Advanced Modalities Flashcards
How do you explain a CT scan?
A process of creating cross-sectional images of any part of the body
Who received the Nobel Prize in medicine and physiology for their work in CT?
Godfry Hounsfield and Allan Cormack
Who developed the fist whole body scanner in 1974?
Dr. Robert Ledley
How are CT scanners categorized and what is the difference between these categories?
- Generations
- Difference between generations related to number and arrangement of detectors
How many detectors did the 1st and 2nd generation CT scanners have and what type of beam resulted?
- 1st (1 to 3 detectors) pencil beam
- 2nd (30 detectors) fan shaped beam
What was unique about the 3rd and 4th generation scanner?
- 3rd gen - tube and detector array rotate simultaneously around patient
- 4th generation - tube rotates but detectors remain stationary
A 5th generation CT scanner is classified as? What is a type or 5th generation scanner?
High speed scanner
Electron beam CT for cardiac studies
The tube rotates around the patient and for every position of the tube, the detectors measure the transmitted x-ray values (known as….) and converts them into an electrical signal
Projections, scan profile, raw data
Define ray, view and profile:
Ray - part of x-ray beam that falls onto one detector
View - a collection of rays for one translation across object
Profile - a view generates a profile
What happens to the electrical signal in CT?
- It is sent to the computer to be digitized where a number is assigned that is directly proportional to signal strength
- Those numbers are arranged in a grid (rows/columns) = Matrix
What is a pixel and a voxel?
Pixel - single square or picture element within matrix
Voxel - volume element; slice thickness to pixel
An _____ in the matrix will cause the pixel to be smaller and allow ___ detail to be seen in the image (better resolution but increase noise in image)
Increase; small
What is a Hounsfield unit?
Each pixel within image is assigned a number (CT number or Hounsfield unit) that is proportional to the degree of attenuation of that tissue
What is the CT number of water?
0
CT number correspond to different shades of ____ in the image
Gray
What is window width?
- The range of CT #s that are displayed as shades of gray
- A wide window width has more shades of gray and a narrow window width has fewer shades of gray
What is window level?
- Determine the CT # that will be the center of the window width
- Midpoint of range of gray levels
What is an algorithm or kernel?
- Mathematical formula designed for computers to carry out complex calculations
- For enhancement of soft tissue, bone and edge resolution
What is a region of interest or ROI?
Measurement of CT numbers within a specified area for evaluation of average tissue density
What are the names used for the preliminary image?
Scanogram, scout, tapogram
What is the barium concentration used for CT procedures?
2% - low concentration to prevent streak artifacts
What is dynamic scanning?
Rapid sequential scanning at the same level to observe contrast filing or incremental scanning of rapid series scanning during bolus injection of contrast
What is spiral or helical scanning? What special technology does it utilize?
- Gantry continuously rotates while table moves through aperture at all the same time
- Slip ring technology replaces high tension cables of earlier scanners
What type of data set is acquired with spiral/helical scanning?
- Volume of data
- Forms a spiral path
- Scans volume rather than group of individual slices
What is the central opening of the gantry?
Aperture
What is a solid state detector?
Composed of photodiodes coupled with scintillation crystal material that converts transmitted x-ray energy into light which is then converted into electrical energy and the into a digital signal
What does the CT collimator assembly consist of?
- Prepatient (at x-ray tube); determines slice thickness
- Postpatient (at detector)
What is another name for the CT table?
- Patient couch
- Designed to move in increments or indexing
What does the DAS consist of?
The data acquisition system consist of the method of scanning (conventional/helical) and image reconstruction (array processor)
What are the components on the operator console?
Keyboard, graphic monitor, touch display screen, mouse
What is the scan field of view?
Includes entire cross section of body part of only a specified region within body part
What is the display field of view?
- Refers to the anatomy that is displayed on the monitor
- Can be smaller than the scan field of view but not larger
How do you define slice thickness?
- Number of millimeters intersecting thickness of x-ray beam
- Partially defined by size of focal spot
- Further refine the thickness by the collimators
What is spatial resolution?
A measure of the size of the smallest object that can be visualized in an image
What is contrast resolution?
The ability to differentiate small differences in density on the image
What is noise?
- A fluctuation in the CT number within a uniform material making the image appear grainy or speckled
- Difficult to see subtle contrast differences or fine details
What is an artifact?
- Beam hardening or streak artifacts
- Metallic “spray” artifact (dental fillings, pacemakers, prosthesis, bone-soft tissue interface)
What is MPR?
- Reconstructing axial images into coronal, sagittal or oblique planes without the need for additional radiation
- Postprocessing technique
