phys. pre break exam

Transient (or inertial, normal) cavitation happens with what?

rarefactional pressure

Define bioeffects.

potential biological change to tissue secondary to interaction with sound wave

What are the 2 principal mechanisms for creating bioeffects while using ultrasound

Thermal and mechanical

__ bioeffects are related to temperature rise

thermal

____ bioeffects are related to cavitation.

mechanical

What is the maximum beam intensity allowed for focused beams?

1,000 mW/cm2 in focused ultrasound or 1W/ cm2

What is the maximum beam intensity allowed for unfocused beams?

100 mW/cm2 in unfocused ultrasound

- Can be used in therapeutic ultrasound

Define denaturing.

process where proteins lose their native structure leading to the disruption of cell function and potentially cell death.

_______________ is a catalyst that controls the rate at which chemical reactions occur in living organisms.

enzyme

although the temperature varies for the denaturing of various proteins, for many proteins denaturing occurs at temperatures above_______________.

41 degree C

cavitation

when bubbles are produced, vibrate or oscillate, and in extreme cases burst or implode

--> Don't always have to explode

what occurs when the oscillation of the micro bubbles does not lead to collapse.

Stable cavitation

An implosion of microbubbles is referred to

Transient (or inertial, normal) cavitation

--> the best science to date indicates that it seems to be a threshold effect.

Temperatures below _____ degrees Celsius seemingly produce no irreversible bioeffects

40

(No thermal bioeffects have resulted in adverse effects when the tissue temp has increased less than 2o degrees Celsius.)

what does the book state that there are definitive thresholds or a verifiable standard that guarantees that ultrasound can always be safe?

The book states that "definitive results relating bioeffects and safety limits do not exist and likely won't ever". This is because so many aspects have to be factored in when saying "always" safe.

- Complicity between tissue types varies greatly, there are too many variables to account for

What happens to the intensity of a beam if the acoustic power is increased from 50 mW to 100 mW and the beam area is doubled?

The two changes would cancel each other out, if both power and area are doubled then it cancels the change out. So, nothing would happen

Define "spatial" with respect to intensity

spatial measurement where the beam peaks in space or the average of the space

Define "temporal and pulse" with respect to intensity

· Temporal - taken during PRP

· Pulse - taken during pulse duration

List the 4 basic concepts the book provides to help you decipher the Common Intensities discussed with ultrasound bioeffects.

- A peak is always greater than or equal to the average.

- The first letter (S) represents the word "spatial" and refers to how the beam is distributed over space

- The first letter of the second letter "cluster" refers to an intensity measurement over time. If this letter is "P" the intensity measurement is taken during the pulse duration

- If the first letter of the second letter cluster is "T" the intensity measurement is taken during the pulse repetition period

Which common intensity has the highest value

SPTP

What is the ratio between the temporal average and the pulse average called

duty factor

To which common intensity are thermal bioeffects most closely linked

ISPTA

A device used to measure the acoustic pressure fields produced by transducers is called a

hydrophone

The display of the mechanical or thermal index on the ultrasound system became required as a result of a 1985 joint committee decision referred to as the _____________________

Output Display Standard (ODS)

Define Thermal Indices

Estimated maximum tissue temperature increase for the current imaging situation

- EX: If there is a thermal index is 3 then we are also saying it can heat up to 3 degrees Celsius

Which index would be used to indicate the risk of a thermal bioeffect when performing a transcranial Doppler?

Thermal index in cranial bone

What is the name of the principle which essentially states that the sonographer is responsible for understanding the risks of bioeffects and minimizing those risks while optimizing clinical information?

ALARA

According to the AIUM, in some applications, such as fetal examinations in which the ultrasound beam passes through a layer of reduced attenuating liquid, such as urine or amniotic fluid, the TI can underestimate ∆Tmax by up to a factor of ________.

2

For non-fetal imaging, what is the thermal index above which scanning is not recommended, no matter how short the time?

6

For fetal imaging, what is the thermal index above which scanning is not recommended, no matter how short the time?

3

Ultrasound bioeffects

potential biological change to tissue secondary to interaction with sound waves.

--> No known adverse effects, when used with intensities intended for diagnostic imaging

In vivo

Research performed within the living body.

In vitro

Research performed outside the living body.

Dosimetry

Scientific practice of measuring ultrasound beam characteristics relevant to biological effects.

Research techniques used to study bioeffects

1. Retrospective studies

2. Mechanistic approach

3. Empirical approach

4. Prospective studies

Mechanistic approach

Searches for relationship between cause and effect.

Empirical approach

Searches for a relationship between exposure and response.

Prospective studies

Individuals are followed over time and data about them is collected as characteristics or circumstances change.

Retrospective studies

Looks backward and evaluates data that has already been gathered.

Epidemiology

Study of health outcomes in populations. Many ultrasound safety studies evaluate fetal exposure during pregnancy.

Epidemiology study limitations

Often retrospective in design

Data interpretation may be unclear

Other factors can influence outcomes

Epidemiology study strong evidence

Strongest evidence comes from prospective, randomized studies

AIUM (American Institute of Ultrasound in Medicine )

Promotes ultrasound research, education, and safe clinical practice.

--> Key researchers

Information from AIUM about scanning

- No confirmed harmful bioeffects in patients or sonographers with the use of standard diagnostic ultrasound at accepted power levels

- Epidemiologic data indicates ultrasound exposure is not associated with adverse fetal outcomes

- Benefits to patient outweigh risks

Food and Drug Administration (FDA)

- Set standards based on AIUM research

- Regulate and enforce set standards

Regulates ultrasound equipment, establishes and enforces safety standards for diagnostic use.

Sonographer Responsibilities for Exposure Limits

•Minimize exposure time—shortest scan time necessary to complete quality diagnostic test

•Scan for medical need, not as entertainment

•Follow ALARA

•Ensure equipment is maintained and properly functioning

→ It is also important to remove any damaged equipment as soon as it is identified.

Force Balance

measures the power of the sound wave

3 Devices measure output of u/s transducers by absorption

Liquid crystals

Calorimeter

Thermocouple

Calorimeter

Measures total power in sound beam through the process of absorption (measures temperature rise and time of heating).

Thermocouple

Measures the beam's power at specific locations in the beam (Tiny electronic thermometer assessing temperature changes).

Liquid crystals

Beam is absorbed by crystals and changes color based on temperature (mood rings). Provides information on beam shape and strength.

What are the units for all intensities?

Watts/cm²

Which intensity measurement has the highest value?

SPTP

Which intensity measurement has the lowest value?

SATA

Which is most relevant intensity with respect to tissue heating?

ISPTA Spatial Peak, Temporal Average

Common Intensities Related to Bioeffects

Intensity Spacial (average or peak), (temporal or pulse ) (average or peak )

Which is almost always smaller, temporal average or pulse average?

TA < PA almost always

When will these TA/PA intensities be the same?

TA = PA with CW

TP

max intensity in time, and max intensity is averaged over the most intense half cycle s

PA

averaged over pulse duration

TA

average during PRP

Thermal influenced by

- Output beam intensity

- Scan time duration

- Tissue absorption characteristics

- Ability of tissue to dissipate heat

- Scanning technique (scanned vs. non- scanned beam)

Mechanical Bioeffects

Related primarily to cavitation (gas bubble activity in tissue).

--> Cavitation is the production, oscillation or destruction of bubbles

- stable or intertial

Stable Mechanical Bioeffects

bubbles oscillate without collapsing

Inertial Mechanical Bioeffects

bubbles expand and collapse

(TI) Thermal Index is ...

estimated maximum tissue temperature increase (prediction) for the current imaging situation

Thermal index for Fetal imaging:

Begin examinations with TI ≤ 0.7

thermal index for Non-fetal and neonatal applications:

Maintain TI < 0.6 when possible.

-->Includes neonatal TCD, spinal, and transorbital imaging

(TI) Why do non-scanned modalities have a higher thermal risk?

Energy remains focused in the same location for longer periods, increasing the potential for tissue heating.

Scanned modalities (TI)

Beam continuously moves across tissue, reducing heat accumulation.

- 2-D (B-Mode)

- Color Doppler

Non-scanned modalities (TI)

Beam repeatedly insonates the same tissue region, increasing thermal exposure.

- CW Doppler

- PW Doppler

- M-mode

mechanical index (MI) calculation

calculated value used to estimate the likelihood of mechanical bioeffects (cavitation) during ultrasound exposure.

MI = peal rare press / /f

MI increases when:

- Ultrasound frequency decreases

- Peak rarefactional (negative) pressure increases

--> Higher MI values indicate a greater potential for cavitation

SPTA (spatial peak temporal average)

- Limited to 100 mW/cm2 in unfocusedultrasound

- Limited to 1,000 mW/cm2 in focused ultrasound

- Typical SPTA output for diagnostic ultrasound systems is 1 mW/cm2

• Monitor exposure time

CW

PD is the same as the PRP, therefore the Pulse Average is equal to the Temporal Averag

SPTA = SPPA

SATA = SAPA

For the same SPTP intensity

CW Doppler produces a higher SPTA than PW Doppler.

For the same SATP intensity

CW Doppler produces a higher SATA than PW Doppler.

Electrical Safety

• Proper grounding

• Remove any equipment with frayed wires or broken components

• Greatest risk from electrical shock comes from a cracked transducer housing or worn covering

AIUM Diagnostic ultrasound has an established ...

safety record and provides a cost-effective imaging option

Prudence should be followed (AIUM)

- No needless scans

- Minimize scan time (time is primary determinant of exposure)

- Follow the ALARA principle by using the lowest acoustic output (higher gain/amplification) that achieves diagnostic quality.

• If there is a risk of missing clinically relevant information by using a lower transmit power, increasing the power is warranted (benefit vs. risks).

Tissue Harmonics

Higher frequency waves generated as a sound pulse propagates through tissue in a nonlinear fashion.

Nonlinear Sound Propagation in Soft Tissue

Sound travels faster during compressions (stiffer tissue) and slower during rarefactions (less stiff).

Harmonic Frequency Multiples

Tissue-generated harmonic frequencies are multiples of the fundamental frequency wave.

Penetration vs. resolution in harmonic imaging

Lower fundamental frequency enables deeper penetration, while higher harmonic frequency improves return signal resolution.

Transducer requirement for harmonic frequency sound waves

Harmonic imaging requires broadband transducers.

Origin of harmonic frequency sound waves

Harmonic waves occur secondary to tissue responses to the fundamental frequency sound wave.

Sound propagation in harmonic imaging

Harmonics arise from non-linear propagation of sound, where wave speed travels irregularly or disproportionately.

Clinical benefits of harmonic frequency sound waves

They reduce artifacts and help improve suboptimal diagnostic images.

Two main types of harmonics in ultrasound

Tissue harmonics and contrast harmonics.

When are tissue harmonics created?

During transmission as the beam travels deeper into tissue.

Why do tissue harmonics reduce surface distortion and noise?

Because the harmonic signal is not present as it leaves the transducer.

Nonlinear sound propagation in tissue harmonics

Sound travels faster during compression than during rarefaction.

Signal strength of tissue harmonics

Weaker harmonic signal compared to contrast harmonics.

What is required for contrast harmonics?

Ultrasound enhancing agents (microbubbles).

When are contrast harmonics created?

During reflection of the sound beam off microbubbles.

Mechanical Index (MI) requirements for contrast harmonics

Occurs at MIs > 0.1, with a recommended MI < 0.3.

Resonance imaging

Another term for contrast harmonics created by non-linear behavior of microbubbles.

Signal strength of contrast harmonics

Stronger harmonic signal than tissue harmonics.

Fundamental frequency

The operating frequency of the transducer, also called the 1st harmonic.

2nd harmonic frequency

2 × fundamental frequency.