Ultrasound Safety 2024 PDF

Specialist Science Education Department Leeds Institute of Cardiovascular and Metabolic Medicine SCHOOL OF MEDICINE Science of Medical Ultrasound: Ultrasound Safety [email protected] Last time. Any Questions?  Algebr...

Specialist Science Education Department Leeds Institute of Cardiovascular and Metabolic Medicine SCHOOL OF MEDICINE Science of Medical Ultrasound: Ultrasound Safety [email protected] Last time. Any Questions?  Algebra review from last time  Review Sound – what have you learned?  Echo – history of word  Speed of sound waves through tissues  Propagation and interactions of sound waves through tissues  Pulse echo principle (calculations) D  Concept of echo location  Multiple reflectors  So why do we need gel? After today you should be able to  Provide an overview of safety issues/guidelines for ultrasound imaging  Name & explain 3 types of potential harm ultrasound  State which parameters impact on safety and understand why  Name and describe, including maths, 2 safety indices  Give examples of when these issues could occur (clinical applications of ultrasound imaging)  State which imaging modes these safety issues apply to  Access and understand published guidelines including safety thresholds  Explain the ALARA concept and suggest how it can be upheld Today’s Topics  Overview of Ultrasound safety  3 main sources of harm from Ultrasound  Which parameters impact on safe use of Ultrasound  Indices for safety and their thresholds  Which modes and applications are of most concern  Standards documents  ALARA  Questions to test your knowledge / further reading Ultrasound Safety - overview Recall – we discussed lack of side-effects, no investigations completed Generally no evidence of harm for decades Equipment has been used wisely only by trained operators Newer equipment is more powerful Ultrasound Safety - overview Ultrasound Safety - overview Hand-held ultrasound equipment for Point-of-Care: How do we get around non-sonographers operators using it? Safety Indices What can cause harm? Ultrasound damage can be caused by: 1. Heating – power / intensity (“thermal”) 2. Cavitation - bubble activity - pressure/frequency 3. Microstreaming - local circulation Anything not to do with heating is referred to as “non-thermal” 1. Heating (Thermal Effects) All about energy transfer Energy is absorbed in tissue With every interaction (scatter/reflection/absorption) there will be transfer of energy And can then be conducted to further tissues Transducer can heat immediate skin Heating potential varies with different imaging modes 1. Heating (Thermal Effects) Heating potential varies with different imaging modes (intensity / power dependent) aside: Contrast Agents in Ultrasound microbubbles to change acoustic properties of tissues Injected into bloodstream Can even be ‘targeted’ to biologically or chemically react with the target 2. Microstreaming Can occur when bubbles oscillate or pulsate under the pressure of the ultrasound This causes new forces, torque, radiating pressure 3. (Acoustic) Cavitation Also a response to bubbles oscillating Acoustic Cavitation - response of gas bubbles in a liquid under the influence of an acoustic wave All about activity of micro-bubbles under pressure Collapsing bubbles are dangerous This is of particular importance around areas with gas i.e. lungs In a nutshell – bubbles and pressure cause damage! (more later on contrast agents and cavitation) Non-Acoustic Cavitation For context Damage here also from implosion of vapor bubbles due to pressure variations Parameter that matter here Amplitude Power Intensity All impact on safety i.e. they determine whether of not one or more of these effects could occur Amplitude A sound wave passing through a medium causes particles to oscillate in the direction of propagation Maximum distance moved by the particle is determined by the amplitude i.e. strength of the wave Unit of measurement? This determines the strength of the sound wave used Amplitude Power Power produce by a source of sound the rate at which it produces energy [Watt] = [joule] per [sec] i.e. j s-1 How quickly can energy be produced? The more quickly, the higher the power Power can produce heat Power can produce pressure which can produce oscillations Power Intensity Intensity is the power flowing per unit area [W m-2] Presented at 90 degrees to eh direction of propagation Power can produce heat Power can produce pressure which can produce oscillations Intensity  Thermal Index Indicates risk of thermal damage i.e. potential for a tissue temperature rise There are separate indices for soft tissue, bone and cranial bone Different clinical applications have different limits Thermal Index = (Output power)/ Wdeg (power emitted by the transducer divided by power required to raise temperature by 1oC) Mechanical Index Indicates risk of mechanical biological effects (from non-thermal mechanisms, such as cavitation damage) Limits vary with clinical application Pressure / √(frequency) (peak negative pressure divided by the square root of the ultrasound frequency) Let’s look at the Standards! Your Tasks: Group 1 - Determine how much temperature elevation is necessary to cause harm. To whom could this harm and which equipment/modes are most likely to cause thermal harm? Use your own words. Group 2 - Tell your classmates in your own words under what circumstances non-thermal damage from ultrasound imaging could occur? Which index should be kept as low as possible to avoid this? Group 3 - Extract relevant TI values to give us your classmates a feel for how the safety thresholds vary from one clinical application or patient to the next Group 4 - Review Guidelines and extract relevant MI values to give us your classmates a feel for how the safety thresholds vary from one clinical application or patient to the next Use wipe boards to prepare what you will present Everyone – take note of key principles for the safe use of ultrasound i.e. general rules to live by for safety’s sake! Thermal only – there are many! Thermal and Mechanical – there are two! ALARA – what can we do? Use maximum receiver gain and minimum output power to achieve diagnostic image. Avoid pulsed Doppler in early pregnancy unless critical. Minimise dwell time. Do not perform scans with no clinical justification. Ensure that operators are adequately trained. What else? Questions to ask yourself  Provide an overview of safety issues/guidelines for ultrasound imaging  Name & explain 3 types of potential harm ultrasound  State which parameters impact on safety and understand why  Name and describe, including maths, 2 safety indices  Give examples of when these issues could occur (clinical applications of ultrasound imaging)  State which imaging modes these safety issues apply to  Access and understand published guidelines including safety thresholds  Explain the ALARA concept and suggest how it can be upheld Further Reading Safety Statements – EFSUMB (this is a link) (lots of publications there) The end of this textbook! Do not get too carried away until you learn about Doppler, contrast agent imaging, etc What you should be able to explain after today  How do sound waves propagate through tissues  (different speeds and interactions)  Demonstrate mathematically why different tissues respond differently  Pulse echo principle – as a general simple concept  Why do we need gel? You should read about this after today If you are struggling to do the above, contact me Further and Recommended Reading Chapters 1 & 2 Any other textbook (everyone is different) A few different descriptions can help