Day 4 - Pulsed Wave Operations and Transducers

Pulsed Wave

creation of images using sound by sending sound waves into the body, timing its return to determine the depth of the reflector

Pulse

a collection of cycles that travel together

• must have a beginning and an end

• an entire pulse moves as a single unit

has 2 components:

• Transmit- “on time", SENDS PULSE OUT

• Receive- listening or “off“ time, ANALYZE RINGING & MAKE SENSE OF IT

• Pulse = 1/f

  • if f ↑ then P ↓

Pulse Duration

(pulse)

the ACTUAL TIME from the start of a pulse to the end of that pulse, considered the transmit time

• determined by the source (freq.)

• clinical imaging PD ranges from 0.3 to 2.0 microseconds (μs)

• # of cycles in a pulse x period of central frequency (PD = n x p)

  • if f↑ then P↓ and PD↓

Pulse Repetition Period (PRP)

(pulse)

the start of one pulse to the start of the next pulse

• includes the "on” (transmit) and “off” (listening) time

• UNITS: msec or μsec

• directly related to depth, can be changed by changing depth

  • determined by imaging depth selected by the sonographer and the sound source 

  • if depth ↑, PRP ↑

  • deeper imaging depth = longer PRP bc sound waves have to travel further (direct relationship)

• directly related to the maximum imaging depth

PRP and PRF are reciprocals

PRP = 1/ PRF (pulse repetition frequency)

differences between PRP and PD

• PRP includes listening time and PD does not

• both have units of time

• PRP can be changed by the operator by changing depth

• PD cannot be changed by the operator, it is determined by the source only

Pulse Repetition Frequency (PRF)

(pulse)

number of pulses transmitted into the body by the US system each second

• UNIT: Hz or per sec

• determined by the sound source; changes with depth

• NOT RELATED TO FREQUENCY

• depth and PRF have an INVERSE RELATIONSHIP

  • depth ↑, PRF ↓

• inversely related to PRP

  • PRF = 1/PRP

• if transducer is excited 500 times, then PRF is 500 Hz

Spatial Pulse Length (SPL)

(pulse)

distance a pulse occupies in space from start to end, the length of a pulse

• SPL = # of cycles in a pulse x wavelength

• UNIT: mm

• determined by both the source and medium

• CAN’T BE CHANGED by the sonographer

• shorter SPL = better image

• ↑ freq = shorter period (time) = shorter pulse duration = shorter wavelength (distance) = shorter SPL

Duty Factor (DF)

(pulse)

the % or fraction of time that the system transmits a pulse

• unit less (1.00 or 100%) for CW bc it is always transmitting

• DF = (PD/PRP) * 100

• DF has a value between 0 & 1

  • clinical US: DF ranges between 0.001 and 0.01 (more time receiving)

  • low transmitting time (less than 1%), for PW mostly “listening” (>99%)

•  Determined by the source and depth

  • PRP is affected by depth => DF affected

  • *ANYTHING THAT AFFECTS PRP AFFECTS DF*

  • ↑ depth = ↑ PRP = ↓ DF

  • freq affects DF

Sound Wave Parameters Chart

*14*

Bandwith

the difference between the ↑ and ↓ frequency emitted by a transducer

• narrow bandwidth emits few frequencies (Continuous Wave- runs on 1 freq)

• wide bandwidth has short pulses and emits more frequencies (Pulsed Wave- short ring time allows freq to be changed)

Damping

(bandwidth)

used to lower ring time

• has wide bandwidth —> more frequencies (PW)

• ring time is shorter

No Damping

(bandwidth)

continuous transducer

• narrow bandwidth —> fewer frequencies (CW)

• ring time is longer

Bandwidth and Ring Time (Output)

*16*

Range Equation

used to determine how far away a reflector is located to be displayed on the screen

• aka “time of flight“ or round trip

• US assumes that the beam is traveling through soft tissue at a speed of 1.54 mm per microsecond

• Depth = (1.54 mm/μs x Go-return time) / 2

• *19*

13 μsec Rule

it takes 13 μsec for sound to travel to a depth of 1 cm and return to transducer (round trip time)

• applies only to soft tissue

• it takes 6.5 μsec to get to reflector and 6.5 μsec to return to transducer —> ~13 μsec per cm of depth

• EX: pulse time of flight = 26 μs when a reflector is 2cm deep

  • 26 μs ÷ 13 μs/cm = 2cm

• EX: pulse traveling 3cm deep = 39 μsec (13μsec + 13μsec + 13μsec)

  • total distance traveled = 6cm (3cm * 2)

Transducer

device that converts one form of energy into another

• electrical energy —> sound (acoustic energy) —> electricity

Piezoelectricity (PZT)

helps convert electrical to acoustic to electrical

• crystals within transducers have positive and negative sides and during the process of PZT, the crystals go through an area of contraction and expansion

Reverse Piezoelectric Effect

the property of certain materials to create a voltage when they are mechanically deformed or when pressure is applied

• PZT materials change shape when a voltage is applied to them causing a reverse piezoelectrical effect

  • (+) on one side and (-) on the other

  • expands in thickness

Ferroelectric Material

other materials that convert sound into electricity and vice versa

• Quartz and Tourmaline- found in nature

• most commonly used in clinical transducers:

  • Lead Zirconate

  • PZT

Curie Point

Curie temp for PZT is ~328°C to 365°C 

• CAN’T heat transducer to these levels bc it would destroy the crystals and lose piezoelectrical properties

Transducer Construction

(1) Active Element - PZT Crystal

(2) Electrode Wires

(3) Backing Material - Damping

(4) Matching Layer

(5) Lens

(6) Insulation

(7) Housing - Case

(1) Active Element(s)/PZT

crystal thickness determines the freq. of a transducer

• thicker crystal = ↓ freq.

• thinner crystal = ↑ freq.

most modern transducers have multiple crystals known as array transducer - crystals are arranged in rows

PZT elements are connected to a motor (shaped like a coin)

(2) Electrode Wires

provides an electrical connection between the PZT and the US system

• each crystal is connected to 2 electrode wires: one (+) and one (-) electrode wire PER crystal 

• causes the crystals to vibrate during transmission to produce an ultrasonic wave

(3) Backing Material - Damping

used to decrease the ring time

• placed in the back of the active element (crystal)

• made of epoxy resin and tungsten

• dampens sound pulse by restricting the extent of PZT deformation

• shorter pulse = improved axial resolution

Disadvantages:

• ↓ sensitivity —> unable to detect low level echoes

Pros:

• wider range of frequencies (bandwidth)

• CW HAS NO DAMPING

• PW YES DAMPING

Damping Relationships

*32*

(4) Matching Layer(s)

located in front of the PZT at the face of the transducer

• made of multiple layers

• ↑ efficiency of sound energy transfer by ↓ impedance mismatch

  • helps match the impedance between the transducer and the skin

• gel is like a matching layer used to remove air between the patient and the transducer

• protects the active element (crystal)

• 1/4 wavelength thick

(5) Acoustic Lens

protects the matching layer

also makes the sound beam thin, reduce the slice thickness

• used to reduce the thickness of our sound beam

• newer transducers don’t need these lenses

(6) Insulation

(7) Housing - Case

Transducer Characteristics

(1) Frequency

(2) Bandwidth

(3) Quality Factor

(4) Sensitivity

(1) Transducer Frequency Continuous Wave

CW produces a continuous electrical signal that constantly excites the active element of the transducer

• frequency transmitted by the probe = frequency of electrical signal

• freq. is determined by the voltage of the probe

  • voltage = 5 MHZ —> transducer freq. = 5 MHz

(1) Transducer Frequency Pulsed Wave Ultrasound

creates short duration electrical spikes (bursts) applied to crystal

• very short pulses required in real time 2D imaging & M-mode

• longer pulses used for PW Doppler and Color Doppler

2 characteristics of the active element in PW:

• speed of sound of the PZT

• thickness of the PZT & direction of freq. = INVERSE RELATIONSHIP

  • thinner crystal = ↑ freq. (shorter wavelength)

  • thicker crystal = ↓ freq. (longer wavelength)

  • diameter of the PZT determines width of US beam

(2) Bandwidth

the range of freq. present within the beam

• allows for wide range of freq. in PW —> “multi-hertz“ transducers

• shorter pulse in PW = WIDER bandwidth (imaging)

• longer pulse in CW = SHORTER bandwidth (no imaging, just listening)

damping limits ringing and shortens pulses —> WIDER bandwidth

• causes low quality imaging

• BW = max freq. - min freq.

long vs short pulse response

high operating freq. = shorter periods/pulse duration = shorter SPL = improved axial resolution

(3) Quality Factor

describes the quality of the sound beam, how pure the sound beam is

• PW transducers have LOW QF due to damping and WIDER bandwidth

• unitless #

  • low QF is no longer pure due to the range of freq.

• non imaging CW probes have the cleanest freq. —> HIGHEST QF (bc it operates at 1 freq.)

• QF = best functioning frequency (MHz)/bandwidth (MHz)

(4) Sensitivity

• WIDE bandwidth (↑ freq.) = LESS sensitivity

• NARROW bandwidth (↓ freq.) = HIGHER sensitivity

CW Beam shape

• transmitted and return pulse meet at the sensitive region

• beam shape is continuous and can be seen all at once like a flashlight

• beam begins as the width of the transducer converges to a narrower region and diverges