Unit 10: Post Processing in CT Flashcards by Courtney Goodhew

What is the image display in CT

last phase in creation of the CT image

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What does display system include in a CT image

patient info
scan protocol data
graphic aids (ROI, measuring tool etc)
designed to assist in image interpretation

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What is window width in CT image display

width determines the number of HU (CT numbers) represented in an image
software assigns shades of grey
anything above the range: white
anything below range: black

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What is it called when you increase window width

widening
more numbers are assigned to each shade of grey

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What is window level

selects the centre CT value of the window width

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What is the window level and width in this range

WW: 300
WL: 200

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How do we determine what window level we want to set

set the level as close to the HU of the tissue we want to see

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Window width and window level for different studies

tissue types with similar densities should have a small window width
tissue types with large densities should have a wider window width (ie chest)

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What is the ROI tool

placed on the image to determine the average HU measure of that area and the standard deviation

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Using the Cursor vs ROI for HU measurement

using the mouse shows value for one pixel
ROI shows the average HU of that area and is more accurate for diagnosis

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When should ROI measurement be used

when HU is involved in diagnosis

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Factors that increase the SD measurement of the ROI HU measurement

mixed attenuation within the ROI (ie calcium flecks within an organ)
an ROI that is not inside the margins of the object being measured

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What is the reference image function

displays the location of slices through the slice lines in corresponding locations on the scout image
- feature aids in localizing slices

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DFOV versus Magnification

decrease in DFOV increases size of displayed image and improved spatial resolution
image mag does not improve magnification, uses image data not raw data

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What type of data is there after CT scan

raw and image

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What is image reconstruction

used when raw data are manipulated to create an image

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What is reformation

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when image data is are assembled to produce images in different planes or produce 3D images

What is Prospective Reconstruction

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steps involved in acquiring and manipulating raw data so initial image can be formed
ie Selecting algorithm, SFOV slice thickness etc and setting DFOV
planned before scanning begins

What is retrospective reconstruction

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Can only be performed on the operator console (scanner).
* Raw data needed.
* Even though many parameters can be changed retrospectively, the resultant image are always in the same plane.
* DFOV, image centering and reconstruction algorithm can be changed.
* Can be applied to both helical and axial data.

What is happening in this image

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retrospective recon of DFOV

What is image Reformation

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uses Image data to generate images in a new plane or orientation from the prospective image

scanned plane may not be optimal for diagnosis thus reformats are valuable

Parameters for image reformation

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source images must have an identical DFOV, image center, gantry tilt, and they must be contiguous or overlapping.
even a small amount of motion can degrade reformation
thinner slices = better reformation
images can be 2D or 3D

What is multi planar reformation

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done to show anatomy in various planes (2D)
preserves original attenuation values
done at the operator console separate workstation
modern software and computing power allows high quality reformations to be generated

Manual vs Scanner MPRs

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manual MPRs requires operator to input the criteria such as thickness of MPR, the plane desired and the incrementation of resulting image
scanner created MPR: automatically generated by the scanner software
advantages: saves the technologist time and ensures they are done

What is Curved Planar Reformations (CPRs)

- type of MPR accomplished by aligning the long axis of the imaging plane with a specific anatomical structure - includes the enture structure on a single image (good for displaying a vessel)

How/ What are 3D reformations

- can require special software - time consuming and not done on the console

What is shaded surface display

- is a form of segmentation - selectively removing or isolating information from the data set referred to as segmentation - voxels located on the edge of the surface are used to show the outline/outside shell of the structure | creates a 3D model looking at bone structures clearer than axial images

What is this image

SSD removed any voxel value different from the shell of bone

What is volume rendering

a 3D representation of the image using all voxels Pixels in the final VR image can be assigned a color, brightness and degree of opacity (0% - 100%).

how does volume rendering and shaded surface display differ

- in VR all voxels contribute to the image versus just the shell of the structure in SSD

What are MIPs

- Maximum Intensity Projections - examines each voxel along a line from the views eyes and selects only the highest values to be displayed - used when objects of interest are the brightest objects in the image (bones and contrast filled structures)

What is endoluminal imaging

- look inside the lumen of a structure - reffered to as virtual endoscopy - Endoluminal imaging visualizes a structure as if it were hollow and the viewer were inside of it.

What is was used to create this image

Maximum Intensity Projections

What is MiniIP

using the voxels with the minimum value in the line of display used for low attenuation structures like the bronchial tree

What is seen in this image

MiniIP

What is the best way to obtain MIP or MiniIP images

limiting the data set so other structures that are present will not obscure

What is region of interest Editing

- most basic method of segmentation - ROI is removed by manually drawing a shape from within the data set using a "virtual scalpel" to cut the defined section - used to remove obscuring structures from 3D images - can be manual automatic or semiautomatic

Factors that degrade reformatted images

1. Segmentation errors 2. Image noise 3. Artifacts

How does segmentation errors affect reformatted images

- Important vessels or structures are inadvertently edited out of data set. - Can be obvious or subtle. - Crucial if a missing structure is unexpected. Example, Accessory artery. - Therefore, important to review unsegmented volumes, MIPs and source data.

How does Image noise affect reformatted images

- excessive noise can limit quality and utility of 3D images - can be avoided by using appropriate scan parameters for patient size

How does artifacts affect reformatted images

- motion

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Unit 10: Post Processing in CT Flashcards

(42 cards)