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CT

CT advanced modaities

QuestionAnswer
How does CT eliminate superimposition that is typically seen in Xray? passes a tightly collimated beam through a part at many different angles
In CT how is a scanned obtained? by a xray tube rotating around the body part
What measures exit radiation in CT? detector assembly
What does the detector assemby do with the radiation measurements? sends the information or primary data to a computer
CT creates what kind of plane? cross-sectional tomographic plane
What is the newest method of data acquisition? spiral/helical CT
How is raw data obtained in the spiral/helical CT? continuous motion forms a spiral path; gantry rotates continuously while the table moves through the aperture at the same time
- scans a volume o f tissue rather than a group of individual slices spiral/helical
What are the advantages of spiral/helical Ct? respiratory motion is minimized useful for detection of small lesions useful for scanning non-cooperative, combative, trauma, and pediatric patients
What provides information on positional relationships and tissue characteristics? CT
What measures exit radiation and converts it to an electronic signal and sends it to a computer? detectors
- measured xray transmission values raw data
After the computer digitizes the signas, what is the next step? a whole number is assigned to each
How does the computer assemble data? Into the matrix to form axial image
- an array of numbers arranged in a grid of rows and coumns matrix
- single square, or picture element, within matrix pixel
- the volume element of the digital image and matrix, product of a pixel area and slice thickness voxel
- represents a piece of anatomy and is displayed on the image monitor as a dot of light in one of the shades of gray a pixel
How are pixe assigned numbers? related to the linear attenuation coefficient of the tissue within each voxel
What are the numbers called that are assigned to the pixels? CT numbers or Hounsfield units
What is the varying factor for the Hounsfield number of tissue? the density of the tissue
What is the range of the Hounsfield number? +1000 HU and -1000
- determines amount of data displayed on monitor field of view (FOV)
What determines pixel size? matrix size and displayed field of view
Pixel size =? dfov / matrix size
- the ability of an imaging system to differentiate among objects resolution
Which would resut in a better image? 1024x1024 or 526x526 1024x1024
Why is water used as the reference material for CT? abundant in the body has a uniform density water assigned arbitrary value of 0
What kind of CT numbers are denser than water? positive CT numbers
What kind of CT numbers are less dense than water? negative
What is the value for air? -1000
What is the value for dense bone? + 14,000
What corresponds to CT number for that pixel? gray level
- allows the technologist to alter the contrast of displayed image by adjusting the window width and window level windowing technique
- range of CT numbers used toi map signals into shades of gray window width
- est the number of gray levels displayed window width
narrow window = ? fewer shades of gray/ high contrast
wide window = ? greater shades of gray
-determines the midpoint of range of the gray range displayed window level
- used to help distinguish normal anatomy from pathology; increase visibility of disease processes contrast media
How is contrast media administered for ct? IV, orally, rectally
- ability to reconstruct axial images into coronal, sagittal, or oblique body planes without additional radiation to patient multiplanar reconstruction (MPR)
Which is better for bone? Which is better for soft tissue? CT is better for bone MRI is better for soft tissue
Created by: erikasmith28