Chapter 24 Flashcards by Lindsey Kaminski

Bioeffects

the effects of the sound wave upon the biologic tissue through which it travels

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Hydrophone

AKA: a microprobe
-needle like shaped with a tiny piece of PZT on the end
-A wire connects the PZT to an oscilloscope (readout meter)
-Pressure and amplitude (when calibrated) at specific locations is determined
-The needle can be moved within the beam, thus beam shape can be determined

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Radiation Force Balance

-The sound beam strikes a float, or a ball
-measured force is communicated to a balance which determines the power in the beam
AKA: a feedback microbalance

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Acousto-Optics

-Based on the interaction of sound and light
-Common instrument = Schlieren
-Is a shadowing system/camera that allows visualization of the shape of the sound beam in a medium
-Sound waves (beam profiles) appear as shadows

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Calorimeter

-Like a thermos filled with absorbing material
-Sound beam is directed into the calorimeter and the total power is calculated by measuring the temperature rise and the time of heating
-Measures the power of the entire beam

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Thermocouple

-A tiny electronic thermometer with a dab of absorbing material on it
-It is inserted into the sound beam and the temperature at that location is measured
-The temperature rise is related to the power or intensity of the sound beam at that particular location

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Liquid Crystals

-Sound beam strikes the crystals, the sound energy is absorbed
-The change in crystal temperature causes a change in color, providing insight into the shape and strength of the sound beam

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Risk-Benefit Relationship

Primary mandate of US, benefits to the patient must outweigh the risks of the exam

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Therapeutic ultrasound

Controlled intensities can be used to create beneficial bioeffects

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Therapeutic ultrasound

Controlled intensities can be used to create beneficial bioeffects

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Dosimetry

measuring the “dosage” of ultrasound (or characteristics of the beam) likely to result in bioeffects

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In vivo (“in living”)

studies are performed within the living body of a plant or animal

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In vitro

-research is performed outside the living body; in an artificial environment
-These studies provide an opportunity to perform research that would be impossible with live human subjects
-The challenge is to meaningfully relate the results to the clinical setting

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AIUM statement on In Vitro Bioeffects

-In vitro research is important
-In vitro bioeffects are real in the artifical environment and can initiate in vivo validation
-In vitro bioeffects which claim direct clinical significance without in vivo validation should be viewed with caution

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Empirical Approach

-Is based on the acquisition and review of records from patients/animals exposed to US
-It seeks to establish a statistically significant exposure response relationship
-No mechanism is proposed

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Mechanistic Approach

-Begins as a proposal that a specific mechanism has the potential to produce bioeffects
-Then a theoretical analysis/study is designed to test the mechanism i.e., the scope of the bioeffect at various exposure levels
-searches for a specific cause and effect

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Thermal Bioeffects

Proposes that bioeffects result from temperature elevation

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Thermal Index

a useful predictor of maximum temperature increase under most clinically relevant conditions

is a power parameter calculated for output power and attenuation

Wdeg

is the estimated power needed to increase tissue temp by 1ºC

TIS

is the thermal index calculated assuming that the sound beam travels through soft tissue
first trimester (bone not mineralized)
-vascular, echo, abdominal

TIB

is the thermal index calculated assuming that bone is at or near the beam’s focus, usually a higher number than the TIS
-second and third trimesters

TIC

is the thermal index calculated assuming that cranial bone is in the sound beams near field
-Transcranial doppler

Maximum heating of tissues is related to SPTA intensity

FDA limit is 720 mW/cm2 SPTA

Thermal Findings

Tissue heating relates to: 1. Output power/temperature rise 2. Exposure time 3. Characteristics of tissue

Temperature elevation becomes progressively greater in this order

2D – color – spectral doppler

Nonthermal or Mechanical Bioeffects

1. Cavitation 2. Radiation Force

Radiation Force

-Force in beam can deform structures -Force can move fluid which creates shear (parallel) stress that deforms structures

Lithotripsy = Therapeutic Ultrasound Example

-uses shock waves to break down stones in the kidney, gallbladder, or ureter. -Intravascular lithotripsy is being proposed for coronary calcium prior to angioplasty

Cavitation

-is the interaction of sound waves with microscopic, stabilized, gas bubbles in the tissues (gaseous nuclei); specifically Gaseous nuclei oscillate (expand and contract)

Two forms of cavitation exist

1. Stable 2. Transient

Stable: at lower MI levels, stable cavitation occurs

-Gaseous nuclei oscillate (expand and contract) -They may double in size but do not burst -Surrounding fluids micro stream -Surrounding cells experience shear stress

Transient Cavitation

-At higher MI levels gaseous nuclei burst AKA: inertial or normal cavitation (TIN) -The pressure threshold for transient cavitation is only 10% higher than that for stable cavitation

Mechanical Index

is related to the likelihood of cavitational bioeffects

Epidemiology (Population) Studies

-Is a branch of medicine that studies prevalence of disease in specific populations -It is empirical: Seeks exposure response relationship

Many epidemiologic studies deal with in utero fetal exposure to US and the response on

Birth weight Length and Head Circumference Apgar Scores Congenital Abnormalities Hearing Infection

Limitations of epidemiologic studies are

-Studies are retrospective, obtained form old medical records -Ambiguities may exist in the data such as reason for exam, gestational age, # of scans, technique, and exposure time -Risk factors other than US exposure may precipitate a bad outcome in the fetus (maternal age, smoking, environmental factors, alcohol, or drug abuse)

The best epidemiologic studies are

1. Prospective 2. Randomized

Prospective studies

are forward looking, a protocol is established, and specific information is systematically obtained

Randomized studies

-create two groups of patients -One group is exposed to US and the other is not -The groups are otherwise similar taking other risk factors out of the equation

Clinical Safety and Prudent Use The conclusions of the AIUM include:

-No confirmed harmful bioeffects from exposure to diagnostic US has ever been reported -It is possible that bioeffects may be identified in the future -The benefit to the patient outweighs the risks -It is appropriate to use diagnostic US prudently to provide benefit to the patient -It is inappropriate to use diagnostic US in a non-medical setting for entertainment

Training and Research The conclusions of the AIUM include:

-No confirmed bioeffects on patients or sonographers have been found with the use of diagnostic US in research and training -Research and training exams may differ from standard exams (longer research exams and greater exposure) -The subject should be informed how the research study differs from standard diagnostic procedures

Electrical Safety

-US systems present no special electrical safety hazard -Basic precautions include: >Proper grounding >Routinely checked for proper condition >Since transducer poses greatest risk, routinely check transducer housing for cracks