Pulse Echo Instrumentation

Principle 1 of Operation

One-to-One correspondence principle

• physical beam forming is directly couples with displayed scan lines

• Each pulse generates one scan line

Principle 2 of Operation

Virtual beam forming does not rely on one-to-one relationship

• Images are in focus throughout, improved quality

Beam former, signal processor, image processor, display

Operating Principle 1 System is composed of what 4 elements

Beam former

Element of Principle 1 instrumentation where action originates and produces sound waves

• pulses

• Pulse delays

• Transmit/recieve switch

• Amplifiers

• Analog-to-digital converters

• Echo delays

• Summer

Sections of the beam former

Generates voltages to drive the transducer, initiates pulse

• determines PRF, coding, frequency, and intensity

• Amplifies returning echos

• Compensates for attenuation

Functions of the beam former

Pulser

Part of the beam former which generates the voltages that drive the transducer

Inversely related

Describe the relationship between depth and PRF

Pulses

What section of beam former automatically adjusts PRF based on imaging depth

Sequencing, phase delays, variations in pulse amplitudes

Functions of Pulser and Pulse delays

• Beam scanning

• Steering

• Transmission focusing

• Aperture

• Apodization

The pulser and pulse delays allows for electronic control of :

Apodization

Sending different voltage amplitudes to minimize grating lobes

Precise control of beam characteristics

what does an increased number of channels allow for

Channel

Independent signal path consisting of a transducer element, delay, and possibly other electronic components

Transmission

During ____, the transmit/recieve switch opens the path from the pulser to the transducer elements

Reception

During ____, the transmit/recieve switch opens the path from the elements to the reception amplifiers

Protects the sensitive input components of the amplifiers

Function of transmit/recieve switch

1

The beam former has ___ amplifier(s) for each channel

Gain

Determines amount of amplification of echoes

X2, 1/2

3dB =

X10, 1/10

10dB =

Time Gain Compensation (TGC)

Amplifies selectively based on arrival time (depth)

• compensates for attenuation

Digitizers in the Beam Former

Converts voltages to numbers for digital signal processing and storage

Reception dynamic focus, steering

Echo delays allow for ___

Reception Apodization, dynamic aperature

The Summer allows for ___

Summer

Signals are added together to produce a scan line

Signal Processor

Recieved digital signals from the beam former

Filtering, detection, compression

Functions of the Signal Processor

Filtering

Rejection of frequencies above and below the range of frequencies

• reduces noise

• Eliminates the fundamental frequency in harmonic imaging

Detection/Demodulation

Conversion of echo voltages from radio frequency form to video form

Is not

Detection is/isn’t operator controlled

Rectification and smoothing

Detection includes ___ and ____

Rectification

Inversion of negative values into positive

Smoothing

Allows for one mean signal to be recorded

Compression

Reducing the dynamic range to a usable range

Few

• black to white, high contrast

Small (narrow) dynamic range means how many shades of gray

More shades of gray

• good contrast resolution

High (broad) dynamic range means how many shades of gray

Image Processor

Converts scan line data into images

• processes images before storing them and as they come out of memory

• scan conversion

• Preprocessing

• Persistence

• Panoramic imaging

• Spatial compounding

• Cine loop

• Post processing

• Gray/color scale

Functions of Image Processor

Preprocessing

Image processing done before storing in memory

Pixel interpolation

assigns brightness based on average brightness of adjacent pixels

Persistence

Reduces noise and smooths the image by frame averaging

Edge enhancement

Sharpens boundaries to make them more detectable and measurements more precise

Write zoom

Increases number of pixels or scan lines while zooming

• persistence

• Pixel interpolation

• Volume imaging

• 4D imaging

• Edge enhancement

• Write zoom

• Panoramic imaging

• Spatial compounding

Preprocessing examples

Scan converter

Converts scan line data into images format data (matrix)

• locates each series of echoes corresponding to each location in depth along the scan line

Finer (improved/better)

More pixels = what kind of spatial detail

Post processing

Which kind of processing is operator controlled

• gray scale maps

• B-color

• Three dimensional presentation

• Read zoom

• Measurement calipers

Post processing examples

Digital to analog converter

Gives the amplitude that determines the brightness of the echoes on the display

Echo strength

Brightness is proportional to ____

Frame rate

Number of images entered into memory per second

A, M, B

3 modes of display

A mode

Amplitude mode

Horizontal : depth

Vertical: amplitude

In A-Mode, what is shown on the horizontal and vertical axis

B Mode

Brightness Mode

• aka gray scale

• 2D or 3D

M Mode

Motion mode

• combines A & B modes

Horizontal: time

Vertical: depth

In M-Mode, what is displayed on the horizontal and vertical axis

Number of bits per pixel in the image memory

What does contrast resolution depend on

Contrast resolution

Ability of gray scale displays to distinguish between echoes of slightly different intensities

Temporal resolution

Ability of display to distinguish closely spaced events in time and to present rapid moving structures correctly

Frame rate

What is temporal resolution dependent on

Frame rate will increase and temporal resolution will improve

If PRF increases, what happens to frame rate and temporal resolution

Same as using multiple foci

• increased penetration

• Reduction of speckle with improved contrast resolution

• More sensitive receiving system

Function of coded excitation

Uses a series of pulses and gaps, rather than a single driving mules, ensembles of pulses drive transducer to create a single scan line

• lateral resolution improvement

• Grating lobes eliminated

• Superficial reverb reduced or eliminated

How does harmonic imaging improve image quality

Spatial compounding

Averaging of frames that view anatomy from different angles

• reduction in speckle/clutter artifacts

• Smooths imaging surfaces

• Visualization of structures behind a highly attenuation structure

How does spatial compounding improve image quality

Frame averaging/ Frequency compounding

Echo frequency spectrum is divided into bands by filters, they are processed separately and the recombined

• reduces noise and improves contrast resolution

Penetration, resolution, tissue texture

Frequency compounding/ frame averaging can be adjusted to emphasize ____,___, &_____

Elastography

Imaging in the means of palpatation, presents qualitative tissue stiffness information

Cardiac strain imaging

Presents info regarding contraction and relaxation strain and strain rate info for the myocardium

Fusion imaging

Combined presentation of a sonographic anatomical image with another imaging form

• usually CT or MRI