Chapter 24 Flashcards
Bioeffects
the effects of the sound wave upon the biologic tissue through which it travels
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
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
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
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
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
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
Risk-Benefit Relationship
Primary mandate of US, benefits to the patient must outweigh the risks of the exam
Therapeutic ultrasound
Controlled intensities can be used to create beneficial bioeffects
Therapeutic ultrasound
Controlled intensities can be used to create beneficial bioeffects
Dosimetry
measuring the “dosage” of ultrasound (or characteristics of the beam) likely to result in bioeffects
In vivo (“in living”)
studies are performed within the living body of a plant or animal
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
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
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
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
Thermal Bioeffects
Proposes that bioeffects result from temperature elevation
Thermal Index
a useful predictor of maximum temperature increase under most clinically relevant conditions
Wp
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
- Cavitation
- 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
- Stable
- 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
- Prospective
- 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
