Diagnostic ultrasound ( PDFDrive )

CHAPTER
2
Biologic Effects and Safety
J. Brian Fowlkes and Christy K. Holland
SUMMARY OF KEY POINTS
• Clinical ultrasound has been found to be an effective
imaging modality with an excellent safety proile when
used appropriately.
• Ultrasound can produce physical effects, which should be
understood as part of the beneit-versus-risk assessment
as with any medical procedure.
• The thermal index (TI) and mechanical index (MI) provide
feedback to the user and should be minimized while
obtaining the requisite medical beneit from the ultrasound
examination.
• Ultrasound exposures during clinical, research, and
educational examinations should be as low as reasonably
achievable (ALARA).
• Users of ultrasound should be appropriately trained and
familiar with the equipment operation and controls that
affect ultrasound exposure.
CHAPTER OUTLINE
REGULATION OF ULTRASOUND
OUTPUT
PHYSICAL EFFECTS OF SOUND
THERMAL EFFECTS
Ultrasound Produces Heat
Factors Controlling Tissue Heating
Spatial Focusing
Temporal Considerations
Tissue Type
Bone Heating
Soft Tissue Heating
Hyperthermia and Ultrasound Safety
Thermal Index
Homogeneous Tissue Model (Soft
Tissue)
Tissue Model With Bone at the
Focus (Fetal Applications)
Tissue Model With Bone at the
Surface (Transcranial
Applications)
Estimate of Thermal Effects
Summary Statement on Thermal
Effects
EFFECTS OF ACOUSTIC CAVITATION
Potential Sources for Bioeffects
Sonochemistry
Evidence of Cavitation From
Lithotripters
Bioeffects in Lung and Intestine
Ultrasound Contrast Agents
Considerations for Increasing Acoustic
Output
Mechanical Index
Summary Statement on Gas Body
Bioeffects
OUTPUT DISPLAY STANDARD
GENERAL AIUM SAFETY
STATEMENTS
EPIDEMIOLOGY
CONTROLLING ULTRASOUND
OUTPUT
ULTRASOUND ENTERTAINMENT
VIDEOS
Ultrasound has provided a wealth of knowledge in diagnostic
medicine and has greatly afected medical practice, particularly
in obstetrics. Millions of sonographic examinations are
performed each year, and ultrasound remains one of the fastest
growing imaging modalities because of its low cost, real-time
interactions, portability, and apparent lack of biologic efects
(bioefects). No casual relationship has been established between
clinical applications of diagnostic ultrasound and bioefects on
the patient or operator.
REGULATION OF
ULTRASOUND OUTPUT
he U.S. Food and Drug Administration (FDA) regulates the
maximum output of ultrasound devices to an established level.
he marketing approval process requires devices to be equivalent
in eicacy and output to those produced before 1976. his historic
regulation of sonography has provided a safety margin for
ultrasound while allowing clinically useful performance. he
mechanism has restricted ultrasound exposure to levels that
apparently produce few, if any, obvious bioefects based on the
epidemiologic evidence, although animal studies have shown
some evidence for biologic efects.
In an efort to increase the eicacy of diagnostic ultrasound,
the maximum acoustic output for some applications has increased
through an additional FDA market approval process termed
“510K Track 3.” he vast majority of ultrasound systems currently
in use were approved through this process. he Track 3 process
provides the potential for better imaging performance and, as
discussed later, requires that additional information be reported
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