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CT 1

Basic Principles of CT

Name the main advantages of CT of conventional radiography. 1. Elimination of superimposition 2. Differentiate small differences in densities
Define root TOMO to cut, section, or layer
Describe axial cuts of early scanners slices, like rings of a tree
CAT (old) Computerized Axial Tomography
CAT (new) Computer Assisted Tomography
Words describing preliminary CT image. Scout (GE), Topogram (Siemens), Scanogram (Toshiba)
Continuous Acquisition Scanning Spiral (Siemens), Helical (GE), Isotropic (Toshiba)
The ability of a system to define small objects distinctly. Spatial Resolution
The ability of a system to differentiate, on the image, objects w/ similar densities. Low-contrast resolution
The speed that the data can be aquired. Temporal Resolution
The thickness of the cross-sectional slice. Z axis
What limits the beam to the z axis? Collimators (small shutters)
Width X
Height Y
Each 2D square Pixel (picture element)
Thousands of _______ creates the CT image Pixels
When the z axis is taken in account w/ Y & X it is a cube Voxel (volume element)
The rows and columns of pixels form a grid known as The Matrix
What s the most common matrix size? 512
512 matrix = 512 rows X 512 columns = 262,144
The degree to which an x-ray beam is reduced by an object is Attenuation
White on an image is where a xray beam is completely absorbed, referred to as high attenuation
How are areas of intermediate attenuations represented Various shades of gray
The degree to which matter is crowded together and concentrated. Density
The # of photons interacting depends on density, thickness, and atomic #
Xray photons that pass thru an object unimpeaded are represented as black areas on image ie air
Low attenuation black
Linear Attenuation Coefficient (greek letter U) The amount of the xray beam that is scattered or absorbed
Water Linear Attenuation Coefficient 0.180cm-1 18% of photons are absorbed in 1 cm of water
To differentiate adjacent objects on a image there must be a density difference between the 2 objects
Linear Attenuation Coefficients Air Fat Water Cerebrospinal Fluid White Matter Gray Matter Blood Dense Bone 0.0003 0.162 0.180 0.181 0.187 0.184 0.182 0.46
Hounsfield Units quantify the degree that a structure attenuates an xray beam. (measurements)
Who is Godfrey Hounsfield? An pioneer of CT and HU is named after.
Approximate HU Bone Blood Brain Matter Water Fat Air 1000 100 50 0 -200 -1000
How does HU help the RAD? By being able to speculate density of a suspicious area. ie cyst is fluid
What contributes to inaccurate HU measurements poor equipment calibration, image artifacts, & volume averaging
What does polychromatic mean? Xray energy (beam) varies , Spectrum changes from weak to strong photons.
What are artifacts that appear as dark streaks or vague areas of decreased densities sometimes referred to as cupping artifacts. Beam hardening artifacts.
homogenous an xray beam that is more uniform (improved image quality)
The process by which different tissue attenuation values are averaged to produce one less accurate pixel reading is volume averaging (partial volume effect)
What has affects on volume averaging Z axis, X & Y ie small pixel size = reduces volume averaging
Retrospective reconstruction Raw data used later to generate new images
Prospective Reconstruction The reconstruction that is already produced during scanning
The process of using raw data to create an image image reconstruction
Step and shoot mode 1980's, entwined cables, must do each slice independently, move table and so on
Helical (spiral) scanning 1990's, cont acquisition scanning, eliminated cables, cont moving gantry, single row detectors
Multidector row scanning 1992, 2 row detectors, further improvements of many row detectors allowing ore data per slice acquired
Anterior Ventral movement forward , toward the face
Posterior Dorsal movement back of body
Inferior Caudal Down (feet)
Superior Cranial Cephalic Up (Head)
Lateral sides
Medial midline
Proximal towards body
Distal away from body
Axial plane Horizontal, transverse plane, (sheet of glass parallel to the floor)
Sagittal Plane Vertical, longitudinal, cut body in right & left sides
Coronal Plane Dividing the body in anterior and posterior planes
Mid sagittal vs Para sagittal Midline vs Rt or Lt sagittal cuts
Changing the field of view, the way that an area is represented on the cross sectional slice by changing the imaging plane
What is produced when a substance is bombarded by fast moving electrons? Xrays
How are xray photons produced? by fast moving electrons slamming into a tungsten target ie anode
kinetic energy energy of motion
The kinetic energy of electrons are transformed into electromagnetic energy
Heat capacity the ability of the tube to withstand the resultant heat
Heat dissipation the ability to rid itself of heat
What are the 3 main segments of the CT process Data Acquisition ---Get data Image Reconstruction---Use Data Image Display---Display Data
Created by: brandyellen