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Flashcards in Advanced Modalities Review Deck (38):

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?

-Godfey Hounsfield
-Allan Cormack


***Who developed the first whole body scanner in 1974?

Dr. Robert Ledley


How are CT scanners categorized and what is the difference between these categories?

-Difference between generations related to number & 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 & 4th generation scanners?

-3rd generation – tube & 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 of 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.

-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 increase in the matrix will cause the pixel to be smaller and _________

allow small details to be seen in the image (better resolution but increase noise in image).


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?



CT numbers correspond to different shades of ____ in the image.



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?

-Determines 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?



***What is the barium concentration used for CT procedures?

2% - low concentration to prevent streak artifacts


What is dynamic scanning?

Rapid sequential scanning @ the same level to observe contrast filling 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?



***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 then to a digital signal


What does the CT collimator assembly consist of?

-Prepatient (@ x-ray tube)
-Postpatient (@ detector)

-Prepatient determines slice thickness


***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?

-Graphic monitor
-Touch display screen


What is the scan field of view?

Includes entire cross section of body part or 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
-Bone-soft tissue interface (skull)


***What is MPR?

-Reconstructing axial images into coronal, sagittal or oblique planes without the need for additional radiation
-Postprocesing technique