Signal Processing (pt. 2)

Scan Converter

Accepts and stores echo signal information, amplitude, and location

Processes information by manipulating target information and assessing grey-scale levels

Outputs signal information in a format for video display

Analog Scan Converter (ASC)

Older technology

Associated with Cathode Ray Tube (CRT)

Images are not stable/fuzzy & needs constant readjustment

Analog Scan Converter Controls

Read Level (reject) - controls noise level & range of displayed signals

Read Zoom

Display Subdivision

Polarity Switch - inverts gray-scale patterns (black on white/white on black)

Raster

File format for storing/displaying digital images

Size is dependent on dimensions

Determines number of pixels in display

Works from top left to right, then bottom left to right

Functions of a Digital Scan Converter

Accepts analog signal - line-by-line B scan information

Converts analog signal to digital for storage

Stores digital signal

Processes information to emphasize or compress amplitude ranges

Converts digital signals to analog for output

Outputs signals in raster format for video display

Advantages of a Digital Scan Converter

Stable/uniform signal storage

Accurate & increased storage

Long life

Quick data access

Can self-check for malfunctions

Disadvantages of a Digital Scan Converter

Image artifacts

Flicker of pixels

Increased costs

Digital Scan Converter Processing Sequence

Input Signal

Analog to Digital Conversion

Pre-Processing

Signal Storage

Image Updating (related to raw data)

Post-Processing

Digital to Analog Conversion

Display

Input Signal

Analog signal leaves receiver and goes to scan converter for processing

Dynamic Range ~ 35-35 DB

Analog to Digital Conversion

Samples are taken from each pixel and that information is converted into binary numbers that represent levels of gray on the display

Digitization

Analog signals are converted into binary numbers

Sampling

Types: center, peak, average of 4+ samples of signal

Occurs every 0.1 mm along echo signal

Can increase processing time

Pre-Processing

Done before image is frozen

Assigns digitized value of each pixel to a range of values that preserves weak/low amplitude information

Compresses & emphasizes signal ranges

Determines the number of shades of gray in image

Ex. TGC, write zoom, panning

Pre-Processing Maps

X-axis = input/incoming signal

Y-axis = output/stored value

Linear Pre-Processing

Straight-line relationship

Logarithmic Compression Pre-Processing

Emphasis on low-level echoes

Fast initial increase, then a slower progression

Exponential Compression Pre-Processing

Emphasis on high-level echoes

Initially increase slowly, then very quickly increase

S-Curve Compression Pre-Processing

Emphasizes mid-range echoes

Signal Storage

Computer memory stored in binary format

Multi-level checkerboard -> more layers = more shades of grey

Permits:

Freeze frame & cine-loop

Frame Insertion - updating/averaging frames

Interlacing - prevents flicker

Converting to Binary Code

Bit

1 or 0

Building blocks of words in binary code

How to calculate total number of bits for storage

Dimensions of a board x number of boards

Usually between 1-3 million

Word

Combination of bits

Treated as single entity

Corresponds to a location in space

Number of bits dictate the number of different configurations/values it can represent

Byte

8 bits

Total number indicates total number of characteristics that the storage device can hold

Address

Location of checkerboard

Guides system to know where each signal should be placed on the display

Pixel

Location on display monitor

Size has inverse relationship with size of matrix board

Contributes to spatial resolution of monitor

Larger number = smaller size = better image resolution

Formula for Pixel Size

Image depth ÷ one depth dimension of matrix

Changing the spatial resolution on the ultrasound display does not change the axial/lateral resolution of the scan line or sound beam

Image Updating

Uses algorithms to determine if new information needs to be displayed or if the information can remain the same

Last Value Algorithm

Total replacement of old signals with new ones

Compound/Integration/Fraction Algorithm

Old signal value is compared to the new one

If old is > new = no change

If old is < new = percent (or total) of new value is added

Interpolation

Averages signals to fill-in missing pixel information

Post-Processing

Occurs after image is frozen

Does not change stored values

Determines gray scale values on displayed image

Signals are read from the raster and stored in the register

Post-Processing Maps

X-axis = input/stored value

Y-axis = Output/displayed gray scale values

Linear Post-Processing

Uniform distribution

No specific range emphasized

Logarithmic Post-Processing

Emphasis on low-level echoes

Exponential Post-Processing

Emphasis on high-level echoes

S-Curve Post-Processing

Emphasis on mid-range echoes

E-Curve Post-Processing

Steeper slope emphasize low-level echoes

Flatter slope compresses low-level echoes

Digital to Analog Conversion

Digital signal leaves register and is converted back into an analog signal for video display

Functions of Displays

Converts electrical energy to light

Signal storage & display

Types of Displays

Light Emitting Diode (LED)

Liquid Crystal Display (LCD)

Cathode Ray Tube (CRT) -> Oscilloscope & TV monitor

Minimum frames/second on a TV monitor

30

Minimum images/second on a TV monitor

16

For continuous, uninterrupted motion

Components of a CRT

Cathode (electron gun)

Control Grid

Focusing anode, grid, cup, coil or collimator

Accelerating anode or coil

Deflecting plates, coil or yoke

Anode (Phosphor screen)

CRT Image Creation

Electrons are released from cathode

Negative electrons move towards positive anode

Electrons pass through control grid

Internal focusing anode, grid, or cup determine beam diameter

Electrons accelerate by anode (internal electrical) or coil (external magnetic)

Internal deflection plates or external magnetic deflection yokes determine beam position

Electrons strike anode/phosphor screen and emit light

Light color is determined by phosphor material

Amount of light is determine by how many electrons strike anode

Film

Light strikes solver bromide crystals and forms image

Developed in darkroom

Laser Printer

Automated film handling & developing

Good gray scale

No image distortion

Expensive

Thermal Printer

Heat sensitive paper

Does not age well

Color Thermal Printer

Color is pressed on thermal paper and heated so that it bleeds/melts onto the paper

Video Tape Recorders (VTR, VCR)

Contain magnetic oxide particles

Optical Discs

Use laser to write information

Magneto-Optical Discs

Use laser to write information

Can be erased and rewritten magnetically

CD-R & DVD

Types of optical discs

Digital Archive Tape (DAT)

Digital Video Recorder (DVR)

Solid State Hard Drive

No moving parts - helps with failure rates