week 2 lecture 1 Flashcards
GR is a form of _____ imaging. What does this mean?
planar imaging.
it means that it will produce 2d images of 3d objects
what are the limitations to GR?
- all structures are superimposed
why is superimposition bad?
- makes it hard to identify precise location of the abnormalities
- makes it difficult to distinguish slight change in density
what is blurred in Linear Tomography?
structures outside of the area of interest
what purpose is the blurring effect in linear tomography?
to improve visualization of the tissue of interest
what is one of the advantages to linear tomography?
enhancement of radiographic contrast
how is linear tomography done?
the patient is stationary while the x-ray tube and image receptor is following the patient’s longitudinal axis
why is the x-ray tube and detector connected?
enables them to move in opposite directions at the same time
focal plane
section that images anatomical structures clearly
focal plane is aka?
section thickness or object plane
the amount of tube motion is =?
size of the section to be imaged clearly
fulcrum
imaginary pivot point about which the x-ray tube and detector move
centre of the object plane (aka focal plane aka section thickness) is determined by what?
the fulcrum
what causes the objects outside the focal plane to blur in the images?
the movement of the x-ray tube and detector during exposure
the objects position on the detector correlates to what?
the x-ray tube’s movement; objects are in the opposite direction
*tube movement causes objects to exhibit varying positions on the detector during the exposure
tomographic angle
amount of tube motion during an exposure which controls the thickness of the section imaged
decreasing in tube motion = (increase/decrease) tomographic angle
decrease tomographic angle
0 degree angle = no tube and detector motion = section thickness to infinity
smaller angles = (thinner/thicker) sections
smaller angles = thicker sections
image blur increases as object distance from the focal plane (increases/decreases)
image blur increases as distance from focal plane increases
what are the advantages of linear tomography over general radiology?
- increase radiographic contrast
- decrease superimposition
what are the disadvantages of linear tomography over general radiology?
- increases patient dose due to x-ray tube being on for the length of tube travel (longer exposure)
- multiple exposures may be done
- requires the use of grids
what are the main advantages of ct?
- low-contrast resolution
- versatility in data acquisition options
- image reconstruction capabilities
- reformatting possibilities
what is the basic principle of computed tomography?
production of sharp, clear, cross-sectional images of the human body
how does CT obtain these cross-sectional images?
use of a rotating tube, detector array and computer reconstruction algorithms
detector array
use of multiple detectors
what are the modifications for ct?
- patient is also moving through the system
- detector array
- uses a computer to collect, organize and create axial image
what are the main disadvantages to ct?
- produces high radiation doses
- susceptible to more image artifacts
- decrease in spatial resolution
what are the 5 components of ct?
- table
- gantry
- power injector
- operating console
- workstation
what are the four steps to creating a ct image?
- data acquisition
- image reconstruction
- image display
- image archive
data acquisition
how we collect information to form an image
steps to data acquisition.
- scouts (planning)
- ct slices
raw data
what is collected by the detectors when the x-ray tube irradiates an area of interest
raw data aka?
scan data
image reconstruction
process of manipulating raw data using algorithms
how is image reconstruction done?
- radiation flux at the detectors create electrical signals that are converted into digital format (HUs)
- these will be put onto a matrix
- data is selected for display
DAS - data acquisition system
measures radiation collected by detectors
calculates beam attenuation
ADC - analog to digital data conversions
converts the raw data into digital format
how is this conversion of raw data to digital format happening?
ADC assigns pixels different HU values
algorithms
set of rules/directions used to get a specific output from specific input
algorithms are required to be:
- simple and well defined
- terminates after finite number of steps
what are common automatic algorithms?
- back projection
- filtered back projection
- iterative reconstruction
- Fourier transform
back projection
raw data is dragged/smeared onto a matrix in order to view the shape of the scanned object
star-pattern artifacts are produced by?
back projection
filtered back projection
uses convolution to remove the blurring caused by back projection = improves image quality
iterative reconstruction
compares scan data with an assumed image dataset then it projects the calculated difference onto the matrix
Fourier transform
used in MRI image reconstruction; based on measuring frequencies
what are the 3 common “selected” algorithms?
- standard
- smoothing
- edge enhancement
smoothing
improves visualization of soft-tissue structures and can help minimize artifacts
when is smoothing usually used?
when it is a brain tissue image
smoothing = (increase/decrease) noise + (increase/decrease) spatial resolution + (increase/decrease) contrast resolution
smoothing = ↓ noise and spatial resolution, ↑ contrast resolution
edge enhancement
helps demonstrates fine detail (smaller objects)
when is edge enhancement usually used?
when imaging small bony fragments and pulmonary structures
edge enhancement = (increase/decrease) noise + (increase/decrease) spatial resolution + (increase/decrease) contrast resolution
edge enhancement = ↑ noise + spatial resolution, ↓contrast resolution
standard
algorithm that balances noise and detail
image display
process where the reconstructed data is averaged for display on a monitor
image data
data of an image that can be viewed on a monitor
DAC - digital to analog data conversion
assigns the pixels different shades of grey based on the pixel’s HUs
why is DAC necessary?
it is needed to convert the reconstructed data into image data that can be seen at the workstation/operating console monitors
can post-processing functions be used to manipulate image data?
yes. as it can improve image viewing
image archiving
how we store CT image data
how are images accessed remotely?
- PACS (picture archival communication system)
- DICOM (digital imaging and communication in medicine)
when does saving images for future viewing occur?
after raw data has been reconstructed and image display is acceptable
smaller fss is used for?
when we want to get finer details (better spatial reso)
larger fss is used when?
regardless of needing detail but the patient is more hypersthenic
the tube and detector needs to be (perpendicular/parallel) to the patient’s longitudinal axis. why?
parallel, because it will help with the avoidance of superimposition and too much blur
scout image aka?
preliminary, pilot, localizer
