Biological effects of UltraSound King Saud University, college of Applied medical sciences, Radiological Sciences Department. Radiological Protection Course Presented by: Shatha Jamal Al Mushayt At 2009-2010
Thermal effects Def: temperature within a medium (locally). How? As the sonic energy is absorbed & converted into heat. Thermal effect depends on: Beam intensity, tissue absorption coefficient, blood flow, exposure parameters(e.g. Duration of exposure, frequency, …) 1.
More likely to occur with endocavity probes where the probe is enclosed within the body & can be almost
stationary for several minutes.
Clearly express thermal injury
e.g. trans-esophageal exams. Endocavity probe
Mechanical(direct) effects e.g.Particledisplacement & fluid streaming: Target particles are pushed away from the transducer acoustic streaming in ﬂuids, cell distortion* and lysis. Non-thermal effects 2.
Non-thermal effects 2. B. Cavitation Regions of compression & rarefaction are created in the medium. increases & decreases in pressure alternatively. Gas bubbles form(how?) & grow until critical size then collapse. generates the energy for mechanical effects.
Cavitation Types Cavitation may be transient or stable.
Transient cavitation : very rapid expansion &
Causing high temp. & pressure, release of free radicals
Stable cavitation : bubbles oscillating with sound
Cause mechanical damage, membrane rupture
& sometimes cell lysis.
Safety Indices Thermal Index (TI) & mechanical Index (MI) Not perfect; but they are the most common & practical measurements available at present. Indicate the probability of thermal & non thermal effects. Assist the sonographer in patient exposure. How? > By keeping these indices as low as possible while obtaining the best possible diagnostic images.
Thermal Index(TI) An indicator of the temp. elevation possible at a particular equipment setting. TI has 3 subdivisions : Soft tissues (TIS); bone (TIB); and adult cranial exposure (TIC).
An indicator of the probability of cavitation events. Generally, MI should be < 1.9 Mechanical Index(MI)
Thermal effects Temperature rise of less than 1.5 degrees C no hazard to human (including fetus). Temp. rise of 4 degrees C, lasting for 5 min or more hazardous specially to a fetus.
Imaging modes and thermal effects In routine practice : B-mode, M-mode and 3D imaging are less likely to give rise to thermal injury. >> figures Doppler US can cause signiﬁcant temp. rises. Doppler image M-mode image B-mode image
No significant cavitation damage in vivo caused by diagnostic or physiotherapy beams.
How did these mechanisms help in developing other techniques The development of new imaging techniques e.g. IV Injection of gas-filled micro bubbles as contrast agents to enhance the echogenicity. New therapeutic applications. e.g. (next slides) Maintaining the safe use of diagnostic US.
Therapeutic U/S Usually continues US wave or pulses of much higher intensities than in diagnostic. Examples of applications: Lithotripsy, (mechanical) Tumor therapy by high intensity focused ultrasound (HIFU): heat tissue (thermal) & produce necrosis.
Diagnosis vs. Therapy Diagnostic exposures are designed to the interaction of US with tissue to avoid potential bioeffects. Therapeutic application depends on the direct interaction of US with tissue to produce the desired beneficial bioeffect. Exposure parameters are often different. Therapeutic intensities, pulse durations far exceed the diagnostic devices output.
receives one or more US scans. Risks & benefits are different depending on: Types of US, stages of pregnancy, machines, centers, & sonographers Each situation must be judged in its own merit. Major centers are preferable for better trained sonographers & powerful machines No long or repeated scans.
Long Term Edverse Effect During Pregnancy Some reported fetal effects of US exposure: Delayed speech, dyslexia*, growth restriction, & non-right-handedness.
BUT up to date(7/2009), there is insufficient justiﬁcation
to conclude that there is a causal relationship between diagnostic US & long-term adverse fetal effects.
Recommendation for clinical practice of diagnostic US No routine US with no clear indications for use. Should only be used when benefits outweigh risks. Users should know the exposure parameters of US equipment they employ. Users must know how to alter machine settings so as to reduce exposure. Instruments must be checked routinely to maintain the capability of obtaining reliable diagnostic information at ALARA exposures.
Recommendation for clinical practice of diagnostic US For US scans for operator training, memorial pictures & videos of the fetus, or research, a lower threshold is recommended ) TI 0.5 , MI 0.3 ( It is not recommended to use colorDoppler mode of the 1sttrimester embryo routinely; as this mode has a potential to produce signiﬁcant temp. rises. Acoustic output from B-mode, M-mode, 3D imaging is safe during all pregnancy stages(if used as needed).