Chapter 8. Ultrasound Safety and Quality Assurance

Although ultrasound is one of the safest imaging modalities, there are safety considerations to consider. In addition to concerns about potential bioeffects, some of the concerns addressed in this chapter have to do with lack of knowledge or skill leading to incorrect clinical judgments, inappropriate use, infection exposure and control, and how to mitigate these with quality assurance plans.

A study published by Wasickanin et al in 2020¹ assessed obstetrics providers’ level of knowledge on ultrasound biosafety.

Of those providers surveyed(physicans, ob specialists and residents, nurse midwives, and advanced practice providers) 70% did not recognize output display standard functions. After education intervention there was a statistically significant improvement in scores, suggesting a need for improved education on ultrasound safety.

By the completion of this chapter the participant will be able to:

Describe the difference between x-ray and ultrasound with respect to ionizing radiation
Cite the World Health Organization’s opinions from 2013 on the safety of ultrasound in pregnancy
Define pulsed versus continuous wave ultrasound and the safety concerns of each
Accurately define thermal index and mechanical index, and know their respective safety ranges for pregnancy
Locate TI and MI indicators on the ultrasound screen
Describe the differences in high-level and low-level disinfection, and accurately describe the appropriate method for each type of transducer
Describe protocols for safe ultrasound practice
Utilize standardized templates to develop quality assurance guidelines

Types of Imaging²

The two most common forms of imaging are Xray and Ultrasound. Ultrasound has become more widely used in recent years due to its relative safety profile, but x-ray is still a necessary imaging modality for many indications. Ultrasound cannot substitute for x-ray for some diagnostics.

Key Differences

X-ray uses ionizing (electromagnetic) radiation.

Benefits of x-ray include greater definition of solid structures such as bones and joints. One definite advantage in women’s imaging, for example, is mammography, which detects solid lesions in the breast. Mammograms can detect earlier densities more easily than ultrasound. Ultrasounds can help refine a diagnosis or differentiate soft tissue abnormalities, but mammography remains the gold standard for breast imaging.

Although x-ray is generally considered safe, there has been concern that over time, with repeated exposure to ionizing radiation, there can be a cumulative effect on the body over a lifetime.

Ultrasound

Ultrasound uses sound transmission, which is non-ionizing and safer, especially for fetal tissues

Benefits of ultrasound include greater definition of soft tissue, such as abdominal and pelvic organs, and pregnancy. These are the systems most pertinent to this course. Ultrasound is also useful for imaging all other areas of the body, and very helpful in studying superficial, delicate and thin-layered tissues such as corneas, skin lesions, nerves, etc.

Ultrasound machinery is less expensive, less cumbersome, and easier to transport than typical x-ray equipment.

Some disadvantages are its limitations with diagnosing bony structures adequately, although there is current investigation on its usefulness to diagnose bone fractures.

Again, ultrasound is not necessarily a substitute for x-ray.

Safety Regulations

Despite years of research on bioeffects of diagnostic ultrasound, there are potential safety concerns given the properties of sound described in earlier chapters. Sound waves have the potential for generating mild temperature increases, as well as minute mechanical vibrations – each of which have a small potential for damage to tissues.

National and international radiology and medical societies have closely examined safety of ultrasound for the past 3 decades and have published numerous papers and guidelines.

In 1991 – the AIUM (American Institute for Ultrasound in Medicine) and NEMA (National Electrical Manufacturers Association) created “Output Display Standard”.³ This meant that all ultrasound machines are required to display on the screen the indices for thermal and mechanical effects for various functions of that machine or application. This display changes in response to changes in settings or exam types selected on the machine. so, a practitioner must monitor these indicators throughout the exam. These will be described in more detail later in this chapter.

ISUOG (International Society of Ultrasound in Obstetrics and Gynecology) issued a safety statement in 2003 ⁴ summarizing the safety of ultrasound relative to ob/gyn application.

For B-mode and M-mode: outputs are generally not high enough to produce ill effects

For Doppler:⁷

Significant temperature increases may be generated by spectral doppler, especially in the vicinity of bone
Use caution when using color doppler with a focused region of interest, since this produces the highest potential for bioeffects

For Pregnancy ultrasound:⁸

Based on current evidence, routine scanning of every woman during pregnancy using B-mode is not contraindicated
ISUOG reiterates that safety education is of utmost importance. Joint responsibility for safety is shared between manufacturers and users.

In 2013 the World Health Organization published its document describing safety and concluded the following:⁵

Use of diagnostic ultrasound generally accepted as safe at this time
Safety with advancing technology cannot rule out the possibility of ultrasound-induced adverse effects
Prudent use is justified even though there may be no concern about most applications
Obstetric applications are of most concern due to rapidly dividing and differentiating fetal tissues

Acoustic Output^6,9 is described as a gradient from the least to the most energy output. B-Mode creates the least amount of energy. Color Doppler is in between. Spectral Doppler (pulsed and continuous wave Doppler) produces the highest energy – this is because the beam is focused at a very small area of a blood vessel, causing the potential for tissue damage at that area due to intensity.

These indices can change if there is any adjustment to focal depth or an increased size of the color box. Users must periodically observe MI and TI onscreen throughout the scan, as was stated earlier.

Thermal Effects9,¹⁰

When discussing the safety aspects of ultrasound, understanding the effect of an ultrasound beam on body tissue is vital. Of primary concern is that ultrasound waves send energy into the body, most of which is converted into modest amounts of heat. Generally, this heat is dispersed through the tissue evenly. However, if focus is applied to viewing a structure, aiming that beam at the structure for an extended period can increase the temperature to a degree that may not be safe.

Figure 1. Thermal effects monitored by ultrasound operator.

The following principles are important to learn:

Thermal Index (TI) expresses the potential for a rise in temperature at the ultrasound beam’s focal point.
Relative absorption of ultrasound energy by body tissue converted to heat
Bones have the highest absorption
Tissue closer to the transducer absorbs more energy
Water or fluid-filled tissue does not absorb as much energy
The greatest rise in temperature occurs at the focal zone (where the focus of the beam is directed)
Static scanning (staying in one place with a transducer) generates more heat than continually moving the transducer
Example: holding a hot iron in place too long instead of moving it

Transducer dwell time

Temperature increases can be minimized by the ultrasound operator and perhaps the most significant way to do this is to minimize transducer dwell time. Transducer dwell time (or dwell time) is defined as the total time that a structure is continuously exposed to the active ultrasound beam during an ultrasound examination. When an image is “frozen”, this stops the transmission of the sound beam. The transducer can be held in place to hold the scanning position without unnecessary exposure.

Thermal effect also depends on beam intensity, transducer focal point and location. The closer the object to the transducer and the smaller the area the beam is hitting, the greater the energy intensity. This can happen when the focal depth is changed while scanning. The TI can also increase if the color box size is increased to see a wider view of vasculature.

The type of tissue also matters – scanning bone will generate more heat than scanning a less dense structure such as a uterus or other softer tissue. Before starting to scan, it is very Important to note the TI setting on the machine and that it is less than 1.0 if scanning a pregnant woman.

NOTE: The overall goal in ultrasound is to get the best image while using the least amount of energy in the least amount of time.

Thermal index is divided into three categories, and this is important to be aware of when scanning either an early pregnancy or a late pregnancy, where the sound may encounter more bony structures.

Types of Thermal Index indicators^9,10

TIS – Thermal index for soft tissue
- Assumes that only soft tissue is being imaged
- Usually used up to 10-12 weeks’ gestation
TIB – Thermal index for bone
- Assumes bone is present at the depth where intensity is greatest
- Adjusts settings to protect bone
TIC – Thermal index for cranial bone
- Not displayed in ob settings
- Assumes the cranial bone is very close to the probe itself

Figure 2. provides general time limits for scanning, showing that the higher the thermal index, the less time you should spend performing the exam.

Figure 2. Recommended scanning time limits based on TI.

Figure 3. Average intensity comparison by modes of ultrasound.

This illustration provides the average intensities reported by manufacturers of ultrasound scanners. It is these intensities that have the most relevance for thermal effects. ^9,15

Non-Thermal Effects

Figure 4. Mechanical bioeffects

Non-thermal or Mechanical Effects

In addition to thermal effects, ultrasound energy waves create other effects in tissue. Mechanical bioeffects include anything that is not thermal, described as the Mechanical Index or MI. Remember the principle that the sound wave causes tiny movement of particles in its path. These vibrations are minute and typically innocuous, but at the cellular level there can be significant impact.

Mechanical effects caused by the pressure of sound waves passing thru or near areas with gas or air pockets, causes tissue motion or vibration. This vibration can result in “mini-explosions” (collapse) of air or gas bubbles in tissue, and this is called ‘cavitation’. Cavitation itself is used for a variety of medical purposes, including use as a contrast agent for tumor identification. It is also used in destruction of fat cells during cosmetic procedures. In theory it is potentially harmful for delicate fetal tissue, but in the safety range used for obstetric ultrasound, there has never been evidence of real damage occurring to a fetus. In obstetrics the risk is considered potential, but awareness of that potential is necessary.

Output display

The output display is the place on the ultrasound screen that shows a numerical value for both thermal and mechanical effects.

The MI and TI are usually located at the top middle or top right of this screen, but on some machines can be found in the upper right corner or right side of the screen.

Figure 5. Output display samples.

The MI and TI are usually located at the top middle or top right of this screen, but on some machines can be found in the upper right corner or right side of the screen.

This is an example of the location of the Thermal and Mechanical Index on a handheld screen.

Figure 6. TI and MI on handheld display.

Adjusting TI or MI

Machine manufacturers must include a default output setting for TI (Thermal Index) and MI (Mechanical Index) based on the selected scan type or application. For instance, when selecting an ‘OB-1’ application for early pregnancy, the default settings are lower compared to other types of scans. These default settings can be adjusted manually if necessary. Why might that be necessary? Increasing the output can sometimes enhance image quality, though this is usually outside the typical scope of point-of-care ultrasound. However, raising the output also increases the energy transmitted into the body, which could potentially affect safety.

ALARA

ALARA is a term that is widely used in the field of imaging. It was originally used for acknowledging safety risks of x-ray exposure, but was adopted as an acronym appropriate for ultrasound as well. This acronym is the hallmark of imaging safety awareness. It is a common test question and helps one to be mindful of ultrasound safety. It stands for “As Low As Reasonably Achievable” and it has to do with energy of beams and scan time (transducer dwell time).

NOTE: This acronym needs to be memorized: It will be referred to and asked throughout this course.

As

Low

As

Reasonably

Achievable

Figure 7. ALARA- the universal imaging safety acronym.

This is a summary of how to achieve ALARA:²⁰

Use lowest power to achieve a satisfactory image
Use the least amount of time to acquire the necessary image
Use power or Color Doppler only as necessary and RARELY in first trimester
Do not use pulsed Doppler in OB since it creates the most amount of energy
TI (Thermal Index) should be < or = 1.0 Typically displayed at the top of the screen on the display

Figure 8. Diagram of ALARA process.

Ultrasound exposure and autism

In 2016 there was a publication in Autism Research²² suggesting that ultrasound in pregnancy correlated with autism. The ISUOG Safety Committee that same year carefully reviewed the evidence and found the study to have medium to poor research quality.²³ Based on available data,no scientifically proven association between ultrasound exposure in first or second trimesters and ASD (Autism Spectrum Disorder) can be determined. However, ISUOG did reaffirm that spectral and color doppler should not routinely be used in early pregnancy due to higher acoustic output. It also affirmed that pulsed Doppler may be used for some clinical indications only when necessary to make a critical diagnosis. The TI should be at or less than 1.0, with minimal exposure time

DOPPLER ULTRASOUND

Pulsed vs Continuous wave^7,8

The concept of pulsed versus continuous wave is applied differently than what was previously reviewed in the discussion of NON-DOPPLER ultrasound.

There are two types of Doppler ultrasound: continuous wave (CW) Doppler and pulsed Doppler.

Continuous wave (CW) Doppler is utilized in the handheld Doppler units used in medical offices for detecting pulses or fetal heartbeats. It is a continuous wave and it is characterized by the fact that it generates a sound.

In pulsed Doppler, there is higher ultrasound intensity, due to longer pulse and higher pulse repetition frequency than simple B-mode ultrasound. Ob settings in ultrasound machines set the thermal and mechanical indices below 1 in pulsed Doppler. The use of pulsed Doppler in the first trimester is recommended by the ISUOG Safety Committee except when necessary.²⁵

Figure 9. Pulsed versus continuous wave Doppler.

It is important to remember the following regarding Pulsed Doppler:

Pulsed Doppler (spectral, power and color flow imaging) ultrasound should not be used routinely in first trimester.
Pulsed Doppler ultrasound may be used for certain clinical situations such as to confirm a loss of cardiac blood flow in a fetal demise.
When performing Doppler ultrasound, the displayed thermal index (TI) should be less than or equal to 1.0 and exposure time should be kept as short as possible (usually no longer than 5-19 minutes, not exceeding 60 minutes).

Ultrasound for non-diagnostic (keepsake) purposes

Figure 10. Ultrasound for non-diagnostic purpose.²⁶

Keepsake imaging is something many women desire. In POCUS during first trimester, for example, the woman may want a picture of the early fetus with the heartbeat to show her partner or family. It is not recommended by FDA, AIUM, ISUOG, or other obstetric and ultrasound societies, for ultrasound to be performed for keepsake imaging alone, but there IS allowance for images to be provided during a medically-indicated exam.^26,27

SUMMARY of ALARA Principles^18-20

Table 1 represents an overview of safety guidelines by type of ultrasound, defined by international ultrasound organizations.

Table 1. Summary of ALARA principles.

Infection Control: Transducer Cleaning

Infection control is a critical concern for both the woman you are scanning and for the practitioner. Transducers should be carefully inspected before use to be certain they are free of any debris or fluid. Transvaginal probes should be covered with an appropriate sheath. Both types of transducer need to be wiped clean with a soft cloth prior to the disinfecting process in order to remove any body fluids or gel. The cord should also be wiped down.

Sidebar 1. Wiping of transducers prior to disinfecting.

Infection Control: Transvaginal Probe cover

The transvaginal probe must be covered with an appropriate sheath prior to its insertion
Commercially-available sheaths are most often used; avoid latex sheaths
Some are plain, some are pre-lubricated on the inside with a small amount of gel
AIUM states leakage through commercial probe covers is 8% to 81%
Condom – provides better protection from infection
- Avoid using a condom with an extra bulge or reservoir on the tip. It can be difficult to eliminate the air from it which cause a darkening or “drop-off” of the image
AIUM states leakage through a condom is 0.9% to 2%

NOTE: When removing the probe cover after scanning, you can invert the glove you are wearing and pull the sheath off with it. This minimizes splattering of gel and body fluids.

Infection Control: Transperineal Probe cover

This slide describes the steps used to cover the curvilinear probe while using in the transperineal approach.

Put gel on probe before applying glove
Put glove on probe and wrap glove fingers around it
Place gel on the outside of the gloved probe
Wear gloves while scanning!
When removing the probe cover after scanning, invert the glove with the used cover inside of it

The probe then must be cleaned with high level disinfectant since it contacted vaginal fluids.

Transducer Disinfection

AIUM provides detailed instructions for transducer cleaning, and these instructions need to be posted in locations where probes are cleaned and disinfected.³¹ Transducers that contact mucus membranes or non-intact skin must be cleaned with High-Level Disinfectant. Each machine manufacturer provides a list of recommended products for their equipment. After disinfecting, the recommendation is for probes to be stored in a clean utility area, or close to the ultrasound machine. Keep the clean probe covered with a cloth or keep it in a protected bin

Figure 11 shows a pyramid describing the levels of disinfection required by endocavity probes versus those used transabdominally. The transvaginal probes are to be cleaned with high level disinfection. ANY OTHER PROBE COMING IN CONTACT WITH BODY FLUIDS REQUIRES HIGH LEVEL DISINFECTION. This means that if a curvilinear probe is used transperineally, it will require high level disinfection. Since curvilinear probes are not constructed to be submerged in gluteraldehyde, they must be cleaned with a UVC machine or with chlorine dioxide wipes (described below).

Figure 11. Levels of disinfection for ultrasound transducers.

This pyramid describes the type of disinfection required by endocavity probes versus those used transabdominally. The transvaginal probes are to be cleaned with high level disinfection. ANY OTHER PROBE COMING IN CONTACT WITH BODY FLUIDS REQUIRES HIGH LEVEL DISINFECTION. This means that if a curvilinear probe is used transperineally, it will require high level disinfection. Since curvilinear probes are not constructed to be submerged in gluteraldehyde, they must be cleaned with a UVC machine or with chlorine dioxide wipes (described below).

Safe Use of Gluteraldehyde³⁴

Glutaraldehyde is one product that is useful for high level disinfecting of probes. Since it is considered toxic in its concentrated form, it is critical that anyone using this chemical be thoroughly familiar with how it is to be handled. This is an example of how to process a probe in a glutaraldehyde bath to provide a chemical high level disinfection. Each brand of this chemical has its own instructions on soaking and rinsing protocol. This chemical destroys most microorganisms.

Figure 12. Safe use of glutaraldehyde.

Since Glutaraldehyde in its concentrated form is a toxic chemical, it must be stored in nonporous containers with tight-fitting lids or in covered soaking bins. The chemical can aerosolize, so it is not recommended to have prolonged exposure to its fumes.

Exposure can occur with:

Agitating the solution while pouring
Handling instruments soaked in it
Flushing
Disposing of the chemical
Inhaling vapor

It is imperative to use PPE (personal protective equipment) while handling, including gloves and eye or face shields and have adequate room ventilation. This chemical must be replaced every 2-3 weeks (each product has a replacement regimen). Although it is toxic in concentrated form, it CAN be diluted with water, rendering it not toxic, and disposed down a sink drain.

It may be necessary to check with your state or country environmental hazard rules to see if there are other recommendations for disposal. Dilute completely, according to instructions, when disposing.

Read the ultrasound manufacturer’s guidelines for use of chemical disinfectants like glutaraldehyde – most manufacturers recommend inserting the probe all the way into the solution up to the cord. DO NOT PLACE THE CORD OR THE CONNECTOR IN THE SOLUTION – This will destroy the probe’s electrical function.

Post an instruction sheet in the dirty utility room, including emergency instructions for spills or splashes.

Post a log to document the solution disposal schedule.

In the past, either UV-C machines or glutaraldehyde solution were considered the only sufficient disinfectant methods for high level disinfecting of endocavitary (transvaginal) probes). Recently a disposable wipe with chlorine dioxide has been shown to be similarly effective.³⁶

Other Safety Considerations

Other safety considerations include regular inspection and cleaning of all parts of the machine, including:

Transducers and connections
Console
Cart and handles
Inspect for:
Non-functioning parts
Cords
Regular inspection and cleaning
Keep off the floor
Keep from draping over patients while scanning
Calibrations and preventive maintenance
Older machines require regular manufacturer maintenance; however, most new machines are constructed with solid state components and do not require this.

COVID Safety Concerns

Since the onset of the COVID-19 pandemic, all medical facilities had to develop rigid infection control policies. The following guidelines were published in response to COVID-19, but these principles can be extrapolated to other highly contagious infections.

In 2020, the ISUOG Safety Committee provided guidelines related to COVID protection for those who perform ultrasound. They addressed the following safety concerns:³⁷

Room Cleaning
Ultrasound equipment
Protection of patients and providers

Room Cleaning

This is a list of the ISUOG recommendations for room cleaning:

Clean each morning
Wipe all surfaces with LLD (low level disinfectant) including ultrasound monitor, console, rails, transducer holder, door handles, cabinet knobs, switches, chairs
Have only 1 transducer connected to the machine
Avoid unnecessary items or people in the room
Replace fabric-covered chairs
Replace all linens with disposable covers
Transducers and cables should be cleaned after each patient
Wipe bed with LLD prior to changing table paper
Wipe console and keyboard after each patient
Remove soiled linen with double gloves
At end of day, do complete terminal room cleaning

Transducer Disinfecting

Recommended steps for cleaning and disinfecting:

Disconnect
Remove cover and dispose in biohazard waste container
Remove foreign material (wiping with cloth or using running water with soap and possibly a brush). Dry before disinfecting
Clean cable/cord with LLD (low level disinfectant) wipe
Disinfect – HLD (high level disinfectant)
Refer to facility’s infection control policies
Store in clean closet or utility room

Protecting patient and providers: General COVID

To minimize spread of infection in the room or unit where ultrasound takes place:

Screen all patients and visitors
Exclude all ultrasound providers with health problems increasing their risk of contracting COVID
Assure appropriate PPE and training on use of PPE
Use latex-free disposable globes for all patient contact
Require handwashing/sanitizer between patients
Limit conversation during the exam (explain to patient)

Scheduling modifications:

Limit patient encounter time
Provide additional time between patients for cleaning

Protecting providers – suspected or confirmed patient infection

All attending staff should wear PPE (respirator), goggles, face protective shield, surgical gown and gloves) prior to entering the exam room.

Use a bedside scanner with the patient in her room when possible
Have one or more dedicated ultrasound machines for patients with suspected/probable/confirmed SARS infection
If scan is done in clinic as non-urgent, scan the patient at the end of the day
Assure adequate hand hygiene once gloves are removed
Staffing should provide a designated non-affected or non-exposed group of providers in reserve

Handwashing

It cannot be overstated that between each patient you should either wash or disinfect your hands, even if you have been wearing gloves.

SUMMARY USD Infection Control General

COVID -19 pandemic created a unique set of circumstances for how ultrasound is performed
Guidelines for extra precautions with patient contact and disinfection of rooms and surfaces have been published by various national and international organizations. Infection control begins with an awareness of how infection is transmitted and the importance of simple activities such as wiping surfaces and washing hands.

SUMMARY Ultrasound Infection Control - Transducers

Transducers that contact only intact skin surfaces need basic or low-level disinfection, as directed by the manufacturer’s guidelines
Transducers that contact body fluids must have an adequate cover or sheath and be disinfected at a high level.
Chemical disinfectants are effective, but many are toxic in their concentrated form; so great caution must be taken when using the products. Specific written instructions must be made visible in dirty cleaning areas for
Appropriate cleaning methods
Emergency procedures for inadvertent exposure

QUALITY ASSURANCE^{24, 25}

Quality assurance is a topic that is often avoided because of its perception as being disciplinary (singling out individual performance errors). However, quality assurance is a necessary way to provide the highest quality of care while recognizing areas for improvement. It has become a mandatory part of almost all medical practice currently. It should be designed to make nonbiased evaluations of individual/team performance or systems of care.

Before implementing any new medical procedure, safety can be reassured by the development of protocols, which are simply written steps or guidelines. In this section a sample protocol will be provided as a template for adaptation to a practice site using women’s health POCUS.

All medical procedures must demonstrate ongoing assurance of quality to fulfil the mission of providing the best and safest care to all women.
Guidelines for monitoring quality indicators related to standard ob/gyn ultrasound also apply to point-of-care women’s ultrasound.
Guidelines in this Section outline steps that can be adapted to your clinical situation for monitoring quality practice.
Electronic record monitoring
Log books
Requires commitment and diligence from all practitioners who perform POCUS.

Protocol Development³⁹

The SDMS (Society of Diagnostic Medical Sonography) published a document titled Scope of Practice and Clinical Standards. The following are key points listed in that document.

Safety is assured by development of protocols. Protocols establish:

Indications for POCUS exam
Who may perform POCUS exam
Required training or certification
Specific types of POCUS that may be performed (gyn, first trimester, second-third trimester, BPP, etc)
Documentation protocol
Reporting and referral mechanisms

Quality Assurance Flow Example

The following section provides samples of quality assurance protocols that can be useful as templates.

Figure 13 presents a diagram illustrating a QA process that was developed by Dowdy and implemented for a practice in the United States for nurse midwives in private practice.

Figure 13. Diagram of sample QA process.

Table 2 presents a template for a QA list that reviews metrics for both Transvaginal and Transabdominal scans

Table 2. Example of a QA checklist.

Table 3 gives a list of quality indicators, or metrics (established by ISUOG), that are used to review POCUS images used for assessing fetal biometry and growth. These provide a checklist to assure completeness.

Table 3. ISUOG Recommended Quality Review for Biometry⁴³

SUMMARY – Quality Assurance

Ultrasound is a safe imaging modality since it uses non-ionizing radiation; however, there are certain safety precautions to observe
International Ultrasound Organizations have established safety parameters for the use of medical ultrasound
ALARA is the acronym used to define minimal ultrasound exposure for patients: As Low As Reasonably Achievable
Machine safety parameters to be monitored include: thermal and mechanical indicators
Infection control assures protection for both patient and provider and includes minimal standards for cleaning environment and equipment.
Quality assurance is an essential component of an ultrasound program, and assures competency for imaging providers as well as patient safety

Foundations of Women's Point-of-Care Ultrasound

Chapter 8 Ultrasound Safety and Quality Assurance.