Biological effects of UltraSoun

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King Saud University, college of Applied medical sciences, Radiological Sciences Department.Radiological Protection Course

Presented by: Shatha Jamal Al

MushaytAt 2009-2010

Main Objectives

What are the biological effects of ultrasound and how do they occur?

How can they be predicted and prevented?

How did they help in developing other techniques?

Examples of some reported effects and if their relationship with ultrasound is proved or not yet.

Introduction

• Ultrasound is a high frequency mechanical waves

that are above the human hearing range(>20,000 Hz).

• They are produced by converting the electrical energy

into mechanical energy.

• When transmission is through biological tissues

& under certain conditions, they may cause

biological effects.

Biological Effects of US

Mechanisms

Thermal Non-thermal

Mechanical Cavitation

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.

Transducer Self-heating

• Electrical energy is converted to thermalenergy instead of sonic energy.

• 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

A. Mechanical(direct) effectse.g.Particle displacement & fluid streaming:

Target particles are pushed away from the transducer acoustic streaming in fluids, cell distortion* and lysis.

Non-thermal effects 2.

gas buble expansion

gas buble compression

bubble rotation & associated fluid movement alongcell membranes

Cavitation Microstreaming

Non-thermal Effects of Ultrasound

Non-thermal effects

B. CavitationRegions 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.

2.

Cavitation Types

Cavitation may be transient or stable. Transient cavitation : very rapid expansion & violent collapse.

Causing high temp. & pressure, release of free radicals

May cause genetic damage in vitro.

Stable cavitation : bubbles oscillating with sound beam.

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)

•MI & TIS are displayed on screen.

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

Doppler image M-mode image

In routine practice :B-mode, M-mode and 3D imaging are less likely to give rise to thermal injury. >> figuresDoppler US can cause significant temp. rises.

B-mode image

Mechanical Effect

No mechanical bioeffects have been reported in humans from currently used exposure in diagnostic US.

Cavitation

Currently, 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.

US Exposure During Pregnancy

Almost 100% of fetuses in the developed world 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 color Doppler mode of the 1st trimester 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).

THANK YOU

References

•MECHANICAL BIOEFFECTS OF ULTRASOUND - Annual Review of Biomedical Engineering and the Rochester Center for Biomedical Ultrasound- D Dalecki - access with KSU password

•Article: Ultrasound safety and collapsing bubbles: http://www.highbeam.com/doc/1G1-4600240.html - access with KSU password

•[PDF] Ultrasonic imaging of the human body P N T Wells.

•Practical radiation protection in healthcare, by Martin & Sutton

•[PDF] Guidelines for the Safe Use of Diagnostic Ultrasound•Ultrasound Physics and instrumentation by Hedrick, hykes, starchman >>image•[PDF] ULTRASOUND: AN OVERVIEW OF THE LITERATURE http://www.sonochemistry.info/Research.htm•http://pain.about.com/od/testingdiagnosis/f/ultrasound_types.htm