chapter 14: pulsed echo instrumentation

what are the 2 major functions of an ultrasound system?

• preparation and transmission of electrical signals

• reception of electrical signals

during transmission, the transducer:

transforms electrical energy into acoustic energy

during reception, the transducer:

transforms the returning acoustic energy into electrical energy

pulser

• creates electrical signals that excite the transducer’s PZT crystals and create sound beams

• functions only during transmission

the pulser determines:

• amplitude

• PRP

• PRF

master synchronizer

maintains and organizes the proper timing and interaction of the system’s components

display

presents processed data

storage

archives the ultrasound studies

changes in transducer output/pulser voltage modify:

the brightness of the entire image displayed on the system’s screen

pulser voltage is also known as:

• transducer output

• output gain

• acoustic power

• pulser power

• energy output

• transmitter output

• power

• gain

can the sonographer adjust the transducer output?

yes

what effect does the transducer output have on the image?

the brightness of the entire image

noise

• a random and persistent disturbance that obscures or reduces a signal’s clarity

• contaminates images with low-level information

signal-to-noise ratio

a comparison of the meaningful info (signal) in an image, compared to the amount of contamination (noise)

when the signal-to-noise ratio is high:

the image is of high quality

when the signal-to-noise ratio is low:

the image contains a larger amount of visible contamination

how are transducer output and signal-to-noise ratio related?

directly

what is the most common way to improve (increase) the signal-to-noise ratio?

increase the output power

shallow imaging:

• shorter listening time

• shorter PRP

• higher PRF

• higher DF

beam former

• determines the firing delay patterns for phased array systems

• part of the transmitter

• functions during transmission and reception

how do the pulser and beam former work together?

the beam former receives the pulser’s signal electrical spike and distributes it to the numerous active elements of an array transducer

with phased, linear, annular and convex array probes, the beam former:

• coordinates the complex electrical signals sent to each active element

• is responsible for apodization

during reception, the beam former:

establishes the correct time delays used for dynamic receive focusing

the beam former controls dynamic aperture by:

varying the number of PZT crystals used during both reception and transmission

digital beam former

use advanced microprocessor technology and produce signals in digital format

advantages of digital beam formers:

• system modifications and updates often require only software programming rather than design and manufacture of new hardware

• extremely stable with no mechanical parts

• versatile, capable of using transducers with a wide range of frequencies

transmit and receive switch

• part of the beam former

• important during transmission and reception

• protects the delicate receiver components from the powerful signals that are created for pulse transmission

• directs the electrical signals from the transducer to the appropriate electronic processing components

channel

made up of a single PZT element in the transducer, the electronics in the beam former/pulser, and the wire that connects them

the number of elements in an array transducer that can be excited simultaneously is determined by:

the number of channels in the ultrasound system

receiver

transforms the electrical signals from the transducer (produced by the reflected sound) into a form suitable for display

what are the 5 receiver operations:

• amplification

• compensation

• compression

• demodulation

• reject

amplification

• AKA receiver gain

• each electronic signal returning from the transducer is made larger

• does not alter the signal-to-noise ratio

what effect does amplification have on the image?

the entire image is made brighter or darker when the receiver gain is adjusted

can the sonographer adjust amplification?

yes

units for amplification:

decibels (dB)

preamplification

the process of improving the quality of a signal before it is amplified and often occurs within the transducer itself

compensation

• corrects for attenuation

• creates an image that is uniformly bright from top to bottom

• AKA time-gain compensation (TGC), depth gain compensation (DGC) and swept gain

can the sonographer adjust compensation?

yes, with the TGC controls

units for compensation:

decibels (dB)

what effect does compensation have on the image?

it treats echoes different depending upon the depth from which they arise

in a TGC curve, the x-axis measures:

the amount of compensation

in a TGC curve, the y-axis measures:

the reflector depth

near gain

at this superficial depth, reflections undergo a small, constant amount of compensation

delay

the depth at which variable compensation begins

slope

at this depth, compensation corrects for the effects of increasing attenuation that result from increasing path length

knee

at this depth, reflections are maximally compensated by the ultrasound system

far gain

at this depth, it indicates the maximum amount of compensation that the receiver can provide

compression

• performed twice without altering the ranking between the signals; the largest signal remains the largest and the smallest signal remains the smallest

• AKA log compression or dynamic range

first process of compression:

keeps the electrical signals levels within the accuracy range of the systems electronics

second process of compression:

keeps an image’s gray scale content within the range of detection by the human eye

can the sonographer adjust the compression?

yes, user-controlled compression modifies the gray scale mapping of the images

what effect does compression have on the image?

it changes the gray scale characteristics of the image

units for compression:

decibels (dB)

why is compression clinically important?

most meaningful backscattered signals from biologic tissues are very weak and the sonographer must be able to see differences in these weak reflections`

demodulation

a two-part process that changes the electrical signals within the receiver into a form more suitable for display on a monitor

what are the 2 processes of demodulation?

• rectification

• smoothing (enveloping)

rectification

• changes the form of the electrical signal by eliminating or correcting for negative voltages so that it is appropriate for the system’s display

• converts all negative voltages into positive voltage

smoothing (enveloping)

places a smooth line around the “bumps” of the voltage signal and evens them out

can the sonographer adjust demodulation?

no

what effect does demodulation have on the image?

none

reject

• allows the sonographer to control whether low-level gray scale info within the data will appear on the displayed image

• AKA threshold or suppression

can the sonographer adjust the reject?

yes

what effect does reject have on the image?

it affects all low-level signals on the image, regardless of their location and it doe snot affect bright echoes

dynamic frequency tuning

systems with this use only the high frequency part of the reflected pulse’s bandwidth to create the superficial portion of the image because higher frequency sound has superior axial resolution

patient exposure to sound energy is affected by:

alterations in output power

The ALARA principle

• as low as reasonably achievable

• states that when modifications to either output power or receiver gain can improve the image’s diagnostic quality, the first and best choice is the one that will minimize the patients ultrasound exposure

if the image is too dark, what should you adjust first, output power or receiver gain?

first increase the receiver gain, which does not increase patient exposure

if the image is too bright, what should you adjust first, output power or receiver gain?

first decrease the output power, which decreases patient exposure