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CT 1
Basic Principles of CT
| Question | Answer |
|---|---|
| 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
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