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SPRING 2024: Principles II > UNIT 2: Digital Image Characteristics and Analog to Digital Conversion > Flashcards

UNIT 2: Digital Image Characteristics and Analog to Digital Conversion Flashcards

1
Q

Matrix

A

A set of pixels arranged in columns and rows

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2
Q

Pixel

A

A picture element, an individual box on the matrix

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3
Q

Voxel

A

A 3-D picture element (used in volume imaging for CT and MRI)

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4
Q

Field of View

A

Total area of the matrix that is used to create the image

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5
Q

What determines the FOV for digital imaging?

A

CR: size of the IR
DR: processing algorithm, affected by Collimation

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6
Q

How does pixel size affect spatial resolution?

A

Decrease in pixel size means an increase in spatial resolution

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7
Q

Pixel size formula

A

FOV(mm)
—————
Matrix size

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8
Q

Object must be _________ than the pixel in order to be imaged

A

Larger

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9
Q

How does Collimation affect spatial resolution?

A

Failing to collimate decreases spatial resolution (increases pixel size)

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10
Q

What is Spatial Frequency

A

• Spatial Resolution is expressed in terms of Spatial Frequency (line pairs/mm)
• How many details can be fit into a space (size of the object being imaged and the space between the objects)
• The lower the SF = fewer lp/mm = reduced spatial resolution

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11
Q

Decrease in spatial frequency means a _________ in spatial resolution

A

decrease

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12
Q

Spatial Frequency formula

A

1
SF= ——————
2(pixel size)

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13
Q

Signal to Noise Ratio (SNR)

A

-Signal-to-Noise Ratio (SNR): ratio of good signal vs. Noise
-Signal = Good information
-Noise= Any unwanted information in the Radiograph
-High SNR = Good signal with low noise

• Affected by exposure factors and noise
-mAs
-Scatter control

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14
Q

How do you ensure the SNR is kept as high as possible?

A

-Collimation: reduces scatter
-Grid
-Optimum kVp
-Appropriate mAs/kVp

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15
Q

4 types of image noise

A

Quantum, System, Ambient, Scatter

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16
Q

Quantum noise is

A

Insufficient exposure

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17
Q

System noise

A

-Background noise, firing of electrical signal in your machine
-Congruent with audio noise, such as the static white noise heard on frequencies between radio stations

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18
Q

Ambient noise

A

-Background radiation (cosmic, terrestrial)

*this is also another term for dark current

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19
Q

Define Contrast to Noise Ratio

A

• Good contrast divided by the noise
• Contrast = image signal difference from the background
• The manifestation of SNR on the image
• Affected by signal and noise
• mAs
• Scatter control

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20
Q

How is CNR measure?

A

Using the ROI (Region of interest)

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21
Q

Detective Quantum Efficiency: (DQE)

A

• The capability of an image receptor to convert the x-ray beam into an image OR the measure of how sensitive and accurate incoming data is converted to output viewing
• Scored from 0 to 1 (always less than 1)
• Affected by IR material, kVp, Spatial Frequency, incudent x-ray energy and noise

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22
Q

DQE and IR Material

A

Cesium iodide highest DQE

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23
Q

DQE and kVp

A

-Optimum kVp ensures penetration of the BODY part but not penetration of the IR
-High KVp = Low DQE

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24
Q

DQE and Spatial Resolution

A

-High SF = Small Objects (harder to image) = Low DQE
-Low SF = Larger Objects (easier to image) = High DQE

25
Q

DQE and Noise

A

Increase in noise means a decrease in DQE

26
Q

Modulation Transfer Function (MTF)

A

• The capability of an imaging system to accurately record anatomic information
• Aka: Fidelity
• Scored from 0.0-1.0
• Affected by Contrast Resolution and Spatial Frequency

-Rate of change aka contrast

27
Q

How does Spatial Frequency affect MTF

A

-High SF = Small Objects= harder to image = LOW MTF
-Low SF = Large Objects= easier to image = HIGH MTF

28
Q

Overall Image Resolution is controlled by what two types of image resolution?

A

-Contrast resolution (visibility) —> MTF
-Spatial resolution (recognizability) —> SF

29
Q

Contrast Resolution

A

• Ability to image differences in object density accurately
• Contrast is essential to make out differences between adjacent structures
• Assessed by CNR and MTF

-the ability of the IR to respond to different energy levels in the remnant beam and transfer these energy levels to gray scale values

30
Q

Spatial Resolution

A

• Ability to image small structures accurately
• Sharpness is essential to visually separate adjacent structures
• Assessed by SF

31
Q

Low SNR means _____ MTF

A

Low

32
Q

Pixel bit depth (gray scale bit depth)

A

The range of values that a pixel can represent

33
Q

Bit depth formula

A

of bit depth system by powers of 2

Ex: What is the grayscale bit depth of 10-Bit system? 2x2x2x2x2x2x2x2x2x2= 1024

34
Q

Pixel size

A

related to the size of the matrix and the area of the detector used for image reconstruction, called the Field of View

35
Q

Pixel pitch

A

measured from the center of one pixel to the center of the adjacent pixel and determines sampling pitch (frequency) expressed as pixels/mm

36
Q

Size of a matrix is calculated by

A

multiplying the number of pixels in each row by the number of pixels in each column

Ex: a typical detector may have 1024 rows by 1024 columns. The number of pixels for this detector would then be 1024 x 1024 = 126,976

37
Q

Other terms used to describe spatial resolution are:

A

• Definition
• Sharpness
• Recorded detail
• Detail

38
Q

Spatial Resolution is controlled by:

A

• Matrix size
• Pixel size
• Pixel bit depth

39
Q

Matrix size and Pixel size

A

Matrix size controls pixel size. As matrix size increases, pixel size decreases= increased resolution

40
Q

As pixel pitch decreases and sampling frequency increase, resolution _________

A

Increases

41
Q

As pixel bit depth increases, the number of gray shade the system can display
increases, so resolution ___________

A

increases

42
Q

The primary measurement used for spatial resolution in digital imaging is

A

Spatial frequency

43
Q

Spatial frequency is measured in

A

line pairs per mm or cm

44
Q

High spatial frequency

A

shorter wavelength/high frequency signal = line pairs close together can be seen = increased or high resolution

45
Q

Low Spatial Frequency

A

long wavelength/low frequency signal = line parts that are further apart can be seen= decreased or low resolution

46
Q

Spatial Resolution Quality Measures

A
47
Q

Low contrast resolution

A

the ability to image objects with similar subject contrast

48
Q

Noise can result from:

A

• Radiation scatter
• Light scatter
• System noise (electronic noise)
• Quantum Mottle

49
Q

Noise is measured by

A

Signal to Noise Ratio (SNR)

50
Q

One way to increase the SNR is to increase the

A

exposure to the IR (kVp, mAs, or both)

51
Q

DQE is affected by:

A

• SNR
• Quantum noise
• System noise
• MTF
• Incident x-ray energy
• Spatial frequency
• Detector material

52
Q

Amplitude

A

defines the height of the sine wave from the x-axis to its highest point (A to B) and is sometimes referred to as its intensity

53
Q

Wavelength

A

is the distance from one maximum crest or high point (B) or one trough lower point (D) to the next crest or trough of the wave

54
Q

Frequency

A

is the interval or how many crests passes a particular point in time.
Frequency is usually measured in cycles per second or hertz as defines the energy of the signal

55
Q

The x-ray attenuation signal is an

A

analog signal

56
Q

Analog signals must be converted to _______ signals for the computer to process and reconstruct the image digitally

A

digital

57
Q

Analog to Digital Converter (ADC)

A

The process of digitizing the analog signal. Done by the analog to digital converter, a piece of hardware that may be either a module inside the computer “box” or a separate “box” connected to the computer.

58
Q

Basic steps involved in the ADC process:

A
  1. Scanning - acquiring the analog signal intensity strength and location on the IR ( this step is left out of some texts)
  2. Sampling - taking “snapshots” of various points in the analog signal
  3. Quantization - assigning values to the “snapshots”
  4. Coding - coding the values in binary form

SPRING 2024: Principles II (13 decks)

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