pulse echo instrumentation

what are the 2 major functions of the ultrasound system?

• preparation and transmission of electrical signals to the transducer which creates a sound beam

• reception of electrical signals from the transducers, with subsequent processing into clinically meaningful images and sounds

transducer

• produces an ultrasound pulse for each electrical pulse applied to it

• for each echo received from the tissues, an electric voltage is produced by the transducer

• during transmission, it transforms electrical energy into acoustic energy and during reception, it converts the returning acoustic energy into electrical energy

voltages go through the beam former to the:

signal processor where they are processed to a form suitable for input to the image processor

electric information from the image processor drives the:

display which produces a visual image of the cross-sectional anatomy interrogated by the system

master synchronizer

• communicates with all of the individual components of the ultrasound system

• organizes and times their functions, so as to operate as a single integrated system

• coordinates all of the components of an ultrasound system

beam former

• produces electrical pulses that drive the transducer and performs initial functions on returning echo voltages from the transducer

• sends digitized echo voltages to the signal processor

• determines the firing delay patterns for phased array systems

the beam former consists of:

• pulser

• delays

• transmit/receive (T/R) switch

• amplifiers

• analog-to-digital converters (ADCs)

• summer

beam former - pulser

• receives timing signal from synchronizer and creates an electrical signal that excites the PZT crystal

• determines the amplitude, PRP and PRF for phased array systems

the beam former - pulser determines:

• PRF

• PRP

• pulse amplitude

when the sonographer increases output power:

higher electrical voltages are created that strike the PZT crystal and this increases the sound intensity created by the transducer and sent into the pt

synonyms for beam former - pulser

• output gain

• acoustic power

• pulser power

• energy output

• transmitter output

increasing transducer pulser’s output improves:

• signal-to-noise ratio (increases)

  • signal strength is increased while the noise level remains unchanged

  • determined by the excitation voltage from the pulser

pulser mode - continuous wave

• constant electrical signal stimulates the PZT

• electrical freq = ultrasound freq

pulser mode - pulsed wave, single crystal

• short duration electrical “spike” smashes into the PZT, which then vibrates at its resonant freq

• 1 electrical spike creates 1 ultrasound pulse

pulser mode - pulsed wave, arrays

• short duration electrical “spikes” stimulate the many crystal that make up the array

• multiple electrical spikes create array

pulser and pulse delays

• control electronic steering, focusing, aperture size and apodization

• distributes a single electrical voltage from the pulser to all crystals of an array transducer

• different amounts of electrical voltages are sent to different crystals at different times

coded excitation

• technique controlled by pulser and pulse delays

• instead of one pulse per scan line, a series of pulses are transmitted

coded excitation improves:

image quality by reducing speckle/noise so signal-to-noise ratio (SNR) increases

signal

meaningful portion of the data

noise

inaccurate portion of the data which degrades the quality of our information

high SNR

meaningful part of data is much stronger than the inaccurate portion and a high quality image is created

low SNR

inaccurate part of data is much stronger than the meaningful part and a poor quality image is created

transmit/receive (T/R) switch

• directs sound from the pulser, through the pulse delays, to the transducer

• directs echoes that returned to the amplifier

T/R switch prevents:

electrical voltage pulse from going to the amplifier

T/R switch protects:

the amplifier/receiver

amplifier

part of the beam former that initiates the increase of the strength of the signal

pre-amplifier

alters the signal before it is amplified, often performed in the transducer

pre-amplification

technique used to reduce system noise

analog-to-digital converters (ADC)/digitizers

after amplification, the echo voltages pass through the ADC where they are converted from analog form to digital form

ADCs covert the analog echo voltage into:

a series of numbers representing the sampled voltage

echo delays

after being amplified and digitized, the digital echo voltage passes through echo delay lines to accomplish reception dynamic focus

summer

after passing through echo delays, signals are added together in the summer to produce a scan line and that scan line will then go though the signal and image processors and will then be displayed

receiver (image processor)

• part of pre-processing

• boosts the strength of returning signals, processes them and prepares them for display

signal processor - receiver order

1. amplification

2. compensation

3. compression

4. demodulation

5. rejection

amplification (receiver gain)

• increases the strength of all signals prior to further processing

• changes the brightness of the entire image

• measured in: dB

when amplification levels change, what happens to the SNR?

SNR is unchanged

what component of the ultrasound system has the greatest dynamic range?

amplifier

output power and receiver gain both do what?

change the brightness of the entire image

output power

affects the brightness by adjusting the strength of the sound pulse sent to the body of the transducer

when the output power is high, what happens to lateral and longitudinal resolution?

the lateral and longitudinal resolution degrade

receiver gain

affects the brightness by changing the amplification of the electronic signals after returning to the receiver

if the image is too bright, what 2 steps must you follow?

• first, you decrease output power

• second, you decrease receiver gain

ALARA

as low as reasonably achievable

signal processor - compensation (TGC, DGC, swept gain)

• used to create image of uniform brightness from top to bottom

• needed for deeper pulses that undergo more attenuation and have lower amplitudes

TGC and higher frequency

• beams from higher frequency transducers undergo more attenuation so more TGC must be used

• TGC curve is shifted upward and to the right

TGC and lower frequency

• beams from lower frequency transducers undergo less attenuation so less TGC is needed

• TGC curve is shifted downward and to the left

TGC - near gain

• superficial depths

• small amount of compensation

TGC - delay

depth at which variable compensation begins

TGC - slope

compensation corrects for the effects of attenuation due to increasing depth

TGC - knee

reflections are maximally compensated by the system

TGC - far gain

indicates the maximum amount of compensation that the receiver can provide

signal processor - compression

• reduces the total range of signals, from smallest to largest

• keeps signals within the operating range of the system’s electronics and the gray scale within the range of what the human eye can see

• decreases the dynamic range of the signals

dynamic range

• technique which reduces the dynamic range of the returning echoes

• amount of info that the ultrasound system can handle and accurately evaluate

• similar shades of black, grey or white are grouped together

fewer shades displayed means:

• few choices

• black and white

• narrow dynamic range

• high contrast

• high compression

more shades displayed means:

• many choices

• grey scale

• wide dynamic range

• low contrast

• low compression

single processor - (amplitude) detection/demodulation

• involves the conversion of echo voltages from radio frequency to video form

• amplitude remains the same even after being detected (converted)

what are the 2 steps of detection/demodulation?

1. rectification

2. smoothing/enveloping

rectification

• turning all of the negative voltages into positive ones

• corrects for or eliminates negative voltages

smoothing/enveloping

putting an envelope around the “bumps” to even them out

signal processor - rejection (suppression, threshold)

• displays low level echoes only when they are clinically meaningful

• accomplished through the use of a tunes amplifier

• eliminates low-level noise in the image

tuned amplifier

an amplifier with a bandpass filter

bandpass filter

allows frequencies within a certain bandwidth to pass through while rejecting frequencies outside that range

filtering

keeping useful echo signals (frequencies), while rejecting frequencies that are not useful