UNIT 2: Digital Image Characteristics and Analog to Digital Conversion Flashcards
Matrix
A set of pixels arranged in columns and rows
Pixel
A picture element, an individual box on the matrix
Voxel
A 3-D picture element (used in volume imaging for CT and MRI)
Field of View
Total area of the matrix that is used to create the image
What determines the FOV for digital imaging?
CR: size of the IR
DR: processing algorithm, affected by Collimation
How does pixel size affect spatial resolution?
Decrease in pixel size means an increase in spatial resolution
Pixel size formula
FOV(mm)
—————
Matrix size
Object must be _________ than the pixel in order to be imaged
Larger
How does Collimation affect spatial resolution?
Failing to collimate decreases spatial resolution (increases pixel size)
What is Spatial Frequency
• 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
Decrease in spatial frequency means a _________ in spatial resolution
decrease
Spatial Frequency formula
1
SF= ——————
2(pixel size)
Signal to Noise Ratio (SNR)
-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
How do you ensure the SNR is kept as high as possible?
-Collimation: reduces scatter
-Grid
-Optimum kVp
-Appropriate mAs/kVp
4 types of image noise
Quantum, System, Ambient, Scatter
Quantum noise is
Insufficient exposure
System noise
-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
Ambient noise
-Background radiation (cosmic, terrestrial)
*this is also another term for dark current
Define Contrast to Noise Ratio
• 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
How is CNR measure?
Using the ROI (Region of interest)
Detective Quantum Efficiency: (DQE)
• 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, incident x-ray energy and noise
DQE and IR Material
Cesium iodide highest DQE
DQE and kVp
-Optimum kVp ensures penetration of the BODY part but not penetration of the IR
-High KVp = Low DQE
DQE and Spatial Resolution
-High SF = Small Objects (harder to image) = Low DQE
-Low SF = Larger Objects (easier to image) = High DQE
DQE and Noise
Increase in noise means a decrease in DQE
Modulation Transfer Function (MTF)
• 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
How does Spatial Frequency affect MTF
-High SF = Small Objects= harder to image = LOW MTF
-Low SF = Large Objects= easier to image = HIGH MTF
Overall Image Resolution is controlled by what two types of image resolution?
-Contrast resolution (visibility) —> MTF
-Spatial resolution (recognizability) —> SF
Contrast Resolution
• 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
Spatial Resolution
• Ability to image small structures accurately
• Sharpness is essential to visually separate adjacent structures
• Assessed by SF
Low SNR means _____ MTF
Low
Pixel bit depth (gray scale bit depth)
The range of values that a pixel can represent
Bit depth formula
of bit depth system by powers of 2
Ex: What is the grayscale bit depth of 10-Bit system? 2x2x2x2x2x2x2x2x2x2= 1024
Pixel size
related to the size of the matrix and the area of the detector used for image reconstruction, called the Field of View
Pixel pitch
measured from the center of one pixel to the center of the adjacent pixel and determines sampling pitch (frequency) expressed as pixels/mm
Size of a matrix is calculated by
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
Other terms used to describe spatial resolution are:
• Definition
• Sharpness
• Recorded detail
• Detail
Spatial Resolution is controlled by:
• Matrix size
• Pixel size
• Pixel bit depth
Matrix size and Pixel size
Matrix size controls pixel size. As matrix size increases, pixel size decreases= increased resolution
As pixel pitch decreases and sampling frequency increase, resolution _________
Increases
As pixel bit depth increases, the number of gray shade the system can display
increases, so resolution ___________
increases
The primary measurement used for spatial resolution in digital imaging is
Spatial frequency
Spatial frequency is measured in
line pairs per mm or cm
High spatial frequency
shorter wavelength/high frequency signal = line pairs close together can be seen = increased or high resolution
Low Spatial Frequency
long wavelength/low frequency signal = line parts that are further apart can be seen= decreased or low resolution
Spatial Resolution Quality Measures
Low contrast resolution
the ability to image objects with similar subject contrast
Noise can result from:
• Radiation scatter
• Light scatter
• System noise (electronic noise)
• Quantum Mottle
Noise is measured by
Signal to Noise Ratio (SNR)
One way to increase the SNR is to increase the
exposure to the IR (kVp, mAs, or both)
DQE is affected by:
• SNR
• Quantum noise
• System noise
• MTF
• Incident x-ray energy
• Spatial frequency
• Detector material
Amplitude
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
Wavelength
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
Frequency
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
The x-ray attenuation signal is an
analog signal
Analog signals must be converted to _______ signals for the computer to process and reconstruct the image digitally
digital
Analog to Digital Converter (ADC)
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.
Basic steps involved in the ADC process:
- Scanning - acquiring the analog signal intensity strength and location on the IR ( this step is left out of some texts)
- Sampling - taking “snapshots” of various points in the analog signal
- Quantization - assigning values to the “snapshots”
- Coding - coding the values in binary form
