Pulsed Echo Instrumentation & Dynamic Range

2 major functions of system

preparation & transmission

reception

six major components

transducer

pulse & beam firmer

receiver

display

storage

master synchronizer
For Dare’s class: also transmitter and scan converter/memory

Whats the pulser?

creates electrical signals exciting PZT crystals to create sound beams during transmission

what’s the transducer output?

adjustable?

effect on image?

changes the pulse voltage to modify the brightness of the entire image on the screen

Yes, adjustable

High pulser voltage = stronger vibration of PZT = Stronger sound beams = stronger echoes = brighter picture

However, lower pulse voltage desired to reduce bioeffects on patient

Synonyms for transducer output

output gain, acoustic power, pulser power, energy output, transmitter output, power, or gain

signal to noise ratio (SNR)

Whats the relationship b/w SNR and TDR output?

signal in image compared to noise level

when ratio is high, signal much stronger than noise and image quality is good

directly related (increases power is most common way to increase SNR)

Pulse repetition period (PRP)

pulser determines the PRP

PRP determines max image depth (depth of view)

PRP short = PRF high

What’s the beam former?

-part of transmitter

-coordinates signals sent to PZT, optimizing transmitted beam

-does apodization, dynamic receive focusing, and dynamic aperture

-amplifies returning echoes

What does beam former consist of?

pulser, delays, transmit/receive switch, amplifiers, analog to digital converters, summer

Advantages of digital beam formers

-System more flexible and only needs software update rather than replacement

-extremely stable w/no mechanical parts to fall out or break

-versatile w/diff. TDRs

Transmit receive switch

-ensures electrical signal travels in right direction

-ensures pulser voltage goes to TDR and received voltages go to signal processor

what’s a channel?

a single PZT crystal, electronics in beam former/pulser, and the wire connecting them

-# of elements in array that can be excited simultaneously is determined by # of channels

Receiver/Signal processor

-prepares info from signals to display on monitor

-signals returning are weak & require receiver processing due to attenuation

Order of receiver operations

Amplification

Compensation

Compression

Demodulation

Reject

*hint: it’s alphabetical

Amplification

adjustable?

units?

AKA “Receiving Gain”

-each returning

-signal undergoes equal amount of amplification

-entire image made brighter (everything’s done equal since amplitude occurs to every single echo)

-operator dependent

Unit: dB

preamplification

process if improving quality of a signal before it is amplified

often occurs w/in TDR (as close as possible to active element)

Compensation

Adjustable?

Units?

ALA “Time Gain Compensation (TGC)” or “Depth Gain Compensation (DGC)” or “swept gain”

-compensates for attenuation due to depth

-creates image that’s uniformly bright from top to bottom

-controlled by sonographer

-use TGC curve

-Higher frequency TDRs have more attenuation die to less absorption

units: dB

Anatomy of TGC curve

x-axis = compensation

y-axis = depth

from top to bottom: near gain, delay, slope, knee, far gain

Compression

Adjustable?

Units?

AKA “log compression or dynamic range”

-changes grayscale characteristics

-decreases difference b/w smallest & largest amplitudes (largest remains largest & smallest remains smallest)

-results in reduction echo range

2 kinds: done in receiver (can’t be changed) & one user adjustable (gray scale mapping)

Units: dB

Demodulation

Adjustable?

2 part process changing signals in receiver to more suitable form on display

Rectification: converts all neg. voltages to pos.

Smoothing/enveloping": smooths bumps around wave/evens them out

NOT adjustable

No visible effect on image; change forms of signal so it’s appropriate for display

Reject

Adjustable?

AKA threshold or suppression

-controls low level grayscale (unwanted noise) being displayed on image

-weak echoes associated w/noise & rejection removes weakest echoes w/o affecting important ones

Has built in NOT adjustable reject & also a user adjustable one

output power effects

changes brightness of image

alters signal to noise ratio

alters patient exposure

bioeffect concerns

decrease first if image too bright

receiver gain effects

changes brightness of image

does not affect signal to noise

does not change patient exposure

no bioeffect concerns

increase first if image too dark

ALARA

as low as reasonably achievable

want to minimize risk of patient’s ultrasound exposure

dynamic range

method of reporting extent to which a signal can vary & still be accurately measured

units: dB

weak signals: below threshold

strong signals: saturate the system

general rule of dynamic range

dynamic range of information decreases the more it’s processed

alternate description of dynamic range

besides compression, think number of choices

fewer shades of gray = few choices, black & white (bistable), narrow dynamic range, high contrast

More shades = many choices, gray scale, wide dynamic range, low contrast