Chapter 7 and 8 Physics Test (range equation and transducers)

How is an Image created?

• sound is transmitted

• bounces off a reflector

• sound returns to the transducer

• speed that sound travels in soft tissue is known

• the travel time of the sound allows the ultrasound system to accurately place the reflector on the image

Time of Flight / Go-Return Time

• the time from pulse creation to pulse reception

  • time determines position of the reflector on the image created

  • superficial reflectors = short time of flight (shallow depths)

  • deeper reflectors = longer time of flight

Time of flight is _____ ______ to how deep a sound pulse travels

directly related

If a reflector is twice as deep, the time of flight is _______

twice as long

Depth Equation

depth = speed * go-return time (𝜇𝑠) / 2

depth = (1.54mm/𝜇𝑠) * go-return time (𝜇𝑠) / 2

13 Microsecond Rule

For every 13 𝜇𝑠 of go-return time, the object creating the reflector is 1cm deeper in the body

• always applies when traveling in soft tissue

What is the Time of Flight / Go-Return Time when a reflector is 2cm deep?

26 𝜇𝑠

total distance travelled is _____ the reflector depth

twice

A sound pulse is emitted from the transducer and returns in 52 µs.

1. How deep is the reflector?

2. What is the total distance traveled?

1. 4 cm

2. 8 cm

A sound pulse is emitted from the transducer and returns in 130 µs.

1. How deep is the reflector?

2. What is the total distance traveled?

1. 10 cm

2. 20 cm

PRP and Maximum Imaging Depth

PRP is the go-return time to the very BOTTOM of the image

• SHallow Depth = SHort PRP

• Deeper Depth = Long PRP

PRP and Maximum Imaging Depth in relation to the 13 Microsecond rule

PRP (µs) = imaging depth (cm) * 13 µs/cm

PRP is ______ to maximum imaging depth

directly related

Speed of sound in soft tissue is

1.54 mm/µs OR 154,000 cm/s

If sound can travel a TOTAL distance of 154,000 cm over one second, the distance of the reflector would be

77,000 cm away

How many pulses can be sent in one second to a depth of 7.7 cm and return to the transducer

10,000

PRF

number of pulses per second

when the depth is set to 7.7 cm, PRF = ?

10,000 Hz or 10kHz

When depth is shallow, PRF is _____

high (inverse relationship)

when depth is deep, PRF is _____

low (inverse relationship)

PRF and Depth Equation

PRF = 77,000 (cm/s) / imaging depth (cm)

Transducer Definition

device that converts one form of energy into another

Ultrasound machines convert ______ to ______ on transmission

electrical to acoustic

Ultrasound machines convert ______ back to _____ on reception

acoustic to electrical

Two Functions of Ultrasound Transducers

1. Transmit

• electrical energy from ultrasound system converted to sound wave

2. Receive

• reflected sound pulse is converted to electrical

Piezoelectrical materials are

deformed resulting in an electrical field

• this is how the ultrasound transducer transforms the electrical energy to sound and then sound to electrical energy

how is a voltage created

when mechanical stress is applied to certain materials

• noted by Curie Brothers in 1880’s

Piezoelectric Effect 2 Parts

1. Transmission (electric to acoustic):

• the reverse piezoelectric effect explains transmission of ultrasound

  • ultrasound machine produces a voltage that is applied to piezoelectric material

  • material changes shape

2. Reception (acoustic to electric):

• the piezoelectric effect explains reception of ultrasound

  • pressure is applied to the piezoelectric material by the return of the sound pulse

  • a voltage is created by the pressure/deformation of material

Another name for Piezoelectric Materials

ferroelectric

Piezoelectric Materials Definition

materials that convert sound into electricity OR electricity into sound

PZT

the material commonly used in ultrasound transducers

• manmade piezoelectric material

PZT is also known as…

• ceramic

• active element

• crystal

Naturally Occurring Piezoelectric Materials

• ceramics

• quartz

• tourmaline

• Rochelle salt

Manufactured Piezoelectric Materials

• barium titanate

• lead zirconate titanate

  • PZT; MOST COMMON IN U.S TRANSDUCERS

• barium lead zirconate

method to improve the characteristics of piezoelectric materials

Poling

Poling Steps

1. high voltage and very high heat are applied to align the poles

2. electric voltage will cause dipoles to vibrate, changing the shape of PZT which in turn creates a sound wave

3. when sound wave returns and hits the crystal, the opposite effect occurs and sound is transferred to electrical energy

Curie Point

the temperature at which transducers piezoelectric material is polarized

• temperature required for polarization

Curie Point for PZT

300-400 degrees Celsius

• heating above this will depolarize the crystals

What will happen if the crystal is reheated to the Curie Point

• depolarization

• crystal will lose its piezoelectric properties

7 Components of the Basic Transducer

1. Case

2. Electric Shield

3. Acoustic Insulator

4. PZT or active element

5. Wire

6. Matching Layer

7. Backing Material AKA Damping Element

Case

the plastic or metal cylindrical tube that covers the crystal and internal components

What does the Case Protect?

• protects the crystals from damage

• protects the patients from being electrically shocked

Cleaning Protocol for Case to not Damage It

• wiped down

• intracavitary transducers are soaked in disinfectant (Trophon)

Electrical Shield

• thin metallic barrier lining the inside of the case

• prevents electrical noise from outside of the reflections from the patient

Acoustic Insulator

• barrier or cork or rubber that separates the internal components of the transducer from the case

• prevents vibrations from returning and interfering with true echoes from the patient

(yellow line)

Active Element (PZT)

• the crystal itself (heart of transducer)

• transmits and receives sound

• coin-shaped

dimensions of the active element determine…

the characteristics that are emitted by the transducer

the thickness of the PZT is…

½ the wavelength

Wire

• connects transducer to PZT element

• provides electrical connection between ultrasound unit and crystal

  • TRANSMISSION: voltages from ultrasound system causes crystals to vibrate and produce an ultrasound wave

  • RECEPTION: crystal vibration produces a voltage that can return to the system to be processed into an image

Matching Layer

• in front of PZT at a face of the transducer

  • increases efficiency of sound energy transfer between active element and the body

  • protects the active element

Matching Layer thickness is….

¼ of the wavelength

Anything that comes between the crystal and the skins surface must have ____ impedances.

similar

as the difference of impedance increases, reflection

increases

The matching layer has an impedance between that of the _____ and the ______

active element and the skin

PZT impedance is ________ than the impedance of the skin

20 times greater

Matching Layer Impedance Benefit

multilayer and allows more sound to transmit into the body by lessening the impedance differences

ex: gel

What Would Happen if the Matching Layer were Not Present to Bridge the Impedance Differences?

the majority of the sound beam would reflect at the interface and never make it into the body

What makes the transducer efficient enough to transfer sound into the body?

the matching layer and gel

Acoustic Impedance high to low

• PZT (high)

• matching layer

• gel

• skin (low)

Backing Material/Damping Element

• bonded to the back of the active element

• reduces “ringing” of the PZT

• made of epoxy resin and tungsten filaments

(green)

what does the backing layer restrict?

the excitement of the PZT from the electrical voltage

the backing material ____ the sound pulse

DAMPENS

• reduces number of cycles per pulse

• shortens the pulse duration and spatial pulse length

  • better image resolution

Characteristics of Damping Material

• high degree of sound absorption

• acoustic impedance similar to PZT

  • allows sound to be absorbed into the backing material and away from the patient

Consequences of Backing Material

1. decreased sensitivity

2. wide bandwidth

3. low quality factor

Decreased Sensitivity

• backing material cannot be removed between transmit and receiving

  • vibration is reduced during reception as well

• reflections back to transducer are tiny

• less able to convert the low-level reflections into electrical signals

RECEPTION = DECREASED SENSITIVITY

Wide Bandwidth

• backing material does not allow the PZT to vibrate freely

• dampening of tone produces sounds at varying frequencies both above and below the main frequency of the transducer

Bandwidth Definition

• the range of frequencies in the pulse, or the difference of the highest and lowest frequencies

longer pulse = ____ bandwidth

narrow bandwidth

fewer frequencies

shorter pulses = _____ bandwidth

wider bandwidth

more frequencies

imaging probes (PW) broadband or Wideband

short duration pulse

wide range of frequencies

CW and therapeutic probes with narrow bandwidth

long duration pulse

one frequency

Low Quality Factor

• a measure of the efficiency of the transducer in converting electrical energy to sound energy and vice versa

  • unitless

  • inversely related to bandwidth

Q factor Equation

Q-Factor = main frequency / bandwidth

high/wider bandwidth probes have

• low Q factor

• shorter pulses

• more frequencies per pulse

• better image quality

  • ex: imaging probes (use damping material)

low/narrow bandwidth probes have

• high Q-factor

• longer pulses

• less frequencies per pulse

  • ex: CW and therapeutic probes (lack of damping material)

Sterilization

destruction of all microorganisms by way of extreme heat, chemicals, or radiation

Disinfection

application of chemicals to reduce or eliminate organisms on an object (reduce germs)

Intruments that penetrate skin need to be ____

sterilized

Ultrasound transducers require

disinfection

should you expose transducers to heat?

no

CW and PW transducer frequencies depend on

the SOURCE (ultrasound machine)

Continuous Wave Transducers

• electrical frequency determines the CW frequency

  • crystal is constantly supplied with an electrical voltage

  • the crystal constantly (or continuously) emits sound waves

• electrical frequency = acoustic frequency

Pulsed Wave Transducers

• short duration electrical spikes travel through the wire to the PZT crystal in the transducer

  • characteristic of THAT element (PZT) determines the frequency

Frequency is determined by

1. speed of sound in the PZT element

2. thickness of the PZT element

Speed of Sound in PZT is ____ ______ to the frequency

directly

when the speed of sound in the PZT is faster, the frequency created by that transducer is ____

higher

when the speed of sound in the PZT is slower, the frequency created by that transducer is ____

lower

PZT Thickness and frequency are _____

inversely related

the thinner the active elements (PZT) the _____ the frequency (_____ wavelengths)

higher

shorter

the thicker the active elements (PZT) the _____ the frequency (_____ wavelengths)

lower

longer

typical thicknesses in diagnostic imaging

.2-1 mm

Thickness of the PZT crystal in pulsed wave transducers = ______

½ the wavelength of sound in the PZT