Bioeffects and ALARA

ALARA

"As Low As Reasonably Achievable is used to reduce unnecessary, potentially hazardous exposure to individuals"

- Sonography Canada

Remember to Practice ALARA!!!

Acoustic Output Indices

• 1976 FDA set limits for thermal and mechanical output:

Cardiac-430

Peripheral vessel-20

Ophthalmic -17

Fetal imaging -94

• 1992 changed limits to 720 mW/cm² except for ophthalmic

Mechanical Effects

Attention!

No current known biological effects but certain hazards needs to be addressed to reduce risks for biological effects that could be identified in the future

Guidelines for TI and MI

• TI should not exceed 39°C ( 2°C above body temperature)

• MI should not exceed 1

Ways to Promote Safety and

Reduce Bioeffects

• License for all diagnostic devices

• Identification of correct patient and procedure

• Fetal ultrasound should be done only when benefits outweigh risks

• Sonographers needs to be certified

• Limit duration of exposure

• Quality assurance

Mechanical Effects

• Radiation force
• Direct force applied by a sound beam on an absorber or a reflector

• Microstreaming
• Rapid movement of flow of surrounding fluid
• Causes shear stresses

• Cavitation

Stable cavitation

• Bubbles expand and contract with variant pressure of sound wave

Inertial/transient cavitation

• Bubbles undergo large size changes then collapse, producing shock waves, high temperatures and light emission

• No demonstrated risk of clinically significant damage in human
• Ultrasonically induced capillary hemorrhaging in lung and intestine of mammals
• Hemorrhaging in other soft tissues if gas contrast agents are used

Thermal Index (TI)

• Proportional to potential of heating
• An estimate of maximum temperature rise which could occur
• Represents probability that ultrasound will produce biological effects on patient
• Unitless
• Normalized to a temperature elevation of 1°C

Categories of TI

• Soft Tissue Thermal Index (TIS)
• Bone Thermal Index (TIB)
• Cranial Bone Thermal Index (TIC)

Objectives

Mechanical Index (MI)

Thermal aka Heating

• To describe mechanisms of bioeffects which include thermal and mechanical
• To demonstrate ways to minimize their effects
• To relate ALARA principles to reflect on bioeffects and their safety

Waves propagate through tissues energy is absorbed

heat is produced

• MI= largest rarefaction pressure (in MPa) of sound beam /square root of center frequency (in MHz) of pulse

• Evaluating likelihood of cavitation-related adverse biological effects for diagnostically relevant exposures

• Thresholds for mechanical effects depend on
• Tissue characteristics
• Exposure duration
• Ultrasound frequency
• Pulse duration
• PRF

metabolic breakdown and cellular damage can occur

MI continued...

• No confirmed biological significant mechanical effects with a MI ≤ 1.0 in mammalian tissues without gas bodies

• Biological effects has been confirmed with diagnostic ultrasound exposure with a MI above 0.4 and a gas body contrast agent present in circulation

Introduction

“There are no known harmful effects associated with the medical use of ultrasound”

- AIUM

What Exactly is Considered Heating?

• Temperature rise surpasses 2°C
• Intensities > a few hundred mW/cm²

References

• Gibbs, V., Cole, D. & Sassano, A. (2011). Ultrasound Physics and Technology: How, Why and When. Churchill Livingstone:

Elsevier Health Sciences.

• Health Canada. (2001). Guidelines for the safe use of diagnostic ultrasound.
• Health Canada (2008). Guidelines for the safe use of diagnostic ultrasound. Retrieved from

http://www.hc-sc.gc.ca/ewh-semt/pubs/radiation/01hecs-secs255/index-eng.php.

• Hedrich, W. (2013). Technology for Diagnostic Sonography (1st ed.). St. Louis: Elsevier Mosby.
• Johnsen, E. Ultrasound-induced pulmonary hemorrhage during diagnostic examination of the lung. Retrieved from

http://mcubed.umich.edu/projects/ultrasound-induced-pulmonary-hemorrhage-during-diagnostic-examination-lung.

• Kremkau, F. (2011). Sonography Principles and instruments (8th ed.). St. Louis: Elsevier Mosby.
• Liu, H.L., Fan, C.H., Ting, C.Y. et al. (2014). Combining Microbubbles and Ultrasound for Drug Delivery to Brain Tumors: Current

Progress and Overview. Theranostics. 4 (4): 432-444.

• Longo, S. (2004). FDA Recommendations for the Safe Use of
Ultrasound in Obstetrics. Retrieved from

https://e-edcredits.com/nursingcredits/article.asp?testID=29.

• Miller, D.L. (2007). Cavitation biology and the safety of contrast-aided diagnostic ultrasound. 19th International Congress On

Acoustics Madrid.

• Olsson, J. (n.d.) Ultrasound Safety: A Review of Current Trends. Retrieved from

https://www.isrrt.org/images/isrrt/11H00%20OLSSON%20MR%2021%20SATURDAY%20SESSION%202.pdf.

• Sonography Canada (2014). Professional practice guidelines and policy statements for sonography. Retrieved from

http://www.sonographycanada.ca/Apps/Sites-Management/FileDownload/DataDownload/28291/ SC_ProfessionalPractice%20Clean%20Final/pdf/1/1033.

• Thomas R.Nelson( 2015) Ultrasound biosafety considerations for the practicing sonographer and sinologist, American institute of

ultrasound in medicine. Retrieved from http://www.jultrasoundmed.org/content/28/2/139.full.

• Umemura, S. I. Non-Invasive Treatment with Focused Ultrasound Enhanced with Nano- to Micro-Particles. Retrieved from

http://www.nanobme.org/en/summeries/summeries11.html.

Factors Affecting Heating

Exposure Time

Attenuation Coefficient

• Highest in tissues with a higher absorption coefficient bone
• Lowest where there is little absorption amniotic fluid

Longer exposure time

Greater absorption

Increased risk for biological effects

• More relevant on spectral Doppler (longer pulses, higher powers, higher PRF rates)

Intensity

Frequency

Bioeffects and ALARA in

Sonography

• Varies along beam
• Greatest at focus where beam is narrowest
• Can be altered by other operator controls (power output, scan depth, mode of operation)
• B mode < M Mode, colour, spectral Doppler

Higher operating frequency

Higher absorption

More potential to cause heat

Presented by: Elizabeth Phan, Natasha Gervais and Ying Wang