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CT
CT advanced modaities
| Question | Answer |
|---|---|
| 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
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