality
+ acquisition variability
contiguous
slices are side-by-side
gapped
slices are spaced apart
overlapped
slices double expose parts of anatomy
helical acquisition aka?
spiral beam geometry
what 3 things are required of the helical acquisition method?
- slip rings to allow the tube to rotate continuously
- constant x-ray output
- continuous table movement
in terms of irradiation, what is the difference between axial and helical?
axial = irradiates slice-by-slice
helical = irradiating a volume of tissue
what are the advantages to helical scanning?
+ *reduces chance of misregistration
+ more data manipulation capabilities
+ decreased scan times
+ less chance for motion artifacts from specific types of patients
+reduces amount of contrast used in enhanced studies
what are the disadvantages to helical scanning?
low image quality because the full 360 degrees’s data is not obtained
extrapolation/interpolation
- removes the “slant” from an image slice
- creates motion-free images
what type of data is only accessible on the CONSOLE COMPUTERS hard disks?
raw data
hard disks are (short/long)-term storage
hard disks = short-term storage
what type of data is needed for retrospective reconstructions?
raw data
image data requires (lots/less) storage space and is stored in (short/long)-term storage.
image data = less storage + long-term
where is image data stored?
PACS or workstation monitors
what are the data manipulation capabilities like for image data?
limited
the limit to data manipulation capabilities for image data, what does that do to retrospective reconstructions?
disables retrospective reconstructions so adjustment to DFOV AFTER a scan acq. isn’t able to be done
matrix size is typically compared between scanners, as the matrix size of a CT system can’t be adjusted
↑ matrix size = (↑/↓) pixel size = (↑/↓) spatial resolution
↑ matrix size = ↓ pixel size = ↑ spatial resolution
↓ matrix size = (↑/↓) pixel size = (↑/↓) spatial resolution
↓ matrix size = ↑ pixel size = ↓ spatial resolution
height of pixel = ___ axis and describes?
height of pixel = y axis and describes the tissue’s thickness
width of pixel = ____ axis
width of pixel = x axis
pixels bit depth affects what?
contrast resolution
voxel
represents a volume of tissue – slice thickness
z axis
thickness of the slice
localizer scans are also used to set the extent of anatomic coverage. t/f
how?
true
by selecting the location of the first and last cross-sectional slices
localizers are used for positioning. t/f
false
it should never be used for positioning as it gives patient a dose
how much can a patient’s dose be increased if you don’t isocenter them?
by 140%
anisotropic
more rectangular in shape
