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Flashcards in Notes From 2nd Weeks Readings Deck (79)
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1

Algorithm

-a precise set of steps to be performed in a specific order to solve a problem
-in CT reconstruction algorithms are used by the computer to solve the many mathematical equations necessary for information from the detector array to be converted to information suitable for image display
-"a set of rules or directions for getting a specific output from a specific input"
-an algorithm must always terminate after a finite number of steps

2

Interpolation

Estimating the value of an unknown function using the known value on either side of the function
-a mathematical method of creating missing data

3

Hard disk (or hard drive)

-essential component of CT systems
-number of images that the hard disk can store varies according to make and model
-saves information (data)

4

What are the principal components in a computer

-input device ex keyboard, mouse, touch sensitive plasma screen
-output device ex monitor, laser camera, printer, archiving equipment such as optical disks or magnetic tape
-central processing unit (CPU)
-memory

5

Input and output devices

Pieces of computer hardware designed to feed data into the computer or accept processed data from the computer
-input devices ex: keyboard, mouse, touch sensitive plasma screen, and CT detector mechanisms
-output devices ex: monitor, laser camera, printer, and archiving equipment such as optical disks or magnetic tape

6

Central processing unit

Component that interprets computer program instructions and sequences tasks
-referred to as the "brain" of the CT system
-gets the data it needs from RAM, processes it, and writes new data back to the RAM in a continuous cycle

7

Scan data and raw data are used interchangeably

-all of the thousands of bouts of data acquired by the system with each scan are called raw data
-used interchangeably to refer to the data sitting in the computer waiting to be made into an image

8

The process of using raw data to create an image is called?

Image reconstruction
-the same raw data may be used later to generate a new image because it includes all measurements obtained from the detector array, a variety of images can be created from the sam data (this is called retrospective reconstruction)
-raw data storage requires much more computer storage space than that of image data

9

Image data

Those which result once the computer has processed the raw data
-one hounsfield unit value is assigned to each pixel (this value, or density number, is the average of all attenuation measurements for that pixel) the proportional amount of x-ray energy that passes through anatomy and strikes the detector
-once the value has been assigned to each pixel an image can be formed
-data manipulation is limited

10

Adaptive statistical iterative reconstruction

-there are a large variety of algorithms used, but each starts with an assumed image, computes projections from the image, compares it with the original projection data, and updates the image on the basis of the difference between the calculated and actual projections
-called adaptive statistical interactive reconstruction algorithms
-used to extract additional image clarity and suppress noise
-improve image quality by improving low-contrast detectability
-shown to reduce patient dose by as much as 50% compared to back projection methods

11

Scan field of view (SFOV)

-selecting the SFOV determines the area, within the gantry, from which raw data is acquired
-scan data always acquired around the Isocenter, the patient must be positioned in the center of the gantry
-determines the number of detector cells collecting data
-anything outside the SFOV is not imaged because no data are collected beyond this circle
-parts of patient located outside the SFOV may cause inaccuracies in the image, called out-of-field artifacts
-manifest in image as streaking, shading, and incorrect HU numbers

12

Display field of view

Determines how much of the collected raw data is used to create an image (the section of data selected for display on the image)
-changing the DFOV will affect image quality by changing the pixel size
-works in a similar way to zoom on a camera and can be used to show the entire area or to display a specific region of interest in greater detail
-the DFOV cannot be larger than the SFOV

13

Choosing the optimal display field improves the detectability of ___

Abnormalities

14

Selecting too large a DFOV makes the image appear unnecessarily ____

Small
-harder to visualize if its smaller
-more data included in each pixel and special resolution decreases

15

If the DFOV is too small it may exclude necessary patient ____

Anatomy

16

_____ refers to a selected circle in the center of the gantry. Raw data are acquired and calibrated for any object that lies within this circle. The entire scan circle or any portion of the circle may be selected to display on the monitor. The size of the circle that is displayed is called the ____

SFOV, DFOV

17

Display monitors

-output device allows the information stored in computer memory to be displayed
-displayed in back and white or color
-display device is usually either a cathode ray tube (CRT) or some form of flat panel such as a TFT LCD (thin-film transistor, liquid crystal display)
-CRT is basically a standard television set with some modifications that improve image resolution (hotter and less durable than LCD)
-LCD produce higher luminance and higher spatial resolutions
-DAC change the digital signal from the computer memory back to an analog format so that the image can be displayed on the monitor

18

Cameras

-output device that transfers the image from the monitor to the film (multi format camera)
-usually a laser camera (bypass the image on the display monitor and transfer the data directly from the computer bypassing the video system entirely, thereby significantly improving image quality)
-film used in CT consists of a single emulsion that is sensitive to either the light-emission spectrum of the video screen phosphor (for the multi format camera) or to the laser light beam

19

Gray scale

-a display processor assigns a certain number of HU to each level of gray
-the number of HU assigned to each level of gray is determined by the window width
-the gray scale is used to display CT images. This system assigns a certain number of HU to each shade of grey
-assigns higher HU values lighter shades of grey
-lower values are represented by darker shades

20

Window width

-determines the number (QUANTITY) of HU represented on a specific image
-the software assigns shades of gray to CT numbers that fall within the range selected
-all values higher than the selected range appear white, and any value lower than the range appears black
-"by widening the width" (increasing the WW) more numbers are assigned to each shade of gray

21

_____ assigns the quantity of pixel values to the gray scale. ____ determines the center pixel value in the gray scale

WW, WL

22

Window level

-selects the center CT value of the WW
-window level and window center mean the same thing
-selects which hounsfield numbers/ values are displayed as shades of gray on the image

23

True or false: the window level should be set at a point that is roughly the same value as the average attenuation number of the tissue of interest

True!

24

Wide window widths

-best for imaging tissue types that vary greatly, when the goal is to see all the various tissues on one image
-encompass greater anatomic diversity, but subtle density discrimination is lost
-decrease image contrast, they suppress display of noise on an image
-common practice to widen window width when patients are obese or when there are metallic artifacts

25

True or false: Tissue types with similar densities should be displayed in a lower, or narrow window width

-provide greater density discrimination and contrast
-great for displaying brain

26

Single detector row systems

-opening or closing the collimator controls the slice thickness by controlling the portion of the detectors width that is exposed to incoming x-rays
-opening the collimation beyond the upper limit would do nothing but increase dose to the patient and scatter radiation
-radiation emitted from the collimated x-ray source is referred to as a fan beam
-each gantry rotation produces data for a single slice
-collimators slightly closed results in a thinner slice

27

Multi-detector row systems

-a single rotation can produce multiple slices
-provides longer and faster z axis coverage per gantry rotation
-faster
-slice thickness is determined by a combination of the x-ray beam width (controlled by the collimators) and the detector configuration (can combine different numbers of individual detector elements together ex if one detector row will provide slices 1.25 mm thick, grouping two detector rows together will provide slices 2.5mm thick)
-radiation emitted from the collimated x-ray source in these systems is referred to as a cone beam
-can be used for either axial or helical data acquisitions
-in some systems the width of the detector rows are uniform
-in others, the width of the detector row is variable, with the rows thinner in the center and wider at the periphery

28

When are axial scans typically used?

-when acquisition speed is not a major concern
-when optimal resolution is required
-used for studies in which slices are gapped, or when the exposure will be interrupted

29

Pitch

-a parameter commonly used to describe the CT table movement throughout a helical scan acquisition
-most commonly defined as the travel distance of the CT scan table per 360 rotation of the x-ray tube, divided by the x-ray beam collimation width

30

Pitch in SDCT

-table speed and slice thickness are directly related in a helical scan process
-when the table moves a distance that is equal to the slice thickness during each gantry rotation, the pitch is described as 1
-describes the relationship of the table speed to the slice thickness
-as the pitch increases, so does the slice angle
-as pitch increases, the effects of interpolation become more pronounced ex image unsharpness and effective slice thickness blooming
-information is collected for each table position regardless of pitch
-as pitch increases fewer data are acquired for each table position