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27 Cards in this Set
- Front
- Back
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Describe Computed Tomography |
Uses Xray technology and sophisticated computers to create images of cross sectional "slices" through the body |
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Define Tomography |
Refers to the ability to view an anatomic secion of slice through the body |
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Benefits to CTs |
Provide a quick overview of pathologies and enable rapid analysis and treatment plans |
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Anatomic Cross sections refer to |
Transverse axial tomography |
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Describe how a CT image is created |
Processofscanning to gather x-ray absorption coefficients taken from thinsections through body Obtaining multiple measurements from these coefficients Reconstructing thesemeasurements into an image that displays that section’s anatomy. |
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What is the Gantry? |
Xray Tube that patient enters Contains: Detectors Function: Data Acquisition |
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What is the operators console |
Select slice thickness Reconstruction algorithms |
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What is a scout image? |
Needed prior to CT scanning Digital radiograph used to localize scanned structures |
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Describe the scanning process |
Xray beam and detectors are housed in a circular scanner Tube moves around the pt and the detectors measure radiation --Designed to produce fan shaped beam of xrays approx as wide as your body |
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How does the CT create an image? |
Xray beams passed through the object from many points across the object and from many angles. CT can differentiate tissue densities --Tissue attenuarion is measured over a large region from one position of the Xray tube |
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What are the basic principles of CT |
-Xrays attenuated by body tissues -Radiodensities of tissue interpreted in shades of gray -CT makes images of axial slices -Made by up to 1000 projections from different angles |
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What is a pixel |
Picture Element 2D square shade of gray |
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What is a voxel |
Volume element 3D volume of gray (3D pixel) Each voxel is about 1mm on a side and has a 2-10mm thickness depending on the depth of the scanning x-ray beam |
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How does the X-ray tube move? |
There is no stop and go action - tube moves continuously creating a helical slice and data is collect by multiple detector rows simultaneously |
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How is data converted? |
-Xray detectors measure remnant radiation as electric current or analog signal -Dataacquisitionsystem converts analogsignalto digital &sends it tocomputer -Computerconvertsdigital signals into matrixmadeup of pixels. -Eachpixel displays shadeofgray representing radiodensity. (Houndfield units) |
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How is the image reconstructed? |
The computer reconstructs images based on geometric plot points, mathematical process called back projection |
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What is windowing? |
Refers to a range of radiodensities displated in an image Each digital image is only "small window" on total data obtained by computer Humans see 32 shades of gray, but a computer can see 100s |
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Describe Hounsfiled Units |
(CT Numbers) Way that shade of gray is assigned Represent the percent difference between the x-ray attenuation coefficient for a voxel and that of water multiplied by 1000 --Water has a CT number of Zero |
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What is contrast resolution? |
The ability to differentiate between different tissues densities in the image |
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Define high contrast |
Ability to see small objects and details that have high density difference compared with background --ability to see small dense lesion in the lungs or soft tissue vs bone |
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What is low contrast? |
Ability to visualize objects that have very little difference in density from one another --better with low noise and visualizing soft tissue lesions in live --Better differentiate gray matter from white matter in brain |
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What are window levels |
Bone window ot soft tissue window Simple way to indicate range of radiodensities that are being chosen Allows discrimination between tissues of similar densities |
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What do CTs image best? |
Loose bodies in the joint Subtle or complex frazctures Degenerative changes Spinal Stenosis Osseous alignment |
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What are imaging artifacts |
Can degrade image quality and affect the perceptibility of detail -Streaks - due to pt motion, metal, noise, mechanical failure -Rings and bands - due to bad detector channels -Shading - can occur due to incomplete projections. |
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Advantages of CTs |
•Less expensivethan MRI •Lessclaustrophobic •Can imagesoft tissues and osseous structures in one series •Desiredimage detail is obtained •Fast imagerendering •Filtersmaysharpen or smooth reconstructed images •Raw datamay be reconstructed post-acquisition with a variety of filters |
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Disadvantages of CTs |
•Volumeaveraging of radiodensities in avoxel •Highradiation •Multiplereconstructions may be required if significant detail is required from areasof thestudy that contain bone and soft tissue •Needforquality detectors and computer software |
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CT for PT use |
Pt Education Evaluation of malunion after fracture Can improve understanding of possible interference of tendon movements over osseous deformities |
