Signal Processing (pt. 1)

Timer

Coordinates the sending, receiving and display of signals

Keeps track of when sound is sent and how long it takes for echoes to return

Range Equation

Distance to reflector = (propagation speed x pulse round trip travel time) ÷ 2

13 µsec

= 1 cm on screen

Beamformer

Essential for electronic transducers

Pulse-Delay sequences produce dynamic focusing, electronic steering, dynamic focusing, and apodization

Electronic Steering

Phased array scan lines

Doppler cursor positioning

Color box adjustments

Apodization

Greatest energy in center of array - sending and receiving signals

Decreased energy on outer edges of array

Produces grating/side lobes

Transmitter/High Voltage Pulse Generator (HVPG)

Generates short electrical pulse that starts vibration of crystal

Determines energy of sound wave

Controlled by adjusting acoustic output/power/intensity

Must be electronically matched to transducer

Exams that use lower voltages/power (MI & TI)

OB

GYN

Exams that use mid to higher voltages/power (MI & TI)

Cardiac

Cirrhotic Liver/Fatty Liver

Obese patients (penetration)

Transducer

Where electrical energy is converted to mechanical energy, then back into electrical energy

Piezoelectric properties - Resonance frequency of crystal, transmits and reflects radiofrequency signals, damping

Resonance Frequency

Natural frequency of crystal

Damping

Removes energy from crystal

Makes pulses shorter - broad bandwidth

Used in PW Doppler

Transmit-Receive Switch

Located between transducer and detector - surge protector that protects electrical components

Only accepts signals < 1 Volt

First component signal moves through after transducer

Frequency Passband Filter

Designed for broadband/multi-frequency transducers and harmonic imaging

Deterioration may harm receiver electronics

May be integrated as part of transmit-receive switch or function separately

Detector

Receives variety of information from returning signals:

Amplitude - basis for brightness

Time of arrival - basis for location

Frequency - basis for Doppler & harmonic imaging

Phase - basis for Doppler direction

Tissue speed - basis for shear wave elastography

Receiver

Processes the electrical voltage signal detected by the crystal

Sequence of Receiver Functions

Amplification

Compensation

Compression

Demodulation/Rectification

Rejection

Pre-Amplification

Happens before true amplification

When small amplifier is placed in transducer assembly

Occurs before signal reaches the cable within transducer

-> Generates low-level noise that affects signal to noise ratio

Not adjustable

Amplification

Uniformly amplifies all returning signals

Converts small received signals to larger ones suitable for processing

-> Determines overall gain

Gain

Ratio of output to input electric power

Compensation

Allows for uniform display of similar tissues despite differences in depth

Compensates for attenuation & more for higher frequency transducers

-> controls TGC, DGC, Near/Far Gain

Compression

Proportional decrease in signal amplitude

-> controls dynamic range (how many shades of grey)

Linear, logarithmic, and exponential methods

Rectification

Converts alteration current to direct current - negative to positive

Half-Wave Rectification

Diode only conducts signal during positive portion of cycle

Full-Wave Rectification

Diode converts all signals into output

Demodulation

Converts voltages in receiver from one form to another

Smoothing

Rejection

Eliminates smaller amplitude signals produced by weak echoes

-> wall filter or reject

Signal to Noise Ratio (SNR)

Strength of echo signal compared to random noise level

Want higher echo signal than noise

Techniques to Improve SNR

Change location of amplifier within system

Internal & external shielding of system

Limited cable length

Cable sheilding

Noise free power source